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.
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Y'all give a cheap boat GPS WAY too much credit for position fixes!
It's only good to about 3 feet, on a sunny day, with no reflecting
airplanes making multipath signals, far out away from any land.

Boat GPSes are NOT GPS surveying instruments like the Geodetic Survey
little Japanese guy who comes to my house to check the fault line I live
on for movement in mm every month. God help any of you that think that
cheap piece of crap in the plastic box is gonna put you within 5 ft of
the bouy in the fog. It's just NOT accurate to inches.....EVER.

Here, test it at the dock. Turn it on and clear its bread trails.
Leave it on sitting dead still at the dock in perfectly flat water until
tomorrow. See if it stays within 5 ft for a day sitting still. It
won't, but you need to know and NOT trust it so much. If you live in a
metro area with an airport, the aluminum clouds flying by will make it
really go crazy over the course of a day, suddenly jumping way down the
dock, then jumping back as the aluminum clouds move around. GPS works
on the phase relationships between precisely pulsed microwave signals
from 3 or more overhead birds. If you change the PATH from the birds to
the GPS, huge errors are introduced into the GPS phase relationships.

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.

All this terror over the motion of the mast is just crazy! The mast,
itself, and all your rigging to any GPS antenna on the deck is causing
multipath signals from the overhead birds....and screwing up the timing.

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