At an altitude of 10,900 nautical miles, each satellite orbits earth every 12h period but repeats the same track approximately each 24h - 4m period. With six orbital planes, (nominally 4 satellites in each) inclined at 55 degrees equatorial plane and equally spaced 60 degree apart, there are between five and eight satellites visible from any point on the earth at any given time.
The GPS constellation of 24 satellites is designed so that a minimum of five are always observable by a users anywhere on earth. The receiver uses data from the best four satellites above the horizon, adding signals from one as it drops signals from another, to continually calculate its position.
GPS operation is based upon the concept of ranging and triangulation from a group of satellites in space which act as precise reference points. A GPS receiver measures distance from a satellite using the travel time of a radio signal. The course/acquisition (CA) code one of the two Pseudo-Random Codes of each satellite, contains information on the satellite's position, the GPS system time, its clock error, and the health and accuracy of the transmitted data. GPS satellites have very accurate atomic clocks in order to calculate signal travel time. Knowing the speed at which the signal traveled (approximately 186,000 miles per second) and the exact broadcast time, the distance traveled by the signal can be computed from the arrival time (velocity x time = distance).
The GPS receiver matches each satellite's CA code with an identical copy of the code contained in the receiver's database. To make the measurement we assume that both the satellite and our receiver are generating the same Pseudo-Random Codes at exactly the same time. By comparing how late the satellite's Pseudo-Random Code appears (shifting its copy of the satellite's code, in a matching process, and comparing this shift with its internal clock compared to the receiver's), the receiver can calculate how long it took the signal to travel from the satellite to the receiver. The distance derived from this method of computing distance is called a pseudo-range because it is not a direct measurement of distance, but a measurement based on time. Pseudo-range is subject to several error sources: for example, an ionospheric delay, and time disparities between the atomic clocks in the satellites and the GPS receiver.
In addition to knowing the distance to a satellite, a receiver needs to know the satellite's exact position in space; this is known as its ephemeris. Each satellite's signal transmits ephemeris information about its exact orbital location. The GPS receiver uses this information to precisely establish the position of the satellite.
Using the calculated pseudo-range and the position information supplied by the satellite, the GPS receiver mathematically determines its position by triangulation. The GPS receiver needs at least three satellites with timing corrections from a fourth satellite to yield an unaided, unique, and true three-dimensional position (latitude, longitude, and altitude) and time solution. The GPS receiver computes navigational values such as distance and bearing to a waypoint, ground speed, etc., by using the GPS receiver's known latitude/longitude and referencing these to a database built into the receiver.
GPS Positioning Services:
Precise Positioning Service (PPS): Authorized users with cryptographic equipment and keys and specially equipped receivers use the Precise Positioning System. U. S. and Allied military, certain U. S. Government agencies, and selected civil users specifically approved by the U. S. Government, can use the PPS.
Standard Positioning Service (SPS): Civil users worldwide use the SPS without charge or restrictions. Most receivers are capable of receiving and using the SPS signal. The SPS accuracy is intentionally degraded by the DOD by the use of Selective Availability.