Conference unifix::sailing

GPS errors are pretty constant over a small area. This is the pricipal on
which differential GPS is based. A fixed transmitter in a known location 
broadcasts it's GPS position constantly, and the differences between it's
actual position and it's perceived one are applied somehow to your GPS
position to arrive at a more accurate position. ( I'm sure this is an 
over-simplification but the principal is right ).

So, assuming that GPS readings don't change greatly over the time that
it takes to set the course ( watch how they change when stationary ), that
should give you a pretty accurate readings.

Chris

    
    	I'd use the Loran, tho either would do. Are these very short legs,
    with inflatable markers?
    
    	We use government marks around Boston, to make it easy on RC. The
    couple times I tried comparing distances of legs by Loran and chart
    with dividers, the Loran seemed fine. Besides, how accurate IS the
    chart? Not very, with government markers, even taking ground tackle
    etc.etc. into account.
    
    	And of course, the PHRF ratings are a scary rathole unto
    themselves!
    
    		Scott.
    

    Yes, we switched to inflatable marks to get "true" courses aligned with
    the wind.  Each leg is about 1-2 miles, with races typically 5-8 miles
    long.
    
    Ah yes, the PHRF rathole....
    Stay tuned for my next note (when I get the time) searching for answers 
    to some Time on Time vs. Time on Distance questions we are debating.

    I am getting ready to get a handheld gps whose main purpose is to 
    get my boat to the next mark on a race course in any weather. These
    marks are all permanent. My plan is to load the waypoints by motoring
    by them before the season starts. 
    
    I think several courses would be nice so I could load all 8 of the 
    predefined courses and just go from one to another. 
    
    The other use is to time a start. I plan on doing this by taking a
    waypoint at the startline at the end I wish to head for. Then when
    the countdown starts, I can calculate time to arrival and compare with
    my stopwatch.
    
    I just noticed a handheld loran for $99. Since it's repeatability is
    better. Would this be better than a gps ?
    
    
       thanks
    
       john

    re -.1  Disregard. The wife got me a Garmin 45 for my birthday.
    
    

Loran repeatability is much better than GPS, since the loran position changes
only because of physical changes in the system, e.g. changing temperature
and humidity in the atmosphere. The GPS position changes because the government
deliberately lies to us about the position of the satellites, and changes the
details of the lie (i.e. the effective offset vector) over time.

I believe the GPS offset vector changes gradually, so you should be able to get
reasonably good short term repeatability. This also implies that you can
reliably compute reasonably accurate course and speed from the GPS fixes.

--RS

re .6:

I have seen the position displayed by my Magellan GPS change quite rapidly 
even when it is not moving. This implies to me that the dithering of the 
GPS position is not gradual at all. The Magellan has variable averaging 
times for speed and course. With the minimum averaging time, the changes 
in displayed speed are large enough to be unrealistic. With 20 second 
averaging, the displayed speed is still occasionally suspect. I have 
seen estimates that the error in displayed speed is something like 8%, 
which isn't very good (our 15 year-old Signet knotlog is better than 
that, I think). The slower your speed, the greater the inaccuracy. 

At six knots, you are moving about 202.5 feet in 20 seconds. If the 
dithering of the GPS signal is such that the GPS thinks you've moved 
222.8 feet, the displayed speed will be 6.6 knots, a 10% difference. A 
20 foot difference in a position fix is a rather small amount of 
dithering (see Note 452.96).

    Speaking of distance measurements....
    
       I just got back from a business trip to HK/Tokyo. I took my Garmin
    45 with me, as I wanted distances between various DEC offices for a
    marketing display.  Anyways, getting lock on satillites is a
    nightmare in these canyons of steel and glass. It took 4 tries in HK
    before I got a successful starting point. I had to stand on a highway
    overpass next to the harbor and abutting a park to do this.
    
       In Tokyo, it took 3 tries. I expected this in Shinjuku, which is
    like Manhattan, but not at the office which is out in the sticks, 15
    miles from downtown. The 2 and 3 story buildings abutting it did me 
    in. Finally found a clean set of signals in a park in Shinjuku after
    walking around the park for 30 minutes looking for a clean site on 
    Saturday.
    
       Finally getting my waypoints, I eagerly asked for a computation back
    to the office in Stow, but it only showed miles. I wanted to see at least
    tenths of a mile, but no luck. Oh well...
    
       I guess the antennea on the Garmin 45 is meant to be out in the
    open ocean, and doing the byways of major cities. Got lots of looks
    from the locals, who never saw one of these electronic "toys" before
    ;>) .
    
        john

    after using the Garmin 45 for other than sailing, I am disappointed. It
    does not lock to anything if the slightest cover exists above about 15
    deg from horizontal. 
    
    excellent on the water, useless for anything else.....
    
    

re .9:

Not necessarily the fault of the Garmin. Our Magellan 5000 has the same 
behavior. The very high frequency and very weak GPS signals are
attenutated beyond use by almost anything. You really do need a clear
view of the sky to quite close to the horizon to get position fixes 
(which is a minor detail that is not usually mentioned when the glories 
of GPS are being touted). A few tree leaves and you're left fixless.