Timing's the thing Think of GPS satellites as floating DME stations. They move along in orbit and that complicates things but forget about that for the moment. Your aircraft DME unit is really a transceiver that interrogates a transponder in a VOR-DME station. When you tune the DME, your unit sends a signal to the ground station, which replies on a different frequency. The receiver multiplies half the total signal transmission time by the speed of light and converts it to distance.
Subsequent fixes allow ground speed and time computations.
And, as we said, the SVs are moving around up there at about five miles a second, so we have to account for that, too. You can see how it begins to get complicated. Like DME, GPS measures the time that it takes the signal to reach the receiver. However, unlike DME, it doesn't have benefit of a returning pulse from an interrogation to act as a baseline. It relies purely on one-way timing.