Assisted GPS, generally abbreviated as A-GPS, enhances the startup performance of a GPS satellite-based positioning system. It is used extensively with GPS-capable cellular phones. Its primary use is for making the location of a cell phone available to emergency call dispatchers.
The assistance server has a good satellite signal, and lots of computation power, so it can compare fragmentary signals relayed to it by cell phones, with the satellite signal it receives directly, and then inform the cell phone or emergency services of the cell phone's position.
GPS is a global navigation satellite system (GNSS). It is the only fully functional GNSS in the world, can be used freely, and is often used by civilians for navigation purposes.
In certain conditions, conventional standalone GPS has difficulty providing reliable positions in poor signal conditions. For example when surrounded by tall buildings (resulting in multi path), or when the satellite signals are weakened when a GPS device is indoors or under trees.
In addition, when first turned on in these conditions, some non-assisted GPS units may not be able to download the almanac and ephemeris information from the GPS satellites, rendering them unable to function until a clear signal can be received continuously for up to 40 seconds.
