Modelling of Drop Size Distribution of Rain from Rain Rate and Attenuation Measurements at Millimeter and Optical Wavelengths

This paper describes a technique for modelling of rain drop size distributions at Calcutta in terms of negative exponential function, from the measurements of rain rate and attenuation over a dual wavelength LOS link at millimeter and optical frequencies. The DSD model obtained is then used to determine the attenuation at 94 GHz, for comparison with experimentally obtained attenuation at 94 GHz. This is also compared with the attenuation calculated by considering other experimentally obtained DSD models. The best fit negative exponential distribution function (modified M-P model) is presented along with some other experimentally obtained and reference models.     

1-Min Rain Rate Statistics Predictions From 1-Hour Rain Rate Statistics Measurements

A new method is presented, which allows the recovery of the cumulative distribution function (CDF) of point rain rate with 1-min integration time (P(R)₁) from rain rate CDFs with longer integration time (in this case, 1-hour time average rain rate statistics P(R)₆₀)- The method generates the scaling factors between P(R)₆₀) and P(R)₁) as a function of the probability level and of the site geographical coordinates. From the scaling factors, a scaling law of the form proposed by Chebil and Raman is devised. The method is applicable worldwide and produces predictions far more accurate than the ones provided by general P(R) estimation techniques.     

Radiometric Measurements of Rain Attenuation at 22.2 and 31.4 GHz Over Calcutta

Measurements of rain attenuation over Calcutta have been made using radiometers operating at frequencies of 22.235 and 31.4 GHz. The results have been correlated with observations of rainfall rates made with a fast-response rain gauge. The distributions of peak rain rates over Calcutta for two different years and the cumulative occurrences of rainfall rates and attenuations at the two frequencies are presented.     

Radar Backscatter Measurement Accuracies Using Digital Doppler Processors in Spaceborne Scatterometers

The normalized standard deviation, Kp, of radar backscatter measurements using digital Doppler processors in spaceborne scatterometers is derived. The Kp expression for analog Doppler filter processors, such as that used in the Seasat scatterometer [7] is shown to be a special case of the derived Kp expression. A connection to Welch's power spectrum estimation results [6] is also made. Tradeoff studies in digital filter design such as hardware complexity, computational speed, and system performance can be performed based on this Kp expression. We briefly discuss a current application in the design of the NASA scatterometer (NSCAT) to be flown in 1990. This derivation should be useful for system design and analysis of other radar remotesensing instruments.     

由实测小时雨量预报地星通信雨致衰减的方法研究

利用１９９１年中国３３３个地面雨量站的小时雨量数据，进行降雨率累积概率对数正态和伽玛分蝗计算比较，得出伽玛分布在较大的降雨范围内能更好地反映实际降雨率累积概率分布。     

Retrieval of Snow and Rain From Combined X- and W-Band Airborne Radar Measurements

Two independent airborne dual-wavelength techniques, based on nadir measurements of radar reflectivity factors and Doppler velocities, respectively, are investigated with respect to their capability of estimating microphysical properties of hydrometeors. The data used to investigate the methods are taken from the ER-2 Doppler radar (X-band) and cloud radar system (W-band) airborne Doppler radars during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment campaign in 2002. Validity is assessed by the degree to which the methods produce consistent retrievals of the microphysics. For deriving snow parameters, the reflectivity-based technique has a clear advantage over the Doppler-velocity-based approach because of the large dynamic range in the dual- frequency ratio (DFR) with respect to the median diameter D₀ and the fact that the difference in mean Doppler velocity at the two frequencies, i.e., the differential Doppler velocity (DDV), in snow is small relative to the measurement errors and is often not uniquely related to D₀. The DFR and DDV can also be used to independently derive D₀ in rain. At W-band, the DFR-based algorithms are highly sensitive to attenuation from rain, cloud water, and water vapor. Thus, the retrieval algorithms depend on various assumptions regarding these components, whereas the DDV-based approach is unaffected by attenuation. In view of the difficulties and ambiguities associated with the attenuation correction at W-band, the DDV approach in rain is more straightforward and potentially more accurate than the DFR method.     

In Search of a Location-Centric Rain Attenuation Model

Wireless Personal Communications - In search of an accurate location-centric rain-induced attenuation model, in this paper, the authors have investigated the measured attenuation and rainfall...     

卫星雷达高度计中准最大似然算法的高度误差估计

利用准最大准然估计算法，可以估计海洋卫星高度计中海平面到卫星平台之间的高度，本文首先讨论了最大似然算法的基本原理，然后根据高度计的测高原理，讨论了采用准最大似然估计带来的高度偏差，并提出了进行计算机仿真的有效方法，分析了均方根高度误差与信噪比，脉冲累积量和均方根波高之间的关系。     

雨衰减的解析近似

本文对Ｌａｗｓ－Ｐａｒｓｏｎｓ和广州雨滴尺寸分布雨衰减和降雨经之间的指数关系中的ａ和ｂ值＾「１」「２」进行了分析和解析回归，给出其与频率的解析近似关系，利用其计算的雨衰减和数值计算结果有很好的一致性。     

紫外光在雨中的传输衰减研究

针对目前雨中衰减模型在紫外光波段的适用局限性,介绍了考虑雨滴对光传输的遮挡作用时光的传输衰减模型,通过与Modtran数据及相关文献比较验证了模型在紫外光波段的适用性,分析了降雨时影响紫外光衰减的主要因素,表明降雨时能见度对紫外光的衰减影响较大,并结合实际的降雨情况考虑能见度的影响提出了紫外光在雨中的传输衰减计算模型。     

Determining Attenuation of Waveguide from Electrical Measurements on Short Samples

improved method for accurately determining the attenuation of waveguide from measurements on very short samples is presented. First, two samples are measured separately and then in tandem. When the measurements are properly made, the sum of the attenuations when the samples are measured separately agrees with the attenuation when measured in tandem at each frequency of measurement. Second, the average effective resistivity is found over a band of frequencies. Using the average effective resistivity, the attenuation at any frequency in the band can be determined. Results for WR159 copper waveguide are shown.     

空载合成孔径雷达技术及展望

空载合成孔径雷达（ＳＡＲ）是一种现代高分辨率侧视成象雷达，它应用合成孔径技术、脉冲压缩技术和数字信息处理技术，获得方位向和距离向的高分辨率。本文将对空载ＳＡＲ的现状及发展趋势进行综述，并主要讨论ＳＡＲ的有关技术，涉及星载ＳＡＲ工作模式和体制，雷达数字图象的信息获取与成象处理等，最后，给出了ＳＥＡＳＡＴ－Ａ卫星ＳＡＲ数字成象结果。     

On the Space-Time Variations of Rain Attenuation

Rain attenuation shows a considerable temporal and spatial variability. To simulate fade mitigation techniques such as route diversity, a space-time channel model which accounts for the spatial and temporal variation of rain attenuation is needed. In this paper we investigate the space-time correlation of rain attenuation utilizing 42 GHz star-like network measurements. By combining the spatial and temporal correlation properties of rain attenuation, a simulation model for generating multiple correlated rain attenuation time series based on the Maseng–Bakken model is developed. The model is validated by comparing the statistical and angular diversity properties of the model with those of measurements and theoretical diversity gain models. Furthermore, parameters for the Maseng–Bakken dynamic rain attenuation model were extracted from the star-like network measurements. In addition, using a systematic multivariable technique a model for the parameter $beta _{s}$ which controls the dynamics of rain attenuation in the Maseng–Bakken model was developed. Moreover, using available rain attenuation measurements the advantage of route diversity with selection combining is investigated.      

Mode Content Determination in a Multimode Waveguide from Radiation Pattern Measurements Using the Genetic Algorithm

This paper presents a novel technique for determining the mode content in a multimode waveguide from the radiation pattern measurements. The technique employs the Genetic Algorithm (GA) for determining the mode content in the waveguide from the measured pattern and a numerical method for simulating radiation patterns for the modes propagating in the waveguide. By numerical experiments, we validate the feasibility and effectiveness of the proposed technique, and illustrate its sensitivity to the number of observation points and random noise in the measurement.     

A Stochastic Dynamic Model of Rain Attenuation

A dynamic mathematical model of rain attenuation has been developed and is presented in this paper. This model permits the expression of analytic relationships between parameters commonly used to describe the properties of interest for communication. The dynamic model is based on the lognormal distribution of rain attenuation and utilizes a memoryless nonlinear device to transform attenuation and rain intensity into a one-dimensional Gaussian stationary Markov process. Hence, only one parameter is required to introduce the dynamic properties of rain attenuation into the model. Experimental results and the known properties of rain have been used to derive and to verify the model; comparative results are presented and demonstrate good correspondence. The application of the model to the statistical analysis of the performance of communications systems is illustrated in the paper. The use of a dynamic rain attenuation model is necessary in order to analyze radio communication systems with transmit power control to offset the effects of rain attenuation, and where the finite response time of the control system affects the performance. An advantage of the model is the simplicity with which it allows simulation of communication link performance under the influence of rain attenuation. Such simulations are of great interest for complex models of adaptive networks where several deteriorating effects, including finite response times, are present.     

Rain Attenuation at Millimeter Wavelength of 1.33 mm

Specific rain attenuation values calculated by using the four empirical raindrop-size distributions, that is, the Marshall and Palmer, the Joss, Thams and Walgvogel, the Ihara, Furuhama and Manabe, and the Weibull raindrop-size distributions. The millimeter wave measurement at 225 GHz (1.33mm) was compared with our calculations. It has been shown that the Weibull distribution has the best agreement with the measurements.     

Measurements of Earth-Space Attenuation at 230 GHz

Measurements of attenuation at 230 GHz through the total atmosphere due to the presence of oxygen and water vapor molecules, clouds, and rain are presented and discussed. The measurements were carried out using a specially designed superhetrodyne receiver mounted on a sun tracker. Simultaneons measurements were also carried out at 13 GHz. For a measuring site close to sea level at Holmdel, NJ, the "clear-sky" zenith attenuation was found to be given by A (dB) = 0.35 rho, where rho was the measured ground water vapor density in g/m/sup 3/. When the ground temperature was below about 7/spl deg/C, most cloud and overcast gave < 0.5 -dB attenuation whereas with a ground temperature greater than 13/spl deg/C, cloud attenuation was 8-10 times greater. Calculations of zenith attenuation in the 230-GHz atmospheric window were also made using the Gross analytic line shape, Schulze-Tolbert empirical line shape, and an empirically modified Gross line shape. These calculations were based on determinations of water vapor density and temperature made at the measurement site, and on radiosonde measurements made at a distance of 80 km away. Measured and calculated results are graphically compared. It is concluded that either the modified Gross line shape or the Schulze-Tolbert line shape gives conservative estimates of zenith attenuation at 230 GHz for clear days, while the Gross line shape gives fair agreement with measured results.     

Ka频段卫星通信系统降雨衰减分析

Ka频段卫星通信因其具有可提供的带宽大(3.5GHz)、通信容量大、波束窄、终端尺寸小,轨道平面内可容纳的卫星多和抗干扰能力强等优势成为未来卫星通信的必然趋势。Ka频段卫星通信面临的一个巨大挑战在于它受气象因素的影响大,这一度使研究人员认为Ka频段卫星通信是不可能实现的。降雨、闪烁、大气吸收等因素都会导致Ka频段地空链路信道质量的恶化。根据Ka频段卫星通信的特点,分析了降雨衰减的特性,提出了几种抗雨衰的办法。     

Rain Measurement Results Derved from a Two-Polarzation Frequency-Diversity S-band Radar at Wallops Island, Virginia

A dual-polarization radar located at the NASA Wallops Flight Facility, Wallops Island, Virginia is described. This radar operates with a slow polarization switch having a cycle time of 0.7 s and also incorporates a frequency diversity technique to achieve independent sampling over short intervals of time. Rain-rate measurements derived from the dual-polarization radar and low-and high-resolution rain gauges located at a remote site are compared. Average percent differences in rainfall of less than 5 and 16 percent were demonstrated when comparing the dual-polarization radar measurement with the low-and high-resolution rain gauges, respectively. Excellent correlation of the rain rates was in evidence during one rain day. All rain measurement cases examined were limited to only light rain rates (less than 7 mm/h).     

卫星广播通信中的雨衰减研究及抗衰落策略

本文介绍了影响I0GHz以上频段卫星广播通信系统面临的主要电波传播问题,重点介绍了无线电气象、雨衰减统计预报、雨衰减动态特性等方面的研究进展.基于我国统计数据建立的电波环境数字地图是进行可靠电波传播效应预测的基础,新提出的雨衰减统计预报模式具有更高的预测精度,基于我国站点实测数据建立的衰落斜率预报模型更能符合我国区域的电波传播特性.文章对当前主要的抗衰落策略进行了介绍和对比.在高频段卫星广播通信系统中更合理的利用电波传播研究成果,采用合理的抗衰落策略,可以优化系统设计和资源配置、提高系统性能.