Remote survey of the Leaning Tower of Pisa by interferometric sensing

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.