Development of the global positioning system, or GPS, began in 1973 under the leadership of Colonel Bradford Parkinson of the US Air Force; the system is still operated and maintained by the US military for military use, but is also available for civilian use. The system includes 24 satellites that orbit the earth at altitudes more than 11,000 miles up, in six orbital planes spaced 60° apart. This constellation allows a GPS receiver to access five to eight satellites from any point on earth at any time.
GPS for civilian use
For civilian use, the satellites broadcast a coded signal on a 1575.42 MHz (L1) carrier frequency. When a GPS receiver receives the coarse acquisition (C/A) coded signal, it compares it to an internal signal in the receiver and uses the phase shift between the two signals to compute the time required for the signal to travel from the satellite to the receiver. Because signals travel at the speed of light (which is known), the receiver can calculate the distance from the satellite to the receiver. The signal from one satellite thus determines that the receiver must be on the surface of a sphere whose radius is equal to the distance to the satellite. Because the intersection of two spheres is a circle, receiving the signal from a second satellite places the receiver on a circle in space. Receiving the signal from a third satellite pinpoints the receiver location to one of two points in space. Normally, the receiver tracks four or more satellites to calculate its position. Typically, a C/A based GPS can determine horizontal position within 100 meters and vertical position within 160 meters. These accuracies and those in the following sections are based on the 2SD accuracy system.
Differential GPS
Differential GPS involves the use of two receivers. One is at a fixed location whose coordinates are accurately known, while the other is a roving receiver. By comparing the GPS reading of the fixed receiver with the known coordinates, the fixed receiver can calculate correction factors that can be applied to the roving receiver. The corrections can be applied in real time using a radio link between the receivers, or in a postprocessing mode. Differential C/A based GPS can determine horizontal positions within a few meters. A number of organizations now provide GPS correction signals for a fee. The FAA provides Wide Area Augmentation System (WAAS) correction signals without a fee. Generally, the roving receivers must be within 160 km (100 miles) or less of the base station receiver for accurate corrections. GPS receivers now available include antennas for both the GPS signals and the correction signals and software that will cause the GPS receiver to automatically display the corrected coordinates.
