Absolute radiometric calibration of the CCRS SAR

Determining the radar scattering coefficients from SAR (synthetic aperture radar) image data requires absolute radiometric calibration of the SAR system. The authors describe an internal calibration methodology for the airborne Canada Centre for Remote Sensing (CCRS) SAR system, based on radar theory, a detailed model of the radar system, and measurements of system parameters. The methodology is verified by analyzing external calibration data acquired over a six-month period in 1988 by the C-band radar using HH polarization. The results indicate that the overall error is ±0.8 dB (1σ) for incidence angles ±20° from antenna boresight. The dominant error contributions are due to the antenna radome and uncertainties in the elevation angle relative to the antenna boresight     

S波段高分辨宽幅SAR辐射定标及误差分析方法

合成孔径雷达(SAR)系统的辐射定标可以构建SAR图像与地物后向散射截面积(RCS)的关系,反演目标物理特性,满足SAR定量化遥感需求。相对于其它波段,S波段SAR的定量化遥感工作罕见报道。该文利用已知SAR及平台参数进行S波段SAR辐射定标处理,首先推得了图像像素值与目标后向散射系数的关系,接下来详细分析了各项误差对定标精度的影响,给出了天线指向误差对定标精度影响的解析表达式。该文的分析有利于建立各参数与辐射定标精度的关系,方便设计时候的误差分配。该文给出了草地、道路和平静水面的S波段后向散射截面积统计值。最终实际数据处理结果表明,该系统利用该定标方法可以在20°的视角范围内实现较高的绝对辐射精度。     

A statistical model for the error bounds of an active phased arrayantenna for SAR applications

A model for the theoretical error bounds for the radiometric calibration of SAR imagery is described. The model is then applied utilizing the radar system parameters that will be used in the project PHARUS (Phased Array Universal SAR), a Dutch polarimetric airborne C -band universal SAR, which is currently under construction. An error model for the phased array antenna pattern is presented. This model was applied to a 16×8 phased array antenna to determine the influence of errors in the T/R modules and angle variations of the beam direction. For a wide range of variances the model is in good agreement with the exact solution, but with far less computer effort. Other, more complicated, antennas can be analyzed in a straightforward way. The model can be used as a design tool for digital phased array radar     

SAR calibration: an overview

Progress in synthetic-aperture radar, (SAR) calibration is reviewed. The difficulties of calibrating both airborne and spaceborne SAR image data are addressed. The quantities measured by a SAR, i.e. radar backscatter, are defined and mathematical formulations for the three basic types of SAR image are developed. The difficulties in establishing science requirements for calibration are discussed. The measurement of SAR image quality is briefly addressed. The problem of radiometric calibration is introduced via the SAR form of the radar equation, with both internal and external calibration approaches considered. The development of algorithms for polarimetric radar calibration is reviewed and the problems involved in phase calibration of interferometric SAR are discussed. Future challenges in the field of SAR calibration are considered     

SAR辐射定标精度设计与分析

合成孔径雷达（SAR）辐射定标是一项关联图像像素值和地物后向散射系数的重要工作。文中在分析SAR系统的误差源和辐射定标原理的基础上,研究了无源辐射定标和有源辐射定标的精度设计原理。以机载SAR、Pi-SAR定标结果为例,结合雷达散射计的测试结果,分析了相同条件相似地物的后向散射系数的差异及相应的原因。理论分析和数据比较表明了辐射定标精度设计和比较的合理性,为定标精度和结果分析提供了理论支持和技术途径。     

星载sAR辐射定标误差分析及成像处理器增益计算

本文对星载SAR辐射定标的各项误差因素进行了分析,提出了计算辐射定标误差的公式,提供了分配星载SAR系统辐射定标误差的工具;对基于CS(Chirp Scaling)算法的成像处理器的增益计算公式进行了推导,并通过计算机仿真得到了验证;定量地分析了成像处理器增益计算误差对SAR系统辐射定标精度的影响。     

Cross-calibration between airborne SAR sensors

A comparative study of data acquired by two different airborne synthetic aperture radar (SAR) sensors from the same site is presented. External, ground-target-based calibration has been performed on the NASA/JPL DC-8 SAR C-band data and the DLR E-SAR C-band data collected over the DLR test site in Oberpfaffenhofen. The consistency of both the polarimetric and radiometric calibration parameters derived from different tracks indicates the stability of DC-8 SAR system during the campaign. Radar cross-sections and backscattering coefficients measured from different tracks under different incidence angles emphasize this stability     

Phase calibration of polarimetric radar images

The problem of phase calibration between polarization channels of an imaging radar is studied. The causes of various types of phase errors due to the radar system architecture and system imperfections are examined. A simple model is introduced to explain the spatial variation in phase error as being due to a displacement between the phase centers of the vertical and horizontal antennas. It is also shown that channel leakage can cause a spatial variation in phase error. Phase calibration using both point and distributed ground targets is discussed and a method for calibrating phase using only distributed target is verified, subject to certain constraints. Experimental measurements using the NADC/ERIM P-3 synthetic-aperture radar (SAR) system and NASA/JPL DC-8 SAR, which operates at C-, L-, and P-bands, are presented. Both of these systems are multifrequency, polarimetric, airborne, SAR systems.<>     

TerraSAR-X Instrument Calibration Results and Extension for TanDEM-X

Spaceborne remote sensing with synthetic aperture radar (SAR) has become an essential source of high-resolution and continuous Earth observation. Modern satellites like the German TerraSAR-X system provide state-of-the-art radar images with respect to operating flexibility and imaging quality. The outstanding performance of TerraSAR-X image products is achieved by an innovative calibration approach that minimizes systematic antenna and instrument characteristics. The active phased array X-band antenna is fed by 384 transmit/receive modules for electronic beam steering and shaping in the azimuth and elevation direction. The flexible radar instrument hosts an internal calibration system which guarantees the high radiometric stability of all SAR products. New techniques for antenna performance control have been successfully implemented, setting a high standard for next-generation SAR missions. This paper summarizes all essential calibration results of TerraSAR-X that cover internal instrument behavior. Furthermore, we give an outlook on the required bistatic calibration techniques for the future TanDEM-X mission that faces additional performance challenges when calibrating two TerraSAR-X satellites flying in close formation.      

基于DPCA的机载SAR系统运动误差及其补偿

基于相位中心偏置天线（DPCA）技术的机栽SAR系统在实际运用中普遍存在着因雷达平台运动不稳定导致DPCA约束条件不满足的问题,这在很大程度上影响了机载SAR系统的杂波抑制性能。针对这个问题,该文以双天线机载SAR系统为模型,通过对DPCA的对消原理和运动误差的分析,结合插值理论,对载机匀加速运动状态下造成的运动误差提出了一种基于三次样条函数的运动补偿算法。通过计算机仿真,验证了该算法的有效性,且算法易于工程实现。     

Polarimetric calibration of the SIR-C C-band channel using activeradar calibrators and polarization selective dihedrals

A polarimetric calibration experiment of Shuttle Imaging Radar C (SIR-C) is carried out using several different calibration targets. These are C-band polarimetric active radar calibrators (PARCs), polarization selective dihedrals (PSDs), 22.5° rotated dihedrals, and a trihedral. A novel polarimetric calibration algorithm is proposed that combines existing algorithms and uses one PARC and two PSDs. An error evaluation example is shown to estimate the typical hardware error value of the calibration targets allowable for a given calibration error. The novel algorithm gives polarimetric calibration results comparable to those obtained using the existing algorithm for three PARCs. Since PSDs work at frequencies lower than design frequency, and hence can be commonly used at multiple frequency bands, the simple addition of just one more frequency band PARC allows polarimetric calibration of a dual-frequency polarimetric synthetic aperture radar (SAR) by means of the proposed algorithm     

利用点目标进行SAR辐射定标的方法研究

SAR辐射定标是一项关联图像像素值和地物后向散射系数的重要工作。由于角反射器具有稳定、大的散射截面积和在较宽的角度范围内散射截面积变化较小,因而成为重要的点目标定标器。研究利用已知散射截面积的三面体角反射器来计算不同地物的后向散射系数、系统定标常数和系统总的传递函数的方法,并通过大量实测SAR数据进行了定标实验。理论分析和实验结果表明,利用点目标进行SAR辐射定标具有简单、实用、精度较高的优点。     

The effect of topography on SAR calibration

During normal synthetic aperture radar (SAR) processing, a flat Earth is assumed when performing radiometric corrections such as antenna pattern and scattering area removal. The authors examine the effects of topographic variations on these corrections. Local slopes will cause the actual scattering area to be different from that calculated using the flat Earth assumption. It is shown that this effect may easily cause calibration errors larger than a decibel. Ignoring the topography during antenna pattern removal may also introduce errors of several decibels in the case of airborne systems. The effect of topography on antenna pattern removal is expected to be negligible for spaceborne SARs. The authors show how these effects can be taken into account if a digital elevation model is available for the imaged area. The errors are quantified for two different types of terrain, a moderate relief area near Tombstone, AZ, and a high relief area near Oetztal in the Austrian Alps. The authors show errors for two well-known radar systems, the C-band ERS-1 spaceborne radar system and the three frequency NASA/JPL airborne SAR system (AIRSAR)     

考虑运动补偿的机载SAR定位误差传递模型及航迹标定方法

机载合成孔径雷达(SAR)定位误差不仅受载机位置/速度测量误差、系统时间误差等的影响,还与运动补偿残余误差有关.然而现有机载SAR定位模型很少考虑运动补偿误差的影响.该文针对实际中普遍存在的含运动误差和载机航迹测量误差的情况,结合运动补偿和频域成像算法,推导了机载SAR图像定位误差传递模型,阐明了运动补偿残余误差影响下...     

Calibration of a polarimetric imaging SAR

Calibration using point targets is discussed. The fourport network calibration technique is used to describe the radar error model. The processor ambiguity function and the radar distortion matrices are combined to form a generalized polarimetric ambiguity function. The polarimetric ambiguity function of the SAR is found using a single point target, namely a trihedral corner reflector. Based on the resultant polarimetric ambiguity function, an estimate for the backscattering coefficient of the terrain is found using a modified version of the single target calibration technique (STCT). A radar image recorded by the JPL aircraft SAR, which includes a variety of point targets, is used for verification of the new calibration method. The calibrated responses of the point targets are compared both with theory and responses based on the POLCAL technique     

Ambiguity-free Doppler centroid estimation technique for airborne SAR using the Radon transform

In synthetic aperture radar (SAR) signal processing, the Doppler centroid estimation technique, called the "clutter-lock", is important because it is related to the signal-to-noise ratio, geometric distortion, and radiometric error of the final SAR image. Conventional algorithms have either ambiguity problems or somewhat high computational load. Using the fact that the Doppler centroid and the squint angle are directly related, we propose an ambiguity-free Doppler centroid estimation technique using Radon transform, named geometry-based Doppler estimator. The proposed algorithm is computationally efficient and shows good performance of estimating the absolute Doppler centroid.     

A new hybrid-beam data acquisition strategy to support ScanSAR radiometric calibration

Wide-swath synthetic aperture radar (SAR) coverage is provided by RADARSAT using a multiple-beam scanning strategy called ScanSAR. Each beam covers a different range, and is allocated a fixed period of time in which to transmit and receive radar pulses. During SAR processing, the data from each beam must be "stitched" together to form a complete image of the scanned area. This data must be radiometrically calibrated to compensate for antenna beam patterns. However, incorrect measurements of the satellite roll angle cause errors in radiometric calibration, and can lead to visible artifacts in the image (e.g. banding). A new ScanSAR data acquisition technique is proposed that improves roll angle estimation through the use of radar pulses, transmitted by one beam and received by another. The new data are called "hybrid beam data" and can be utilized with modified versions of existing roll estimation algorithms. This paper shows how the hybrid beam data are collected, accommodating pulse repetition frequency, range gate delay, and other timing changes as beams are switched.     

一种新颖的星载SAR无线内定标方法研究

内定标利用雷达系统内部设备和定标通路来测量系统各部分幅度和相位在成像过程中的相对变化,是保证雷达图像辐射精度的重要手段。该文针对传统有线内定标方案定标通路未覆盖相控阵天线TR输出端至无源阵面路径、定标网络庞大且自身误差控制难等不足,提出了一种新颖的利用辅助天线的无线内定标方法,给出了定标原理和分析模型,推导了SAR天线TR通道幅相特性和系统传递函数的标定方法,并在典型星载SAR系统参数下对标定误差进行了仿真分析,仿真结果表明,辅助天线支撑杆位置引起的TR通道幅度标定误差在10–3 dB量级,可以忽略;引起相位标定误差与支撑杆位置偏差密切相关,可依据文中给出的仿真曲线得到。支持杆位置引起的系统传递函数幅度标定误差小于0.1 dB;引起的相位标定误差对支撑杆位置偏差不敏感。最后在实际相控阵天线上对无线内定标方法进行了验证,获取了TR通道幅相特性标定的实测结果,表明了该方案的可行性和有效性。      

A digital calibration method for synthetic aperture radar systems

A basic method to calibrate imagery from synthetic aperture radar (SAR) systems is presented. SAR images are calibrated by monitoring all the terms of the radar equation. This procedure includes the use of both external (calibrated reference reflectors) and internal (system-generated calibration signals) sources to monitor the total SAR system transfer function. To illustrate the implementation of the procedure, two calibrated SAR images (X-band, 3.2-cm wavelength) are presented, along with the radar cross-section measurements of specific scenes within each image. The sources of error within the SAR image calibration procedure are identified     

Terrain influences in SAR backscatter and attempts to theircorrection

Synthetic aperture radar (SAR) images reveal radiometric image distortions that are caused by terrain undulations. The authors present the results of a study extracting and investigating the various components of these terrain influences. An imaging model, is set up for the geometric rectification of the SAR image and for a reconstruction of the imaging geometry. A prerequisite for the setup of this model is the use of a digital elevation model. Eight different geometric parameters are derived and investigated for their influence on grey-value variations in the geocoded SAR image. Image grey-value variations of three major land-use classes-forest, agricultural land, and urban/suburban areas-are examined. Empirical models of the SAR-backscatter variations are used to describe the relations between image grey values and various geometric parameters