Xiaolan Qiu

An Improved NLCS Algorithm With Capability Analysis for One-Stationary BiSAR

This paper deals with the imaging problem of one-stationary bistatic SAR (BiSAR) with large bistatic angle. An improved nonlinear chirp scaling (NLCS) algorithm is proposed for this BiSAR. The main work here includes three aspects. First, a range chirp scaling function for correcting the differential range cell migration correction is derived. Then, the azimuth perturbation is generated by local fit method, which makes the NLCS algorithm suitable for the large bistatic angle case. Furthermore, the negative effects introduced by the perturbation (including phase error and locality error) are discussed, and some compensation methods are proposed to enhance the capability of the algorithm. The simulating results exhibited at the end of this paper validate the correctness of the analysis and the feasibility of the algorithm.

Some Reflections on Bistatic SAR of Forward-Looking Configuration

Forward-looking imaging has many potential applications, but it is impossible with the usual monostatic synthetic aperture radar (SAR) principle. Through the bistatic SAR configuration, forward-looking imaging can be realized for one of the bistatic platforms. This letter designs a bistatic configuration with a stationary transmitter and a forward-looking airborne receiver. It then analyzes the 2-D resolution and finds out which geometric parameter affects the imaging ability mostly. Besides, it gives out the signal formulation in the frequency domain and shows its imaging characteristics. Then, an imaging method is chosen for this special configuration, and the simulation results are exhibited, which validate the correctness of the analysis and prove the 2-D imaging ability of forward-looking bistatic SAR.

A Novel Motion Parameter Estimation Algorithm of Fast Moving Targets via Single-Antenna Airborne SAR System

A novel parameter estimation algorithm of fast moving targets using single-antenna airborne synthetic aperture radar (SAR) databased on desampling and Radon transform (RT) is introduced in this letter. First, the dual-channel data are constructed by desampling the single-antenna airborne SAR data in the azimuth direction. Then, the clutter and the spectrum aliasing of the moving target can be cancelled by coherent subtracting. As a result, the moving target trajectory exhibits a single curve in both the range-compressed and the range-Doppler domains. Second, range cell migration correction is adopted to eliminate the range curve and parts of the range walk. Owing to the Doppler ambiguity, the moving target trajectory becomes a straight line. Third, the desampled Doppler ambiguity number and the Doppler rate of the moving target can be calculated by the slope of the line, which is measured by RT. Finally, along- and across-track velocities of the moving target are further obtained. The effectiveness of the proposed scheme is validated by the simulated and real data.

Focusing of Medium-Earth-Orbit SAR With Advanced Nonlinear Chirp Scaling Algorithm

The signal processing of the medium-Earth-orbit synthetic aperture radar (SAR) is more challenging than that of the current low-Earth-orbit SAR because the imaging geometry is more complicated, and the range and azimuth variances are more severe. This paper deals with these imaging problems in three aspects. First, an advanced hyperbolic range equation (AHRE) is proposed for the first time, which is more precise for a spaceborne SAR than the conventional hyperbolic range equation (CHRE). Second, the point target spectrum based on the AHRE is analytically derived, which is useful for developing efficient SAR processing algorithms. Third, the well-known nonlinear chirp scaling (NLCS) algorithm is modified according to this new spectrum, and the so-called AHRE-based advanced NLCS (A-NLCS) algorithm is established. The simulation results validate the correctness of our method for L-band SAR systems at altitudes from 1000 to 10000 km with an azimuth resolution around 3 m. It is also shown that the A-NLCS algorithm has better performance than the CHRE-based algorithms in longer integration time cases. Therefore, we recommend the A-NLCS algorithm for a spaceborne SAR with a lower frequency, finer resolution, and higher satellite altitude.

A Bistatic SAR Raw Data Simulator Based on Inverse

A synthetic aperture radar (SAR) raw data simulator is an important tool for testing the system parameters and the imaging algorithms. In this paper, a scene raw data simulator based on an inverse ω-k algorithm for bistatic SAR of a translational invariant case is proposed. The differences between simulations of monostatic and bistatic SAR are also described. The algorithm proposed has high precision and can be used in long-baseline configuration and for single-pass interferometry. Implementation details are described, and plenty of simulation results are provided to validate the algorithm.

\omega{-}k

This paper first shows the 3D property of bistatic synthetic aperture radar (BiSAR) geometry, which clarifies that the algorithms for bistatic SAR should be deduced in 3D space. It then models the bistatic echo according to the 3D geometry and obtains the signal spectrum in the wavenumber domain. Based on the spectrum, the formula for the wavenumber-domain interpolation of the omega-K algorithm is deduced, and the residual phase is obtained. Then, the impacts of the residual phase, including position displacement, range, and azimuth defocusing, and a constant phase for each pixel, are explicated. Finally, the simulating results exhibited at the end of this paper validate the correctness of the analysis and the feasibility of the algorithm.

Algorithm

This study aims to detect vessels with lengths ranging from about 70 to 300 m, in Gaofen-3 (GF-3) SAR images with ultrafine strip-map (UFS) mode as fast as possible. Based on the analysis of the characteristics of vessels in GF-3 SAR imagery, an effective vessel detection method is proposed in this paper. Firstly, the iterative constant false alarm rate (CFAR) method is employed to detect the potential ship pixels. Secondly, the mean-shift operation is applied on each potential ship pixel to identify the candidate target region. During the mean-shift process, we maintain a selection matrix recording which pixels can be taken, and these pixels are called as the valid points of the candidate target. The l1 norm regression is used to extract the principal axis and detect the valid points. Finally, two kinds of false alarms, the bright line and the azimuth ambiguity, are removed by comparing the valid area of the candidate target with a pre-defined value and computing the displacement between the true target and the corresponding replicas respectively. Experimental results on three GF-3 SAR images with UFS mode demonstrate the effectiveness and efficiency of the proposed method.

An Omega-K Algorithm With Phase Error Compensation for Bistatic SAR of a Translational Invariant Case

Multichannel synthetic aperture radar (SAR) is a breakthrough given the inherent limitation between high-resolution and wide-swath (HRWS) faced with conventional SAR. This paper aims to obtain unambiguous imaging of static scenes and moving targets with the first Chinese dual-channel spaceborne SAR sensor. We propose an integrated imaging scheme with the dual-channel echoes. In the imaging scheme, the subspace-based error estimation algorithm is first applied to the spaceborne multichannel SAR system, followed by the reconstruction algorithm prior to imaging. The motion-adapted reconstruction algorithm for moving target imaging is initially achieved with the spaceborne multichannel SAR system. The results exhibit an effective suppression of azimuth ambiguities and false targets with the proposed process. This paper verifies the accuracy of the subspace-based channel error estimator and the feasibility of the motion-adapted reconstruction algorithm. The proposed imaging process has prospects for future HRWS SAR systems with more channels.

Fast Vessel Detection in Gaofen-3 SAR Images with Ultrafine Strip-Map Mode

Target recognition in synthetic aperture radar (SAR) images has become a hotspot in recent years. The backscattering characteristic of target is a significant issue taken into consideration in SAR applications. Almost all of the previous work focus on the scatter point extraction to depict the backscattering characteristic of the target; however, a point-target corresponds to a region rather than a single point due to the convolution during the imaging. Based on this fact, we first analyze the extent to how a point-target spreads, then propose a novel scatter cluster extraction (SCE) method, and utilize the scatter cluster as the feature to solve the airplane recognition problem in SAR images. In practice, there often exist interfering objects near the target to be classified. To overcome this issue, we design a reweighted sparse representation (RSR)-based automatic purifying method by assigning a weight to each element of the feature iteratively according to the representation error. Since the element with large representation error always corresponds to the interfering objects, we give it a small weight, consequently suppressing the influence of the interference. Experimental results demonstrate that the proposed SCE method outperforms the traditional scatter point extraction-based method as well as some state-of-the-art methods. The comparison result also validates the effectiveness of the proposed RSR method.

Unambiguous Imaging of Static Scenes and Moving Targets with the First Chinese Dual-Channel Spaceborne SAR Sensor

GF-3 is the Chinese Synthetic Aperture Radar (SAR) satellite mission with scientific and commercial applications, which will be launched in 2016. It allows operation in Stripmap, ScanSAR and Sliding Spotlight modes in different polarizations. The Sliding Spotlight Mode is a new imaging mode widely used in high resolution SAR imaging. Considering the imaging processing model, we have modified the equivalent parameters on the base of the Range-Doppler model to improve the geometric accuracy for the sliding spotlight mode of GF-3. The simulation and TerraSAR-X experimental results show the the validity of the method.