<html><tt>From:</tt> <tt>
Robert Kirk <kirk@neptune.gsfc.nasa.gov></tt>
<br><br>
George Dinwiddie wondered:<br>
<br>
<blockquote type=cite cite>So you think the GPS can measure 5 knots based
on the doppler shift<br>
of the relative movement between your boat and the satellite?
I<br>
don't think so. [...]</blockquote><br>
It sure does. And the satellites are whizzing around at thousands of
miles an hour while your boat is doing 5 knots. Isn't modern
technology nice? Your GPS receiver is using a couple of phase lock
loops to (1) receive the message data, (2) measure the travel time
(Pseudo Range), and track the phase shift (Pseudo Range Rate or
DOPPLER). All this on up to 12 satellites at a time. It needs 4
satellites for an x,y,z solution and uses any additional ones to improve
the solution. It can measure the doppler offset to about <u>1/20
Hertz</u>. It uses the velocity information to update the position and
the positional changes to help calculate the doppler in a boot strappy
way. It throws all this into a Kalman filter and outputs position, time,
& speed. Since your GPS doesn't show negative (vector) speeds, the
stationary speed won't average out to zero.<br>
<br>
Even with SA on, the max probable speed error is less than a knot, and in
practice is better. The spec for the current Garmins is 1/10 knot
RMS.<br>
<br>
You can check the GPS speed against wood chips thrown over the side if
you want real precision.<br>
<br>
Cheers,<br>
Bob<br>
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