Do I need internet/3G to use GPS? 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?
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.