Pengbo Wang

A Novel Image Formation Algorithm for High-Resolution Wide-Swath Spaceborne SAR Using Compressed Sensing on Azimuth Displacement Phase Center Antenna

High-resolution wide-swath (HRWS) imaging with space- borne synthetic aperture radar (SAR) can be achieved by using az- imuth displacement phase center antenna (DPCA) technique. How- ever, it will consequently leads to extremely high data rate on satellite downlink system. A novel sparse sampling scheme based on compressed sensing (CS) theory for azimuth DPCA SAR was proposed, by which only a small proportion of radar echoes are utilized for imaging to re- duce data rate. The corresponding image formation algorithm for the proposed scheme was presented in the paper. The SAR echo signal of each channel can be reconstructed with high probability by using orthogonal matching pursuit (OMP) algorithm in Doppler frequency domain. The reconstructed echo signals of each channel are jointly processed by means of spectrum reconstructing fllter for compensat- ing Doppler spectrum aliasing resulting from non-uniform sampling in azimuth direction. The high quality SAR image can be obtained by us- ing chirp scaling algorithm. The efiectiveness of the proposed approach was validated by computer simulations using both point targets and distributed targets.

A High-Order Imaging Algorithm for High-Resolution Spaceborne SAR Based on a Modified Equivalent Squint Range Model

Two challenges have been faced in signal processing of ultrahigh-resolution spaceborne synthetic aperture radar (SAR). The first challenge is constructing a precise range model, and the second one is to develop an efficient imaging algorithm since traditional algorithms fail to process ultrahigh-resolution spaceborne SAR data effectively. In this paper, a novel high-order imaging algorithm for high-resolution spaceborne SAR is presented. First, a modified equivalent squint range model (MESRM) is developed by introducing equivalent radar acceleration into the equivalent squint range model, and it is more suitable for high-resolution spaceborne SAR. The signal model based on the MESRM is also presented. Second, a novel high-order imaging algorithm is derived. The insufficient pulse-repetition frequency problem is solved by an improved subaperture method, and accurate focusing is achieved through an extended hybrid correlation algorithm. Simulations are performed to validate the presented algorithm.

SAR Image Despeckling by Selective 3D Filtering of Multiple Compressive Reconstructed Images

A despeckling technique based on multiple image reconstruction and selective 3-dimensional flltering is proposed. Multiple SAR images are reconstructed from a single SAR image by employing compressive sensing (CS) theory. In order to obtain multiple images from single SAR image, multiple subsets of pixels are selected from input SAR image by imposing restriction that each subset has at least 20% difierent pixels from any other subset. These subsets are taken as measurement vectors in CS framework to obtain multiple SAR images. A despeckled image is obtained by employing selective 3-dimensional flltering to multiple reconstructed SAR image. The proposed technique is tested on single look complex TerraSAT-X data set, and experimental results exhibit that the proposed technique outperformed benchmark despekling methods in terms of visual quality and despeckling quality metrics.

A NOVEL THREE-STEP IMAGE FORMATION SCHEME FOR UNIFIED FOCUSING ON SPACEBORNE SAR DATA

Current advanced spaceborne synthetic aperture radar (SAR) systems may operate at multiple imaging modes, including conventional modes as stripmap, ScanSAR and spotlight, as well as the state-of-the-art SAR modes, e.g., sliding spotlight, TOPS (Terrain Observation by Progressive Scans) and inverse TOPS etc. A novel image formation scheme for unifled processing spaceborne SAR data was proposed, which signiflcantly simplifled complexity of SAR processor sub-system. The unifled-model-coe-cient (UMC) was deflned for modeling all SAR modes by means of analyzing both imaging geometry and time-frequency diagram corresponding to each imaging mode, respectively. The unifled mathematical formula for modeling all SAR modes echo signal was derived as a function of UMC. Consequently, a unifled image formation scheme for accurately focusing spaceborne SAR data in an arbitrary mode was proposed, which integrates all of SAR image formation procedures into a standard three-step processing framework, namely, de-rotation, data focusing and re-sampling, which evidently improve e-ciency and robustness of data processing sub-system. Computer simulation experiment results verify the efiectiveness of the proposed scheme.

Mitigation of Azimuth Ambiguities in Spaceborne Stripmap SAR Images Using Selective Restoration

A novel framework is proposed for mitigating azimuth ambiguities in spaceborne stripmap synthetic aperture radar (SAR) images. The azimuth ambiguities in SAR images are localized by using a local mean SAR image, SAR system parameters, and a defined metric derived from azimuth antenna pattern. The defined metric helps isolate targets lying at locations of ambiguities. The mechanism for restoration of ambiguity regions is selected on the basis of size of ambiguity regions. A compressive imaging technique is employed to restore isolated ambiguity regions (smaller regions of interconnected pixels), whereas clustered regions (relatively bigger regions of interconnected pixels) are filled by using exemplar-based inpainting. The simulation results on a real TerraSAR-X data set demonstrated that the proposed scheme can effectively remove azimuth ambiguities and enhance SAR image quality.

A Novel General Imaging Formation Algorithm for GNSS-Based Bistatic SAR.

Global Navigation Satellite System (GNSS)-based bistatic Synthetic Aperture Radar (SAR) recently plays a more and more significant role in remote sensing applications for its low-cost and real-time global coverage capability. In this paper, a general imaging formation algorithm was proposed for accurately and efficiently focusing GNSS-based bistatic SAR data, which avoids the interpolation processing in traditional back projection algorithms (BPAs). A two-dimensional point target spectrum model was firstly presented, and the bulk range cell migration correction (RCMC) was consequently derived for reducing range cell migration (RCM) and coarse focusing. As the bulk RCMC seriously changes the range history of the radar signal, a modified and much more efficient hybrid correlation operation was introduced for compensating residual phase errors. Simulation results were presented based on a general geometric topology with non-parallel trajectories and unequal velocities for both transmitter and receiver platforms, showing a satisfactory performance by the proposed method.

A Synthetic Bandwidth Method for High-Resolution SAR Based on PGA in the Range Dimension.

The synthetic bandwidth technique is an effective method to achieve ultra-high range resolution in an SAR system. There are mainly two challenges in its implementation. The first one is the estimation and compensation of system errors, such as the timing deviation and the amplitude-phase error. Due to precision limitation of the radar instrument, construction of the sub-band signals becomes much more complicated with these errors. The second challenge lies in the combination method, that is how to fit the sub-band signals together into a much wider bandwidth. In this paper, a novel synthetic bandwidth approach is presented. It considers two main errors of the multi-sub-band SAR system and compensates them by a two-order PGA (phase gradient auto-focus)-based method, named TRPGA. Furthermore, an improved cut-paste method is proposed to combine the signals in the frequency domain. It exploits the redundancy of errors and requires only a limited amount of data in the azimuth direction for error estimation. Moreover, the up-sampling operation can be avoided in the combination process. Imaging results based on both simulated and real data are presented to validate the proposed approach.

Kalman Filter for Removal of Scalloping and Inter-Scan Banding in Scansar Images

Spaceborne synthetic aperture radar (SAR) plays more and more important role in Earth observation science, especially with ScanSAR mode which provides wide-swath coverage with moderate resolution. However, scalloping and inter-scan banding (ISB) are two major artifacts, which signiflcantly degrade the quality of ScanSAR images. In this paper, a novel technique for removal of scalloping and ISB in ScanSAR images is proposed. Scalloping and ISB artifacts are modeled by two-dimensional gain and ofiset parameters varying as function of both azimuth time and range position. The gain and ofiset parameters can be split into azimuth and range components. The variations of gain/ofiset with respect to azimuth and range positions would represent scalloping and ISB artifacts respectively. In the proposed technique, recursive and minimum mean square error (MMSE) estimates of azimuth gain/ofiset parameters are found out by using Kalman fllter for each azimuth location in a subswath by considering corresponding range samples as observation vector. Subsequently, range gain/ofiset parameters causing ISB artifacts are estimated by using Kalman fllter for each range positions by considering azimuth samples as observation vector. The MMSE estimates of gain/ofiset parameters are used to directly remove scalloping and ISB artifacts. The proposed scheme was applied on simulated as well as calibrated real ScanSAR images. The experimental results exhibited the potential of proposed technique to be used as post processing tool for enhancing ScanSAR image quality.

Extended three-step focusing algorithm for sliding spotlight and tops data image formation

This paper conducted thorough research to the theory of space-borne Sliding spotlight and TOPS data processing. First, the unified definition of hybrid factor of both TOPS and sliding spotlight mode was given. Then, the hybrid factor was analyzed and amended along the range direction combined the geometry model. Combined with amended hybrid factor, azimuthal time-frequency characteristic of was analyzed and discussed in detail by mathematic derivation. Based on the analysis, an efficient and precise extended three-step image formation algorithm was presented for sliding spotlight and TOPS image formation. Finally, the imaging results justify the effectiveness of the extended three-step algorithm.

A novel three-step focusing algorithm for TOPSAR image formation

This paper conducted thorough research to the theory of space-borne TOPSAR data processing. The time-frequency characteristic of TOPSAR mode signal was analyzed and discussed in detail by mathematic derivation combined with the TOPSAR factor. Based on the analysis, an efficient and precise three-step image formation algorithm was presented for TOPSAR image formation without data division, and the operation in every step was theoretically proved effective. The de-rotation operation in the first step and deramp operation in the third step are adopted to finish the stretch in both frequency and time domains respectively, by which the azimuth folding effects in the time and frequency domains are overcome.