Range cell migration correction for bistatic SAR image formation

A novel synthetic aperture radar (SAR) constellation called Constellation of Geostationary and Low Earth Orbit SAR (ConGaLSAR), including one geostationary illuminator and several low Earth orbit CubeSat transponders, was previously proposed with outstanding revisiting. The MirrorSAR technology used in transponder significantly simplifies the satellite; however, it brings complexity to the imaging geometry, resulting in failures of the traditional bistatic imaging method. The introduction of probe signals achieves accurate measurement of the additional transponding distance. In this article, a three-step imaging method is proposed to realize accurate imaging processing aiming at the transponding mechanism and wide swath observation: 1) linear time variation for range cell migration grid correction; 2) second range compression for the additional squint angle introduced in the previous step; and 3) range cell migration correction in the range-Doppler domain for nonuniform cell grid caused by bistatic SAR (BiSAR). Digital simulations and real BiSAR data experiments are conducted to verify the effectiveness and accuracy of the algorithm. Moreover, the influence of the elevation is analyzed, confirming its excellent tolerance of elevation errors under the condition of a 3-m resolution and a 40-km swath width, which conforms to the system design of ConGaLSAR.

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 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.

Geosynchronous synthetic aperture radar (GEO SAR) data focusing is a more challenging and difficult task than the low earth orbit (LEO) SAR due to the strong 2-D coupling of echo signal induced by the orbital trajectory curvature. The range cell migration (RCM) in the GEO SAR configuration is space variant in both range and azimuth directions, hence standard RCM correction (RCMC) functions developed for LEO SAR are inadequate for GEO. In this letter, a curved trajectory model is proposed, taking into consideration the impacts of "stop-and-go" assumption. Based on the range model, a new data transform is derived to deal with the complicated coupling in GEO SAR. From the derivation, we find that the original Stolt mapping is a special case of the proposed "generalized Omega-k" algorithm. In comparison with the original Omega-k algorithm, this new algorithm can correct more complicated RCM effectively. Finally, simulation results show that the proposed imaging algorithm performs well for large scene focusing in an L-Band GEO SAR system.

An effective focusing approach for azimuth invariant bistatic SAR processing

A space-variant chirp scaling algorithm based on the range cell migration (RCM) equalization and azimuth subband synthesis has been studied to process simulated geosynchronous synthetic aperture radar (GEO-SAR) data. The acceptable order of terms in polynomials for the slant range models in the RCM correction and phase error compensation, division of subband, and suppression of grating lobes of the subbands was investigated. Qualitatively and quantitatively, the method was able to focus simulated GEO-SAR signals well. Finally, the constraint on the spatial extent of azimuth and range dimensions using the algorithm was assessed.

This paper proposes a new ground moving target imaging algorithm for high-resolution airborne circular stripmap synthetic aperture radar (CSSAR)-ground moving target indication (GMTI) systems. In the proposed algorithm, the range cell migration correction is performed in the 2D frequency domain via a phase multiplication. The azimuth compression is performed in the range Doppler domain. A key step of the proposed algorithm is to utilize an iterative strategy to achieve an accurate correction of the range cell migration. This step makes the proposed algorithm work well for high-resolution CSSAR-GMTI systems. Numerical simulations are conducted to validate the proposed algorithm.

Based on the principle of planar wavefront assumption, a novel point of view in processing the spotlight synthetic aperture radar (SAR) data is proposed in this letter. After match filtering and motion compensation to the scene center, from the perspective of range cell migration correction (RCMC), two scaling transforms (a range-frequency scaling with a subsequent azimuth-time scaling) are proposed to correct the range cell migration caused by the coupling between the range-frequency and azimuth-time. Moreover, to reduce the spectrum loss (the loss of image resolution), two constant scaling factors are introduced to optimize the range-frequency and azimuth-time scaling transforms, which enable the presented algorithm to realize the flexible and efficient imaging ability. Simulation results are conducted to validate the efficacy of this algorithm.

This paper investigates the imaging problems in squint beam-steering synthetic aperture radar (SBS-SAR) mounted on high-speed platforms with constant acceleration. The cross-range-dependent range cell migration (RCM) is compensated by keystone transform (KT) and time domain RCM correction (RCMC). By derotation and phase compensation, the KT of Doppler folded signal is achieved without zero-padding. For azimuth processing, the signal is reconstructed by the nonlinear phase and range-dependent derotation. Then, the space-variant (SV) Doppler chirp rate is corrected by time domain azimuth nonlinear chirp scaling (ANCS). After frequency domain matched filtering, the full aperture signal is focused in the 2-D time domain. The algorithm is validated by simulated SAR data, including the evaluation of RCMC with KT, geometric correction, and the focusing performance.

Imaging approach based on range-Doppler algorithm for monostatic SAR is extended to bistatic case. Because of that the transmitter and receiver are located separately, the range cell migration phenomena of bistatic SAR is more complicated than that of monostatic case. In this paper, a simple method of range cell migration correction is used instead of interpolation operation in traditional range-Doppler algorithm for monostatic SAR. Point scatter response, range and azimuth resolutions are also discussed. The simulation results are presented.

Nonlinear RCMC method for spaceborne/airborne forward-looking bistatic SAR

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 imaging at high resolution and highly squint angle is confronted with several difficulties, such as the spatial variance of range cell migration (RCM) and the azimuth-dependent property of high-order terms. In order to address the above-mentioned problems, an improved non-linear chirp scaling algorithm based on keystone transform (KT) and linear range walk correction (LRWC) is proposed. First, KT is applied to correct the second-order RCM in range frequency and azimuth time domain. Then the spatial variance of linear RCM (LRCM) is analysed, and range–azimuth coupling is alleviated through LRWC, in this way, azimuth-variant RCM can be removed by phase multiplication, which avoids the approximate error of RCMC in two-dimensional frequency domain. Taking both the spatial variance of LRCM and the difference of the minimum bistatic slant range caused by LRWC into consideration, high-precision approximation of azimuth-dependent coefficients for azimuth signal are deduced, based on which the Doppler centroid, azimuth FM rate and cubic phase can be equalised by selecting proper perturbation coefficients. The proposed algorithm leads to a large depth of focusing in both range direction and azimuth direction. Simulation results prove the effectiveness of the proposed method.

This paper presents a bistatic focusing solution for bistatic forward-looking SAR (BFL-SAR) in translational invariant case. The approach is based on the spectrum which is derived by time domain equivalency. According to strong range cell migration (RCM) of BFL-SAR, high order phase terms compensation is added to the algorithm. Simulation results show that the extended SIFFT algorithm can focus the BFL-SAR signal.

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.