Unlike a TV broadcast satellite, GPS satellites are not stationary, but are in near-circular Earth-orbits. They take approximately 12 hours to circle the globe, but because the Earth is also rotating, a stationary Antenna will see a particular satellite reappear at intervals of around 24 hours.
There are up to 24 satellites in six different orbits equispaced around the equator, which have been described as being reminiscent of a spherical birdcage. Their orbits are inclined to the earth's axis, and therefore the satellites do not pass overhead at the poles. There are three or four satellites spaced around each orbital plane.

A GPS Antenna is to a first approximation, omnidirectional above the horizontal plane, and it is capable of receiving signals from as many satellites as are above its visible horizon at a particular time, provided that its view of them is not obstructed by local objects.
Satellites that appear near the horizon will provide only weak signals, and the Antenna and receiver are usually programmed to reject those that are below an angle of 5 to 10 degrees from the horizontal plane.
The number of satellites visible at a particular time will vary from as few as 4 to as many as 10. Because they are in different orbits, some will appear for a short time by just rising briefly over the horizon and others may track almost overhead and therefore be visible for several hours,

The signal level available from a GPS Antenna sited anywhere near the Earth's surface does not vary significantly with the Antennas' height above the local ground level. In a flat desert region it would work well if mounted only a few centimetres above the ground, and there would be no advantage in mounting it on a tall mast.
However it is essential that the Antenna has a 'line-of-sight' relationship with available satellites, because at the frequencies used for GPS (around 1.6GHz), signals are blocked by solid or semi-solid objects in the same way that light is blocked.
In typical urban locations it is therefore frequently necessary to mount the Antenna in a high position in order that it can 'see' a lot of the sky, despite the presence of signal-opaque objects such as adjacent buildings.

When viewed from the Northern Hemisphere, and because of the inclined orbits mentioned above, the sky-tracks described by satellites viewed from the Earth's surface tend to concentrate in the Southern sky. If, at a particular site, a large obstruction to an Antennas view is unavoidable, it is usually advantageous to site the Antenna so that the obstruction is on its Northern side.

Although apparently complex, the visibility of all the satellites at a particular point on the Earth is precisely predictable, and there are (PC) computer programs available that will accurately forecast the situation at any point in time, past, present or future, and at any place on the globe.

In addition to the basic geometry of the GPS system, it is also necessary to consider the likelihood of the Antenna receiving signals that have bounced off nearby reflecting surfaces, concrete walls for example. This multi-path effect, as it is known, can seriously degrade the performance of a GPS system, and may cause complete loss of signals from otherwise 'good' satellites, due to the time-delayed reflected signal cancelling out the direct signal. Whilst nothing can usually be done about reflection from distant objects, it is wise to avoid siting an Antenna at the same horizontal level as a wall or metallic structure such as a lift-motor housing or water tank, which are common objects on many large buildings.

Unlike the short-term 'flutter' caused by passing aircraft, that affects VHF-FM radio reception, the multi-path symptoms in stationary GPS installations are frequently very long-term effects caused by the movement of the satellites and not the reflecting objects. Signal levels can be observed to fluctuate with periods of several hours as the satellites position changes relative to the reflecting surface.
In addition the amount of energy reflected from a particular surface may vary for example according to whether it is dry or wet, so the situation can change from day-to-day.

Finally, it is important to remember that the signal power received by a GPS receiver is very low indeed, and it is prudent to avoid, as far as possible, siting Antennas next to significant sources of interference; electrical switchgear, and Radio (transmission) Antennas are obvious candidates.
Also, power transformers, particularly large ones used in sub-stations, frequently appear to be a 'magnet' for rf energy from various sources and are items to be viewed with suspicion.

The notes above are intended to illustrate problems that may occur, rather than those that will occur.

GPS receivers are designed to work with small fluctuating signals, to survive sudden obscuration of a satellites signal, and in Timing applications, can even operate for long periods with only one satellite in view. It is because of these remarkable capabilites that the vast majority of Antenna sites, if chosen intelligently and well engineered will operate correctly from the day of installation onwards.