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You do not need to be in the air.
Are you operating off of a saved mission that has a purple dot with it? This is the location the simulator sets the aircraft's location to. Otherwise you need to make sure that you have enough satellites to get a GPS fix.
We do a ton of simulating but no longer test with the Phantom 3 series of aircraft so we can't really verify operation for you.
You also might want to make sure that your aircraft has the most recent firmware on it and that you are using the most recent version of Map Pilot. Also, make sure all other drone related apps are FULLY closed.