Assisted GPS is a system that is often able to significantly improve the startup performance,
or time-to-first-fix, 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 units depend solely
on radio indicators from satellites.
A-GPS develops that by likewise using cell tower data to enhance unwavering quality and
precision when in poor satellite signal conditions.
In exceptionally poor signal conditions, for example, in urban territories, satellite signs
may endure multipath propagation where signals skip off structures, or are weakened by barometrical
conditions, dividers, or tree spread.
Some standalone GPS route gadgets, when utilized within poor conditions, will be unable to
fix a position because of satellite sign fracture and must hold up for a finer satellite indicator.
A GPS unit may require as long as 12.5 minutes to resolve the problem and be able to provide
a correct location.
An assisted GPS system can address these problems by using data available from a network.
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 more precise, "A-GPS" features are mostly dependent on an internet network and/or connection
to an ISP.
A mobile device featured with "A-GPS" can work only when there is an internet link/connection
to an ISP/CNP - it is useless in areas with no coverage of internet link to connect to
those A-GPS servers.
On the contrary, a "S-GPS" device/feature requires no connection to internet/network
to obtain GPS data since it connects directly to GPS satellites swarming overhead in Low
Earth Orbit.
Assistance falls into two categories: Mobile Station Based: 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: 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 will use a data connection 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.
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.
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 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 from the GPS satellites using
a A-GPS server in its system.
With the help of the above data the A-GPS server calculates the position and sends it
back to the A-GPS device.
Mobile Station Based 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.
Standards A-GPS protocols are part of Positioning Protocol
defined by two different standardization bodies, 3GPP and Open Mobile Alliance.
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
See also List of devices with Assisted GPS
Mobile phone tracking References