Processing bistatic forward-looking spotlight SAR data using fast backprojection algorithm

This study presents a fast backprojection algorithm (FBPA) for one-stationary bistatic circular synthetic aperture radar (OS-BCSAR) imaging. This method reconstructs an illuminated SAR scene in a ground plane instead of a slant-range plane. First, the imaging geometry of the OS-BCSAR is given. Then, based on the subaperture imaging model, the sampling requirements of the subimage polar grid are derived from the bandwidth angle. Finally, the implementation of the proposed FBPA is discussed. Simulation results are shown to demonstrate its validity.

The fast time-domain algorithm (FTDA) for the bistatic forward-looking synthetic aperture radar (BFSAR) imaging is presented in this paper. Here, the subimage is represented in the slant range plane instead of the ground plane. The FTDA includes the fast backprojection algorithm (FBPA) with the subaperture and elliptical polar grid processing, and the FBPA with the subaperture and polar grid processing. Experimental results are given to prove the validity of the proposed FBPA.

With appropriate geometry configurations, bistatic Synthetic Aperture Radar (SAR) can break through the limitations of monostatic SAR on forward-looking imaging. Thanks to such a capability, bistatic forward-looking SAR (BFSAR) has extensive potential applications. However, the current BFSAR imaging algorithms always involve the time-consuming interpolation to carry out range migration correction. This paper develops a Chirp Scaling type imaging algorithm, which just needs the complex multiplication and FFT, for BFSAR with two parallel moving stations. The simulation results validate the effectiveness of the method.

The bistatic SAR has forward-looking imaging capabilities if a opportunistic illuminator is selected to form the synthetic aperture. This would enable it to perform some inspection tasks that traditional SAR cannot perform. The system resolution of a bistatic SAR is related to its configuration. Due to the special configuration of the bistatic forward-looking SAR, the traditional system trajectory design method is no longer applicable. Therefore, a trajectory design method used for bistatic forward-looking SAR is proposed in this paper. First, the original trajectory of the receiver is determined according to the system which is always cannot be changed. Then, the slant range and the pitch angle of the transmitter are determined at each time. Next, the resolution of the transmitter under different squint angles at different times is calculated. Then, the available area of squint angle is obtained by drawing the results into a diagram. Finally, a position curve of the squint angle that is contained within the available area is found, and then the position of the transmitter and the resolution are calculated based on the position curve. The method can make the resolution of the forward-looking SAR system meet the requirements at all times under the condition that the changes to the original trajectory are at the minimum. The simulation results verify the validity of the proposed method.

In this article, a novel sinogram decimation fast back-projection (SD-FBP) algorithm for strip-map synthetic aperture radar (SAR) imaging is proposed. We first review the back-projection (BP) and the fast factorized BP (FFBP) algorithms, then analyze the cause of the FFBP imaging degradation theoretically based on the combination of SAR imaging and the sinogram concept of computed tomography, illustrating that the degradation is much more severe for strip-map mode. Based on the SAR sinogram, the SD-FBP algorithm is proposed to mitigate the FFBP degradation for strip-map SAR imaging. The complexity of the SD-FBP is analyzed and compared with that of the FFBP and the Cartesian factorized BP (CFBP). The superiority of the SD-FBP algorithm is validated using the simulation and real spaceborne strip-map SAR data, e.g., Sentinel-1, Gaofen-3, and LuTan-1. The results show that the SD-FBP can tackle the FFBP imaging degradation for strip-map SAR, and runs faster than the FFBP and the CFBP.

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.

Left/right ambiguity and low angular (azimuth) resolution are severe problems for monostatic forward-looking SAR imaging. It is strongly believed that these technical issues can definitely be solved with bistatic forward-looking SAR. The analysis presented in this paper points out that the left/right ambiguity problem still exits. However, an appropriate selection of the position of bistatic base line and antenna beamwidth allows us to conceal it. The paper also gives some recommendations which can be considred for the forward-looking SAR imaging.

Bistatic forward-looking SAR is a synthetic aperture radar system with an independently moving transmitter and receiver breaks through the limitations of the conventional monostatic SAR on the forward-looking imaging. However, the existence of a double hyperbola in the bistatic range equation makes the severe two-Dimension (2-D) coupling. Based on the bistatic SAR space geometrical model and the point target spectrum form the series reversion, this paper introduces the result of bistatic forward-looking SAR raw data processed using polynomial NCS algorithm is given to confirmed the applicability of mentioned algorithm. Furthermore, an analysis about the SAR image feature is discussed, then a produce is defined to slove the problem which may reduce the SNR of bistatic SAR is proposed.

Due to the variation of target electromagnetic characteristics with the bistatic synthetic aperture radar (BiSAR) observation angle and frequency, extended targets often appear as multiple discrete points in the imaging results, leading to a loss of structural information. Existing studies have not yet addressed this phenomenon, which can significantly impact the subsequent interpretation and analysis of BiSAR images. To clarify the imaging characteristics of BiSAR, this paper analyzes the morphology and distribution of targets in BiSAR imaging results, considering the electromagnetic scattering characteristics of typical targets, such as linear targets, plates, and dihedrals. Initially, the two-dimensional wavenumber relationship of typical targets is applied to directly derive the imaging morphology and distribution of linear targets in the BiSAR configuration through the wavenumber mapping of the envelope and phase. Next, using the imaging results of the linear target and applying a first-order Taylor approximation for wavenumber inversion, the imaging morphology and distribution of plates and dihedrals are derived. To improve the accuracy of the model in describing dihedrals, we update the scattering model of dihedrals under the BiSAR configuration, building upon the existing model. Additionally, by considering the coupling relationship between target size, BiSAR configuration, and imaging results, the variations in morphology and distribution of these three typical targets with configuration and target size are explored. Furthermore, based on the imaging characteristics of extended targets, the structural information of the targets is estimated, resulting in structured imaging outputs. Numerical simulations are conducted to validate the accuracy of the derived imaging results and characteristics. This paper establishes the relationship between the BiSAR imaging principles and target scattering characteristics, and, for the first time, analyzes the morphology and distribution of extended targets in BiSAR imaging results. These insights enhance the interpretability of BiSAR images and lay the groundwork for more refined image interpretation.

Forward-looking imaging has extensive potential applications, such as self-navigation and self-landing. By choosing proper geometry, bistatic synthetic aperture radar (BiSAR) can break through the limitations of monostatic SAR on forward-looking imaging and provide possibility of the forward- looking imaging. In this special bistatic conflguration, two problems involving large range cell migration (RCM) and large range-azimuth coupling are introduced by the forward-looking beam, which make it di-cult to use traditional data focusing algorithms. To address these problems, a novel Omega-K algorithm based on two-dimensional non-uniform FFT (2-D NUFFT) for translational variant (TV) bistatic forward-looking SAR (BFSAR) imaging is proposed in this paper. In this study, we derive an accurate spectrum expression based on two-dimensional principle of stationary phase (2-D POSP). 2-D NUFFT is utilized to eliminate the range-variant term, which can make full use of the data and improve the computational e-ciency as well. The experimental results, presented herein, demonstrate the efiectiveness and advantages of the proposed algorithm.

Synthetic aperture radar (SAR) raw data simulator is an important tool for parameter-optimizing and algorithm-testing, particularly for those complicated configurations in which real raw data is difficult to obtain. As a new and special imaging mode, bistatic forward-looking SAR with constant acceleration (BFCA-SAR) can perform two-dimensional imaging for targets in the straight-ahead position over mono-static SAR. But there exist more complicated square roots and high-order terms in range history owing to high velocities and accelerations from both platforms. In addition, space variances in phase terms of two-dimensional frequency spectrum (2-D FS) make it difficult to gain echo data accurately. In this paper, a fast scene raw data simulator for BFCA-SAR based on quantitative analysis and effective correction of phase space variance is proposed. With high precision, our method can generate raw data more efficiently than traditional algorithms.

Bistatic forward-looking synthetic aperture radar (BFL-SAR) is a kind of bistatic SAR system that can image forward-looking terrain in the flight direction of an aircraft. Until now, BFL-SAR imaging theories and methods have been researched for stationary targets. Unlike the stationary target, the motion of a ground-moving target (GMT) induces unknown range cell migration and additional modulation of the azimuth signal. Thus, to finely image the GMT, one must obtain its velocity parameters accurately, but they are usually unknown. In this paper, a novel GMT imaging and velocity estimation method, which is based on mismatched compression, is proposed for BFL-SAR without a priori knowledge of the GMT's velocity parameters. The main idea behind mismatched compression is to use a presumed azimuth reference function for performing correlated operation with the azimuth signal of the GMT. In general, the Doppler parameters of the presumed azimuth reference function are different from those of the GMT's azimuth signal because the velocity parameters of the GMT are unknown. Therefore, the correlation operation referred to earlier is actually mismatched compression, and the resulting image is shifted and defocused. The shifted and defocused image is utilized to get the real Doppler and velocity parameters of the GMT. The advantage of this method is that not only the GMT can be well focused but also the GMT's velocity can be simultaneously obtained. In addition, this method needs only monochannel antenna. The proposed BFL-SAR GMT imaging and velocity estimation method is validated by numerical simulations.

Fractional Fourier transform (FrFT) is a generalized form of the well-known Fourier Transform that offers superior detection and recognition of chirp signals. In this paper, a chirp scaling algorithm (CSA) for forward-looking linear-array synthetic aperture radar (FLLA-SAR) imaging is presented using the FrFT based on an equivalent squint model. Simulated results demonstrate the validity and the efficiency of the proposed algorithm.

The Geosynchronous Satellite Synthetic Aperture Radar (GEO SAR) operates at a high orbital altitude, resulting in an extended imaging time and substantial variations in slant range. Additionally, the GEO SAR satellite orbit experiences a bending effect, and the target’s movement, caused by the Earth’s rotation, is influenced by the Earth’s curvature. The back-projection (BP) algorithm has been proven to be a highly effective technique for precise imaging with GEO SAR by processing these specific echo signals. However, this approach necessitates considerable computational resources. Existing BP algorithms, such as the fast BP algorithm, do not consider the “Stop-and-Go” error present in GEO SAR. Consequently, we developed a Precise Slant Range Model that considers the motion of both the satellite and targets. The model incorporates velocity and acceleration factors to accurately represent the signal transmission from transmission to reception. Additionally, we propose a Rapid Range History Construction Method to lessen the computational burden of generating the three-dimensional range history array. By utilizing the Precise Slant Range Model and the Rapid Range History Construction Method, and employing parallel processing through aperture segmentation, we propose an Accurate and Efficient BP imaging algorithm suitable for GEO SAR applications. To validate its effectiveness, simulations were conducted using the parameters of a GEO SAR system. The results indicated that the proposed algorithm enhances the imaging quality of GEO SAR, reduces the processing time, and achieves high-precision rapid imaging, thereby improving operational efficiency.

The bistatic configuration of frequency-modulated continuous wave (FMCW) synthetic aperture radar (SAR) offers considerable capabilities, reliability and flexibility in designing the compact size and low cost remote sensing systems. Furthermore, the spacial separation between transmitter and receiver avoids the antenna isolation problem. A signal model is proposed here to accurately formulate the bistatic slant range history in FMCW SAR configuration, where the motion during the long pulse duration is taken into account. Moreover, based on the preceding model, an optimization approach is presented to deal with the double square root (DSR) term, and thus an accurate FMCW bistatic point target reference spectrum (BPTRS) is derived. The proposed spectrum can not only address the signal characteristics of bistatic FMCW SAR, but also significantly simplify the bistatic FMCW SAR processing, which will really push the applications of FMCW SAR in the remote sensing field. Furthermore, based on the proposed signal model, the application of an improved range-Doppler algorithm (RDA) is further proposed to focus the bistatic FMCW SAR data. Simulation experiments are carried out to verify the accuracy of this novel formulation and highlight the performance of the proposed focusing approach.