Three-dimensional vector modeling and restoration of flat finite wave tank radiometric measurements

In this paper, a three-dimensional Fourier transform inversion method describing the interaction between water surface emitted radiation from a flat finite wave tank and antenna radiation characteristics is reported. The transform technique represents the scanning of the antenna mathematically as a correlation. Computation time is reduced by using the efficient and economical fast Fourier transform algorithm, To verify the inversion method, computations have been made and compared with known data and other avaliable results. The technique has been used to restore data of the finite wave tank system and other available antenna temperature measurements made at the Cape Cod Canal. The restored brightness temperatures serve as better representations of the emitted radiation than the measured antenna temperatures.     

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     

Absolute brightness temperature measurements at 3.5-mm wavelength

Careful observations have been made at 86.1 GHz to derive the absolute brightness temperatures of the Sun (7914 pm 192K), Venus (357.5 pm 13.1K), Jupiter (179.4 pm 4.7K), and Saturn (153.4 pm 4.8K) with a standard error of about three percent. This is a significant improvement in accuracy over previous results at millimeter wavelengths. A stable transmitter and novel superheterodyne receiver were constructed and used to determine the effective collecting area of the Millimeter Wave Observatory (MWO) 4.9-m antenna relative to a previously calibrated standard gain horn. The thermal scale was set by calibrating the radiometer with carefully constructed and tested hot and cold loads. The brightness temperatures may be used to establish an absolute calibration scale and to determine the antenna aperture and beam efficiencies of other radio telescopes at 3.5-mm wavelength.     

A Simplified Microwave Model and Its Application to the Determination of Some Oceanic Environmental Parametric Values, Using Multichannel Microwave Radiometric Observations

This paper proposes a simplified microwave model, linerized by Taylor expansion and incorporating observed fundamental microwave emissive properties, and discusses its application along with the least square method for determining some oceanic environmental parametric values from multichannel observation data. As a case study, microwave radiometric observations were made in vertical and horizontal polarizations at 6.7 and 18.6 GHz at incidence angles from 200 to 700 at steps of 100, and sea-surface temperature, wind speed, and sky brightness temperatures at the above dual frequencies at zenith angles from 200 to 700 were determined from a set of observed data at each incidence angle. The algorithm-derived results were compared with observed ones and showed good agreements within 10 percent of errors at incidence angles from 200 to 600; however, no good agreement was attained at large incidence angles such as 70°, except sky brightness temperature at 6.7 GHz. This disagreement may be attributed mainly to the difficulty and incompleteness in corrections such as estimation of reflected sky radiation and antenna characteristics, both for retrieval of the antenna temperature averaged over the main lobe and observation errors. This fact, however, may also suggest that observations at such larger incidence angles are of less utility.     

毫米波辐射探测目标亮温的估计

毫米波近感技术是利用物体的毫米波辐射特性探测识别目标，在许多领域有着重要应用。为在毫米波辐射探测中求得精确的真实目标亮温，必须建立天线电磁辐射与环境电磁辐射交会的数学模型，此模型是第一类病态Fredho1m积分方程。寻找适当的方法能得到一个尽可能好的或同精确值相当接近的近似计算方法，为此，本文给出毫米波辐射计目标辐射亮温的估计方法。     

Sky brightness temperature measurements at 135 GHz and 215 GHz

Zenith sky brightness temperature measurements at 135 GHz and 215 GHz have been made on a semicontinuous basis for a period of seven months in the Gunston Cove area of Northern Virginia. These measurements were made using Dicke receivers with noise figures of 8 dB and 14 dB, respectively. A liquid nitrogen cooled load was used to calibrate the measurements. The 215 GHz sky temperature was on the average about 80 K greater than that at 135 GHz. Clouds were found to cause the sky temperatures to fluctuate as much as 150 K in a few minutes. Graphs are presented to outline general trends of the data as well as representative days, including the blizzard on February 11, 1983. In addition, empirical relations between precipitable water vapor, atmospheric water density at the surface, sky brightness temperatures, and zenith attenuation are given for visually clear days.     

装甲目标毫米波辐射亮温解的优化控制

毫米波系统有着较微波、红外、光学系统不同特性的优点.毫米波辐射探测技术在许多领域有着重要应用与发展.为能求得真实的目标物体亮温,从测得的天线温度数据或天线温度分布函数中反演出装甲目标的亮温分布,需解第一类Fredholm积分方程.该积分方程是一病态方程.DFP法算法是无约束最优化方法中最有效的方法之一.本文尝试用DFP法优化控制毫米波装甲目标辐射亮温,得到较为稳定的反演解.     

Airborne imaging radiometer scan simulation

An imaging radiometer scan simulation program is developed for comparing the performance of different scan patterns and process behavior under varying circumstances. The program gives the radiometer's antenna beam the desired scan motion over an artificial target scene, computes the antenna radiometric temperature as the convolution of the antenna pattern with the scene brightness temperature deviation, computes a moving average of the antenna output, and records the data values as well as the corresponding coordinates at the sampling moments. The simulation indicated that a helicopter-borne imager needs at least passive attitude stabilization. In addition, the state-of-the-art sampling rate was found to be too slow, if the sampling period is set equal to the integration time. A detailed study revealed the achievable spatial resolution (line pairs/length unit-definition) to be 1.0-1.2×footprint dimensions, but the integration and sampling periods should be as short as 0.2-0.4×footprint dimensions     

G/T predictions of communication satellites based on a new earth brightness model

The G/T predictions of communications satellites in the past usually stipulated that an antenna noise temperature of 290 K be used in the calculations. This, when compared to in-orbit derived data, has resulted in large discrepancies (up to 3 dB). These discrepancies result directly from the use of a pessimistic value of the antenna noise temperature. The actual antenna noise temperature is lower because a portion of the spacecraft receive antenna pattern falls in deep space and because the brightness temperatures of large parts of the Earth are lower than 290 K. This discrepancy has become more apparent as technology of low noise amplifiers advances, thus making the antenna temperature the predominant factor when budgeting the system noise temperature of the spacecraft. This paper focuses on a model to determine the antenna noise temperature and especially how this model was directly verified in orbit during the acceptance campaign of EUTELSAT II-F6 (Hot Bird 1). The methodology used for the determination of the integrated antenna temperature is also fully discussed. Full results are also presented.     

Passive active L- and S-band (PALS) microwave sensor for oceansalinity and soil moisture measurements

A passive/active WS-band (PALS) microwave aircraft instrument to measure ocean salinity and soil moisture has been built and tested. Because the L-band brightness temperatures associated with salinity changes are expected to be small, it was necessary to build a very sensitive and stable system. This new instrument has dual-frequency, dual polarization radiometer and radar sensors. The antenna is a high beam efficiency conical horn. The PALS instrument was installed on the NCAR C-130 aircraft and soil moisture measurements were made in support of the Southern Great Plains 1999 experiment in Oklahoma from July 8-14, 1999. Data taken before and after a rainstorm showed significant changes in the brightness temperatures, polarization ratios and radar backscatter, as a function of soil moisture. Salinity measurement missions were flown on July 17-19, 1999, southeast of Norfolk, VA, over the Gulf Stream. The measurements indicated a clear and repeatable salinity signal during these three days, which was in good agreement with the Cape Hatteras ship salinity data. Data were also taken in the open ocean and a small decrease of 0.2 K was measured in the brightness temperature, which corresponded to the salinity increase of 0.4 psu measured by the M/V Oleander vessel     

A 16-foot diameter millimeter wavelength antenna system, its characteristics and its applications

This paper describes the characteristics of the 16-foot diameter millimeter wavelength antenna system at the Electrical Engineering Research Laboratory, University of Texas, Austin, Tex. Aperture efficiencies of the antenna at the frequencies of 35, 70 and 94 Gc are 59 per cent, 55 per cent and 52 per cent, respectively, corresponding to gain and beamwidths of 62.5 dB and0.118degat 35 Gc/s, of 68.5 dB and of 70.9 dB and0.048degat 94 Gc/s. First sidelobes and other sidelobes over the 35- to 94-Gc/s frequency interval are -18 dB and -25 dB, respectively. The 70-Gc/s radiation temperatures of Jupiter and the earth's moon have been measured. The brightness temperature of Jupiter was found to be112 pm 22degK and the maximum brightness temperature contour of the moon was270degK. Relatively higher emission temperatures were observed from the maria and the craters of Copernicus and Tycho than from surrounding lunar areas.     

The effect of the half-width of the 22-GHz water vapor line on retrievals of temperature and water vapor profiles with a 12-channel microwave radiometer

We show that observed biases in retrievals of temperature and water vapor profiles from a 12-channel microwave radiometer arise from systematic differences between the observed and model-calculated brightness temperatures at five measurement frequencies between 22 and 30 GHz. Replacing the value for the air-broadened half-width of the 22-GHz water vapor line used in the Rosenkranz absorption model with the 5% smaller half-width from the HITRAN compilation largely eliminated the systematic differences in brightness temperatures. An a priori statistical retrieval based on the revised model demonstrated significant improvements in the accuracy and vertical resolution of the retrieved temperature and water vapor profiles. Additional improvements were demonstrated by combining the MWRP retrievals with those from the GOES-8 sounder and by incorporating brightness temperature measurements at off-zenith angles in the retrievals.     

Retrieval of land and sea brightness temperatures from mixedcoastal pixels in passive microwave data

A technique is presented to separate uncontaminated land and sea brightness temperatures from mixed coastal pixels in 37-GHz vertically polarized passive microwave data from the Special Sensor Microwave Imager (SSM/I) instrument. Combining a mathematical model of the instrument response over several neighboring footprints with a GIS representation of the coastline yields a relationship between land and sea brightness temperatures and radiation measurements made at the satellite. Inverting this relationship allows separate land and sea brightness temperature values to be derived for each mixed coastal pixel in the original image. The technique has been successfully applied to 37-GHz vertically polarized SSM/I imagery for test areas covering the Gulf of Aden and the British Isles. Errors in the retrieved brightness temperatures were estimated to be of the order of 1-2 K     

天线口面的平均亮温度

本文提出了天线口面平均亮度的概念,给出了天线终端接有匹配噪声源和非匹配噪声源时天线口面平均亮温度的表达式。该式表明,无耗天线口面的平均亮温度仅与三个因素有关:噪声源的噪声温度,噪声源的反射系数和天线的面积利用系数。     

Two-load radiometer precision and accuracy

The advent of fast computers and digital signal processing permits many aspects of radiometer operation to be decided in real time so that precision and accuracy are maximized. Closed-form expressions for precision, optimal antenna/load dwell times, and maximum chopping period are derived for a class of radiometers that uses two loads to calibrate its measurements of antenna brightness temperature. The importance of load brightness temperature selection emphasizes the need to develop "cold" calibration loads that can operate at ambient temperature. A method of incorporating critical thermistor data into the radiometer brightness temperature calibration can be used to help stabilize measurements against drift and may permit radiometers to operate without thermal control in some low-accuracy applications.     

Wet Reflectors in Millimeter-Wave Radiometry-Expenrment and Theory

Microwave radiometers are often required to operate under all weather conditions, including those under which an exposed surface of the antenna system becomes wetted by rain. Here we discuss the special case of a wetted flat reflector, with energy at 20.60 and 31.65 GHz linearly and orthogonally polarized at an incidence angle of 45°. The brightness temperatures produced by the wet reflector are computed from conventional theory, and are measured for various thicknesses of water layer. Both theory and experiment show that the brightness temperatures produced by the wet reflector have the same behavior throughout a range of water-layer thicknesses. The impact of these results on design of radiometers for research and operational application is discussed.     

Interferometric synthetic aperture microwave radiometry for theremote sensing of the Earth

Interferometric aperture synthesis is presented as an alternative to real aperture measurements of the Earth's brightness temperature from low Earth orbit. The signal-to-noise performance of a single interferometric measurement is considered, and the noise characteristics of the brightness temperature image produced from the interferometer measurements are discussed. The sampling requirements of the measurements and the resulting effects of the noise in the measurements on the image are described. The specific case of the electronically steered thinned array radiometer (ESTAR) currently under construction is examined. The ESTAR prototype is described in detail sufficient to permit a performance evaluation of its spatial and temperature resolution. Critical aspects of an extension of the ESTAR sensor to a larger spaceborne system are considered. Of particular important are the number and placement of antenna elements in the imaging array     

A broadband microwave radiometer technique at X-band for rain and drop size distribution estimation

Radiometric brightness temperatures below about 12 GHz provide accurate estimates of path attenuation through precipitation and cloud water. Multiple brightness temperature measurements at X-band frequencies can be used to estimate rainfall rate and parameters of the drop size distribution once correction for cloud water attenuation is made. Employing a stratiform storm model, calculations of the brightness temperatures at 9.5, 10, and 12 GHz are used to simulate estimates of path-averaged median mass diameter, number concentration, and rainfall rate. The results indicate that reasonably accurate estimates of rainfall rate and information on the drop size distribution can be derived over ocean under low to moderate wind speed conditions.     

综合孔径微波辐射计天线与信道模型仿真

首先建立用于微波被动遥感的一维综合孔径微波辐射计的理想信道仿真模型,在此基础上通过分析单元天线方向图的不一致、信道的幅度相位不平衡与信道间串扰对反演亮温的影响来对综合孔径微波辐射计的天线和信道部分的误差进行分析,并建立系统仿真模型.建立的仿真模型所反演的目标亮温能够更真实地反映被观测目标的实际情况.