Have a look at this:

http://marcopeluso.com/2009/08/23/how-to-prevent-iphone-from-deep-sleeping/

It s a drop in piece of code used to disable deep sleep. Your screen can still shut off while your application continues to run in the background.

You have to leave the screen unlocked. But that does not mean you have to leave GPS enabled. You can shut down the GPS monitor and awaken it every few minutes, that and an all-black "screensaver" that comes on after a minute should cut down on the drain quite a lot.

If your application plays (or records) sound, even if you play silence, it would continue running even when the screen is locked. So you might want to try to play silence via audio queue services to keep your app running.

Note that I don t know any GPS-related specifics with regards to this question. Another commenter says that GPS is specifically prohibited from running when the screen is locked (which might be a sensible decision on Apple s side, given the possible privacy issues). I can t advise whether that s true or not.

I am not 100% sure, but I think I heard somewhere that the GPS module only works when the screen is not locked. (Unless you have a jailbroken phone of course...)

I think I read this when I was downloading a running application that kept track of my location while I was out running. The app had its own "lock screen" that I had to have up for it to work, which drained the battery quite fast. If I were to lock the screen normally then unlock it again, it would have to reacquire my location.

I don t think its possible (its something I ve wanted to be able to do in the past). I know for instance that the safari browser, which can access GPS, doesn t do so when the screen is locked, although that could be by design rather than limitation.