Focus Improvement of Spaceborne-Missile Bistatic SAR Data Using the Modified NLCS Algorithm Based on the Method of Series Reversion

Abstract

The speed and direction of a missile shifts sharply in the dive phase, making the azimuth frequency modulation (FM) rate change with the azimuthal position, leading to azimuth ambiguities and image distortion. To solve this problem, a modified nonlinear chirp scaling (NLCS) algorithm was adopted to compensate for the azimuth FM rate. First, the geometric configuration and echo signal model of the spaceborne missile bistatic synthetic aperture radar (SAR) were built, and then the Doppler frequency correction was performed, and the 2-D spectrum of the signal was derived by the method of series reversion. Next, range migration correction and range compression were finished in the 2-D frequency domain. Following this, a modified NLCS algorithm was proposed to solve the space variance of Doppler phase problem. After compensating for the azimuth FM rate, the azimuth compression focusing was completed and the imaging result was obtained. Finally, by comparing the calculation amount, imaging effect, and performance index with the traditional NLCS algorithm, it can be concluded that the algorithm reduced the calculation amount by 1.0128 × 108 floating point operations per second (FLOPs) compared with the traditional NLCS algorithm, and the azimuth focusing effect of the edge point was greatly improved. Its resolution, peak sidelobe ratio (PSLR), and integrated sidelobe ratio (ISLR) were improved by 0.87 m, 3.32 dB, and 1.79 dB, respectively, which proved the effectiveness and feasibility of this method.

An Extended Time Scaling Algorithm Based on the Spatial Inverse Mapping for Bistatic SAR Imaging With Complex Trajectory

“…The third term in KT  includes components such as RCC and secondary range compression (SRC), which has a small magnitude but still affects the imaging processing in high resolution mode. We expand the third term in (29) with respect to the range frequency r f in the McLaughlin series and construct the RCC and SRC compensation function, which can be expressed as: (31) where 0 i k represents the i-th McLaughlin expansion coefficient of bistatic slant range history at the scene center point O.…”

“…Aiming at this issue, a widely adopted strategy is to correct the linear reference component of RCM primarily [27], [30]- [33]. Subsequently, [27], [31], [32] have chosen to perform the range cell curvature (RCC) correction in the 2-D frequency domain, but this method may result in incomplete migration correction. A more suitable method for TV-BiSAR imaging is to introduce the keystone transform (KT) to correct the residual linear RCM (LRCM), followed by RCC correction to achieve deeply decoupling for range and azimuth dimensions [33].…”

“…However, the higher-order spatial variant components of Doppler parameters cannot be ignored and affect the azimuth focusing quality in CT-BiSAR imaging, and the performance of NCS is limited in this condition. Extended NCS (ENCS) algorithms and modified NCS (MNCS) algorithms are analyzed in [27], [29], [31], [33], [35], which are applicable to full-aperture imaging data. These algorithms are unsuitable for CT-BiSAR imaging due to the large amount of data to be processed and ineffective for sub-aperture imaging processing.…”

An Improved NLCS Algorithm Based on Series Reversion and Elliptical Model Using Geosynchronous Spaceborne–Airborne UHF UWB Bistatic SAR for Oceanic Scene Imaging

“…Furthermore, Li et al introduced a method based on the Lagrange inversion theorem (LIT) for deriving the point target spectra, incorporating the NLCS algorithm [25]. The NLCS algorithm has also been extensively applied in spaceborne illuminator BiSAR imaging [26][27][28]. These studies provide valuable insights for our research; however, they predominantly operate in high frequencies, with limited exploration in UHF UWB BiSAR imaging.…”

Modified Auto-Focusing Algorithm for High Squint Diving SAR Imaging Based on the Back-Projection Algorithm with Spectrum Alignment and Truncation

“…However, the high squint diving mode has always been a challenging problem since the range and azimuth coupling of the echo is severe, which brings many difficulties to imaging. Although various algorithms have been proposed to solve the coupling problem [1][2][3][4][5][6][7][8][9][10], the complicated implementation of the phase filters limits their applications. In contrast, time-domain algorithms have garnered increasing attention in high squint SAR mode when compared to frequency domain algorithms [11][12][13][14].…”