Real Aperture Radar Interferometry for Static Monitoring of Heritage Structures: Application to the Leaning Tower of Pisa

Abstract

Displacement monitoring represents an important component of structural control and preservation of cultural heritage assets, especially when foundation settlements are involved. The continuous acquisition of displacement data, when integrated with environmental information, allows for the distinction of deformation components associated with environmental variations and those related to structural modifications. Traditional structural monitoring systems generally rely on the installation of contact sensors directly on the building surface. While this approach is well established, and can provide high-accuracy measurements, its efficacy largely depends on the number, spatial distribution, and proper calibration of the sensors. However, deploying and maintaining a dense network of contact instruments can be costly and logistically demanding. In addition, many historical and artistic monuments are subject to strict conservation regulations that limit or prohibit the installation of invasive instrumentation, thus promoting the development of non-contact monitoring solutions capable of ensuring reliable measurements without altering the physical or aesthetic integrity of the structure. Although non-contact techniques are already mature for large-deformation applications (e.g., slopes and infrastructure), their use in heritage contexts, typically characterized by small displacements and complex geometries, still requires further consolidation and methodological refinement to achieve comparable accuracy and long-term reliability. Within this framework, the present study investigates the application of Ground-Based Interferometric Real Aperture Radar (GBInRAR) technology as a non-invasive method for the structural monitoring of masonry towers. This technique, developed and refined over the past few decades, enables displacement measurements with sub-millimetric precision by comparing the phase differences of radar signals acquired at successive time intervals. The radar system, positioned at a distance from the target, allows for continuous observation of multiple points along the line of sight, offering a global view of the structural response and temporal evolution of the monitored asset. The bell tower of the Cathedral of Santa Maria Assunta in Pisa, universally known as the Leaning Tower, was selected as the case study. Over the centuries, the monument has experienced significant settlements and deformations, prompting extensive stabilization works and the establishment of a long-term contact-based monitoring system to ensure its safety. Despite these efforts, several aspects of its structural behavior, and their coupling with environmental variables, remain insufficiently characterized. Motivated by these gaps, the study presents the results of acquisition campaigns conducted with GBInRAR technology during different periods of the year, along with a comparative analysis of these data and those obtained from the existing monitoring system, while also exploring correlations between observed displacements and environmental parameters. The results aim to highlight the main advantages, limitations, and potential applications of radar-based monitoring in cultural heritage conservation, supporting the development of reliable and non-invasive strategies for the long-term protection and structural behavior assessment of historical monuments.