[alberg30] GPS Doppler Velocity

[Alan P. Kefauver]

From: "Alan P. Kefauver" <apk2 at home.com>

I used the Garmin GPS to calibrate the speedometer and make a deviation chart on my Cobra Convertible. The trick is watching the Garmin and driving
safely at the same time:)
Alan
*********** REPLY SEPARATOR  ***********

On 3/2/2000 at 2:31 PM Robert Kirk wrote:

>George Dinwiddie wondered:
>
>>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. [...]
>
>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 1/20 Hertz. 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.
>
>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.
>
>You can check the GPS speed against wood chips thrown over the side if you 
>want real precision.
>
>Cheers,
>Bob




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