GPS

(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). It was designed so that signals from at least four satellites would be on the horizon at all times, which is sufficient for the GPS receiver to compute the current latitude and longitude (map location) and elevation anywhere on earth to within a range of 15 to 70 feet (see latitude).

MEOs in Six Orbits
In six different orbits approximately 12,500 miles above the earth, the system's 24 MEO satellites circle the earth every 12 hours. The satellites 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.

There are also numerous government and commercial monitoring networks around the world that use earth-based reference stations to improve accuracy (see GPS augmentation system).

Whether installed in vehicles or carried by hand, 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 a given moment, the receiver uses triangulation, the navigation technique of ship captains for centuries, to pinpoint its own location. See triangulation.

At Least Three Satellites
If four satellites are picked up, map location and elevation can be computed. If three are available, only location can be derived, and if less than three signals are received, the system cannot compute its current location unless it uses the speed of the vehicle (see in-dash navigation).

No More Asking Directions
Widely used in the military, GPS is used in many other applications, including agriculture, oil, gas, mining and construction.

In the mid-1990s, GPS car navigation systems emerged, and by 2000, in-dash systems were either standard or an option in luxury cars. Subsequently, after-market units were made available for all vehicles. Portable units can be taken from car to car, and navigation software can be added to a laptop computer. Small handhelds help hikers and hunters find their way. See vehicle tracking, GPS augmentation system, mobile positioning, GIS glossary, MEO, GNSS and Galileo.

GPS in the Car

In 1996, Sony's NVX-F160 was one of the first navigation systems for the road. It could direct you to the nearest restaurant, hotel or other point of interest. (Image courtesy of Sony Corporation.)

GPS in the Woods

Portable navigation systems work anywhere. The flat object pointing up at the top of this unit is the antenna.

GPS On a Laptop

Delorme's Street Atlas turns any laptop into a "nav," and the software is extremely informative. The bottom clip shows all satellites in range (it just lost #15), their elevation (Elv), azimuth (Az) and signal-to-noise ratio (dB). NET means (N) the satellite is used for navigation, (E) ephemeris data is available, and (T) it is being tracked. The yellow object on the dash is the antenna.

---

GPS - Global Positioning System