High squint mode SAR imaging using modified RD algorithm

SAR imaging with high squint angle in the terahertz band has the advantages of high resolution and unique application prospects. But the increase of frequency and squint angle leads to the deterioration of the imaging quality. The errors of THz SAR imaging are analyzed first in this paper. Then the complexities of SAR imaging with high squint angle in the terahertz band are analyzed. Finally, the impact of high squint angle and frequency on SAR imaging through simulation is verified. It is demonstrated that there are many unsolved complex problems in the SAR imaging with high squint angle in the terahertz band. It provides reference and help for THz SAR imaging with high squint angle in the future.

During the data acquisition of a squint terrain observation by progressive scan (TOPS) synthetic aperture radar (SAR), the steering of the antenna main beam increases the azimuth bandwidth and results in the azimuth signal aliasing in the Doppler domain. Besides, the range curvature and the Doppler frequency modulation (FM) rates after linear range walk correction are azimuth-variant for squint TOPS SAR. These problems may cause some difficulties for the SAR data processing. To deal with the problems, a new imaging algorithm for the squint TOPS SAR is introduced in this paper. After linear range walk correction, the azimuth preprocessing is implemented to achieve the two-dimensional frequency spectrum without aliasing. Then, utilizing a modified chirp scaling algorithm, we complete the range cell migration correction (RCMC) and range compression to the TOPS SAR data without subaperture. Finally, the frequency nonlinear chirp scaling (FNCS) is proposed to correct the variation of the FM rates and the azimuth signal is focused in the Doppler domain via the spectral analysis (SPECAN) method. Both simulation and real data results show the effectiveness of the proposed algorithm.

A nonlinear chirp scaling algorithm (NCSA) is proposed for bistatic SAR data processing with high squint angles in tandem configuration. Besides the dependence of the azimuth frequency, the dependence of range is considered for the Doppler chirp rate of tandem bistatic SAR. The proposed algorithm can compensate the range dependence of both the range cell migration (RCM) and the second range compression (SRC) terms well by appropriately setting the coefficients of the phase term, which is obtained after the nonlinear chirp scaling process in the two-dimensional wavenumber domain. Only fast Fourier transforms (FFTs) and phase multiplies are required, and fast imaging is implemented in the frequency domain. Satisfying focusing qualities verify the effectiveness of the proposed algorithm by simulations.

With appropriate geometry configurations, bistatic Synthetic Aperture Radar (SAR) can break through the limitations of monostatic SAR for forward-looking imaging. Thanks to such a capability, bistatic forward-looking SAR (BFSAR) has extensive potential applications. This paper develops a frequency-domain imaging algorithm for translational invariant BFSAR. The algorithm uses the method of Lengendre polynomials expansion to compute the two dimensional point target reference spectrum, and this spectrum is used to perform the range cell migration correction (RCMC), secondary range compression and azimuth compression. In particular, the Doppler-centroid and bistatic-range dependent interpolation for residual RCMC is presented in detail. In addition, a method that combines the ambiguity and resolution theories to determine the forward-looking imaging swath is also presented in this paper.

In order to solve the problems that the existing high squint missile-borne synthetic aperture radar( SAR) algorithm can not deal with too high squint angle,and that the calculated amount of traditional back-projection( BP) imaging algorithm is too large,as well as that the current fast BP algorithm can not process the echo data in parallel,an improved approximate forward looking missile-borne SAR back-projection imaging algorithm which can process echo data in parallel was proposed. According to geometrical relationship of approximate forward looking missile-borne SAR,an echo signal model was established. And the imaging area was divided to equidistance strip in range along the distance direction. The image was split when merging sub aperture,and the splitting and merging did not stop until the accuracy of image met the requirements.Next the raw echo data was back projected to each strip for coherent integration,and then the missile-borne SAR image was obtained. If these strips were processed for imaging in parallel,the imaging speed would be further improved. Finally,results of simulation echo data and real echo data validate that the improved algorithm can deal with approximate forward looking scene with 86° squint angle,and it can greatly improve the computation speed compared with that of traditional BP algorithm. Moreover,the computation speed is faster than that of other improved BP algorithms by processing those strips in parallel.

The FMCW radar has lots of advantages such as compact, energy effective and low cost. It is suitable for mounting on the vehicle and also on small sized UAV. In this paper, the high squint angle effect is considered for both the FMCW synthetic aperture radar (SAR) and the inverse synthetic aperture radar (ISAR) in the case of the high resolution vehicle radar. The 2nd range compression is performed for removing the range and the azimuth coupling effect under the high squint angle situation. The polar reformatting is performed for improving the radar resolution in the ISAR. By keeping practical situation in mind, the image qualities under the squint angle with random error is investigated. In addition, the simulation software for the SAR and the ISAR are developed.

The wavenumber domain algorithm is an ideal solution for high-resolution and highly squinted (HRHS) synthetic aperture radar (SAR) imaging in the case of an ideal straight trajectory. However, for airborne HRHS SAR imaging, the Stolt mapping leads to nonsystematic range cell migration (NsRCM) and secondary range compression (SRC) for the HRHS SAR, which causes the HRHS SAR image to defocus severely. To obtain a well-focused HRHS SAR image, a new autofocusing algorithm for HRHS SAR imagery based on an equivalent broadside model is proposed in this article. After coarse motion compensation, linear range walk correction and azimuth resampling are employed to transform the HRHS SAR data into the equivalent broadside SAR data, which has been proved to greatly reduce NsRCM and SRC. After that, the motion error prior structure in the 2-D wavenumber domain is revealed. According to the structure, a novel 2-D wavenumber domain autofocusing algorithm is proposed by the relationship between the 1-D azimuth phase error and the 2-D wavenumber domain phase error correction. Finally, the well-focused HRHS SAR imagery is obtained. Experiments based on simulated and acquired data are carried out to verify the necessity and effectiveness of the proposed algorithm for HRHS SAR imaging.

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 difficult.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 efficient 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.

In synthetic aperture radar (SAR) imagery, images of ground moving targets (GMTs) are smeared, distorted, and shifted. Current GMT imaging methods are mostly based on range-Doppler algorithms, which have two main drawbacks: 1) coupling between range cell migration correction (RCMC) and Doppler parameter estimation and 2) cross terms degrade the performance of the nonlinear estimation methods. In this paper, an optimal 2-D spectrum matching method for SAR GMT imaging is proposed. The main innovation or advantage of this method is that the GMT imaging problem is transformed into a constrained optimization problem, and differential evolution is applied to guarantee a high-processing efficiency. As they are associated with the Doppler centroid variation compensation and range shift compensation processing, all GMT point scatterers can be well focused and well located. Compared with the current methods, the improvements exhibited by this method include three main benefits: 1) RCMC and Doppler parameter estimation can be simultaneously accomplished; 2) both along- and cross-track GMT velocities can be simultaneously estimated; and 3) this method can be applied to both monostatic and bistatic SARs. Numerical simulations and experimental data processing have verified the effectiveness and robustness of the proposed method.

In squinted synthetic aperture radar (SAR) imaging, the range-azimuth coupling requires precise range cell migration correction (RCMC). Moreover, for high-resolution airborne SAR, motion compensation (MOCO) becomes complicated as the squint angle increases, thereby degrading the performance of Doppler-domain imaging algorithms. On the other hand, time-domain back-projection (BP) SAR imaging approaches are considered as optimal solutions to performing precise image focusing and MOCO. Among current BP algorithms, the fast factorized back-projection (FFBP) algorithm is one of the most essential representatives that achieve high-resolution images in an efficient manner. In this paper, the principle and applications of the FFBP algorithm are investigated through the derivation of the azimuth impulse response function (AIRF) of the resulting image. The phenomenon of spectrum displacement induced by motion errors in the BP image is presented and analyzed. Based on rigorous mathematical derivations, a modified FFBP algorithm is proposed to facilitate a seamless integration with motion compensation and accurate imagery of high-resolution highly squinted airborne SAR. Real data results confirm the effectiveness of the proposed approaches.

The algorithms that processes electromagnetic back scattered echo holds importance in Synthetic Aperture Radar (SAR) imaging. The selection criteria of an appropriate algorithm depend on application requirement of SAR technique. The computational complexity of Range Doppler Algorithm (RDA) and Chirp Scaling Algorithm (CSA) differs in terms of the procedure adopted for Range Cell Migration Correction (RCMC). In this paper, computational complexities and structural similarities between processed images by these two algorithms are studied through their processing steps on varying pixel size images with the help of MATLAB tool.

Range cell migration correction (RCMC) is the key process in the imaging formation of the bistatic SAR. Due to the existence of double-square-root in the bistatic range equation, the RCMC in the general bistatic SAR configuration is more complicated than the traditional monostatic method. In this letter, the range cell migration of bistatic SAR system is analysed in detail, and a novel data transformation is derived. From the derivation, we find that the original keystone transform is only a special case of our proposed transformation, so that we called it the generalized keystone transform. Compared with the original keystone transform, this generalized method can correct more complex range cell migration nicely. After the generalized keystone transform processing, the range cell migration is eliminated closely, and the focused image with full range and Doppler resolution can be obtained using the fast Fourier transform. Finally, a hybrid spaceborne/airborne bistatic SAR numerical simulation is performed to validate the proposed processing procedure.

This thesis examines a new algorithm, to be called secondary range compression (SRC), for significantly improving the range resolution of the range/Doppler synthetic aperture radar (SAR) processing algorithm when the radar antenna is significantly squinted away from the zero Doppler direction. The algorithm was recently introduced by Jin and Wu [13] for application to the SEASAT SAR sensor. Significant extensions of their algorithm and models are presented. First the model of range broadening in the basic range/Doppler algorithm is extended by using a more general form for the range compressed profile. A mathematical theory is developed to examine more closely the approximations involved in both basic range/Doppler processing and SRC and to explore alternate SRC implementations. The theory is used to derive the SRC algorithm as a matched filter directly from the point target response model rather from the simplified range compressed response used by Jin and Wu. Two new discrete implementations (azimuth SRC and range SRC) are developed for both single-look and multilook processing. In addition two new alternate methods of multilook SRC are presented : fixed SRC and look-dependent SRC. The sensitivity of the SRC algorithms to parameter errors is investigated. Extensive simulations are developed to quantify the image quality produced by each algorithm for a variety of processing parameters. The simulation results with nominal RADARSAT parameters show that the SRC algorithms can significantly extend the range of squint angles which can be processed with the range/Doppler type of algorithm.

In this paper, an extended step transform algorithm (EST) that is capable of focusing SAR data at extremely high squint angles is presented. To avoid the large walk that is inherent in high squint data, the EST algorithm removes this linear term in its envelope by taking data along the linear range term line. However, by doing this, targets of different ranges may fall onto the same range cell. To solve the range focus problem, a reference function of varying chirp rate to deramp the input signal is used to accommodate the change in the azimuth chirp rate along each range cell. Suitable phase compensation is also necessary to correct for the residual phase error. The EST has been implemented on a SUN workstation and used to process simulated data. The results of the simulations at 55 degrees squint angle are presented.

The wavenumber-domain SAR algorithm is studied for the case of a high squint angle with particular application to runway imaging from an approaching aircraft. The feasibility of using the algorithm for this application is shown by point target simulations which indicate that aberrations due to the high squint angle can be eliminated with finer interpolation in the processing.