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I'm a professional helicopter pilot (and was a aircraft navigation systems engineer before that) and all of the GPS documentation that I have talks about calculating velocity (ie speed and direction) from consecutive GPS fixes. TomDavison was correct when he said earlier in the thread that the error from the same satellites should be consistent and so the speed between consecutive fixes (with the same error) can be calculated accurately.
To disprove the Doppler-based speed theory, if you turn on any GPS receiver in a poor reception area where satellites are being picked up and lost continuously, you will generally see a speed of one or two mph even if you are not moving. This is because the fixes from different satellite combinations are slightly different and the receiver is calculating the distance and time between these fixes and assumes you are moving (which is why you also see the direction changing). If the receiver used Doppler, since you are not actually moving there would be no Doppler shift and, hence, no speed would be displayed.
Just a quickie regarding something I've read in the above posts. There's a few statements regarding the speed of the satellites and how difficult it would be to work out certain measurements from them. Yes they are moving at a very fast speed indeed, but I was under the impression that as they're 'fixed' in space in the upper atmosphere, they're moving at the same speed as the Earth's rotation and as such they appear stationary to an observer on the ground, so to speak. Or is this talk of the speed of the satellites something different?
Geo-stationary satellites are different. They are the ones that TV dishes point at. If you look at the GO satellite/configurtion screen it will show you where the satellites are. They are always on the move. _________________ TTGO 700 (dead)
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Kenwood DNX-4210DAB with Garmin maps
I'm a professional helicopter pilot (and was a aircraft navigation systems engineer before that) and all of the GPS documentation that I have talks about calculating velocity (ie speed and direction) from consecutive GPS fixes. TomDavison was correct when he said earlier in the thread that the error from the same satellites should be consistent and so the speed between consecutive fixes (with the same error) can be calculated accurately.
To disprove the Doppler-based speed theory, if you turn on any GPS receiver in a poor reception area where satellites are being picked up and lost continuously, you will generally see a speed of one or two mph even if you are not moving. This is because the fixes from different satellite combinations are slightly different and the receiver is calculating the distance and time between these fixes and assumes you are moving (which is why you also see the direction changing). If the receiver used Doppler, since you are not actually moving there would be no Doppler shift and, hence, no speed would be displayed.
Error from the same satellite wont be consistent, if it was you could get 100% accuracy. Errors come from many sources, including the inherent limits of accuracy in the transmitter and the receiver, and the atmosphere.
Whilst stationary, my magellan Gold would show my speed as 0MPH, even though it would show my location as fluctuating. If it calculated the speed as due to differences in position over time, it would have displayed a varying speed as well.
Of course, there is nothing to stop someone using the position differences over time to calculate speed, but since, at least for this model, it obviously didnt do that, and must therefore have calculated it in some other way, using doppler seems to be the easiest, eg the chipset must be transmitting it directly as calculated by doppler. Or put it this way, since it thought I was moving location (and displayed this on the screen) why did it never show my speed changing?
I have tried both a Road Angel and a PDA based GPS system in variety of vehicles, on factory tyres, and compared the readouts on a straight, level road at a constant speed. The most accurate speedos were on BMWs. Both a 3-series and an X5 matched exactly the GPS unit speeds. The worst was a VW Polo which overread by 5 mph. All the others (Land Rover, Ford, Saab) overread by 3 to 4 mph.
The ACPO guidelines for speeding prosecutions (not necessarily followed by certain constabularies) allow a latitude of 10% plus 2 mph for any given speed limit. So 35 in a 30, 46 in a 40, 57 in a 50, 68 in NSL, and 79 on a DCWAY/MWAY.
So, the moral is: pay extra heed to your POI warnings when driving Beemers in North Wales!
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