| T.R | Title | User | Personal
Name | Date | Lines |
|---|
| 452.1 | TRANSIT and GPS | PRORAT::BERENS | Alan Berens | Wed Dec 17 1986 16:55 | 17 |
| The Global Positioning System (GPS) system will be a replacement for the
existing system (TRANSIT) of navigation satellites. It will be capable
of giving extremely precise fixes (for military purposes), but for
civilian use the accuracy will be reduced considerably. At present
there are few GPS receivers available, and they cost on the order of
$20000. The 18 GPS satellites were to have been launched from the US space
shuttle by 1990. Due to the Challenger disaster, it is unknown when the
GPS satellites will be launched (they are too large to launch by other
means).
A full complement of TRANSIT satellites is in orbit, and there is a
complete set of spares. The TRANSIT system was to have been discontinued
in 1994, but now it is likely that it will be in use until 2000 or
later. Buying a SatNav (TRANSIT) receiver is still the best option if
you want to use satellite navigation.
|
| 452.2 | | GRAMPS::WCLARK | Walt Clark | Wed Dec 17 1986 17:25 | 15 |
| The Loran C net in the North sea is also exceptional (I put
several years there and the North Atlantic on our 'boomers).
My personal guess is that Loran C will be the system of choice
for coastal navigation and NAVSAT (or Transit Sattelite) for offshore
navigation, and both will be maintained well into the 90's. GPS
should be on its feet and the receivers may be price close to Loran C
receivers by then too.
Walt
(Sperry - or is it Sperr-ough now - the prime submarine navigation
contractor for the US Navy may be the only one making them now,
for the navy only of course)
|
| 452.3 | inside scoop on GPS/Loran developments | DNEAST::BELTON_TRAVI | Travis Belton | Thu Mar 15 1990 07:20 | 150 |
| Not much discussion of GPS lately. This note pulled from Flying Notes
(with permission of author), has some interesting comments on both
Loran and GPS---extension of Loran coverage across continental U.S.,
accuracy of GPS, etc.
Travis
-----------------------------------------------------------------------
<<< MEIS::PUB$:[NOTES$LIBRARY]FLYING.NOTE;4 >>>
-< General Aviation >-
================================================================================
Note 2126.8 Navigation: GPS, global positioning system 8 of 9
MAMIE::LAMIA "Real Customers buy with Real Money" 136 lines 22-JAN-1990 21:15
-< From a Usenet posting, by a "Coastie" >-
--------------------------------------------------------------------------------
From: [email protected] (Rex A. Buddenberg)
Subject: Radionavigation system developments
Message-ID: <[email protected]>
Date: 8 Jan 90 05:11:24 GMT
Organization: USCG Headquarters
Several developments in the business of radionavigation should interest
readers in this newsgroup, so what follows is a review including current
events as I understand them. Because of my position, the disclaimer:
I am the planning officer in the Electronic Systems Division in USCG
Headquarters -- that is engineering, not policy. What follows is hopefully
informed, but spiced with some opinions and explanations that are definitely
personal and decidedly not official.
Loran:
The Mid-Continent Expansion project continues. Our stated goal is
to have the FAA-mandated service up by the end of 1990. Unfortunately
we lost a contractor recently for the New Mexico station and on-air service
will probably slip a few months. This project entails building 4 new
transmitting stations, 5 new monitor stations and dual-rating of 5
existing stations. The result will be expansion of the existing
coverage (which provides coastal coverage around North America and
incidentally terrestrial coverage over about half the continental
United States) to complete coverage of the continental US and significan
increases in coverage in Canada, especially over the population
concentrations in the southern tier.
GPS:
Recent policy change by the Joint Program Office will result in a
reconfiguration of the constellation. Currently the satellites are bunched
so you get complete 3-dimensional coverage part of the day and nothing
during other parts. Great for R&D, poor for operational use. The change,
plus a few more satellites launched (need twelve total, if I recall correctly)
will provide 2-d coverage continuously by the end of 1990 (although the
system will not be declared operational for some months after).
Coupled with the advent of reasonably priced receivers, this will make
GPS a viable system for real users fairly soon.
Zap-Sat. One of the defects in GPS that the FAA has been vocal about
for some time is the inability to block a sick satellite's signal quickly.
System integrity problem. JPO has agreed to field a monitor system that
will detect satellite bad data and scramble the 'gold code' which has
the effect of making a user's receiver drop the satellite from his nav
equation entirely. Not sure what the schedule for this is, but the
policy change was announced last month.
Loran/GPS integration. For aircraft use, system integrity is the critical
unfilled requirement -- both Loran and GPS are inherently accurate enough
for airway navigation and non-precision approaches. The problem is that
neither system is robust enough to shield the user from intermittent failures
at least some of the time. This is not a major problem for the mariner as
ships don't go that far in a minute of bad data, but an aircraft on approach
is a different story.
Part of the solution is user notification -- unannounced bad data is
worst of all. The Zap-Sat noted above brings GPS to the place where
Loran has been for years in this area (although we must tighten the
grace time up over the next few years).
The other part is to make Loran and GPS interoperable so that failures
in one system degrade, but do not destroy, the positional integrity.
In order to do this, Loran master stations are being synchronized more closely
to UTC than previously. By changing operating procedures, we feel we have
reliably reduced the variance from 2.5 useconds to 0.5 useconds. Engineering
changes in the time transfer mechansim will be required to reduce this
variance further to the goal of 100 nseconds. This should happen over
the next few years -- active engineering project now.
Availability numbers. Both Loran and GPS provide availabilities in the
neighborhood of 99.99, or 10e2 (the exponent equals the number of 9s after
the decimal point). Aviation folks are saying they need 10e8 [!!] kinds of
figures for the system to work right, safely, and to retire other obsolete
and costly radionav systems (VOR/DME). If the user (aircraft) has both
a Loran and a GPS receiver (no integration), then he boost his availability
figures a couple orders of magnitude -- to around 10e4. With the time
synchronization, then the user can take all of the data from both the Loran
stations (several chains worth -- anything he can track) and all visible
satellites. Throw them all into a Kalman filter and throw away any bad
data (sick satellites or Loran transmitters). This gets one or two more
orders of magnitude -- the analysts that I've talked to indicate 10e6 is
realizable without major changes to either of the systems (expensive).
Differential GPS:
dGPS operates by placing a (slightly specialized) GPS receiver over
a surveyed benchmark. This reference receiver tracks the GPS satellites
and compares where the satellites tell him he is and where he knows he is.
This <differential> is then communicated to a user 'in the neighborhood'
who applies it to what the satellites are telling him.
This technique (which also works for Loran) will correct GPS fixes
to within 10 meters (2dRMS). Current CG research is focussing on how
far away 'in the neighborhood' is with estimates running from 300NM to
3000NM (at which point you effectively fall off the edge of the earth).
While there is no requirement to provide public differential service,
the Coast Guard has in-house requirements (buoy tender positioning) and
we are talking with the Navy's mine warfare folks. (We do, informally,
have a weather eye out expecting that public requirements will eventually
get stated and validated -- so we are considering this eventuality in our
engineering.)
Differential techniques correct for several errors in the GPS signals
including orbit wobbles, clock drift ... and selective availability.
GPS selective availability:
In order to frustrate enemy use of GPS positioning, DoD invented
a concept of selective availability -- the satellite 'lies' about its
time base. This degrades the position to 100 meters unless you are
an 'authorized user' and have the crypto code to correct for the
satellite's lying. if you have this p-code, then you get positioning
accuracy to 18 meters (all these figures are 2 sigma).
Selective availability has aroused a lot of controversy for several
years and it seams to be drawing to a head for several reasons:
- we've demonstrated that we can defeat selective availability
by differential techniques. If a bunch of dumb Coasties can do it,
Ivan can do it.
- who follows the minesweeper (authorized user) through the swept
minefield? The convoy of merchant ships is who -- all unauthorized users.
These guys need the same reference and the same accuracy as the minesweepers
lest they start sweeping mines themselves (any ship can be a minesweeper...
once).
- would any Ivan in his right mind use the yanks' positioning system
to target ICBMs? The soviets we've talked to laugh uproriouly over that one.
There is some interesting history in the Loran world that is instructive.
Initial Loran deployments were for DoD use and the Coast Guard maintained
a reconfiguration schemes for wartime use. Our DoD customers (like FBM
submarines) has Loran tables so they could use the reconfigured chains.
After a few years, it became apparant that our sealift and especially
our airlift folks would also be dependent on Loran, so the base of users
needing the reconfiguration plans expanded by an order of magnitude. Then
the burgeoning commercial, allied and neutral user base started to get
considered. The last Security of Aids to Navigation (SCATANA) plan was
unclassified and said: don't change anything... My opinion is that
GPS selective availability is heading down this same road.
Shipboard developments.
I'll save this discussion, which is an active topic, for another
posting. Since the Coast Guard is both a service provider and a navigation
service consumer, we have a unique position in the business where we can
influence both. Look for a little feedback before I compose this one.
Rex Buddenberg
|
| 452.4 | My GPS Accurate to +/1 100 ft. | CARLSN::WACKER | | Tue Feb 26 1991 09:52 | 26 |
| Just wanted to add to .0 about GPS accuracy NOW with
Selective Availability turned off.
I have had my Trimble TransPak hand held GPS receiver for two
weeks now and when I put it into STATUS mode it tells me that the accuracy
is +/- 100 ft. There are times when it falls back and tells me that
the accuracy is +/- 300 ft. This takes place when one or two of the
three or more satellites it is tracking are low on the horizon.
Just to check it out I took a fix in my car in the DEC parking
lot and stored it as waypoint 1. I drive home with GPS receiver on
the dash giving me 1 second updates as to my speed, heading , bearing and
distance from way point 1. Get home enter waypoint 2 as the fix while
sitting in my driveway. Next day, put in NAV mode , to WPT 1 from WPT 2 ,
the unit gives me Heading, Range & Bearing to WPT 1 , crosstrack error left
or right, speed, eta and time to go.
When I get to the Same parking space as the day before the unit
reads the SAME Lat/Long and Range +/- 0.003 miles, that's +/- 16 ft. I
am running it in land mode now, also has sea or air modes.
I am as HAPPY AS A LARK with it.. Can't wait to use it on the
boat. Happy Navigating.....
Hank, (:-)
|
| 452.5 | nifty device | MSCSSE::BERENS | Alan Berens | Tue Feb 26 1991 12:23 | 25 |
| re .7:
>>> ... when I put it into STATUS mode it tells me that the accuracy
>>> is +/- 100 ft.
This is not quite the same as actually being accurate to +/- 100 feet.
It is more of an advertising claim, one which may or may not be true.
After all, a loran displaying lat/long in deg.min.sec is in effect
claiming to be accurate to about 100 feet (a claim I doubt any of us
would believe). I will certainly not argue that the error is small,
however.
>>> When I get to the Same parking space as the day before the unit
>>> reads the SAME Lat/Long and Range +/- 0.003 miles, that's +/- 16 ft.
This simply says that its measurements are highly repeatable, which is
certainly good. The position it gives could still be in error by
considerably more.
>>> I am as HAPPY AS A LARK with it.
I would be too. How much does Trimble want for one of these little
marvels?
Alan
|
| 452.6 | More on Accuracy of GPS | CARLSN::WACKER | | Tue Feb 26 1991 17:22 | 103 |
|
Re: .8 GPS accuracy read-out
The STATUS mode on this unit tells me several things....
1: Tracking: Number of satellites (sv's) it is tracking.
This unit will track up to 7 sv's. It needs 3 SV's to
do 2D fixes and 4 SV's for 3D fixes. ( This unit is a
three-channel receiver that can track two SV's on two
channels while sequencing on the third channel. )
2: Current State Display:
OK - Indicates the unit is making position fixes.
N/A- Indicates insufficient SV's are visible
to make a fix.
BAD- Indicates either that sufficient SV's are
available but they are positioned such that
a fix is not possible or that a receiver
error has occurred.
Estimated Accuracy- +/- xxxx number of ft.
More on that in a min.
Satellite Availability- Indicates the window
of satellite availability. ( only 22.5 hrs
coverage in my area at present ) If you are
presently within a window of availability,
( sufficient SV's for a fix ) presents the
amount of time remaining in the current GPS
window and the start of the next window.
I/E ... REM 2:30 NEXT 14:35
If you are not now within such a window it
shows the starting time of next window and
its duration. (I/E NEXT FIX 14:35 22:40)
More on Estimated Accuracy:
This read-out is calculated base on some fixed data
stored in the unit and variable data received by the unit.
The ultimate accuracy of GPS is determined by the sum of
several sources of error. The contribution of each source
may vary depending on atmospheric and equipment conditions.
In addition, the accuracy of GPS can purposefully be degraded
by the D.O.D. ( RE: Base Note ) This Selective Availability
Error when implemented will be the largest component of GPS
error. The following is a list of TYPICAL ERROR RATES..
Error Source Error
Satellite clock error 2 ft.
Ephemeris error 2 ft.
Receiver errors 4 ft.
Atmospheric/ionospheric 12 ft.
Worst case S/A(if implemented) 25 ft. ! off at present
Total ( root-square sum ) 15 to 30 ft.
The above is sort of the FIXED data...
Now to calculate ACTUAL error !! Multiply the total above
( 15 to 30 ) by the GDOP ( Geometric Dilution of Precision )
GDOPs under good conditions range from 4 to 6. SO the total
accuracy that you can expect would be ..
Typical - good reviver 60 - 100 ft.
- worst case 200 ft.
- if S/A on 350 ft.
More on GDOP ; To get the best possible accuracy this unit
takes into account a subtle principle of geometry called
Geometric Dilution of Precision. It refers to the fact that
your fix solutions can be better or worse depending on which
satellites you use to make a position measurement. Not that
one satellite is better than another. It's just that depending
on their relative angles in the sky, the geometry can magnify
or lessen all the error rates mentioned above. Same as when
taking a visual fix, if the two or three objects are close
together the relative error could be large, but if the objects
are far apart, lets say >~90 degrees, the better the fix.
This GPS receiver has routines which analyze the relative
positions of all the satellites available and choose the four
best candidates. Those best positioned to to reduce GDOP are
then used to calculate a fix. The GDOP times the other listed
errors gives the read-out of +/- xxxx on the unit.
As you said this isn't an absolute +/- error factor but it
is a best guess error factor, ball park error.
To find out its actual accuracy I will post the readings of
the GPS unit verse chart Lat/Lon fixes on a charter in the Keys
coming up April 1 - 7 , OR if I get to the local map store
and get a U.S. Geological Survey map of the city and compare
known Lat/Lon positions on map with GPS unit read-outs.
Cost: LWL * Draft * Beam / sqrt Disp.
From $ 2600.00 to $ 3800.00 depending on what marina/shop
you call. In short supply because of Gulf War.
Hank (:^)
|
| 452.7 | Chart verses GPS Receiver | CARLSN::WACKER | | Fri Mar 01 1991 11:35 | 27 |
| Some feedback based on my hand held GPS receiver.
I was able to get a U.S. Geological Map of my area. Charted two
fixes in my sub-division. Took the GPS reveiver to thoes locations
and let the receiver sit there for 2 or 3 min.'s Here is what I
came up with....
Chart Data GPS read-out Error
First test location Readings Between
N38 07.615' N38 07.609' & .613' - 36 to 12 ft.
W85 38.000' W85 37.997' & .999' - 18 to 6 ft.
Second test location Readings Between
N38 07.511' N38 07.507' & .514' +/- 24 ft.
W85 38.188' w85 38.186' & .192' +/- 36 ft.
The accuracy of my chart fix could be off because of lack of calibrated
eye-balls. The first location with its long. of W85 38.000' is right
on the money as it was marked on the chart and a line up from it
gave me the exact spot on the street where another street intersected.
This chart was a 1 to 24,000 so the scale wasn't too bad.
Trimble clames its receivers to be accurate to 15 meters. So far
so good. Will post my findings of accuracy of this unit after my
Fla. Keys sailing trip.
Hank
|
| 452.8 | Fla. Trip Report | CARLSN::WACKER | | Mon Apr 08 1991 11:56 | 12 |
| This is the follow-up report I said I would give after my charter
in the Fla. Keys. This is a report on the accuracy of the Trimble
portable hand held GPS reveiver.
Took locations that had fixed markers that had there Lat/Lon
marked on the Chart Kit Chart, checked it with dividers, entered
it as a way point. Sailed to several of these positions/locations
and found that the worst error factor was 0.043 min's. off. In other
cases it was less than 0.020 min's. of the actual tower/day marker.
That makes the worst about 260 ft. off, and the others within 120
ft.
That's the way it was 4-1 to 4-6-1991.
hank
|
| 452.9 | as always, be cautious | MSCSSE::BERENS | Alan Berens | Wed May 22 1991 16:27 | 16 |
| re .7:
According to an article in the latest issue of Ocean Navigator, you need
to be very cautious about comparing the lat/long from a GPS or loran to
the charted lat/long of a bouy, ledge, harbor entrance, etc. It seems
that charts are based on various datums (reference points), some dating
back to 1927 and even earlier. Not all of these datums agree with each
other. Loran and GPS receivers assume some datum, and that datum may not
be the chart datum. The error from this is quite small on the US east
coast, but is on the order of 100 to 200 yards in Alaska and Hawaii, for
example. In addition to this, many charts are based on old surveys (some
of my 1980s Nova Scotia charts are drawn from surveys made in the
1850s), and charted positions could well be in error. The error from
using different chart datums is easily corrected in a loran by simply
entering the proper TD corrections. I don't know if something similar
can be done with a GPS receiver.
|
| 452.10 | PRONAV comments | MAMTS3::HAJOHNSON | | Wed May 22 1991 22:04 | 19 |
| I bought a PRONAV earlier this year and have been pleased with it so
far. I have used to AUTOSTORE feature to program my harbor and it is
quite able to return me to these points.
When using it for route planning, I assume nothing; e.g. the buoys are
always right where they are charted. I chose the LAT/LONG near the
mark but with any error on the favored side of the channel/creek.
So far I have used the unit on two full weekends and had only about 20
minutes of poor coverage at a time. I am only using the small antenna
provided on the unit itself. Some of the features. like the anchor
watch, cannot be used due to the alarm going off when coverage is lost.
I sailed for over ten years without any sophisticated aids to
navigiation of the electronic variety. I could live without it and so
far the only negative is the constant reminder from my "partner" about
the 1800 cost.
|
| 452.11 | Built in Datum ( 44 ) | CSOA1::WACKER | Hank Wacker | Fri Aug 16 1991 03:21 | 9 |
| re .9
Just found that .7 & .8 had been moved here.
About datum and my Trimble Transpak portable GPS receiver.
This unit has built into its setup display a selection of
44 datums to pick from. The chart kit I was using was based on the
NAD-27, CONUS datum, so thats what I selected in the setup.
All seemed to be working just fine.
|
| 452.12 | Handheld GPS w/computer interface? | CSC32::VISAGE | | Tue Apr 13 1993 14:58 | 6 |
| Is anyone aware of a handheld GPS receiver capable of feeding data to a
computer? Ideally the interface would be something relatively
standard and easy to come by such as RS232.
Thanks,
Jerry
|
| 452.13 | Open up that expansion slot! | MILKWY::WAGNER | Scott | Wed Apr 14 1993 12:17 | 13 |
|
I haven't seen any connectors that big in a handheld, but I've only
seen a few units-
I _have_ seen advertised, a card that you slide into one of the
slots in your PC, which gives you GPS. Maybe even cheaper- should be,
since you are providing power, display and weathertightness...
It would be REALLY neat as a hardwired part of a laptop (sys.
integrators take note) for not only sailors, but anyone who's on the
road a lot- a location stamp, on top of a time stamp, think of that!
Scott
|
| 452.14 | Coast Guard GPS/LORAN BBS | CSC32::VISAGE | | Thu Apr 15 1993 13:01 | 12 |
| In looking into information about GPS I ran across the fact that
the Coast Guard is running an electronic BBS with all sorts of
information about GPS and LORAN.
The BBS phone number is (703)313-5910. They advertise 2400bps,
I didn't try any higher speeds.
I'm going to post an article as the next reply that I pulled off the
BBS that I think will be of particular interest to the followers of
this conference. It is a rather lengthy article on the Coast Guard's
Differential GPS program. (aprox 900 lines)
|
| 452.15 | The Coast Guard's Differential GPS Program | CSC32::VISAGE | | Thu Apr 15 1993 13:02 | 915 |
| The Coast Guard's
Differential GPS Program
D. H. ALSIP, J. M. BUTLER, and J. T. RADICE
USCG Headquarters
Office of Navigation Safety and Waterway Services
Radionavigation Division
June 29, 1992
ABSTRACT
The U.S. Coast Guard has a project to provide Differential Global
Positioning System service for harbor and harbor approach (HHA) areas of
the coastal United States. The Great Lakes, Puerto Rico and most of Alaska
and Hawaii will also be covered by the service. The Coast Guard's DGPS
system will fulfill the 8-20 meter navigation accuracy requirement for HHA
with an availability of up to 99.9%. The Coast Guard intends to provide
this service to the general public and other government agencies, as well
as use the system for its own missions. This capability is expected to
enhance maritime safety in keeping with the National Transportation Policy
by providing an all-weather radionavigation service to supplement existing
radar and visual techniques, as well as a highly accurate position sensor
for future electronic chart displays. This paper describes the Coast
Guard's program. Background and historical information on the development
of pseudorange differential GPS is presented first, followed by a
description of currently available technology. Various aspects of the
Coast Guard's plan for implementing DGPS are then described, concluding
with a rough project time line and a statement concerning Federal DGPS
policy.
INTRODUCTION
In order to aid navigation and to prevent disasters, collisions, and wrecks
of vessels and aircraft, the Coast Guard is charged under 14 USC 81 with
establishing, maintaining, and operating electronic aids to navigation to
serve the needs of U.S. armed forces, maritime commerce, and air commerce
(as requested by the Federal Aviation Administrator). Starting back in
1921 with the introduction of radiobeacons [1], the first electronic aid to
navigation, and continuing through the development of Loran-A, Loran-C, and
Omega, those responsibilities have been met to the extent that technology
would allow. With the introduction of each new system, navigators were
able to improve their efficiency while increasing the safety of navigation.
Electronic or radio aids provided all-weather navigation services that met
the accuracy requirements for ocean crossings through Omega and for
off-shore and coastal navigation using Loran-C. But these systems did not
meet the required navigational accuracies of 8-20 meters for harbor
approach and harbor navigation as described in the Federal Radionavigation
Plan (FRP) [2]. To meet these needs, differential techniques were
developed to provide corrections to the existing signals, resulting in much
higher accuracies. Due to the nature of both spatial and temporal
propagation anomalies in Loran and Omega signals, differential corrections
were valid for only a small area, requiring extensive ground equipment to
provide coverage for all harbors and harbor approaches. It was not
economically feasible to build such an extensive system. Also, the
accuracies achieved with these systems fell short of most requirements.
Differential Omega achieved approximately 1/4 mile accuracy, and
Differential Loran-C could do no better than approach 20 meters accuracy
for limited periods of time.
==== Table 1 ====
ACCURACIES OF GPS SERVICES VS. REQUIREMENTS
----------------------------------------------
| REQUIREMENTS | H/HA | Aids | VTS | NOAA |
---------------------------------------------------------------
| Accuracy | 8 - | 10 | 10 | 60 |
SERVICES | (meters 2drms) | 20 | | | |
---------------------------------==============================
SPS with S/A | 100 || NO | NO | NO | NO |
---------------------------------------------------------------
SPS without S/A | 30 || NO | NO | NO | YES |
---------------------------------------------------------------
PPS | 17.8 || NO | NO | NO | YES |
---------------------------------------------------------------
DGPS | < 10 || YES | YES | YES | YES |
---------------------------------------------------------------
H/HA: Harbor/Harbor Approach
Aids: Aids to Navigation Positioning
VTS: Vessel Traffic Services
NOAA: NOAA Offshore Surveying
==== (end of table 1) ====
Once again, technological advances have provided a new method of meeting
navigational requirements through the development of the Global Positioning
System (GPS). Developed and operated by the U.S. Department of Defense,
GPS provides two levels of service: Standard Positioning Service (SPS) and
Precise Positioning Service (PPS). While SPS accuracy can be better than
30 meters (2drms), with the activation of Selective Availability (SA) that
accuracy is degraded to 100 meters (2drms) for civil users. Nevertheless,
GPS provides all-weather global coverage, 24 hrs/day at unprecedented
accuracies. PPS will provide 17.8 meter (2drms) accuracy to military and
approved civil users. Yet even GPS, with its remarkable accuracies, still
does not meet the needs of harbor and harbor approach navigation. But by
applying differential techniques, as used with Omega and Loran systems,
navigational accuracies of better than 10 meters can be achieved. For the
first time, an all-weather system is possible to meet all the marine
navigator's needs. Table 1 outlines the accuracies required by four
maritime applications versus the ability of the various levels of GPS
service to meet those needs.
In addition to providing a highly accurate navigational signal, DGPS also
provides a continuous integrity check on satellite health. System
integrity is a real concern with GPS. With the design of the ground
segment of GPS, a satellite can be transmitting an unhealthy signal for 2
to 6 hours before it can be detected and corrected by the Master Control
Station or before users can be warned not to use the signal. But with the
continuous, real-time messages generated by DGPS, unhealthy satellites can
still be used, or the navigator's receiver is directed not to use a
particular satellite. This can eliminate the danger of the navigator
relying on an erroneous signal.
In addition to the use of DGPS for harbor and harbor approach navigation,
DGPS will serve a number of other needs:
Integration of precise navigational information from DGPS with the radar
picture and electronic charts will revolutionize maritime navigation safety
and efficiency.
Coast Guard units can position aids to navigation with DGPS in less than
one tenth of the time that manual methods require.
Hydrographic surveys by the Coast and Geodetic Survey can be done faster
and more accurately.
Using DGPS coupled with transponders, Vessel Traffic Services in major
ports can use dependent surveillance to monitor and control port traffic.
Underwater mine disposal teams can increase efficiency and safety of
operations through precisely locating and returning to the mine site for
disposal.
Rescue Centers can vector search and rescue units to a distressed vessel.
Ice breaking activities can be carried out in a more accurate and efficient
manner.
There are numerous other applications for DGPS, including a myriad of land
uses in surveying, mapping, tracking forest fires, etc. As with other new
systems, users always find innovative and often surprising applications.
The unprecedented accuracies that DGPS will provide will yield applications
that will revolutionize industry, services, and operations.
HISTORY OF DOT/USCG DGPS RESEARCH
After GPS was made available to the world by Presidential Proclamation in
1983, the Department of Defense proposed that the SPS accuracy offered to
the civil users would be at the 500 meter (2drms) level (later revised to
100m). Since 500m provided no better accuracy than existing
radionavigation services (and in some areas less accuracy), the Coast Guard
Office of Research and Development (R&D) initiated a project to investigate
Differential GPS. Investigation into DGPS had already been undertaken with
testing at the Yuma Proving Ground in 1980 [3] and NASA sponsored research
into the concept in 1981 [4-7]. In 1983, Coast Guard R&D and the
Department of Transportation's (DOT) Transportation Systems Center (TSC)
cosponsored research into DGPS for the purpose of determining achievable
accuracies and developing system design alternatives [8-9]. The research
examined various methods of computing corrections, methods of transmission,
the theoretical applicable range of the corrections, in addition to the
format and frequency of the corrections. The study concluded that
differential corrections could improve the accuracy of positioning using
SPS to about 14 meters (2drms), recommended an update frequency of 28
seconds for high-accuracy marine applications, and determined that manual
entry of the DGPS corrections while SA is activated is infeasible.
To investigate requirements for the DGPS correction message and make
recommendations for a standard format, a workshop was held at TSC in June
1983. The workshop produced a strawman format that was flexible to
accommodate various communications systems and robust enough to provide a
high-accuracy and reliable service for a variety of air, marine and land
users.[10] The Radio Technical Commission for Maritime Services (RTCM)
followed through by establishing Special Committee (SC) 104 in November
1983. Coast Guard and TSC personnel worked closely with this committee in
the development of the recommended standards. By 1986, a set of
recommendations was made available [11]. The recommended standards
considered the widest possible uses of DGPS, not just marine navigation, to
provide a format with flexibility as well as uniformity. The committee
also examined the communication of the corrections to the users. Based on
data supplied to DOT by the GPS Joint Program Office concerning SA
variations, a transmission rate of 50 bits per second was determined to be
the minimum transmission rate for differential corrections. The committee
went on to review the radio frequencies allocated in the U.S. for
radionavigation and evaluate their suitability for use with DGPS. The
committee concluded that the radiobeacon band of 285-325 Khz was the only
band that met the needs of DGPS for radionavigation use without requiring
changes in international frequency allocations. This conclusion was
supported by the results of a 1984 Radiobeacon Data Link Workshop held at
Coast Guard Headquarters [12]. This workshop evaluated the suitability of
radiobeacons for DGPS and developed a preliminary design for test and
demonstration hardware. The radiobeacon network operated by the Coast
Guard promised to be a convenient conduit for the DGPS corrections. The
beacons were already located in sites where marine navigators needed
coverage, the effects of blockage and multipath were small in this band
compared to higher frequencies, radiobeacons had already been used
successfully for other differential navigation applications, and the range
of the radiobeacon signal roughly corresponded to the applicable range of
the DGPS corrections.
Building on research into the design of a DGPS reference station by TSC
[13] and a DGPS communications link by USCG R&D [14], in 1987, the Coast
Guard Research and Development Center conducted field tests of DGPS using
the recommended RTCM SC-104 data format and a VHF data link at 50 bits per
second. The Coast Guard field tests (described in [15]) demonstrated that
the navigation requirements for harbor and harbor approach areas can be met
with DGPS. In each test, the DGPS positions were accurate within 10 meters
and usually were shown to be better than 8 meters. The testing also
validated the capabilities of the RTCM SC-104 format and the 50 bits per
second data link. In 1989, the Coast Guard tested the use of a marine
radiobeacon located at Montauk Point, Long Island for the transmission of
DGPS signals. This initial test was only temporary, but its success
prompted the need for further operational testing. During this time frame,
further refinements were made to the SC 104 recommendations and in January
1990, the RTCM published a document describing SC-104 format version 2.0
[16].
Starting on August 15, 1990, Montauk Point began transmitting the first
prototype DGPS service publicly available and one of the first operations
of DGPS using a marine radiobeacon as the broadcast facility. The
prototype and operational testing are described in [17]. Results of this
testing demonstrated that marine radiobeacons can be used effectively for a
public DGPS service. The DGPS service is suitable for radio positioning
applications, as well as radionavigation, and the DGPS reference station
and broadcast equipment are easy to install, maintain, and operate.
DGPS TECHNOLOGY
Differential GPS is based upon knowledge of the accurate geographic
location of a reference station, which is used to compute corrections to
GPS parameters, error sources, and/or resultant positions. These
differential corrections are then transmitted to GPS users, who apply the
corrections to their received GPS signals or computed position. For a
civil user of SPS, differential corrections can improve navigational
accuracy from 100 meters (2drms) to better than 10 meters (2drms). A DGPS
reference station is fixed at a geodetically surveyed position. From this
position, the reference station tracks all satellites in view, downloads
ephemeris data from them, and computes corrections based on its
measurements and geodetic position. These corrections are then broadcast
to GPS users to improve their navigation solution. There are two
well-developed methods of handling this:
Computing and transmitting a position correction in x-y-z coordinates,
which is then applied to the user's GPS solution for a more accurate
position, and
Computing pseudorange corrections to each satellite, which are then
broadcast to the user and applied to the user's pseudorange measurements
before the GPS position is calculated by the receiver, resulting in a
highly accurate navigation solution.
The first method, in which the correction terms for the x-y-z coordinates
are broadcast, requires less data in the broadcast than the second method,
but the validity of those correction terms decreases rapidly as the
distance from the reference station increases. Additionally, this method
requires that both the reference station and the user compute the
navigation solution using the same set of satellites, when that is not
necessarily the case.
Using the second method, an all-in-view receiver at the reference site
receives signals from all visible satellites and measures the pseudorange
to each. Since the satellite signal contains information on the precise
satellite orbits and the reference receiver knows its position, the true
range to each satellite can be calculated. By comparing the calculated
true range and the measured pseudorange, a correction term can be
determined for each satellite. These corrections are then broadcast, and
the corrections are applied to the pseudorange measurements of the
satellites received at each user's location. This method provides the best
navigation solution for the user and is the preferred method. It is the
method being employed by the Coast Guard DGPS Service. Figure 1 shows the
system concept using pseudorange corrections. The reference station's and
the mariner's pseudorange calculations are strongly correlated.
Pseudorange corrections computed by the reference station, when transmitted
to the mariner in a timely manner, can be directly applied to the mariner's
pseudorange computation to dramatically increase the resultant accuracy of
the pseudorange measurement before it is applied within the mariner's
navigation solution. The major elements of a DGPS system are briefly
described below:
Reference Station. Generates corrections to pseudoranges for users within
its coverage area. This consists of a high quality, all-in-view GPS
receiver with the additional computational capability to generate RTCM
messages for the broadcast and to communicate with a remote control system.
It performs an integrity check on GPS satellites. It also accepts a "watch
dog" signal from its integrity monitor, and is prepared to broadcast an
integrity warning in the event of loss of monitoring, or when ordered by
the integrity monitor. Co-location with the broadcast transmitter as
depicted here is not required. However, the Coast Guard's system design is
proceeding under the assumption that reference stations will be co-located
with broadcast transmitters.
Broadcast Transmitter. A marine radiobeacon that has been modified to
accept MSK modulation [18]. Real time differential GPS correction data are
input in the RTCM SC-104 format and broadcast to all users capable of
receiving the signals. The Coast Guard does not plan to use data
encryption. Radiobeacons were chosen because of existing infrastructure,
compatibility with the useful range of DGPS corrections, international
radio conventions, international acceptance [19], commercial availability
of equipment and highly successful field tests. [17, 20-27]
Control Station. Two computerized control systems operated by live
watchstanders will be established, one on the East and the other on the
West coast. Each will be connected to all Monitor Stations in its area by
a dedicated data communications network and to all Reference/Broadcast
sites in its area by dial-up data communications. The Control Stations
perform system level monitoring and configuration control of the data
communications network and equipment at the individual sites. Each station
will be capable of handling the entire network in an emergency, or for
maintenance purposes.
Integrity Monitor. This will consist of an MSK receiver, DGPS receiver and
a computer. The computer will provide a check on the GPS broadcast, the
DGPS correction data and the MSK broadcast signal. There will be one of
these for each Reference/Broadcast site. The integrity monitor will be
connected to the GPS reference receiver with a real time landline data
link, and it will continuously inform the reference receiver of the status
of the broadcast as seen in the coverage area. Integrity monitors will be
placed in the outer 10% of the coverage area and where possible, will
define the outer bound of the coverage area.
Shipboard Equipment. Ships must have an MSK/Radiobeacon receiver, a DGPS
receiver, and a chart display. The DGPS receiver is a GPS receiver that is
capable of incorporating DGPS correction data.
The Coast Guard's DGPS system will broadcast corrections to the user in the
RTCM SC- 104 format [16]. The RTCM has defined data messages and an
interface between the DGPS receiver and the data link receiver. Several
different messages were defined, some "fixed" and some "tentative". The
fixed messages are:
Type 1 Differential GPS Corrections. This message contains the
pseudorange corrections (PRC) and range-rate corrections (RRC) for all
satellites in view of the reference station. The message also indicates
the nominal time (shown below as t0) for which this data was valid. The
user computes the current differential correction as follows:
PRC(t) = PRC(t0) + RRC (t-t0) ,
where PRC(t0) is the PRC value in the type 1 message. The user then
applies the PRC by adding it to their pseudorange measurement. The RRC is
included in an attempt to extend the life of the PRC, as the RRC is a
"rate" term which is used to propagate PRC's in time.
Type 2 Delta Differential GPS Corrections. Special Committee 104
considered that oftentimes a reference station may update its ephemeris
earlier than the users. This message provides "delta" PRC's and RRC's for
each satellite. The user applies them if its "issue of data" is different
than that indicated in the type 1 message, but identical to that indicated
in the type 2 message. The delta corrections are added to corrections
found in the current type 1 (or type 9) message. The reference station
will broadcast type 2 messages for the first several minutes after a change
in satellite ephemeris data. Because this message adds considerable
latency to the corrections that can be applied by the user, a review of the
need for this message is being conducted. A cost/benefit analysis will be
conducted, considering the additional latency (and attendant degradation of
accuracy) imparted by this message vs. the additional accuracy it provides
on its own.
Type 3 Reference Station Parameters. The Earth-Centered-Earth-Fixed
(ECEF) coordinates of the reference station with a resolution of 0.01 meter
are found here. This message will nominally be broadcast every five
minutes. User derived atmospheric corrections are possible through use of
this message type.
Type 6 Null Frame. This message is used to maintain data link
synchronization in the event there are no other RTCM messages to transmit.
In the operational GPS scenario, transmission of this message will be rare
indeed.
Type 9 High Rate Differential GPS Corrections. This message is similar to
the type 1 message, but individual type 9 messages will be generated only
for those satellites with high pseudorange rates. This is likely to be
needed with "operational" S/A, when one or several satellites exhibit very
fast acceleration due to S/A effects. The use and frequency of this
message is the subject of current research and it may fully replace the
type 1 message.
Type 16 Special Message. This is an ASCII message up to 90 characters
long. It can be sent by service providers to broadcast warning
information, such as scheduled outages. User equipment should have the
ability to display this information to the navigator, with audible warning
of receipt. The type 16 message will be supplemented by the proposed type
22 message.
The following "tentative" messages will be used by the Coast Guard:
Type 5 Constellation Health. The main use of this message type will be to
notify the user equipment suite that a satellite which is deemed unhealthy
by its current navigation message is usable for DGPS Navigation.
Type 7 Radiobeacon Almanac. This message provides location, frequency,
service range and health information for adjacent broadcast transmitters.
When broadcast from a given radiobeacon, it can be used to acquire the next
transmitter when in transit down the coast. This message will nominally be
sent every 10 minutes.
Type 22 Integrity Message. This message will provide the user with
information on both the current and future status of the broadcast [28].
This message is being submitted to the RTCM SC- 104 by the Coast Guard.
CURRENT EXPERIMENTS
Several experimental and developmental efforts are presently being
conducted in order to optimize the performance of the USCG DGPS Navigation
Service. These efforts include the operation of several prototype
installations, development and verification of a coverage prediction tool,
and the development and testing of message scheduling processes.
Over the last two years, the Coast Guard has deployed prototype reference
station installations in the Northeast (Portsmouth, New Hampshire, Montauk
Point, New York, Cape Henlopen, Delaware and Cape Henry, Virginia), the
western portion of the Gulf of Mexico (Corpus Christi and Galveston,
Texas), and the upper Great Lakes Region (Whitefish Point, Michigan).
These regions were chosen to provide contrasting atmospheric noise and
propagation characteristics and are providing valuable data for network
planning purposes. Evaluation of the coverage provided by a radiobeacon is
a major reason for fielding prototypes. Figures 2, 3 and 4 show the
approximate coverage of these sites.
It has been found that the 10 meter (2drms) position accuracy goal can be
achieved at the required availability level when the horizontal dilution of
precision (HDOP) is less than 2.3. HDOP is a measure of the effect of the
geometrical relationship of the satellites vs the user's location on the
accuracy of the position solution using those satellites. As the level of
accuracy is improving with the better DOP distribution and receiver
technologies, a detailed evaluation of recently collected data from the
Whitefish Point and Cape Henlopen prototype installations is ongoing. In
the actual network, consideration has been given to the placement of
reference station/broadcast sites in close proximity to severely
constricted waterways which experience the transit of large vessels. For
these areas, it is anticipated that the accuracy will approach five meters
(2drms). Though studies have shown that accuracy better than eight meters
provides no additional advantage to the mariner, the added margin provided
by these sites will enhance network reliability [29].
A requirement for velocity accuracy of 0.05 knots exists in Vessel Traffic
Service (VTS) coverage areas which employ dependent surveillance. (Prince
William Sound, Alaska is the only such VTS planned at this time.) The
major emphasis in fielding prototypes is focusing on evaluating the network
control dynamics involved in operating the DGPS Network. Slated for
completion in July of this year are several prototype sites, which will
cover the northeastern portion of the United States, and will be controlled
from a central location. Referred to as the Northeast United States (NEUS)
Testbed, data and lessons learned from its operation will be incorporated
into the final operational requirements covering network control and data
logging functions.
A prototype control center which is designed to support a live watchstander
is presently under development and the human- machine interaction aspects
will be extensively evaluated. The prototype will also allow the
evaluation of DGPS transmissions when used on actual user platforms. To
date this has included a number of Coast Guard and NOAA vessels, and this
summer, the evaluation aboard a one thousand foot Great Lakes ore carrier
will be added to the list of user platforms.
A software tool for coverage prediction is under development, with
completion expected in July of this year. It will undergo an extensive
evaluation effort using detailed noise and propagation data presently being
collected for verification and optimization of the coverage tool. As more
accurate and extensive noise databases are gathered over time, the coverage
tool parameters and tables will be updated. In addition to assisting in
network planning, the coverage tool will provide a graphical representation
of the coverage which will be provided by the USCG DGPS Navigation Service.
This feature will be used in the publication of coverage charts.
Several message scheduling schemes involving the use of Type 9 messages to
combat atmospheric noise are presently undergoing evaluation. Updated
receiver firmware has recently been delivered by Magnavox to the Coast
Guard which incorporates the Type 9 message into its navigation solution.
It is anticipated that extensive use will be made of the Type 9 message
broadcast to the point where it may fully replace the Type 1 message. If
this proves beneficial, the minimum data transmission rate will probably be
increased to 100 bits per second. Several MSK receiver impulse noise
suppression techniques will be evaluated under actual and simulated
Gaussian and impulse noise conditions.
STANDARDS
Several documents are being developed in support of the DGPS Navigation
Service. Included are standards, user equipment interface recommendations,
and procurement requirements. In draft form at the present time, the
Broadcast Standard [28] is being circulated among industry, government, and
user communities in order to gather comments from as wide a base of
interested parties as possible. The Broadcast Standard is a reference
document which describes the format, information content, modulation
parameters, coverage area, and use of the signal which is broadcast from
the marine radiobeacons in the USCG DGPS Navigation Service. Additionally,
the Broadcast Standard specifies the system performance which can be
achieved in conjunction with the proper user equipment. The Coast Guard
will continue to work with the International Association of Lighthouse
Authorities in assuring that a level of commonality is achieved that will
allow the low cost development of globally compatible equipment. Official
release of the final document to the general public is slated for fall
1992.
The GPS Reference receiver used in the Coast Guard's system will have two
interfaces: one to a remote control device, and one to the MSK modulator.
General requirements for these are described here:
Remote Control Interface. This will connect the GPS reference receiver
with the integrity monitor. All setup parameters must be loadable over
this interface, which must also accept commands such as message scheduling,
satellite health override, transmission of type 7 and 16 messages or the
new type 22 integrity message. The interface must also be capable of
forwarding RTCM corrections for archival or integrity analysis purposes.
Modulator Interface. The reference receiver will be connected to an MSK
modulator, which will convert RS-232 RTCM data into an MSK- modulated
radiobeacon carrier. The reference station must be capable of setting up
the modulator over this interface, perhaps through the use of "pass
through" commands, where commands received over the remote control
interface are passed through without action by the reference receiver, to
the modulator.
Work is getting underway on a user equipment performance standard which
includes DGPS and MSK receivers. Again, the Coast Guard plans to work
closely with industry in drafting this document. Due to more stringent
regulations on radiobeacon interference outside of the European Maritime
Area (EMA), a more hospitable operating environment exists in North America
for MSK radiobeacon receivers. As a result, the Coast Guard is considering
developing two sets of specifications for MSK receivers: One being a
requirement for operation in U.S waters and the other being a
recommendation for vessels which operate worldwide - the rationale being
that vessels operating only within the U.S. DGPS Navigation Service would
be able to acquire lower cost, higher performance MSK receivers. These
receivers would be single down conversion or fully digital receivers. It
is hoped that the protection ratios listed in the draft of the Broadcast
Standard will gain international acceptance as they allow the use of
receivers with a superior impulse noise processing capability. This would
allow EMA receivers to adequately function outside of the EMA. The user
equipment standard will address accuracy and integrity issues in order to
specify the contribution of the user equipment suite to the overall
performance of the system.
The Coast Guard does not plan to regulate equipment interfaces, but it sees
the need to aid the vendor and user communities in insuring the
interoperability of equipments which make up the user equipment suite. To
this end, the USCG Research and Development Center has begun an effort to
develop a set of recommendations in this area based on industry-wide
meetings and discussions. It is hoped that the Coast Guard will be able to
handoff this effort to a government- industry organization which will
develop a set of equipment interface recommendations.
THE DGPS IMPLEMENTATION PROJECT
==== Table 2 ====
Usage Accuracy (2drms) Availability
----- ---------------- ------------
Harbor/Harbor Approach 8-20 meters 99.7%
Aids to Navigation Positioning 10 meters 95%
Vessel Traffic Services 10 meters 99.9%
NOAA Offshore Surveying 60 meters 95%
==== Table 3 ====
DGPS BROADCAST SITES
------
FORMAT
------
NAME
LAT.
LONG.
RANGE
------------------
GREAT LAKES REGION
------------------
EAGLE HARBOR, MI WHITEFISH PT., MI LOOKOUT 4, MI
47 27.7 46 46.3 46 17.1
88 09.5 84 57.5 84 12.7
220mi 80mi 30mi
SEUL CHOIX PT. MI STURGEON BAY, WI MILWAUKEE, WI
45 55.3 47 47.7 43 01.6
85 54.7 87 18.8 87 52.9
120mi 60mi 140mi
PRESQUE ISLE LT. MI GRAVELY SHOAL, MI FORT GRATIOT, MI
45 21.4 44 01.2 43 00.3
83 29.5 83 32.3 82 52.4
80mi 40mi 140mi
SANDUSKY, OH BUFFALO, NY DETROIT(BELLE IS.), MI
41 30.0 42 52.2 42 20.4
82 40.5 78 54.2 82 57.6
130mi 120mi 70mi
ROCHESTER, NY TIBBETS PT., NY
43 15.4 44 06.1
77 36.2 76 22.2
100mi 40+mi
------------------------
ATLANTIC AND GULF COASTS
------------------------
ARANSAS PASS, TX GALVESTON, TX MOBILE PT. AL
27 50.0 29 19.7 30 13.6
97 03.5 94 44.3 88 01.4
175mi 175mi 165mi
TBD - NEW ORLEANS EGMONT KEY, FL KEY WEST, FL
TBD 27 36.0 TBD
TBD 82 45.7 TBD
TBD 210mi 150mi
TBD - PUERTO RICO JUPITER INLET, FL CAPE CANAVERAL, FL
TBD 26 56.9 28 27.6
TBD 80 04.9 80 32.6
TBD 125mi 250mi
CHARLESTON, SC FORT MACON, NC CAPE HENRY, VA
32 45.5 34 41.5 36 55.6
79 50.6 76 41.0 76 00.5
150mi 130mi 125mi
CAPE HENLOPEN, DE TBD - NY HARBOR MONTAUK PT.,NY
38 46.6 TBD 41 04.0
75 05.3 TBD 71 51.8
175mi TBD 125mi
PORTSMOUTH, NH TBD - CEN. MAINE
43 04.3 TBD
70 42.5 TBD
TBD TBD
-------------------------------
PACIFIC COAST, ALASKA, & HAWAII
-------------------------------
BARBERS POINT, HI UPOLO POINT, HI POINT LOMA, CA
21 18.0 20 14.8 32 40.0
156 06.5 155 53.2 117 14.6
170mi 170mi 150mi
POINT ARGUELLO, CA SF BAY, PT. BLUNT POINT ARENA, CA
34 34.7 37 51.2 38 57.3
120 38.6 122 25.2 123 48.6
190mi 30mi 125mi
CAPE BLANCO, OR GRAYS HARBOR, WA EDIZ HOOK, WA
42 50.3 46 54.2 48 08.4
124 33.8 124 07.8 123 24.1
130mi 150mi 70mi
ROBINSON POINT, WA GUARD ISLAND, AK CAPE SPENCER, AK
TBD 55 26.8 58 12.0
TBD 131 52.8 136 38.3
TBD 300mi 300mi
C.HINCHENBROOK,AK POTATO POINT, AK COOK INLET, AK
60 14.3 61 03. TBD
146 38.8 146 42. TBD
TBD TBD 250mi
KODIAK, AK COLD BAY, AK
59 00. TBD
156 30. TBD
275mi 300mi
==== (end of table 3) ====
The Coast Guard plans to provide accurate radionavigation service to U.S.
harbors and harbor approaches (HHA) by 1996. The system will employ
current DGPS technology to meet the accuracy and availability requirements
for HHA navigation identified in the FRP. The project includes design,
procurement, installation, operation, and maintenance of DGPS reference,
transmitter, monitor and control stations. The system is expected to cost
approximately $15 million to acquire and about $5 million per year to
operate and maintain. The system will also be designed to meet existing
operational requirements for buoy positioning and vessel traffic services,
and whenever feasible, will meet future operational requirements of the
Coast Guard or other cooperating agencies. Table 2 shows some of these
requirements. Relatively new, previously unaddressed numerical
requirements are being researched. For example, integrity and reliability
requirements for HHA have never been adequately addressed by the FRP. The
DGPS Project will define methods of calculating these, and other system
characteristics, and determine the performance of various candidate system
architectures. The reliability and especially integrity of DGPS are
expected to be quite high. Integrity should be in the vicinity of 99.999%.
More on this in a future paper. The planned DGPS Broadcast Station sites
are listed in table 3, and figure 5 shows the expected DGPS coverage of the
Continental U.S., Alaska, Hawaii and Puerto Rico.
The time line for the project is shown in figure 6. The Northeast U.S.
Testbed, along with system design efforts, is expected to yield the
information necessary to procure the components for the system. This
procurement should begin in mid-1993, with hardware sets being delivered by
July, 1994. Installations in the field will be conducted from that time
through the end of 1995. In parallel paths, monitor and control station
design will take place. An important part of those stations is software,
which need not be complete until early-mid 1995. However, the software
design must be far enough along to allow procurement of the monitor and
control hardware by late 1993. Monitor and control stations will be
installed in 1995.
There has been a great deal of discussion concerning the government's
policy regarding DGPS. The FRP, jointly developed by the DOD and DOT,
outlines the federal policies regarding federally provided radionavigation
systems. While DGPS is not addressed in the current plan (1990), the
following is the draft Federal maritime DGPS policy statement for inclusion
in the next FRP: "The USCG plans to provide Differential GPS (DGPS)
service for the harbor and harbor approach phase of maritime navigation.
Maritime DGPS will use fixed GPS reference stations which will broadcast
satellite pseudo-range corrections using maritime radiobeacons. The USCG
DGPS system will provide radionavigation accuracy better than 10 meters
2drms for U.S. harbor and harbor approach areas by 1996, free of charge.
Until the service is declared operational by the USCG, users are cautioned
that signal availability and accuracy are subject to change due to the
availability of GPS, testing of this developing service, and the
unreliability of prototype equipment. While all licensed communication
links, including those used to transmit differential GPS corrections, are
subject to the direction of the National Command Authority,
radiopositioning services are not directly regulated by Federal law and are
therefore beyond the purview of this plan."
FUTURE RESEARCH
Even though the USCG is implementing a DGPS service, it will continue to
conduct research for the next few years. The following areas will undergo
research and development:
Integrity Monitor. The Coast Guard R&D Center has been working on an
integrity monitor for DGPS broadcasts. An operational monitor should be
complete by summer, 1994.
DGPS/ECDIS Integration. Policy, accuracy and integrity issues regarding
radionavigation and Electronic Chart Display and Information Systems
(ECDIS) require further investigation.
Heading Determination with DGPS. Large vessels using an accurate
radionavigation sensor such as DGPS also need real-time heading information
(vessel attitude), that should be displayed in the form of an icon
representing the ship on the ECDIS display.
Reduced Visibility Navigation with DGPS. With the availability of such an
accurate and reliable navigation sensor, vessel owners and operators will
want to extend their envelope of operation to include reduced visibility
conditions. Further study in this area is ongoing.
MSK Radiobeacon Broadcast Receivers. Further investigation into enhancing
the technology of these receivers is needed. All- digital techniques
should be examined to assess their potential to reduce the size, cost and
complexity of MSK receivers.
Wide Area DGPS Techniques. These techniques, involving combinations of
multiple reference station data, are not considered mature enough for
implementation at this time. However, this is a fertile area for further
research. As the Coast Guard's network is intended to be adaptable to
wide-area use in the future, monitoring and participation in this research
is needed. Also, use of data from multiple MSK broadcasts by the user
needs investigation.
Spatial Decorrelation Effects. Further investigation into these effects is
needed, including quantifying them. [30,31]
SUMMARY
The background of the Coast Guard's DGPS Program has been described,
followed by the technology of DGPS. The project elements, time frames,
ongoing research necessary for project completion, and future research
projects for follow-on work with DGPS have been presented. Although the
program is ambitious in scope and time frame, the theory and technology are
sound and the benefits to navigators are extensive. The program has the
support of the Secretary of Transportation and the keen interest of
industry, potential users, and other agencies. The Coast Guard looks
forward to continued work with all of these groups as the development
continues.
REFERENCES
1. O'Brien, Arthur E., History and Status of U.S. Marine Radiobeacon
System, U.S. Department of Transportation, DOT-CG-N-1-82, National
Technical Information Service, Springfield, VA, February 1983
2. 1990 Federal Radionavigation Plan, U.S. Department of Defense,
DOD-4650.4 and U.S. Department of Transportation, DOT-TSC-RSPA-90- 3,
National Technical Information Service, Springfield, VA, January 1990
3. Teasley, Stewart P., Hoover, Wayne M., Johnson, Charles R.,
Differential GPS Navigation, Texas Instruments, Inc., PLANS Symposium,
December 1980
4. Beser, Jacques, Parkinson, Bradford W., The Application of NAVSTAR GPS
in the Civilian Community, Navigation, Vol 29, No. 2, 1982
5. Cnossen, R., Cardall, J., DeVito, D., Park, K., Gilbert, G., Civil
Application of Differential GPS Using a Single Channel Sequencing Receiver,
Magnavox Co., NASA CR 166168, May 1981
6. Ruedger, W. H., Feasibility of Collision Warning, Precision Approach
and Landing Using the GPS, Research Triangle Institute, NASA CR 165675,
March 1981
7. Beser, J., Parkinson, B. W., The Application of NAVSTAR GPS to Civil
Helicopter Operations, Intermetrics, Inc., NASA CR 166169, June 1981
8. Kalafus, Rudolph, NAVSTAR GPS Accuracy Studies, Proceedings of the
Surface Transportation Users Conference on Navigation, (DOT-TSC-RSPA-83-1),
Washington, DC, November 16-17, 1982
9. Kalafus, Rudolph M., Vilcans, Janis, Knable, Norman, Differential
Operation of NAVSTAR GPS, Navigation, Vol 30, No. 3, 1983
10. Kalafus, R.M., Synopsis and Recommendations of the TSC Workshop on
Differential Operation of NAVSTAR GPS -June 1983, U.S. Department of
Transportation, DOT-TSC-RSPA-83-10, National Technical Information Service,
Springfield, VA, October 1983
11. Kalafus, R. M., Van Dierendonck, A. J., Pealer, N. A., Special
Committee 104 Recommendationsd for Differential GPS Service, Navigation,
Vol 33, No. 1, 1986
12. Proposed Radiobeacon Data Link For Differential GPS Corrections,
Report of Radiobeacon Data Link Workshop, USCG Headquarters, November 8-9,
1984
13. Denaro, R. P., Quill, J., Kalafus, R. M., Differential GPS Reference
Station Design and Development, Institute of Navigation National Technical
Meeting, Anaheim, CA, January 1987
14. Quill, J., U.S. Coast Guard Differential GPS System Development,
Radio Technical Commission for Maritime Services Assembly Meeting, Boston,
MA, April/May 1986
15. Pietraszewski, D., Spalding, J., Viehweg, C., Luft L., U.S. Coast
Guard Differential GPS Navigation Field Test Findings, Navigation, Vol 35,
No. 1, 1988
16. Recommended Standards for Differential GPS Service, Version 2.0, Radio
Technical Commission for Maritime Services Special Committee 104,
Washington, DC, January 1990
17. Spalding, J., Krammes, S., Pietraszewski, D., Status of U.S. Coast
Guard DGPS Prototype Service, Institute of Navigation, DGPS '91
Proceedings, Albuquerque, NM, September 1991
18. Pasupathy, S., Minimum Shift Keying: A Spectrally Efficient
Modulation, IEEE Communications Magazine, July 1979
19. Recommendations for Government Provided DGPS Service, International
Association of Lighthouse Authorities, Oct 1991
20. Enge, P., Megapulse, Inc., Wheeler, A., Racal- Decca, The Transmission
of Differential NAVSTAR GPS Corrections Using Low Frequencies, June 1983
21. Handley, D., Racal-Decca, A Dedicated Low Frequency Radio Link for
Communicating Differential GPS Corrections, January 1984
22. Enge, P., Ruane, M., Boston Univ., Radiobeacon Data Link Planning,
August 1985
23. Boston University, Medium-Frequency Data Link for Differential NAVSTAR
GPS Broadcasts, U.S. Department of Transportation, DOT-TSC-CG- 86-1,
National Technical Information Service, Springfield, VA, June 1986
24. Castillo, F., Cost Analysis of a Differential GPS Station, U.S.
Department of Transportation, DOT- TSC-CG-945-PM-88-23, National Technical
Information Service, Springfield, VA, September 1988
25. Enge, P., Ruane, M., Development of Radiobeacon Data Link, U.S.
Department of Transportation, DOT-TSC-CG-945-PM-89-2, National Technical
Information Service, Springfield, VA, February 1989
26. Enge, P., Ruane, M., Radiobeacon Network Design, U.S. Department of
Transportation, DOT- TSC-CG-945-PM-89-3, National Technical Information
Service, Springfield, VA, March 1989
27. Enge, P., Ruane, M., Olson, K., Code Design, Data Analysis and Signal
Availability for the DGPS Radiobeacon Data Link, U.S. Department of
Transportation, DOT-TSC-CG-945-PM-89-3, National Technical Information
Service, Springfield, VA, March 1989
28. Broadcast Standard for the USCG DGPS Navigation Service (Draft), U. S.
Coast Guard COMDTINST M16577.1, May 18, 1992
29. Gynther, J., Smith, M., Radio Aids to Navigation Requirements 1988
Simulator Experiment, U.S. Coast Guard, Washington, DC, June 1989
30. Brown, A., Extended Differential GPS, Navigation, Vol. 36, No. 3, Fall
1989
31. Kremer, G., Kalafus, R., Loomis, P., Reynolds, J., The Effect of
Selective Availability on Differential GPS Corrections, Navigation, Vol.
37, No. 1, Spring 1990
|
| 452.16 | Moto has the output | MILKWY::WAGNER | Scott | Fri Apr 16 1993 12:54 | 10 |
|
OK, I've already proved myself wrong! There's a Motorola Traxar
which is their handheld, and it said it has the standard marine-type
output. I forget the name- NMEA such&such- same as comes out the back
of my LORAN.
Now, how to interface that to your serial port... it HAS been done;
if you find a copy of the standard, let me know.
Scott_who_wants_more_electronic_toys
|
| 452.17 | NEMA 183 to RS-232 | HIBOB::MALOY | | Wed Apr 21 1993 16:16 | 38 |
|
> OK, I've already proved myself wrong! There's a Motorola Traxar
> which is their handheld, and it said it has the standard marine-type
> output. I forget the name- NMEA such&such- same as comes out the back
> of my LORAN.
The Magellan and Garman have NEMA outputs too. The Garman has selectable
NEMA 180, 181 or 183.
> Now, how to interface that to your serial port... it HAS been done;
> if you find a copy of the standard, let me know.
I looked at the output from my Garman GPS-50, it's at TTL logic levels (0-5V),
so you would need a TTL to RS-232 level (+-12V) converter. The data comes in
bursts at about 2 second intervals.
The following company lists two software packages with NEMA 183 to RS-232
interfaces included.
"NAVLINK" says it does the following:
Waypoints and routes entered and displayed on screen.
3D perspective display.
Shows: time to go, ETA, speed made good, cross track error...
Mercator plotter: start and end points, vessel position, all waypoints
in the route, Lat, Lon, zoom, pan...
Displays: CGA, EGA or VGA.
US Coast Guard and DMA "Light Lists on a Disk"
Self contained NEMA 183 to RS-232 interface, connects to your serial
port.
Price: $99.00
Available from:
DF Crane Associates Inc.
Nautical Computing
2535 Kettner Blvd
P.O. Box 87531
San Diego, CA 87531
Phone: 619-233-0223 Fax: 619-233-1280
|
| 452.18 | Magellan has models with RS-232C. | CSC32::VISAGE | | Fri Apr 23 1993 14:10 | 14 |
| re .17
Thanks for the info about the NMEA-RS232 interface. I'll get in touch
with them.
I've requested literature from Magellan, Motorola, Garmin and Trimble.
Of those only Magellan's material has arrived. They actually have two
models that have a built in RS-232 interface. The NAV 5000A is
targeted as an aviation model and its data output is listed as "RS-232C,
ARGUS map compatible". They also have the NAV 5000 Pro. It is a
survey grade handheld capable of downloading the actual carrier phase
data from the GPS satellites for post-processing on a PC rendering
sub-meter accuracy. This of course is what I would love to have, too
bad it lists at $3,700.
|
| 452.19 | Try Trimble | BOSEPM::WALKER_K | Ken Walker @TTB | Fri Apr 23 1993 17:55 | 1 |
| The Trimble Transpac GPS has an RS-232 output.
|
| 452.20 | Handheld recommendations? | VERGA::FACHON | | Thu May 13 1993 11:48 | 14 |
| Glad to see this note is active! My Dad is interested
in buying a handheld GPS. He asked me if I knew any
"gurus" who might recommend what unit to buy and where
to get a good price. Do I?
My Dad doesn't need any serial ports, nor does be need
differential GPS. Just a rugged, easy to use unit. I'd put
simplicity ahead of bells and whistles, but rugged and reliable
is a must. And price, as long as it's "good" for the unit he
gets, is not a real issue.
What and where? Thanks in advance for your suggestions!
Dean F.
|
| 452.21 | PS Says Trimble Ensign | BOSEPM::WALKER_K | Ken Walker @TTB | Thu May 13 1993 12:59 | 9 |
| Practical Sailor just did and evaluation of hand-held GPS receivers.
They rated the Trimble Ensign best, just ahead of the Motorola Traxar.
The Trimble unit was clearly the most accurate and had the fastest
"time-to-position" (acquired multiple signals and calculated a
position after initialization).
I haven't personally used any but the Magellan hand-held. Seemed okay
but with no criteria for comparison so I wouldn't venture a
recommendation.
|
| 452.22 | I like Trimble | OTOOA::MOWBRAY | This isn't a job its an Adventure | Fri May 14 1993 08:38 | 5 |
| For what it is worth, I was lent a trimble ensign for a trip .... but
no manual (the lender is a paper miser) ... I was very impressed with
the easy-to-learn controls on it. It was
intuitive and simple. Very important in that kind of unit. (tell that
to my Loran!)
|
| 452.23 | Another vote for Timble | PIHIA::ARLINGTON | | Sat May 15 1993 09:42 | 8 |
| for what it is worth we where recently loaned a trimble ensign hand
held GPs from the boys off YAMAHA round the world whitbread 60, Great
instrument but it chews through battery power. On a 35miles tight
reach on a black night we blitzed passed a lot of boats without the use
of one.
Cheers
revel
|
| 452.24 | fix installed GPS | HAEXLI::PMAIER | | Wed May 19 1993 04:56 | 49 |
| I'm still using my 10 year old Satnav. Since GPS, they switched on
the spare satellites in orbit and I have now much more fixes. But I will
buy a GPS next year to have the added functionality.
( speed over ground, course over ground))
Everybody speaks about "handheld" GPS. Update every second, 10ft max
error (but Galapagos Islands are 2 nm off, Zackinthos Island in Greece is
1nm off etc.) 8 Satellites, 5 second warm up etc etc etc. Are this
really the features I'm looking for ?
Jimmy Cornell wrote in his wrapup of "Europe 1991" (the round the
world rally for cruisers in 18 month) that most participant did start
with a handheld GPS, but all of them changed during the rally to a fix
installed GPS, keeping the handheld as spare.
But I have not seen one single article in any newspaper or yachting
magazines mentioning the pro's and contra's of fixed installed GPS.
I plot every 2 hours my position in the chart, with the log, speed
and barometer in the logbook. So if the electronic should fail (it
did once in 10 years) I could carry on with dead reckoning. I will
do that also with a GPS.
I will need the data from the GPS at the chart table, and not in the
cockpit.
I have some questions:
What means "handheld" ? Can I operate a Trimble or Magellan or Garmin
below deck without using a separate outside antenna ? Does it work in
the wheelhouse ? I have heard, it must be in the cockpit. Is that true?
And my second question: numerical keypad or not. My Walker Satnav has
a numerical keypad to enter the numbers, but I have to scroll for
waypoints, magnetic variations etc, in total 10 choices.
I'm not very fond of "scrolling". Most GPS use scrolling for all
input. Any comments on that ?
So what I'm looking for is a GPS for below deck, numerical
keypad, no scrolling for the functions and should not be to big and
if possible no LCD display. (LCD displays go black if the temparatur
goes ever below -20 degrees C). And it should be only a GPS. Not
a GPS, radar, chartreader, fishfinder and Loran in the same box.
Is there any such thing on the market ?
Peter
|
| 452.25 | Trimble -Magellan | TFH::KTISTAKIS | Mike K. | Wed May 19 1993 09:22 | 12 |
| Peter, I have a page from a mag. that evaluates 6 handheld GPS.
Two of them,Magellan and Trimble,operate belowdeck.They all have
keypads that are rated for ease of comprehension,I don't see any
scroll reading used, but they all use LCD reading,tested good in
sunshine,low light and artificial light.Doesn't say anything about
temparture readability.
Personally I would buy a Trimble handheald first.It rated the best on this
evaluation and someplace in this notesfile are some good words about
it.
Then if I was going to Tahiti or Europe I add a fixed one just in case.
The handheld also can be taken in a lifeboat if the need arises.
|
| 452.26 | | UNIFIX::BERENS | Alan Berens | Wed May 19 1993 10:15 | 14 |
| re .24:
My impression is that the battery life for all handhelds is quite short
-- a very few hours at most. On a long voyage this would be a serious
problem. I would not buy a handheld unless it could run off ship's power
and use an enternal, deck-mounted antenna (eg, a Magellan 5000).
re .25:
I'm not at all sure I see the utility of a GPS in a liferaft. Unless, of
course, you have some means of propelling yourself toward safety rather
than just drifting about.
Alan
|
| 452.27 | | HAEXLI::PMAIER | | Wed May 19 1993 11:03 | 16 |
| I will not carry a spare GPS. If it breaks, it breaks. then we
go back to manual operation: compass, log, sextant and book. (no
computer)
My operator manual from my Satnav insists on mounting the antenna
as high as possible, outside of the rigging.
I had it always that way until the cable broke. Now I have installed
the Satnav antenna inside the boat and it works perfect, does not get
wet and the chance that the (activ)antenna is breaking down due
to static discharge during a storm is much smaller.
So I'm wondering, if the handheld GPS may as well work inside the
boat. I have some information, that the Garmin 50 must be in the
open.
Peter
|
| 452.28 | | TFH::KTISTAKIS | Mike K. | Wed May 19 1993 12:00 | 19 |
| re. 26
Alan, usually you cover all the safety angles :^).
If I ever go in a liferaft I will take with me my survival bag,extra
food and water an EPIRB,my handheald VHF and GPS if I had one.
The last two items just to help the may be resquers if they ever are
close enough but cannot see me.There are currents in the oceans too.
re. 27
Peter, According to this evaluation I have ( I can send it to you if I
knew your mail Stn.) only Magellan and Trimble handhealds could operate
without extra antenna belowdeck.Not Carmin 50 neither Micrologic,
Panasonic or Motorola.The evaluation on handheld GPS's was done by
a recent Sail or Cruising magazine,it was kind of extensive comparison
but I only kept the half page synopsis.In addition in small print
states "Belowdeck operation was tested aboard the author's 41 foot
cutter.Receivers were held in the optimum position at the nav station
next to the open companionway,beneath a balsa-cored fiberglas
coachroof"
|
| 452.29 | 7 fixed GPS sets on test | RDGENG::BEVAN | | Thu May 20 1993 05:09 | 3 |
| RE .24 Peter, there is a 3 page article in PBO on 7 fixed (below deck)
GPS sets. I put a copy in the internal mail.
Regards Steve
|
| 452.30 | | CSC32::VISAGE | | Wed May 26 1993 14:30 | 15 |
|
Well I have my new toy, a Magellan NAV 5000D. Among the lower cost
handhelds I was most impressed by its features. I should get my first
opportunity to see how it works for land use next week when I'll be doing a
little prospecting near my in-laws home in the California gold country.
I wanted to thank you folks for putting up with a landlubber like
me. I expect to see land use of GPS receivers become quite
a lot more popular as the prices continue to drop (heck, I got mine
for a pretty reasonable price). At this point and time though, this
conference turned out to provide the best source of information.
Thanks
Jerry
|
| 452.31 | | HAEXLI::PMAIER | | Thu Jun 03 1993 11:34 | 9 |
| Sorry, was away for the last two weeks. Varnishing the teak, replacing
the leaking toilet (the new one is leaking as well...)
re .28 Mike, my mailstop is @RLE 3A20
re .29 Steve, thanks. I'm really curious, what they write.
Regards Peter
|
| 452.32 | PBO GPS article is in 3 parts | RDGENG::BEVAN | | Thu Jun 03 1993 11:47 | 10 |
| re .31
Peter, the article is in 3 parts. Part 1 is about handheld units
(so I didn't send that). You should receive part 2 now-ish and part 3
about a week after part 2.
I tried out the handheld Sony Pixit (sp?) over the weekend and it would
not read below deck, but worked fine in the cockpit.
Regards Steve
|
| 452.33 | Garmin GPS 75 | CSOA1::GELO | | Thu Jun 03 1993 16:07 | 13 |
| Just bought a Garmin GPS75. Easy to learn, Hi-res display, great
graphics (plots, bar graphs, sky views), water proof. Comes with
detachable antenna, swivel/service mount, battery pack, carrying case,
power/data cable (can connect to ships power and interface with
plotters, autohelm, etc.). Available options include cigarette lighter
adapter, rechargeable battery pack with charger/dc converter, remote
marine antenna (for those who want to use the GPS below deck), and
mobile antenna. (cost: $ 875).
Batteries last about 7 hours. While on the boat, I run off ships power.
I plan to get the rechargable batteries (2 packs) and charger, as well
as the remote marine antenna. Anybody have a couple hundred dollars
they don't need?
|
| 452.34 | | HAEXLI::PMAIER | | Mon Jun 07 1993 08:43 | 30 |
| re .32:
thanks Steve, just received part 3. (2 is probably still on its way)
Very interresting reading. For those of you having no access:
The Philips AP Navigator Mk8 seems to be the best. Almost all plots
inside 50 meters (24 hour, ploted every 30 seconds) but no numerical
keypad, only scroll. Consumption 4.1 Watts.
The Eagle and Lowrance had wrong fixes up to 1/3 nm, the Lowrance has
a numerical keypad, both have only one chartdatum and very powerhungry.
(9.9 / 11.9 Watts)
The Apelco, the one I have seen most on the ARC fleet: relativly cheap,
medium accuracy, not many features, medium power (6.0 Watts), single
channel receiver.
Especially the plotted accuracy of the 8 tested GPS is very
interesting. The single error was up to 950 Meters. (1 nautical mile has
1854 Meters).
The old rule (Satnav) of giving on a strange coast every obstacle in the
water a 2 nm wide berth is still valid. Landfall always 2nm to seaward.
re -1: Have you tried your Garmin GPS75 below deck ?
Peter
|
| 452.35 | GPS | CSOA1::GELO | | Tue Jun 08 1993 15:19 | 8 |
| re .34:
Have you tried the GPS75 below deck yet?
No. However since Garmin offers a remote antenna, I doubt it will work
below deck. One more option to buy........
Carl
|
| 452.36 | | LARVAE::CSOONE::BARKER | NE1410IS | Wed Jun 09 1993 05:22 | 20 |
| The Magellean 5000 doesn't work below decks. The manual says that the ariel
must have an unobstructed view of the sky.
During a quiet moment in the a recent regatta, I did the following...
Sitting on the Visitors moorings in St Peter Port, Guernsey,I plotted our position
from GPS, Decca and compass bearings, on a very large scale chart of the
port.
Not surprisingly, the Compass Bearings agreed with reality, but the GPS and
DECCA were about 150 yards out. However, when I applied the Chart Datum
corrections ( under the heading "Satellite Derived Positions" on the chart
information ) the GPS was only about 20yds ( less than 2 boat lengths ) out.
The Magellen GPS in question allows input of Datum corrections, but nobody
had bothered with this. GPS and SATNAV gives positions based on a datum
called WSG84 ( or something similar ). Charts tend to be on someting different,
depending on the locality. This is worth remembering.
Chris
|
| 452.37 | GPS-50 | HIBOB::MALOY | | Wed Jun 09 1993 14:24 | 70 |
|
I purchased a Garman GPS-50 last year and used it to navigate the Southern
Cross from Annapolis to Miami last Fall.
The GPS-50 was mounted on the binnacle next to the ships compass and
connected into the ships power. The GPS-50 comes with a snap-in plastic
mount that can be bolted to a bulkhead or mounted to a horizontal surface to
have a free standing mount with tilt and swivel. The mount will accept the
GPS with or without the battery pack attached to the unit. The recessed
connector on the back of the unit mates with a connector in the mount to
supply power and also contains the NEMA output as well as an external alarm
connection. When the unit is removed from the mount, the batteries in the
removable battery pack take over without interruption of service. The power
input can range from 4-40V DC. The power drain is minimal with 4 AA NiCad's
lasting about 6 hrs. I use NiCad's in the standard battery pack which will
also accept 4 AA dry cells, (Garman has a special NiCad battery pack which
will automatically recharge when connected to ships power for about $50).
The GPS-50 performed perfectly for the entire trip except for about 2 minutes
right outside the entrance to Charleston when it lost all satellites and had
to re-acquire (no explanation). We left the unit on 24 hrs a day and found
it to be very useful in choosing the fastest track to the next waypoint, ie.
Speed Made Good, and showing us when we should tack back towards shore to
try and stay out of the Gulf Stream. The Off Track and Direction to Steer
functions were also useful. Most of the crew found themselves looking at the
GPS more than the compass, (especially at night when the compass light kept
burning out).
The GPS-50 would not work below decks on the Southern Cross but works just
fine on my Valiant 32. The Southern Cross has a teak deck with a thick
plywood core, the Valiant is FRP wit a balsa core. The standard Garman
antenna is connected to the unit with a common BNC connector and is
removable. I have successfully used the GPS-50 with a 6 ft length of 50 Ohm
coax between the unit and antenna without problems. The reason for this is
the Garman antenna is an active antenna, ie, the GHz frequencies received are
mixed down in the antenna and only a low frequency is carried in the coax.
This means you don't have to buy the special external antenna, for about
$200, if you want/need an external antenna. I don't believe this can be
done with any of the other handhelds.
Depending on how much night sailing you do, I found the backlit keypad on
the GPS-50 to be a great feature. I believe the Garman is the only handheld
that has this capability. The backlight has a variable off timer and is
turned on by touching the keypad or it may be locked on.
The waypoint name limitation in the GPS-50 of 4 characters could be
detracting to some, but if your used to using numbers for waypoints, it's no
big deal. The full keypad does allow the entry of any character with a
maximum of two keystrokes. I haven't used a scrolling type entry but I
think it would get tiresome real quick, (we had over 100 waypoints to enter
on the trip South).
The unit appears to be water tight and corrosion resistant. It was sprayed
with salt water may times during the trip and left to dry. When we reached
port, I just rinsed it under the faucet with fresh water and dried it off.
The connectors for both the battery and ships power are gold plated and are
molded into the case. The battery pack also has O-rings to seal out water.
The Practical Sailor article that compares portables to fixed GPS units that
I have is from the 1992 Buying Guide and doesn't include the Garman GPS-50,
but does have the Pronav GPS-100, an older unit made by the same company.
Check out the other GPS reviews in Practical Sailor: June 1, 1992,
September, 1992, and January 1, 1993.
I really don't see the reason for buying fixed mount GPS vs a handheld. The
performance difference is negligible and the portables can be snapped into a
mount at the nav station and used like a fixed unit or moved to another mount
in the cockpit or even taken off the boat entirely. I like the versatility
of of the handhelds.
|
| 452.38 | Works for me! | BTOVT::HILTON | SYS-F-UNIVCRASH% REALITY.SYS Corrupted - Reboot Universe? (Y/N/Q | Thu Jun 10 1993 10:25 | 15 |
|
as dave ahs noted, the garman gps unit works great. we used it for
the entire trip from Miami to Burlington Vt. It had an added handy
feature during the trip through the 10+ locks up to lake champlain.
We set a waypoint for whitehall NY and used the unit to check speed
made good while dealing with locks and currents in the canal.
georgia
PS: we also had an Ensign GPS which works well but is not has handy
to use as the garman. (both being hand-held with external mounts)
(both also failed to work below decks on the Southern Cross.
|
| 452.39 | Is it the safety lines causing the GPS-portable-below-decks problem ? | RDGENG::BEVAN | | Fri Jun 11 1993 09:24 | 20 |
|
-.1
I have a sneaky suspision that the Southern Cross has a continuous wire loop
running around the deck, formed by the safety lines (wires running along the
stanchions), pulpit and pushpit.
If this is so, it effectively shorts-out the radio signal below deck. A simple
test to see if this is the problem is to open both safety lines at the midships
boarding point, then try taking the GPS below to see if it still looses the
satellites.
If this fixes it just modify the safety-lines. Replace the wire-end shakles
with whipping in one place in both lines where they fix to the pushpit. This
allowed the RDF to work below deck on my boat, so may do the trick.
On the other hand of course it could be a zillion other reasons.........
Don't you just lurvvvve problems like this ;^)
St
|
| 452.40 | GPS Weak Signals by Design | SNOC01::RADKEHOWARD | | Sun Jun 13 1993 20:17 | 23 |
| Re: last few
I suggest that the difficulty with receiving GPS signals below deck can
be attributed to two reasons, 1) the weak signals inherent from the
satellite (a few watts at most) and the compromised antennas found on
most hand-held GPS receivers and, 2) the extremely high frequency of
GPS transmissions tends to make it difficult for the radio waves to
penetrate solids such as the typical deck material.
The very high frequency used creates extremely short "microwave" length
signals which tend to travel in straight lines and bounce off solids
(much like radar). Some of them may sneak into the cabin, but most are
deflected. In the cellular telephone business thay are using passive
antennas to receive the signals outdoors, then connect to indoor
antennas to re-radiate the signal in difficult areas such as steel
buildings and basements. There may be an idea here for some marine
oriented entrepreneur!
Until receiver/antenna technology improves or someone comes up with an
idea such as the passive antenna I don't expect that GPS reception will
be reliable inside the cabin.
Howard
|
| 452.41 | Wet deck as shield | SUBSYS::CHESTER | | Sat Jun 19 1993 10:24 | 4 |
| Possible other reason is the wet deck acts as a shield.
KC
|
| 452.42 | Garmin 50 | HAEXLI::PMAIER | | Mon Nov 08 1993 03:01 | 37 |
| Ok, I finally decided on a Garmin 50.
On a recent boatshow, I checked out most of the GPS (Steve, I could not
find the Philips).
I decided to drop the fix installed GPS. The once I have seen, did have
part of the electronic inside the antenna. As an electronic engineer,
I hate to have valuable electronics unprotected facing an electric
storm. Furuno has a very nice unit: all functions, very large and
clear display). But a very ugly, big antenna with all the GPS electronic
in it. Apelco has a cheap display and I belief, the electronics and
software is first generation.
So I had a look at the handhelds. Most of them have unreadable
displays, full of unwanted information in too small letters. Toys !
So I decided for the Garmin, based on the good quality display, large
letter, removable antenna (can be extended with a simple cable with
BNC connectors) and numerical keypad.
Coming from a second generation satnav, I must say the third generation
GPS is simply said: unsave for navigation.
My Satnav has a button for : current position in lon and lat
a button for : waypoint range and course to steer
The GPS needs to many key strokes to enter the information or to
retrive the information.
We used or satnav for apprx. 23'000 nm and we have made during that time
3 fatal programming errors.
Ha.... any comments ?
Peter
|
| 452.43 | fatal is a relative term ?? | WRKSYS::SCHUMANN | | Mon Nov 08 1993 08:49 | 6 |
| > We used or satnav for apprx. 23'000 nm and we have made during that time
> 3 fatal programming errors.
Well, apparently not completely fatal :-)
--RS
|
| 452.44 | GPS Article in Spectrum | CFSCTC::CLAFLIN | | Thu Dec 16 1993 13:32 | 29 |
| The December 1993 issue of IEEE's SPECTRUM has an article on GPS. It includes
a digest history of the developement of radio navigation, culminating with GPS.
The overal article is about seven pages long.
Page 1:
Shortcomings of Loran and star navigation.
Page 2:
The requirements of 3D navigation. (To drop bombs and missles dummy!).
These are after all milatry inspired and developed systems.
Page 3,4
History of radio navigation.
Page 5
How GPS works.
Page 6
Civilain applications.
Page 7
The future and additional references.
If you talk to me real nice, I may post office mail you a hard copy. I am no
where near the artist to render the drawings, nor the typest to enter the text.
Doug Claflin
dtn 244-7042
hm 508 635-0460
|
| 452.45 | GPS deal being assembled | 3D::SEARS | Paul Sears, ML05-2, 223-0559, Pole 10a | Thu Mar 31 1994 13:28 | 10 |
| Apparently someone in the Flying notes file (VMSZOO::FLYING) is getting
together a group purchase of a Moterola GPS receiver. See note 4826.
For ~$150 it might be a good buy. It does not display or have nav
functions; it only outputs NMEA position sentences. But if you have a
display such as Radar, Fishfinders, MNEA readouts, DSC VHF, etc. that
show such positions, you can use it to provide positions on your net.
It would also work quite well with a laptop.
fwiw,
paul
|
| 452.46 | Free Garmin ROM Upgrades | CSOA1::GELO | | Wed Feb 22 1995 09:50 | 12 |
| 2 years ago I bought a Garmin GPS 75. It has been working fine. Last
week a co-worker showed me his new GPS 45, and I noticed the software
version different. I called Garmin and found that they will upgrade my
software, verify the oscillator is correct, and give an overall check
free! All you pay is shipping. Use the 800 number (1-800-800-1020) to
get a return authorization number first if anyone else plans to have
this done.
The Garmin tech told me the changes in software provide internal
enhancements and you will not see any display changes. I'm very
satisfied with the performance of the GPS 75, and now I'm even happier
I purchased from Garmin. Unmatched factory support!
|
| 452.47 | | LEEL::LINDQUIST | Luke 2:4; Patriots 200:1 | Wed Feb 22 1995 13:10 | 17 |
| �� <<< Note 452.46 by CSOA1::GELO >>>
�� -< Free Garmin ROM Upgrades >-
��
�� The Garmin tech told me the changes in software provide internal
�� enhancements and you will not see any display changes. I'm very
�� satisfied with the performance of the GPS 75, and now I'm even happier
�� I purchased from Garmin. Unmatched factory support!
Last week, at the Miami Boat Show, I asked the Garmin rep
what was in the 'PC Kit' for the model 45 (which I have).
He went out of his way to explain how I could wire up the
the data cable I have, and how I really didn't need to buy
the PC Kit, unless I wanted the software in it. I thought he
was very gracious, and honest in suggesting that I not buy
something.
- Lee
|
| 452.48 | Not so free | CSOA1::GELO | | Tue Mar 07 1995 09:13 | 8 |
| Re: .46
Oops! The jury is out on the free ROM upgrades from Garmin. My friend
called Garmin to get the upgrade and was told it would cost him about
$150. He's calling Garmin back to remind them of MY free upgrade. It is
possible that older ROM versions are being purged. Mine was 2.14.
Updates to follow.....
|
| 452.49 | Free for me... | EYLAK::BATES | Ken Bates | Tue Mar 07 1995 10:08 | 11 |
| > Oops! The jury is out on the free ROM upgrades from Garmin. My friend
> called Garmin to get the upgrade and was told it would cost him about
> $150. He's calling Garmin back to remind them of MY free upgrade. It is
> possible that older ROM versions are being purged. Mine was 2.14.
I called last week, and they said just ship it back and they would update it
for free (mine was 2.21, although I know that the latest is at least 2.23).
They gave me an RA number and never mentioned a charge, asked for a credit
card, or anything.
- Ken
|
| 452.50 | I was quoted $150 also... | WONDER::BRODEUR | | Tue Mar 07 1995 11:50 | 4 |
| Hmmmm, I too called and was told $150! Can't remember now just what
version I currently have but it is NOT the latest that they told me
about. Maybe I'll call back and get a better answer.
|
| 452.51 | It was free... | EYLAK::BATES | Ken Bates | Thu Mar 09 1995 14:57 | 21 |
| > Hmmmm, I too called and was told $150! Can't remember now just what
> version I currently have but it is NOT the latest that they told me
> about. Maybe I'll call back and get a better answer.
Mine just arrived back from Garmin today; updated to version 2.33, and no
charge. Total time from when I sent it in to when I received it back was 6
days. Two new features in 2.33 (from my earlier version 2.21):
o There's an "AutoLocate" mode, which is new under the OPMODE settings.
You use it when you first arrive somewhere. Punch in the approximate
Lat/Lon, and it finds the satellites very quickly. Probably useful when
you travel via plane from one place to another (I just went from
Colorado to Miami, and it would have helped then).
o On the CDI, the "road" is clear, but outside of the road is all
black. Helps locate exactly where the road is, especially if you're
coming in at right angles to the track. before, it was just horizontal
line, and you couldn't tell whether you were above or below the road.
Much easier now.
- Ken
|
| 452.52 | Whats hot in GPS ? | OTOOA::MOWBRAY | This isn't a job its an Adventure | Fri Mar 10 1995 08:53 | 15 |
| I noticed the other day that a marine catalog was open in front of me
at the GPS page, same thing happened this morning .... I deduce that I
am starting to think seriously about one of these things.
Price range seems ($us) to be about $350 - 600 for a hand held and
about $150 for an external mount antenna etc.
I notice that the big difference seems to be number of satelites,
display features and keypad - is that true (other than quality, name
etc.). What else does one need to worry about ?
I had briefly used one of the original Trimble's that had a pretty
arcane GUI but I got used to it fairly quickly, however one other
person that used it still complains that the GUI was tought to use ...
my point being that maybe more keys is imporatant for the casual user.
|
| 452.53 | | SX4GTO::WANNOOR | | Wed Mar 29 1995 20:56 | 3 |
| There's talk on CompuServe's Sailing Forum today about a bug in
Garmin's GPS75, not initializing and freezing up. Three have just
turned up with similar problems...
|
| 452.54 | Low batteries in Garmin 75 | WONDER::BRODEUR | | Thu Mar 30 1995 13:56 | 12 |
| Don't know about that bug but one thing that "bugs" me about mine is
the fact that the batteries can get in a state where they do NOT give a
batteries low message but if you switch to a menu that requires a
little more power to drive the display or turn on the backlight it just
shuts off! Need to power on and go through the entire process of
picking up the satellites etc. I had my first one replaced for this
reason and the new one does the same. New batteries fixes it. Call to
Garmin and they just say replace the batteries. Real PITA is you ask
me!
Paul
|
| 452.55 | Garmin GPS 45 for $300 | RANGER::BAZ | Tom Bazarnick | Mon Apr 10 1995 20:50 | 18 |
| FYI, the Garmin GPS 45 has a list price of $299 at Bliss Discount
Marine in Massachusetts. Bass Pro Shops wants $399.
The Bliss price for the GPS 40 is $349, the same as Cabela's.
The salesman at Bliss said the firmware in the GPS 45 is identical to
that in the GPS 40, and the only difference between the two models is
the superior external (but permanently attached) antenna on the 45.
Does anyone know if that's true? I'm curious why Bliss charges more
for a presumably superseded model.
Based on the talk about ROM version upgrades in previous replies, I'm
also concerned about whether all GPS 40 and GPS 45 units have the most
up to date ROM version. Does anyone know about that, or know how to
find out?
Thanks,
Tom
|
| 452.56 | | OTOOA::MOWBRAY | Wish I didn't know now what I didn't know then | Tue Apr 11 1995 09:33 | 6 |
| Any chance you could post the phone number for Bliss marine ?
or you mail it to me at Otooa::mowbray.
I would appreciate that. By the way, does anyone out there have a 45 ?
what do you think about it ?
|
| 452.57 | I thought the 40 was a scaled down version, not a supercede | MILKWY::HEADSL::SAMPSON | Driven by the wind | Tue Apr 11 1995 10:06 | 33 |
| I thought the 40 was a step down from the 45 and I've seen them priced
that way as well. After reading the PS review and looking at prices we've
picked up a 45 but have yet to use it in a navigation application.
COnsidering the unit cost less than my Loran 5 years ago the price
is pretty good. I've played with it quite a bit and I'm impressed. Entering
a waypoint is unbelievebly simply in comparison to the microprocessor routine
of my loran. I already have more points loaded into the GPS than I ever spent
time to load on my Loran. I am looking forward to having navigation info in my
hand in the cockpit. I have programmed in two routes which we often sail and the
entry was equally as simple as the waypoint entry. My waypoints are 6 character
alpha-numeric which will be a lot easier on the fly than asking for "I think it
was waypoint 07, no that doesn't look good try 06".
From a user standpoint the 45 unit is excellent. I have watched it work
by driving to work with it in my car. With a glass sunroof I can recieve enough
satalites for 3d navigation. Selecting statute readout the speed tends to vary
within a range of about 3 mph below my speedometer but had closer absolute
accuracey when I was stopped or moving slow. VMG is not something you can
take advantage of in a car, but I look forward to using that option on the boat
and can see it as being very useful in reaching any destination whether a
turning mark or a cruising destination. It occured to me that the route option
would be very adventageous when racing on preset courses.
The ability to cary the unit home and program it while planning a route
or cruise is a distinct advantage over the Loran. The compactness of the unit
is also much nicer than what I had seen of handheld Lorans. The battery
consumption is low, but I would not look forward to a season of navigation
without access to ship power.
By the way, nearly everyone is selling the unit for $299, Boat/US, West
but I thought the bliss price was higher.
I really like the unit, I look frward to using it in it's intended
application.
Geoff
|
| 452.58 | worries | OTOOA::MOWBRAY | Wish I didn't know now what I didn't know then | Tue Apr 11 1995 13:24 | 8 |
| I have 2 concerns (read perhaps unfounded worries) about a handheld
GPS. First is that I am still awaiting, after 30 years on boats, my
first major expensive splash and this could be it. Second I am
concerned about the ability of the system to work down below (when
navigating at the chart table) if it has a limited view of the sky.
I know that Garmin or anyone cannot help me if I splash it, however
what about using a handheld down below without an ext. antenna ?
|
| 452.59 | Too weak | CSOA1::GELO | | Tue Apr 11 1995 13:34 | 8 |
| They won't work very well, if at all. Garmin's antenna has a BNC
connection so you can hook up to their external antenna if you go
below. It is an expensive option, so some people fabricate their own
external mount and coax lead below, then hook up inside. I've never
tried it, but I hear it works fine.
Why not borrow a GPS like the type you have interest in, and see if it
works below?
|
| 452.60 | Could you elaborate on the antenna | MILKWY::HEADSL::SAMPSON | Driven by the wind | Tue Apr 11 1995 14:04 | 7 |
| I was wondering about the simple coax extention as a cheap alternative
to the external antenna. It seems like a simple thing to do, but I wasn't sure
of the match or if lead length would weaken the signal. Do you personally know
of people who have done this and had success. It is an alternative I would like
to have.
Geoff
|
| 452.61 | Some more info | EYLAK::BATES | Ken Bates | Tue Apr 11 1995 14:20 | 18 |
| > I was wondering about the simple coax extention as a cheap alternative
>to the external antenna. It seems like a simple thing to do, but I wasn't sure
>of the match or if lead length would weaken the signal. Do you personally know
>of people who have done this and had success. It is an alternative I would like
>to have.
The Garmin (and presumably all others) use an active antenna, so there's no
appreciable loss over the coax. Dave Maloy has a Valiant 32, with the antenna
for his Garmin 50 mounted on the stern railing, and around 20' or so of coax
running to the Garmin, which is mounted on the nav station.
FYI, you should try the unit belowdecks before installing an external antenna.
My GPS-75 worked belowdecks on a chartered Hunter 355, Dave's Valiant, the
Southern Cross (a 65' glass over wood ketch), and my own puny Catalina 22.
There's no doubt that the signal is better if you have an external antenna, but
one might not be absolutely necessary.
- Ken
|
| 452.62 | My life with my handheld GPS | TINCUP::CLAFLIN | | Tue Apr 11 1995 15:44 | 18 |
| My Garmin works about half of the time below decks. I suspect that it partially
depends on whether the satelites can see in through the open hatch.
All last year I used the GPS without any modification. The biggest short coming
I found was battery life. So this year I intend to add the external power to
two locations, one on the mizzen so the helmsman can see it, the other is in the
nav area below decks.
My Scottish blood will not allow me to pay $175.00 (Boat US) for the Garmin
external antenna. Since I can almost use the handheld below decks as is,
degradation from a less than perfect antenna is not a major problem. Any
details on jury rigged systems would be appreciated.
Now I have to go home and take the antenna off. Never don that. Perhaps
instead I should start my taxes. Decisions decisions.
Doug
dtn 592-4787
|
| 452.63 | Extending Cable Worked for Me | ORCAS::JESCHKE_AN | | Tue Apr 11 1995 18:54 | 13 |
|
re .60
My Garmin '75 would not work below on my Lancer 30, but I was
sucessful in fabricating an extension cable to put the antenna
outside. If your careful that the external BNC connection stays dry,
I think you'll be ok. (BTW, I packed silicone sealant into heat
shrink tubing for the external connection).
Good Luck,
Andy
|
| 452.64 | Nice idea | OTOOA::MOWBRAY | Wish I didn't know now what I didn't know then | Wed Apr 12 1995 09:19 | 14 |
| Are you saying that the samll antenna on the Garmin is removable and by
running coax from the unit to a mount for the antenna that you can jury
rig an external antenna ?
If that is the case, my question would be how "repairable" would this
be ? I would still like to be able to take the hand held unit with me,
perhaps X-Country skiing or perhasp canoeing - would that be possible
?
Also does anyone know if there are appreciable differences between the
locations and the signal strngths ? I know from a datacom point of
view that in Newfoundland and Labrador, we sometimes get weak signals
because we are the extreme limit of the satelite available.
|
| 452.65 | Easy | EYLAK::BATES | Ken Bates | Wed Apr 12 1995 11:58 | 20 |
| > Are you saying that the samll antenna on the Garmin is removable and by
> running coax from the unit to a mount for the antenna that you can jury
> rig an external antenna ?
The Garmin antenna connects to the main unit via a BNC connector attached to a
swivel. You remove the antenna by twisting the BNC connector. You could then
mount the antenna "somewhere", and run coax with BNC connectors on each end
from the antenna to the unit.
> If that is the case, my question would be how "repairable" would this
> be ? I would still like to be able to take the hand held unit with me,
> perhaps X-Country skiing or perhasp canoeing - would that be possible
> ?
Assuming you've got the antenna mounted remotely, you would then untwist (?)
the coax BNC connector on the antenna end, remove the antenna from its mount
(that you somehow fabricated), untwist the coax BNC from the main unit, and
plug the antenna in.
- Ken
|
| 452.66 | Interesting ... | OTOOA::MOWBRAY | Wish I didn't know now what I didn't know then | Wed Apr 12 1995 12:03 | 8 |
| Pardon this dumb one then but if its that simple, why do Garmin make an
external antenna ? If I was them I would sell a small external
mounting bracket with the unit and then market it as the first handheld
with its own external antenna .... damn I'd be rich !
Is there anyone out there that knows of a marine supply centre near
Portland Oregon ? My wife is going to a meeting there in June and
could buy a GPS there and carry it back duty free.
|
| 452.67 | It really is that easy | EYLAK::BATES | Ken Bates | Wed Apr 12 1995 13:45 | 8 |
| > Pardon this dumb one then but if its that simple, why do Garmin make an
> external antenna ? If I was them I would sell a small external
> mounting bracket with the unit and then market it as the first handheld
> with its own external antenna .... damn I'd be rich !
Sounds like an opportunity. And yes, it really is that simple.
- Ken
|
| 452.68 | Re .66, .64 | ORCAS::JESCHKE_AN | | Wed Apr 12 1995 14:18 | 16 |
| Re .66
Try West Marine. I've always liked their no questions asked
return policy, and their located across the U.S..
West Marine
1176 N. Hayden Meadows
Portland, Or.
503-289-9822
Re .64
Make sure that the outside connection stays dry. I used heat shrink
over that connection so I could remove it and replace it for mobility.
|
| 452.70 | positive on the price | MILKWY::MILKWY::SAMPSON | Driven by the wind | Thu Apr 13 1995 15:56 | 10 |
| We just bought ours a few weeks ago at boat/US.the price was 299, then
we had a 10% discountmaking the cost 269. That's the bare bones unit
and add ons can add up. But with a piece of coax and $20 cigaret lighter
power plug we wont need a whole lot more for accessories. Unless of
course we want to spend lots more money, buy a laptop and feed the info
into a chart program. That's a good way to get lazy, but I think we'll
buy sails first.
It's a pretty cool tool!
Geoff
|
| 452.71 | A bit of history | TINCUP::CLAFLIN | | Thu Apr 13 1995 16:33 | 15 |
| I can not help but mention some interesting history on GPS's here.
I just read the original replies in this topic. Way back then, GPS was not
really trusted and VERY expensive. Can you say $20k? The Navy Satnav was the
hot open water navigation system. Loran was thway to go for almost everyone
both then and forever more.
Time marches on. Is there anyone out there with Satnav. I still have an
RDF, but a GPS is now cheaper than an RDF is. GPS is now actively in competition
with Loran for the low end, and it owns the high end.
Read the notes, it is interesting.
Doug
dtn 592-4787
|
| 452.72 | Fix note to correct price | STAR::KENNEY | | Thu Apr 13 1995 17:57 | 25 |
| <<< UNIFIX::DISK$AUX_SYSTEM:[NOTES$LIBRARY]SAILING.NOTE;1 >>>
-< SAILING >-
================================================================================
Note 452.69 GPS 69 of 71
STAR::KENNEY 17 lines 13-APR-1995 14:48
-< Curious...... >-
--------------------------------------------------------------------------------
RE: .55
Are you sure the GPS 45 is $299 I have the latest flyer from West
Marine, and Boat US and they both list it at $399. I had been thinking
of a handheld to carry in case the fog rolled in.
The Rhodes 19 fleet won't allow it while racing but does not
prohibit carrying one for emergencies. At $300 it may just fit the
budget at $400 it gets a little harder to justify. I am surprised
that Bliss is $100 cheaper than these other places.
I still have to wait to see if the budget can stand it. The baot
needs new standing rigging, sails, and lord knows what other expensive
items to get it race ready......
Forrest
|
| 452.73 | OK thanks | STAR::KENNEY | | Thu Apr 13 1995 17:59 | 7 |
|
Thanks will call Boat US and see if they will mail order it at that
price...... Thanks.....
Forrest
|
| 452.74 | Boat US confirmed 298.50 | STAR::KENNEY | | Fri Apr 14 1995 15:34 | 7 |
|
I called and the current price is 298.50 this is not a sale price
according to the home office. Thanks to notes I now have another tool
to consider for this season.
Forrest
|
| 452.75 | | LEEL::LINDQUIST | Pluggin' prey | Sun Apr 16 1995 10:40 | 14 |
|
I have the Garmin GPS 45. While it is possible to disconnect
the antenna, and use a piece of coax between the antenna and
the gps; when I tried this, it didn't work.
Originally, the GPS 45 was sold with a mounting bracket, and
a power/data cable included. This, and the removeable
antenna differentiated it from the 40. Recently, the 45 has
been sold without the bracket and the cable. I think this
may account for the wide range of prices between vendors.
I'm still waiting for the casio gps watch...
- Lee
|
| 452.76 | why ? | OTOOA::MOWBRAY | Wish I didn't know now what I didn't know then | Mon Apr 17 1995 08:48 | 1 |
| Any thoughts on why it did not work ?
|
| 452.77 | Is the antenna waterproof? | RANGER::BAZ | Tom Bazarnick | Tue Apr 18 1995 14:19 | 7 |
| The instructional video might have been part of the $400 price also.
The Garmin 40 and 45 are advertized as waterproof and nigrogen filled,
but what about the swivel attachment on the 45 antenna? Can salt water
possibly get in there and corrode the connection?
Tom
|
| 452.78 | GPS 75 ANTENNA | TINCUP::CLAFLIN | | Mon Apr 24 1995 13:44 | 28 |
| Well I had some time this weekend for two trips to Radio Shack. The first trip
resluted in a standard size BNC connector on a 6' cable. Connector is too
large. Back for the smaller size. Since cable TV typically uses the smaller
size this was also a shelf item.
The 6' cable was a male/male (jack/jack) type. I needed male/female
(jack/socket). So I bought a sex converter (coket/sokect). Coax was rated at
52 ohms. Cable and converter cost about $8.00, tax included.
I still have no idea what the impedance of the antenna is. In theory a perfect
match would result in no signal attenuation. That night I tried to figure out
the empirical loss. I did not get a noticable change in signal strength when
adding the COAX. In any event this will improve my below decks recption. Looks
like a go to me.
My below deck nav station is located by a dorade. So the cable is going through
the vent with and the antenna gets mounted to the doarad box on deck. No new
holes in the boat. I hope to protect the antenna connector inside the dorade
box.
Ideally I can buy a second "portable antenna" from Garmin on the cheap. If so,
I will leave the one permanently mounted. If not, it is a simple twist to
remove it from the coax.
Thanks to Ken for starting me on this.
Doug
dtn 592-7487
|
| 452.79 | Satellite S/N numbers | EYLAK::BATES | Ken Bates | Mon Apr 24 1995 14:20 | 22 |
| >I still have no idea what the impedance of the antenna is. In theory a perfect
>match would result in no signal attenuation. That night I tried to figure out
>the empirical loss. I did not get a noticable change in signal strength when
>adding the COAX. In any event this will improve my below decks recption. Looks
>like a go to me.
Among other things, the NMEA output for a GPS includes the SNR for each
satellite in view, duplets of the satellite number and SNR (from 0 to 99 db).
I'm writing a Mac program to capture/display the NMEA data, but you could also
hook up a terminal emulator (or even a terminal) to see the NMEA sentences,
since they're plain ASCII.
If you monitored the SNR of specific satellites with and without the remote
antenna, above and belowdecks, you might be able to get a more objective look
at any signal strength issues. Of course, you would have to switch things
quickly, since the satellite strengths change, but over a period of time, you
should be able to get a good estimate.
I don't have the NMEA spec with me, but if anyone is interested, I could post
the specific NMEA sentence structure relating to satellite strengths.
- Ken
|
| 452.80 | | LEEL::LINDQUIST | Pluggin' prey | Mon Apr 24 1995 17:25 | 11 |
| �� <<< Note 452.79 by EYLAK::BATES "Ken Bates" >>>
�� -< Satellite S/N numbers >-
��Among other things, the NMEA output for a GPS includes the SNR for each
��satellite in view, duplets of the satellite number and SNR (from 0 to 99 db).
��I'm writing a Mac program to capture/display the NMEA data, but you could also
��hook up a terminal emulator (or even a terminal) to see the NMEA sentences,
��since they're plain ASCII.
The garmin 45 displays bar graphs showing the strength of
each satellite being tracked.
|
| 452.81 | | LEEL::LINDQUIST | Pluggin' prey | Wed Apr 26 1995 10:22 | 19 |
|
I was at radio shack yesterday, and they had a:
"DUAL SUCTION-CUP BNC ANTENNA MOUNT"
6' of rg58 cable with a male bnc connector on one
end, and a female on the other. And, the antenna
end has to suction cups to stick it to something.
I claimed to be perfect for use with handheld scanners and
all walkie talkies.
I tried it with the garmin 45, and it worked! I 'navigated'
into zko this morning, with the antenna stuck to the back
window of my truck. It's hard to correct the cross track
error, if you stick to the roads...
Not bad for $11.
- Lee
|
| 452.82 | | POBOX::ROGERS | hard on the wind again | Wed Apr 26 1995 12:15 | 7 |
| Not if you have a Jeep....... :>)
I have review and reviewed this string. sounds like I just must get a
Garmin 45.
who needs a VHF anyway...... :>)
|
| 452.83 | New Garmin 45 price at BPS | RANGER::BAZ | Tom Bazarnick | Thu Apr 27 1995 16:02 | 6 |
| I just got the latest Bass Pro Shops sale catalog, and the price
for the Garmin GPS 45 has been reduced from $399 down to $299.
So it looks like word has gotten around and this will be the
new stable price.
Tom
|
| 452.84 | Aviation version of Garmin (GPS55) | PDMOPS::LOKEN | | Thu Apr 27 1995 16:27 | 13 |
| I have a Garmin 55 I use for flying. This unit is virtually identical
to the model 50 except I have a database of all the airports and radio
navigation beacons. This unit comes with (no extra charge) a detachable
antenna, six feet of RG58 and a suction cup mount with the male BNC
connector. If you don't want to make up your own I suspect you can
purchase it directly from Garmin. It also comes with a Yoke mount which
the unit can be plugged into, including external power connections.
This yoke mount would be perfect topside at a steering station.
If anyone is interested I can bring in the paperwork and try to get
some order numbers.
--Harlan
|
| 452.85 | GPS 45/55 product retirement? | CHICHZ::EDELMANN | | Fri May 05 1995 17:15 | 17 |
| The recent drop in prices for the GARMIN GPS 45 and 55 is a result of
the introduction of a new GARMIN GPS unit. The Aviation version of
this is called the GARMIN GPS 90, and includes a moving map and
aviation database with all airports, VORs, NDBs and intersections in
Canada, North America, Central America and South America. The GARMIN
GPS 90 was selling for ~$670.00 at Sun 'n Fun, the spring EAA fly-in
down in Florida. As a comparison point, I paid $1400.00 for the GARMIN
GPS 95 2 years ago. The new GARMIN 90 is about 6 inches high, 2 inches
wide, and 1 inch deep. It also has a moving map display with aviation
database identical to that of the 95. One striking improvement was
battery life. The new unit runs 15-20 hours on 4AA Alkaline batteries.
The drop in price of the 45 could easily signal eventual product
retirement, with a replacement marine unit in the form factor of the
GPS 90.
John
|
| 452.86 | I thought it was an ASIC | MILKWY::HEADSL::SAMPSON | Driven by the wind | Mon May 08 1995 09:35 | 18 |
| I'm not so positive about the GPS 90 replacing the 45 and that driving
the price down. Your description of the 90 and, both dimentional and
battery life, is remarkably similar to the 40 and 45. I am led to believe
that there are two factors that have brought the price down. I think the
first is competion, several units are in this price range now. The other
factor which I believe has contributed to reduction in price, geometry,
and power consumption, is the development of an ASIC component. Building
an Application Specific IC would allow for fewer drivers for data communitcation
and driving the signal is a major power consumer. Once a variety of components
can be built in to one chip, the cost of material and aqusition comes down
and it fits in less space. From your description of the 90, Id suspect the
basic unit is the same with a different data base installed in the ROM
which is probably on the same ASIC.
The 45 has been selling to rapidly for it to be end of lifed, the stock
would be depleted.
Geoff
|
| 452.87 | Got it ! | OTOOA::MOWBRAY | set profile /presonal_name= '';EXIT | Mon Jun 12 1995 09:45 | 18 |
| Garmin GPS 45 - $ 289 in the M&E catalog. West honoured their price
and so I am now the proud owner of a lovely little unit.
I just got it yesterday when my wife returned form Portland Oregon, she
wanted to unpack and tell me about her trip, while I was checking the
location of the back yard so the compromise was that I have not fully
checked it out. The User interface seems very good, especially Vs. my
PL 99 hand held Loran which requires you to remember function codes
etc.
This device will be perfect for Sailing as well as Cross-Country Skiing
etc. I cant remember if I mentioned this before but I have a friend
who has the Trimble Ensign and he sets his rabbit snares (in the
winter) as waypoints so he can navigate around and check them even in a
snowstorm.
Now if someone would only invent a GPS that has a built in Cellular
phone and electric razor it would be great !
|
| 452.88 | | LEEL::LINDQUIST | Pluggin' prey | Mon Jun 12 1995 13:57 | 19 |
| �� <<< Note 452.87 by OTOOA::MOWBRAY "set profile /presonal_name= '';EXIT" >>>
�� -< Got it ! >-
�� Garmin GPS 45 - $ 289 in the M&E catalog. West honoured their price
�� and so I am now the proud owner of a lovely little unit.
For a short time, I've placed a shareware program at:
tle::FORD5$:[LINDQUIST.PUBLIC]GARDOWN5.zip
This will let you up/download waypoints and routes to the
garmin 45.
You do need the overpriced garmin cable, which I don't think
ships in the $299 version.
You can also use a terminal emulator (or gasp, a terminal) to
see the nmea 'sentences'...
|
| 452.89 | be gentle, I'm in sales ! | OTOOA::MOWBRAY | set profile /presonal_name= '';EXIT | Tue Jun 13 1995 13:34 | 11 |
| Thanks I appreciate that.
??Gasp a Terminal ?? .... I live my life through a VT220 and a
multiplexed line at 9600 baud, you would not believe the technical
gyrations I need to go through to get things like the .zip file you put
on the net for me, however I love it.
I did get the cable with the NMEA output, now for a dumb question
though, how do I read ascii from that ? What I mean is is it a simple
as connecting the output wire to a DB25 and plug it into the VT220 ?
nah couldn't be that simple ?
|
| 452.90 | | LEEL::LINDQUIST | Pluggin' prey | Tue Jun 13 1995 18:16 | 33 |
| �� <<< Note 452.89 by OTOOA::MOWBRAY "set profile /presonal_name= '';EXIT" >>>
�� -< be gentle, I'm in sales ! >-
^^^^^^^^^^^^^^^^^ Great line.
�� I did get the cable with the NMEA output, now for a dumb question
�� though, how do I read ascii from that ? What I mean is is it a simple
�� as connecting the output wire to a DB25 and plug it into the VT220 ?
�� nah couldn't be that simple ?
The garmin outputs rs232 level signals at 4800bps. So, if
you set your terminal [emulator] to 4800bps, and wire up
the cable to the terminal, you will see the nmea sentences.
They will show lat, lon, time. Also route information
(time/distance to go, etc.) if you are on a route. The
garmin pin outs are: data-in data-out, ground, and power.
You only need data-out and ground. Data-in if you want
to upload. The gps-45 accepts +5-+40 as an external power
source, so you could potentially use the +5 volts from a
pc to power it, through the same cable. I didn't bother
with power.
One of the garmin setup menus is for I/O. For this, is has
to be set to NMEA/NMEA. If you want to upload/download
waypoints, you have to set the I/O to GRMN/GRMN to use their
private protocol.
Not much more than fun on a terminal, but if you get this
stuff into a pc, there is lots of play value.
Time-stamped postions of your day, moving map displays, etc.
I hope the above doesn't offend anyone, as this IS the
sailing conference, where folks scoffed at the idea of
electronic charts a year or two ago.
|
| 452.91 | Thanks Again, Good Stuff | OTOOA::MOWBRAY | set profile /presonal_name= '';EXIT | Wed Jun 14 1995 08:11 | 14 |
| Thanks again for all the info. I got the software yesterday and
unfortunately I have a bunch of meetings today, else I would take the
day off and play with the cable !
I'll look forward to connecting the GPS to my PC and then play. I must
admit though that I find that I keep thinking about things like hooking
the GPS to the autopilot etc. - yet I know in another note somewhere I
agreed that it was overkill.
The ultimate aim now seems to be to be able to fire up a PC, cast off
the lines and send the boat out for a sail by itself while I can work
late!
|
| 452.92 | changes to differential GPS coming | DELNI::CARTER | | Wed Jun 14 1995 10:31 | 13 |
| I was reading the latest Local Notice to Mariners from District One of the
USCG. One of the first articles in the notice is notification that as of
Sept 1, 1995 the differential GPS corrections broadcast from Portsmouth
N.H. and Montauk N.Y. will change from Type 1 to type 9-3. The notice
indicates that "some equipment' will have to be returned to its
manufacturer for a firmware upgrade.
Is the impication that the differential receivers will have to be
modified to accept both Type 1 and type 9-3? Or, have the other
differential transmissions already been in 9-3 format?
djc
|
| 452.93 | Article on GPS type 9-3 in Ocean Navigator | UNIFIX::FRENCH | Bill French 381-1859 | Wed Jun 14 1995 11:05 | 13 |
| There is an article on GPS types in this month's ocean navigator.
If I recall correctly, type 9-3 had been defined before GPS went on
line and it was stated early on that type 9-3 would be used.
It sounded like most manufacturers should have been able to interpret
9-3 from the early days but it's possible that some did not.
The article made it sound like it was going to be more of a theoretical
problem rather than actual, but it might be necessary for some to
send their DGPS receivers in for new firmware.
Bill
|
| 452.94 | Long term use of GPS is very good | MCS873::KALINOWSKI | | Mon Jul 10 1995 16:25 | 4 |
| Talking to a friend Friday night. He told me he was working on some
software to give better accuracy for positioning satillites. The
numbers he was throwing around would make even Alan happy for finding
those rocky channels in the fog. Designed on a VAX of course ;>)
|
| 452.95 | Just a note | STNKBG::MELENDEZ | | Tue Jul 11 1995 12:35 | 7 |
| Software to fix the problem with the GPS accuracy may be
illigal, The navy places in the error to provide for national
security.
Check first.
Joe who worked at Naval Satellite Operations Center.
|
| 452.96 | | UNIFIX::BERENS | Alan Berens | Tue Jul 11 1995 13:33 | 17 |
| re .95:
Gee, I certainly hope that more than just a bit of software is needed to
correct the (intentional) positioning errors in GPS positions. If not,
why bother degrading the accuracy? I suspect that what .94 is referring
to is software that collects many, many GPS (inaccurate) positions and
averages them in some way to give a more accurate position.
Practical Sailor just reported on the positional accuracy of DGPS. They
showed a plot of the positions given by a fixed-in-one-place GPS. The
data scattered rather widely (almost 0.1 degree in latitude and
longitude) but when plotted one could see that the scatter was around
the actual location of the GPS receiver. Trouble is, of course, that
when we're sailing, we don't have the opportunity to average hours of
data. We have to use whatever position our GPS gives and hope for the
best.
|
| 452.97 | | MCS873::KALINOWSKI | | Tue Jul 11 1995 14:27 | 4 |
| re .94 He is working for the US gov at an R&D lab, so there is
nothing illegal about this.
.95, Yup, it uses lots of gps positions.
|
| 452.98 | | STNKBG::MELENDEZ | | Tue Jul 11 1995 20:40 | 10 |
| Not trying to stir up trouble here, gang!
I still think a sextant is a good investment.
even on a lake it can be used with a good topomap to
determine relative position. i will admit not much
good in a fog though.
BTW: military GPS equipment doesnt see the error we do!
Joe
|
| 452.99 | gps vs loran | STARCH::HAGERMAN | Flames to /dev/null | Thu Jul 13 1995 15:44 | 13 |
| I heard an interesting theory the other day. The claim was made
that, if the goal is to get to a specific place, LORAN is better,
while if the goal is to get back to the same place as you were
before, GPS is better. (Which sounds like saying that LORAN is
more accurate while GPS is more precise, although it wasn't
described in those terms.) According to this theory, LORAN fixes
would scatter around a point some given distance from the correct
point, while GPS fixes would scatter in a smaller pattern around
a point that is further from the correct location.
Just a claim propounded by an old LORAN enthusiast, FWIW.
Doug.
|
| 452.100 | | TINCUP::CLAFLIN | | Thu Jul 13 1995 15:57 | 15 |
| Actually my understanding is just the opposite.
If you enter Lat and Lon GPS will get you "there" better than Loran. However,
if you are some place and store its location in Loran, you will do better at
returning to you fishing site. I think this has to do with the inaccurracies
introduced by selective availability.
I am like a man with one watch, (who always knows what time it is). I use
traditional coastal piloting (charts compas etc), and enjoy the luxury of a GPS.
I do not want an instrument rating for Holiday II.
Anyone know for sure. I bet PS had an article in the last couple of years,
or West Marine.
a differnt Doug
|
| 452.101 | | UNIFIX::BERENS | Alan Berens | Thu Jul 13 1995 16:25 | 31 |
| re .99:
Hmmmm. My understanding is that GPS is more accurate than loran in the
sense that a random position measurement is more likely to be correct.
If many GPS measurements are plotted, they will scatter around the
correct or true location. The scatter may be fairly large (see .96). I
have observed this behavior with my Magellan GPS.
Lorans, on the other hand, seem to give more consistent measurements,
though the measurement may be less accurate. What loran does do well is
give repeatable measurements over long time periods. When I sailed to
Provincetown in 1989, I measured the TDs for the Race Point bouy. I
measured them again two weeks ago. The measurements differed by 0.1
microsecond.
In practice, both loran and GPS can be very accurate, but one does need
to understand their limitations and where errors might occur. Some of
our charts are based on the NA1927 datum, some on a later datum. If I
forget to enter the proper datum in our GPS, the positions it gives can
be rather wrong. Gotta be careful. Loran doesn't care about chart
datums. As long as I've been somewhere before, my loran gets me back
there again (plus/minus very little).
By the way, accuracy and precision aren't the same thing at all. A
precise measurement (eg, my GPS gives lat/long to .001 deg) may still be
very inaccurate. That is, the precisely displayed GPS position may be a
long ways from where the GPS receiver really is. A less precise
measurement (eg, my loran only display lat/long to 0.1 deg and TDs to
0.1 microsecond) may actually be more accurate (nearer reality).
Alan
|
| 452.102 | If ya need more, stay home..... | MCS873::KALINOWSKI | | Thu Jul 13 1995 16:32 | 4 |
| Lets put this gps scatter factor into perspective. In the P.S. article
they said take your normal 1:40000 chart. do a dot with a pencil on
your chart.
There's your avg scatter.
|
| 452.103 | what if you use a sharper pencil? | UNIFIX::BERENS | Alan Berens | Thu Jul 13 1995 18:25 | 10 |
| re .102
Er ..... I use a 0.5 mm pencil, which draws a line approximately 0.02
inch wide. On a 40000:1 to chart, a line .02 inch wide is about 67 feet.
The scatter reported by PS is about .05 minute, which is (in latitude)
about 300 feet, or about 5 pencil line widths. Yeah, offshore so what?
If you're navigating using larger scale charts, the scatter/inaccuracy
of GPS positions may matter.
|
| 452.104 | | POBOX::ROGERS | hard on the wind again | Fri Jul 14 1995 01:54 | 8 |
| In my recent voyage, GPS and Loran never deviated from each other more
than .2nm. There were places in northern lake Michigan where the Loran
kept crashing until I put the antenna extension on. My Apelco has never
been so well behaved as since I got the Garmin.
Why is that?
|
| 452.105 | Lat/Long to UTM? | KAOFS::LOCKYER | | Fri Jul 14 1995 14:59 | 9 |
| Good Day All (especially Mr. Kalinowski)!!
Just bought a GPS (for ballooning) and hooked it up to my PC. All the
position data from the GPS is in lat and long, but we use UTM for
balloon competitions. Anyway know how to convert lat and long to UTM?
Regards,
Garry, who sails the skys!
|
| 452.106 | is UTM a TLA? | WRKSYS::SCHUMANN | Never tested on vegetables | Fri Jul 14 1995 15:15 | 3 |
| What's UTM?
--RS
|
| 452.107 | UTM = Universal Transverse Mercator | KAOFS::LOCKYER | | Fri Jul 14 1995 15:32 | 7 |
| Sorry,
UTM = Universal Transverse Mercator projection. On Canadian (and
others) 1:50,000 topographic maps, there is a 1,000 metre UTM grid that
is much easier to use than lat and long.
Garry
|
| 452.108 | | UNIFIX::BERENS | Alan Berens | Fri Jul 14 1995 15:41 | 6 |
| re .105:
On my Magellean 5000, you rummage around in the setup mode and
eventually find where you can set the output display to UTM. Dunno about
other models/manufacturers.
|
| 452.109 | GPS Displays UTM, Sends Lat/Long To Data Out | KAOFS::LOCKYER | | Fri Jul 14 1995 17:50 | 3 |
| I have a Garmin GPS 75 and I can get it to display UTM, however
position data from the GPS to the PC is in lat/long, not UTM. So, it
looks like I have to do the conversion myself (in the PC)...
|
| 452.110 | SMOP ? :-) | MOVIES::WIDDOWSON | Brought to you from an F64 disk | Sat Jul 15 1995 06:36 | 1 |
|
|
| 452.111 | Check with the NEMA folks | MCS873::KALINOWSKI | | Mon Jul 17 1995 11:52 | 13 |
| re .109 Gary, just get a copy of the core_aes$utm_conv.bas function from
Bevan and recompile ;>) ;>) (inside joke).
Actually the output of the Garmin is for the NEMA commands. I would
suggest a call to garmin or maptech to see if they know of a peice of
public domain software to make the correction.
IF they cann't do it, maybe a local call the Canadian foresty or
wilderness IS folks in Ottawa to see how they do it .
Good Luck.
john
|
| 452.112 | Don't Need No Stinkin' PD Code - At Least, Not Yet!! | KAOFS::LOCKYER | | Mon Jul 17 1995 13:41 | 16 |
| G'Day John!
Just talked to Bevan about core_aes$utm_conv.bas and he says that
you always meant to write it, but never delivered. Mutterred
something about FLS too...... :^))
Anyway, as luck would have it, I had a textbook about cartography on
the shelf at home and I now know more about UTM than I ever wanted to.
After a couple hours of hacking, I think I've almost got it - just need
a littel more work to make it better agree with the GPS. Should be
done tonight....
Regards,
Garry
|
| 452.113 | | MCS873::KALINOWSKI | | Mon Jul 17 1995 14:18 | 8 |
| Watching some TV last night, there was a show on about deep sea
fishing in alaska. The person asked the captain if he used Loran. He
replied he used to but got rid of it for GPS. Evidently in remote
areas, the power for the land transmitters sometimes goes down for up
to 20 minutes. He also said certain conditions (ie fog and heavy seas)
could screw up the fix. With GPS, all these problems have disappeared.
Just a datapoint.
|
| 452.114 | Loran accuracy? | OFOSS1::GINGER | Ron Ginger | Tue Jul 18 1995 22:04 | 6 |
| Ive been playing with a Loran in Maine. If I take its reading while
tied to my dock, and plot it on my chart it shows me about 1/2 mile inland.
How accurate should I expect Loran to be? I understand the point that
Loran will be very repeatable, and I am planning to run thru my safe
route home and record the actual way points.
|
| 452.115 | | UNIFIX::BERENS | Alan Berens | Wed Jul 19 1995 09:55 | 30 |
| re .114:
Ron, I'd guess that the answer to your question is buried somewhere in
one of the notes about lorans. Loran signals travel at slightly varying
speeds over water and land. The TD to lat/long conversion algorithms in
lorans assume a model of the earth that is locally inaccurate. The
magnitude of the local inaccuracy is location dependent. Inaccuracies
are greatest close to land. The half-mile error you have is not
unlikely for an uncorrected loran.
For my early 1980s TI loran, I enter corrections to the TDs. These are
added to/subtracted from the measured TDs prior to the lat/long
conversion. The corrections result in the proper lat/long being given.
At the Manana Is whistle '14M' off Monhegan, the corrections are +0.7
microsecond for the 13000 line and +3.4 microseconds for the 26000 line.
Actually, my loran will determine the needed corrections for me. The
corrections are generally good for a largish area, though I have found
some very local and very significant errors (eg, off Winter Harbor past
Mt Desert). The corrections in Rockland Harbor for the 13000 and 26000
lines are +4.0 and +4.4 microseconds.
Over the years I've measured the TDs of the bouys and other points of
interest to me, so I don't have to worry about whether or not the
lat/long conversion is correct.
By the way, I find it MUCH easier to plot a rough loran position on
charts with loran TDs than to plot lat/long positions since the TB lines
are printed so close together. Another good reason to keep using my loran.
Alan
|
| 452.116 | | LEEL::LINDQUIST | Pluggin' prey | Wed Jul 19 1995 12:43 | 22 |
| ��By the way, I find it MUCH easier to plot a rough loran position on
��charts with loran TDs than to plot lat/long positions since the TB lines
��are printed so close together. Another good reason to keep using my loran.
I've heard this from quite a few people.
The current charts are just not set up to use lat/long
easily.
I'm beta testing some electronic charts that run under win95.
(I know this file has a distaste for electronic charts) But,
it's pretty nice to just motor around with the mouse cursor,
and have the lat/lon windows scroll to reflect the position.
Zoom in to whatever accuracy you'd like, and it's at least as
accurate as the paper method. (Although visting the point,
and marking the location still seems to be the best.)
I can well imagine plotting every point of interest into a
file, and then uploading only those of interest for a
particular cruise into my GPS.
I can also imagine paper charts with lat/lon overprinting. Maybe
this is a product idea!
|
| 452.117 | | SHIPS::GOUGH_P | Pete Gough | Thu Jul 20 1995 04:33 | 8 |
| I am a tad confused but do I get the impression that your charts of all
scales do not have lat/long around the edges and if they why the
problem in plotting?
just curious.
Pete
|
| 452.118 | | UNIFIX::BERENS | Alan Berens | Thu Jul 20 1995 09:48 | 16 |
| re .117:
Pete, yes, the charts do have lat/long scales along the edges and
lat/long line across the chart, but a very large intervals. To plot a
lat/long fix, I need to lay the chart flat enough to be able to use a
parallel rule to do the plotting. This isn't always easy at night or in
rough weather and it always does take some time. And we have a better
than most chart table to do the work on.
The loran lines are printed at fairly close intervals -- usually 10
microsecond spacing -- all across the chart. I can very quickly locate
the two appropriate lines and visually interpolate between them to get a
fairly accurate fix plot in just a few seconds.
Alan
|
| 452.119 | | SHIPS::GOUGH_P | Pete Gough | Thu Jul 20 1995 11:43 | 12 |
| Alan et al over here (U.K.) instead of parallel rulers we have a device
called a Breton Plotter which once used means you tend to throw the
parallel ruler away!!! My chart table takes half a standard Admiralty
chart and I find that whatever the conditions I can plot quickly and
effectively.
If any one wants further details mail me and I will endeavor to get
details for you. They cost about 5 pounds Stirling.
Pete
|
| 452.120 | | LEEL::LINDQUIST | Pluggin' prey | Thu Jul 20 1995 11:49 | 13 |
|
Clearly the solution is to have a boat/chart-table large
enough to accomadate a drafting machine. I just need to
convince Bob Palmer that this is a necessity, and I'll get
the needed raise.
It would seem to be straight forward to over-print lat/lon
lines on charts, just the way the loran td's are overprinted.
But, I don't expect to see it happen on the NOS charts, as
they seem to be doing everything possible to cut costs.
Maybe on some of the chart kit knock-offs...
|
| 452.121 | | LEEL::LINDQUIST | Pluggin' prey | Thu Jul 20 1995 11:53 | 15 |
| �� <<< Note 452.114 by OFOSS1::GINGER "Ron Ginger" >>>
�� -< Loran accuracy? >-
�� Ive been playing with a Loran in Maine. If I take its reading while
�� tied to my dock, and plot it on my chart it shows me about 1/2 mile inland.
Are you plotting the loran displayed time-delays, or letting
the loran convert to lat/lon?
If you're plotting td's, I dont' have a clue.
If you're using lat/lon, it's possible that your loran and
your chart are using different datum.
In the meantime, I wouldn't rely too heavily on a waypoint
arrival alarm!
|
| 452.122 | | TINCUP::CLAFLIN | | Thu Jul 20 1995 14:31 | 6 |
| Since the topic of parallel rules and drafting tables came up, I thought I would
offer another twist. I beleive the product is called the Yoeman. This is
strictly a mechanical devie (there is an electronic version from BBA(?). It is
essentially a small drafting table. Holds about 1/2 of a NOAA chart. I have
never used one. The only one I have seen in real life belonged to Mike
Ktistaskis. I think it costs around $120.00.
|
| 452.123 | check your charts.. | HIGHD::MELENDEZ | | Thu Jul 20 1995 14:40 | 9 |
| This may sound dumb, so forgive if the implication is offensive.
Out here in Califronia, the pole is moving all the time (earthquakes?)
So I have to buy new charts every couple of years. I assume that this
is the case where you are at. Make sure that your not useing an old
chart. I would hate to run aground when I think Im out 4 miles.
Also I would think the tides there would make maters worse.
Joe
|
| 452.124 | Yeoman Plotters | LARVAE::SMARIO::BARKER | | Fri Jul 21 1995 06:22 | 28 |
| Re -1,
I have used a Yeoman Plotter quite extensively on Offshore races, and it is the
next best thing to electronic charts.
The idea is that you clip an ordinary paper chart onto the base, which is
flexible plastic, about the size of an average chart table ( there is a larger
version too ). There is an electronic 'puck' which you slide around the chart.
You register the chart by pointing the puck at three points, and telling it the
long. and lat. of them. Once you have done that, moving the puck around the
chart changes the long. and lat. displayed on the top of the puck.
The other key feature is an NMEA input, which downloads position information
from a GPS/Loran/Decca. There are 4 little arrow shaped LEDs on the 4 sides of
the puck, which show which direction to move the puck to in order to get to your
current position. The puck has a little hole in the middle, through which you
can stick your pencil.
There is also a course/distance feature, and the ability to upload waypoints
from the plotter to the GPS.
Yeoman was originally designed and built by a small English company, which has
just been bought out by a larger English company ( Autohelm ), which in turn is
part of a larger US outfit ( Raytheon ?? ). This should lead to greater
worldwide marketing of this excellent product.
Chris
|
| 452.125 | | SX4GTO::WANNOOR | | Sun Jul 23 1995 15:40 | 13 |
| Re -.2
The magnetic poles may indeed be moving around (a little bit), but all
that affects is the magnetic North and the variation (usually a few
minutes) on the chart. The rate of change of the variation is small and
fairly consistent, and is absolutely no reason whatever to buy a new
chart every two years: neither the latitude, longitude nor Loran-C TDs
change at all, except for continental drift (a few millimeters per
year) or shaoling/coastal erosion.
Indeed, if you get a chart updated at good chart shops, you can quite
happily sail in such well-charted waters as the continental USA with
charts 20 years old or more.
|
| 452.126 | | OFOSS1::GINGER | Ron Ginger | Mon Jul 24 1995 12:09 | 8 |
| Ive done a bit more playing and I guess the problem is being to far
inland. My dock is about 4 miles up the Damarascotta river. I assumed
that was close enough to the ocean to be OK, but apparently not.
I really wanted to be able to work my way up the river past the
narrows, with Loran. Guess Ill have to get a GPS
Anyone want to buy an almost unused Apelco Loran :-)
|
| 452.127 | don't give up the loran | UNIFIX::BERENS | Alan Berens | Mon Jul 24 1995 12:36 | 14 |
| re .126:
Ron, don't be hasty. Loran TDs are quite consistent with location.
Simply go up the river and note the TDs at various places of interest.
Then when visibility is limited use the TDs to navigate, not lat/long.
My loran happily accepts waypoints in either TDs or lat/long, and I
assume yours does too.
My guess is that the TDs will be more repeatable than will GPS lat/long
positions. Remember that GPS positions can easily vary several hundred
feet, which is not what you want navigating a river in limited
visibility.
Alan
|
| 452.128 | I use a GPS | TINCUP::CLAFLIN | | Mon Jul 24 1995 15:53 | 18 |
| Like a man with one watch (who always knows the time), I have one primary
electronic navigation aid (GPS), my RDF is far cruder. I have marked waypoints
bringing me into Manchester. However, my understanding is that a Loran
would actually do a better job (in terms of repeatability).
Enter the way points by going to them in good weather. Probably also a good
idea to WRITE down the TD. Do this for each way point up the river. I have
marked my identified way points on the charts also. By physically going to
them, you will avoid most local varaitation in TD's due to land mass etc. There
is still a very small variation intoduced by things like air density.
Loran does not produce as accurrate a Lat/Lon as GPS, however, it is supposed to
be more exact at repeating a position.
Give it a try and let me know if the thin air out here is destroying my last two
brain cells.
Doug
|
| 452.129 | | POBOX::ROGERS | hard on the wind again | Mon Jul 24 1995 17:39 | 7 |
| and another thing........
once you have a gps, the Apelco will behave impeccably....
smiles......
|
| 452.130 | | OFOSS1::GINGER | Ron Ginger | Tue Jul 25 1995 10:21 | 5 |
| thanks for the tips. I shall continue to work this. I have ignored Tds,
because my chart doesnt happen to have them, but its worth a try.
I have started the process of recording all major points in good
visibility.
|
| 452.131 | Loran accuracy dependent on boat speed? | QUICHE::NEALE | Who can, do - who can't, consult | Tue Aug 01 1995 08:48 | 31 |
| Re: .130 et al
My understanding was that Loran was very repeatable, although to prone to land
effects close inshore/up river estuaries (just like Decca, which has been much
more popular in Europe until the advent of GPS at affordable prices). However, I
was with Ron a couple of weeks back on the Maine coast (in beautifully clear
weather where there was no possible doubt of our position) and the Loran did not
give anything like the repeatability I would have expected - maybe 100-200 yds?
Maybe worse? Certainly I would not have trusted it to navigate in fog back
upriver.
I wonder if speed has an effect at all? I do not know how long a measuring
interval ther Loran receiver uses to determine position, but with a boat speed
of 15kts could this cause problems?
This discussion is very interesting but of no relevance to me - at home on this
side of the Atlantic the main reason for buying Decca is that sets are being
sold cheaply as it will be going out of service in maybe 10 years or so, and GPS
is dropping quickly in price (cheapest I have noticed recently is �300 for a
handheld). GPS is taking over around UK coasts based not on performance but
purely cost and availability.
My only personal experience as navigator using Decca was not actually very
successful. Two years running we used (tried to use...) Desmond the Decca to
navigate cross-channel some 60 miles from the UK south coast to the Channel
Islands. Desmond would work fine for an hour or so from switch on, and then
steadily go into a sulk and claim "not enough signal" until we left him alone
for some time (like overnight). Personally I think it was just sea-sickness, but
everyone else blamed the operator...
- Brian
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| 452.132 | the usual moral to the story applies ..... | UNIFIX::BERENS | Alan Berens | Thu Nov 09 1995 15:26 | 5 |
| A brief article in today's Boston Globe reports that a Maine Marine
Patrol boat (LOA not given) headed from Rockland to Vinalhaven hit a
ledge and sank. The three people on board survived unhurt. Cause of the
loss: Depending on a GPS waypoint that was improperly entered (about 900
yards off).
|
| 452.133 | Let's make GPS more sensitive... | MCS873::KALINOWSKI | | Fri Nov 10 1995 12:03 | 9 |
| re .132 Don't any of these guys run depth sounders with the alarms
on anymore ??? ;>) ;>)
In a airline magazine I was reading a couple weeks ago, there was
a blurb about how the FAA had come up with this new world wide Jetliner
tracking system based upon GPS. The European equivilent authorities to
the FAA told them to go pound sand if the sensitivity was not turned
back on GPS first. So it looks like the FAA may be on our side soon.
|
| 452.134 | | ACISS1::ROGERSR | hard on the wind again | Fri Nov 10 1995 13:00 | 9 |
| We almost won that one with Wilcox Electric. Only Alpha had the CPU
horsepower to do the crunching of over 900 inputs simultaneously. This
thing augments ILS to get to 0/0 landing capability. GPS coords within
9" even with selective availability.
Applications did us in. Need a NetExpert app that does not run on Alpha
but does on AIX .... IBM came first, we're second... Wilcox won the bid
from the FAA.
|
| 452.135 | | ACISS1::ROGERSR | hard on the wind again | Fri Nov 10 1995 13:06 | 15 |
| BTW, could the obstruction have moved?
In 1989, I nearly knocked my keel off with a full speed strike on a
rock that was on the rhumbline. Jib reaching at 7.2knts. This rock, the
size of a dumptruck, was moved by the spring ice from the upper great
lakes. My depthfinder said 14ft (like always) until we found the rock.
And yes, 14ft is 'deep' in lake Erie.
After moving to lake Michigan, I still cringe at sailing races where
the typical depth was 8-9 ft in a fin keel boat that draws 6ft. I use
to punch in the offset rather than the true depth just to get
reactions. Blasting along on a screaming reach with the depthfinder
saying 1.6ft was always worth a laugh.
|