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).[2]
An Assisted GPS system can address these problems by using data available from a network.
Assistance falls into two categories:
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 and openBmap.)
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
