GPS-based navigation systems have become very popular,mainly because they let people rapidly explore unknown areas. However, GPS works only outdoors, because the required satellite links are blocked or unreliable inside buildings. For indoor location sensing, many types of sensor sexist, including ultrasonic, infrared, magnetic, and radio sensors. But they all require a permanent electronic infrastructure to facilitate measurements, and localizable objects relying on this infrastructure need special sensors or actuators. Practical problems such as power consumption, wiring, and overall infrastructure cost have inhibited such technologyâ„¢s deployment in entire buildings.