Accurate Water Level Measurement in the Bridge Using X-Band SAR

Bridges are an important infrastructure during natural hazards; however, the water levels of reservoirs on which exist bridges have often been restricted in practice owing to the lack of gauging stations. This letter presents a method for measuring the relative water level at bridges using synthetic aperture radar (SAR) single-look complex data with an X-band. Unique multiscattering between the bridge and water surface provides a basis for the range shifting of the strong backscattering in the SAR data according to the water level change. The performance was evaluated using multitemporal Constellation of Small Satellites for Mediterranean Basin Observation (COSMO-SkyMed) images with strong backscatter related to the bridge. A time-series of water level is estimated from double-and triple-bounced backscatters and compared with the gauged data. The estimated relative water levels at the subpixel shift aided by oversampling of Hough image have a squared correlation coefficient of 0.985 and 0.999 for the double and triple bounce, respectively. The triple bounce was more consistent with the gauged water level, as the sensitivity of the range distance change is twice that of the double bounce, and the disturbance by the bridge substructure is low. In particular, the standard deviation of the relative water level change using the triple bounce was 0.17 m. The results demonstrated accurate water level measurements with subpixel precision. Therefore, it can be effectively used to monitor hydrological changes in remote areas with bridges.