From: wejones@crdec7.apgea.army.mil (Bill Jones)
Newsgroups: rec.radio.amateur.misc
Subject: Re:  GPS Systems
Date: 23 Jul 1992 17:37:35 -0700
Organization: UCSD Usenet Gateway
NNTP-Posting-Host: ucsd.edu

> 
> >The GPS 50 sells for about $899, and I can't imagine what the extra $400
> >would buy.  
> 
> Two things.  One, a difference in quality.  "The Garmin GPS50 isn't as
> ruggedly constructed as the Magellan Nav 5000 and is a bit more difficult
> to use" (Practical Sailor, June 1, 1992). 

The Magellan does "look" to be more rugged, but it's hard to say, the Garmin
has received some fairly rough treatment, with no problem.  It is a bit
confusing to program too.

> Second, the Magellan provides
> for multiple "map datums" (I know, the plural of datum is data, but that's
> what they call it).  If you're using it for navigation, and you're using a
> single map datum that's different from the chart you're using, you can 
> end up a mile or more off course--and on the rocks.
> 
The GPS50 also has multiple (about 80-90 different options) map datums.

> >The thing is very
> >accurate, the 50-100 feet seems to be correct, although altitude is less
> >accurate, but I can't see how it can be so accurate with respect to where it
> >is and be so far off with respect to where the sats are, although it is
> >possible that the error is in the sat tracking programs, since I don't think
> >they are quite so good on sats that far away (these are nearly out as far as
> >geostationary sats), traksat sometimes bombs out on geosync sats.
> >  Another thing that's interesting is that if I walk reasonably fast around
> >my yard carrying the GPS 50, it will tell me what direction I am walking
> >and how fast, with updates about once per second, and yet I obviously
> >haven't moved 50 feet in that second, so it must have a relative precision
> >that is better than the 50-100 foot accuracy.  
> 
> "Selective Availability" (SA) has been turned on almost continuously since
> the Gulf War (when they had to turn it off, because they were using so 
> many "civilian" units!).  SA "degrades GPS accuracy to only 100 meters
> 95 percent of the time and 300 meters the other five percent of the time.
> Despite claims of considerably better accuracy by GPS manufacturers, that
> is the best accuracy you will be able to realize on a consistent basis"
> (Practical Sailor, March 1, 1992).
> 
> SA also affects the speed and bearing readings you get, and the log as 
> well.  A friend accidentally left his on while his boat was at the dock,
> and when he returned discovered that the boat had travelled (supposedly)
> several miles, simply because the GPS kept getting slightly different 
> readings!
As you say, the manufacturers claims I have seen are better than this.  I
haven't done any exhaustive study of the thing, but I took the GPS50 on
vacation, and passed a few roadside markers marking latittude, and the
thing was accurate at least within 100 feet.  I passed one marker three times,
and it was right on every time.  Also, I have programmed in the location of
my home, work, and a few other places, and have been watching the thing for
a couple of months now, and it gives me the same reading every day.  I don't
know if the absolute positions are correct, but it has never shown any
variation in position of more than a couple hundred feet, if that much.
Anyway, I am impressed with the thing.  It may not be quite as good as
the claims, but I have never seen it off by 300 meters.
Of course I don't use it for anything important (like sailing through
channels), so if I had to depend on it I might be more critical of its
performance.