Zaoyu Sun

Modeling and Processing of Two-Dimensional Spatial-Variant Geosynchronous SAR Data

Imaging of spatial-variant geosynchronous synthetic aperture radar (GEO SAR) in the L band with long integration time is discussed. To compensate for spatial variances in both range and azimuth directions, a new algorithm based on improved omega-K (ωK) and three-time azimuth chirp scaling (3ACS) is proposed. First, the integration time and the slant range model were analyzed. Second, the two-dimensional (2-D) spectrum was used for range cell migration correction (RCMC), secondary range compression (SRC), and azimuth compression, and the influences of spatial variances on each term were considered. Third, the improved ωK was used to compensate for the range variance, and 3ACS was used to compensate for the azimuth variance. The scope of 2-D focusing in high-resolution GEO SAR imaging was clearly enlarged. Finally, the performance of the algorithm was demonstrated using simulations based on a spaceborne radar advance simulator (SBRAS).

Processing of Ultrahigh-Resolution Spaceborne Sliding Spotlight SAR Data on Curved Orbit

In high-resolution spaceborne synthetic aperture radars (SARs), with increasing synthetic aperture length on curved orbits, the hyperbolic range history (HRH) assumption deteriorates, and simple analytic transfer functions in the Doppler domain used in SAR processing do not exist. A subsection HRH model is proposed. Ultra- high-resolution sliding spotlight SAR data can be handled satisfactorily, and simple analytic transform functions are used all the same. The proposed range history model is also applicable to bistatic SARs.

A multi-mode space-borne SAR simulator based on SBRAS

A multi-mode space-borne Synthetic Aperture Radar (SAR) simulator is presented in this paper. The simulator is based on the Space-borne Radar Advance Simulator (SBRAS), and supports the parameter design, raw data simulation, data focusing and performance evaluation of strip-map mode, spotlight mode, sliding spotlight mode, ScanSAR mode, Terrain Observation by Progressive Scans (TOPS) mode and mosaic mode. The hardware architecture and software composition are described below. At last, an application example of high resolution sliding spotlight with point targets and another application example of wide swath TOPS mode with distributed targets are provided.

Radar Signal Level Fusion Imaging

In this paper, we consider bandwidth fusion to improve the range resolution. We propose a method based on spectral estimation and amplitude-phase error model to compensate for the lack of mutual coherent between radar subbands. Then we consider two-dimensional fusion to improve the range and cross-range resolution. A new algorithm, called EMEMP, is developed to resolve the poles mismatch occurred in traditional fusion method based on modified Root-Music. By simulations, good performance of these methods will be show.

Azimuth ambiguity of multi-channel SAR

Azimuth multi-channel synthetic aperture radar has been widely studied to realize high resolution wide swath imaging. Spectrum reconstruction algorithm deals with the main-lobe aliasing while the side-lobe spectrum still exists and is weighted by the reconstruction network as well. In the present paper, azimuth ambiguity of multi-channel strip SAR is investigated, an analytical expression of azimuth ambiguity to signal ratio is derived based on established linear model of the multi-channel signal in Doppler domain. The analysis will definitely benefit multi-channel system design.

Improved geometrical SAR image registration based on elevation correction

Accurate co-registration of SAR image is a crucial step in interferometry SAR (InSAR) processing, and lower registration accuracy will lead to more interferometry phase error. The most commonly used is 2-D polynomial function registration method (2-DR), which is simple and works well in most conditions. But in the case of long baselines and complex terrain scene, (2-DR) can not meet the precision requirements. The DEM-assisted geometrical SAR image registration method (GeoR), which use the external DEM and orbit data to calculate offsets, can theoretically achieve an ideal precision for any baseline and terrain. In GeoR, corresponding elevation value of each pixel in the master image needs to extracted from the external DEM. Unfortunately, there are errors in the DEM data and the orbit data, which often result in approximate horizontal shift error and mismatch between DEM and SAR image, caused low precision elevation value extraction and eventually degrade the registration performance. In this paper, the influences of DEM error and orbital error in elevation value extraction are firstly analyzed, and then an improved geometrical SAR image registration method based on elevation correction (I-GeoR) is proposed. In elevation correction, Elevation value in DEM can be correctly assigned to each pixel of the image, which is achieved by deformation extraction that based on SAR image simulation. Finally, two ALOS-PALSAR images are processed to vaidate the new method. The result shows that I-GeoR can effectively correct the elevation and has a higher registration precision.

Ananlysis and compensation of vibration error of high frequency synthetic aperture radar

Synthetic Aperture Radar (SAR) system of high frequency, high resolution and high frame rate is one of the hot research areas. At present, the analysis of vibration error influence mainly embarks from the theoretical derivation, and simulated in level model. In order to accurately analyze the influence of high frequency vibration to high frequency SAR, it is necessary to simulate in signal level. Due to the short wave length of high frequency SAR, impact of high frequency vibration which is almost negligible to the conventional SAR of microwave band must be finely processed. Phase Adjustment by Contrast Enhancement (PACE) is a better SAR imaging autofocusing algorithm in the case of low Signal Noise Ratio (SNR), which can precisely estimate high frequency phase errors. In actual implementation, interpolation method is an effective means to improve the efficiency of the algorithm.

Spectrum reconstruction and the performance analysis for multichannel bidirectional SAR

Single-channel bidirectional synthetic aperture radar (BiDi-SAR) systems require the pulse repetition frequency (PRF) to be cautiously designed. Firstly, it should be higher than the total bandwidth of the two Doppler subbands; secondly, it should be carefully selected so that the two Doppler subbands are properly located. We introduce multichannel BiDi-SAR imaging mode to realize bidirectional high resolution wide swath (HRWS) imaging. Acquisition geometry of multichannel BiDi-SAR is interpreted. Doppler spectrum of BiDi-SAR consists of two subbands with limited bandwidth. Based on a prior knowledge about the location of the Doppler subbands, we derive the method for separating the aliased Doppler spectra for general PRFs. Then, we analyze the reconstruction performance in terms of azimuth ambiguity to signal ratio (AASR) and signal to noise ratio (SNR). Besides, we propose to use a properly designed sparse array antenna to improve the azimuth ambiguity performance. Validity of the signal processing method is verified by simulation results.创新点单通道双指向合成孔径雷达 (BiDi-SAR) 对系统PRF有着较为苛刻的要求: 首先, 系统PRF必须高于多普勒谱总带宽; 其次, 所选择的PRF应当能够使得卷绕到基带的两个多普勒子频带正好错开, 从而通过带通滤波将与前后两个波束想对应的两个多普勒子频带予以分离。本文提出多通道BiDi-SAR观测模式以实现双指向高分辨率宽测绘带 (HRWS) 宽测。对多通道BiDi-SAR模式的工作原理进行了阐释。基于BiDi-SAR多普勒谱的各子频带位置的先验性息, 推导了适用于一般PRF条件的多普勒谱分离方法。对方位模糊比 (AASR) 以及信噪比 (SNR) 等关键的重建性能进行分析, 给出了解析表达式。此外, 提出利用稀疏阵列天线实现天线方向图的优化从而实现BiDi-SAR AASR性能的提升。

A Novel Array Error Estimation Method for Azimuth Multichannel SAR

Minimum side-zone power to center-zone power ratio (MSCR) method is presented to estimate array errors of azimuth multichannel synthetic aperture radar (SAR). Spaceborne azimuth multichannel SAR is one of the most promising candidates for achieving high-resolution wide-swath imaging. However, array errors brought in by instrument in∞uences and aperture position errors need to be compensated. MSCR method is designed to obtain phase error estimates by minimizing side-zone power to center-zone power ratio, where the side-zone and the center-zone indicate the intervals far from and around Doppler centroid respectively. The proposed method achieves signiflcantly improved performance on phase error estimation especially when signal to noise ratio is low. Experiment results conflrm the validity and solidity of the method.

WYPSAR: Wide-swath yields by partial signal of synthetic aperture radar

A novel and promising type of Synthetic Aperture Radar (SAR) concept for High Resolution and Wide Swath (HRWS) observation is presented. Different from conventional SAR systems, the proposed WYPSAR transmits a long pulse of Linear Frequency Modulation (LFM), but receives only partial signals of echoes, however, the focused image of the short received signals can be nearly the same length as the transmit pulse. And the rest time of the pulse interval can be used to receive echoes from other swaths. By WYPSAR, the HRWS observation is carried out much simpler than using the technology of multi-channel systems. After an introduction to the new concept, a simple example is presented. At last, the signal processing technique is discussed and a simulation result follows.