Assisted GPS ( A-GPS )

- for mobile or cell phone location services

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.