Focusing one-stationary bistatic circular SAR data using fast backprojection algorithm

This study presents a fast backprojection algorithm (FBPA) for bistatic forward-looking spotlight synthetic aperture radar (BFSSAR) imaging. It represents the subimages on the polar grids to reduce the computational burden. First, the backprojection algorithm for the BFSSAR imaging is introduced. Then, FBPA for the BFSSAR imaging based on the subaperture processing is provided, and the sampling requirements for the polar grids of subimages are derived from the bandwidth angle. 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.

Bistatic radar imaging can overcome limitations of monostatic radar imaging and obtain abundant target feature information; thus, it is followed with interest. Different from bistatic inverse synthetic aperture radar (Bi-ISAR) imaging, bistatic radar coincidence imaging (Bi-RCI) provides a new tack on the bistatic radar imaging technique. In this paper, a Bi-RCI based on multiple measurement vectors (MMV) for rotating cone-shaped targets is proposed to realize Bi-RCI coherent processing and improve imaging performance. Based on the mixed mode signals, a MMV parametric model is established and measurement number coarse selection is proposed. Finally, a modified sparse Bayesian learning (MSBL) algorithm is introduced to reconstruct the target image. Simulation results demonstrate the validity and the superiority of the proposed method.

Based on the analysis of the airborne bistatic synthetic aperture radar (SAR) imaging geometric mode, an extended nonlinear chirp scaling algorithm is employed to simulate and verify the imaging effect of the bistatic SARs. A gradient theory-based two-dimensional resolution bistatic SAR model is proposed, and the constraints of the multi-platform flight trajectory parameters meeting the imaging accuracy of the bistatic SAR are analyzed. Finally, through the bistatic SAR imaging simulation of cooperative flight trajectories under various situations, the spatial configuration constraint envelope between the flight vehicles to achieve the optimal resolution is revealed. The results of this paper will provide a theoretical reference for the SAR application in formation flight control.

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.

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.

In Radar remote sensing, the electromagnetic (em) modeling of scattering contributions from natural or man-made scatterers has often to deal with coupling scattering mechanisms due to the underlying reflecting surface (ground or water). For the monostatic configuration, this contribution is commonly associated with the so-called double bounce scattering mechanism, which is also known to be located in synthetic aperture radar (SAR) images at the vertical projection of the scatterer onto the ground. To explore this phenomenon in the more general case of bistatic SAR but also for triple bounce scattering, a new formalism using virtual scatterers is introduced in this paper. Based upon image theory and on far-field assumption, these virtual points are defined by simple interaction scatterers located on the reflecting surface, and involving the same traveling wave path than the considered scattering mechanism. Depending only on the incidence and scattering angles, elegant formulae in their simplicity are highlighted to construct these fictitious scatterers. The resulting reduction of multiple to simple interaction scattering mechanisms simplifies the finding of their location within bistatic SAR images, generalizing thereby migration effects associated to coupling mechanisms in the bistatic configuration.

The attitude variation of the platforms for airborne bistatic synthetic aperture radar (BiSAR) will cause the unsteadiness of the antenna phase center (APC) and the phase error of the echo due to inevitable installation error of the antenna, which will result in the spatial synchronization error. In this paper, the spatial synchronization error compensation based on attitude is proposed for BiSAR imaging. Compared to the conventional SAR imaging using R-D algorithm without concerning the spatial synchronization error, the compensation of the spatial synchronization error can produce higher quality images with reduced sidelobes and increased resolution. The simulation and experimental results demonstrate the effectiveness of the proposed method.

The paper introduces an algorithm for Ultrawideband Ultrawidebeam (UWB) bistatic Synthetic Aperture Radar (SAR). The algorithm works in time-domain and therefore inherits time-domain characteristics such as unlimited scene size, local processing and manageable motion compensation. The proposed algorithm is not limited by any configuration of bistatic SAR. The algorithm processes the UWB bistatic SAR data on a subaperture and subimage basis. This means, instead of backprojecting directly the SAR data to a ground image plane, the algorithm handles the data in two stages: beam forming and local backprojection. The algorithm is named Bistatic Fast Backprojection (BiFBP) and has been tested successfully with the simulated UWB bistatic SAR data.

Bistatic inverse synthetic aperture radar (ISAR) can help image the high-speed target for its advantage in tracking and more observation angle to provide more information than monostatic ISAR. However, the complex image geometry makes it difficult to achieve a clear image of the target with geometry distortion correction and calibration. Furthermore, high-speed motion will make the image blurred or defocussed. To address these problems, a bistatic ISAR (B-ISAR) imaging method for high-speed motion target with geometric distortion correction and calibration is proposed. According to the motion decomposition idea, we established the B-ISAR echo model of the high-speed motion target. Then, based on the range Doppler algorithm, we deduce the analytic formula of the geometric distortion factor and calibration factor, and transform the imaging problem into a parameter estimation problem. With the sparsity of the scattering points, the required parameters are solved using the expectation maximization algorithm based on the maximum a posteriori probability criterion. With the estimated parameters, a clear B-ISAR image of a high-speed motion target with geometric correction and calibration is obtained. The simulations show that the proposed method has better resolution and simultaneously attains geometric distortion correction and calibration of the image.

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 concept of a passive receiving small satellite flying in formation with an active satellite for bistatic Synthetic Aperture Radar (SAR) imaging has been the subject of numerous national and international studies. In 2013, ESA received an offer from the National Commission for Space Activities of Argentina (CONAE) to launch a small satellite with the SAOCOM-1b satellite, and to collaborate during the mission exploitation phase. The main SAOCOM-1b payload is an L-band polarimetric SAR instrument. This offer, provides a unique opportunity to the Earth observation science community to demonstrate at global scale the science capabilities of an L-band active-passive satellite constellation through dedicated in-orbit experiments and to secure valuable feedback within a short time frame on the scientific potential of L-band SAR mission constellations.

In an azimuth time-invariant configuration, the transmitter and receiver move with the same relative positions. This configuration simplifies the complicated bistatic synthetic aperture radar (SAR) imaging geometries and provides an opportunity to deduce the imaging algorithm. This chapter introduces several imaging algorithms, which can be categorized into three types: the first transforms the signal model in the bistatic case to that in the monostatic case; the second is based on an analytical spectrum, such as the two-dimensional (2D), inverse-scaled fast Fourier transform (FFT) and the bistatic chirp scaling algorithm; and the third is based on the numerical method and the typical approach is the NuSAR algorithm. The detailed derivation and processing procedure are shown for each algorithm. To show the effectiveness of each, some simulations or real images processed by the presented approach are given. It should be noted that each algorithm has its own merits and a pertinent algorithm should be chosen for specific system parameters.

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 authors consider Low Earth Orbited Satellite (LEOS) systems for personal communications. Their rather high power spectral density near the Earth surface could be used for aircraft detection. Bistatic radars that utilize communication signal transmitting from LEOS could operate in a passive mode. Moreover, the targets will be characterized by their bistatic radar cross-section (RCS) that could be 20 to 40 dB larger than the monostatic RCS. Another LEOS peculiarity is their high speed relative to the surface. This fact can be used for radar applications, namely: to design a bistatic synthetic aperture radar (SAR). Such a bistatic radar has a receiving subsystem near the acquisition scene (restricted usually by the horizon line) and the transmitter is the satellite transmitting communication signals. The direct communication line could be used for the heterodyne synchronization. It is important to mention, that a heterodyne signal should be formed that way in order to avoid or maximally reduce the Doppler frequency shift in the heterodyne circuit. In the present paper, using a particular example, the authors demonstrate the feasibility of this approach and evaluate parameters for system development. The example system demonstrates the feasibility of vessel detection at sea.