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Assisted GPS, generally abbreviated as A-GPS, is a system which can, under certain conditions, improve the startup performance, or TTFF (Time To First Fix) of a GPS satellite-based positioning system. It is used extensively with GPS-capable cellular phones as its development was accelerated by the U.S. FCC's 911 mandate making the location of a cell phone available to emergency call dispatchers.^[1]

Assistance falls into two categories:

1. Information used to more quickly acquire satellites
   • It can supply orbital data or almanac for the GPS satellites to the GPS receiver, enabling the GPS receiver to lock to the satellites more rapidly in some cases.
   • The network can provide precise time.
   • The device captures a snapshot of the GPS signal, with approximate time, for the server to later process into a position.
   • Accurate, surveyed coordinates for the cell site towers allow better knowledge of local ionospheric conditions and other conditions affecting the GPS signal than the GPS receiver alone, enabling more precise calculation of position. (See also Wide Area Augmentation System and CellHunter.)
2. Calculation of position by the server using information from the GPS receiver
   • The assistance server has a good satellite signal, and plentiful computation power, so it can compare fragmentary signals relayed to it by GPS receivers, with the satellite signal it receives directly, and then inform the GPS receiver or emergency services of the GPS receiver's position.

Simultaneous GPS or S-GPS is a method to enhance a mobile phone's satellite-based position reporting ability to a carrier.

Ordinarily, a built-in GPS device is used to determine the location of an E911 call made from CDMA phones. By using a time-multiplexed scheme called TM-GPS, the reception of the telephone call and the GPS signal are alternated one after the other, requiring only one RF radio.