One-stationary Bistatic forward-looking SAR imaging based on improved nonlinear chirp scaling algorithm

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.

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 nonlinear chirp scaling (NLCS) algorithm is proposed to process data of highly squinted, high-resolution, missile-borne synthetic aperture radar (SAR) diving with a constant acceleration. Due to the complex diving movement, the traditional signal model and focusing algorithm are no longer suited for missile-borne SAR signal processing. Therefore, an accurate range equation is presented, named as the equivalent hyperbolic range model (EHRM), which is more accurate and concise compared with the conventional fourth-order polynomial range equation. Based on the EHRM, a two-dimensional point target reference spectrum is derived, and an extended NLCS algorithm for missile-borne SAR image formation is developed. In the algorithm, a linear range walk correction is used to significantly remove the range-azimuth cross coupling, and an azimuth NLCS processing is adopted to solve the azimuth space variant focusing problem. Moreover, the operations of the proposed algorithm are carried out without any interpolation, thus having small computational loads. Finally, the simulation results and real-data processing results validate the proposed focusing algorithm.

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.

A disadvantage of monostatic SAR is that it cannot image the forward-looking area. In this paper, the possibility of forward-looking imaging with bistatic configuration is verified using DVB-T signals as transmitting signals. The Back Projection (BP) method is used to perform the imaging in each configuration and the point spread functions (PSFs) are presented. The forward-looking imaging ability is analyzed in terms of the resolution for each configuration. From the simulation, we realized forward-looking imaging with bistatic Radar in configuration 3, that is, the squint angle has been reduced to zero. The result show that bistatic forward-looking imaging is feasible and the resolution can achieve the requirement.

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.

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.

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.

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.

ABSTRACTBistatic synthetic aperture radar (SAR) is a new type of SAR system which is capable of achieving multi‐angle target detection and flexible configuration. For spaceborne bistatic SAR, when operating at low frequencies, it is susceptible to the ionosphere. The ionospheric Faraday rotation effect will cause a rotation of polarisation plane of electromagnetic waves, leading to amplitude and phase errors between different polarisation channels of SAR. This effect can be compensated by estimating the Faraday rotation angle. Currently, Faraday rotation angle estimation methods for spaceborne SAR are all applicable to monostatic SAR. However, due to the different signal propagation paths between bistatic SAR and monostatic SAR, the Faraday rotation angle estimation method differs for bistatic SAR. This paper proposes a Faraday rotation angle estimation method for bistatic spaceborne SAR, which is applicable to the alternating bistatic mode. The effectiveness of the method is verified through simulation experiments and the performance of the method is analysed. Under different signal‐to‐noise ratios (SNR) and filtering window sizes, the estimation error range is 0.02°‐0.26°.

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 deals with above problems with the Second-Keystone and the nonlinear chirp scaling (NCS) algorithm. The results of simulation verify the feasibility of the method.

The spaceborne stripmap range sweep synthetic aperture radar (SS-RSSAR) is a new concept spaceborne SAR system that images the region of interest (ROI) with ROI-orientated strips, which, unlike the traditional spaceborne SAR, are allowed to be not parallel with the satellite orbit. The SS-RSSAR imaging is a challenging problem because echoes of a wide region have strong spatial varieties, especially in high-squint geometries, and are hard to be focused by a single swath. The traditional imaging algorithms could solve this problem by cost-ineffectively dividing an ROI into many subswaths for separate processing. In this paper, a new wide nonlinear chirp scaling (W-NLCS) algorithm is proposed to efficiently image the SS-RSSAR data in a single swath. Comparing with the traditional nonlinear chirp scaling algorithm, the W-NLCS algorithm is superior in three major aspects: the nonlinear bulk range migration compensation (RMC), the interpolation-based residual RMC, and the modified azimuth frequency perturbation. Specifically, the interpolation for the residual RMC, the most significant step in achieving the wide-swath imaging performance, is made innovatively in the time domain. The derivation of the W-NLCS algorithm, as well as the performance analyses of the W-NLCS algorithm in aspects of the azimuth resolution, accuracy, and complexity, are all provided. The presented approach is evaluated by the point target simulations.

Radio Frequency Interference (RFI) is a growing problem in Synthetic Aperture Radar (SAR). The choice of operational frequency of remote sensing instruments is dictated by the physics of the parameters that are to be observed. On the other hand, the frequency spectrum becomes more crowded with the increasing demand for wireless services. This requires cohabitation of multiple systems at the same or neighbouring frequency bands. As a result, RFI can introduce artefacts and degrade the derived SAR products. A mitigation of RFI is critical. New removal techniques can be implemented with Digital Beamforming (DBF) radars. In this paper, we present a Pulse and Range-Dependent Time Minimum Variance Distortionless Response (PRDTMVDR) Beamformer. The antenna pattern (AP) is adaptively changed for each pixel based on the inherent imaging geometry of SAR and thus the RFI suppression is improved compared to an AP that is fixed for the pulse duration.

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.

Bistatic Synthetic Aperture radar (BiSAr) is named for its unique separated transmitting and receiving platforms, which has the advantages of high security, flexible configuration, and strong information acquisition capabilities, and has broad application prospects. Aiming at the problem that the performance of traditional BiSAr imaging algorithms degrades sharply when facing complex squint and space-variant scenes, this paper proposes a Near-range Wide-Swath imaging algorithm for NP BiSAr based on a three-dimensional ellipsoid model. Firstly, a range pre-processing operation is used to eliminate the range cell migration (rCM) space-variant error introduced by the squint angle. Then, aiming at the high-order rCM residual amount after processing, a correction method based on a three-dimensional ellipsoid model is explored, and the improvement in accuracy of the three-dimensional ellipsoid model is verified by comparing the simulation with the second-order phase error of the traditional ellipsoid model. In response to the consistency requirement of Doppler parameters during azimuth compression, the three-dimensional ellipsoid model is combined into the Extended Frequency Nonlinear Chirp Scaling (EFNLCS) algorithm with residual Doppler centroid correction capability, which eliminates the residual Doppler centroid while improving the space-variant equalization capability of Doppler phase parameters. Finally, simulation experiments verify that the algorithm has better focusing processing capability and better applicability to near-range wide-swath scenes.