[alberg30] GPS mount

[Robert Kirk]

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

At 02:07 PM 2/26/00 -0500, Timothy C. Lackey wrote:

>All GPS--whether differential or not--provide very relaible and accurate
>SPEED OVER GROUND readings.  They do not provide speed through the water as
>a knotmeter does.  Differential GPS provides a correction to the satellite
>signal to correct for the government's random downgrading of the original
>signal and therefore may provide more accuracy (assuming that the original
>signal is even downgraded, which it probably isn't much of the time; it's a
>random (called "selective availability" or SA) manipulation of the signal,
>and the amount, if any, varies from nothing to within a reasonable limit).


The speed calculation is pretty much immune to the SA degradation, so 
differential isn't too helpful there - though of course it's useful for 
positioning.

Bob Kirk
Isobar #181





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