Great article. But there are even more types of solutions than this article mentions. Like sensor fusion motion sensing, otherwise known as inertial navigation, where phones uses their accelerometers and compases and other sensors to track their movement away from GPS coverage. Or network centric approaches, where wi-fi networks track the phones from the cloud. Or more esoteric signal analysis where phones use signal changes to track their movement. This report covers all the ongoing research in this exciting field It's going to be an exciting year or two for indoor location!