Assisted GPS
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Assisted GPS
Assisted GPS (abbreviated generally as
A-GPS and less commonly as
aGPS) is a system that often significantly improves startup performance—i.e.,
time-to-first-fix (TTFF), of a
GPS satellite-based positioning system. A-GPS is extensively used with GPS-capable
cellular phones, as its development was accelerated by the U.S.
FCC's
911 requirement to make cell phone location data available to emergency call dispatchers.
Description
Standalone/self-ruling
GPS devices depend solely on information from
satellites. A-GPS augments that by using cell tower data to enhance quality and precision when in poor satellite signal conditions. In exceptionally poor signal conditions, for example in urban areas, satellite signals may exhibit
multipath propagation where signals skip off structures, or are
weakened by meteorological conditions or tree canopy. Some
standalone GPS navigators used in poor conditions can't fix a position because of satellite
signal fracture, and must wait for better satellite reception. A GPS unit may need as long as 12.5 minutes (the time needed to download the
GPS almanac and ephemerides) to resolve the problem and be able to provide a correct location.
An assisted GPS system can address these problems by using external data. Utilizing this system can come at a cost to the user. For billing purposes, network providers often count this as a
data access, which can cost money depending on the plan.
To be precise, A-GPS features depend mostly on an internet network or connection to an
ISP (or CNP, in the case of CP/mobile-phone device linked to a Cellular Network Provider data service). A mobile (cell phone, smart phone) device with just an
L1 front-end radio receiver and no GPS acquisition, tracking, and positioning engine only works when it has an internet connection to an ISP/CNP where the position fix is calculated offboard the device itself. It doesn't work in areas with no coverage or internet link (or nearby BTS towers, in the case on CNP service coverage area). Without one of those resources, it can't connect to the A-GPS servers usually provided by CNPs. On the other hand, a mobile device with a GPS chipset requires no data connection to capture and process GPS data into a position solution, since it receives data directly from the GPS satellites and is able to calculate a position fix itself. However the availability of a data connection can provide assistance to improve the performance of the GPS chip on the mobile device.
Assistance falls into two categories:
- Mobile Station Based (MSB): Information used to acquire satellites more quickly.
- 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.
- Mobile Station Assisted (MSA): Calculation of position by the server using information from the GPS receiver.
- The device captures a snapshot of the GPS signal, with approximate time, for the server to later process into a position.
- The assistance server has a good satellite signal, and plentiful computation power, so it can compare fragmentary signals relayed to it.
- 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.
As an additional benefit, in mobile station assisted implementations, the amount of processing and software required for a GPS receiver can be reduced by offloading most of the work onto the assistance server.
A typical A-GPS-enabled receiver uses 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 A-GPS 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 multilateration and sometimes a
hybrid positioning system.
[4]
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.
Basic concepts
Standalone GPS provides first position in approximately 30–40 seconds. A standalone
GPS needs orbital information of the satellites to calculate the current position. The data rate of the satellite signal is only 50 bit/s, so downloading orbital information like ephemerides and the almanac directly from satellites typically takes a long time, and if the satellite signals are lost during the acquisition of this information, it is discarded and the standalone system has to start from scratch. In A-GPS, the
network operator deploys an A-GPS
server, a
cache server for GPS data. These A-GPS servers download the orbital information from the satellite and store it in the database. An A-GPS capable device can connect to these servers and download this information using mobile network radio bearers such as
GSM,
CDMA,
WCDMA,
LTE or even using other wireless radio bearers such as
Wi-Fi. Usually the data rate of these bearers is high, hence downloading orbital information takes less time.
Modes of operation
A-GPS has two modes of operation:
Mobile Station Assisted (MSA)
In MSA mode A-GPS operation, the A-GPS capable device receives acquisition assistance, reference time and other optional assistance data from a mobile service provider. The mobile service provider continuously logs GPS information (mainly the
almanac) from the GPS satellites using an A-GPS server in its system. With the help of the above data (the data received from the mobile device and the data already present in A-GPS server) the A-GPS server calculates the position and sends it back to the A-GPS device.
Mobile Station Based (MSB)
In MSB mode A-GPS operation, the A-GPS device receives ephemeris, reference location, reference time and other optional assistance data from the A-GPS server. With the help of the above data, the A-GPS device receives signals from the visible satellites and calculates the position
