Staggered SAR: Imaging a Wide Continuous Swath by Continuous PRI Variation

Abstract

Synthetic aperture radar (SAR) remote sensing allows high-resolution imaging independent of weather conditions and sunlight illumination and is therefore very attractive for the systematic observation of dynamic processes on the Earth’s surface. However, conventional SAR systems are limited, in that a wide swath can only be achieved at the expense of a degraded azimuth resolution. This limitation can be overcome by using systems with multiple receive apertures, displaced in along-track, but a very long antenna is required to map a wide swath. If a relatively short antenna with a single aperture in along-track is available, it is still possible to map a wide area: Multiple swaths can be, in fact, simultaneously imaged using digital beamforming in elevation, but “blind ranges” are present between adjacent swaths, as the radar cannot receive while it is transmitting. Staggered SAR overcomes the problem of blind ranges by continuously varying the pulse repetition interval (PRI). If the sequence of PRIs is properly chosen, the samples, missing because the radar is transmitting, are distributed across the swath and along azimuth, such that they can be then recovered by interpolation of neighboring azimuth samples. This concept therefore allows high-resolution imaging of a wide continuous swath without the need for a long antenna with multiple apertures. In order to provide satisfactory suppression of azimuth ambiguities, some azimuth oversampling is required. This may cause (1) increased range ambiguities, which can be suppressed by jointly processing the data acquired by the available multiple elevation beams, and (2) an increased data volume, which can be reduced by on-board Doppler filtering and decimation. A design example for a fully-polarimetric high-resolution wide-swath SAR system is presented, based on a 15 m reflector. The impact of staggered-SAR operation on image quality is furthermore assessed with experiments with real data. As a first step, highly oversampled F-SAR airborne data have been used to generate equivalent staggered SAR data sets and evaluate the performance for different oversampling rates. Then, the German satellite TerraSAR-X has been commanded to acquire data over the Lake of Constance in staggered SAR mode. Measurements on data show very good agreement with predictions from simulations. The staggered SAR concept is being considered for Tandem-L which is a proposal for a polarimetric and interferometric satellite mission to monitor dynamic processes over the Earth’s surface with unprecedented accuracy and resolution.