Assisted GPS

Assisted GPS (abbreviated generally as A-GPS and less commonly as aGPS) is a system that often significantly improves startup performance—i.e., time-to-first-fix (TTFF), of a GPS satellite-based positioning system. A-GPS is extensively used with GPS-capable cellular phones, as its development was accelerated by the U.S. FCC's 911 requirement to make cell phone location data available to emergency call dispatchers.

Standalone/self-ruling GPS devices depend solely on information from satellites. A-GPS augments that by using cell tower data to enhance quality and precision when in poor satellite signal conditions. In exceptionally poor signal conditions, for example in urban areas, satellite signals may exhibit multipath propagation where signals skip off structures, or are weakened by meteorological conditions or tree canopy. Some standalone GPS navigators used in poor conditions can't fix a position because of satellite signal fracture, and must wait for better satellite reception. A GPS unit may need as long as 12.5 minutes (the time needed to download the GPS almanac and ephemerides) to resolve the problem and be able to provide a correct location.

An assisted GPS system can address these problems by using external data. Utilizing this system can come at a cost to the user. For billing purposes, network providers often count this as a data access, which can cost money depending on the plan.

To be precise, A-GPS features depend mostly on an internet network or connection to an ISP (or CNP, in the case of CP/mobile-phone device linked to a Cellular Network Provider data service). A mobile (cell phone, smart phone) device with just an L1 front-end radio receiver and no GPS acquisition, tracking, and positioning engine only works when it has an internet connection to an ISP/CNP where the position fix is calculated offboard the device itself. It doesn't work in areas with no coverage or internet link (or nearby BTS towers, in the case on CNP service coverage area). Without one of those resources, it can't connect to the A-GPS servers usually provided by CNPs. On the other hand, a mobile device with a GPS chipset requires no data connection to capture and process GPS data into a position solution, since it receives data directly from the GPS satellites and is able to calculate a position fix itself. However the availability of a data connection can provide assistance to improve the performance of the GPS chip on the mobile device.

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