Conference unifix::sailing

Similar "relative accuracy" question occurs when you ask your GPS to watch
if your boat's anchor still grips ... In some cases the alarm shouldn't arise
while moving 40 meters, in other cases a 10 meters position change is critical.

    You can buy a nearly fully featured GPS unit from catalogs at 
    aroung $ 800.00. The Sony Pyxis has received rather poor reviews
    from Practical Sailor. It appears that the Garmin 50 (?) portable
    unit is a top choice at this time.
    
    I dont think that you can expect 1 s response from GPS. At best
    5 to 10 seconds in ideal conditions may be possible. So you still
    should have a conventional knotmeter/log for time resolution. Besides
    you must have some means to DR so a log is a must. With respect
    to accuracy in speed, selective availability, i.e. deliberate
    degrading of positional accuracy, is not constant in space and time.
    So, in principle, there is some velocity component introduced
    by it. I dont know how large it is and how it compares with the
    random noise inherent to the system. I guess the 0.1 kts is 
    probably the limit of accuracy you may expect under ideal conditions.
    
    /D

So far as I can tell, the manufacturer's of marine electronics
(loran, GPS, knotmeters, depthsounders, anemometers, etc) do a miserable 
job of explaining the accuracy limits of what they build. 

Example: The boat speeds displayed by lorans and GPS boxes. For a 
loran/GPS to determine speed, it must use the basic equation

   speed = (distance traveled)/(travel time) 

or 

   s = d/t

Time is no problem. Loran/GPS can measure time intervals very 
accurately. Distance is problematic, however. Assume a boat speed of
6.00 knots and a travel time of 30 seconds. The distance traveled is 
about 303.80 feet. Now assume a boat speed of 6.10 knots. Distance 
traveled is now about 308.86 feet, a difference of only 5.06 feet. 

To measure distance traveled, the loran/GPS must determine your position 
at time zero and again some time later. I think it should be obvious that 
both of these positions will have to be incredibly accurate for a speed 
reading to have even a 10% accuracy. 

Note that this speed is really average speed over a 30 second period. If 
you want to detect speed changes in say, a second, the difference in 
distance traveled at 6.1 knots vs 6.0 knots becomes on the order of 0.2 
feet. 

I've watched loran displays when the loran is not moving. The displayed 
TDs and calculated positions jump around a little, say 0.01 minute of 
latitude and longitude (my older Texas Instruments loran jumps around by
0.1 minute). 0.01 minute of latitude is about 60 feet. 

Without belaboring the point further, it would seem that speeds 
determined by lorans/GPS must be from averaging over a significant time, 
say several minutes, and even then they probably aren't very accurate.
(The manual for my TI doesn't give the averaging time for speed 
calculations, but it seem to five minutes or more.)

It would seem that a conventional knotmeter would be a significantly 
better choice for measuring speed over short time periods.

I am currently in the process of purchasing a new GPS
system. I have decided to go hand-held...

	there are 3 excellent systems for around $1100.

	BUT...

	The Magellian can interface with a Diferential
	system based on land. Together with 5 channel
	satillite connectivity... it seems to be able
	to be accurate to 10 meters or less.. 
	(PS: thats under the US self inflicted max accuracy limits)

	The basic unit is accurate up to 40 meters 
	, sometimes even better...


	Another option is the Ensign unit... I have
	borrowed one and played with it for a few days
	while trying to decide... its also is an
	excellent choise...

	BTW all give very accurate speed, Ground speed
	direction etc...

	I used on in my car the other day just for fun.
	It worked very well and even at high speeds it
	kept up pretty well.

				Georgia

    My experience using Loran speeds led me to believe that the speed given
    is pretty useless.  On a big reach across a bay last year, I saw speeds
    in excess of 9 knots on one hand and speeds of less than 4 on the
    other.  The boat did not slow or speed up appreciably and the dead
    reckoning speed was about 5.75 kts.  
    
    I don't believe that the loran was accurate for speed in such small
    increments but it kind of made me feel good to have a 26 foot waterline
    going through the waves at a (pretend) seed of 9 knots !

    My Loran seems pretty accurate as long as my knot meter isn't too far
    off. When I'm sailing with the tide the Loran is reading higher,
    against it lower. This makes perfect sense to me My knot meter has been
    close enough that before I had a Loran, dead reckoning across the bay
    in a heavy fog, I got anxious to find my mark at about the time I
    sailed just south of it. 
    	If the knot meter is close, I surfed my 22'er up to 9.24!!! I could
    feel I was on top of a wave and had been surfing faster and faster. I
    was psyched to see it go into the 9s. You don't have to believe it,
    but I like to!
    
    Geoff

    I have a NorthStar (Digital Marine Electronics) Loran.  I do not have
    a GPS.  I have carefully callibrated my knot meter by towing a log
    for several hours.  I have an OCKAM instrument system that is
    interfaced to the Loran, a CETREK electronic compass and a PC.  The OCKAM 
    system keeps a ded reckoning position based on the knot log, the wind 
    angle (to calculate leeway), the compass course, etc.  The OCKAM 
    system also compares its calculated ded reconing position with the
    Loran position.  OCKAM assumes the difference is current set and drift.
    The PC (among other things, like plotting true wind direction and speed
    over time) logs all instrument readings every 1/10th of a second and
    can play them back at a later time.
    
    We have used this setup to navigate in competition for both short
    (around the cans) and long distance races to great advantage.  We
    have gotten a 1st and 2nd for the Chapman Bowl (90 miles), 2nd for
    the Corinthean 200 (200 miles), and amassed a pretty good record
    around Mass Bay.
    
    In my estimation, both the knot meter and the Loran have been spot
    on in every situation I can remember.  The Loran does not give good
    instantaneaos speed readings, since it takes a couple of minutes to
    "average" out the jitter.  However, the calculated current set and
    drift is very good.  GPS in low resolution mode, as a previouis reply
    pointed out, has a randomizing factor built in.  This must be averaged
    over some period of time (probably half what a Loran takes) to be
    accurate.  Differential GPS is believed to be extremely accurate by
    the Americas Cup people, but this ins't readily available yet and is
    still very expensive (>$5,000), if you have access to a secondary
    "ground" station.
    
    So, you cannot use GPS or Loran for instantaneous speed.  E.G., not
    good for "let's ease the backstay 100 pounds and see what it does for
    our speed."  However, when holding a course for a few minutes, the
    Loran reading given is almost certainly an accurate indication of your
    speed over ground and course over ground (which, due to current,
    leeway, etc., are almost certainly different than your speed/course
    through the water).  Speed/course over ground and speed/course through
    the water should be similar going dead down wind (no leeway) in no
    current and calm seas.
    
    If not, I would suspect callibration.  Loran's can be "out of
    callibration".  This does not mean that they give a "wrong" answer, but
    that it takes them longer to average out the jitter.  Lorans have a
    built in frequency standard (oscillator) to sync up with the incoming
    signals and calculate time differences.  If the oscillator is of poor
    quality (isn't temperature controlled) or has drifted, your loran's
    ability to "average" out both the jitter due to the incoming signals
    and due to the oscillator will suffer.
    
    Also, your knot meter could be significantly off.  Its base
    callibration, its position on the hull of the boat, differences between
    tacks, etc. can make large (>1 knot) differences.  I would strongly
    recommend getting a known good log to tow around for a day.  You
    probably will be as surprised as I was at the results!

Re  .5  

I think Geoff in .6 is correct here. Ignoring all the instrument errors 
there are at 2 seperate  measurements. 

	*The Loran/GPS whatever is measuring speed over the ground. 

	*The log is measuring speed of hull through water. 


Tidal set/drift won't affect the log speed unless you anchor ;^)  but 
will change your speed over the ground ie the Loran/GPS measurement.

OK, now you can switch on the instrument errors again (which just blurrs 
the picture a little but you see the point). Look at the "speed" fluctuations 
across a bay when the boat is steady on the wind (ie not surfing). 
The speed through the water is more or less constant (ie a steady log reading 
if you had a log). But what of the tidal circulation in the bay and how does 
it change in the hour (or whatever) it takes to cross it?  Maybe soemtimes its 
helping and sometimes its heading. Only takes a 2 knot current added and 
subtracted to your speed through the water + instrument errors etc to make 
your sums add up. I doubt if your water speed was 9 knots  ;^)



re .6 Boat surfing. 

Most logs have very short integration times, maybe a few seconds, so 
they respond to short-term changes in the water speed. My 22 foot boat 
sure accelerates well when picked up by a wave. You can read it on the 
log, hear it on the bow-wave, see it as the front lifts and feel it 
in your gut !  If you don't believe it, surf alongside some big heavy 
old thing that isn't !

The 9+ knots is (somewhere) near it for a few seconds and then is probably 
followed by a few seconds of 3 knots before the whole thing starts again. 
The average speed through the water is probably only one knot higher than the 
displacement hull max speed ie non-surfing, but isn't it a great feeling ! 

If you have a trailing log it exagerated the whole thing (cable wind-up 
and the spinner in a different wave-chain)

 St

    I like my Loran best when going under a bridge!  Wow - my 26' Pearson
    doing 4, 6, 9, 15, 22, 33, 45 knots!  Wow, is it exhilerating!
    
    	Ron

    Hi everyone.
    
    A friend of mine is going to States and would like to buy a
    Garmin 50 portable. Could you pls let me know more infos about
    price and places where he can find it?
    He plans to go to NY, Frisco and San Antonio.
    
    Thanks a lot and bye, Arrigo.
    

    Arrigo Hi !
    
    Your friend can buy the Garmin 50 from many places.
    Here are a couple of them. They both are catalog sales outfits also 
    and can ship if you want.or you can visit them at:
    
    Defender Industries                         Boat US
    255 Main str.                          tel: 800-937-2628
    New Rochelle,N.Y.10802-0820            Fax:703-461-2854
    tel. (914)-632-3001
    Fax: (914)-632-6544                    Price $747.00
    
    Just called for a price quote:         Also you have to be a member.
                                           Another $ 12.00 I believe.
    $690.00                                Locations of Boat US
                                           are in Houston TX,Holmdel,NJ
                                           and San Diego,Culver City and
                                           Fountain Valley in CA but
                                           nothing in NY,SF or San Antonio.
    
    If you need more information let us know.
    
    Regards 
    
    Mike K.

    Defender's 1992 catalog (1993 should be out very soon, if not
    already) has the following:
    
    Garmin 50-05 (List $1495) sells for  $899.95
    Garmin 50-10 (List $1695) sells for $1029.95
    
    Defender is in New Rochelle NY which is a very short drive (perhaps
    10 miles from downtown Manhattan) northeast of NYC. The store should
    also be reasonably easy to get to by train and/or taxi.
    
    Otherwise, he should consider a West Marine store when in San
    Francisco. I will check prices and store locations for you tonight.
    
    John

    re. 2
    John, I called Defender for a current price of Garmin 50-05 an hour ago.
    $690.00.
    Also if you have Overton's 1993 catalog it sells for $ 699.95
    
    Personaly I wait till goes to $400.

    A friend and I were out Saturday. It was blowing like stink. Decided to
    try to calibrate the new knotmeter with the new handheld GPS. Waited
    until we were out of Salem Sound and broad reached for Glouster, just
    shy of 6 nm (G9 to nun off eastern point).
    
    The knotmeter was registaring really low numbers. We had never gotten
    past 4 knts this season. The GPS (Garmin 45) had us doing 6.7-7.2 with
    some blasts to 8.3. I reset the knotmeter to run about .5 knts lower.
    This also made it much more sensitive to speed changes. 
    
    From there on, the units stayed in sync pretty well. On the way back
    we did a beat (or should I say beating) between the 2 marks. The knot
    meter was averaging 5.5, the Garmin 5 knts. The time came out to 1:00:45
    for 5.75 nm. Close enough as the boat never stopped heeling less than
    17 degrees and the seas were pretty good size. 
    
    Retested on Monday, and both say in sync pretty well in calmer seas
    until we get to speeds greater than 4.5 knots. Then the knotmeter is
    real hesitant to admit we are starting to fly. But it is working better
    than before.
    
    With prices going down to less than $300 for some of these units, these
    handheld gps units are very handy. My Loran didn't hold a candle next
    to either.

    Remember that your GPS measures the speed over the ground, while your
    knotmeter is reading through the water. It gets real interesting going
    against a current.

Re: .14

This looks like very impressive GPS performance.  Perhaps selective availability
was turned off when you were doing your tests.  I offen see random speeds of
more than 1 kt will I am stationary, on my GArmin 45.  Have you ever observed
the same?

/Dimitri Antoniadis

    re .16
    
        Well when I booted on a run the other day, I got 17.3 for 5
    seconds before it corrected to true speed. I find I get a corrected
    knotmeter match for current within 15 seconds, so I am pretty happy with 
    the system. Selective availability will hopefully be a thing of the past in
    the near future if the FAA gets its way.
    

    re: .14 and velocities of 1 kt when stationary - depending upon who you
    believe, GPS receivers calculate velocity from the Doppler shift of the
    L1 carrier frequency (I don't think lowcost handheld receivers do, but
    don't know for sure) or they do it from succesive positions (this is
    what I think happens).  In any event, receiver error ("noise"),
    multipath errors (shouldn't be much in the middle of the ocean...) and
    other random errors can easily account for a few metres (1 to 10) of
    error, even when there is no error because of Selective Availability. 
    If the GPS receiver thinks (due to random error) that it is moving
    by just 1 m between position calculations, you will see a velocity of
    about 2 kts.
    
    Re: .17 and Selective Availability - SA is a bias error that changes
    relatively slowly - over hours not seconds.  While there will be
    some affect on velocity calculations, it should be relatively constant
    and not great.
    
    When SA is turned off, position error will still be 10 to 25 metres,
    depending upon receiver "quality".  Differential GPS will still be
    needed to get you into the 1 to 10 metre range.  I least I convinced
    myself it will be useful 'cause I just spent more money on a DGPS
    receiver than I did on the d**n GPS!
    
    Regards,
    
    Garry

Another possibility and it saves yo money on a differential receiver that will
be obsolete hopefully soon.

SA  strips off some number of least significant bits (6?). on the clock.  This
gives a position error.  However, if your speed is calculated using change in
position over time S = d * t, then position error remains biased, but relatively
the same for selections close together.

Thus yourr speed will still be pretty good, but your position is lousy.

Conversely, if the DoD salts bogus numbers into the time's least significant
bits,  you can get wildly sperated positions between any two samples.  In this
case, your GPS may make a low pass filter by averaging several postions over
time.  The bogus positions would be too far from the others and therefore
dropped.  If this firmware is in effect, then you still get an accurate speed.
It just takes longer to settle down.

I have three speeds on Holiday II, slow, stopped, trying to catch up to that
speeding island.  So it doesn't really matter to me.

Doug
dtn 381 6355

    Regardless of how SA is implemented, it is a relatively slowly changing 
    bias, so relatively fast/often velocity calculations using positions is 
    effectively a "differential" measurement - SA is effectively
    eliminated.  Absolute position on the other hand is greatly affected by
    SA.
    
    DGPS receivers will not be obsolete after SA is removed because
    significant error sources will still remain, most notably ionospheric
    delays.  Ionospheric delays are handled in military GPS receiver's by
    using two carrier frequencies which are affected differently by the
    ionosphere.
    
    So, if you want better than 10 - 25 m accuracy, it's DGPS or military
    (2 frequency) receivers.... 

Gee silly me,  I thought all the buggers used the two frequencies.  Guess you learn
something every day.

On the other hand, differential GPS gives superior location SA not withstanding.  I think
I have seen reports of position to within a meeter close to the differential transmitter
(i.e. major harbors).  This allows for the possibility of things like bringing a ship to
dock in a thick fog, using the GPS (an instrument docking).  I do not know if it is good
enough to actually land an airliner (I think so).  If so, airport capacity is far less
effected by weather conditions.

In my case, wandering around in the fog with 10 meter positioning information, is better
than I can plot.  On most charts the pencil dot is bigger than that.  Besides if I can
not see one or two boat lengths around me, perhaps I should drop the hook and read a
book. 

    I use my GPS for competitive ballooning - if I get within 10 metres, I
    should be able to throw my marker the rest of the way....  We'll see
    when I make my "big comeback" (haven't competed serioulsy for many
    years) this summer...
    
    Re: landing an airliner - there is yet another form of GPS that
    measures the PHASE of the GPS carrier frequency.  I think real-time
    carrier phase GPS can get to within 10 cm. and with post-processing to
    1 cm.  I hope a GPS ILS doesn't rely on post-processing...
         

	While reading sci.geo.satellite-navigation (I think that was it) over
the winter, it sounded like surveyors have access to some very sophisticatedand
expensive equipment that bosts accuracy measured in centimeters. This may be
DGPS and or combined with the two carrier scheme. The recievers were in a
completely different class and price range than what we use on our boats. 

	I wouldn't want to come in to a dock with GPS in thick fog, but with 30
feet of visibility it might be okay. Robyn and I motored Scarlet into Hadleys
Harbor last year in thick fog at about 23:00. I was driving to GPS and it was
hard to stay on target. When Robyn found the nun (With a highly inadequate
flashlight which has since been upgraded, to a true, give me some photons dammit
:-), light) we were far enough off course to bee in danger. The nun was off our
port side and I know the water is extremely thin there. We made it in without
incident, but I'd prefer not to have to do that again. A more thought out
approach would have been better. We should have come in more perpendicular  to
the shallow water that the shallow angle I chose. 

	If we had to I'd do it again. but I'd come in differently. 

	But the GPS took us up to Maine and every place without a problem. The
only unreasonable error we encountered all season was my own improporly entered
coordinates. 


Geoff

    
    
    In a light air race last night the gps earned it's first "atta-boy!" . 
    
    As the zephers disappeared and the darkness came, we were not moving
    much. The gps said we had traveled .03 miles in 25 minutes. But as a light
    breeze started is fill, it told us we did .2 miles in the next 20
    minutes. You could not feel this on the boat, nor did the knotmeter
    move.    Not much speed, but it told us not to bail out, as something was
    building. Sure enough, several boats quit the race, just before the wind
    filled in.  Since it was dark out, we confirmed the shortened finish
    line with bearings and distance on the Garmin.
    
    Just like the big boys eh???  ;>)

	The wind filled in?  That must have been on a different part of the
course.  I just checked the finish times, and saw that you finished before we
decided to withdraw.  Back where we were (by R12), there was no perceptable
breeze, and the current was pushing us backwards while our sails hung limp.  It
would have been interesting to see how we were moving, but the instruments on
Holiday II seem to like to insulate us from a sense of radical change.  The GPS
showed us being stationary in Colorado Springs.

	Congrats on the #2 finish.

Mark

    Mark
    
       You guys bailed out about 15 minutes too early. It came up nicely.
    After finishing, we reached back to the mooring at 5 knts and beat
    half the C class boats that motored back.The wind has a way of coming
    back about an hour after dark. We were bobbing less than a quarter of a
    mile ahead of you.
    
    
    john
    

Back to the original question about GPS vs Log. readings.

If your log starts under-reading at high speeds, it may be due
to cavitation ( something to do with air bubbles, not sure what ).
This may be caused by bad impeller design or bad placement, both of which
are quite hard to fix. Also check for barnacles on the blades, or other
fouling.

Also, if your log impeller is not centrally mounted ( and they usually
aren't ) is it actually greater angles of heal, on only one tack that
causes the log to under-read.

Chris

    chris
    
        I set it up per Standard's instructions, 18" ahead of the keel, and
    within 2" of center. I know the lighter density of air will slow it
    down. We occationally see it as we crash off of waves going hard on the
    wind when it gets real wild out.
    
        But straight line on a calm sound on a broad reach says the
    sensivity is off. A simple adjustment and a check to make sure we are
    still in sync at low speeds shows all is well.

    
    
        I raced the Multiple Scrlosis cup (sp?) on Sunday. We knew it would be
    bad when we had trouble getting out of the mooring field and trying to
    find the channel out to the Sound....
    
         On the course, it was about 15-50 foot of visibility after the
    start. We ran the whole race on GPS points either locked in earlier or
    from a race booklet. At one point I had put a lot of time on
    another boat and even reeled in and left a much larger boat that left 10
    minutes before us. When we got to a windward turning mark, we could not
    find it.  We double checked the coordinates to no avail and then
    overstood where we thought it should be by .15 of a mile before heading
    back to be sure we  had rounded, losing 10 minutes. As we sighted the
    mark and gybed to  round it properly, the big boat came out of the murk
    to round just before us. Obvisously their radar is handy in such fog.
    
    Going back was a good time with 3 islands, and about 5 shoals just
    waiting to eat us. Using some bouys coordinates not normally used for
    racing, but in waypoint memory made navigation easier as we could use 
    bearings to keep us out of trouble. 
    
    Twice a Pearson 424 came out of the murk and immediately disappeared
    even when just in front of us (not a easy boat to make disappear). At
    the finish, they had race committee boats on both sides of a 30 yard line
    and still had trouble making out if anyone was crossing. On the run
    back to the club, I found we hadn't put in any of the bouys for
    channel, so the ride back was a slow one.
    
    So the GPS shows it is one nice machine, but the variances without
    differental can be a real problem. I also need to load the bouys into
    my loran. Finally, I need to load the way home into both the loran and
    GPS. Heading west for 3 miles is a recipe for disaster...

    You pointed out the only reason I would consider a differential
    unit-dense fog. I can usually get within 90ft of a waypoint. That's
    close enough to get an eyeball on the destination...but not in fog.
    
    
    The cost of a differential is as much as the Garmin 75 I own. So far,
    I've been able to avoid really heavy fog situations. I wonder how
    expensive radar would be?
    
    Glad you made it without scratching your paint!
    
    Carl 

    The thing that REALLY bugs me is that they degrade the reading
    unless we go to war.  I would think you would want to turn off the
    selectivity when you are are about to go to war or are in one, though the
    old saying "close enough is for horseshoes, handgrenades and hydrogen
    bombs" comes to mind.

Actually I think the reason they turned off SA during Desert Storm wsa
because they had bought up civilian GPS units because there were not
enough military ones.

Military, governement in general,logic is largely an oxymoron.  One
group degrades a signal, another finds a way around the signal
degradation.  Both programs are funded in the name of national
security and vital interests.  Go figure.

Doug

    Although I think that SA should be done away with, there is a logic to
    the following:
    
    > One group degrades a signal, another finds a way around the signal
    > degradation.  Both programs are funded in the name of national
    > security and vital interests.
    
    The DGPS beacons are low power and only uesful for a few hundred miles,
    therefore there might be some logic to their maddness, but I doubt if
    I'll have DGPS any time in the near future, due to price.
    (and the fact that my LORAN still works well.) The feds finally
    smartened up a but with respect to FCC licenses (after taking their
    pound of flesh from many), mebbe there is hope for GPS, too.
    
    Barnacle

I really think this is a matter of a true snafu.

DOD orginally made GPS to give navigation in a hostile envronment
(i.e. with with the Soviet Union).  SA was implimented to deny the
Soviets the same accuracy using our GPS.  Accuraccy needed to be
something like 10 meters for B52 bomb drops.

The Soviet xenophobia wouldn't use our system anyway.  they
implimented their own system (which I think gets widespread use in
europe).  

From an attack standpoint, the high degree of positional accuracy
is really only important when approaching target.  A cruise missle
a hundred miles out really doesn't care if it is off by a 100
meters.  However, as it appraches its target, this becomes more
important.  The close range is also where DGPS comes in.  so the
original concern that the DOD had with accurate positioning using
GPS has been accomplished through the USCG. 

Hence SA no longer makes much sense for three reasons.

	Soviet Union no longer exists, and its decendents have 
	more important and immediate problems than WW III.

	There currently exists an alternative to GPS (the Soviet
	and now Russain system).  So hostile units can gain the
	needed accuracy independent of GPS.

	DGPS bypasses SA at close range.  This is where targeting
	accuracy is most important.

Strangely enough, even if SA were dropped tomorrow, DGPS still has
its place.  My numbers are shakey, but GPS gives something like 3
meter accruacy.  DGPS can provide better than 1 foot to no worse
than 1 meter, depending on how the ground wave reaches you.  The 1
meter accurracy was desisired for commercail shipping especailly
inside harbors.  I guess this raises the possibility of brining in
ships and planes in zero zero visibility.  Ultimately, ground
control could modify your waypoints to avoid collisions etc.

So I think DGPS is here to stay.  I predict that SA will not make
it to the year 2000.

Loran of course was discontinued around 1990.  The added Loran
chains both domestically and abroad confirms this.

Spectrum a couple of years back had a nice history of Loran,
navstar, GPS etc.  Lots of good thinking on the technical side.

Cruise missiles are rather off the subject of sailing, but .....

My understanding is that cruise missiles do not use GPS or DGPS. Rather, 
they are given a picture (optical or radar) of the target and terrain to 
the target and they simply (using very clever algorithms) follow that 
picture. Military satellites are rumored to be achieving 1 meter 
resolution in their photographs, and these can be used to program cruise 
missiles. I rather doubt that you'd get the GPS or DGPS coordinates of 
most military targets accurate to 1 meter. And rather many targets of 
interest don't conveniently stay in one place while you try to destroy 
them.

I read a report in a UK technical paper about a month back that said that
Rockwell were very interested in building chipsets, etc, to work with the
Russian "GPS" system (not, as far as I know, currently used in Europe, and I
have never seen sets for it advertised in the UK - perhaps someone knows more
about this?).

The reason that Rockwell were interested was to provide customers with DGPS-like
accuracy, or maybe just no SA,  without any dependence on the US Military (why
should that organisation be considered unreliable?).

The last paragraph of the article mentioned that Rockwell were one of the
original contractors for the current GPS satellite system...

- Brian

Alan is this is too much of a digress, please feel free to edit it to
keep in the spirit of the notes file.

Doug

[-.2]
Point well made alan.  I hang my head in hyperbole shame.

Years ago, I came across a statement of legal limit to civilian sattelite
resolution.  At that time (mid 70's) it was 10 meters.  I was reading
about agricultural pest control.  3 meter resolution was desired (i.e. one
tree).  While acheivable with the then current technology, the government
would not allow it to be launched.  At that time the acknowledged military
resolution was 10 cm.  Incidently this is an order of magnitude better
than the spy sattelite pictures released recently.

10 years ago, ground based cameras could certainly not only spot, but
identify which specific heat tiles were missing on the space shuttle as it
flew over Hawaii.  The tiles were all basically rectangular and about 3"
on a side.  To identify specific tiles implies a resolution of at least 1
cm at about 90 miles.  

I would guess that military resolution is now significantly better.  
However, you can get drowned in the detailed information.  So there is
some sort of limit.

I will have to check into articles from the Navel Institute some time. 
But I think there is a good chance that initial guidance is intertial (or
perhaps GPS) on a cruise missle.  Terrain following I think is used on
closer approach.  

[-.1]

I honestly did not know how widespread the use of the Russain system is. 
My understanding is that it is very similar to GPS.  I think it is
supposed to be accurate to about 3 meters.  This would put it between GPS
(without SA) and DGPS.

Personally, I use my GPS as a friendly confirmer of my pioloting. 
Traditional skills sort of go with my boat.

    re .36    The reason may be the fact that the Europeans are pushing for
    a GPS like 3D system to keep track of airplanes. Using full blown GPS is
    great, but the rub is the US Dept of Defense is saying NO. So the
    European Aviation organizations are probably moving to the Russian
    package as they had threatened. If so, Rockwell well understands that they
    can get a lot of money building guidence systems for all commerical
    airlines rather than handhelds for boats/hunters. 
    
    re .37    You can buy your own local transmitter that ties out with the
    master clocks and have sensitivity of 1cm for about $6,000. This is
    used in the contruction industry when putting up skyscapers (no more
    plumb bobs). A friend was working on a package for NASA last year for
    the  guidence system of the space shuttle. Using an Alpha platform
    (good 'ole 64 bits), they were doing some serious precision for
    automating docking manueavers at 1,000s of miles an hour. That is 3D by
    the way. So the technology is there. It is a matter of telling the
    generals the war is over and to lighten up for the good of all us. 

    Regular single frequency GPS is good to about 25 metres (no SA).  DGPS
    can easily get to 10 m and 1 m service is available - I have 10 m
    service over broadcast FM from DCI.  DGPS will definitely be useful if 
    and when SA is turned off.
    
    Military or dual frequency GPS gets you to about 3 m (I think, if my
    memory is correct).  It is essentially a differential system that uses
    two different carrier frequencies to allow ionispheric delays to be
    accounted for.
    
    Sub metre accuracy is accomplished by measuring the phase of the
    carrier frequencies.  These units are quite expensive, require
    relatively lengthy amounts of time to take a measurement (don't do well
    in a dynamic environment) and may require post processing of results
    gathered in the field.
    
    At least one manuafacturer (Ashland?) has a dual Navstar/Glonass
    receiver that uses the DOD and Russian systems together as a
    differential system.
    
    Regards,
    
    Garry

    I am new to big boat ownership. I recently purchased a Pearson 30. It
    comes with a Loran. Some friends suggest I also pickup a GPS. I
    understand how GPS works and the difference between GPS and Loran. I
    just get overwelmed when I walk into the marine store and see so many
    models to choose from. What I would like is some input on picking a
    GPS.
    
    Does anyone know of any Mag. articles that might compair different
    GPS's?
    
    Is there one brand that is better than another?
    
    What are the features that I should look for?
    
    There  are a lot of these things on the market for under $300. and they
    seem to go up to $1000. Are the $300 models worth it or am I wasting my
    time with one in that price range?
    
    Thanks for the help.
    Joe Vlcek
    LASSIE::VLCEK
    [email protected]
    

I haven't compared GPS units since buyiny a Magellan 5000DLX a few years 
ago. My impression is that the various Magellan and Garmin models are 
all good and all likely have more features than you'll neeed or use. 
Some are designed more for marine use than others. Visit a good marine 
electronics dealer and you probably learn more than you want to know.

A couple of minor points: Few keys (ie, no numeric keypad) means many 
more keystrokes to enter data, which means more chances for error. Our 
Magellan has a numeric keypad and since I could afford the extra cost, I 
bought and am glad I did. Also be sure that the GPS can be run off 
ship's power. Most models use up batteries rather quickly (4 to 8 hour 
life is common). An external, remote antenna is also desirable. While 
our Magellan is a (largish) handheld, it is more or less permanently 
mounted at the nav table.

Alan

    I've bought a Garmin GPS 75 and a couple of other things from West
    Marine.  Their home page is:
    
    http://www.gsn.com/sports/boating/westmar/westhome.htm
    
    They have an "advisor" about GPSs you might want to read.
    
    There is a newsgroup - sci.geo.satellit-nav - that talks lots & lots
    about GPSs.  They like to b*tch about Garmins, mostly 'cause Garmin
    imposes a 90 kt speed limit on their non-aviation models and some folks
    say Garmin's are not as waterproof as advertised.  As above, I own a
    Garmin, it hasn't melted yet, I don't expect to go more that 90 kts in
    my balloon and I've got better things to do than stare at a GPS while
    flying on a commercial plane...
    
    Garmin's newest products are the GPS38 and the GPS45XL - niether has a
    "full" keypad.  The 38 has an internal antennae while the 45XL has an
    external antennae.  Both cost under $300 US.  The Garmin 75 is a little
    older, a little bigger, has an external antennae and "full" keypad and
    can handle a wide range of input voltage - up to 40 volts DC.
    
    All GPSs (OK, civilian, single frequency ones) are about the same - the
    big difference concerns how the receiver tracks multiple satellites -
    does it switch (or sequence) from satellite to satellite using 1 or
    perhaps 2 channels, or can it actually track mulitple satellites
    simultaneously.  Sequencing receivers tend to be cheaper and less power
    hungry - parallel receivers do better in a more dynamic (high velocity,
    high rate of position change) environment.  Some say parallel receivers
    maintain lock better.  Most Garmins (including the 38 & 45XL) are fast 
    switching sequencing receivers, despite the "MultiTrac" name.  Their 
    newest recievers use a 12 channel parallel design, but I don't know any 
    model numbers.
    
    A couple of points to consider are map datums and coordinate systems -
    you want to pick a GPS that has map datums that match the maps/charts
    you intend to use.  A mismatch in map datums can cause several
    (hundreds in some cases!) of position error.  You also want the GPS to
    display position data in the coordinate system you like to use - you
    probably want Lat & Long, but I like UTM.
    
    Other than the good advice in the previous note, that's all I
    can offer, 'cause beyond that, you're into a religious argument about
    which is the best GPS - see sci.geo.satellite-nav!
    
    Regards,
    
    Garry
    
    PS: Regardless of which GPS you buy, and assuming you're into PCs.
    consider buying the data cable and software package (most likely an
    option).  It makes managing waypoints much easier and you'll be able to
    save and display your track, etc.

I understand that Sailing? had an article recently on GPS's.  Most
interesting thing that I heard was the battery life difference
between a Garmin 75 and a Garmin 45.  The Garmin 75 is good for only
a couple of hours, the Garmin 45 much longer.

My experience is similar to Alan's.

I have a Garmin 75.  I tend to use only the basic functions such as
position and occasionally speed.  The Garmin 75 comes battery ready
and with a kit to mount it to a wall and run off of ship's power. 
This is one of my projects.

A neat thing about the Garmins is the antenna.  They will sell you an
external antenna for a second on your house and your first born. 
However Ken Bates entered a note here about a $6 alternattive.  Buy
the appropriate coax from Radio Shack.  Unclip your antenna and
insert coax.  Perhaps not as high a gain as a matched antenna, but
the price can not be beat and it works.

I intend to mount my Garmin by the nav station and route my "own"
external antenna.

As others here will testify, I tend to head towards the lower end of
the price spectrum.  For coastal cruising I think a low end unit will
do just fine, especailly since you already have a Loran.

I think the big advantage in higher priced models will be such things
as more sattelite tracking (quicker and more accurate location?),
greater ergonomics etc.

West Marine often has evening talks about various product lines.  A
couple of months ago a Trimble rep was in talking about GPS.  Give
them a call.  These talks are freebies.

And don't let your friends talk you into a GPS because THEY think
YOU need one.

Unless you sail offshore, LORAN is quite satisfactory for confirming
your position and measuring your speed over the bottom. A backup
unit is nice to have, but if you are a typical fair weather sailor,
your primary navigation will be by chart and eye, and the LORAN
serves as your backup.

I presume you have a long (and growing) wish list of other gear for
your boat. If so, there no compelling need to put the GPS near the
top of the list, IMO. 

--RS

    I would second .44's question. Why do you think you need a GPS
    if you have a working LORAN?  If you really want one, fine,
    but do you really need one.
    
    As one who manages to sail lake (Winnipesaukee) and coast (Maine, 3
    weeks a year) on a (relative) shoestring (for around $2K a year)
    I find that my 5-year-old LORAN meets my needs just fine. When someday,
    it goes belly up, I'm strongly suspect I'll replace it with GPS but
    I wont rationalize having both at once.
    
    Not trying to start a debate here, just the low end point of view.
    
    Bill
    

Actaully, I have a GPS and no Loran.  I would also agree.  I like
mine, great toy.  

I grew up in the mountains. Survival ability is important in any
hostile enviroment (try breathing watr).  Knowledge and awareness
are your best protectors.  Get good at traditional piloting.  Then
use the Loran to confirm what you already "know".  Going to the Cape
this summer we were off twice.  Not seriously, but it is still
embarrassing.

Now taking that $300 how would I spend it.  The answer is an easy
one for me.  i bought a set of 7x50 Tasco Offshore binocculars.

These have a built in bearing compass, and range finder.  The
glasses are of good quality.  Once you have a pair, it is amazing
how often you actually use them.  Most uses are mundane, making it
easier to see a bouy number etc.  The compass acts as a crude back
up to your ship compass (I have a total of three acceptable ship's,
hand, binoccular and one more rotten - autopilot- compass on board).

Got a compass, get some inflating life vests.  They run about $120 a
pop.  These are high on my list.  I want a set for Amy and I with
the built in harnesses.  Then my ucrrent harnesses would be for
other crew.  We are both strong swimmers, but it is easy to be
multiple miles off shore.  I can probably do 3+ miles, but
hypothermia would certainly get me.

Got that, get a hand held to back up the VHF.

Aw heck, you get the idea.