Thought i'd write this simple post since i had the sudden concern over the network data charges when using garmin xt on my E71.
In case you're new to this, GPS is basically a global positioning system which allows the GPS device to track and lock your location (well the device's location) by triangulating your location from satellites in space (You can check it out further here); GPS is readily available on my E71 and i honestly believe it is such a waste to have a smartphone which has GPS but not use it. I salute whoever the smart fella who invented this GPS device!
A-GPS which stands for Assisted-GPS, is where the GPS device connects to a satellite tower via the phone's internet network, to assist the GPS to calculate the location and speed up the signal locking of the GPS device. This is the part where users will be charged by their network provider for the data usage. The GPS connects to the satellite tower via GPRS or other internet means such as EDGE, 3G or other mobile internet (Wifi should not be considered as you're on the move).
Question is, how does A-GPS works?
To sum it all up, upon starting the GPS (Garmin software), it will first try to lock the satellite signal, at same time it sends small packets of data via the GPRS or mobile internet to assist the GPS device in doing its job. So the satellite tower shall assist to provide info of the nearest location of the GPS and assist as a triangulating point. Basically its only a 1 time connection and just to lock the signal, thereafter navigation should rely only on the internal GPS, therefore no other data charges will be affected. However, be cautioned - if the GPS device looses its signal lock due to bad weather or was restarted, and requires the GPS to search for the signal lock again, there may be instances of the A-GPS reconnecting, but i am not sure about this and have yet to confirm it.
How about the data usage?
For every signal lock with A-GPS, generally less than 10KB of data is used.
Let's try this, clear counter of your packet data log from your phone's MENU>LOG>PACKET DATA, turn on your garmin software, hide it in background and go back to MENU>LOG>LIST. You may notice the actual data which was received and whether via GPRS,3G,EDGE or others. I myself tried this and received about 3KB - 5KB on my 3G for every lock. I believe the satellite signal strength and length of time to lock to the satellites determines the amount of data transferred.
Anyway base on my data plan, i'm charged about RM0.10 for every signal lock, hardly a bother to me but surely a free piece of mind as i can almost immediately get a GPS lock rather than waiting few minutes for it (if A-GPS is turned off).
U may want to consider turning off A-GPS if you go overseas, as by then data roaming charges shall apply. Get to know the charges first, some countries may have minimum usage such as 100KB and such, which will surely cost alot due to currency exchange. GPS will still work fine without A-GPS, only may take slightly longer time for the first signal lock.
If you have a GPS device which disconnects a lot and reactivates A-GPS many times, you may also want to consider turning it off. Do a trial run, clear counters, run garmin and navigate it for a while, once done check the log to see how much data was transferred. If you can live with it, i'd definitely recommend having it turned on!
Just in case you need to turn it off, go to MENU>TOOLS>SETTINGS>GENERAL>POSITIONING>POSITIONING METHOD>Untick 'Assisted GPS'