I have a Samsung Galaxy S2 that has some problems with the GPS:
If I use internet my navigation works but when I turn the 3G off, GPS does not work.
Do I need internet/3G to use GPS?
You are confusing two different concepts: "GPS" and "Navigation", that are used for two distinct scenarios on your device.
GPS is used to pin-point your location in the globe. Navigation is used to plan and track your movements from point A to point B.
While Navigation may depend on 3G or Wi-Fi to access the internet and retrieve maps and other information as to graphically present your location in relation to the designated route, GPS doesn't use the Internet, it works communicating directly with antennas and in turn with satellites to ascertain you global position.
There are two phone settings regarding GPS, one is the GPS on/off toggle, the other one is the A-GPS (Assisted GPS), that actually relies on Internet Connectivity as to provide faster and some times more accurate results:
If you need navigation capabilities while offline, you can use an app that relies on maps downloaded to your phone. Personally I'm happy with Navfree: Free GPS Navigation by Navmii available from Google Play Store. I've downloaded some maps, for Portugal, around 90Mb, but afterwards I'm fully capable of navigating without any internet connection.
Maps are provided by OpenStreetMap.org free of charge and updated by a large community.
The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. DoD. There are no subscription fees or setup charges to use GPS.
Read about it at:
Navigation is the process of accurately ascertaining one's position, planning and following a route.
Read about it at:
GPS and navigation are separate concepts.
GPS itself does not require Internet connectivity. However, many navigation apps (e.g. Google Maps or Waze) require an active connection in order to access map data on-the-fly, compute directions, look up traffic details, search for points of interest, etc.
There are other (usually paid) apps that don't require Internet connectivity for navigation because they provide this data as a separate download. Directions, turn-by-turn navigation, POIs, etc. can then be accessed from data that is stored on SD Card, while the device is offline.
In both cases GPS is only used to place your current position on the map.
In short: GPS works without an active connection, it's completely passive.
GPS has however an addon feature called A-GPS (assisted GPS) which speeds up the startup (time to first fix) considerably. It basically warms up the receiver with GPS status data such as time, coarse location and most of all GPS satellite orbit location data (ephemeris data download). Everything of this is also included in the GPS signal but a cold start needs longer if the receiver needs to fetch it itself. With A-GPS this data is downloaded from the internet and fed to the receiver.
Note: The A-GPS data is good for some days but deteriorates over time until the phone needs longer to (once!) cold start again (or a internet re-download is necessary). You can trigger a manual redownload with some apps, for example GPS status.