The microwave radiometer aboard ERS-1.1. Characteristics andperformances

The microwave radiometer onboard ERS-1 has two channels at 23.8 and 35.6 GHz. It is a nadir pointing sensor, which is aimed at the correction of the altimeter tropospheric path delay. The characteristics of the instrument are described, and its performance is established from ground calibration results. Using early data obtained after launch, the in-flight engineering calibration is presented, showing that the radiometer behavior is nominal. The gain stability permits noise reduction, and the radiometer performance appears to be as good as specified     

Tropospheric wet path-delay measurements

A dual-channel microwave radiometer measuring the sky brightness temperature at the frequencies 21.0 and 31.4 GHz, an infrared spectral hygrometer (IRSH) measuring the ratio of the radiation from the sun at the wavelengths 931 and 880 nm, and radiosondes have been used simultaneously to determine the excess path length due to water vapor (wet path delay) of radio waves propagating through the troposphere. By a least squares fit of the measured parameters from the microwave radiometer and the infrared spectral hygrometer, respectively, to the wet path delay calculated from the radiosonde profiles, the following root mean square (rms) differences of the wet path delay in the zenith direction were obtained: infrared spectral hygrometer-radiosondes, 1.1 cm; microwave radiometer-radiosondes, 0.7 cm; and 0.5 cm for a selected group of "good weather" data. The wet path delay was also calculated from surface meteorological measurements alone and the rms difference compared with corresponding radiosonde data was 2.0 cm in the zenith direction.     

TOPEX/Poseidon Microwave Radiometer (TMR). I. Instrumentdescription and antenna temperature calibration

The TOPEX/Poseidon microwave radiometer (TMR) is a three-frequency radiometer flown on the TOPEX/Poseidon (T/P) satellite in low Earth orbit. It operates at 18, 21, and 37 GHz in a nadir-only viewing direction which is co-aligned with the T/P radar altimeters. The TMR monitors and corrects for the propagation path delay of the altimeter radar signal due to water vapor and nonprecipitating liquid water in the atmosphere. The paper describes the TMR instrument and the radiometric instrument calibration required to derive antenna temperature (T_A ) from the raw digital data. T_A precision of 0.4 K is predicted on orbit in all expected thermal environments, T_A accuracy of 0.5-0.6 K is expected following a post-launch field calibration campaign. These performance figures represent a significant improvement over those of the Seasat and Nimbus-G Scanning Multichannel Microwave Radiometer on which TMR is based. The improvements are the result of specific hardware design and calibration changes. Hardware changes include a redesigned feed horn, to reduce impedance mismatches, and the addition of radomes over the feed and sky horns, to reduce thermal variations. Calibration changes involve more extensive temperature cycling and data analysis during thermal/vacuum testing     

TOPEX/Poseidon microwave radiometer (TMR). III. Wet troposphererange correction algorithm and pre-launch error budget

For pt.II see ibid., vol.33, no.1, p.138-46 (1995). The sole mission function of the TOPEX/Poseidon microwave radiometer (TMR) is to provide corrections for the altimeter range errors induced by the highly variable atmospheric water vapor content. The three TMR frequencies are shown to be near-optimum for measuring the vapor-induced path delay within an environment of variable cloud cover and variable sea surface flux background. After a review of the underlying physics relevant to the prediction of 5-40 GHz nadir-viewing microwave brightness temperatures, the authors describe the development of the statistical, two-step algorithm used for the TMR retrieval of path delay. Test simulations are presented which demonstrate the uniformity of algorithm performance over a range of cloud liquid and sea surface wind speed conditions. The results indicate that the inherent algorithm error (assuming noise free measurements and an exact physical model) is less than 0.4 cm of retrieved path delay for a global representation of atmospheric conditions. An algorithm error budget is developed which predicts an overall algorithm accuracy of 0.9 cm when modeling uncertainties are included. When combined with expected TMR antenna and brightness temperature accuracies, an overall measurement accuracy of 1.2 cm for the wet troposphere range correction is predicted     

TOPEX/Poseidon Microwave Radiometer (TMR). II. Antenna patterncorrection and brightness temperature algorithm

For pt.I see ibid., vol.33, no.1, p.125-37 (1995). The calibrated antenna temperatures measured by the TOPEX Microwave Radiometer are used to derive radiometric brightness temperatures in the vicinity of the altimeter footprint. The basis for the procedure devised to do this-the antenna pattern correction and brightness temperature algorithm-is described in the paper, along with its associated uncertainties. The algorithm is based on knowledge of the antenna pattern, the ground-based measurements of which are presented along with their analyses. Using the results of these measurements, the authors perform an error analysis that yields the net uncertainties in the derived TMR footprint brightness temperatures. The net brightness temperature uncertainties range from 0.79 to 0.88 K for the three TMR frequencies, and include the radiometer calibration uncertainties which range from 0.54 to 0.57 K. the authors also derive an estimate of the uncertainty incurred by using brightness temperatures measured in the ~40 km TMR footprint to estimate path delay in the ~3 km altimeter footprint. The RMS difference in path delay averaged over the largest TMR footprint relative to that in the altimeter footprint is estimated to be about 0.3 cm. Finally, the authors discuss the error associated with using unequal beams at the three TMR frequencies to derive path delays, and describe an approach using along-track averaging of the algorithm brightness temperatures to reduce this error     

用微波辐射计测量大气积分水汽的改进算法

提出选择具有最佳频率组合的双通道微波辐射计,通过测量路径上的湿延迟来解算积分水汽含量的改进算法。由于在湿延迟测量模式中,消除了云中液态水对湿延迟测量的"过量"影响,湿延迟测量具有很高的精度。因此,改进算法可提高云天大气积分水汽的测量精度,与由辐射亮温直接解算的传统算法相比测量误差可减小35%以上。     

WVR-GPS comparison measurements and calibration of the 20-32 GHz tropospheric water vapor absorption model

Collocated measurements of opacity (from water vapor radiometer brightness temperatures) and wet path delay (from ground-based tracking of global positioning satellites) are used to constrain the model of atmospheric water vapor absorption in the 20-32 GHz band. A differential approach is presented in which the slope of opacity-versus-wet delay data is used as the absorption model constraint. This technique minimizes the effects of radiometric calibration errors and oxygen model uncertainties in the derivation of a best-fit vapor absorption model. A total of approximately five months of data was obtained from two experiment sites. At the Cloud and Radiation Testbed (CART) site near Lamont, Oklahoma, three independent water vapor radiometers (WVRs) provided near-continuous opacity measurements over the interval July-September 1998. At the NASA/Goldstone tracking station in the California desert two WVRs; obtained opacity data over the September-October 1997 interval. At both sites a Global Positioning Satellite (GPS) receiver and surface barometer obtained the data required for deriving the zenith wet delays over the same time frames. Measured values of the opacity-versus-wet delay slope parameter were obtained at four WVR frequencies (20.7, 22.2, 23.8, and 31.4 GHz) and compared with predictions of four candidate absorption models referenced in the literature. With one exception, all three models provide agreement within 5% of the opacity-versus-wet delay slope measurements at all WVR frequencies at both sites. One model provides agreement for all channels at both sites to the 2-3% level. This absorption model accuracy level represents a significant improvement over that attainable using radiosondes.     

对流层电波折射误差高精度实时修正

介绍了利用对大气中水汽含量敏感的微波辐射计，在电波传播路径上实时测量大气辐射亮温直接进行折 射误差修正的方法。该方法是在电波传播路径上直接获得折射修正信息样品的，不受大气时变特性和水平不均匀性的 影响，因而可提高修正精度。在天顶方向湿项折射误差实时修正精度可达厘米级。     

Microwave radiometric technique to retrieve vapor, liquid and ice.I. Development of a neural network-based inversion method

With the advent of the microwave radiometer, passive remote sensing of clouds and precipitation has become an indispensable tool in a variety of meteorological and oceanographical applications. There is wide interest in the quantitative retrieval of water vapor, cloud liquid, and ice using brightness temperature observations in scientific studies such as Earth's radiation budget and microphysical processes of winter and summer clouds. Emission and scattering characteristics of hydrometeors depend on the frequency of observation. Thus, a multifrequency radiometer has the capability of profiling cloud microphysics. Sensitivities of vapor, liquid, and ice with respect to 20.6, 31.65 and 90 GHz brightness temperatures are studied. For the model studies, the atmosphere is characterized by vapor density and temperature profiles and layers of liquid and ice components. A parameterized radiative transfer model is used to quantify radiation emanating from the atmosphere. It is shown that downwelling scattering of radiation by an ice layer results in enhancement at 90 GHz brightness temperature. Once absorptive components such as vapor and liquid are estimated accurately, then it is shown that the ice water path can be retrieved using ground-based three-channel radiometer observations. In this paper the authors developed two- and three-channel neural network-based inversion models. Success of a neural network-based approach is demonstrated using a simulated time series of vapor, liquid, and ice. Performance of the standard explicit inversion model is compared with an iterative inversion model. In part II of this paper, actual radiometer, and radar field measurements are utilized to show practical applicability of the inverse models     

基于地基微波辐射计的大气参数廓线遥感探测

介绍了实时连续遥感探测大气温度廓线、湿度廓线和液态水含量廓线的地基微波辐射计。通过无线电气球探空测量的温度、湿度廓线、太阳辐射计测量的晴天水汽总量与微波辐射计探测结果的对比,证明该微波辐射计可以可靠地探测大气温度、湿度和水汽总量。通过对两个典型天气事件微波辐射计的观测数据分析表明：微波辐射计还可以监测云和天气变化的过程信息。     

PIn. I. An operational nonlinear physical inversion algorithm forprecipitable and cloud liquid water estimate in nonraining conditionsover sea

For pt.II see ibid., vol.39, no.12, p.2575-86 (2001). An operational nonlinear physical inversion (PIn) algorithm for precipitable and cloud liquid water estimate is described. It is suited for a generic conical scanning satellite microwave radiometer acquisition over sea in nonraining conditions. The algorithm does not need any calibration phase and is independent of the availability of in situ data, being consistent in different geographical and climatological situations. Adopted formulation is addressed to provide observational data to help in validating water vapor and cloud fields produced by a numerical weather prediction model. Furthermore, such a technique can be utilized for the purpose of global reanalysis, improving estimates of primary fields of the hydrological cycle. A sensitivity study of the forward model and a comparison between output brightness temperatures and those from a robust numerical code are also reported. The discrepancies that result are considered acceptable with respect to instrumental constraints and computation time     

A New Instrument for the Determination of Radio Path Delay due to Atmospheric Water Vapor

A new water-vapor radiometer is being developed to determine atmospheric path-delay corrections for geodynamics applications. The radiometer has microwave channels at 20.7, 22.2, and 31.4 GHz, and includes new features which should promote radiometric precision while at the same time reduce instrument complexity. The instrument will be compact, portable, and will possess a self-diagnostic capability to ensure reliability. The performance objective is the determination of the line-of-sight path delay though the atmosphere due to water vapor with a one-sigma error less than 0.5 cm at the zenith.     

A three-year study of radio wave propagating delays due to tropospheric water vapor

Measurements of the radio emission from the sky at 21.0 and 31.4 GHz have been made from May 1980 to April 1983 at the Onsala Space Observatory on the west coast of Sweden. A total of 483 such observations have been used to calculate the water vapor induced propagation delay of radiowaves penetrating the troposphere. A comparison with simultaneous radiosonde launches made at Gothenburg-Landvetter Airport (37 km away from Ousala) gives a root mean square (rms) difference of 1.3 cm in the zenith direction and 1.1 cm for a certain group of stable weather data. The zenith wet path delays are also calculated with three different models based on traditional meteorological observations at the surface. The lowest rms difference obtained by using a model and compare the results with radiosonde data at the same site was 2.1 cm. A third site on the Swedish west coast was compared with the other two by using old monthly means of radiosonde data together with the three models. The offsets between the models and the radiosonde/water vapor radiometer (WVR) data are dependent on the model, on the site, and on the time of the year.     

用微波辐射计测量大气湿延迟和频率选择

采用具有最佳频率组合的双通道微波辐射计测量大气湿延迟,由于是在测量路径上实时获得测量信息的,测 量结果可反映大气时变特性和水平不均匀性,同时可消除云中液态水对湿延迟的“过量”估计。因此,用于电波折射误 差修正和解算大气积分水汽(或可降水量PWV )可有很高的精度。由历史气象探空数据分析,得出权函数随高度变化小 的频率对在不同气候区或地区不同,工作在最佳频率对时,在5km 以下高度权函数相对变化可小于5%。     

哥德堡地区基于无线通讯网络的水汽密度监测分析

利用无线通讯网络中的微波链路来监测降雨和水汽等是大气环境监测的新技术之一。这个技术可以测量近地面的降雨强度和水汽密度等气象参数,具有时空分辨率高、成本低等优势。利用瑞典爱立信公司(Ericsson)提供的位于哥德堡地区E频段的微波通讯链路资料、位于链路一端的气象站1资料和由瑞典气象水文研究所(SMHI)气象网站提供的气象站2资料,对2017年06月13日至2017年07月13日近1个月的水汽密度进行反演计算和分析。结果表明:同一区域的不同地点处的气象要素有一定的差异性,同一区域的温度会有一定的浮动(0~4℃),两者之间的相关性为0. 87;微波通讯链路反演的水汽密度结果与研究区域的地面气象站1和气象站2测量结果有很好的一致性,两者之间的相关性分别为0. 89和0. 97,均方根误分别差为0. 75 g m3和0. 79 g m3;利用微波链路,与现有的湿度监测方法相比,可以为现有的天气监测网络提供额外的丰富的数据源。     

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.     

SSM/I instrument evaluation

The Special Sensor Microwave/Imager (SSM/I) instrument and scan geometry are briefly described. The results of investigations of the stability of the gain, calibration targets and spin rate, the radiometer noise and sensitivity, the coregistration, the beam width and main-beam efficiency of the antenna beams, and the absolute calibration and geolocation of the instrument are presented. The results of this effort demonstrate that the SSM/I is a stable, sensitive, and well-calibrated microwave radiometric system capable of providing accurate brightness temperatures for microwave images of the Earth and for use by environmental product retrieval algorithms. It is predicted that this SSM/I and the 11 future ones currently built or to be built will provide high-performance microwave measurements for determination of global weather and critical atmospheric, oceanographic, and land parameters to operational forecasters and users and the research community for the next two decades     

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.     

Ground-Based Microwave Radiometric Observations of the Temporal Vanation of Atmospheric Geopotential Height and Thickness

Since 1981, the Wave Propagation Laboratory of NOAA has operated a ground-based zenith-viewing microwave radiometer. This radiometer, designed to measure precipitable water vapor, cloud liquid, and temperature profiles, has two moisture-sensing channels and four temperature-sounding channels. Data from this system, taken at Denver, Colorado, are used to derive geopotential heights and thicknesses from the surface (about 830 mbar) to 300 mbar. Time series and spectra of several directly measured and inferred quantities are analyzed for different meteorological situations: a period of unusual calm in surface pressure, a frontal passage, and a gravity wave event. The three cases presented illustrate how rapid variations in meteorological variables can be studied using ground-based radiometers. These radiometers provide temporal continuity not hitherto available. The performance of the radiometer, both in observing a blackbody target and during an unusually calm pressure event, shows high sensitivity to changes in geopotential height and thickness and to integrated water vapor. Consequently, the combination of high temporal resolution and high sensitivity allows unique monitoring of rapidly changing conditions, such as frontal passages and gravity wave events. Comparisons of these data with various sources of ground truth, including radiosondes, satellite cloud observations, and arrays of microbarographs, show excellent agreement.     

Soil moisture and rainfall estimation over a semiarid environmentwith the ESTAR microwave radiometer

The application of an airborne electronically steered thinned array L-band radiometer (ESTAR) for soil moisture mapping was investigated over the semiarid rangeland Walnut Gulch Watershed in southeastern Arizona. During the experiment, antecedent rainfall and evaporation were very different and resulted in a wide range of soil moisture conditions. The high spatial variability of rainfall events within this region resulted in moisture conditions with distinct spatial patterns. Analysis showed a correlation between the decrease in brightness temperature after a rainfall and the amount of rain. The sensor's performance was verified using two approaches. First, the microwave data were used to predict soil moisture, and the predictions were compared to ground observations of soil moisture. A second verification used an extensive data set collected the previous year at the same site with a conventional L-band push broom microwave radiometer (PBMR). Both tests showed that the ESTAR is capable of providing soil moisture with the same level of accuracy as existing systems