[XPERIA X1 Themes and Apps] pielealbers:
Assisted GPS, generally abbreviated as A-GPS, is a system which can improve the startup performance 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.[1]

[edit]

Description

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 confusingly off buildings, or be weakened by passing through walls or tree cover. When first turned on in these conditions, some non-assisted 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 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:

Using information known to the assistance server but not the GPS receiver

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.)

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 by GPS receivers, with the satellite signal it receives directly, and then inform the GPS receiver or emergency services of the GPS receiver's position.

As an additional benefit, in some A-GPS device implementations, the amount of CPU and programming required for a GPS receiver is reduced by offloading most of the work onto the assistance server. For modern hardware this is not a major consideration but many 20th century GPS navigation devices used Intel 80386-class 16 MHz CPUs[citation needed] or similar hardware.

A typical A-GPS-enabled receiver will use a data connection (Internet or other) to contact the assistance server for A-GPS information. Some A-GPS devices cannot fall back to standard GPS, needing cell tower or internet signal as these A-GPS devices won't function with only GPS satellite signal.

Many mobile phones combine A-GPS and other location services including Wi-Fi Positioning System and cell-site triangulation in a hybrid positioning system.[2]

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.