IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 62, NO. 3, MARCH 2013 599 A Synchronous TDMA Ultrasonic TOF Measurement System for Low-Power Wireless Sensor Networks Carlos Medina, José C. Segura, Senior Member, IEEE, and Ángel de la Torre Abstract—This paper presents the design and evaluation of an ultrasonic time-of-flight (TOF) measurement system in the context of a smart sensor wireless network. In particular, the ZigBee protocol is used for data transmission and synchronization purposes. Low-cost and low-power restrictions are taken into account in the design. A synchronous measurement scheduling approach is used to minimize the network traffic and, therefore, the power consumption, while avoiding the need of wired connec- tions between the nodes or the use of specific radio link to provide synchronization. A theoretical model that describes the accuracy of the proposed system is derived. This model takes into account both clock drift effects and finite clock resolution of the network nodes. According to the model, the estimation of the TOF is biased due to the clock drifts, and a solution is proposed to compensate this bias. The compensation is based on an accurate estimation that each node performs for its own clock drift. An error analysis of this estimation procedure is also developed, and its effects on the TOF accuracy are presented. A theoretical model of the system that predicts the system performance in terms of TOF accuracies is proposed. An implementation of the TOF measurement system is presented, from which experimental results that validate the theoretical derivations and the effect of the clock drift compen- sation are obtained. Experimental evaluation of the system also demonstrates that TOF accuracies better than 2 μs are achievable, which will be more than adequate for achieving subcentimetric or even submillimetric precisions in ultrasound-TOF-based distance measurement systems. Even though a particular approach for TOF estimation is considered in this work, most of the derived results are also applicable to other systems involving time syn- chronous measurements. Index Terms—Local positioning systems (LPSs), ultrasonic pseudorange measurement, wireless sensor networks. I. I NTRODUCTION T HE USE of location information and its potential in the de- velopment of ambient intelligence applications has led in recent years the design and implementation of location systems. The main differences between local positioning systems (LPSs) are related to the type of technology used, conditioned by the Manuscript received March 8, 2012; revised July 5, 2012; accepted August 7, 2012. Date of publication October 5, 2012; date of current version February 5, 2013. This work was supported in part by the Junta de Andalucía under Research Project P08-TIC-03886. The Associate Editor coordinating the review process for this paper was Dr. Serge Demidenko. The authors are with the Department of Signal Theory, Telematics and Com- munications (TSTC) and the Information and Communications Technology Research Centre (CITIC), University of Granada (UGR), 18010 Granada, Spain (e-mail: cmedina@ugr.es; segura@ugr.es; atv@ugr.es). Digital Object Identifier 10.1109/TIM.2012.2218056 requirements of infrastructure and system accuracy. Systems based on radio signals require fewer infrastructures than other technologies but offer lower precision: from tens of centimeters in ultrawideband systems using time-of-arrival measurements [1] to several meters for systems using the received-signal- strength-indicator measurements (Wi-Fi [2], ZigBee [3], and radio-frequency identification [4]). Advances in machine vision make achieving accuracies of several centimeters possible [5] at the cost of using an expensive infrastructure, low modularity, and high processing requirements. An alternative solution is the use of ultrasonic signals. Unlike other technologies, ultrasound technology has certain advan- tages such as a slow signal propagation speed, no penetration of walls, lack of regulatory control, or low-cost transducers. This technology allows obtaining centimetric or even subcentimetric accuracies with a relatively low processing resource demand. In the literature, we find several examples of ultrasonic LPSs based on the measurement of the time of flight (TOF) of ultrasonic signals, the most representative being Active Bat [6], Cricket [7], Dolphin [8], and 3D-Locus [9], [10]. The time for the ultrasonic signal to travel the distance be- tween a sending node and a receiving node is used for the esti- mation of the distance between these two nodes considering the sound propagation speed. Accurate TOF measurements require a proper synchronization of transmitter and receiver devices. In a wireless system architecture, the use of radio-frequency or infrared signals to provide such synchronization is common. Recent developments on low-power smart sensor networks technology facilitate the implementation of low-cost low- energy ad hoc networks that can be used as the basic infrastruc- ture for indoor location [11]. Therefore, adding ultrasonic TOF measurement capabilities to a smart sensor network opens the possibility to develop precise, low-cost, and versatile LPS or in- dustrial positioning systems [9] with the advantage of removing the need of wired connections between ultrasonic sensors. Several alternative solutions have been proposed for ultra- sonic TOF measurement [12] with different levels of com- plexity and precision. For the present work, we have selected a TOF measurement technique based on a digital quadrature correlation receiver. A quadrature bandpass sampling [13], [14] scheme allows one to implement this technique with the limited memory and computational resources of the smart network nodes. In this paper, we focus on the design and evaluation of an ultrasonic TOF measurement system to be used in a low-power 0018-9456/$31.00 © 2012 IEEE