The rumor is that aGPS is worse then GPS, or even, is no GPS at all.
So when a spec sheet of a new device comes along and it lists aGPS, people freak and curse that it won't have GPS and that they wish it had and might buy a bluetooth GPS to go with it, etc.
Ok, let's try and set the record straight.
aGPS is always full GPS
aGPS is better then GPS
aGPS is a hardware GPS chip combined with "intelligent" software
aGPS does not require 3G, but is helped by it
There you go!
Some explanation. GPS uses satellites to triangulate it's position. To do this it needs to "see" (direct line of sight is not required) multiple satellites to get a fix. The more satellites it can see, the better. I believe that most GPS chips can see a maximum of 12 to 13 satellites at the same time. Depending on where you are on earth, the maximum satellites you can see ranges from 9 to 15. It is influenced by the curvature of the earth. About 3 or 4 are required to get accurate data. More satellites means better accuracy.
The problem with this system is that the initial fix can take 2 minutes or more. This is because the GPS has no idea where in the world it is, so it needs time to figure this out.
THAT is why aGPS was invented. The "a" in aGPS is software in your device which tries to figure out where you are in the world, quicker then your GPS hardware chip can.
In general it uses 2 methods to do this:
1. It will try to do this using your external internet IP (very rough, but it can pick your country quite easily because of your IP subnet or provider or DNS suffix).
2. It can also leverage your 3G connection to ask the tower it's connected to it's cell tower identification number. Using a database of these towers it will look up the coordinates of that tower and it instantly knows where you approximately are.
It then relays this to your hardware GPS chip so that it can narrow it's search region considerably and give you a quicker fix. Often within 10 to 20 seconds.
That is all it does, and ever will do. Spread the word!