A-GPS, or Assisted GPS, is a system that cuts down the time needed for initial localisation when using a GPS terminal. It shows its usefulness especially in urban 'canyons', such as narrow streets or heavily tree-lined avenues, where it is difficult to acquire the signal from satellites and decode the navigation message containing ephemeris and almanac. This system is showing considerable popularity, and is normally associated with location-based service (LBS) systems based on mobile telephony.


One of the main problems of classic GPS terminals is related to the first localisation (fixing), since a terminal, when switched on, has to obtain the list of satellites in view at that moment, in order to lock on to them and try to detect its position. This process is generally quite expensive in terms of time and resources, and the A-GPS system was designed to cut these costs, also in view of its use on terminals with low processing capacity or limited energy resources, such as mobile phones.
The main purpose of this system is to 'assist' the GPS receiver in calculating its position, providing it with information useful in acquiring the signal and calculating the fix (approximate time and position of the receiver itself, almanac and ephemeris of the satellites in view).


The basic idea is as follows: since every mobile phone cell in the territory has a fixed position, the cell is made to find out which GPS satellites are in view of it, moment by moment. When an A-GPS terminal wants to know its position, it connects via the cellular network to an Assistance Server (which can also be managed by the operator itself), to which the information on the cell to which the user is hooked is also sent.
Since the satellites in view of the cell are known, it can be reasonably assumed that the A-GPS terminal also sees the same satellites. The server therefore compiles a list with the satellites in view, and sends it via the cellular network to the terminal, which thus already knows which satellites it will be able to connect to, and thus derives its position from them.