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Guest phog

Speed Question

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Guest phog

Hello everybody

Just a technical question :How the speed is calculated ? the "distance/time" way ( the time between two different points divided by the time spent to link them or by mesuring the doppler effect on GPS signal caused by moving?

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Guest Alex

Interesting question. I have always presumed time/distance calculation and never thought of doppler. What do the experts say?

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Guest malates2

I can help with this a bit...

 

In order to use the doppler effect, the GPS would have to be able to tell if the Electromagnetic Waves coming in are blue shifted or red shifted, and by how much... Now for such small speeds it would be virtually impossible for the GPS to measure any such doppler effect...

 

In astrophysics, we look at the doppler shift of E/M Waves from stars... a relatively simple process, but one that requires a decent source...

 

So the answer must be that the GPS averages a bunch of distances over a bunch of times (a very simple and quite accurate process)

 

If any GPS designers are out there that know differently, please enlighten us...

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Guest iFixThings

Im not a GPS tech, but I have a good idea how the speed is calculated. The GPS knows where you are and where you have just been. Since it keeps very accurate time, all it has to do is find the distance from "Im here now" and "I was just there", and the time it took to get there. Then it uses Distance X Time = Speed. Now with the speed and the distance from "Im here now" to the targets loaction, it can use the same equation to find out the amount of time it will take to get to the target.

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Guest Mike_Teague

th methods.. position and doppler:

http://joe.mehaffey.com/gpsspeed.htm

 

From the NAVSTAR GPS User Equipment Introduction document Section 3.7:

 

GPS receivers typically calculate velocity by measuring the frequency shift (Doppler shift) of the GPS D-band carrier(s). Velocity accuracy can be scenario dependent, (multipath, obstructed sky view from the dash of a car, mountains, city canyons, bad DOP) but 0.2 m/sec per axis (95%) is achievable for PPS and SPS velocity accuracy is the same as PPS when SA is off.

 

Velocity measured by a GPS is inherently 3 dimension, but consumer GPS receivers only report 2D (horizontal) speed on their readout. Garmin's specifications quote 0.1mph accuracy but due to signal degredation problems noted above, perhaps 0.5mph accuracy in typical automobile applications would be what you can count on.

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