3284 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 44, NO. 11, NOVEMBER 2006 Dynamic Monitoring of Bridges Using a High-Speed Coherent Radar Massimiliano Pieraccini, Matteo Fratini, Filippo Parrini, and Carlo Atzeni Abstract—Remote dynamic monitoring of bridges by a high- speed interferometric radar is proposed. The equipment is a continuous-wave step-frequency radar with very fast frequency hopping that is capable of sampling the structure at a rate high enough for transient analysis of motion through phase comparison of successively acquired images. An experimental test carried out on a highway bridge forced by vehicular traffic is presented. Index Terms—Dynamic monitoring, interferometry, radar, remote sensing, structural test. I. I NTRODUCTION D YNAMIC tests and monitoring systems of bridges are currently employed in order to: 1) validate design spec- ifications; 2) evaluate in situ the effects on the structure of the actual dynamic load (e.g., traffic and wind); 3) evaluate the effectiveness of maintenance, retrofit, and repair works; 4) provide diagnostic evidence for planning rehabilitation, maintenance, and repair; and 5) provide real-time information for safety assessment immediately after disasters and extreme events. Dynamic testing is currently implemented by networks of accelerometers installed on the structure. Such sensors are accurate and reliable but need to be positioned in contact with the surveyed structure. Settling the optimal sensor placement is a common problem encountered in many engineering applica- tions and is a critical issue in the implementation of effective structural health monitoring [1]. Furthermore, the monitoring of large structures can give rise to accessibility problems, often requiring the use of costly and cumbersome scaffolding. In a number of situations, the placing of contact sensors may be not possible; this is the case, for example, in buildings with symp- toms of impending collapse after a seismic shock or a blast. The capability of performing in-service monitoring is a key requirement for planning survey campaign aimed at the early identification of structural problems in order to enable low-cost maintenance remedial actions to be taken [2]. Another open challenge is the direct measurement of the deflection (i.e., absolute displacement) of long-span bridges [3]. Accelerometers can provide the displacement measurement only after a double integration in time, and traditional trans- ducers can only be used for relative displacement measurement. Laser sensors [4] have been proven often unpractical for in-field applications. A proposed solution is to use a global positioning Manuscript received March 6, 2006; revised April 28, 2006. The authors are with the Department of Electronics and Telecommunica- tions, University of Florence, 50139 Florence, Italy. Digital Object Identifier 10.1109/TGRS.2006.879112 system (GPS), but its accuracy is not good enough to meet the strict measurement requirements. In 1999, Farrar et al. [5] proposed a microwave sensor for noncontact measurements of vibration frequency in dynamic testing of structures. This sensor was able to measure punctual vibration frequency with satisfactory performance but did not provide imaging capability. Images of static deformation of a variety of structures, such as concrete girders in anechoic chamber [6], dams [7], bridges [8], and buildings [9] have been obtained by a ground-based synthetic aperture radar. While this technique offered both imaging capability and displacement measurement, it was too slow to detect the dynamic behavior of a structure. Recently, we designed and realized an interferometric radar that is able to image a scenario with a sampling rate that is high enough to track the movements of great architectural structures [10], [11]. In this paper, a description of this novel technique is presented and experimentally demonstrated in an in-field test on a long-span bridge in northern Italy. II. WORKING PRINCIPLE As known, the range resolution of a radar is determined by bandwidth B of the transmitted signal; a conventional radar transmits pulses of duration τ =1/B, but the same resolu- tion can be achieved by transmitting longer modulated signals with the same bandwidth, provided that the received signal is “compressed” in a short pulse of duration 1/B by proper processing [12]. The radar described in this paper makes use of a continuous- wave step-frequency (CW-SF) transceiver. It transmits, step by step, continuous waves at discrete frequency values, sampling bandwidth B at a constant interval Δf . The measured data set is a one-dimensional array of complex values, in which each one is relative to a single frequency. Pulse compression is achieved simply by performing the inverse fast Fourier transform (IFFT) of the data array. It can be easily found [12] that IFFT converts the response in frequency domain, which is relative to a single point scatterer at distance R 0 , into a synthetic pulse in time domain I (t) of width 1/B, as given by I (t)= e j 4πfc c R 0 sinc  πB  t - 2R 0 c  (1) where f c is central frequency of the employed band, B is the bandwidth, and c is the speed of light. 0196-2892/$20.00 © 2006 IEEE