GPS, a satellite-based positioning system, is operated by the United States Department of Defense and was officially put into operation in 1995. Using the difference in the radio signal propagation times of at least three or more of the 32 GPS satellites, a GPS receiver can accurately determine its position worldwide to within a few meters using trilateration. The planned European Galileo system will be largely compatible with GPS.
Assisted GPS (A-GPS) is mainly used in mobile terminals to enhance performance by means of faster localization (time to first fix, TTFF) and higher accuracy. A-GPS uses the coordinates of the base station serving the radio cell in which the mobile terminal is located and transmits this information to the mobile terminal via an assisted channel. An initial approximation of the position of the mobile terminal is possible, thus considerably shortening the time until exact GPS position data is available. The assisted data is transmitted via an IP channel (secure user plane location, SUPL) as defined by the Open Mobile Alliance (OMA).