Bistatic TerraSAR-X/F-SAR Spaceborne–Airborne SAR Experiment: Description, Data Processing, and Results

We report about the first X-band spaceborne–airborne bistatic synthetic aperture radar (SAR) experiment, conducted early November 2007, using the German satellite TerraSAR-X as transmitter and the German Aerospace Center's (DLR) new airborne radar system F-SAR as receiver. The importance of the experiment resides in both its pioneering character and its potential to serve as a test bed for the validation of nonstationary bistatic acquisitions, novel calibration and synchronization algorithms, and advanced imaging techniques. Due to the independent operation of the transmitter and receiver, an accurate synchronization procedure was needed during processing to make high-resolution imaging feasible. Precise phase-preserving bistatic focusing can only be achieved if time and phase synchronization exist. The synchronization approach, based on the evaluation of the range histories of several reference targets, was verified through a separate analysis of the range and Doppler contributions. After successful synchronization, nonstationary focusing was performed using a bistatic backprojection algorithm. During the campaign, stand-alone TerraSAR-X monostatic as well as interoperated TerraSAR-X/F-SAR bistatic data sets were recorded. As expected, the bistatic image shows a space-variant behavior in spatial resolution and in signal-to-noise ratio. Due to the selected configuration, the bistatic image outperforms its monostatic counterpart in almost the complete imaged scene. A detailed comparison between monostatic and bistatic images is given, illustrating the complementarity of both measurements in terms of backscatter and Doppler information. The results are of fundamental importance for the development of future nonsynchronized bistatic SAR systems.      

Noise-Related Radiometric Correction in the TerraSAR-X Multimode SAR Processor

Synthetic aperture radar (SAR) image intensity is disturbed by additive system noise. During SAR focusing, pattern corrections that are adapted to the characteristics of the wanted signal, but not to the characteristics of the noise, influence the spatial distribution of the noise power. Particularly in the case of ScanSAR, a distinct residual noise pattern in low backscatter areas results. This necessitates a noise-adapted radiometric correction of the focused image for almost all applications except interferometry. In this paper, we thoroughly investigate this topic. Based on signal theoretical and stochastic considerations, we develop a radiometric correction scheme. Simulations and the application of the algorithm to TerraSAR-X datatakes support the theoretical results.      

Generalized Frequency-Domain SAR Processing

The range-Doppler algorithm and the chirp-scaling algorithm (CSA) process synthetic aperture radar (SAR) data with approximations to ideal SAR processing. These approximations are invalid for data from systems with wide beamwidths, large bandwidths, and/or low center frequencies. While simple and efficient, these frequency-domain methods are thus limited by the SAR parameters. This paper explores these limits and proposes a generalized chirp-scaling approach for extending the utility of frequency-domain processing. We demonstrate how different order approximations of the SAR signal in the 2-D frequency domain affect image focusing for varying SAR parameters. From these results, a guideline is set forth, which suggests the required order of approximation terms for proper focusing. A proposed generalized frequency-domain processing approach is derived. This method is an efficient arbitrary-order CSA that processes the data using the appropriate number of approximation terms. The new method is demonstrated using simulated data.      

The TerraSAR-X Satellite

TerraSAR-X is a versatile synthetic aperture radar (SAR) satellite with active phased array antenna technology and represents the backbone of the German national radar Earth observation mission. With its large variety of different SAR imaging modes and its high operational flexibility, TerraSAR-X ideally serves the scientific community and users from the industrial sector and governmental institutions. The innovative satellite system design combines the rich experience from past German and European SAR space missions like X-SAR, SRTM, ERS 1 and 2, and Envisat combined with state-of-the-art Earth observation bus technology as used, e.g., on the CHAMP and GRACE satellites.      

调频连续波SAR非线性处理方法研究

合成孔径雷达系统畸变引入的幅相误差严重影响了雷达的成像效果,必须予以校正才能获得高质量的图像。该文针对调频连续波SAR系统频率响应非理想特性引入的幅相误差以及信号扫频非线性误差对系统性能的影响,分析建立了存在系统误差的调频连续波SAR系统回波信号模型,研究了系统误差估计与校正的问题,并考虑小型化调频连续波SAR实时成像处理的需求,提出了一种改进的适合实时成像处理的调频连续波SAR的高精度非线性距离-多普勒成像处理算法。最后,通过理论推导和仿真分析,验证了算法的可行性和有效性。     

去调频处理的合成孔径雷达成像

针对合成孔径雷达(SAR)成像SPECAN算法中距离向采样率过高的问题,将去斜率原理用于SPECAN算法的距离向处理,先以STRETCH(去斜率)方法将宽带信号转化为单频信号,然后对单频信号做快速傅里叶变换(FFT).这种对距离向的处理方式,降低了回波信号的采样率,并且只需要一次FFT便可将回波信号压缩到与目标距离相关的位置上,减少了数据运算量和存储量.原理分析和仿真实验均表明了该算法的正确性和有效性.     

去斜率线性调频合成孔径雷达成像研究

为获得高距离分辨率 ,雷达一般发射宽带或超宽带信号 ,这给数据采集带来了很大的困难。A/ D采样是数据采集的瓶颈 ,所以必须降低大时间带宽积信号的采样频率。当雷达发射线性调频信号时 ,去斜率方法能有效降低接收机中频带宽 ,从而减小数据采集难度和数据量。本文将去斜率原理用于高分辨率合成孔径雷达 ,通过原理分析和仿真实验证明了该方法的正确性和有效性 ,并给出了成像步骤和实验结果     

圆轨迹SAR快速成像处理

由于圆轨迹SAR特殊的运动轨迹,直线SAR的成像算法不能直接应用于圆轨迹SAR数据处理。然而,对于圆轨迹SAR系统而言,其响应函数具有沿角度维平移不变的特性。因此可以利用这一特性,借鉴条带SAR成像算法的思想,在频域研究圆轨迹SAR数据的快速成像方法。该文推导了完全精确的柱面坐标系下圆轨迹SAR回波频谱解析表达式。利用条带SAR成像算法的思想,通过对频谱的合理近似,给出了圆轨迹SAR的快速成像算法。仿真实验验证了算法的有效性。     

用于高分辨率宽测绘带SAR系统的SAR/GMTI处理方法研究

随着科技的不断发展,高分辨宽测绘(High-Resolution and Wide-Swath, HRWS)SAR成像已经越来越受到人们的关注,具有地面动目标指示(Ground Moving Target Indication, GMTI)功能的SAR成像系统因其具有对静止场景进行高分辨成像和动目标检测能力在很多军用和民用领域受到广泛运用。具有HRWS成像能力的沿方位多通道SAR系统,其可以有效地解决高分辨和低脉冲重复频率(Pulse Repetition Frequency, PRF)的矛盾,该矛盾在HRWS成像处理过程中经常遇到。由于方位空域自由度可以用来进行杂波抑制,因此多通道构型具有提供GMTI潜能。该文提出一种新的杂波抑制和动目标的成像方法,使得在低PRF的HRWS系统进行SAR成像的同时可以完成动目标的检测与成像处理,而不需要单独的高PRF系统操作模式。      

Influence of Atmospheric Path Delay on the Absolute Geolocation Accuracy of TerraSAR-X High-Resolution Products

Two coupled investigations of TerraSAR-X (TSX) high-resolution data are described in this paper: geometric validation, and estimation of the tropospheric path delay using measurements of corner reflectors (CRs) placed at different altitudes but nearly identical ranges. The CRs were placed within Alpine and valley sites in Switzerland, where terrain diversity provides ideal territory for geometric validation studies. Geometric validation was conducted using slant-range complex products from the spotlight and stripmap (SM) modes in ascending and descending configurations. Based on the delivered product annotations, the CR image positions were predicted, and these predictions were compared to their measured image positions. To isolate path delays caused by the atmosphere, six TSX SM scenes $(sim!!hbox{35} times hbox{50} hbox{km})$ were examined containing four identical CRs with the same ranges and an altitude difference of $sim$ 3000 m. The CR arrangement made it possible to verify the annotated TSX atmospheric path delay by comparing the predicted slant range with the slant range obtained by measuring the reflector image coordinates. Range differences between the high- and low-altitude reflectors helped to quantify small variations in the path delay. Both SM and spotlight TSX products were verified to meet the specified accuracy requirements, even for scenes with extreme terrain variations, in spite of the simplicity of the atmospheric model currently integrated into the processor. Small potential improvements of the geolocation accuracy through the implementation of more comprehensive atmospheric modeling were demonstrated.      

Knowledge-Based SAR Processing and Geocoding: The Elementary Components of the German Processing and Archiving Facility for High Throughput and High-Precision Processing of ERS-1 SAR Data

Future microwave sensors such as the active microwave instrument (AMI) of the first European remote sensing satellite (ERS-1) to be launched in 1990 will be increasingly conducted in an operational mode. This fact implies an increased amount of synthetic aperture radar (SAR) data to be processed systematically, and, furthermore, combines with the growing needs of the user community in terms of various product levels, adequate production, and organization schemes. Among these needs are the requirements regarding much improved radiometric and absolute geometric accuracy of the generated SAR images. Therefore, an end-to-end SAR processing system has been designed that is significantly different from existing ones. The challenging requirements for high precision combined with high throughput of the generated products will be met by two systems: the knowledge-based intelligent SAR processor (ISAR) and the geocoding system. They represent the backbone of a distributed computer network engaged in SAR processing and image restitution within the German Processing and Archiving Facility (D-PAF) for ERS-1, which shall be built up and operated by the German Aerospace Research Establishment (DFVLR). This paper describes the requirements for the system as well as the specific hardware and software concepts.     

The TerraSAR-X Ground Segment

TerraSAR-X, the first national German remote-sensing satellite, was launched on June 15, 2007. It carries an X-band high-resolution synthetic aperture radar (SAR) instrument featuring imaging modes like StripMap, ScanSAR, and, particularly, SpotLight in a variety of different polarization modes. Primary mission goal is the provision of both science and commercial users with a variety of products from advanced SAR modes. The TerraSAR-X Ground Segment, which is provided by the German Aerospace Center (DLR), constitutes the central element for controlling and operating the TerraSAR-X satellite, for calibrating its SAR instrument, and for archiving the SAR data, as well as generating and distributing the basic data products. This paper depicts the ground-segment layout and describes its major elements. The ordering and product-generation workflow is presented. It introduces the applied prelaunch integration, testing, verification, and validation approach, a major key to the completion not only of the SAR technical-verification program but also the operational qualification of the ground segment itself within the commissioning phase.      

聚束式SAR的宽场景成像算法

 提出了基于SPECAN处理的宽场景聚束式SAR成像算法.首先对距离Dechirp后的信号在方位上作谱分析(SPECAN)处理消除方位模糊并对信号支撑区进行伸缩与平移变换,二次相位补偿后选择合适的插值方法恢复信号理想支撑区,最后二维IFFT成像.讨论并给出了算法对脉冲重复频率选择的限制.仿真实验验证了该方法的有效性.     

调频连续波SAR MCM成像算法

针对调频连续波SAR实时成像问题,提出了一种新的"乘法-卷积-乘法"(MCM)成像算法.在详细分析调频连续波SAR信号特征的基础上,算法采用MCM方法完成距离向处理,仅需FFT和乘法运算即可实现去斜率信号的时间尺度变换,从而实现距离徙动的精确校正.采用步进变换实现方位向压缩,不需要几何校正即可获得高质量的压缩效果.算法运算效率高,适合实时成像处理.给出了算法的推导过程和实现步骤,通过仿真试验证明了算法的正确性和有效性.     

弹载侧视合成孔径雷达信号分析及成像研究

本文探讨了装载在导弹上的侧视合成孔径雷达(SAR)在导弹下降飞行过程中回波信号的特性和成像处理问题.根据导弹在要求合成孔径雷达成像期间内的飞行特点,建立了SAR工作的空间几何模型和目标距离表达式,分析了目标距离的时间变化特性和工作过程中多普勒参数的变化.由于要求成像的过程中,导弹的高度在不断减小,多普勒参数变化大,不能用同一参数对整个成像过程进行处理.分析和仿真表明,通过采用子孔径处理,能够得到聚焦的子孔径图像.     

The TerraSAR-X Mission and System Design

This paper describes the TerraSAR-X mission concept within the context of a public–private partnership (PPP) agreement between the German Aerospace Center (DLR) and the industry. It briefly describes the PPP concept as well as the overall project organization. This paper then gives an overview of the satellite design and the corresponding ground segment, as well as the main mission parameters. After a short introduction to the scientific and commercial exploitation scheme, this paper finally focuses on the mission accomplishments achieved so far during the ongoing mission.      

SAR的动目标成像与定位处理

合成孔径雷达（SAR）是一种高分辨成像雷达。当今雷达成像技术已经非常成熟，在国民经济与军事领域中有着广泛的应用。但在SAR成像过程中如何对动目标成像并定位依然是一个值得探讨的课题。该文提出通过求取动目标调频斜率的方法能对动目标更清晰地成像，然后通过多普勒频谱搬移法得到动目标在雷达图像上的定位，使成像质量较高的动目标图像叠加到静止地物雷达图像上。最后通过雷达实测数据进行处理，得到较为满意的结果。     

TerraSAR-X Commissioning Phase Execution Summary

This paper provides an overview of the TerraSAR-X commissioning phase (CP). The overall CP planning and preparation is presented. The strategy for data-take (DT) command generation is discussed, and statistical reports summarize the acquired CP DTs. An overview summary on the main results in the different characterization and verification areas is provided together with synthetic aperture radar image examples.      

TerraSAR-X System Performance Characterization and Verification

This paper presents results from the synthetic aperture radar (SAR) system performance characterization, optimization, and verification as carried out during the TerraSAR-X commissioning phase. Starting from the acquisition geometry and instrument performance, fundamental acquisition parameters such as elevation beam definition, range timing, receiving gain, and block adaptive quantization setting are presented. The verification of the key performance parameters—ambiguities, impulse-response function, noise, and radiometric resolution—is discussed. ScanSAR and Spotlight particularities are described.      

基于FPGA的星载SAR成像信号处理技术

通过现场可编程门阵列(FPGA)设计并实现星载合成孔径雷达(SAR)成像处理,能够完成对多种模式星载SAR回波数据的高速处理。该文搭建的成像处理系统以子孔径极坐标格式算法(PFA)为核心,首先,在精确的多普勒中心频率估计基础上,采用基于尺度变换原理的距离向处理(PCS)与方位向高精度sinc插值级联的算法处理流程实现子孔径数据域重采样,极大地提升了处理精度与运算效率;然后,对各子孔径图像进行辐射校正消除扇贝效应,并采用加权平均算法获得了全孔径拼接图像;最后,基于Sentinel-1卫星实测数据对本系统进行了验证和分析,在系统工作频率200 MHz情况下,能够在5.92 s内实现8 192×8 192像素点的32位单精度浮点成像处理,实测数据成像结果验证了该系统的有效性,从而为实时处理奠定了技术基础。