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
A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows satellite navigation devices to determine their location (longitude, latitude, and altitude/elevation) to high pre ...
(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
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotop ...
to calculate its 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 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
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
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
Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communication ...
,
WCDMA,
LTE
LTE may refer to:
Science and technology
* LTE (telecommunication) (Long-Term Evolution), a telephone and mobile broadband standard
** LTE Advanced, an enhancement
*** LTE Advanced Pro
* Compaq LTE, a line of laptop computers produced by Compaq
* ...
or even using other radio bearers such as
Wi-Fi
Wi-Fi () is a family of wireless network protocols, based on the IEEE 802.11 family of standards, which are commonly used for local area networking of devices and Internet access, allowing nearby digital devices to exchange data by radio w ...
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
A tariff is a tax imposed by the government of a country or by a supranational union on imports or exports of goods. Besides being a source of revenue for the government, import duties can also be a form of regulation of foreign trade and p ...
.
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 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
base transceiver station (BTS) towers, in the case on CNP service coverage area). Without any 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.
Modes of operation
Assistance falls into two categories:
; Mobile Station Based (MSB)
: Information used to acquire satellites more quickly.
:* It can supply
orbital data
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...
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
A cell site, cell tower, or cellular base station is a cellular-enabled mobile device site where antennas and electronic communications equipment are placed (typically on a radio mast, tower, or other raised structure) to create a cell, or adj ...
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.
Not every A-GNSS server provides MSA mode operation due to the computational cost and the declining number of mobile terminals incapable of performing their own calculations. Google's SUPL server is one that doesn't.
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.
Related technologies
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
CDMA2000 (also known as C2K or IMT Multi‑Carrier (IMT‑MC)) is a family of 3G mobile technology standards for sending voice, data, and Signaling (telecommunication), signaling data between mobile phones and cell sites. It is developed by 3GP ...
family defined this protocol for CDMA 2000 networks.
:* RRC position protocol – 3GPP defined this protocol as part of the
RRC standard for
UMTS
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile cellular system for networks based on the GSM standard. Developed and maintained by the 3GPP (3rd Generation Partnership Project), UMTS is a component of the In ...
network.
:* LPP – 3GPP defined LPP or LTE positioning protocol for
LTE
LTE may refer to:
Science and technology
* LTE (telecommunication) (Long-Term Evolution), a telephone and mobile broadband standard
** LTE Advanced, an enhancement
*** LTE Advanced Pro
* Compaq LTE, a line of laptop computers produced by Compaq
* ...
networks.
;
User Plane Protocol
:Defined by the OMA to support positioning protocols in
packet switched
In telecommunications, packet switching is a method of grouping Data (computing), data into ''network packet, packets'' that are transmitted over a digital Telecommunications network, network. Packets are made of a header (computing), header and ...
networks. Three generations of
Secure User Plane Location
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 th ...
(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
The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consists ...
's
TCP/IP infrastructure. As a result, its application goes beyond the original intended use of mobile devices and may be used by general-purpose computers.
SUPL 3.0 legitimizes such use by adding allowance for WLAN and broadband connections.
suite of all standards
Actions defined by SUPL 3.0 include a wide range of services such as
geofencing and billing. The A-GNSS functions are defined in the SUPL Positioning Functional Group. It includes:
[
* SUPL Assistance Delivery Function (SADF), which provides the basic information sent to the device in both A-GNSS modes.
* SUPL Reference Retrieval Function (SRRF), which tells the server to prepare the information mentioned above by receiving from the satellites.
* SUPL Position Calculation Function (SPCF), which lets the client or the server ask for the client's location. The server-generated location may result from MSA or from mobile cell. If a MSB (SET based) mode is used, the client reports its location to the server instead.
The specifics of communication is defined in the ULP (Userplane Location Protocol) substandard of SUPL suite. As of December 2018, GNSS systems supported include GPS, Galileo, GLONASS, and BeiDou.][
]
See also
* Mobile phone tracking
Mobile phone tracking is a process for identifying the location of a mobile phone, whether stationary or moving. Localization may be effected by a number of technologies, such as the multilateration of radio signals between (several) cell towers ...
* GNSS enhancement
GNSS enhancement refers to techniques used to improve the accuracy of positioning information provided by the Global Positioning System or other global navigation satellite systems in general, a network of satellites used for navigation.
Enhanceme ...
References
{{DEFAULTSORT:Assisted Gps
Mobile technology
Global Positioning System