IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 7, NO. 1, JANUARY 2010 185 Remote Survey of the Leaning Tower of Pisa by Interferometric Sensing Carlo Atzeni, Alberto Bicci, Devis Dei, Matteo Fratini, and Massimiliano Pieraccini Abstract—The Leaning Tower of Pisa, one of the world-famous architectural marvels of Italian heritage, needs continuous sur- veying to assess its stability. In this letter, remote-sensing equip- ment recently developed by the authors, based on the principle of microwave radar interferometry, has been experimented to measure the frequency response of the Tower without requiring any contact with its structure. Wind and human traffic were used as natural excitation sources, allowing the natural frequencies of the first vibration mode of the Tower to be measured in the north–south and in the west–east directions. Modal shapes of the Tower vibrations were also obtained from data acquired by the radar. Index Terms—Architectural heritage, interferometry, Leaning Tower of Pisa, modal analysis, radar, remote sensing, resonance frequency, structural health monitoring. I. I NTRODUCTION M EASUREMENT of the dynamic response to ambient excitation contributes to the procedures used to assess the condition of architectural structures and to identify possi- ble damage [1]. High-speed radar interferometry [2] has been recently demonstrated to be a powerful sensing technique for remote dynamic surveying of structures such as bridges, viaducts, and towers [3], [4], providing performances compa- rable with those ensured by traditional contact accelerometers [5]. The basic technique of high-speed interferometry is similar to that developed for spaceborne and airborne differential SAR interferometry (D-InSAR), employed in subsidence monitor- ing, and for ground-based InSAR, for monitoring quasi-static movement of structures and landslides [6]. Synthetic aperture processing, however, cannot be used for dynamic monitoring of structures, as the acquisition rate would be too low to track the structure vibration. Thus, while the high-speed radar sensor can achieve acquisition rates as high as 100 Hz, it can provide only range resolution. The proposed technique appears particularly attractive for surveying the health of architectural heritage structures [7], where the use of contact sensors, such as accelerometers, would be invasive or unpractical. Application of the microwave interferometric technique to heritage was first reported by the Manuscript received March 26, 2009; revised June 26, 2009. First published October 20, 2009; current version published January 13, 2010. C. Atzeni, D. Dei, M. Fratini, and M. Pieraccini are with the Department of Electronics and Telecommunications, University of Florence, 50139 Florence, Italy (e-mail: carlo.atzeni@unifi.it; devis.dei@unifi.it; matteo.fratini@unifi.it; massimiliano.pieraccini@unifi.it). A. Bicci is with the Georadar Division, Ingegneria Dei Sistemi SpA, 56121 Pisa, Italy (e-mail: a.bicci@ids-spa.it). Digital Object Identifier 10.1109/LGRS.2009.2030903 authors with their monitoring of Giotto’s Tower in Florence, demonstrating how the Tower oscillation, caused by wind, bell ringing, or simply vehicular traffic noise, can be detected from distances as far away as 1 km, allowing the measurement of the resonance frequency [8]. In recent years, exhaustive consolidation and restoration in- terventions have been carried out on the world-famous Leaning Tower of Pisa, Italy, after concern about an increase in its leaning angle. After a long period of substantial interventions, during which it was closed to public, the Tower has been equipped with the most advanced monitoring tools for the continuous controlling of its health condition [9]. Microwave interferometry can hopefully offer a contribution to the survey- ing of the Tower by remotely detecting the Tower movements and measuring the related resonance frequencies. Similar techniques able to carry out remote dynamic mea- surements without any contact with the structure under test can rely on the use of laser Doppler sensors [10]. Laser vibrometers, however, can only detect the movement of a single point at a time, not the deformation of the whole structure. A laser interferometer was used for instance to determine the natural frequencies of the individual columns in the loggias of Pisa’s Leaning Tower [9]. However, laser is sensitive to dust and environmental changes, typical of in-field applications; thus, it is not suitable for long-term monitoring, whereas radar sensors are robust instruments that can be employed for continuative surveying. II. I NTERFEROMETRIC SENSOR The high-speed interferometric sensor has been described in a number of previous papers [2]–[4]; therefore, only features required for the reader’s understanding of the following results are reported in this letter. The sensor basically consists of a co- herent radar, allowing the measurement of the phase of a signal received from a reflecting target. It operates by transmitting a set of continuous waves (CWs) at frequencies incremented by a constant step Δf , scanning a bandwidth B, using a technique referred to as CW step frequency (CW SF). A fast frequency-hopping direct digital synthesizer and a fixed-frequency precision oscillator were used to generate the CW-SF waveforms. The radar is equipped with separate trans- mitting and receiving horn antennas. After processing of the received CW signals, a 1-D range profile of the illuminated structure is provided, with a resolution given by ΔR = c 2B (1) 1545-598X/$26.00 © 2009 IEEE