Assisted GPS, A-GPS
- an overview, information or turorial about the basics of Assisted GPS
(Global Positioning System) or A-GPS used to provide location based services
used for cellualr technology and cellualr networks.
In recent years the importance of location services for
mobile phone / cellular telecommunications has risen significantly. In
particular Assisted GPS ( A-GPS ) has been chosen as the front runner of the
possible location services to be used for cell phone applications where the E911
directive has mandated the accurate location of mobile phones by the emergency
services.
A number of techniques are available by cell phone operators
including TDOA (time difference of arrival), EOTD (Enhanced Observed Time
Difference), AFLT (Advanced Forward Link Trilateration), and others. However it
is assisted GPS ( A-GPS ) that has emerged as the front runner for cell phone
applications.
Although the GPS or Global Positioning System is a
particularly attractive option for providing accurate location, it requires some
assistance in order to be able to provide the required service. As a result the
Assisted GPS ( A-GPS ) technology has been developed, and in this way it is able
to provide the required level of service for the mobile phone networks and
operators.
GPS System
The GPS or Global Positioning System is a satellite based
system that enables receivers or terminals on the Earth to gain accurate
location information. The system uses a total of 24 active satellites which have
been placed in six different equally spaced orbital planes with four satellites
in each. This provides the earth user with a view of between five and eight
satellites at any time from any point on the Earth. When four satellites are
visible, sufficient information is available to be able to calculate the
position on Earth.
The satellites transmit signals that are received by the GPS
receivers it and using this they are able to deduce their position. Each GPS
satellite transmits data that includes information about its location and the
current time. All GPS satellites synchronize operations so that these repeating
signals are transmitted at effectively the same instant. The signals, moving at
the speed of light, arrive at a GPS receiver at slightly different times because
some satellites are further away than others. The distance to the GPS satellites
can be determined by calculating the time it takes for the signals from the
satellites to reach the receiver. When the receiver is able to calculate the
distance from at least four GPS satellites, it is possible to determine the
position of the GPS receiver in three dimensions.
The satellite transmits a variety of information. Some of the
chief elements are known as ephemeris and almanac data. The ephemeris data is
information that enables the precise orbit of the satellite to be calculated.
The almanac data gives the approximate position of all the satellites in the
constellation and from this the GPS receiver is able to discover which
satellites are in view.
The problem with GPS for mobile phone applications is that
signal levels are low, and the receiver needs to have a direct view of the
satellite. This can cause problems when the phone is used in a building, or even
in an urban area where a direct view of the satellite is masked. Additionally
the time taken for the receiver to lock - Time To First Fix (TTFF) can be as
much as ten minutes or more from switch on. This is not acceptable when
emergency calls are being made as a much faster acquisition time is required. To
achieve this assistance is required for the GPS receiver, thereby giving rise to
the requirement for Assisted GPS.
Assisted GPS
The system known as Assisted GPS or A-GPS uses the mobile
phone network to assist the GPS receiver in the mobile phone to overcome the
problems associated with TTFF and the low signal levels that are encountered
under some situations.
For A-GPS, the network provides the Ephemeris data to the
cell phone GPS receiver and this improves the TTFF. This can be achieved by
incorporating a GPS receiver into the base station itself, and as this is
sufficiently close in position to the mobile the data received by the base
station is sufficiently accurate to be transmitted on to the mobiles. The base
station receiver is obviously on all the time, and will be located in a position
where it can "see" the satellites.
The information provided can be either the Ephemeris data for
visible satellites or, more helpfully the code phase and Doppler ranges over
which the mobile has to search, i.e. 'acquisition data'. These ranges can be
estimated as the position of the mobile is bounded because it must be within the
cell served by the particular base station. This technique is able to improve
the TTFF by many orders of magnitude.
Assisted GPS or A-GPS is also used to improve the performance
within buildings where the GPS signals are by 20 dB or possibly more. Again by
providing information to the GPS receiver in the mobile it is able to better
correlate the signal being received from the satellite when the signal is low in
strength. Using this technique it is possible to gain considerable increases in
sensitivity and some manufacturers have claimed it is possible to receive
signals down to power levels of around -159dBm. The base station supplies the
receiver with navigation message bits - 'sensitivity data'.
Summary
It is anticipated that the location services now being
implemented, particularly within the USA will enable the emergency call centres
to provide a greatly improved services. With many millions of emergency calls
being made each year from cell phones, this new technique will greatly assist in
saving lives and providing swift support to exactly the correct location when it
is needed.
|
