Daoxiang An

Extended Nonlinear Chirp Scaling Algorithm for High-Resolution Highly Squint SAR Data Focusing

In this paper, an extended nonlinear chirp scaling (ENLCS) algorithm for focusing synthetic aperture radar data acquired at high resolution and highly squint angle is proposed. The whole processing of the ENLCS consists of the following three steps. First, a linear range walk correction is used to remove the linear component of target range cell migration (RCM) and to mitigate the range-azimuth coupling of the 2-D spectrum. Second, a bulk second range compression (SRC) is performed in the 2-D frequency domain for compensating the residual RCM, SRC term, and higher order range-azimuth coupling terms. Third, a modified azimuth NLCS (ANLCS) operation is applied to equalize the azimuth frequency modulation rate for azimuth compression. By adopting higher order approximation processing and by properly selecting the scaling coefficients, the proposed modified ANLCS operation has better accuracy and little image misregistration. The overall focusing procedure of the ENLCS algorithm only involves fast Fourier transform and complex multiplication, which means easier implementation and higher efficiency. The experimental results with simulated data prove the effectiveness of the proposed algorithm.

Extended Two-Step Focusing Approach for Squinted Spotlight SAR Imaging

An extended two-step focusing approach (ETSFA) for processing the squinted spotlight synthetic aperture radar (SAR) data is proposed in this paper. The effect of the squint angle on the azimuth coarse focusing is analyzed and discussed. Based on the analysis results, a nonlinear shift preprocessing method is introduced, which can completely remove the squint angle impacts on the azimuth coarse focusing. Furthermore, based on the squinted spotlight SAR imaging model and the preprocessed echo data, derivations of the azimuth coarse focusing with the deramping-based technique and precise focusing with the modified Stolt-based technique are carried out in detail. Moreover, to produce an acceptable image by the proposed ETSFA for high-resolution (<; 3 m) squinted spotlight SAR with large scene, a subscene processing method is introduced. The experimental results on simulated data prove the validity of the whole analysis and the proposed methods.

Performance Evaluation of Frequency-Domain Algorithms for Chirped Low Frequency UWB SAR Data Processing

This paper studies the performance of the frequency-domain algorithms (FDAs) for low-frequency ultra-wideband synthetic aperture radar (UWB SAR) data processing. First, a generalized theoretical derivation of the FDAs is presented from the viewpoint of SAR signal processing. The derivation not only provides a deeper understanding to the imaging principle of the extended Omega-K algorithm (EOKA), but also makes it compatible and comparable with the other FDAs. Second, the performance comparison on different FDAs is made based on theoretical analysis, simulation and experimental data. The comparison results show that the Omega-K algorithm (ωKA) has the highest imaging precision in the ideal case (i.e, no motion error), but its application is limited by the poor ability of compensating motion errors. In contrast, the EOKA and nonlinear chirp scaling algorithm (NCSA) have excellent performance on dealing with the motion error, but they can only be applied under specific preconditions. Besides, as cetner frequency gets lower, the fractional bandwidth and integration angle get larger, the imaging precision of NCSA greatly decreases, while the ωKA and EOKA still keep high precision.

Fast Time-Domain Imaging in Elliptical Polar Coordinate for General Bistatic VHF/UHF Ultra-Wideband SAR With Arbitrary Motion

In this paper, a fast time-domain imaging algorithm called bistatic fast factorized back projection algorithm (BFFBPA) is proposed for the general bistatic VHF/UHF ultra-wideband synthetic aperture radar. It cannot only accurately dispose the large spatial variant range cell migrations, serious range-azimuth coupling and complicated motion error, but also achieve the imaging efficiency similar to frequency-domain algorithms. It represents subimages in elliptical polar coordinate to reduce the computational load. The imaging geometry with arbitrary motion in this coordinate system is provided, and the bistatic back projection algorithm (BPA) is derived to provide a basis for the proposed BFFBPA. Considering motion errors, the more accurate sampling requirements for elliptical subimages is deduced to offer the near-optimum tradeoff between the imaging quality and efficiency, and the constraint of motion errors for acceptable sampling requirements is discussed. Based on this sampling requirement, the advantage of using elliptical subimage grids for this BFFBPA is analyzed. A phase error correction is performed to reduce the impact of phase errors caused by interpolations in the BFFBPA. The speed-up factor of this BFFBPA with respect to the bistatic BPA is derived. Simulation results and evaluations are given to prove the correctness of the theory analysis and validity of the proposed method.

Research on Spatial Resolution of One-Stationary Bistatic Ultrahigh Frequency Ultrawidebeam–Ultrawideband SAR Based on Scattering Target Wavenumber Domain Support

Compared with the traditional one-stationary bistatic narrowbeam-narrowband (NB) synthetic aperture radar (SAR), the coupling between the azimuth and range wavenumbers for the one-stationary bistatic ultrahigh frequency (UHF) ultrawidebeam-ultrawideband (UWB) SAR is much larger due to the large fractional signal bandwidth and wide antenna beamwidth, which may affect the behavior of spatial resolutions. Considering the wavenumber coupling, the more accurate spatial resolutions for the one-stationary bistatic UHF UWB SAR are proposed based on the wavenumber domain support of the scattering target in this paper. First, the one-stationary bistatic SAR imaging geometry is provided, and then the spatial wavenumber of the scattering target for the one-stationary bistatic UHF UWB SAR is analyzed. Second, based on the spatial wavenumber spectrum, spatial resolutions of the one-stationary bistatic UHF UWB SAR are derived in detail. In addition, the squint angles of radars are considered in the derivation of the spatial resolutions. Besides, the narrowing/broadening factors defined as the ratio of the proposed spatial resolutions to traditional spatial resolutions are presented, which describe the effects of the fractional bandwidth and associated integration angle on the spatial resolutions. Finally, simulation results are given to prove the correctness and validity of the proposed resolutions.

Fast Factorized Backprojection Algorithm for One-Stationary Bistatic Spotlight Circular SAR Image Formation

In this paper, a fast factorized backprojection (FFBP) algorithm is proposed for the one-stationary bistatic spotlight circular synthetic aperture radar (OS-BSCSAR) data processing. This method represents the subimages on polar grids in the slant-range plane instead of the ground plane, which can be accurately referenced to the tracks of both transmitter and receiver. It can not only accurately accommodate the complicated circular track including the motion error, scene topographic information, large spatial variances and significant range-azimuth coupling of the echo data, but also improve the imaging efficiency compared with the backprojection (BP) algorithm. First, OS-BSCSAR imaging geometry is provided, and then the bistatic BP algorithm for the OS-BSCSAR imaging is derived to provide a basis for the proposed FFBP algorithm. Second, based on the subaperture imaging model, the polar grids for calculating the subimages are defined, and the sampling requirements for the polar grids are derived from the viewpoint of the bandwidth, which can offer the near-optimum tradeoff between the imaging quality and the imaging efficiency. Finally, implementation and computational burden of the proposed FFBP algorithm is discussed, and then the speed-up factor of the proposed FFBP algorithm with respect to the bistatic BP algorithm is derived. Experiment results are given to prove the correctness of the theory analysis and validity of the proposed FFBP algorithm.

An Efficient Minimum-Discontinuity Phase-Unwrapping Method

Phase unwrapping (PU) is significant in reconstructing the digital elevation model of a scene from its interferometric synthetic aperture radar (InSAR) data. The classical minimum discontinuity (MD) PU algorithm by Flynn is highly accurate but involves time-consuming computation. To overcome this shortcoming, we proposed a preunwrapping-assisted MD (PAMD) PU method, which has high efficiency and the same unwrapping quality as Flynns algorithm. In the PAMD method, the phase image is first unwrapped by the preunwrapping algorithm, which generates and optimizes the dipole cuts. The preunwrapping algorithm is fairly fast, and the obtained result is much closer to the optimal solution of MD; therefore, it can be further optimized by Flynns algorithm efficiently. Tests on simulated and real InSAR data confirm the accuracy and the efficiency of the proposed method.

Phase Unwrapping for Large-Scale P-Band UWB SAR Interferometry

Phase unwrapping (PU) for large-scale images is a new challenging problem in reconstructing a digital elevation model from synthetic aperture radar interferometry (InSAR) data. In this letter, we proposed a region-partition-based PU method that could improve the efficiency and decrease the processing memory of the large-scale PU problem for P-band ultrawideband (UWB) InSAR. The interferometric phase is flattened by the reference phase, which is generated from the shift estimation in the registration step. We refer to the residual phase as the misregistration phase (MRP), which corresponds to the error of the shift estimation. The MRP is unambiguous in most of the area with high coherence because of the large fractional bandwidth; thus, the regions that assuredly have the unambiguous MRP are partitioned out from the MRP image and referred to as the high-quality area. Meanwhile, the remaining low-quality areas are separated by the high-quality areas and are considered irregular tiles. After unwrapping the tiles by a minimum discontinuity PU algorithm either in parallel or in series, the full-size unwrapped result is obtained. The test performed on real P-band UWB InSAR data confirms the effectiveness and efficiency of our method.

A Backprojection-Based Imaging for Circular Synthetic Aperture Radar

Circular synthetic aperture radar (CSAR) has attracted much attention in the field of high-resolution SAR imaging. However, the CSAR image focusing is affected by the motion deviations of platform. In the processing of experimental CSAR data to deal with motion errors, the main way is using setup calibrators, which restricts its widespread applications. In this paper, based on the estimation of motion errors, an autofocus CSAR imaging strategy is proposed without using any setup calibrator. The first step is to split the entire aperture into several subapertures, the second step is to process the data in subapertures with an autofocus backprojection method, and the last step is to obtain the final CSAR image by merging the subimages obtained from the subaperture processing. The CSAR data processing results prove that the proposed strategy can remove the motion errors accurately and acquire well-focused CSAR images.

A fast back-projection algorithm for bistatic forward-looking low frequency UWB SAR imaging

Bistatic forward-looking low frequency ultrawideband synthetic aperture radar (UWB SAR) has the complicated range-azimuth coupling, which limits the application of frequency-domain algorithms. In this paper, an extended fast back-projection algorithm is presented to focus the bistatic forward-looking low frequency UWB SAR data. First, the imaging geometry and signal model are established. Then, the phase error of the fast back-projection (FBP) algorithm for processing the bistatic forward-looking SAR (BFSAR) is analyzed, and the implementation of the FBP algorithm is given. Finally, the simulation experiment is carried out to prove the validity of the proposed approach.