Assisted GPS, generally abbreviated as A-GPS, is a system which can improve the startup performance of a GPS satellite-based positioning system.

GPS operation uses radio signals from satellites. In very poor signal conditions, for example in a city, these signals may suffer multipath where signals bounce off buildings, or be weakened by passing through walls or tree cover. When first turned on in these conditions, some non-assisted GPS navigation device 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 40 seconds (the time needed to download the GPS ephemeris).

An Assisted GPS system can address these problems by using data available from a network.

Assistance falls into two categories:

It supplies orbital data and almanac for the GPS satellites in the cell phone, enabling the cell phone to lock to the satellites more rapidly in some cases.
The network can provide atomic time (Accurate Time Assistance)
 

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