Cloud Attenuation in Millimeter Wave and Microwave Frequencies for Satellite Applications over Equatorial Climate

A propagation experiment has been carried out at Penang using the SUPERBIRD-C satellite beacon. Cloud occurrences were observed during different months and it is seen that the low cloud occurrences over Penang is very significant from October to January. The cloud attenuation results that are presented, which include the testing of models, have been obtained from the data gathered over five years. The specific attenuation of radio wave due to clouds at various frequencies 12 GHz, 20 GHz, 75 GHz, 50 GHz and 100 GHz has been estimated whereby the values varies from 0.14 dB/km at 12 GHz to 10.1 dB/km at 100 GHz.     

Cloud Morphology Over Three Indian Tropical Stations for Earth Space Communication

Microwave and millimeter wave frequency bands are in demand for requirement of larger bandwidth for various applications of radio systems. In future in India too, microwave and millimeter wave frequencies will be in use very extensively for radio communication purposes and remote sensing applications. But, the attenuation due to cloud as well as thermal noise associated with could in millimeter wave and microwave frequency bands are of great concern to system engineers and radio researchers. Both cloud attenuation and cloud noise temperature lead to degradation in the performance of microwave communication and radar propagation. The effects of rain on radio wave are more than cloud but the occurrence of cloud is more than rain. In some parts of India cloud occurs for weeks together. The cloud morphology particularly in relation to radio wave propagation over different geographical region of India is therefore very essential and important. In view of this, systematic studies on cloud occurrence morphology over different geographical locations in India have been undertaken. In recent past some results on cloud occurrences over different stations in India have been reported. In this paper, cloud characteristics and cloud attenuation over three more stations are presented. Based on low level cloud coverage observations, cloud occurrences frequency over Mumbai (19.07°N, 72.50°E), Nagpur (21.06°N, 79.03°E) and Ahmadabad (23.04°N, 72.38°E) situated in Indian tropical west coast, central plain and semi arid region of western India in different months during daytime and nighttime has been deduced. The low-level cloud over Mumbai, Nagpur and Ahamedabad has been found to occur for many days and nights and particularly in the months of June, July, August and September. The attenuation of radio wave due to clouds at various frequencies ～10, 18, 32, 44 and 70 GHz over the aforesaid three stations also been deduced.     

Cloud and Rain Effects on AltiKa/SARAL Ka-Band Radar Altimeter—Part I: Modeling and Mean Annual Data Availability

The AltiKa project, developed by the French Centre National d'Etudes Spatiales, is based on a wideband Ka-band altimeter (35.75 GHz). The technical characteristic of the instrument will offer higher performance both in terms of spatial and vertical resolutions that will lead to the improved observation of ice, coastal areas, inland waters, and wave height. An Indian Space Research Organization satellite, called Satellite with ARgos and AltiKa, will embark the AltiKa altimeter. The launch is scheduled at the end of 2010. The major drawback of Ka-band use is the attenuation of the radar signal by atmospheric liquid water. Clouds and rain effects will thus be a strong constraining factor, because the altimeter link budget imposes an attenuation of less than 3 dB. The impact of rain and clouds on Ka-band altimeter data is analyzed and quantified using an analytical model that computes AltiKa waveforms in the presence of rain or clouds. The results are then used to quantify the waveform attenuation and distortion, as well as the error induced on the altimeter geophysical parameter estimates. Because of the nonlinearity of attenuation relations, the impact of clouds/rain depends more on the cloud/rain variability within the altimeter footprint than on the mean characteristics, which makes correction using coincident rain or cloud data difficult. Small rain cell and small dense clouds can thus strongly distort the waveforms and lead to erroneous geophysical parameter estimates. The probability of 20 Hz and 1-s averaged data loss are computed from the model results and from cloud and rain climatologies. On a global scale, about 3.5% of the 20-Hz data will be lost because of rain and clouds and 2.5% of the 1-s averaged data. However, the probability strongly varies over the global ocean and can exceed 10% in the Tropics.      

Centimeter and millimeter wave attenuation by atmospheric gases and rainfall at a tropical station

The radiowave attenuation due to oxygen and water vapour has been computed over the frequency range 3–350 GHz making use of the mean surface air pressure, temperature and water vapour at Ile-Ife (geog. lat 7.5°N, long 4.5°E) in Southern Nigeria. It is observed that the attenuation at this tropical location is generally higher than at temperate climates. A similar analysis was performed for rainfall attenuation using rainfall intensity measurements. The results obtained with three different expressions for the rain height showed that a rain height of 3 km is a reasonable assumption for estimating earth-space rainfall attenuation at this location. It is found that for frequencies above 200 GHz, the polarization dependence of the specific attenuation due to rainfal becomes negligible. The computed attenuation is lower that that predicted using the corresponding CCIR rain climate data. The results show that whilst the contribution of oxygen and water vapour to the total atmospheric attenuation could be neglected when compared with rainfall attenuation up to about 150 GHz, the contribution becomes significant for frequencies above 190 GHz.     

Effect of Fog and Clouds on the Image Quality in Millimeter Communications

The effect of fog and clouds in millimeter communication is discussed, and the attenuation caused by fog and clouds is reviewed. Signal-to-noise ratio (SNR) of image is derived using relating models of fog and clouds attenuation. According to the relation of image quality and its signal-to-noise ratio, the system behavior is forecasted theoretically. It is shown that the signal-to-noise ratio of receiver at certain transmitter power is inverse with radio wave frequency, from about 70dB at 10GHz to 48dB for fog and 49dB for clouds. The image quality of received signal at certain transmitter power is inverse with radio wave frequency, from about 7 grade at 10GHz to 5.27 grade for fog and 5.37 grade for clouds. The above calculated results are consistent with experimental results.     

Attenuation of 8.6 and 3.2 mm radio waves by clouds

Measured attenuations associated with a variety of cloud conditions at wavelengths near 8.6 and 3.2 mm are reported. Two specific events, during which heavy rain clouds covered the sky, are examined and statistical data collected over a six-month period on a variety of cloud types are presented. The number of observations of some cloud types was not large and it was not possible to account for the gaseous attenuation with sufficient accuracy to get reliable values for the attenuation by the cloud droplets for a number of cloud types. The clouds causing the largest attenuations were the rain-bearing cumulonimbus ones. Of the nonrain clouds the two types for which the sample sizes are adequate and attenuations are sufficient for meaningful conclusions are stratocumulus and cumulus, their 35 GHz/95 GHz mean attenuation values being 0.18/0.61 dB and 0.12/0.34 dB, respectively.     

The main results of cloud and rain remote sensing by multichannel microwave radiometry

The basic results have been considered for multiwave remote sounding of the troposphere with clouds and rain from ground-based station in the zenith direction. The radio wave scattering by rain drops as well as variation of vapour content in the atmosphere during measurements are taken into account when processing of the experimental data. A separation of the complete attenuation in clouds with rain has been made over three components: due to vapour, cloud and rain. A relation is considered of millimeter and centimeter wave attenuation in clouds with rain between each other and with the rain intensity. A behaviour of the relation of attenuation structural functions is explained. The Diagnostic problems of millimeter wave attenuation are considered.     

Cloud Height,Cloud Temperature and Cloud Attenuation in Microwave and Millimeterwave Frequency Bands over Indian Tropical East Coast

Based on radar RHI (Range height indicator) measurements, cloud height has been deduced during the worst months (July–August) over Kolkata. Such cloud height results have been utilized to estimate cloud temperature. The attenuation of radiowave due to cloud in various probability levels has been determined in millimeter wave and microwave frequency bands. Such results on different probability levels are useful for satellite communication and remote sensing application over the aforesaid station in tropical India.     

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。     

Proposed Model for Radio Wave Attenuation due to Rain (RWAR)

Radio wave attenuation is primarily caused by the absorption of a radio signals by some atmospheric phenomenon like rain, snow or ice, clouds, dust etc. These losses are more prevalent in the frequency ranges above 10 GHz. Attenuation caused by rain is not only limited to satellite up-link and down-link but it can also affect the point-to-point terrestrial microwave links above 10 GHz. This paper briefly discussed about the work done by researchers at different parts of the world regarding the attenuation caused by the rain for higher frequencies. It then proposes a mathematical model for prediction of radio wave attenuation due to rain. The implementation results of proposed model were also compared with the ITU-R model.

     

Backscatter and attenuation by falling snow and rain at 96, 140,and 225 GHz

The results of measurements are presented for backscatter cross section per unit volume and attenuation for falling snow and rain at 96, 140, and 225 GHz. The attenuation due to rain is almost independent of the measurement frequency, but for snow the attenuation is considerably greater at 225 GHz than at 96 GHz. The rain attenuation generally varies with the rain accumulation rate in accordance with an aR^b relationship for a Laws and Parsons drop-size distribution where R is the rain rate and a and b are constants. The attenuation at all three frequencies is about 3 dB/km for a rain rate of 4 mm/h. The attenuation due to snow varies with airborne snow-mass concentration, with the average rates of increase being 0.9, 2.5, and 8.7 (dB/km)(g/m³) at 96, 140, and 225 GHz, respectively. Generally the attenuation for snow is lower than that for rain. The backscatter cross section per unit volume for rain at 96 GHz is about -35 dB m²/m³ for a rain rate of 4 mm/h. The backscatter from snow at 96 GHz is much lower than that from rain under equivalent accumulation rates or airborne mass concentrations. Snow backscatter at 140 GHz is comparable but higher than that at 96 GHz     

Atmospheric emission measurements of attenuation by microwaveradiometer at 19.4 GHz

Attenuation measurements on an Earth space path are presented using a passive microwave radiometer operating at 19.4 GHz in the emission mode. Attenuation measured under clear weather showed variation between 0.2 to 1.1 dB, whereas for cloud conditions attenuation as high as 1.0 dB have been recorded. Attenuation measurements for rain events have been correlated with rainfall rate using a fast-response 10 seconds opto-electronic rain gauge. The values of attenuation versus rainfall rate varied between A (dB)=0.01+0.18 R(mm/h) at the minimum and A (dB)=0.01+0.25 R(mm/h) at the maximum, showing considerable variability in the values of attenuation from year to year. The attenuation statistics for different seasons have also been computed and they show considerable changes from season to season-the largest attenuation in excess of 10 dB recorded in July-August-September, whereas minimum attenuation in excess of 2 dB recorded in December-March for nearly two years of data. Comparison of measurements made over New Delhi with those reported elsewhere show that for 0.02% of time attenuation values lie between those of Slough, England, and Crawford Hill, NJ. The concept of effective path length has been discussed based on the relationship between effective path length and the rain rate     

Attenuation in melting snow on microwave- and millimetre-wave terrestrial radio links

The scattering properties of melting snow on microwave and millimetre-wave terrestrial radio links are predicted using a new model for melting which includes coalescence. Attenuation, differential attenuation and differential phase are calculated for a horizontal path, with results at 36.25 GHz presented. Peak specific attenuation in the range 8?13 dB/km is expected for underspread rain with 10?15 mm/h rain rates.     

Microwave and Millimeter Wave Characteristics and Attenuation of Clouds over some Malaysian Equatorial Stations

Based on radar range height indicator (RHI) measurements, cloud characteristics in relation to radiowave propagation over three locations in different geographical region in western Malaysia have been presented. It is seen that low cloud occurrence over these locations are quite significant. Cloud attenuation and noise temperature can result in serious degradation of telecommunication link performances. This paper presents cloud coverage in different months, 0°C isotherm height and cloud attenuation results at 12 GHz, 20 GHz, 36 GHz, 50 GHz, 70 GHz and 100 GHz over measurement site. The low level cloud over the measurement sites has been found to occur for many days and nights and particularly in the months of April to May and October to December. Such results are useful for satellite communication and remote sensing application in Malaysia.     

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.     

Attenuation/rain-rate relationships on terrestrial microwave links in the frequency range 10?40 GHz

Attenuations of radio transmissions at 10.7, 19.4 and 36.05 GHz by rain on a 7.5 km path have been compared on a statistical basis with attenuations derived from rainfall rates, measured simultaneously along the path, using the empirical relationship ? = ?R?, where ? is the attenuation coefficient in dB km?1, R is the rainfall rate in mm h?1 and ? and ? are parameters dependent on polarisation and frequency.     

Rain Height in Relation to 0°C Isotherm Height Over Some Indian Tropical Locations and Rain Attenuation for an Indian South Coastal Station for Microwave and Millimeter Wave Communication Systems

There is a dearth of results on rain height over Indian tropical stations.The results on rain height in relation to 0°C isotherm height over four stations having different latitudes are presented in this paper. Four stations have been chosen in such a way that all have different latitudes and are located in different geographical regions having different local weather conditions. The seaonal variation of rain height in relation to 0°C isotherm height has been found to be appreciable over the station located in Indian east coast and Gujarat region, while seasonal variation is not significant at lower and intermediate probability levels over the stations located in Indian south-east coast and island. The prevailing local weather conditions over different stations also have been discussed. Based on observed rain heights and rain rates, the attenuation of radio wave at different frequencies lying in the range from 10 GHz to 150 GHz for different probability levels over Indian south-east coastal station have been deduced and presented in this paper.     

Rain attenuation measurements over New Delhi with a microwave radiometer at 11 GHz

Rain attenuation measurements over New Delhi carried out with a microwave radiometer installed at the National Physical Laboratory (NPL), New Delhi and operating on 11 GHz for a period of more than three years are presented. For 0.01 percent of time for the period June 1977-April 1978, the attenuation exceeded for the monsoon period is 14.0 dB whereas for the whole year, it exceeds 10.4 dB. During the winter for the same percentage of time, the attenuation exceeded 1.5 dB, whereas for March-April it exceeds 0.5 dB. For the period May 1978-June 1980, it is observed that for 0.01 percent of time the attenuation for the whole year exceeds 9.0 dB. During the winter for the same percentage of time, the attenuation exceeds 1.4 dB whereas for March-April it exceeds 0.4 dB. A comparison of attenuation over New Delhi and those reported elsewhere are discussed. Yearly and worst month time ratio over New Delhi are given also as the values reported for the European region. Comparison of the attenuation distribution and the rate of surface rainfall measured with a rapid-response rain gauge are also given. The comparison shows that for the monsoon period and for 0.01 percent of time, the attenuation value exceeded for 14 dB corresponds to the surface rainfall rate of 140 mm/h. For the monsoon of 1978, 1979, comparison shows that for 0.1 percent of time, the attenuation value exceeded for 9.0 dB corresponds to the surface rainfall rate of 90 mm/h. Variation of attenuation and effective path length for various rainfall rates and elevation angles are also given.     

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.     

采用两个级联外部调制器产生四倍频光载毫米波的光纤无线通信系统

研究了一种采用两个级联外部调制器基于光载波抑制原理产生四倍频毫米波的光纤无线通信(ROF)系统.在中心站利用电混频器产生副载波复用信号,通过第一个外部调制器产生两倍射频(RF)信号的光载毫米波信号,再通过第二个外部调制器产生四倍射频信号的光载毫米波.实验显示采用频率为10 GHz的射频信号源和2.5 Gbit/s的数据基带信号混频通过两个级联外部凋制器后产生毫米波的频率为40 GHz,并且在单模光纤中传输距离达20 km,功率代价小于2 dB.