Accelerometer-aided millimeter-wave radar interferometry for uninterrupted bridge displacement estimation considering intermittent radar target occlusion

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Monitoring bridge displacement is necessary to detect early signs of damage, prevent catastrophic failures, and maintain public confidence in the safety of critical transportation infrastructure. Radar interferometry is attractive for long-term continuous monitoring of bridge displacements, given its resilience to weather and lighting conditions. However, intermittent radar target occlusions caused by moving vehicles beneath bridges pose challenges during extended continuous monitoring periods. In this study, accelerometer-aided millimeter-wave radar interferometry was explored with an explicit consideration of intermittent radar target occlusion. Radar and accelerometer measurements were first recorded over a short period. Multiple good targets were selected among all radar-detected targets, and their direction-converting factors were estimated using short-period measurements. Subsequently, bridge displacement was continuously estimated by overcoming intermittent radar target occlusions. A displacement reconstruction algorithm was proposed for target occlusion periods, and a multitarget-based phase-unwrapping algorithm was proposed to remove the displacement drift caused by radar target occlusion. The proposed technique was experimentally validated through laboratory tests on a 10-meter-long bridge structure and field tests on two different spans of a footbridge. The technique accurately estimated displacements with overall errors below 0.1 mm, allowing it to be widely applied to bridges with millimeter-scale displacements.