全极化辐射计的2GHz带宽数字相关系统设计

全极化微波辐射计是进行海面风场测量的新型被动式遥感器。数字极化辐射计通过数字相关器对两个极化通道做相关处理,完成四个Stokes矢量参数的测量,反演海面风场。文中提出辐射计核心器件宽带数字相关器的设计方法。双极化通道通过IQ变换转化为两对正交信号,四片国内自主研制的A/D转换器以2.2Ghz采样率采样四路信号,采样结果通过多路复用DEMUX芯片复用后,传输到新一代现场可编程门阵列(FPGA)-Virtex5芯片中作进一步多路复用和相关运算。文中详细介绍了系统结构,数据处理流程和测试结果。     

Faraday rotation correction in the polarimetric mode of MIRAS

This paper describes a new method to compensate the Faraday rotation in polarimetric radiometric measurements under the assumption that the polarimetric brightness temperature matrix of the target is diagonal. Simulations results of how this method can be used within the Soil Moisture and Ocean Salinity mission and the Microwave Imaging Radiometer with Aperture Synthesis imaging radiometer are presented.     

Fully polarimetric microwave radiometer for remote sensing

The design, characteristics, and operation of the Helsinki University of Technology Fully Polarimetric Radiometer (FPoR) are described. The developed 36.5-GHz radiometer can be used for airborne remote sensing; however, ground-based and laboratory measurements are also possible. A direct cross-correlation technique with analog correlators, which measures all four Stokes parameters simultaneously, is applied. This paper is the first successful demonstration of an analog direct cross-correlation technique for polarimetric remote sensing radiometry. The radiometer was subjected to a variety of laboratory tests, and considerable attention is given to analysis of the characteristics of the instrument. Owing to the effective active temperature control system of the receiver, the radiometric stability of the instrument was found to be very high; test results showing stabilities below 10 mK and of 4-40 mK on time scales of 800 and 8000 s, respectively, are presented. Furthermore, the absolute accuracy of the system is analyzed to be at a sub-Kelvin level for most measurement conditions. A maritime wind vector experiment was carried out over the Gulf of Finland. The feasibility and performance of the applied correlation technique and the whole radiometer system were verified for fully polarimetric airborne measurements. The obtained brightness temperatures of the first three Stokes parameters show typical harmonic behavior with respect to the surface wind; the results suggest, however, that the model coefficients presented earlier for oceans may not be directly applicable for different conditions.     

A fully polarimetric optical time-domain reflectometer

We report the implementation of the first fully polarization sensitive optical time domain reflectometer (POTDR). This nondestructive measurement technique, requiring access to one fiber end only, enables the determination of both the linear and twist induced circular birefringence distribution along the fiber. This information, combined with a knowledge of the mode coupling length, enables the evaluation of the polarization mode dispersion (PMD) properties of fibers and optical cables exhibiting both types of birefringence, as well as identifying fiber sections with and without twist. The POTDR, which has a spatial resolution of 0.3 m, could also be used for monitoring the birefringence properties of fibers at the manufacturing stage, leading to optimization of fabrication techniques to minimize PMD     

The WindSat spaceborne polarimetric microwave radiometer: sensor description and early orbit performance

The global ocean surface wind vector is a key parameter for short-term weather forecasting, the issuing of timely weather warnings, and the gathering of general climatological data. In addition, it affects a broad range of naval missions, including strategic ship movement and positioning, aircraft carrier operations, aircraft deployment, effective weapons use, underway replenishment, and littoral operations. WindSat is a satellite-based multifrequency polarimetric microwave radiometer developed by the Naval Research Laboratory for the U.S. Navy and the National Polar-orbiting Operational Environmental Satellite System Integrated Program Office. It is designed to demonstrate the capability of polarimetric microwave radiometry to measure the ocean surface wind vector from space. The sensor provides risk reduction for the development of the Conical Microwave Imager Sounder, which is planned to provide wind vector data operationally starting in 2010. WindSat is the primary payload on the Department of Defense Coriolis satellite, which was launched on January 6, 2003. It is in an 840-km circular sun-synchronous orbit. The WindSat payload is performing well and is currently undergoing rigorous calibration and validation to verify mission success.     

全极化雷达极化标校技术研究

受各种非理想因素的影响,极化测量雷达难以直接测得目标准确的极化散射矩阵（polarization scattering matrix,PSM）,因此,高精度极化标定是准确获取和利用目标极化信息的基础.本文从极化标校的基本原理出发,建立了PSM测量的误差模型,分析了主要误差因素对PSM测量结果的影响,阐述了无源和有源极化标定方法并对其性能和适用场景进行比较,分析了极化标校器的参数对极化标校精度的影响.分析结果表明,目标的交叉极化分量与极化测量雷达系统的极化隔离度处在相同水平时,目标的交叉极化分量测量结果会明显偏离其真值,通过极化隔离度优于测量系统隔离度无源或有源极化定标体对系统进行标校可以减小系统极化隔离度带来的测量误差.本文工作为极化定标体的研制和全极化测量雷达的系统标校奠定了基础.     

An emissivity-based wind vector retrieval algorithm for the WindSat polarimetric radiometer

The Naval Research Laboratory WindSat polarimetric radiometer was launched on January 6, 2003 and is the first fully polarimetric radiometer to be flown in space. WindSat has three fully polarimetric channels at 10.7, 18.7, and 37.0 GHz and vertically and horizontally polarized channels at 6.8 and 23.8 GHz. A first-generation wind vector retrieval algorithm for the WindSat polarimetric radiometer is developed in this study. An atmospheric clearing algorithm is used to estimate the surface emissivity from the measured WindSat brightness temperature at each channel. A specular correction factor is introduced in the radiative transfer equation to account for excess reflected atmospheric brightness, compared to the specular assumption, as a function wind speed. An empirical geophysical model function relating the surface emissivity to the wind vector is derived using coincident QuikSCAT scatterometer wind vector measurements. The confidence in the derived harmonics for the polarimetric channels is high and should be considered suitable to validate analytical surface scattering models for polarized ocean surface emission. The performance of the retrieval algorithm is assessed with comparisons to Global Data Assimilation System (GDAS) wind vector outputs. The root mean square (RMS) uncertainty of the closest wind direction ambiguity is less than 20/spl deg/ for wind speeds greater than 6 m/s and less than 15/spl deg/ at 10 m/s and greater. The retrieval skill, the percentage of retrievals in which the first-rank solution is the closest to the GDAS reference, is 75% at 7 m/s and 85% or higher above 10 m/s. The wind speed is retrieved with an RMS uncertainty of 1.5 m/s.     

全极化微波辐射计数字相关器下变频算法设计

全极化微波辐射计是一种用于海洋表面风场测量的新型被动微波遥感器,数字相关器是全极化辐射计的核心部件。针对全极化微波辐射计接受到的数字信号采样率较高的问题,提出应用多相滤波的算法以降低数字信号速率,从而满足FPGA时序约束的要求。之后为进一步节省乘法器资源和提高处理速率,滤波部分采用分布式算法,两种方法结合完成下变频模块处理,通过Matlab软件对下变频算法进行了仿真。仿真结果表明算法是可行的。     

HUT fully polarimetric calibration standard for microwave radiometry

This paper describes the Helsinki University of Technology's Fully Polarimetric Calibration Standard (FPCS). The developed standard generates a complete Stokes reference vector and it is applied for the end-to-end absolute calibration of a fully polarimetric microwave radiometer at 36.5 GHz. The FPCS is based on the function principle of a Gasiewski-Kunkee linearly polarized (tripolarimetric) standard, with an additional phase retardation plate to generate the fourth Stokes parameter. Design considerations and operational aspects of the standard are discussed in this paper. An advanced calibration procedure, which takes advantage of both the tripolarimetric and fully polarimetric calibration scenes to suppress calibration uncertainties, is introduced. The feasibility of the standard has been verified and the generated brightness temperatures in a sample calibration are presented. An extensive set of tests has been performed to evaluate the characteristics and performance of the calibration standard. Furthermore, the use of the advanced calibration procedure to measure the characteristics of the phase retardation plate has been successfully demonstrated. The achievable calibration accuracy is analyzed and discussed relative to requirements for maritime wind vector measurements; the results indicate that the pixel-to-pixel retrieval of the wind speed is possible with high accuracy and the retrieval of the wind direction with at least moderate accuracy. In addition to calibration of a fully polarimetric radiometer, other potential applications, e.g., linearity measurements, are discussed.     

A calibration method for fully polarimetric microwave radiometers

A technique for absolute end-to-end calibration of a fully polarimetric microwave radiometer is presented. The technique is based on the tripolarimetric calibration technique of Gasiewski and Kunkee, but is extended to provide a means of calibrating all four Stokes parameters. The extension is facilitated using a biaxial phase-retarding microwave plate to provide a precisely known fourth Stokes signal from the Gasiewski-Kunkee (GK) linearly polarized standard. The relations needed to determine the Stokes vector produced by the augmented standard are presented, and the effects of nonidealities in the various components are discussed. The application of the extended standard to determining the complete set of radiometer constants (the calibration matrix elements) for the National Oceanic and Atmospheric Administration Polarimetric Scanning Radiometer in a laboratory environment is illustrated. A calibration matrix inversion technique and error analysis are described, as well. The uncertainties associated with practical implementation of the fully polarimetric standard for spaceborne wind vector measurements are discussed relative to error thresholds anticipated for wind vector retrieval from the U.S. National Polar-Orbiting Environmental Satellite System.     

Calibration of WindSat polarimetric channels with a vicarious cold reference

Absolute calibration of WindSat's third and fourth Stokes brightness temperatures (T/sub 3/ and T/sub 4/) is needed at the tenth of Kelvin level in order to adequately resolve their dependence on wind direction. Previous aircraft based fully polarimetric microwave radiometers have generally relied on "circle flights", during which a single area of the ocean is observed at all azimuth angles, to estimate residual biases in the calibration of its polarimetric channels. WindSat, the first spaceborne fully polarimetric microwave radiometer, operates in low Earth orbit and thus cannot execute this traditional calibration technique. A new method is presented to estimate the residual biases that are present in WindSat's T/sub 3/ and T/sub 4/ estimates. The method uses a vicarious cold reference brightness temperature applied to measurements made by WindSat at /spl plusmn/45/spl deg/ slant linear (T/sub P/ and T/sub M/) and left- and right-hand circular (T/sub L/ and T/sub R/) polarization. WindSat derives the third and fourth Stokes brightness temperatures by the differences T/sub P/-T/sub M/ and T/sub L/-T/sub R/, respectively. The method is demonstrated by applying it to the 10.7-GHz WindSat observations. Calibration biases of 0.2-0.6 K are determined with a precision of 0.04 K.     

Wind direction over the ocean determined by an airborne, imaging,polarimetric radiometer system

The speed and direction of winds over the ocean can be determined by polarimetric radiometers. This has been established by theoretical work and demonstrated experimentally using airborne radiometers carrying out circle flights and thus measuring the full 360° azimuthal response from the sea surface. An airborne experiment, with the aim of measuring wind direction over the ocean using an imaging polarimetric radiometer, is described. A polarimetric radiometer system of the correlation type, measuring all four Stokes brightness parameters, is used. Imaging is achieved using a 1-m aperture conically scanning antenna. The polarimetric azimuthal signature of the ocean is known from modeling and circle flight experiments. Combining the signature with the measured brightness data from just a single flight track enables the wind direction to be determined on a pixel-by-pixel basis in the radiometer imagery     

Soil moisture retrieval using the C-band polarimetric scanning radiometer during the Southern Great Plains 1999 Experiment

The Advanced Microwave Scanning Radiometer (AMSR) holds promise for retrieving soil moisture in regions with low levels of vegetation. Algorithms for this purpose have been proposed, but none have been rigorously evaluated due to a lack of datasets. Accordingly, the Southern Great Plains 1999 Experiment (SGP99) was designed to provide C-band datasets for AMSR algorithm development and validation. Ground observations of soil moisture and related variables were collected in conjunction with aircraft measurements using a C-band radiometer similar to the AMSR sensor (6.92 GHz), the Polarimetric Scanning Radiometer with its C-band scanhead (PSR/C). The study region has been the focus of several previous remote sensing field experiments and contains vegetation conditions compatible with the expected capabilities of C-band for soil moisture retrieval. Flights were conducted under a wide range of soil moisture conditions, thus providing a robust dataset for validation. A significant issue found in data processing was the removal of anthropogenic radio-frequency interference. Several approaches to estimating the parameters of a single-channel soil moisture retrieval algorithm were used. PSR/C soil moisture images show spatial and temporal patterns consistent with meteorological and soil conditions, and the dynamic range of the PSR/C observations indicates that the AMSR instrument can provide useful soil moisture information.     

全极化频谱外推的合成孔径雷达成像分辨率增强方法

全极化合成孔径雷达可对不同极化通道分别独立进行频谱外推来增强分辨率,但分别独立处理难以利用极化信息的冗余性与互补性．将目标全极化数据分解到反映三种独立散射机理的Pauli基上,对分解后的全极化数据进行三维联合频谱外推处理,并利用实测数据进行了对比分析．结果表明：极化分解后,通过三维联合频谱外推的方法相比传统方法具有更高的预测模型匹配精度,可获得更好的分辨率增强性能．     

基于实测数据的分时全极化雷达海杂波分布研究

首先基于IPIX分时全极化雷达的海杂波实测数据,分析了瑞利分布、韦布尔分布、对数正态分布和K分布对实测数据的拟合效果,发现对数正态分布拟合水平极化接收通道的杂波数据效果较好,K分布拟合垂直极化接收通道的杂波数据效果较好,但是仍然存在海杂波"拖尾"区域拟合不理想的情况.为此,假设海杂波中的布拉格散射体的回波服从K分布,离散海尖峰的回波服从对数正态分布,海杂波整体服从K分布和对数正态分布的叠加混合分布,提出了一种具有闭式表达式的混合分布,以改善海杂波分布"拖尾"部分的拟合效果.实测数据分析表明,本文提出的混合分布对海杂波幅度分布的拟合效果优于对数正态分布和K分布.     

Analysis of correlation and total power radiometer front-ends using noise waves

A complete and systematic noise analysis of radiometer front-ends, including both total power and correlation measurements, is presented. The procedure uses the concepts of noise waves and S-parameters, widely used in microwave systems design and takes into account full noise characterization of receivers including mismatch effects. The general formulation is compatible with known total power radiometer analysis and is specially appropriate in correlation radiometers for which the effect of nonideal components, such as input isolators, is analyzed. Along with numerical simulations, simple formulas are given to compute the measured visibility in nonideal conditions. The analysis is validated using experimental results consisting of correlation measurements of four receivers placed inside an anechoic chamber. Good agreement between theoretical predictions and experimental data is observed.     

全极化雷达相干两点源角度欺骗干扰识别方法

相干两点源能够对单脉冲体制雷达形成有效的角度欺骗干扰,通过干扰机之间协同,达到诱偏雷达波束、掩护目标突防的目的．根据两点源角度欺骗干扰原理,利用相干两点源和雷达目标全极化回波特性差异,设计了识别相干两点源角度欺骗干扰识的二元假设检验方法,理论分析和仿真结果证实了方法的有效性．     

External calibration of a polarimetric synthetic-aperture radar for a limited number of types of reference reflectors

Capabilities of the external calibration of a polarimetric synthetic-aperture radar (SAR) and the monitoring of the radar parameters under flight conditions for a limited number of types of reference calibration targets are determined. An analytic form of the algorithm for neutralization of SAR hardware distortions in the measurement of the scattering matrix of the observed object is proposed and possibilities for application of this algorithm under the conditions of unrealizable total polarimetric calibration are analyzed. The possibilities of determining the particular calibration parameters and their combinations with the use of one or two reference reflectors are analyzed. It is shown that, in the measurement of the scattering matrix of a reciprocal object, it is sufficient to use two types of reference reflectors.     

The radiometer: a 130-year-old mystery

Radiometers are today just considered to be a novelty, but once these devices constituted serious science. Most commercial radiometers are constructed of a machine-blown, lightbulb-shaped glass envelope, which encloses four vanes spinning on a central pivot within a partial vacuum. When set near a sunny window it spins rapidly around in circles. There are a few commercial variations in the design but the principle of operation is the same; the problem is that no one knows for sure what that principle is. Many theories exist and a good number of scientists have grappled with the problem over the years. Fortunately for us, most of the current theories can be easily disproved using a little logic and some basic 21st century science. However, the question remains as to what makes it spin. This article looks at past and present theories, and suggests some experimental approaches