A-GPS or Assisted GPS, generally abbreviated as A-GPS or aGPS, is a system that 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 requirement to make 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 use the satellites 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 are 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 fix a position due to the fragmentary signal, rendering them unable to function until a clearer signal can be received continuously for a long enough period of time. A fix may take as long as 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 acquire satellites more quickly [Mobile Station Based(MSB)]
- 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.
- Calculation of position by the server using information from the GPS receiver [Mobile Station Assisted(MSA)]
- 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 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 A-GPS devices do not have the option of falling back to standalone or autonomous GPS.
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.
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 bit/s, so downloading orbital information like ephemeris and 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 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 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 the A-GPS server. With the help of the above data, the A-GPS device receives signals from the visible satellites and sends the measurements to the 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.
- Control Plane Protocol
- It is defined by 3gpp for various generations of mobile phone system. These protocols are defined for Circuit Switched Networks. Following positioning protocol has been defined.
- RRLP – 3gpp defined RRLP or Radio resource location protocol to support positioning protocol on GSM networks.
- TIA 801 – CDMA2000 family defined this protocol for CDMA 2000 networks.
- RRC position protocol – 3gpp defined this protocol as part of the RRC standard for UMTS network.
- LPP – 3gpp defined LPP or LTE positioning protocol for LTE Networks.
- User Plane Protocol
- It is defined by OMA to support positioning protocols in Packet Switched Networks. Two generations of User plane Protocol have evolved.
- SUPL V1.0
- SUPL V2.0