STLRN,
Back at Joint Firepower Control Course in '83, I saw a film taken from the sensors of an AC-130 gunship over the Ho Chi Ming Trail. It was a line of flickering lights that suddenly blossomed into explosions. The flickering lights were the ignition systems of the trucks in a convoy moving down the trail. The blossoms of exposions were just that, 105mm rounds from the AC-130 hitting the trucks. That was 1970 technology. I'm sure we can track about any electronic emission now.
I think Edward may have thought that the GPS somehow returned a signal to the satellite.
I guess if a person wants to be completely invisible, he needs to not emit any electronic signature (no radios, GPSs, electric razors, flashlights, etc.) and figure a way to keep his body temperature the same as his surroundings.
Jeff