“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 receiver
- 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 server.
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.
Basic Concepts of AGPS
Standalone GPS provides first position in approximately 30-40 seconds. A Standalone GPS system needs orbital information of the satellites to calculate the current position. The data rate of the satellite signal is only 50 b/s, so downloading orbital information like ephemeris and almanac directly from satellites typically takes a long time. In AGPS, the Network Operator deploys an AGPS server. These AGPS servers download the orbital information from the satellite and store it in the database. An AGPS 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 WIFI. Usually the data rate of these bearers is high, hence downloading orbital information takes less time.