Time measurement is essential to GPS surveying in several ways. For example, the determination of ranges, like distance measurement in a modern trilateration survey, is done electronically. In both cases, distance is a function of the speed of an electromagnetic signal of stable frequency and elapsed time.
Both GPS surveys and trilateration surveys begin from control points. In GPS, the control points are the satellites themselves; therefore, knowledge of the satellite's position is critical.
A Passive System
The ranges are measured with signals that are broadcast from the GPS satellites to the GPS receivers in the microwave part of the electromagnetic spectrum; this is sometimes called a passive system. GPS is passive in the general sense that the satellites transmit signals; the users receive them. That situation at a GPS receiver is similar to a car radio receiver. It doesn't send and, in that sense, is passive.
I mentioned that time is one of the unknowns that needs to be resolved to provide a position on the Earth using GPS. The measurement of time is essential. For example, the time elapsed for the electromagnetic signal to travel from the satellite to the receiver is important. Moments of time are also important. There are several clocks (oscillators) associated with the systems both in GPS satellites and in GPS receivers.
As mentioned earlier, both GPS and trilateration surveys on the terrestrial side, have to have control points. In the image here, the satellites themselves are the control points so it's important to know where the satellite is in the sky at the moment that a measurement is taken. This is the purpose of the ephemeris of the satellite.