Accuracy of GPS as a speedometer

[ledfoot]

I have seen my Road Angel showing 20mph more than my actual speed due to interference from other onboard electrical equipment.

I would not rely on GPS for speed information.

[dnb]

GPS speed is pretty good most of the time. Accuracy depends on the number of satalites the system can see, so it's not fixed, and it may under read whereas a conventional speedo is unlikely to do so.

[g_attrill]

kevinday said:

The GPS speed is calculated from the position coordinates as an average, not as an exact speed at that point in time. In general they should be pretty good, unless the Americans turn down the satellite accuracy from 10 metres to 100.

The very oldest commercial GPS receivers use this method, anything made in the past few years uses a calculation based on the doppler shift of the GPS signal and so is pretty accurate, although I believe it is smoothed somewhat so the effect is the same.

Gareth

[nickwilcock]

Indeed, the calculation of position and the calculation of velocity in GPS receivers use different techniques.

Using Microsoft Autoroute 2006, there is some GPS position error when stationary and occasionally whilst on the move. For my car, an indicated 71mph at 2500 rpm corresponds to a GPS value of 70 mph.

[kevinday]

Gareth, thanks for the tech update, always something new to learn!