If you have a GPS phone, does the GPS work even if you don't get any reception? And if so, why? Does it operate off a different signal?
There are two types of GPS with mobile phones, AGPS and GPS. AGPS is assisted GPS, and required a connection to work. I had a Motorola E1000 that had that, it was rubbish. Most new phones these days, from iPhone 3G and new Nokias upwards have proper GPS, so you can use them anywhere, in a country or out on a boat. The GPS in the iPhone is pretty good as well, as long as you have a good line of site.
Hi, if you have a way of downloading the sattelite data to your phone, this helps the GPS know where to look. However, if you don't have that, it doesn't mean that you wont find a satellite, only that it might take longer. All GPS radios are not equal and generally the cellular phone ones are less sensitive than the dedicated gpses. Downloading data exists merely to bridge the gap and compensate for the lower quality built-in phone GPS.
If you have a GPS chip in your phone it will work in places that you don't have signal as long as you are outside.
However, If you don't have cell reception, it might take longer to find satellite reception because cell phone network assists to get you connected, at least that is the case with my Nokia E71. Btw, I'm not talking about having a data plan, of course that helps a lot to get a signal quick.
You can use a gps phone with no reception...but obviously only in map mode.
You need to connect to sattelite in order to view yourself moving around the maps... but with no connection you can only browse a map... not navigate through.
The advantage of a smart phone with a GPS chip is that when they can see the Satellites they use them but when they can't they use other methods to approximate your location.
If you are connected to a WIFI network for instance they can approximate your location using the IP address of your WIFI network. If you are just on a cellular connection (be it 3G or otherwise) then they can triangulate your position using the phone masts which is still pretty accurate (at least to within about 100 metres - or less depending on your connection and how many mobile phone masts your phone can see at the time).
The only problem with the phones that have a dedicated GPS chip is that it tends to drain the battery quite fast (although less of an issue if you are in a car as you could just get an in car charger and plug it in to the 12V power supply)
Both my Garmin GPS and Motorola Droid X2 with Google Maps are accurate and at the same time each has its own limitations. I find Garmin to be more current with traffic but Google maps is more detailed with what is around the destination.
Also, the GPS has trouble getting a signal when leaving an underground garage (which sometimes sucks when you need to know to go left or right out of garage) but the Droid X2 (Android 2.3.5) has a much stronger signal underground. Same issue at times in the Holland/Lincoln/Brooklyn tunnels where the phone lose signal but the GPS receives full signal.
On the iPhone the GPS will work without phone reception but only partially. The GPS chip is separate for the phone chip and therefore does not rely on the phone or wifi to determine your position. However the iPhone does rely on the phone (more specifically a data connection - either WIFI or 3G - in order to download the map and route data).
So if you have no phone reception and want to start a new route the iPhone will not be able to work it out for you. However if you have started a route when you did have a data connection it will be able to continue even when you lose data connection as the phone has already downloaded the route and stored it locally (temporarily at least) and it then only needs to know your location to provide you with directions. It gets this location from the GPS chip.
This applies to the maps app that is supplied with iOS6.
If however you have a proper GPS app on your iPhone then it will always work without phone signal. This works because part of the app is all the map and route information which is then stored on your iPhone (for this reason these apps are large and can only be installed to your iPhone either over WIFI or via iTunes on a computer). This means that these apps only need a GPS signal provided by the GPS chip in order to function and there is no need for a phone signal.