Assisted GPS, generally abbreviated as A-GPS, enhances 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]

Description

In certain conditions, Conventional Standalone GPS has difficulty providing reliable positions in poor signal conditions. For example, when surrounded by tall buildings (resulting in multipath), or when the satellite signals are weakened when a GPS device is used indoors or under trees. Some newer receivers are better at handling these situations.
In addition, when first turned on in these conditions, some non-assisted 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 40 seconds.
An A-GPS receiver can address these problems in several ways, using network elements such as either an assistance server or other data from a network. That assistance generally falls into two categories: a) information used to more quickly acquire satellites, or b) calculations done remotely:
The assistance server can locate the phone roughly by which 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 and/or almanac for the GPS satellites to the cell phone, enabling the cell phone to lock to the satellites faster in some cases.
The network can provide atomic time (Accurate Time Assistance)
Simply capturing a brief snapshot of the GPS signal, with approximate time, for the server to later process into a position.[2]
By having accurate, surveyed coordinates for the cell site towers, it has 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)
As an additional benefit, in certain types of A-GPS, both the amount of CPU and programming required for a GPS phone is reduced by offloading most of the work onto the assistance server. (This is not a large amount for a basic GPS – many early GPSs utilized Intel 80386-class 16 MHz CPUs or similar hardware.) Also most modern GPS chipsets embed ARM or similar processors which have enough computational power and offload the main phone CPU from GPS computations and free cell phone firmware developers from related programming, hence such advantage is questionable. Also the data sent to assistance servers opens the potential for high precision tracking of a cell phones location, raising privacy concerns.
A typical A-GPS-enabled cell phone will use a data connection (internet, or other) to contact the assistance server or a standard network connection for A-GPS information. If it also has functioning autonomous or standalone GPS, it may use standard GPS, which is sometimes slower on Time To First Fix, but does not lead to network dependent downsides, such as failure to work outside of network range, or charges for data traffic.[3] Some A-GPS solutions do not have the option of falling back to standalone or autonomous GPS.
High Sensitivity GPS is an allied technology that addresses some of the same issues in a way that does not require additional infrastructure. However, unlike some forms of A-GPS, high sensitivity GPS cannot provide instant fixes when the phone has been off for some time.