Originally Posted by bellweatherman
And is this GPS Sats thing accurate when calculating your speed? I mean, theoretically, it calculates your speed based on where and how far you've traveled on earth, right?
No, it's all about Doppler effect, not distance travelled. From gpsinformation.net:
"HOW ACCURATE IS THE SPEED READING?
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."
http://gpsinformation.net/main/gpsspeed.htm
Should apply to bike applications too!
Erling.