Studying The Influence Of SAR Azimuth Angle On The Imaging Sea Bridge

SAR uses the side-looking geometry for imaging.A bridge on a SAR image can appear as three bright,narrow,and continuous segments.The segments are created by the direct return from the bridge road surface(single),the water-bridge/bridge-water(doublebounced)interactions,and water-bridge-water/bridge-water-bridge(triple-bounced)interactions.The segments are strongly affected by the angle between the flight direction and bridge orientation.To investigate the influence of the angle or azimuth angle on the segments,we simulated SAR images under a wide range of azimuth angles and variable water/ocean surface roughness determined by wind speeds.In the simulation,the bridge was metallic.The radar wavelength was L-band.The azimuth angle was from 0° to 45°.The wind speed ranged from 0 m/s to 40m/s.The spatial spectral energy distribution in a given direction of the water/ocean surface was modeled using the Pierson-Moskowitz(PM)spectra.Backscatter coefficient was derived using the Kirchhoff approximation algorithm.With the assumption that the noise floor of a SAR system was at –30 d B,three regions on the plane of the azimuth angle and wind speed were delineated.In region I,segments of both double-and triple-bounced returns were observed.There were only segments of double-bounced results in region II.None of the double-and triple-bounced returns was observable in region III.Then,SAR images of a bay bridge with variable clearance and azimuth orientation were analyzed to further the understanding of the impact of the azimuth angle on SAR imagery.The bridge was the curved Coronado Bridge over San Diego Bay,California,USA.On the SAR images,the bridge was shown with three types of features.Feature I consisted of nearly even distributed dots and a short curved segment,feature II dots and a curved segment,and feature III a continuous curved segment.With the SAR image geometry and the polarimetric decomposition method,the dots of feature I were produced by the double-bounced interactions of the bridge surface and light poles on the far side of the bridge toward the SAR.The dots of feature II were from the double-bounced interactions of the ocean surface and light poles on the near side toward the SAR.Feature III was the multiple interactions of the ocean surface and bottom of the bridge.When the bridge side become parallel to the flight direction,three segments of features I,II,and III become magenta or yellow color in the Pauli RGB image indicating the presence of double-bounced backscattering component.Indeed,the segment of feature I came from double-bounced interaction of the side railing on the far side of the bridge toward the SAR and bridge surface.The segment of feature II came from the double-bounced interactions of the ocean surface and side of the bridge facing the SAR.The segment of feature III formed by the multiple interactions of the ocean surface and bottom of the bridge,where a dihedral corner is formed by the extended surface and steel beam or by each steel beam and bottom of the bridge roadway.Finally,the SAR imaging mechanism from the bay bridge was studied.The distance of echoes at variable non-zero azimuth angles were constant.An algorithm to estimate the clearance and width of the bridge that has variable azimuth angles was developed.The estimated bridge height and width were acceptable since the errors were less than one cell size of the SAR datasets(e.g.,ALOS-PALSAR and UAVSAR).