Assisted Global Positioning System

By Zahid Ghadialy (zahidtg@yahoo.com)

Last Updated: 20/03/2007

What is GPS?

The worldwide positioning system "GPS" (Global Positioning System) was installed by the US military in the year 1978. It consists of 24 satellites orbiting around the earth. It is possible today to know the exact place one is around the world at any point of time or place, dependent upon the development of equipment to report it. The purposeful masking of the satellite signals for civilian use was switched off on May 2, 2000. Since this time it has been possible to calculate locations with an accuracy of 10-30 meters. The latest devices can give locations upto an accuracy of 3 metres.

What is AGPS?

AGPS or Assisted Global Positioning System is a technique enabling both mobiles and cellular networks to establish accurate positioning information. The system utilizes both the GPS (Global Positioning System) and the terrestrial cellular network and is one of the LCS (Location Services) methodologies.

OK. How does it actually work?

In case of GPS, the GPS receiver has to receive signals from at least 4 satellites to determine it's location. The GPS receiver then determines its own location. This is very good from privacy point of view; for some other entity to know the location of the GPS, your GPS needs to share the information with that entity.
So who knows that location: GPS receiver on your cellphone/PDA.
Where is the information stored: on the cellphone/PDA

In a self-contained GPS receiver, the receiver contains such pieces a radio, signal processor, geospatial almanac, conversion functions, and optional positional reference data (e.g. maps), all of which together provide the viewing user with understandable information about their position e.g. latitude and longitude.

In AGPS however, the handheld device only (necessarily) contains a radio and signal processor. The results of the GPS signal receipt and processing are then passed over the wireless network, to be interpreted by the assistance server located elsewhere. The assistance server then performs the necessary math and conversions to determine the map location of the handheld.

So In the case of the A-GPS whoever controls the assistance server knows your current location.

So, in practice, GPS is inevitably A-GPS?

Not necessarily. A-GPS is designed for phones with limited power. The idea is that you embed a smaller chip that can collect ephemeral data and then transmit to an assistance server to do the math required. (The math is what drives up battery consumption).

So there are essentially two types - 1) Full blown GPS, does all the computations and 2) The assisted GPS that collects the data and then forwards it to an assistance server for processing.

In case of A-GPS, who knows the locationof A-GPS device?

In the case of the pure GPS, only the GPS device (PDA) knows the current location. There's no "back channel" unless an application is telling a server where you are.

In the case of the A-GPS whoever controls the assistance server knows your current location. Now in the case of Nextel (operator in USA) there are applications that do just that. On Verizon (another USA network operator) applications have to ask your permission to provide location info upstream.

Whats the advantage of AGPS over GPS?

AGPS minimizes the amount of memory and hardware that must be integrated into mobile devices in order to provide GPS-quality device locating ability as required by [[E911]]. This keeps the Mobile device simple and allows longer battery time.

Whats the disadvantage of AGPS over GPS?

GPS is realtime whereas AGPS is not. Network interaction is required everytime you move out of the service area. Useful only for locating a particular place in small area. Also in AGPS privacy is not as good as GPS since the Assistance server knows the location of the device. Another disadvantage is that there needs to be communication over the wireless for processing of GPS data so this could be expensive.

Whats the granularity of the results obtaines over AGPS?

Around 15-30 meteres depending on the cell size.

Do all mobile phones have AGPS?

No, not all phones have AGPS. Most of the phones that support AGPS are by motorola. Please check the specification of your 3G phone to see if it supports AGPS.

Can the entity which provides the Assistance server be non Operator?

Absolutely - it could be an MVNO or other service.

Where can i find more information about AGPS?

Please see the following links








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