GPS is a satellite based positioning system. Assisted GPS, or A-GPS was introduced to enhance performance. The development of A-GPS was accelerated by the U.S. FCC's E911 mandate requiring the position of a cell phone to be available to emergency call dispatchers. blank">http://www.gpsworld.com/gpsworld/article/articleDetail.jsp?id=12287

Conventional GPS then had difficulty providing reliable positions in poor signal conditions. For example when surrounded by tall buildings (as a result of multipath), or when the satellite signals are weakened by being indoors or under trees. Some newer receivers are far better.

In addition, when first turned on in these conditions, some non-A-GPS units may not be able to download the almanac and ephemeris information from the GPS satellites, rendering them unable to function until a clear signal can be received continuously for up to one minute.

An A-GPS receiver can address these problems in several ways, using an Assistance Server:

• The Assistance Server can locate the phone roughly by what cell site it is connected to on the cellular network.
• The Assistance Server has a good satellite signal, and lots of computation power, so it can compare fragmentary signals relayed to it by cell phones, with the satellite signal it receives directly, and then inform the cell phone or emergency services of the cell phone's position.
• It can supply orbital data for the GPS satellites to the cell phone, enabling the cell phone to lock to the satellites when it otherwise could not, and autonomously calculate its position.
• It can have better knowledge of ionospheric conditions and other errors affecting the GPS signal than the cell phone alone, enabling more precise calculation of position. (See also _Wide Area Augmentation System)

Some A-GPS solutions require an active connection to a cell phone (or other data) network to function, in others Frameset.htm" target="_blank">http://www.globallocate.com/A-GPS/A-GPS_Frameset.htm http://www.globallocate.com/SEMICONDUCTORS/SEMI_MAIN_Frameset.htm it simply makes positioning faster and more accurate, but is not required.

As an additional benefit, it can reduce both the amount of _CPU and programming required for a GPS Phone by offloading most of the work onto the assistance server. (This is not a large amount for a basic GPS - many early GPSs ran on 386/16 or similar hardware).

High Sensitivity GPS is an allied technology, that addresses some of the same issues in a way that does not require additional infrastructure. It notably cannot provide instant fixes when the phone has been off for some time, that some forms of A-GPS can.