In geosynchronous (GEO) synthetic aperture radar (SAR), the satellite track is curvilinear and the typical assumption of 'stop-and-go' breaks down due to the increase of orbit altitude. Thus, standard imaging algorithms based on the conventional hyperbolic range equation (CHRE) become inadequate. A four-order range equation taking the 'stop-and-go' errors into consideration is adopted to improve the accuracy in the approximation of the range history for GEO SAR. Then, an accurate two-dimensional spectrum is analytically deduced under the curved trajectory model, and a modified nonlinear chirp scaling (NLCS) algorithm is proposed according to this new spectrum. Simulation results validate that the proposed algorithm has better performance than the CHRE-based NLCS algorithm in the GEO cases.
Sea clutter has a major impact on the detection performance of a shipborne high-frequency surface wave radar (HFSWR) system. Due to the platform motion of shipborne HFSWR, the Doppler spectrum of the first-order sea clutter suffers from some broadening so that the targets submerged in this broadening Doppler spectrum can be hardly detected. In this letter, an improved oblique projection (IOP) method, combining the oblique projection (OP) algorithm and the method of sea clutter suppression in the Doppler domain, is proposed to suppress sea clutter in both Doppler domain and spatial domain for shipborne HFSWR. Compared with the OP and the orthogonal weighting algorithms, the proposed IOP algorithm is shown to give far superior suppression results in the Doppler domain and can achieve better azimuth estimation results based on real data.Index Terms-Oblique projection (OP), sea clutter suppression, shipborne high-frequency surface wave radar (HFSWR).
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