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.     

微波辐射计反演海表面温度和风场研究进展

海表面温度（SST）和海面风场是重要的海面气象水文参数,与海面的大部分物理过程关系密切。卫星遥感探测技术可以同时获得全球范围内的海表面温度和海面风场数据。其中,微波辐射计遥感海表面温度和风场具有不受云层遮挡的优势,本文就其发展过程作简要介绍。     

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.     

综合孔径微波辐射计互耦模型与校正方法

利用阻抗参数法分析天线互耦,根据综合孔径微波辐射成像原理,通过耦合矩阵建立了综合孔径微波辐射计互耦模型。用给出的模型,对互耦引起的反演亮温失真和校正方法进行了深入的理论分析,并给出通过优化天线匹配网络减小互耦影响的方法。仿真结果表明了互耦模型和校正方法的有效性,清晰地反映了互耦造成的反演误差。     

An L-band geophysical model function for SAR wind retrieval using JERS-1 SAR

An L-band geophysical model function is developed using Japanese Earth Resources Satellite-1 (JERS-1) synthetic aperture radar (SAR) data. First, we estimate the SAR system noise, which has been a serious problem peculiar to the JERS-1 SAR. It is found that the system noise has a feature common in all the SAR images and that the azimuth-averaged profile of noise can be expressed as a parabolic function of range. By subtracting the estimated noise from the SAR images, we can extract the relatively calibrated ocean signals. Second, using the noise-removed SAR data and wind vector data from the NASA Scatterometer and buoys operated by the Japan Meteorological Agency, we generate a match-up dataset, which consists of the SAR sigma-0, the incidence angle, the surface wind speed, and wind direction. Third, we investigate the sigma-0 dependence on incidence angle, wind speed, and wind direction. While the incidence angle dependence is negligible in the present results, we can derive distinct sigma-0 dependence on wind speed and direction. For wind speeds below 8 m/s, the wind direction dependence is not significant. However, for higher wind speeds, the upwind-downwind asymmetry becomes very large. Finally, taking into account these characteristics, a new L-band-HH geophysical model function is produced for the SAR wind retrieval using a third-order harmonics formula. Resultant estimates of SAR-derived wind speed have an rms error of 2.09 m/s with a negligible bias against the truth wind speed. This result enables us to convert JERS-1 SAR images into the reliable wind-speed maps.     

A nonlinear optimization algorithm for WindSat wind vector retrievals

WindSat is a space-based polarimetric microwave radiometer designed to demonstrate the capability to measure the ocean surface wind vector using a radiometer. We describe a nonlinear iterative algorithm for simultaneous retrieval of sea surface temperature, columnar water vapor, columnar cloud liquid water, and the ocean surface wind vector from WindSat measurements. The algorithm uses a physically based forward model function for the WindSat brightness temperatures. Empirical corrections to the physically based model are discussed. We present evaluations of initial retrieval performance using a six-month dataset of WindSat measurements and collocated data from other satellites and a numerical weather model. We focus primarily on the application to wind vector retrievals.     

New correlation radiometer for microwave thermography

We describe a new radiometric method capable of reaching a better control of the volume of the tissue under investigation in microwave thermography (this technique is mainly used in biomedical applications). Our process is based on the fact that two (or several) probes with a volume under investigation in common are collecting thermal noise signals which are correlated.     

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.     

激光辐射器的性能测试系统

介绍了激光辐射器的基本工作原理和性能参数,根据实际应用进行了激光性能测试系统的研究,对激光性能测试系统的结构设计、光路设计、测量控制设计以及图像处理和测量分析软件作了详细论述.经误差分析认为设计能够满足实际应用中对激光器件性能的测试要求.     

Discrete scatter model for microwave radar and radiometer responseto corn: comparison of theory and data

As part of the Multisensor Aircraft Campaign, MACHYDRO, two microwave sensors, NASA's Airborne Synthetic Aperture Radar (AIRSAR) and Pushbroom Microwave Radiometer (PBMR) collected data over the same corn fields during the summer of 1990. During these flights, measurements were made on the ground of soil moisture and plant parameters. In this paper the measured canopy and soil parameters are used in a discrete scatter model to predict the response of both sensors (radar and radiometer). A distorted Born approximation is used to compute the scattering coefficient for the corn canopy. The backscatter coefficient gives the radar response and the radiometer response is obtained by integrating the bistatic coefficient over all scattering angles above ground. The objective of this analysis is to test the model and, in particular, to determine how well a single set of plant parameters and single model can yield agreement with both the radar and radiometer measurements. The model values are in reasonably good agreement with the measurements at horizontal polarization and reflect observed changes in soil moisture     

综合孔径微波辐射计光学实时信号处理系统

分析了综合孔径微波辐射计光学实时信号处理系统的基本原理及关键技术,研制了一套X波段6单元综合孔径红外实时信号处理系统样机,并进行了系统测试及点辐射源的一维实时成像实验,获得了清晰的干涉条纹图像.初步的实验结果验证了综合孔径辐射计光学实时信号处理的可行性及系统设计的正确性.     

ESTAR: a synthetic aperture microwave radiometer for remote sensingapplications

ESTAR represents a new technology being developed for passive microwave remote sensing of the environment from space. The instrument employs an interferometric technique called aperture synthesis in which the coherent product from pairs of antennas is measured as a function of pair spacing. Substantial reductions in the antenna aperture needed for a given spatial resolution can be achieved with this technique. As a result, aperture synthesis could lead to practical passive microwave remote sensing instruments in space to measure parameters such as soil moisture and ocean salinity which require observations at long wavelengths and, therefore, large antennas. ESTAR is an L-band, aircraft built as part of research to develop this technique ESTAR is a hybrid real-and-synthetic aperture radiometer which employs stick antennas to achieve resolution along track and uses aperture synthesis to achieve resolution across track. Experiments to validate the instrument's ability to measure soil moisture have recently been conducted at the USDA watersheds at Walnut Gulch in Arizona and the Little Washita River in Oklahoma. The results of both experiments indicate that a valid image reconstruction and calibration have been obtained for this remote sensing technique     

基于双谱模型的被动微波遥感土壤湿度反演

利用随机粗糙表面的双谱散射模型计算了粗糙裸土壤表面的发射率,并建立了基于双谱模型的被动微波遥感的土壤湿度的人工神经网络反演方法.结果表明,基于双谱散射模型计算粗糙裸土壤表面发射率是可行的,利用人工神经网络反演方法,仅采用两个发射率的数据组合便可以反演出精度较高的土壤湿度值,且反演方法具有一定的抗噪性能.     

Surface wind speed retrieval using passive microwave polarimetry:the dependence on atmospheric stability

The results of the Joint US/Russia Internal Wave Remote Sensing Experiment are presented. An airborne Ka-band radiometer-polarimeter was used for surface wind vector retrieval based upon the polarizational anisotropy effect. The anisotropy versus wind speed curve was noted to be approximately twice as steep under unstable atmospheric stratification than under stable or neutral stratification. The dependence of anisotropy on both friction velocity and stability parameter z/L was examined. The correlation between anisotropy and stability parameter is significant, especially for slightly stable stratification. The paper concludes that atmospheric stratification should be taken into consideration to improve the wind speed retrieval algorithm     

K波段微波辐射计通道优化和定标研究

利用K波段20~30 GHz探测大气水汽廓线,深入分析地基微波辐射计的关键技术与性能指标,确定通道频率和个数的优化组合,并对所选通道利用非线性定标方程外定标,确定地基微波辐射不同通道的定标曲线和系统注入噪声,分析各通道定标误差。进而,采用奇偶周期方式的大气定标法进行参数校正。试验证明:以全球定位系统(GPS)无线电探空进行性能为标准,定标后的数据反演均方差为0.52 g/m³,相关系数为0.965.     

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。     

Optimum frequencies of a passive microwave radiometer for tropospheric path-length correction

Radio-astronomical observations require accurate calibration of tropospheric path length. Such calibration can be achieved by microwave radiometers operating near the 22-GHz water vapor line. However, the performances of current passive microwave radiometers are meteorology-profile dependent. This is shown due mainly to incorrect frequency combinations and to saturation of brightness temperatures. By properly selecting an optimum frequency pair and removing the saturation effect, the dependency is alleviated and can be further adjusted by surface measurements alone. Hence, a universal calibration equation is applicable to all environmental conditions. Optimum frequency pairs are systematically searched. Simulation analysis indicates that calibration for the tropospheric water-vapor path-length error is better than 0.3 cm at zenith and better than 2 cm for an elevation angle as low as10deg.     

星载微波辐射计定标及误差分析

微波辐射计是气象卫星对地观测微波遥感重要仪器之一,通过接收被观测场景辐射的微波能量来探测目标的特性.而微波辐射定标是微波遥感探测资料定量化应用的基础.以目前气象卫星的微波辐射计以此为例,对微波辐射计的定标方法进行了概述,并对微波辐射计的定标误差进行了分析和估算.     

云雷达联合微波辐射计的水凝物识别研究

为降低飞机飞行时的积冰风险,利用模糊逻辑算法和神经网络算法对云雷达联合微波辐射计探测数据进行水凝物分类研究.研究结果表明,两种算法均能有效地完成水凝物分类:模糊逻辑算法对退极化比极为敏感;神经网络算法可以区分水凝物是否存在和进行小粒径粒子分类,但对大粒径非球形粒子不敏感.本研究可从水凝物类型的角度对飞机积冰提供预警服务.