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Assisted GPS, generally abbreviated as A-GPS or aGPS, is a system which
can, under certain conditions, improve the startup performance, or time-
to-first-fix (TTFF) 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.
"Standalone" or "Autonomous" GPS operation uses radio signals from
satellites alone. A-GPS additionally uses network resources to locate and
utilize the satellites faster as well as better in poor signal
conditions. In very poor signal conditions, for example in a city, these
signals may suffer multipath propagation where signals bounce off
buildings, or be weakened by passing through atmospheric conditions,
walls or tree cover. When first turned on in these conditions, some
standalone GPS navigation devices 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 12.5 minutes (the
time needed to download the GPS almanac and ephemeris).
An Assisted GPS system can address these problems by using data available
from a network. For billing purposes, network providers often count this
as a data access, which can cost money depending on the plan.
Assistance falls into two categories:
Information used to more quickly acquire satellites
It can supply orbital data or almanac for the GPS satellites to the GPS
receiver, enabling the GPS receiver to lock to the satellites more
rapidly in some cases.
The network can provide precise time.
The device captures 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 conditions affecting
the GPS signal than the GPS receiver alone, enabling more precise
calculation of position. (See also Wide Area Augmentation System)
Calculation of position by the server using information from the GPS
The assistance server has a good satellite signal, and plentiful
computation power, so it can compare fragmentary signals relayed to it
As an additional benefit, in some A-GPS device implementations, known as
"MS-Assisted," the amount of CPU and programming required for a GPS
receiver is reduced by offloading most of the work onto the assistance
A typical A-GPS-enabled receiver will use a data connection (Internet or
other) to contact the assistance server for aGPS information. If it also
has functioning autonomous GPS, it may use standalone GPS, which is
sometimes slower on time to first fix, but does not depend on the
network, and therefore can work beyond network range, and without
incurring data usage fees. Some aGPS devices do not have the option of
falling back to standalone or autonomous GPS.
Many mobile phones combine A-GPS and other location services including
Wi-Fi Positioning System and cell-site triangulation and sometimes a
hybrid positioning system.
High Sensitivity GPS is an allied technology that addresses some of these
issues in a way that does not require additional infrastructure. However,
unlike some forms of A-GPS, high-sensitivity GPS cannot provide a fix
instantaneously when the GPS receiver has been off for some time.