On Sun, 27 Jan 2008 15:15:32 +0000, larry wrote in
:

JohnW wrote in
:

If you are pitching and rolling, the antenna will be moving
relative to the boat so the GPS will include that motion in
with the boat's forward velocity in its speed calculation.

Plus interference with direction over ground calculations due to rocking
from side to side.

If you have a handheld GPS, carry it into the burger joint on a busy
road and let it bread trail on close range. The signal can't get
through the roof so what the GPS receives are signals bouncing off
objects outside, like passing vehicles and stationary (we hope)
buildings through the big windows. Let it run an hour and its fix will
cover the whole shopping center....many hundred feet! This same effect
happens in a HARBOR or the ICW! Signals bouncing off nearby conductive
objects, especially overhead bridges, just eats it alive. Anywhere near
shore a GPS fix gets wider and wider in accuracy because of multipath,
the same signal bouncing that tears up a UHF TV signal on an old analog
TV with "ghosts", signals arriving later than the main signal which
ALWAYS make ghosts to the RIGHT of the main signal, because they arrive
later...we scan from left to right, top to bottom like reading a page in
a book....except every other line, called interlacing to make it flicker
less.

I record NMEA output from my Magellan Sportrak Color GPS on my laptop,
and I'm not seeing that kind of error -- my tracks are quite accurate
when checked on the charts on my laptop.

Ever wonder why it only updates every second? It's trying to average
out the MULTIPATH MOVEMENT ITS MEASURING!

It's actually feeding valuable real-time data to my laptop, which is
automatically computing and displaying target speed polars in real time.

--
Best regards,
John Navas http:/navasgroup.com