Trip Screen

No, it's usually caused by position error, the GPS thinks you are moving.

it is normal!

Today's high sensitivity GPS chipsets will show your position as peridocally "jumping around" even though you are stationary. The SiRFstarIII is more prone to this jumping that the current MediaTek MTK v1 GPS chipset that is now in use in later 7x0's and in the newest 7x5's.

Have it too.

Thanks, that's what I thought, but just wanted to make sure...
Al

This is normal behavior for all consumer GPS units. Afterall, the satellites are all moving, the earth is moving and the GPS uses a model of the earth as the map. Notice that elevation readings are even more inaccurate and change constantly.

A gps determines your position by comparing when satellites say they sent the signals to when you receive them. That can be influenced by atmospheric distortion, other moisture (heavy clouds, leaves, etc.) and multipath (bouncing off of things). Then the question is how does your gps know what time it really is? So it is filtering through almanac data from the satellites and current timing to figure out the time and triagulate your position. And as it gets more up to date info it compensates.

It was much less accurate before May 2000 when the US gov't intentionally floated the signals by as much as 100 meters (+/- 50 meters) to discourage accuracy of enemy forces (military gps had data to compensate).

Last weekend I was in New York City and my in-dash nav exhibited some interesting behaviours. Like suddenly telling me to make a turn one way and then telling me to turn another way in a matter of a few seconds. Other times it would tell me to turn after the intersection had been passed. I realized this was most likely due to GPS signals bouncing off the buildings as well as being blocked which would be an example of multipath which is the result of signals bouncing off of buildings. Technology is being implemented in more advanced systems for commerchial airliners to block jamming intended to fool GPS navigation systems and perhaps some of this technology may eventually find its way into our lower cost GPS systems. However, this is probably not the reason for your GPS thinking it is moving while standing still. While the government does not currently add random biases into the signal but the signal that most GPS receivers use is not as accurate as accurate as the signal used by military GPS recievers. Also as pointed out earlier the altitude even in the military GPS receivers is not very inaccurate.