Re: [Paddlewise] GPS - FYI

Bill wrote (regarding a Garmin GPS receiver):
> 
> ...<snip>...  The tech rep said the
> tracking function does not work at speeds lower than about 3.3 knots. ...
> The track function on my unit was still wildly inaccurate
> while paddling at a  steady 4 kn speed, so I think the tech rep is being
> "disingenuous", as the diplomats say.

I don't think that he was being disingenuous, but he was
oversimplifying. There is no given speed at which the track function
suddenly "works". The primary problem with tracking function accuracy at
slow speeds is error introduced by "Selective Availability", which is
the constantly varying error that the Department of Defense imposes to
degrade the accuracy of non-military GPS receivers.  This can cause
apparent movement even when you are standing still. Whether and how much
this throws off the tracking function depends on the direction and rate
of the "false" movement relative to your actual movement. The faster you
are traveling, the more accurately your receiver will be able to discern
your actual direction of travel. Since the effects of SA vary, there are
times when a 4-knot speed is sufficient to give you a reasonably
accurate estimate of your actual direction of travel, and other times
when it will not. The faster you are traveling, the more frequently the
directional error will less than a given amount. 

As you have discovered, a way to work around this problem is to use the
GPS to provide a compass bearing to your destination, and then use a
deck compass to steer the course.  Take an occasional glance at the GPS
to see if the the bearing to your destination changes *persistently*, in
which case you are drifting off course. The only way to "fix" this
problem (i.e., to dramatically reduce the error at low speeds) is to add
a differential GPS receiver. DGPS will significantly mitigate the
effects of SA (and other errors). 

> 
> Also, he pointed out that the map will only display the 12 nearest waypoints...

I have never found this limitation to be a problem so long as I am
cognizant of this when I lay out waypoints. Of course you can still use
the GOTO function and other functions with waypoints that do not show up
on the screen (if for whatever reason the waypoint you are interested in
is not one of the 12 nearest). 

Dan Hagen
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