Re: [Paddlewise] GPS

>>However, we did receive feedback that GPS could be quite badly in
>>error when satellites were low on the horizon and cliffs affected the
>>signal. I'ld be interested to know if anyone has experienced this?
>>    
>>
>
>GPS normally displays its range of precision, which depends on the number of
>satellites that can provide a good signal at given conditions, i.e.
>geographical position, obstructions etc.  I don't remember what is the
>lowest precision of my Garmin Legend (when it still gives some location),
>but must be better than 900m error of dead reckoning.  I've heard of
>possible fog-induced distortion of signal, but haven't had a chance to check
>it personally, so don't know whether it is just "honest" weakening of signal
>(= honest lower displayed precision), or really distortion of data beyond
>stated range of precision (this would be much worse).
>
I've used my GPS receiver on a daily basis for the last 6 years and have 
had a chance to see a number
of anomalous indications - none of which made themselves known through 
exceptionally high EPE
(estimated position error) or accuracy indications.  These included my 
all-time high indicated speed of
4032.3 mph achieved while pedaling my bike in a canyon area near a 
medical complex.  The tracklog indicated
a sudden jump in position shortly followed by a loss of signal lock.  I 
believe the hills were blocking most
of the satellites and I was probably getting a momentary multi-path 
reflection off the medical buildings
which the unit interpreted as my having rapidly moved to a new 
position.   There was also the 5-minute
period when the GPS indicated that I was at an elevation of 1500' over 
the north Pacific approaching
Seattle at 734 mph while my other navigational tools (eyes) indicated 
that I was on a
residential bike path in the SF Bay area and heading south at about 15 
mph.  The satellite page showed
a lock on just 3 satellites (i.e. a 2D lock) and I suspect that this was 
a case where with only 3 signals the
unit couldn't properly resolve between possible solutions of the pvt 
equations and picked the wrong one.
Turning it off and back on quickly resolved the problem.

But despite these and a few other momentary glitches, I've found GPS to 
be a highly reliable tool,
especially if care is taken to check the indications on the satellite 
page showing the number and
positions of the satellites being tracked.  I have never had a problem 
with reception due to weather
conditions and that includes thick fog and intense thunderstorms.  But 
there can be a problem due
to any film of water forming over the antenna - so condensation on the 
unit as a result of fog should
be wiped off.

Although I always have my GPS turned on when kayaking, I rarely use it 
for navigation.  The
main uses are to keep a record of the trip, incl. speeds and distances 
traveled, give an immediate
indication of actual speed and track including the effects of wind and 
currents, and to show me where
on shore appropriate restaurants, parks, restrooms, etc. may be located 
to help in deciding on
landing spots.  It's rarely used for navigation since I normally know 
where I am just by looking around.
But under unusual conditions, such as the fog coming in unexpectedly, 
the GPS can be a valuable
addition to other navigational aids.
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