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Assisted GPS, generally abbreviated as A-GPS, is a carrier network dependent system which can, under certain conditions, 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.
Conventional or "Standalone 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 device 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.) Some newer receivers are better at handling these situations.
An A-GPS system can address these problems in several ways, using an assistance server or other data from a network.
Assistance falls into two categories:
Using information known to the assistance server but not the phone.
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)
The device capturing 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 errors affecting the GPS signal than the cell phone alone, enabling more precise calculation of position. (See also Wide Area Augmentation System)
Calculation of position by the server using information from the phone.
The assistance server has a good satellite signal, and plentiful 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.
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.)
A typical A-GPS-enabled cell phone will use a data connection (internet, or other) to contact the assistance server 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. 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.
Some A-GPS devices lack standalone GPS capability. They may not function at all unless the device has an active subscription to a network, and is in range of that network. Others offer additional Local Positioning Systems and use whatever ones are available at the moment. Conventional GPS receivers are only limited in the opposite sense, offering service whenever good GPS signals are available.
Some A-GPS solutions inherently give the users position very accurately to the assistance server. However, for mobile phones, the privacy concern this represents is somewhat diluted by the fact that the network already knows the position of the phone to within several hundred meters or better.
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