thumb|320px|Assisted GPS

Assisted GNSS (A-GNSS) is a GNSS augmentation system that often significantly improves the startup performance—i.e., time-to-first-fix (TTFF)—of a global navigation satellite system (GNSS). A-GNSS works by providing the necessary data to the device via a radio network instead of the slow satellite link, essentially "warming up" the receiver for a fix. When applied to GPS, it is known as assisted GPS or augmented GPS (abbreviated generally as A-GPS and less commonly as aGPS). Other local names include A-GANSS for Galileo and A-Beidou for BeiDou.

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.

Background

Every GPS device requires orbital data about the satellites to calculate its position. The data rate of the satellite signal is only 50 bit/s, so downloading orbital information like ephemerides and/or 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 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 canopies. 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 regular 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.

Operation

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 radio bearers such as Wi-Fi or LoRa. Usually the data rate of these bearers is high, hence downloading orbital information takes less time. 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 tariff. 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.

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.

Standards

A-GPS protocols are part of Positioning Protocol defined by two different standardization bodies, 3GPP and Open Mobile Alliance (OMA).

;Control Plane Protocol

: Defined by the 3GPP for various generations of mobile phone systems. These protocols are defined for circuit switched networks. The following positioning protocols have been defined.

:* RRLP – 3GPP defined RRLP (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

:Defined by the OMA to support positioning protocols in packet switched networks. Three generations of Secure User Plane Location (SUPL) protocol have been defined, from version 1.0 to 3.0.

SUPL

The SUPL (Secure User Plane Location) protocol, unlike its control-plane equivalents restricted to mobile networks, runs on the Internet's TCP/IP infrastructure. Consequently, its application extends beyond the original intended use of mobile devices and may be used by general-purpose computers. SUPL 3.0 legitimizes such use by adding admission for WLAN and broadband connections.

Actions defined by SUPL 3.0 include a wide range of services like geofencing and billing. The A-GNSS functions are defined in the SUPL Positioning Functional Group. It includes: Hardware manufacturers such as Qualcomm (gpsOneXTRA), Samsung, and Broadcom each run their own servers and use their own data formats. GrapheneOS runs a caching proxy for these services to help mask user identity.

See also

  • Mobile phone tracking
  • GNSS enhancement

References