how reliable is the GPS Speed on windows phone 7

Question

I am developing a Windows Phone 7 app and I want to tell the user how fast they are going. Something simple like...

Current Speed: 20 km/h Max Speed: 33 km/h Average Speed: 25 km/h

I seem to have two options: 1) use accelerometer 2) use GPS

I'm wondering if people have suggestions on which method to use.

I would prefer to use the GPS (as that is much simpler to extract), but I know that GPS position is often not accurate for various reasons, but how accurate is the speed?

Answer 1

Speed and direction tend to be more accurate than absolute position - this is because speed and direction are not calculated by tracking successive position measurements (as you might expect). Rather, these values are generally estimated by using the Doppler shift of the satellite signals, which for technical reasons is less affected by the factors that tend to distort absolute position.

By contrast, the accelerometer probably won't be very useful for measuring speed, as speed and acceleration are two different things.