Large Area Ultrasound Positioning System

Outdoor positioning systems have been matured for many years. Now days, people enjoy the service of GPS (Global Positioning System) almost everywhere at any time. However, the indoor positioning systems are still lacking.
Ultrasonic devices own the advantages of low cost, simple operation etc. Unfortunately, the strength of sound wave decays quickly in the air. To overcome this disadvantage, we propose the direct sequence spread spectrum (DSSS) approach for range detection. The 4KHz rate pseudo random noise (PRN) code of 1023 chips are binary phase modulated to a 40KHz ultrasonic carrier as transmitting signal. At the receiving side, 320KHz sampler is used to collect all needed data for ranging. The
complex number demodulation, correlation computation, peak detection, etc. will be applied to determine the travelling time of the signal from the transmitter to the receiver. The above value being multiplied with speed of sound will be the distance.
Finally, TDOA (Time Difference of Arrival) model is used to get the position coordinate of the receiver when 4 ranging data are measured from 4 transmitters; respectively. The main contributions of spread spectrum technology are two. The first one is the high processing gain, so that the very weak ultrasonic signal can be detected even its power is less than that of environmental background noise. The second one is the fine ranging resolution, so that the positioning accuracy is improved. The proposed approach is confirmed by the indoor experiment results.