# Generalized Nonlinear Chirp Scaling Algorithm for High-Resolution Highly Squint SAR Imaging
## 1 Introduction  
Synthetic aperture radar (SAR) has been widely used as a remote sensing tool as it can provide the high-resolution images of the interested area during a mission, nearly regardless of weather and time. Side-looking SAR, with the restriction of the antenna beam pointing direction, can only be applied in some special areas [1]. The squint SAR, which has an offset angle between the antenna beam pointing direction and flight path, can be flexibly applied in forehead observation [2; 3; 4; 5; 6], for example. The squint SAR can enhance the survival probability since it accomplishes the monitoring mission without going through the battlefield. It also has the advantages of associating with multi-mode to achieve some special applications, such as video SAR [7; 8; 9; 10; 11; 12] and increasing revisiting times of the interested area [11]. When compared to the perpendicular broadside SAR, the squint SAR is more attractive and widely used in the monostatic system, because it provides more flexibility for observing missions of SAR [13]. A number of outstanding and efficient frequency-domain algorithms have been proposed to solve the perpendicular broadside stripmap SAR imaging in the last fifty years, such as range Doppler algorithm (RDA), \(\omega-k\) algorithm, chirp scaling algorithm (CSA), and frequency scaling algorithm (FSA) [14]. Both the RDA and \(\omega-k\) have a low efficiency because of the interpolation process to accomplish the range cell migration correction (RCMC) [15; 16]. By using the chirp scaling function, CSA and FSA reach a relative faster imaging process compared to RDA and \(\omega-k\). Since the RDA, CSA, and FSA are built and derived under the zero Doppler assumption, they can only process the echo data with a squint angle varying from \(0^{\circ}\) to \(5^{\circ}\). The focusing performances of the aforementioned three conventional imaging algorithms decrease with the increasing of the squint angle. In comparison with the perpendicular broadside SAR, new complexities of signal properties are introduced in thesquint SAR [1]. The squint SAR geometry is also different from the conventional broadside one, which results in the difficulties in focusing precisely. The key problems of processing squint SAR data are the compensations of two-dimensional (2D) spatial-variant range cell migration (RCM), and azimuth-variance of Doppler coefficients [17]. A number of imaging algorithms have been presented for the squint SAR data. Back projection (BP) algorithm is the most precise imaging algorithm when assuming that the trajectory is obtained accurately. However, it has the heaviest computational burden among the imaging algorithms [18]. Extended \(\omega-k\) algorithm can achieve the highest accuracy among all of the frequency domain processing algorithms. However, its computational burden is much heavier than that of the nonlinear CS algorithm (NLCSA) [19]. Although the NLCSA is capable of processing the squint SAR data, it is only available to low/medium squint angle SAR [20]. Then, the extended NLCSA (ENLCSA), which introduces a new perturbation function to equalize the frequency modulation (FM) rates [1], is proposed to deal with the highly squint data whose squint angle can be up to \(65^{\circ}\). However, the azimuth depth of focus (ADOF) for the ENLCSA decreases when the distance of targets deviating from the reference azimuth position increases. With a \(60^{\circ}\) squint angle and 1m resolution, the ADOF can only reach 200 m by NLCSA. It is obvious that the ENLCSA cannot be used in the SAR mission with high resolution and high squint angle requirements. In this paper, a new method is proposed to solve the deteriorations caused by azimuth variant FM rates for high-resolution and highly squint SAR. Furthermore, modification is also carried out in range direction to avoid the range sub-block process with wide swath data. The remaining part of the paper is organized as follows.