A simple expression for estimating attenuation by fog at millimeter wavelengths

At millimeter wavelengths, fog attenuation is a function of the fog density, extent, index of refraction of the fog medium, and wavelength. The attenuation is usually determined by first estimating the index of refraction of water for the wavelength and temperature of interest and then calculating the attenuation using the Rayleigh approximation. In this communication fog attenuation is computed for a large set of wavelengths and indices of refraction. A regression analysis of the attenuation is then conducted as a function of wavelength and temperature. It is shown that an almost perfect fit can be obtained with a four-term regression on wavelength and temperature for the ranges of 3 mm <lambda < 3cm and-8degC< T < 25degC, respectively 5666. This expression produces a normalized fog attenuation; the total attenuation is easily computed by multiplying the normalized attenuation by the fog density and extent. If fog density data are not available, a formula for estimating the density from fog visibility is given.     

A More Precise Empirical Formula for Estimating Normalized Fog Attenuation in the Millimeter-Wave Frequency Range 30 ~ 100 GHz

At millimeter wavelengths, normalized fog attenuation (NFA) in units of (dB/km)/ (g/m³) is generally calculated by the Rayleigh approximation when working wavelengths are much larger than the average diameter of fog droplets. The calculations of the Rayleigh approximation are much less than those of Mie scattering theory, but still complex and heavy. To solve the above problem and facilitate the engineering applications of the Rayleigh approximation, a new empirical formula is discussed to estimate NFA in the frequency range 30?~?100 GHz and the fog common temperature range ?8?~?20 °C. The simulation results of the new formula are compared with those got by other three empirical formulae: the Altshuler empirical formula, the Liebe empirical formula and the Zhao empirical formula. Maximal absolute value of the relative errors (MAVRE) and Pearson correlation coefficient (PCC) indicate the largest deviation of estimated results and the fitting performance of an empirical formula, respectively. Comparisons show that the MAVRE of the new formula is only 4.482 %, which is much smaller than those of the other three formulae. The mean value of the Pearson correlation coefficients (PCCs) of the proposed formula is 0.999943, larger than those of other methods. Additionally, relative error (RE) curves of the four empirical formulae are given at four certain temperatures ?8 °C, 0 °C, 10 °C and 20 °C.     

Frequency dependence of rain attenuation measurements at microwave frequencies

The values of attenuation versus frequency for 10 mm/h, 25 mm/h, and 40 mm/h rain rates for frequencies of 11, 18, and 22.2 GHz are presented. On the basis of these observations the attenuation at frequencies below 10 GHz and above 22.2 GHz have been obtained. The values obtained at various frequencies show an agreement with those calculated on the basis of Oguchi's work. Comparison of the above values in dB/km (assuming a path length of 2.5 km) have been made and they show an agreement with International Radio Consultative Committee (CCIR) values. Also cumulative distributions of attenuation at various frequencies have been given taking 11 GHz results as the reference point.     

海雾对8mm的衰减和闪烁研究

在中国青岛地区进行了海雾对８ｍｍ信号视距传播时所产生的衰减和闪烁测量实验。１ｍｉｎ平均衰减统计表明浓海雾所引起的衰减可接的１ｄＢ／ｋｍ;并对信号的功率谱密度进行了统计分析,分析了海雾湍流团的快变化和慢变化特性;海雾湍流结构符合Ｈｏｌｍｏｇｏｒｏｖ理论;对信号起伏方差的统计结果推算海雾湍流结构常数在１０＾－１１量级。     

电磁波传播的云雾衰减特性研究

研究了云雾对电磁波传输的衰减特性,在云雾衰减理论模型基础上,给出了云雾衰减工程计算模型,该模型所需参数少、计算简便,且误差符合工程计算要求,仿真分析了电波频率、温度、发射角度及成雾类型等参数对雾特征衰减率的影响。当无线电波的频率高于10GHz时,需要考虑云和雾对电波的衰减;当频率高于50GHz时,云、雾对电波的衰减才显得重要。云雾对电磁波传播的衰减随着电波频率、温度、发射角及水汽含量的增大而增大。     

Millimeter-Wave Attenuation Due to Fog and Clouds

The parameters of gamma drop size distribution model of fog and clouds are derived based on the liquid water content and optical visibility, the attenuation are calculated and discussed with this model and empirical relations of the liquid water content and the visibility or other parameters of fog and clouds. A new empirical formula to estimate fog and clouds attenuation is presented based on the Reyleigh absorption approximation, which is more accurate in general and can be used in wider frequency and temperature range than other empirical formulas.     

复杂大气条件对微波传播衰减的影响研究

为了进一步提高大气对微波传播衰减影响的描述精度,为微波链路遥感反演大气新应用提供理论基础,该文系统性研究了大气主要吸收气体和各种大气粒子对微波传播的衰减情况。利用ITU-R模型计算大气主要气体成分对微波的吸收衰减,然后在降水粒子、云雾粒子和沙尘粒子的介电模型、形状、相态和谱分布的基础上,计算得到群粒子对微波的衰减特性,系统讨论降水强度、相态、含水量、谱分布、气压和温度等因素对微波传播衰减的影响。数值模拟结果表明,大气主要气体成分在60 GHz, 180 GHz和320 GHz附近存在强烈的吸收带,其衰减系数与水蒸气含量和气压呈正相关,与温度呈负相关;降水强度、谱分布、相态以及冰水比例对降水的微波衰减存在不同程度的影响,云雾的含水量和相态,沙尘的数密度、谱分布和含水量是影响微波衰减的主要因素,而温度的影响较小;大气各因素的衰减系数从大到小依次为爆炸沙尘、降水、气体吸收、水雾、冰雾和自然沙尘。     

Attenuation and space diversity statistics calculated from radar reflectivity data of rain

During the summer of 1973 the rain reflectivity environment in three-dimensional space was routinely recorded on digital tape at Wallops Island, Va. A mode of operation consisted of sampling periodically60degazimuth intervals over regions in which the rain activities were most intense and widespread. A series of plan-position indicator (PPI) sweeps over these intervals were implemented at a sequence of elevation angles starting from0.5degup io an angle above which the reflectivity values were below a designated threshold level. Approximately 500 such raster scans were acquired in which each scan was obtained in less than 4 min and covered a range interval of 10 to 140 km. Using the above data base, reflectivity profiles along representative earth-satellite paths were determined from which attenuation and space diversity statistics were calculated at the frequencies of 13 and 18 GHz. Specifically, the formk = aZ^{b}was used to deduce the total path attenuation, wherekis the attenuation coefficient (dB/km), andZis the reflectivity factor (mm⁶/m³). The constantsaandbwere calculated using the raindrop distribution for thunderstorm activity as proposed by Joss. Probabilities that the attenuations exceed given fade depths, diversity gain as a function of fade depths, and diversity gain as a function of site separation distances are characterized and compared at the various frequencies. For the space diversity mode, a site spacing of 15 km is shown to give rise to a near optimum condition for the diversity gain. Furthermore, the diversity gain has been demonstrated to be minimally influenced by the transmitter frequency.     

海雾对3毫米波传播特性的影响研究

进行了海雾对3mm传播特性影响的测量实验．浓度大的海雾对3mm信号衰减达7．55dB／km,推算了海雾的特性参数,对不同能见度下的测量结果和用ITU模式计算的结果进行了比较,并对信号闪烁特性进行了分析．     

Propagation Considerations for Satellite Broadcasting at Frequencies Above 10 GHz

At frequencies above 10 GHz, tropospheric propagation effects can significantly alter signal transmission performance. Important phenomena on earth-satellite (slant) paths include attenuation and sky noise increases resulting from atmospheric gases, clouds, and precipitation, depolarization by raindrops and ice crystals, and refractive effects, particularly at low path elevation angles. Various practical aspects of propagation phenomena for satellite broadcasting at frequencies above 10 GHz are reviewed in this paper, with emphasis on the near-term implementation Of services atKu-band.     

Attenuation and backscatter from a derived two-dimensionalduststorm model

Fundamentals of attenuation and radar backscatter through duststorms are reviewed. A duststorm is modeled as circularly symmetric having a visibility with a minimum at its center (e.g., maximum mass loading) and which exponentially increases radially to a fixed maximum visibility threshold level (minimum mass loading). This model enables the convenient calculation of the two dimensional (2-D) structure of radar backscatter and path attenuation. As an example, the parameters of the exponential function describing the visibility distribution for a particular duststorm was derived using measurements made in the Sudan by other investigators operating a 10.5 GHz, 25 km link. A comparison of the calculated and measured attenuation time-series showed relatively close agreement. Both attenuation levels and backscatter levels due to even intense duststorms are expected to be relatively small for frequencies up to 10 GHz. For example, the peak attenuation for the duststorm that contained visibilities smaller than 2 m was less than 6.5 dB. Modeled backscatter due to this duststorm gave levels smaller than that obtained by an equivalent rainrate of 0.6 mm/h. Although the calculations were obtained for X-band, they may be extended to higher frequencies. Frequency scaling at 37 GHz, for example, showed a peak equivalent path attenuation level of at least 26.6 and 48 dB under varying assumptions     

Cloud Characteristics and Cloud Attenuation in Millimeter Wave and Microwave Frequency Bands for Satellite and Remote Sensing Applications over a Northern Indian Tropical Station

Microwave and millimeter wave frequency bands are in demand for requirement of more channels in radio communication systems. It has also been recognized that microwave and millimeterwave frequency radiometers on board satellites as promising tools for remote sensing. The frequency more than 10 GHz is affected by rain and cloud. Though the effects of rain on radiowave is more than cloud but the occurence of cloud is more than rain. Cloud has been found to occur for weeks together over this part of the world. It is therefore essential to study cloud morphology over different geographical region. In this paper, an attempt has been made to the cloud occurrences over an Indian tropical station, Delhi (28.35°N, 77.12°E) observed during different months and daytime and nighttime. It is seen that low clouds occurrence over Delhi is very significant and particularly during July, August and September. The specific attenuation of radiowave due to clouds at various frequencies 10 GHz, 20 GHz, 50 GHz and 100 GHz has been deduced. The specific attenuation of radio wave due to cloud at 10 GHz varies from 0.0608 dB/km to 0.1190 dB/km while at 100 GHz the specific attenuation varies from 6.8460 dB/km to 11.9810 dB/km     

Propagation observations at 3.2 millimeters

A millimeter-wave system for the transmission and reception of television signals has been constructed. The propagation path is 450 meters above sea level at the transmitter and traverses an 18.95-km path to the receiver, which is at an elevation of 39 meters atop a two-story building in El Segundo, Calif. The elevation angle is 1.17 degrees when corrected for curvature and refraction. Received picture quality and tropospheric scintillation and attenuation for various weather conditions are discussed. Tropospheric attenuation ranged from about 13 dB on a typical day to approximately 36 dB when moderate rainfall (∼4 mm/hr) occurred over much of the 18.95-km path. Tropospheric turbulence effects were almost nonexistent on days of heavy fog but reached peak-to-peak magnitudes of 20 dB or more in received signal fluctuations on dry, windy days. High quality television and voice reception were obtained over this link even during light to moderate rainfall periods (∼3 mm/hr). A transmitter output of approximately 100 mW, 0.61-m parabolas at each end of the link, and a receiver noise figure of 25 dB were the main system characteristics. Tropospheric attenuation measurements are in close agreement with values calculated from modified versions of the Van Vleck expressions for attenuation due to oxygen and water vapor. The average of measured tropospheric attenuation rates was approximately 0.7 dB/km for July 1965.     

Frequency response of distributed fiber-optic vibration sensor based on nonbalanced Mach-Zehnder interferometer

We set up the analytical model for the frequency response of a distributed fiber-optic vibration sensor based on the nonbalanced Mach-Zehnder interferometer, the frequency response of which is related to both of the length difference of the delay fibers and the location of the vibration. In addition, a series of null frequencies, in which the vibration cannot be detected, is also found. The distributed fiber-optic vibration sensor with the delay fiber of 25 km and the sensing fiber of 10 km is built experimentally. Two null frequencies of 8013 Hz and 5011 Hz, which are induced from the delay fibers and the location of vibration respectively, are obtained. The results can serve as an important guidance for designing the sensor.     

Circular Waveguide System for Trunk Communications

9-km waveguides of 70-ram inner diameter (ID) have been inserted from manholes into ducts of polyvinal chloride (PVC). Expansion joints are provided for taking up length variations. Only a slight increase in attenuation was measured for the helix waveguide after the laying works. Phase-shift-keyed (PSK) signals with bit rates of up to 640 Mbit/s have been transmitted via this line. A new approximation method for calculating the attenuation of practical line routes has been developed. Improved design of lined waveguides should, from theory, reduce the H01route and curvature loss to below 1 dB/km. Experimental repeaters work with a double conversion scheme. They have mm-wave branching networks in oversize rectangular waveguide, IMPATT oscillators and p-i-n modulators for two-phase modulation. Printed or folded tape meanderlines serve for delay equalization.     

预测毫米波雾衰减的抛物方程模型研究

雾是影响毫米波通信系统性能的典型气象条件之一.针对传统经验模型无法精确预测多径效应下的电波传播问题,给出了基于抛物方程的雾衰减预测模型.以自由空间雾特征衰减的预测为例,将本文模型与Rayleigh近似及经验模型的计算结果进行对比,验证了该方法的可靠性.最后将该模型应用于预测35 GHz和94 GHz毫米波在分别含有平流雾和辐射雾的复杂环境中的传播特性,仿真结果表明该模型有效地反映了地形绕射、地表反射等对电波传播的影响,为快速准确地预测复杂地理环境及特殊气象条件中的电波传播特性提供了一种有效的预测模型.     

A Future Millimeter/Sub–Millimeter Radiometer for Satellite Observation of Ice Clouds

The instrument concept of a future spaceborne millimeter/sub-millimeter radiometer is proposed in this paper for the remote sensing of ice clouds from satellite. The proposed radiometer is expected to operate at a series of frequencies within the millimeter and sub-millimeter wave range from 150 to about 900 GHz. Five frequencies are selected in the atmospheric windows, i.e., 150, 220, 463, 683, 874 GHz, and at each frequency there are two orthogonally polarized channels. Three water vapor channels located close to 183.31 GHz are also included in this instrument, since they can provide water vapor information, which is needed for ice cloud parameter retrieval. To simplify system design and test, a modular design philosophy is followed in the receiver frontend design and two antennas are used separately for the millimeter and sub-millimeter channels. Overall, the instrument requirements can be met with today's technology, except for the channels at the highest frequencies, where the radiometric sensitivity can be larger than the required 1.0 K for the 10 km spatial resolution (2.5 ms integration time). However, this situation can be improved by averaging neighboring pixels in data processing if certain compromise in the spatial resolution can be made at these frequencies.     

Effects of Birefringence Within Ice Sheets on Obliquely Propagating Radio Waves

In this paper, effects of birefringence on radio waves obliquely propagating though polar ice sheets are examined to facilitate interpretations of bistatic and side-looking radar data. A formalism applicable for arbitrary radar configurations is developed to predict the returned power from within and beneath the ice sheets that have arbitrary alignments of ice crystals (ice fabrics). We applied this formalism to a range of ice fabrics found in ice cores and assessed the effects of birefringence in terms of ray-path configurations, ice fabrics, and radar frequency. Predicted frequency dependence of the bed return power replicates prominent features observed at Greenland NGRIP ice-core site. Results show that birefringence in ice of 1 km or more thickness with strong (weak) fabric can reduce the power returned from the bed 2 dB or more at frequencies higher than 200 MHz (20 MHz) as compared to isotropic ice. This suggests that quantitative interpretation of the power returned from the bed requires careful assessment of birefringence almost everywhere over the ice sheets. Application of this formalism also suggests a radar-frequency range usable for attenuation measurements, possible effects of fabric on synthetic aperture radar processing, and a feasibility of remote sensing of ice fabric.     

An Analysis of the Tensor Dielectnc Constant of Sea Ice at Microwave Frequencies

The bilocal approximation of strong fuctuation theory is used to develop a model for the tensor dielectric constant of sea ice that is treated as a random anisotropic medium. The application is a twofold one: first the individual ice grains are treated as pure ice with embedded elongated brine cells with random spacings and sizes and secondly as the combined ice-brine system, which has tensor dielectric properties, with random orientations of the principal axes and random embedded air bubbles. Comparisons with measurements on a variety of ice types over the temperature range 0 to -32 ° C and frequencies from 0.1 to 40 GHz show that the theory is able to account quantitatively for large parts of the published data including expected scattering corrections to the quasi-static dielectric constant at low salinities and high frequencies.     

A vertical-plasma-slab model for determining the lower limit toplasma density in sprite columns from VLF scatter measurements

VLF to MF backscatter from “red sprites” is investigated by the use of a one-dimensional model, in which the sprite is considered as an infinite vertical slab of plasma. The reflection (backscatter) and transmission (forward-scatter) coefficients are found for a wide range of slab parameters. For a slab in which such parameters vary with altitude, the approximation is made that the scatter coefficients vary with altitude accordingly. A conductivity of 10^-4 S/m is found sufficient to explain the observed magnitudes of backscatter (large angles up to 180°), provided this or higher conductivity prevails over a substantial part of the cloud-ionosphere altitude range. The effect of the geomagnetic field on the conductivity at altitudes up to 80 km, and wave frequencies up to 10 MHz, is found to be of little consequence