Assisted GPS, generally abbreviated as A-GPS or aGPS, 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.
"Standalone" or "Autonomous" GPS operation use radio signals from satellites alone. A-GPS additionally uses network resources to locate and utilize the satellites faster as well as better in poor signal conditions. In very poor signal conditions, for example in a city, these signals may suffer multipath propagation where signals bounce off buildings, or be weakened by passing through atmospheric conditions, walls or tree cover. When first turned on in these conditions, some standalone GPS navigation devices may not be able to work out a position due to the fragmentary signal, rendering them unable to function until a clear signal can be received continuously for up to 12.5 minutes (the time needed to download the GPS almanac and ephemeris).
An Assisted GPS system can address these problems by using data available from a network.
Assistance falls into two categories:
As an additional benefit, in some A-GPS device implementations, known as "MS-Assisted," the amount of CPU and programming required for a GPS receiver is reduced by offloading most of the work onto the assistance server.
A typical A-GPS-enabled receiver will use a data connection (Internet or other) to contact the assistance server for aGPS information. If it also has functioning autonomous GPS, it may use standalone GPS, which is sometimes slower on time to first fix, but does not depend on the network, and therefore can work beyond network range, and without incurring data usage fees. Some aGPS devices do not have the option of falling back to standalone or autonomous GPS.
High Sensitivity GPS is an allied technology that addresses some of these issues in a way that does not require additional infrastructure. However, unlike some forms of A-GPS, high-sensitivity GPS cannot provide a fix instantaneously when the GPS receiver has been off for some time.