In this paper, we present an indoor ultrasonic locationtracking system that can utilize off-the-shelf audio speakers(potentially already in place) to provide fine-grained indoorposition data to modern mobile devices like smartphones andtablets. We design and evaluate a communication primitivebased on rate-adaptive wide-band linear frequency modulated chirp pulses that utilizes the audio bandwidth just abovethe human hearing frequency range where mobile devicesare still sensitive. Typically transmitting data, even outsideof this range, introduces broadband human audible noises(clicks) due to the non-ideal impulse response of speakers.

Unlike existing audio modulation schemes, our scheme isoptimized based on psychoacoustic properties. For example, all tones exhibit slowly changing power-levels and gradual frequency changes so as to minimize human perceivableartifacts. Chirps also bring the benefit of Pulse Compression, which greatly improves ranging resolution and makesthem resilient to both Doppler shifts as well as multi-pathpropagation that typically plague indoor environments. Thescheme also supports the decoding of multiple unique identifier packets being transmitted simultaneously. By applyinga Time-Difference-of-Arrival (TDOA) pseudo-ranging technique the mobile devices can localize themselves withouttight out-of-band synchronization with the broadcasting infrastructure. This design is not only scalable with respect tothe number of transmitters and tracked devices, but also improves user privacy since the mobile devices compute theirpositions locally. We show through user studies and experimentation on smartphones that we are able to provide submeter (95% < 10cm) accurate indoor positioning in a mannerthat is imperceptible to humans.