An extended two-step focusing approach (ETSFA) for processing the squinted spotlight synthetic aperture radar (SAR) data is proposed in this paper. The effect of the squint angle on the azimuth coarse focusing is analyzed and discussed. Based on the analysis results, a nonlinear shift preprocessing method is introduced, which can completely remove the squint angle impacts on the azimuth coarse focusing. Furthermore, based on the squinted spotlight SAR imaging model and the preprocessed echo data, derivations of the azimuth coarse focusing with the deramping-based technique and precise focusing with the modified Stolt-based technique are carried out in detail. Moreover, to produce an acceptable image by the proposed ETSFA for high-resolution (< 3 m) squinted spotlight SAR with large scene, a subscene processing method is introduced. The experimental results on simulated data prove the validity of the whole analysis and the proposed methods.Index Terms-Deramping-based technique, squinted spotlight synthetic aperture radar (SAR), subscene processing.

As an efficient image contrast enhancement (CE) tool, adaptive gamma correction (AGC) was previously proposed by relating gamma parameter with cumulative distribution function (CDF) of the pixel gray levels within an image. ACG deals well with most dimmed images, but fails for globally bright images and the dimmed images with local bright regions. Such two categories of brightness-distorted images are universal in real scenarios, such as improper exposure and white object regions. In order to attenuate such deficiencies, here we propose an improved AGC algorithm. The novel strategy of negative images is used to realize CE of the bright images, and the gamma correction modulated by truncated CDF is employed to enhance the dimmed ones. As such, local over-enhancement and structure distortion can be alleviated. Both qualitative and quantitative experimental results show that our proposed method yields consistently good CE results.

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Abstract-The imaging problem of spotlight synthetic aperture radar (SAR) in the presence of azimuth spectrum folding phenomenon can be resolved by adopting the azimuth deramping-based technique and traditional stripmap SAR imaging algorithm, and this method is the so-called two-step processing approach. However, when the spotlight SAR operates on squinted mode, the echo two-dimensional (2D) spectrum is shifted and skewed due to the squint angle. In such case, the original two-step processing approach is not suitable anymore. This paper presents a novel imaging algorithm using the deramping-based technique and azimuth nonlinear chirp scaling (ANLCS) technique. First, the problem of azimuth spectrum folding phenomenon in squinted spotlight SAR is analyzed. Subsequently, based on the analysis results, the linear range walk correction (LRWC) is applied for removing the squint angle impacts on signal azimuth coarse focusing. At last, a modified azimuth NLCS algorithm is proposed for overcoming the depth of focus (DOF) limitation problem that induced by the LRWC preprocessing. Point targets simulation results are presented to validate the effectiveness of the proposed algorithm to process squinted spotlight SAR data with azimuth spectrum folding phenomenon.

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