Answer 3—A GPS receiver doesn’t have direct access to the transmitted signal, so it measures the time differences between pairs of signals received from different satellites. Because the mixing preserves phase relationships, this makes it possible to decode both the phase-modulated navigation information signal and measure relative carrier phase at a lower frequency—and using less power—than the microwave carrier itself. As long as the local oscillator is stable and its frequency is known relative to the GPS timebase, full accuracy is preserved.

Contributor: David Tweed