A GPS signal must somehow communicate to its receiver:
- what time it is on the satellite,
- the instantaneous position of a moving satellite,
- some information about necessary atmospheric corrections, and
- some sort of satellite identification system to tell the receiver where it came from and where the receiver may find the other satellites.
If we are to measure distances from the satellite to the receiver, and that is the foundation of GPS survey, some information needs to be communicated from the satellite to the receiver and that information needs to come along with the signal from the satellite to the receiver.
One aspect is the time on the satellite, because, of course, the elapsed time that the signal spends going from one place to the other is the basis of the distance measurement - ranging. Therefore, it is important to know the time on the satellite the instant that the signal left.
Secondly, the position of the moving satellite at an instant is critical. The coordinate of the satellite at that moment of measurement is important so that it can be used to derive the position of the receiver. In a terrestrial survey, instantaneous position hardly comes into it because the instrument on the control point is stationary on the Earth's surface. Satellites, on the other hand, are moving at a pretty tremendous rate of speed relative to the GPS receiver, so the ephemeris needs to provide the coordinates of the satellites at an instant of time. This is another way that time is important.
Some information about the atmosphere needs to be communicated to the receiver, too. If you're familiar with electronic distance measurement (EDM) surveying, you know that when an electromagnetic signal goes through atmosphere, it is attenuated by the humidity, the temperature, and the barometric pressure. Therefore, these data are introduced into the processing of the distances that are measured with EDM instruments.
The GPS signal is going through a good deal more of the atmosphere than even the longest EDM shot. The first component of the atmosphere that the GPS signal encounters is the ionosphere. The ionosphere has some characteristics that differ from the next atmospheric layer the signal encounters, the troposphere. In any case, the signal can be attenuated rather dramatically during its trip. It follows that it is important to have some representation of the atmosphere through which the signal is passing communicated to the GPS receiver from the satellite. This is so that the resultant delays can be introduced into the calculation of the GPS derived position of the receiver.
Some sort of satellite identification system is required, too. Each distance that the receiver measures from each satellite must be correlated to that satellite. Since the receiver will need to have at least four distances from at least four different satellites, it needs to be able to assign the appropriate range, the appropriate distance or length, to the correct satellite. It needs to identify the origin of each signal.
This is just some of the information that needs to come down on that signal from the satellite to the receiver. It's quite a list and is actually even a little bit longer than this.