Re: estimating error in my GPS position.

*** wrote:
"Mike Coon" <mjcoon@@connectfee.co.uk> wrote in message news:13p68fr1ef29u1d@xxxxxxxxxxxxxxxxxxxxx
Adrian Jansen wrote:
Many GPS units will output a velocity measurement which is independent
of the position, done by using Doppler shift on the signals
I have read this assertion many times, and I have no reason to doubt it. Except that of course the Doppler shift will be largely caused by the motion of the satellite, there will be at least four different ones because four satellites are needed for a good fix, and they just add up to result in an instantaneous varying voltage at the antenna. So what is needed is a very accurate freequency analysis of at least four varying carriers, of sufficient precision to subtract the satellite speed (component along the line joining satellite and receiver) to leave the receiver speed. I find this very impressive!

Mike.
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The calculations will also have to account for the directions of travel of the satellites and of the GPS receiver in 3 dimensions. It is my belief that most receivers use distance travelled and time interval, possibly using more that one time step, to calculate velocity.


No, at least every receiver I ever investigated ( at least 10 ) uses Doppler for the velocity. In fact the receiver HAS to measure the doppler shift to each sat anyway, just to maintain signal tracking. So its a small additional calculation to extract the earth referenced velocity as well as the position.

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Regards,

Adrian Jansen adrianjansen at internode dot on dot net
Design Engineer J & K Micro Systems
Microcomputer solutions for industrial control
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