Assisted GNSS
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Assisted GPS, or A-GPS, is a technology that uses an assistance server to cut down the time needed to determine a location using GPS. It is useful in urban areas, when the user is located in "urban canyons", under heavy tree cover, or even indoors. It is becoming more common and it's commonly associated with Location Based Services (LBS) over cellular networks.
The development of these services is fuelled, in part, by the U.S. Federal Communications Commission's E911 mandate requiring the position of a cell phone to be available to emergency call dispatchers.
A-GPS differs from regular GPS by adding another element to the equation, the Assistance Server. In regular GPS networks there are only GPS satellites and GPS receivers. In A-GPS networks, the receiver, being limited in processing power and normally under less than ideal locations for position fixing, communicates with the assistance server that has high processing power and access to a reference network. Since the A-GPS receiver and the Assistance Server share tasks, the process is quicker and more efficient than regular GPS, albeit dependent on cellular coverage.
Assisted GPS describes a system where outside sources, such as an assistance server (Mobile Location Server) via a network, help a GPS receiver perform the tasks required to make range measurements and calculate position solutions. The assistance server has the ability to access information from the reference network and also has computing power far beyond that of the GPS receiver. In such a system, the assistance server communicates with the GPS receiver on the mobile phone on a cellular network. With assistance from the network, the receiver can operate more quickly and efficiently than it would unassisted, because a set of tasks that it would normally handle is shared with the assistance server. The resulting AGPS system boosts performance beyond that of the same receiver in a stand-alone mode.
Ordinarily, a standard GPS device needs to have a clear line-of-sight to at least four GPS satellites before it can calculate its position. In addition, it needs enough processing power to transform the data streams from the satellites into a position. In one mode of A-GPS, the mobile receiver takes a snapshot of the satellite signals and transmits these to a cell tower to relay the data to an assistance server that performs the necessary calculations for a position fix. The server may send the fix back to the mobile receiver or to a 911 dispatcher. Some mobile phones will accept converted data streams to compute a position themselves.
One of the main purposes of A-GPS is to provide municipalities with location-based emergency phone service, such as E911 service. Another is to provide mobile carriers with end-user, location-based services such as a turn-by-turn navigation aid.