Global Positioning System (GPS)

The GPS composite clock is a network of atomic clocks related by measured signal transit times which defines GPS time, the timescale used today in GPS operations. Simulated GPS clock phase and frequency deviations, simulated GPS pseudo-range measurements, are used to understand GPS time in terms of Kalman filter estimation errors.

In essence, GPS composite clock specifically receive times measured by ground station clocks minus transmit times measured by satellite clocks. Such measurements allow both orbit and clock estimation, but since clocks appear only in differences, any error common to all the clocks would remain unobservable.