Staggered Synthetic Aperture Radar

Kurzfassung

Synthetic aperture radar (SAR) remote sensing allows high-resolution imaging independently 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 imaged at the expense of a degraded azimuth resolu-tion. This limitation can be overcome by using systems with multiple receive subapertures dis-placed 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 subswaths 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). A proper selection of the PRIs, together with an aver-age oversampling in azimuth, allows an accurate interpolation of the non-uniformly sampled raw data on a uniform grid, so that resampled data can be then focused with a conventional SAR processor. This concept therefore allows high-resolution imaging of a wide continuous swath without the need for a long antenna with multiple subapertures. As an additional benefit, the energy of range and azimuth ambiguities is spread over large areas: Ambiguities therefore appear in the image as a noise-like disturbance rather than localized artifacts. In this thesis, the impact on performance of the selected sequence of PRIs, the adopted interpolation method and the processing strategy are addressed. Design examples for single-, dual-, and fully-polarimetric staggered SAR systems are presented, based on both planar and reflector antennas, in L band and C band. The impact of staggered SAR operation on image quality is furthermore assessed with experiments using 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. Moreover, the German satellite TerraSAR X has been commanded to acquire data over the Lake Constance in staggered SAR mode. Measurements on data show very good agreement with predictions from simulations. Furthermore, a data volume reduction strategy, based on on-board Doppler filtering and decimation, has been developed to cope with the increased azimuth oversampling of staggered SAR. Finally, a patented extension of the staggered SAR concept is provided, where the phase centers are continuously varied as well, so that it is possible to transmit pulses according to a sequence of different PRIs and acquire uniformly sampled data without the need of any interpolation. The staggered SAR concept is currently being considered as the baseline acquisition mode for Tandem L, a proposal for a polarimetric and interferometric spaceborne SAR mission to monitor dynamic processes on the Earth’s surface with unprecedented accuracy and resolution.