Motorola 031431a ユーザーズマニュアル

Learning to Use Your Handset - 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.