GPS

The Global Positioning System (GPS) is a worldwide navigation system developed by the U.S. Department of Defense. A constellation of 24 NAVSTAR satellites enables users to determine their position anywhere on the earth in three dimensions (latitude, longitude, and altitude) at any time of day and in any kind of weather. The GPS was declared fully operational on April 27, 1995.

Using the simple mathematical principle called trilateration, the location of a GPS receiver can be determined by computing its distance to at least four GPS satellites whose precise orbital positions are known. The distance to each satellite is determined accurately by measuring the time it takes for the signal transmitted by each GPS satellite to reach the receiver. Because these signals propagate at the speed of light, it is possible to convert the measured time to the distance between the receiver and each satellite. Because only one point on the earth can be at those precise distances from the satellites, the location of the GPS receiver can be determined.

The GPS consists of three major segments. The space segment consists of a constellation of 24 satellites circling the globe on four orbital paths and is designed so that, barring obstructions, a minimum of 5 satellites can be viewed to determine a position. The control segment tracks the satellites' orbits, monitors their status, and frequently relays updates to the satellites, including corrections to their on-board atomic clocks. The user segment consists of GPS receivers and auxiliary equipment such as antennas.

The GPS is designed for dual military and civilian use. The location accuracy available for civilian applications, called the Standard Positioning Service (SPS), originally was lower than the Precise Positioning Service (PPS) used for military applications. This was accomplished by intentionally degrading the signal using a procedure known as selective availability that ended on May 2, 2000. Since then, typical GPS accuracies are on the order of 15 meters horizontally and 25 meters vertically.

A number of factors, including satellite clock errors, orbital uncertainties, and atmospheric effects, contribute to GPS positional errors. These errors can be reduced using differential GPS (DGPS) techniques. Using DGPS, it is possible to determine a location to within 1 meter horizontally and within a few meters vertically. In DGPS, this is accomplished by continually comparing the position of a known location, called a base station, with its position determined by GPS that changes over time. The difference in these two positions measured at a base station can then be used to correct GPS positions measured at other locations. These differential corrections can be transmitted at radio frequencies to specially equipped[Page 199] DGPS receivers and then can be applied in real time as the GPS coordinates are being collected or can be applied after GPS coordinates are collected using postprocessing techniques.

The GPS is used for a wide range of applications, including surveying, aircraft navigation, and even the game of geocaching. As GPS modernization continues and the European Galileo system is deployed, global navigation satellite systems will continue to improve.