An indoor ultrasonic system for autonomous 3D positioning

Indoor positioning is an emerging technology with wide applications. Augmented and mixed reality need accurate and real-time positioning of user's limbs or direction of sight in real time, and reference points within the real environment. Positioning is also required for mall navigation, elderly people movements monitoring, and many others contexts. Indoor positioning of mobile units (MUs) can be provided by multilateration techniques that compute the location of an MU starting from distance measurements between the MU and a set of beacons. In this paper, a nonlinear closed-form solution for the trilateration problem is employed to avoid heavy numerical iterative algorithms. The closed-form solution is allowed by a particular arrangement of the beacons, placed at the vertex of a square. A positioning system is presented where MUs, exploiting the light algorithm of the closed-form positioning, are able to autonomously and privately calculate their own positions. A prototype of the positioning system has been designed, realized, and characterized for an average 4× 4 ×3m3 home or office room. It includes four beacons that emit a sequence of ultrasonic chirp signals, several MUs, and a master unit that provides time synchronization via ANT transceivers onboard each MU and the master unit. Thanks to the adopted closed form solution, each tiny battery-operated MU is able to carry out all the computations onboard, including analog and digital signal processing, beacon-MU distance estimation, and finally MU positioning at a rate of 2 Hz.