The problem with all of them is that Android lets apps check if location is being mocked via the mock location API. A good mocking API should not let the thing

The problem with all of them is that Android lets apps check if location is being mocked via the mock location API. A good mocking API should not let the things being mocked know that they are being mocked at. You can modify a fork of Android to do this at the OS level, I suppose. But I wonder if I could make a phone case that emits GPS signals that make my phone think it is in an arbitrary location but without interfering with anyone standing a reasonable distance away from me.