GPS uses a network of 24 satellites to triangulate a receiver’s position and provide latitude and longitude coordinates. Although GPS provides excellent position accuracy, position fixes require lines of sight to the satellites. In urban areas, when the user is located in "urban canyons", under heavy tree cover, or even indoors, Assisted GPS, or A-GPS, is a technology that can be used to triangulate his position. It is becoming more common and it's commonly associated to Location Based Services (LBS) over cellular networks .
The development of these services is fuelled, in part, by the U.S. Federal Communications Commission's E-911 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 is 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 dependant on cellular coverage.
External Links
Assisted GPS: A Low-Infrastructure Approach