Echo-Driven Doppler Parameter Estimation for High-Resolution Squinted Spaceborne Sliding Spotlight SAR

High-resolution squinted spaceborne sliding spotlight synthetic aperture radar (SAR) imaging requires accurate Doppler parameters. In conventional SAR processing, Doppler parameters are generally derived from prior ancillary information or estimated from the received echoes. However, Doppler parameters derived from prior information are sensitive to ancillary-measurement errors, while existing echo-based methods remain insufficient for ambiguity-resolved Doppler-centroid estimation and Doppler frequency modulation (FM)-rate refinement in high-resolution squinted sliding spotlight SAR. To reduce the reliance on prior ancillary information for Doppler-parameter determination, this paper proposes an echo-driven Doppler-parameter estimation (ED-DPE) method. The proposed method first estimates the baseband Doppler centroid using adjacent-echo cross-correlation and then resolves the Doppler ambiguity number based on the range-migration slope, thereby obtaining the absolute Doppler centroid fd. The estimated absolute Doppler centroid is subsequently used as a physical constraint on an attitude-equivalent observation geometry, which is refined through contrast-based full-aperture image formation to obtain the Doppler FM rate fr and the imaging parameters. Point-target simulations and real spaceborne SAR experiments demonstrate that the proposed ED-DPE method supports accurate focusing of 0.25-m-resolution squinted sliding spotlight SAR.