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I am not sure if I used the right forum to ask this question. If it is wrong, please point me the right one.

I understand in general how A-GPS works with cell towers, and how it depends on the data signal. But there are some thing I don't understand fully yet.

I have one phone primarily used in cities, and it is equipped with A-GPS only. My service provider has certain coverage. But even I drive outside my service provider's coverage I am still able to navigate with offline map.

I have another phone equipped with A-GPS only, I used it to travel in other countries thus I don't even bother to insert a SIM card or use roaming service. Basically it is a just a mini tablet. But in cities I was still able to find myself on offline maps, see myself moving on the maps and so on. In one case I was deep is the Amazon rain forest I had no way to locate myself.

So my questions is if A-GPS depends on cell towers to obtain the locations, does it mean as long as there are towers around, it does not matter I can have service with them or not, then I can use A-GPS?

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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.

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  • \$\begingroup\$ I think my question is does A-GPS work with cell towers belonging to providers I don't have service with? \$\endgroup\$ – hardywang Dec 10 '14 at 14:32
  • \$\begingroup\$ In general I believe you will require access to the provider's network in order to request A-GPS data from their servers. That means you will have to be logged in to a cell tower using a suitable SIM. \$\endgroup\$ – Majenko Dec 10 '14 at 14:35
  • \$\begingroup\$ Majenko, this was my understanding, but how does it explain my situation described in my original post? \$\endgroup\$ – hardywang Dec 10 '14 at 14:46
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    \$\begingroup\$ Because the A part of the A-GPS is only used to improve the normal GPS, not replace it completely. You still have GPS, it's just slow and hard to get a fix in cities. \$\endgroup\$ – Majenko Dec 10 '14 at 14:54
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    \$\begingroup\$ In general, yes. It is possible to have a GPS that ONLY uses the A-GPS information, but these are few and far between (ultra-cheap, and no one really wants them anyway these days), and 99% of them use the A-GPS data to enhance real GPS. \$\endgroup\$ – Majenko Dec 10 '14 at 15:49

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