(1) (General Print Server) An IBM mainframe feature that lets TN3270 clients access LPD/LPR printers via the SNA/VTAM network. See TN3270 and VTAM.|
(2) (Global Positioning System) A satellite-based radio navigation system run by the U.S. Department of Defense, officially known as NAVSTAR GPS (see also GLONASS, Galileo and Beidou). Created for the military, the GPS system is not only used by consumers, but in numerous industries, including agriculture, oil, gas, mining and construction.
For consumers, it means no more asking directions with GPS receivers either installed permanently in vehicles (see in-dash navigation) or portable and handheld units that can be taken anywhere (see portable GPS).
Three or Four Satellite Signals Required
The GPS system was designed so that signals from at least four satellites would be on the horizon at all times, sufficient for the GPS receiver to compute the current latitude, longitude and elevation anywhere on earth to within a range of 15 to 70 feet (see latitude). If signals from only three satellites are received, the location can be derived, but not elevation. If less than three, the system cannot compute its current location unless it is tied to the speed of the vehicle (see in-dash navigation).
The Satellite System
In six different orbits approximately 12,500 miles above the earth, the system's 24 medium-earth orbit (MEO) satellites circle the earth every 12 hours. They constantly transmit their current time based on atomic clocks and current location on two frequencies in the L-band labeled L1 (1575.2 MHz) and L2 (1227.6 MHz). Most receivers pick up L1, while more advanced receivers pick up both signals for greater accuracy by determining and removing ionospheric delays.
A GPS receiver calculates the distance to the satellites by comparing the times the transmitted signals were sent with the times received. By knowing the precise locations of the satellites at any given moment, the receiver uses trilateration, the navigation technique of ship captains for centuries, to pinpoint its own location. See triangulation.
First launched in 1978, GPS satellites have been replaced several times with newer models. There are also numerous government and commercial monitoring networks around the world that use earth-based reference stations to improve accuracy. For example, in agriculture, such systems enable farm equipment to automatically plant ultra-precise rows of crops within inches (see GPS augmentation system). See vehicle tracking, reality view, GPS augmentation system, mobile positioning, LORAN, MEO, GNSS, Galileo and geocaching.
In 1996, Sony's NVX-F160 was one of the first GPS navigation systems for the road. It could direct you to the nearest restaurant, hotel or other point of interest. (Image courtesy of Sony Corporation.)
Portable navigation systems work anywhere. The flat object pointing up at the top of this unit is the antenna. See
Delorme's Street Atlas turns a laptop into a "nav" with a lot of satellite detail. It shows all satellites in range (#15 just faded out), their elevation (Elv), azimuth (Az) and signal-to-noise ratio (dB). NET means the satellite is used for navigation (N), ephemeris data are available (E), and it is being tracked (T). The yellow object on the car's dashboard is the antenna.