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 light, 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 sense that only the satellites transmit signals; the users simply receive them.
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 to GPS surveying in many ways. For example, the elapsed time it take the electromagnetic signal to travel from the satellite to the receiver is important. Please note also that there are several clocks or oscillators associated with the systems in GPS. There are clocks (oscillators) in the satellites and in the receivers, as well.
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. This is a point that will be mentioned more than once. Therefore, 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.
GPS is a passive system. That means that the receivers don't send signals to the satellites. The satellites broadcast and the receivers receive; in some ways, the situation is similar to a car radio. The radio receives, but doesn't send. In that sense, it is passive.