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OT: GPS speed measurement
Posted By: Jeroen Dorrestein (c529cf5ba.cable.wanadoo.nl)
Date: 11 DEC 2004 - 06:45z
This is somewhat off topic, but I was wandering if somebody could help out:
I have recently bought myself a nice new GPS set for in my car. (TomTomgo). I'm familiar with how GPS works, in terms of how it obtains and calculates it's position etc.
I'm just curious how it calculates the speed shown on the GPS, and what sort of accuracy would it have. The impression I'm getting is it appears quite accurate. I had my speedometer properly calibrated and it's gives a very similar reading. When I compare it to other car's speedometers, not calibrated, the GPS speed is on average 3-5% lower. Which is understable as car manafacturers like to show a slightly higher speed than what it really is.
The most basic way of calculating speed would be, of course to compare distance travelled to time travelled. But then the GPS accuracy does come into play somewhat. My GPS appears to update it's speed readings fairly quickly.
I've asked this question on other (GPS) forums as well, but answered ranged from people who claimed that their yacht's log was more accurate then a GPS to persons claiming GPS uses doppler to determine the speed.
Anybody can help me with this OT trivial question?
Jeroen
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