318 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 58, NO. 2, FEBRUARY 2009 A Novel Method of Distance Measurement Based on Pulse Position Modulation and Synchronization of Chaotic Signals Using Ultrasonic Radar Systems Francesco Alonge, Member, IEEE, Marco Branciforte, and Francesco Motta Abstract—This paper deals with a novel method of transmission and receipt of a signal based on both the property of two chaotic systems generating the same chaotic signal when they are syn- chronized and the property of pulse position modulation (PPM) to be insensitive to the distortions of the transmission channel. The method is discussed in the context of ultrasonic radar systems, in which the transmitter and receiver, which consist of ultrasonic sensors, are near each other, and the received signal consists of the transmitted signal reflected by an obstacle. A reference sinusoidal signal is superimposed to a chaotic signal generated by a master chaotic system, and the whole signal is modulated according to the PPM method and transmitted by the sensor. The received signal is demodulated, and the demodulated signal forces a slave chaotic system to generate the chaotic signal embedded in it, which allows recovery of the sinusoidal signal by subtracting this chaotic signal from the demodulated echo. The difference of the phases of the reference sinusoidal signal and the recovered sinusoidal signal allows computation of the time of flight of the signal and, consequently, the distance of the radar system from the obstacle. The novel method is illustrated and tested by both simulation and experiments. The interference problem between the considered radar system and other radar systems (crosstalk) is also addressed, and a solution is proposed to avoid it. Index Terms—Chaotic pulse position modulation (CPPM), chaotic system synchronization, crosstalk, distance measurement, multipath fading, ultrasonic sensors. I. I NTRODUCTION I N THE FIELD of the design and practical realization of mobile vehicles, the problem of avoiding impacts with either obstacles or other vehicles has to be considered to be of primary importance. In this paper, it is assumed that the vehicles in question are equipped with ultrasonic sensors, which, as is well known, emit a beam that returns back after the interception of an obstacle that is able to reflect it; the measurement of the time of flight (TOF) of the beam allows the measurement of the distance of the sensor emitting the beam from the obstacle. Manuscript received August 5, 2007; revised May 28, 2008. First published September 12, 2008; current version published January 5, 2009. F. Alonge is with the Department of Systems and Control Engineering, University of Palermo, 90128 Palermo, Italy (e-mail: alonge@unipa.it). M. Branciforte is with the Automotive Product Group, ST Microelectronics, 95121 Catania, Italy (e-mail: marco.branciforte@st.com). F. Motta is with Acheia s.r.l., 91018 Salemi, Italy (e-mail: motta.fra@ libero.it). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIM.2008.2003309 As is well known, the mobile vehicles are usually equipped with several sensors that are placed as such, so that each of them emits a beam in a certain direction, which allows exploration of the whole neighboring space. In these cases, a problem arises due to the interference between the sensors, which causes the degradation of the quality of the information captured. Another problem that can cause the aforementioned degradation is the presence of closed vehicles that are equipped with sensors of the same type, some of which receive beams emitted by sensors placed on other vehicles. It follows that the use of sensors that improve the naviga- tional safety of mobile vehicles requires that the following two problems be addressed: 1) crosstalk, i.e., the problem of each sensor in distinguish- ing its echo from that due to signals produced by other sensors placed on either the same or other vehicles; 2) multipath fading, i.e., the problem of distinguishing the echo produced by direct reflection of the emitted beam that has an impact on the obstacle from that produced by multiple reflections due to impacts to walls or other ob- stacles. This generates uncertainty about the true distance of the obstacle itself. The multipath fading problem has been addressed by several authors using data fusion techniques; in particular, good results have been obtained using methods based on fuzzy logic (cf., for example, [1]–[3]). The crosstalk problem has also been treated in the literature for obstacle avoidance using mobile robots and as a guide cane for the blind (cf., for example, [4]–[7]). In [4], the authors illustrate a method (error-eliminating rapid ultrasonic firing) in which the firing sequence of the sensors is alternated, so that a sensor cannot receive the beam emitted by another sensor in two consecutive sequences; thus, the sensor compares two successive readings and accepts only the readings near each other. In [5], a different approach is considered based on the frequency modulation of the sensor signals, thus giving to each sensor an easily recognizable label. In [7], a method that uses the crosstalk to calculate the relative position of the reflecting surfaces by means of triangulation is presented. In this paper, a novel measurement method of the distance of a vehicle from an obstacle is proposed based on the property of chaotic systems [8] to generate the same chaotic signal when they are synchronized and the insensitivity property of the pulse position modulation (PPM) technique to the distortions of the transmission channel. Practically, the proposed idea is 0018-9456/$25.00 © 2009 IEEE