Discussion: Garmin

That's pretty common, and others have replicated that experiment. There's a scientific explanation, but I can't provide it.

Isn't it just the lack of accuracy of the signal? The watch thinks it's somewhere then 4 seconds later (when it samples again) it thinks it moved 20 ft, so it adds that to the accumulated total. Over 7 hours, that adds up.

What it all shows is that talking the distance date from our watches to the hundredth of a mile is just plain in accurate. Accuracy of about 10% is probably the way we should look at it.

It's communicating with a satellite in orbit, so accuracy to the meter isn't consistent. Even if the watch isn't moving the satellite is and has to do math to figure out the new angle. Good enough accuracy for the layman; the military probably (I hope) has a better algorithm (for targeting missiles).

I think that's the simple explanation as I understand it. Someone will probably prove me wrong.

I thought that array of GPS satellites was geostationary. That is, they "hover" over the same spot on the surface of the Earth, and do not move, relative to the Earth, and are uniformly 22 000 miles up, where the laws of physics allow geosynchronicity. We calculated that height in my UC-Berkeley engineering class, back in 1964 at the dawn of the space age, when even then my Silicon Valley-bound classmates were foreseeing GPS.

But as the Wikipedia article points out, nothing is perfect. And even those satellites can drift as a result of "solar wind," and other effects. So perhaps Janet is correct; those satellites move too.

I thought it was just that the receivers in the consumer devices we use have limited accuracy. A good receiving device (including, presumably, ones used by the military) would use those same satellite signals and get much greater accuracy.