A-GPS, cell tower, and provider service
A-GPS relies on cell towers to improve the fix of GPS.  In cities GPS by itself isn't good since the buildings get in the way. Without the cell towers it will be harder to get a fix on your location in a city, and sometimes impossible, depending on the height and density of the buildings around you.

A-GPS is received through the mobile network from the provider's A-GPS server.  You will require access to that mobile network in order to communicate with that A-GPS server.


  A typical A-GPS-enabled receiver will use a data connection (Internet or other) to contact the assistance server for aGPS information. If it also has functioning autonomous GPS, it may use standalone GPS, which is sometimes slower on time to first fix, but does not depend on the network, and therefore can work beyond network range, and without incurring data usage fees. Some A-GPS devices do not have the option of falling back to standalone or autonomous GPS.
  
  -- Wikipedia


The key to your problem is in the name Assisted GPS.  The assisted part is used to get a faster fix on the satellites by providing a rough idea of where you are (from the cell tower you're connected to) and the ephemeris data provided by the A-GPS server.  Without that data the GPS can still function if it has a clear view of the sky - it will just take considerably longer to get a fix, and in some cases the fix will fail if it can't see enough satellites due to the buildings around you.