Indoor pseudo-ranging of mobile devices using ultrasonic chirps

In this demonstration, we show an indoor location tracking system that broadcasts ranging data to mobile phones using ultrasonic signals. The system capitalizes on the ability for many smart-phones to detect audio above the human hearing range. This approach provides enhanced ranging capabilities to mobile devices without adding to or modifying their hardware. In [1], we describe a modulation scheme based on rate-adaptive wide-band linear frequency modulated chirp pulses that can be transmitted from standard audio tweeters at just above the human hearing frequency range. Acoustic data transmissions (even outside of the human hearing range), typically introduce audible noises (clicks) due to the non-ideal impulse response of speakers. Our scheme is optimized to avoid these artifacts by using slowly changing power-levels and gradual shifts in frequency. Each speaker simultaneously transmits a uniquely identifiably signature that can be geolocated on a map. By then applying a time-difference-of-arrival (TDOA) pseudo-ranging technique the mobile devices can localize themselves without synchronizing with the broadcasting infrastructure. This design is not only scalable with respect to the number of transmitters and tracked devices, but also improves user privacy since the mobile device can compute its position locally.