A CDE Definition
(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). Originally created for the military, today, GPS receivers are built into all smartphones and millions of cars and trucks. See in-dash navigation, CarPlay, Android Auto and portable GPS.
Three or Four Signals Are Required
At least four satellites are on the horizon at all times, sufficient for the GPS receiver to compute the current latitude, longitude and elevation anywhere on earth to within 15 to 70 feet (see latitude). If due to interference, only three signals are present, there is no elevation. If less than three, the current location cannot be computed unless it is built into and 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 (L1: 1575.2 MHz; L2: 1227.6 MHz). Most receivers pick up L1, while more advanced receivers pick up both 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 GPS receiver computes the current coordinates using trilateration, a method similar to how ship captains navigated for centuries (see triangulation).
First Launched in 1978
In this decades-old system, satellites are periodically replaced. In addition, worldwide government and commercial monitoring networks use earth stations to improve accuracy; for example, enabling farm equipment to plant ultra-precise rows of crops (see GPS augmentation system). See social navigation, vehicle tracking, reality view, GPS augmentation system, mobile positioning, LORAN, MEO, GNSS, Galileo and geocaching.
An Early Car Navigation System
GPS in the Woods
Navigation App in a Laptop
It Doesn't Get Better Than This
Before/After Your Search Term
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Terms By Topic
Click any of the following categories for a list of fundamental terms.