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 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. 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.

 

      To locate precisely, we need accurate distance between the receiver and transmitters. We use direct sequence spread spectrum (DSSS) technique, so that the distances between receiver and transmitters can be calculated. Code correlation is used to estimate TOF in signal propagation, and the code is implemented with PRN sequences. The auto-correlation of PRN sequences is around zero when misaligned and get a peak when aligned. The position is located by time difference of arrival (TDOA), and the advantage of this algorithm is that it doesn't need time synchronization between receiver and transmitters. The ultrasonic based positioning system is shown in Fig. 1.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

            The system architecture can be separated into five blocks, broadcast signal, complex demodulation, auto-correlation, detect real peak, and TDOA positioning algorithm. There are four independent PRN codes broadcasted by each transmitter at the same time. The time interval of a complete PRN code is 0.25575s. The receiver architecture is shown in Fig. 2.

 

         Its code generator structure is shown in Fig. 3. The PRN code is a periodic binary code whcih looks like noise, and has high correlation with itself. Therefore, we can measure how many bits dose the code shift or detect if the receive signal is the same PRN code or not. The PRN code is composed of linear feedback shift register and XOR gate, and each of all register renews its value according to state of previous arrow. Therefore, the initial condition, register length, and feedback tap connection are related to the PRN code. The different feedback tap connections will generate independent codes. Making sure four PRN codes are independent to each other. The correlation results of different parts are shown in Fig. 4. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

            The following Fig. 5&6 show the results of 1000 times positioning at the fixed point. The Table I shows the mean and standard deviation of positioning error. Although the positioning results in x and y directions are close to the true position, the positioning results of Z direction ar not so good. The reason is that the transmitters are set on nearly same height. The Fig. 7&8 show the results of dynamic positioning. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

REFERENCES 

[1] R.Queiro, P.S.Gira and A.Cruz Serra, “Cross-Correlation and Sine-Fitting Techniques for High Resolution Ultrasonic Ranging,” IMTC 2006 Instrumentation and Measurement Technology Conference, Sorrento, Italy, April 2006. 

[2] P.Misra and P.Enge, “Global Positioning System Signals, Measurements, and Performance.” Ganga-Jamuna Press, Lincoln, Massachusetts 01773. 

[3] D.Webster, “A Pulsed Ultrasonic Distance Measurement System based upon Phase Digitizing,” IEEE Transactions on Instrumentation and Measurement, Vol.43, No. 4, August 1994. 

[4] I.C. Lu, P.H. Jau, W.J.Cheng, Y.T.Chiang and F.R.Chang, “The Ultrasonic Ranging System Based on Direct Sequence Spread Spectrum,” The 5th International Conference on Positioning Technology, Kaohsiung, Taiwan, November 2012. 

[5] M.Scherhaufl, R.Pfeil, M.Pichler and A.Berger, “A novel unrestricted center-biased diamond search algorithm for block motion estimation,” 2012 IEEE Topical Conference Wireless Sensors and Sensor Networks, January 2012. 

[6] A.Yazici, U.Yayan and H.Yucel, “An Ultrasonic Based Indoor Positioning System,” 2011 International Symposium Innovations in Intelligent Systems and Applications, Istanbul, June 2011. 

 

H.Yucel, T.Ozkir, R.Edizkan and A.Yazici, “Development of Indoor Positioning System With Ultrasonic And Infrared Signals,” 2012 International Symposium Innovations in Intelligent Systems and Applications, July 2012.