On the Suitability of Distributed Scatterers for Bridge Monitoring in Very High Resolution SAR Data

Abstract. Satellite-based bridge monitoring with Interferometric Synthetic Aperture Radar (InSAR) offers a cost-effective solution for continuous deformation analysis of critical transport infrastructure. For very high resolution VHR SAR data, distributed scatterers DS suitability for bridge monitoring remains uncertain because elevated decks may suffer layover and partial pixel mixing with underlying terrain. This study evaluates the applicability of DS for bridge monitoring using 23 TerraSAR-X Staring Spotlight over two bridges near Regensburg, Germany: a 930 m long bridge crossing the Danube and agricultural land, and a low overpass crossing the A3 motorway. Persistent Scatterer Interferometry (PSI) was performed with a combined PS/DS workflow, and residual terrain error (RTE) estimates were validated against airborne laser scanning data. The results show that DS can increase measurement density compared with pure persistent scatterers (PS), but their reliability strongly depends on the radiometric contrast between the bridge and its surroundings. For the large bridge over heterogeneous terrain, DS and PS on structural elements yielded physically plausible RTE estimates. In contrast, for the overpass above asphalt surfaces with similar backscatter characteristics, DS were frequently affected by layover between bridge deck and ground, producing large vertical biases, with mean residuals reaching -6.09 m on the carriageway and remaining at -2.12 m even after filtering to above-ground points. The results demonstrate that DS-based bridge monitoring in VHR SAR is feasible where surrounding surfaces are radiometrically distinct; otherwise, DS may lead to erroneous RTE estimation and deformation interpretation.