The U.S. Department of Defense (DOD) originally established the NAVSTAR Global Positioning System (GPS) for military purposes. Fortuitously, GPS has proven to have a great side benefit for society at large. This system includes a constellation of 24 satellites orbiting the earth at an altitude of 20,200 kilometers, in six orbital planes. This configuration allows for world-wide, round-the-clock simultaneous reception of radio signals from five to eight satellites to determine precise position, velocity, and time.

Both GPS satellites and GPS receivers can be thought of as very sophisticated clocks. The clocks in the satellites are extraordinarily accurate and are used to form highly variable codes which are then transmitted by the satellites. GPS receivers are able to receive and interpret the coded signals sent by the GPS satellites.

GPS receivers also contain clocks and are capable of receiving and decoding GPS satellite signals. The clocks in the receivers are very good clocks, but not nearly as good as the clocks in the satellites. GPS receivers convert the GPS satellite signal into position, velocity, and time estimates. At least four GPS satellites are required to solve the GPS receiver's four unknowns: latitude, longitude, height, and clock error.

There are two positioning systems provided by the DOD. These are the Standard and Precise Positioning Systems.

The Standard Positioning System (SPS) is broadcast with unrestricted world-wide access. All GPS receivers are able to receive and interpret SPS signals. The SPS accuracy is intentionally degraded by the DOD through the application of Selective Availability (SA). SPS predicted accuracies are (at 95% probability):

• 100 meters (330 feet) horizontally
• 156 meters (512 feet) vertically
• 340 nanoseconds in time

The Precise Positioning System (PPS) requires receivers to have special cryptographic equipment to decode the signals and nullify Selective Availability. PPS is only available for the U.S. and allied military, certain U.S. Government agencies, and selected civil users specifically approved by the U.S. Government. Using PPS the following accuracies are achievable (at 95% probability):

• 22 meters (72 feet) horizontally
• 22.7 meters (75 feet) vertically
• 100 nanoseconds in time

The accuracies indicated above for both SPS and PPS are achievable using only one GPS receiver in a technique known as point positioning. Various unavoidable and unintentional factors may affect the signals being transmitted by the satellites resulting in degraded positional accuracy. These are primarily clock errors and the refractive effects of the signals travelling from the vacuum of space through the earth's dense atmosphere to the receiver.

By augmenting the point positioning procedure with another GPS receiver set at a base station of accurately known location, positional accuracies can be greatly increased. To achieve greater levels of accuracy a technique called differential GPS (DGPS) can be employed. DGPS is a positioning technique that reduces the errors in the signal at the location of the user's GPS receiver by using corrections to the received signals which are computed at a nearby (within 100 kilometers) reference station at a known location.

Because the location (coordinates of the reference receiver) is already known, rather than computing its position the receiver at the reference station can compute corrections to the satellite signals. These corrections can be applied to the same satellite signals being received simultaneously by the user's receiver. DGPS when properly employed can regularly result in real-time positional accuracies in the 1-10 meter range. Using post-processing techniques can result in 1-3 centimeter accuracies in all three dimensions.