A system and methods for estimating the location of a mobile device are disclosed. In accordance with one embodiment, a mobile device receives an ultrasound signal lacking an identification of the source of the ultrasound signal. One or both of the mobile device or a server identify which of a plurality of beacons transmitted the ultrasound signal based on the time at which the mobile device received the ultrasound signal and the time at which the mobile device received a wireless electromagnetic signal prior to receiving the ultrasound signal. One or both of the mobile device or the server estimate a location of the mobile device based on the identified beacon and on the times at which the mobile device received the wireless electromagnetic and ultrasound signals.