Motorola Mobile Phone User Manual page 51

Within the signal from each satellite there is a very accurate "time
stamp" and information specifying exactly where in space each
satellite is currently positioned.
Using this time stamp the AGPS unit can then calculate how long
each received signal took to reach the handset and consequently
exactly how far away each satellite is from the handset.
From these distances and the position of each satellite in space the
AGPS unit can calculate the position of the handset.
To save your battery power the AGPS hardware is only turned on
when a AGPS location fix is required by either a local application on
your handset or by an application within the 3 network.
When the AGPS hardware is turned on it needs to search for the
satellite signals which are available and evaluate the information
contained in the signal before calculating the position. This process
can take up to approximately 60 seconds. If the handset is currently
connected to the 3 network then the AGPS will receive some
assistance data which will reduce this time to less than
approximately 20 seconds.
If the handset is being charged via the mains adapter or via the in-
car adapter when location fixes are requested then the first will take
the times detailed above but all subsequent ones will take less than
approximately 5 seconds.
The accuracy of and the time taken to perform each AGPS position
calculation is very dependent on both:
1 The number of different satellite signals which can be received
2 How strong each received satellite signal is.
The signal received on earth from AGPS satellites is very weak and
are further weakened or even blocked completely by buildings or
other objects.
So best AGPS performance in terms of accuracy and time required
to do the calculation is achieved when the handset is outdoors with
very little of the sky obstructed.
Learning to Use Your Handset - 51