Originally Posted by
Master Splinter
Just like your car GPS system. A GPS receiver listens for a psudorandom timing signal from the GPS satellites and triangulates your position by seeing how much each GPS satellite's timing signal varies from both its own timing count and that of the other satellites.
IE:
Phone GPS chip: "1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12"
Satellite A: "2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12"
Satellite B: "16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12"
Satellite C: "3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14"
Phone GPS: "Hummm, I'm 1 off from satellite 1, -1 off sat 2, and 2 off from sat 3. Now if I check in my handy ephemeris of satellite position and velocity data, add in propagation delay caused by conditions in the ionosphere, then calculating the speed-of-light induced delay from each satellite's position gives me my position.
"Now, I'll just pull out my accurate-to-within-14-nanoseconds-stopwatch, time the delay, and it looks like I'm....just entering the tunnel under Sydney Harbour...I don't know why I bother. La la la, not listening...."
A phone GPS will be able to show your position on the map if it happens to have the map data for that area still in memory. Once you hit a location that you don't have map data for, it can't place you on a map.
Since the sim is out and there's no wireless data connection, it can't download the map data for the new area. It'll still know where you are (with reference to satellite locations), but until it gets new map data, it can't draw the map for you.
(Trivia: The theoretical accuracy of the current GPS system is about 3-4mm....but that's with lab-grade equipment, not something you bung on a chip and sell for under $5.)