[alberg30] GPS mount

[Robert Kirk]

From: Robert Kirk <kirk at neptune.gsfc.nasa.gov>

George Dinwiddie commented on GPS speed calculations:

>Nope, in fact, you can watch the speed go all over the place
>when they're doing a lot of dithering and the GPS is stationary.
>The speed is just a distance over time calculation.  Distance is
>change in location.  The location moves because of the dithering.

George... That's wrong; you must have missed my other message where I say:

>GPS speed is calculated (by all the manufacturers, now, I believe) from the
>satellites' doppler and not by differentiating their positions. So, to
>first order, SA is not a factor in the speed calculation once you get
>underway with more than a knot or two of speed.

You're right that the GPS position (and speed) does wander when you're 
stationary due to SA effects.  (If you get differential correction, this 
wander nearly disappears.)  But as I said, the speed is rock solid once you 
get underway. The reason is that the GPS does not do a distance over time 
calculation, but calculates speed from the satellites measured doppler 
which is accurate and not affected by SA.

Cheers,
Bob



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