Generation of wide-swath and high-resolution SAR images from multichannel small spaceborne SAR systems

Future spaceborne synthetic aperture radar (SAR) systems will be required to produce high-resolution imagery over a wide area of surveillance. However, the minimum antenna area constraint makes it a contradiction to simultaneously obtain both unambiguous wide-area and high azimuth resolution. To overcome this limitation, a technique has been suggested that combines a broad illumination source with multiple receiving channels. Then, the coherent combination of the recorded multichannel signals will allow for the unambiguous SAR mapping of a wide ground area with fine azimuth resolution. This letter first gives an overview of current research work carried out about the generation of wide-swath and high-resolution SAR images from multichannel small spaceborne SAR systems, and then a space-time adaptive processing (STAP) approach combined with conventional SAR imaging algorithms is presented, which could be of help to overcome the existing difficulties in data processing. The main idea of the approach is to use a STAP-based method to properly overcome the aliasing affect caused by the lower pulse repetition frequency and thereby retrieve the unambiguous azimuth wide (full) spectrum signal from the received signal. Following this operation, conventional SAR data processing tools can be applied to fully focus the SAR images. The performance of the approach is also discussed in this letter. The approach has the advantages of simplicity, robustness, and high efficiency.     

采用调零天线阵测向解GNSS姿态仪的整周模糊度

由多个单元天线组成的GNSS调零天线阵通常用于在大功率干扰信号来波方向上形成天线波束方向图零点,以此来抗压制性干扰。采用此调零天线阵所自然形成的二维短基线,按照电子侦察中多基线干涉仪测向原理,通过短基线的相位差测量值来解GNSS姿态仪长基线的相位差模糊,从而实现了GNSS姿态仪的整周模糊度的实时解算。该方法通过GNSS姿态仪与调零天线阵的组合应用,不仅提高了解模糊的性能,而且增强了整个系统抗干扰反欺骗的能力,从而为GNSS姿态仪和调零天线的拓展应用提供了重要参考。      

Antenna Mask Design for SAR Performance Optimization

In this letter, an effective technique for synthetic aperture radar (SAR) antenna mask design is presented for optimizing the system performance of an active phased array SAR. The SAR antenna radiation pattern has an important effect on the system performance. Therefore, the authors derived the quantitative equations for the SAR antenna mainlobe and sidelobe mask design on the basis of the system performance measures such as the range-to-ambiguity ratio (RAR), the noise-equivalent sigma zero (NESZ), and the radiometric accuracy. The antenna mask template should be designed to minimize the ambiguous signals reflected from the antenna sidelobes and maximize the system sensitivity, i.e., the NESZ, determined by the antenna mainlobe. The simple iterative method such as random-mutation hill climbing was utilized to successfully assign the sidelobe level at each ambiguous area using the derived equations. Finally, the antenna patterns were synthesized with reference to the optimized antenna mask templates using the particle swarm optimization, and the swath width, RAR, and NESZ performances were evaluated in order to confirm the effectiveness of the proposed technique.      

低成本GNSS模块RTK性能比较分析

针对低成本GNSS模块及天线类型直接影响RTK性能的问题,该文选取了具有代表性的3种GNSS定位模块华大HD9310、UBLOX NEO-M8T、UBLOX ZED-F9P,以及测地型天线U35和螺旋天线BT-3070两种不同类型的天线。采集GNSS天线和模块不同组合的原始观测数据,深入分析了不同组合的原始GNSS数据的信噪比、模糊度解算初始化时间、伪距与载波相位的双差残差以及定位精度。结果表明:U35天线性能优于BT-3070天线;ZED-F9P模块性能优于NEO-M8T模块和HD9310模块。实验结果可为无人设备、工程测量等领域选取合适的RTK组合提供参考。     

An optimal determination of variable antenna height and power for in-orbit SAR operations

An optimal power and antenna height determination over a wide range of incidence angles is proposed for in-orbit operations of the spaceborne stripmap synthetic aperture radar (SAR) with variable antenna height and variable radiating element power. The antenna height and the average transmitted radio-frequency power of a radiating element for each incidence angle are determined so that the mapping area can be maximized with the constraints on the signal-to-noise ratio, ambiguities, and swath width, given the SAR geometry (an altitude and incidence angles) and radar wavelength. With the receive-chain power consumption of one transmit/receive module (P/sub r/=10 mW) and the SAR instrument power consumption without the phased array antenna (P/sub I/=5 W), the illustrative design example shows that the mapping area of the proposed SAR for the maximum incidence angle (/spl eta//sup max/=75/spl deg/) is about 269.1% of that resulted from utilizing only the variable antenna height. The proposed optimization would have a more beneficial effect on applications that need the high-incidence-angle operations.     

星载SAR海浪遥感中波向确定的一种新方法

海浪传播方向的 180°不确定性是海浪遥感的固有问题。根据海浪传播理论 ,提出了解决该问题的一种新方法。利用该方法可直接由合成孔径雷达 (SAR)图像确定海浪的传播方向 ,消除波向 180°不确定性。实例研究表明 ,利用该方法确定的海浪传播方向与同步海浪数据有很好的一致性     

A Chirp Transform Algorithm for Processing Squint Mode FMCW SAR Data

Frequency-modulated continuous-wave (FMCW) synthetic aperture radar (SAR) is a lightweight cost-effective high-resolution airborne imaging radar. In squint case, the frequency scaling algorithm, which is suitable for processing nonchirped raw data, cannot be used directly in FMCW SAR data processing because of low system sampling frequency. On the other hand, the continuous antenna motion of FMCW SAR can cause serious distortions in the reconstructed images. In this letter, an improved algorithm called the chirp transform algorithm is proposed. When the effects of the residual video phase are negligible, the algorithm uses a chirp transform to perform the time scaling operation to alleviate the sampling frequency problem. It requires only fast Fourier transforms and multiplications. The range cell migration introduced by the continuous motion is also compensated completely in range-Doppler domain. The algorithm performances are analyzed and are supported by point target simulation experiments.     

机载SAR影像主动定位的数学模型研究

本文基于差分GPS(Differential Global Positioning System,DGPS)和惯性导航系统(Inertial Navigation System,INS)数据,在无控制点的情况下,导出了一种进行机载SAR影像主动定位的数学模型。此模型包括DGPS/INS数据进行坐标系转换、天线动态偏心改正、雷达天线相位中心插值和距离-多普勒(Range-Doppler,R-D)模型及解算。根据距离-多普勒(Range-Doppler,R-D)模型和数字高程模型(Digital Elevation Model,DEM)数据,获得机载SAR影像上每点所对应的地理坐标,重采样生成正射影像图。本文通过成都测区1m分辨率的机载SAR影像主动定位试验,验证了此数学模型的正确性,分析了主要系统误差源及系统误差的改正方法。     

On the Existence of Coverage and Integration Time Regimes in Bistatic SAR Configurations

Bistatic synthetic aperture radar (SAR) is an extension of traditional monostatic SAR, which increases the flexibility in designing SAR missions. We describe a scheme for the computation of integration time and azimuth coverage of bistatic SARs based on space-time diagrams. A classification of bistatic SAR configurations is introduced in terms of size and velocity on the ground of antenna footprints. Bistatic SAR regimes are also identified.     

机载InSAR系统精度分析

结合SAR成像原理,在提出了基于DGPS/IMU数据的机载InSAR系统DEM三维坐标求解的基础上,推导出了基线长度、基线倾角、相位差、雷达天线到地面目标点的距离、雷达天线位置、中心多普勒频率及飞机飞行速度等因素误差对DEM精度的影响公式,分析讨论了机载InSAR系统影响精度的主要因素及系统可实现的性能情况,提出了机载InSAR系统部分设计建议,并使用实际的机载InSAR数据进行了试验。结果表明,本方法结果与实际试验统计的精度基本一致,影响精度的分析结论正确。     

玉树地震灾情SAR遥感监测与信息服务系统

在玉树地震中,中国首套自主知识产权的机载多波段多极化干涉SAR测图系统,发挥全天时主动遥感的优势,成功服务于抗震救灾中。依据玉树地震灾情综合地理信息监测与评估工作,着重探讨SAR遥感影像用于地震灾情监测与评估中的处理方法和技术路线,研究制定了地震灾区灾情综合地理信息监测指标,通过快速几何处理、快速变化信息提取、快速目标判读和灾害空间危险性评估,实现了玉树地震震区灾情综合地理信息的解译、制图和统计评估。在此基础上,开发了玉树地震灾情综合地理信息服务系统,实现了灾情监测信息的综合管理、可视化查询和统计分析。     

一种基于曲线SAR的三维目标特征提取与自聚焦新算法

曲线合成孔径雷达（Curvilinear Synthetic Aperture Radar,简称曲线SAR）是一种新的三维成像模式，通过让载机作曲线飞行，曲线SAR系统能同时在方位维和高度维形成大的合成孔径。本文提出一种曲线SAR的三维目标特征提取和自聚焦新方法。新方法是建立在一种灵活的接收数据模型之上的参数化的新方法。仿真实验证明了所提方法的有效性。     

零基线约束的参考站间模糊度固定方法研究

论文提出一种GNSS零基线约束的参考站间模糊度快速固定方法,该方法基于“一天线+双接收机”设计进行参考站间模糊度固定。首先,利用两个参考站四台接收机观测值形成四对参考站间无电离层组合双差观测值。其次,结合无电离层组合观测值模糊度和宽巷整周模糊度进行卡尔曼滤波和N1模糊度实数解计算。然后在模糊度域内进行线性变换,单历元固定零基线模糊度;利用零基线模糊度约束解算四组参考站间模糊度中的一组模糊度。最后,利用固定的模糊度还原所有原始双差整周模糊度。实验证明:与传统参考站间模糊度固定方法比,充分利用了零基线模糊度固定的约束条件进行参考站间模糊度固定,加快了模糊度收敛速度,极大地提高了模糊度固定的速度和成功率。      

In-season crop classification using elements of the Kennaugh matrix derived from polarimetric RADARSAT-2 SAR data

Accurate spatio-temporal classification of crops is of prime importance for in-season crop monitoring. Synthetic Aperture Radar (SAR) data provides diverse physical information about crop morphology. In the present work, we propose a day-wise and a time-series approach for crop classification using full-polarimetric SAR data. In this context, the 4 × 4 real Kennaugh matrix representation of a full-polarimetric SAR data is utilized, which can provide valuable information about various morphological and dielectric attributes of a scatterer. The elements of the Kennaugh matrix are used as the parameters for the classification of crop types using the random forest and the extreme gradient boosting classifiers.The time-series approach uses data patterns throughout the whole growth period, while the day-wise approach analyzes the PolSAR data from each acquisition into a single data stack for training and validation. The main advantage of this approach is the possibility of generating an intermediate crop map, whenever a SAR acquisition is available for any particular day. Besides, the day-wise approach has the least climatic influence as compared to the time series approach. However, as time-series data retains the crop growth signature in the entire growth cycle, the classification accuracy is usually higher than the day-wise data.Within the Joint Experiment for Crop Assessment and Monitoring (JECAM) initiative, in situ measurements collected over the Canadian and Indian test sites and C-band full-polarimetric RADARSAT-2 data are used for the training and validation of the classifiers. Besides, the sensitivity of the Kennaugh matrix elements to crop morphology is apparent in this study. The overall classification accuracies of 87.75% and 80.41% are achieved for the time-series data over the Indian and Canadian test sites, respectively. However, for the day-wise data, a ∼6% decrease in the overall accuracy is observed for both the classifiers.     

基于多历元递推最小二乘卡尔曼滤波方法的模糊度解算

针对GPS动态定位,通过使用递推最小二乘技术,提出基于递推最小二乘的仅含有模糊度参数的卡尔曼滤波方法,实现了利用多历元载波相位观测信息解算模糊度。同时针对多历元定位的特点,讨论了动态定位中的单频周跳探测与修复,并提出将卫星重新出现的情况按照周跳的处理方法,有效地提高了解算效果。     

Eigensubspace-Based Filtering With Application in Narrow-Band Interference Suppression for SAR

Synthetic aperture radar (SAR) has found wide applications in many areas, e.g., battlefield awareness. However, SAR is vulnerable to various kinds of interference, among which narrow-band interference (NBI) is commonly used. In this letter, an eigensubspace-based filtering approach is proposed for NBI suppression in SAR without using passive-sniff data as the reference signal. Moreover, the proposed method can deal with smart or interrupted NBI. Both simulation and experimental results are provided to illustrate the performance of the proposed approach     

Single-frequency L5 attitude determination from IRNSS/NavIC and GPS: a single- and dual-system analysis

The Indian Regional Navigation Satellite System (IRNSS) has recently (May 2016) reached its full operational capability. In this contribution, we provide the very first L5 attitude determination analyses of the fully operational IRNSS as a standalone system and also in combination with the fully operational GPS Block IIF along with the corresponding ambiguity resolution results. Our analyses are carried out for both a linear array of two antennas and a planar array of three antennas at Curtin University, Perth, Australia. We study the noise characteristics (carrier-to-noise density, measurement precision, time correlation), the integer ambiguity resolution performance (LAMBDA, MC-LAMBDA) and the attitude determination performance (ambiguity float and ambiguity fixed). A prerequisite for precise and fast IRNSS attitude determination is the successful resolution of the double-differenced integer carrier-phase ambiguities. In this contribution, we will compare the performance of the unconstrained and the multivariate-constrained LAMBDA method. It is therefore also shown what improvements are achieved when the known body geometry of the antenna array is rigorously incorporated into the ambiguity objective function. As our ambiguity-fixed outcomes show consistency between empirical and formal results, we also formally assess the precise attitude determination performance for several locations within the IRNSS service area.     

机载X波段极化干涉SAR数据非监督分类方法研究

针对经典极化分类算法在处理机载X波段SAR数据时将过多地物分为体散射类型,并且容易受噪声影响,分类结果存在大量误分现象的问题,通过对机载X波段SAR数据非监督分类方法的研究,提出将极化干涉信息用于机载X波段极化干涉SAR数据的分类。通过运用极化干涉数据进行目标分解得到参数A1和A2对数据进行初始分类,然后结合改进的Wishart最大似然分类算法来进行地物的自适应分类。实验结果表明,该方法能有效避免平地效应的影响,抗噪性好,能正确区分三种典型散射类型,分类效果明显优于极化分类效果。     

The affine constrained GNSS attitude model and its multivariate integer least-squares solution

A new global navigation satellite system (GNSS) carrier-phase attitude model and its solution are introduced in this contribution. This affine-constrained GNSS attitude model has the advantage that it avoids the computational complexity of the orthonormality-constrained GNSS attitude model, while it still has a significantly improved ambiguity resolution performance over its unconstrained counterpart. The functional and stochastic model is formulated in multivariate form, for one-, two- and three-dimensional antenna arrays, tracking GNSS signals on an arbitrary number of frequencies with two or more antennas. The stochastic model includes the capability to capture variations in the antenna-quality within the array. The multivariate integer least-squares solution of the model parameters is given and the model’s ambiguity success-rate is analysed by means of the ambiguity dilution of precision (ADOP). A general closed-form expression for the affine-constrained ADOP is derived, thus providing an easy-to-use and insightful rule-of-thumb for the ambiguity resolution capabilities of the affine constrained GNSS attitude model.     

MiniSAR与无人机系统集成应用研究

近年来无人机机载合成孔径雷达（SAR）技术发展迅速,成为全天时、全天候数据获取的重要手段.本文对无人机机载合成孔径雷达国外发展应用和国内研究进展进行了阐述,对Mini SAR载荷与无人机飞行平台系统集成进行了测试研究,并验证了在黄河冰情凌汛应急监测的适用性,最后指出了无人机机载Mini SAR在行业应用中的价值.