Squint SAR Research Articles

Focusing of Highly Squinted SAR Data With Frequency Nonlinear Chirp Scaling

In this letter, several important aspects are addressed for highly squinted synthetic aperture radar data with subaperture (SA) processing. First, the influences of the reference linear range walk correction and the resulting residual Doppler centroid are addressed. Then, the differences between the full-aperture data and SA data in the azimuth focusing are investigated in detail. Finally, a novel frequency-nonlinear-chirp-scaling algorithm with an addition of highly varying residual Doppler centroid correction over azimuth is proposed to equalize the azimuth-variant Doppler parameters so that the uniform azimuth processing can be realized. Compared with the previous works, this addition greatly improves the focused quality of the final image. Airborne real data processing validates the effectiveness of the proposed algorithm.

A Fast Method of Motion Errors Compensation for High Squint Millimeter Wave SAR

High squint millimeter wave SAR can be applied in missile-borne imaging seeker, but it’s very sensitive to the motion errors. Both the high squint SAR imaging algorithm and the motion errors compensation method are very important for achieving well focused SAR image. Therefore, a motion compensation method integrated with the imaging algorithm is proposed. Firstly, the expression of instantaneous squint range is given according to the imaging geometry model of high squint SAR. Then, the components of motion errors are separated and estimated, using the parameters of inertia navigation system (INS). Finally, the envelope and phase corrections are performed in high squint SAR data through the obtained motion errors. The effectiveness of the proposed approach can be verified by the imaging results of measured millimeter high squint SAR data.

A VARIABLE PRF IMAGING METHOD FOR HIGH SQUINT DIVING SAR

The high squint diving SAR is widely used to provide the information in advance. Large squint angle deduces the deeper coupling of range and azimuth of SAR echoes which makes SAR imaging more di-cult. Especially, the large range migration of the deep couple heavily burdens the imaging processing time and storage units. The diving motion of platform worsens the situation. This paper proposes the varied azimuth sample frequency (Pulse Repeat Frequency, PRF) to implement the high squint diving SAR imaging. Based on the signal model of the diving squint SAR, it is analyzed that the range walk is the prominent component of range migration in the high squint SAR. The varied PRF imaging method dramatically decreases the range walk of echoes by shifting the beginning position of transmitted pulses and received echoes and the shift is implemented by the PRF variation. Then the range migration is decreased and the couple of range and azimuth of SAR echoes is reduced. The PRF variation law is deduced and the applicable condition of varied PRF is presented. The simulation results show that the variable PRF method is e-cient to reduce range walk of echoes. Comparison to the traditional constant PRF, the novel variable PRF method for high squint needs less storage and less time expense, which is helpful to real time SAR imaging. The non-uniform FFT can be used for the azimuth compressing of the variable PRF SAR. It will simplify the implementation of the variable PRF SAR imaging.

AN EXTENDED INVERSE CHIRP-Z TRANSFORM ALGORITHM TO PROCESS HIGH SQUINT SAR DATA

This paper proposes an Extended Inverse Chirp-Z Transform (EICZT) algorithm to handle the high squint FMCW SAR data, where the conventional Inverse Chirp-Z Transform (ICZT) cannot work due to the failure in dealing with the range-variance of second- and higher-order range-azimuth coupling terms. A pre- processing operation is implemented in the azimuth-Doppler and range-time (Doppler-time) domain, where a perturbation function consisting of second-order and third-order range time variables is implemented to compensate the range variance of the second order range terms. Moreover, a new scaling factor is formulated to represent the Range Cell Migration (RCM), and further corrected by the presented EICZT approach. The proposed approach is analyzed and compared with the conventional ICZT. The simulated high squint SAR scene with nine targets is well focused by the proposed approach and the quality is greatly improved with respect the conventional ICZT. The proposed algorithm is also validated by the X-band high-resolution real SAR data.

AZIMUTH NONLINEAR CHIRP SCALING INTEGRATED WITH RANGE CHIRP SCALING ALGORITHM FOR HIGHLY SQUINTED SAR IMAGING

The di-culty of focusing high-resolution highly squinted SAR data comes from the serious azimuth-range coupling, which needs to be compensated in the procedure of imaging. Generally, the linear range walk correction (LRWC) can reduce the coupling efiectively, however, it also induces the problem of azimuth-dependence of residual range cell migration (RCM) and quadratic phase. A novel algorithm is proposed to solve this problem in this paper. In this algorithm, the azimuth nonlinear chirp scaling (ANCS) operation is used, which can not only eliminate the azimuth space variation of residual RCM and frequency modulation (FM) rate but also remove the azimuth misregistration. In addition, the range chirp scaling operation is applied to correct the range-dependent RCM. After implementing the unifled RCM correction, range compression and azimuth compression sequentially, the focused SAR image is acquired flnally. The experimental results with simulated data demonstrate that the proposed algorithm outperforms the existing algorithms.

Extended Nonlinear Chirp Scaling Algorithm for High-Resolution Highly Squint SAR Data Focusing

In this paper, an extended nonlinear chirp scaling (ENLCS) algorithm for focusing synthetic aperture radar data acquired at high resolution and highly squint angle is proposed. The whole processing of the ENLCS consists of the following three steps. First, a linear range walk correction is used to remove the linear component of target range cell migration (RCM) and to mitigate the range-azimuth coupling of the 2-D spectrum. Second, a bulk second range compression (SRC) is performed in the 2-D frequency domain for compensating the residual RCM, SRC term, and higher order range-azimuth coupling terms. Third, a modified azimuth NLCS (ANLCS) operation is applied to equalize the azimuth frequency modulation rate for azimuth compression. By adopting higher order approximation processing and by properly selecting the scaling coefficients, the proposed modified ANLCS operation has better accuracy and little image misregistration. The overall focusing procedure of the ENLCS algorithm only involves fast Fourier transform and complex multiplication, which means easier implementation and higher efficiency. The experimental results with simulated data prove the effectiveness of the proposed algorithm.

A Novel Imaging Algorithm for Airborne Bistatic Squint SAR with Unparallel Trajectories

A novel analytical imaging algorithm is proposed for the strip-map mode of airborne bistatic squint SAR with unparallel trajectories. The algorithm derives the two-dimensional (2D) spectrum formula of the point target echo by using the contribution ratios of Doppler frequency modulation ratios of the transmitter and receiver as the weighting coefficients. Through decoupling the target position against the tracks of the transmitter and receiver, the range parameter and the azimuth one in the spectrum formula are separated. In 2D frequency domain, 2D Chirp-Z transform (2D-CZT) is applied to correcting the migrations of the echo along the range and azimuth after the bistatic deformation term has been compensated, so the target image is precisely focused. The advantage of the algorithm is easy to be expanded to the virtual wide swath by blocking the radar data along the range and azimuth to limit the 2D residual migrations. Simulation results confirm the validity of the 2D-CZT algorithm.

A Modified Wave-number Domain Algorithm for Missile-borne Squinted SAR Data Processing

导弹非匀速直线运动且速度快的特点，使得弹载SAR回波信号的多普勒参数随斜距的变化较大，因此在波数域进行相位补偿的波数域算法很难实现弹载SAR测绘带内的高精度成像处理。该文在经典波数域算法及相关文献的基础上对波数域成像算法进行改进，将2维波数域中的方位压缩处理变换至时域或空域进行，从而可完成成像过程中多普勒参数适应距离的变化，降低原有算法使用同一参数带来的相位误差，因此改进算法能适用于宽测绘带的弹载SAR精确成像。仿真实验验证了改进算法的有效性。

Extended NCS Based on Method of Series Reversion for Imaging of Highly Squinted SAR

In the case of high range resolution and squint angle, current chirp scaling algorithm (CSA) and nonlinear CSA (NCSA) have a finite ability to achieve high-quality images. The problem stems from a range-dependent (i.e., space-variant) cubic- and higher order terms of range frequency, which require sufficient compensation or space-variant filtering, in the phase of the synthetic aperture radar transfer function, and this letter aims at dealing with this problem. First, an inequation is introduced to evaluate the highest order of range frequency terms whose space-variant coefficient has to be taken into account. Then, based on the method of series reversion, this letter proposes the extended NCS which can weaken the range dependence of the considered range frequency terms and achieve accurate range cell migration correction and range compression. Simulation results are presented to validate the proposed method.

Missile-borne Large Region Squint SAR Algorithm Based on a Curve Trajectory

摘要: 该文提出一种曲线轨迹下获取前向目标图像的新算法。考虑到曲线轨迹下严重的距离耦合，算法首先通过时域距离走动校正有效减少了距离耦合，然后采用级数反演法获得2维频率域表达式，求取ECS算法的相关系数，利用ECS获得了精确的成像结果。文章最后通过仿真实验验证了算法的有效性。 关键词: 合成孔径雷达 / ECS算法 / 曲线轨迹 / 弹载 / 斜视 / 级数反演法

Nonlinear Frequency Scaling Algorithm for High Squint Spotlight SAR Data Processing

This paper presents a new approach for the squint-mode spotlight SAR imaging. Like the frequency scaling algorithm, this method starts with the received signal dechirped in range. According to the geometry for the squint mode, the reference range of the dechirping function is defined as the range between the scene center and the synthetic aperture center. In our work, the residual video phase is compensated firstly to facilitate the following processing. Then the range-cell migration with a high-order range-azimuth coupling form is processed by a nonlinear frequency scaling operation, which is different from the original frequency scaling one. Due to these improvements, the algorithm can be used to process high squint SAR data with a wide swath and a high resolution. In addition, some simulation results are given at the end of this paper to demonstrate the validity of the proposed method.

Efficient and Precise Processing for Squinted Spotlight SAR through a Modified Stolt Mapping

Processing of squinted SAR spotlight data is a challenge because of the significant range migration effects of the raw data over the coherent aperture time. Although in theory the ()-algorithm takes care of these aspects, its digital implementation requires a time-consuming interpolation step. Moreover, the limited precision of this interpolation can introduce distortions at the edges of the final image especially for squinted geometries. A wave number domain processing using a modified Stolt mapping will be developed and analyzed to enhance the quality of the final SAR image. Additionally, the proposed algorithm has a decreased computational load compared to the original ()-algorithm. Simulation results will validate the focusing and efficiency performances of the modified wave number domain algorithm.

Instantaneous Doppler frequency for squint SAR imaging

Instantaneous Doppler frequency for squint SAR imaging has been found with Chirp Scaling Algorithm (CSA). Because the azimuth sample is not perpendicular to the range sample, the range signal must impact on the azimuth signal in the squint SAR data processing, and the different slant range targets have different Doppler frequencies. From the mathematical model of SAR echo signal, this paper carefully analyzes the instantaneous azimuth frequency, the instantaneous Doppler frequency component of the azimuth frequency and the impact of range chirp on azimuth frequency, which explains that Doppler frequency should be properly selected for correct SAR imaging in the squint SAR. The results of point target simulation experiments show that the way is reasonable for the squint SAR and can effectively complete range compression and azimuth focusing, and improve images’ quality.