Global Positioning System

The Global Positioning System (GPS), officially NAVSTAR GPS, is a global navigation satellite system for position determination. It was developed since the 1970s by the U.S. Department of Defense and replaced the old satellite navigation system NNSS (transit) of the U.S. Navy from about 1985, as well as the Vela satellites for locating nuclear weapon explosions. GPS has been fully operational since the mid-1990s and, since the deactivation of artificial signal degradation (Selective Availability) on 2 May 2000, has also enabled civilian users to achieve an accuracy of often better than 10 metres. The accuracy can be increased to values in the centimetre range or better by differential methods (differential GPS/DGPS) in the vicinity of a reference receiver. Satellite-based augmentation systems (SBAS), which disseminate correction data via geostationary satellites that cannot be received in polar regions and also belong to the class of DGPS systems, achieve accuracies of one meter continent-wide. GPS has established itself as the world's most important positioning method and is widely used in navigation systems.

The official name is "Navigational Satellite Timing and Ranging - Global Positioning System" (NAVSTAR GPS). NAVSTAR is sometimes also used as an abbreviation for "Navigation System using Timing and Ranging" (without GPS). The system was officially launched on 17 July 1995.

The abbreviation GPS is now used colloquially, and sometimes even technically, as a generic term or pars pro toto for all satellite navigation systems, which are correctly grouped under the abbreviation GNSS (Global Navigation(al) Satellite System).

Bradford W. Parkinson, Hugo Fruehauf, and Richard Schwartz received the 2019 Queen Elizabeth Prize for Engineering for their development of GPS.

Areas of application

GPS was originally intended for positioning and navigation in the military sector (in weapons systems, warships, aircraft, etc.). In contrast to mobile radio devices, GPS devices can only receive signals but not actively transmit them. This makes it possible to navigate without third parties receiving information about one's own location. Today, GPS is also used throughout the civilian sector: For spatial orientation in seafaring, aviation and road traffic as well as when spending time in nature; for position determination and tracking in rescue and firefighting services, in public transport as well as in the logistics sector.

Following the establishment of the satellite positioning service of the German national survey (SAPOS), DGPS methods are of particular importance in geodesy in Germany, as they can be used to carry out nationwide surveys with cm accuracy. In agriculture, it is used in precision farming to determine the position of machines in the field.

The Assisted Global Positioning System (A-GPS) was developed especially for use in mobile phones.

Structure and operation of the locating function

The general principle of GPS satellite positioning is described in the article Global Navigation Satellite System.

GPS is based on satellites that constantly broadcast their current position and the exact time using coded radio signals. Special receivers (GNSS) can calculate their own position and speed from the signal propagation times. Theoretically, the signals from three satellites, which must be above their cut-off angle, are sufficient to determine the exact position and altitude. In practice, GPS receivers do not have a sufficiently accurate clock to correctly measure travel times against GPS time. Therefore the signal of a fourth satellite is needed, which can be used to determine the reference time in the receiver. For the number of satellites required, see also: GPS technology

The GPS signals can be used to determine not only the position, but also the speed and direction of movement of the receiver, which can then be displayed on a digital map or as a compass. Since this is generally done by measuring the Doppler effect or differentiating the location according to time (measure and direction of the detected local change), the compass measurement is only possible if the receiver has moved.

The satellite constellation was set so that it is usually possible for a GPS receiver to have contact with at least four satellites. Six orbital planes are inclined 55° to the equator and cover almost the entire world. GPS devices cannot be used in the polar regions, but other satellite navigation systems whose satellites move in other orbits can.

According to the basic GPS configuration, at least four satellites in each of the six orbital planes should orbit the Earth twice at an altitude of 20,200 km on each sidereal day. A satellite of the IIR version, for example, is designed for an operational lifetime of 7.5 years. In order to avoid failures due to technical defects, additional satellites are kept ready. Some of them are placed in extended slots of the constellation and play an active role there. Further inactive spare satellites are waiting in orbit for their deployment. A resulting gap in the constellation does not lead to a reduction in signal availability if an immediately adjacent extended slot is occupied. To fill a gap, a new satellite can be launched, a dormant satellite already in orbit can be activated, or an active satellite can be maneuvered to another position. All of these actions are time consuming. It takes months to move a satellite to the position required for deployment. Within an orbital plane, repositioning can be accomplished by a sequence of braking and acceleration maneuvers as long as the fuel supply is sufficient to do so, which is usually only used to maintain the exact position. Because of its weak engine, a satellite cannot propel itself to a higher orbital plane.

Stationary GPS receiving antenna for time-critical scientific measurements