Abstract
In this paper, a modified azimuth nonlinear chirp scaling (NLCS) algorithm is derived for high-squint bistatic synthetic aperture radar (BiSAR) imaging to solve its inherent difficult issues, including the large range cell migration (RCM), azimuth-dependent Doppler parameters, and the sensibility of the higher order terms. First, using the Lagrange inversion theorem, an accurate spectrum suitable for processing airborne high-squint BiSAR data is introduced. Different from the spectrum that is based on the method of series reversion (MSR), it is allowed to derive the bistatic stationary phase point while retaining the double square root (DSR) of the slant range history. Based the spectrum, a linear RCM correction is used to remove the most of the linear RCM components and mitigate the range-azimuth coupling, and, then, bulk secondary range compression is implemented to compensate the residual RCM and cross-coupling terms. Following this, a modified azimuth NLCS operation is applied to eliminate the azimuth-dependence of Doppler parameters and equalize the azimuth frequency modulation for azimuth compression. The experimental results, with better focusing performance, prove the high accuracy and effectiveness of the proposed algorithm.
Highlights
The above compensation function is similar to the bulk range processing transfer function in [33], which can perform range compression, residual range cell migration (RCM) correction, secondary range compression, and the compensation of all higher-order phase terms for all points located at the reference range
While the bulk secondary range compression (BSRC) that corrects the phase at the scene center is often sufficient for the whole scene, for wider range swaths, it may be necessary to segment the scene into range invariance regions whose width is selected by the quadratic phase errors (QPEs)
lagrange inversion theorem (LIT) spectrum that is suitable for processing high-squint bistatic synthetic aperture radar (BiSAR) data
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The accuracy of LRCMC operation is weakly range dependent [27], which originates from the weak range dependence of the Doppler center (or the squint angle) of MonoSAR or BiSAR geometry This problem can be solved or alleviated through the use of the 2D attitude-steering strategy [22,31] or invariant area [16,20]. A more accurate bistatic slant range approximation is introduced in order to apply LRCMC operation and improve the imaging accuracy of the algorithm. NLCS operation was first proposed in [16], by multiplying a cubic phase perturbation function in azimuth time domain It does not take the azimuth dependent higher order quadratic FM rate term and cubic phase term into account, which will cause defocusing and asymmetry to appear in the final image of the high-squint BiSAR.
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