[alberg30] GPS mount

[George Dinwiddie]

From: George Dinwiddie <gdinwiddie at min.net>

Bob,

So you think the GPS can measure 5 knots based on the doppler shift
of the relative movement between your boat and the satellite?  I
don't think so.  And if you could measure your speed unaffected by
the SA, you'd also be able to measure your location unaffected by
the SA.  Time, speed, distance.  They're related.  I think the 
better performance you're seeing is the fact that the GPS manufacturers
have better algorithms for averaging to avoid obvious jitters.

 - George

Robert Kirk wrote:
> 
> 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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