Well, not bad... the bad thing is: I am not familiar with any APIs about the stuff you wrote here. Of course, if you are, you are welcome to contribute. Apart from being an overkill for a simple notes app, this stuff would blow up the battery no matter how you try. Anyways, the network cell ID seems to be the least power hog among the ideas - the problem is, you don't know the cell ID near the shop (unless you are there), and also there is no guarantee that the cell ID won't change until the next time you get there.