Global Navigation Satellite Systems (GNSS) work by measuring the time it takes for a signal to travel between a GNSS satellite and a receiver.
Determining an accurate position on earth requires two pieces of information from at least 4 satellites. The precise orbit of the GNSS satellite (called the ephemeris) and the time it has taken for the signal to reach the receiver.
The GNSS Receiver uses the time a signal is received minus the time the signal is sent to determine the elapsed time. This elapsed time is then multipied by the speed of the signal (speed of light) to estimate the distance.