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Enhancing SAR ship wake detection with motion estimates from along-track interferometry Ngo, Terry


A difficult problem in remote sensing is the detection of ships on the open ocean. For almost two decades, experimental spaceborne and airborne Synthetic Aperture Radars (SAR) have been used to image ships. However, SAR is inherently limited in (automated) ship detection because it relies on a single parameter, the radar Backscatter, to distinguish faint, small ship signatures from background. Along-Track Interferometry, a recently developed technique that uses two time displaced SAR images (from two alongtrack displaced antennas) to extract velocity estimates of the surface, holds some promise in enhancing the capability of SAR. In this study, we examined the Along-Track Interferometry technique in detail and attempted to assign quantitative measures of its enhancement potential to SAR in ship wake detection. Specifically, we used representative data from an experimental airborne SAR ATI, the Canada Centre for Remote Sensing (CCRS) CV580 C-Band SAR ATI, to examine the effects of ATI processing on: a) the visual appearance of Backscatter and motion signatures of ships and ship wakes in magnitude and phase components of ATI images b) the (quantitative) detectability of ships and their wakes by Backscatter and motion; and c) the characteristics of the Backscatter and motion signatures, such as their spectra, coherence, and signal distribution. And we attempted to determine from our analysis and simulation/modeling, a quantitative measure of the potential of ATI to enhance SAR's performance in ship detection. The results of this study suggest that Along Track Interferometry is capable of enhancing ship wake detection. Specifically, we found that: a) airborne SAR ATI is able to estimate differential (radial) velocity with sufficient accuracy to detect appreciable disturbances of the velocity field of the ocean, and that b) ship wakes, under the conditions of the data collected, were able to sufficiently disturb the velocity field of the ocean surface to be discriminated from ambient ocean structure. This study also suggests that in some circumstances, certain ocean features which may not have been detectable from its Backscatter by a conventional SAR, could be detected from its motion by ATI. And lastly, this study demonstrates that the additional, and in some cases singularly unique, motion information provided by ATI, when fused or combined with conventional Backscatter information, can indeed yield improved detection of ships and wakes overall by the SAR sensor.

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