Demonstration of Staggered Ambiguous SAR Mode for Ship Monitoring Using TerraSAR-X

Maritime surveillance employing synthetic aperture radar (SAR) necessitates the simultaneous achievement of a wide swath and high resolution, enabling effective monitoring of vast areas with enhanced detection capabilities. However, conventional SAR modes encounter limitations as widening the swath compromises azimuth resolution. Ambiguous SAR modes, such as the staggered ambiguous mode utilizing continuous variation of short pulse repetition intervals and low pulse repetition frequency mode, surmount this constraint by enabling the imaging of a wide swath with high resolution. Both modes ensure remarkable detection performance even for small ships without relying on digital beamforming or multiple receive apertures. This paper presents the demonstration of the staggered ambiguous mode through experimental data acquired by the TerraSAR-X satellite over the North Sea. Despite technical limitations inherent in the SAR instrument, a swath of 110 km was successfully imaged with an azimuth resolution of 2.2 m. A probability of detection greater than 0.8 was achieved for small ships measuring 21 m × 6 m. Range ambiguities of the ships were identified based on their characteristics and position, and subsequently rejected. The detected ships were validated with maritime positioning data obtained from their automatic identification system. These findings provide compelling evidence and rationale for adopting the ambiguous SAR modes in current and future SAR systems and missions.