Variability of millimetrewave rain attenuation and

Indian Journal of Radio & Space Physics Vol. 36, August 2007, pp. 325-344

Variability of millimetrewave rain attenuation and rain rate prediction: A survey R Bhattacharya, R Das, R Guha & S Deb Barman Department of Environmental Science, University of Kalyani, Kalyani 741 235, India and

A B Bhattacharya Department of Physics, University of Kalyani, Kalyani 741 235, India Received 24 May 2006; revised 31 January 2007; accepted 3 May 2007 This paper reviews the literature on attenuation of mm-wave due to rain and rain rate prediction methods to analyze the performance of the various systems proposed by different workers at different parts of the globe under varying meteorological and topographical conditions with an emphasis on the observational reports made in the tropics, particularly in the Indian subcontinent. In this comprehensive review, besides considering the various features related to rain rate, the various methods proposed for prediction of rain attenuation have been examined and thereby a comparison of those prediction methods is made. Finally, scopes for future investigation have been critically focused. Key words: Rain attenuation, rain rate, millimetrewave PACS No.: 92.60. Ta

1 Introduction Radio wave propagation is a complex phenomenon. The international regulations for tropospheric radio wave communication are based on set of propagation models. Such models are developed by making physical and statistical analyses of collected data. The more we become rich in the understanding of different phenomena involved, the more we can improve our models. Several propagation models through the troposphere for frequencies above 1 GHz can be thought of. Existing models may be improved for rain attenuation and for trans-horizon tropospheric scatter (troposcatter) propagation. Rain plays a significant role in the undesired absorption of microwave, millimetre and centimetre wave propagation in the lower atmosphere. Besides rain, other major contributors are water vapour, liquid water clouds and fog. Such absorptions cause variations in the signal strengths. Attenuation value exceeded less than 2% of the year is due to rain or cloud except at 22 GHz water vapour absorption line and 60 GHz oxygen line. The model cloud gives attenuation comparable to rain at 2% of the year. At smaller percentages, rain is considered as the sole cause of attenuation to the communication system designer.

In the past, several theoretical and experimental studies were made in respect of rain attenuation to obtain better service reliability in communication. Crane1 theoretically calculated attenuation for a known rainfall intensity along a path. During 1980 and onwards, prediction techniques for statistical estimation of attenuation probability for a particular path have been studied. Two approaches are made − one based on the use of large number of attenuation statistics at different frequencies, locations and path geometries and the other based on the synthesis of attenuation values from meteorological data. Latter approach is of great interest in modelling since a large number of data are available. There is no established theory of statistical variations of rainfall intensity, specific attenuation and attenuation along a path which are linked with each other in a complex manner. Rice and Holmberg2, Lee3 and Dutton and Dougherty4 carried out statistical analysis of U.S. National Weather Service rain accumulation data and provided an estimate of yearto-year variations of rain rate distribution. Rain shows prominent spatial and temporal variation along a horizontal path, which prompts statistical estimation of instantaneous rain rate profile along the path. Several workers have proposed different models for calculation of attenuation along a

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path. Most of the models were developed using point rain rate and path attenuation measurements. Model rainstorms have also been developed from weather radar data analysis to convert point rainfall statistics to path attenuation estimates5. A very useful new technique, viz., ‘observation of phase delay from GPS satellites’ has emerged; it allows continuous monitoring of atmospheric water vapour content along a slant path using millimetrewave radio propagation6-11. Water vapour, rain, cloud and oxygen attenuate radiowaves very significantly. Attenuation due to water vapour along earth-space radio links increase prominently above 50 GHz and becomes higher than the peak absorption found at 22.235 GHz water vapour resonance line12. Moreover, oxygen, cloud and rain (especially, light rain) play a very important role on propagation above 50 GHz. Water vapour contributes to both attenuation and tropospheric scintillation on earth-space paths13; also a relation has been established between the occurrence of water vapour and cloud attenuation14. Due to continuous presence of water vapour in the atmosphere and a greater occurrence rate of cloud than rain, these two factors need to be properly assessed. Using GPS technique, a comparison of attenuation has been made between the radiosonde and radiometer data15 to support millimetrewave propagation in the study of attenuation. Lastly, a suitable model is needed for predicting the attenuation on a slant path with the variation of rain intensity. Below 60 GHz, the attenuation due to snow or ice is very small and hence neglected16. Liquid rain drop is the only cause of attenuation. Studies on the variation of attenuation with height show that major attenuation occurs below the height of the bright band17 (0○C isotherm); this fact is also supported from comparisons between radar based attenuation prediction and simultaneous path attenuation measurements18,19. The rain intensity, on the average, does not vary with height between the surface and the base of the bright band as evident from weather radar data20,21. This leads one to model the specific attenuation as constant from the surface up to a height near 0○C isotherm level which also needs to be estimated statistically. An attenuation prediction model should contain estimates of expected year-to-year and station-tostation variation and also expected attenuation at the same exceedence probability. A model on such estimates has been developed by Crane16. The

attenuation was compared between predictions from model and available attenuation observations made on a number of propagation paths and obtained agreement within expected uncertainty. The growing demand of communication services has congested the currently available radio spectrum to such an extent that a need has been felt for larger bandwidth and new frequency band above 20 GHz is being explored. However, the rain attenuates the upper spectrum and frequencies suffer strong impairment as the rain drop diameter approaches the size of operating wavelength. In tropical countries like India, a great diversity of climatic conditions has been witnessed in the recent past and a search is on to find out a prediction model, useful for all ranges of rain fall rate. 2 Troposcatter propagation Rain scatter is considered to be partly due to interference. However, it is unlikely that there would be interference due to rain. At frequencies slightly below 10 GHz, rain scatter prediction technique gives an excellent estimate of the strength of scattered signal. Rain scatter model suffers a problem above 10 GHz when rain attenuation along signal path and from scattering volume reduces signal level, whereas simultaneous scattering within the volume boosts up the signal level. Attenuation and scattering need not be modelled together, since the former is much more important, and troposcatter link systems should be designed at lower frequencies. In fact, modelling of transhorizon scatter propagation is undergoing a rapid change due to lot of research work and in the coming days revised models are likely to revolutionize our knowledge of troposcatter processes. 2.1 Rain attenuation

Another part of rain scatter process is the attenuation of signal by rain along the scatter path. Way back in 1947 Ryde22 presented a rain attenuation model. After two decades Medhurst23 published a revised model of Ryde & Ryde and he concluded that “… (the) agreement is not entirely satisfactory; there is a tendency for the measured attenuations to exceed the maximum possible levels predicted by the theory”. A decade later, Crane24 looked afresh at the model predictions and compared them with the measured values taking the data available to Medhurst and new data published after that. He found an average matching between model predictions and measurements.

BHATTACHARYA et al.: MILLIMETREWAVE RAIN ATTENUATION & RAIN RATE PREDICTION

A rain attenuation model should predict both the median distribution for a number of yearly attenuation distributions as well as the amount of expected deviations of distribution of attenuation for any single year of observation from the long term median distribution. Old and early models considered raindrop sizes, shapes, uniform rain rates along a path, etc. For uniformity in rain rate, the time average of rain rate at a point was considered by Bussey25 to approximate the path average rain rate along a link. Considering the specific attenuation to be proportional to rain rate, such a model works well, but the physics involved in the process was very much complicated. Earlier models give estimation of point rain rate statistics and the statistics of variation of rain along a path. Statistics of path length through rain has also been modelled with the advent of satellite communication. Initially, the models were physical observation based, but with the availability of attenuation statistics such models were adjusted. Today, enough data are available for a standard model. However, the practical methods, which are adopted to measure the attenuation statistics do not give a reliable model and compels one to turn back to the models that are physical in nature. Development and evaluation of models require a method to control the quality of data observation in the model26-28. The ITUR (formerly known as CCIR) gathers round the clock global data, which measures attenuation. Crane29 used such data and noticed that the statistical deviations of

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attenuation from model predictions were log-normal and the parameters used in log-normal process practically did not change significantly for different models. The ITUR (CCIR) recommended models did show the smallest deviations. The deviations of observations from USA in the CCIR database were analyzed for a physically based model by Crane30. Figure 1 shows the cumulative distribution function (CDF) for log ratio of measured and modelled attenuation at 0.01% of a year. As the statistical distribution for path-to-path variation is lognormal, the log ratios were plotted on a normal probability paper. The CDF must lie on a straight line provided the log-normal hypothesis is correct. In order to display all the observations on a single graph, the attenuation prediction model was used to normalize the data. The same general result is valid for a different prediction model. Only the bias value (intercept with zero value for reduced variate) changes, but the slopes of best fit curve to the deviations remain unaltered. 2.2 Analysis of moderate and intense rainfall rates

With the increasing demand in recent years, there is greater congestion of communication spectrum which leads to allocation of greater bandwidth and hence the increasing need for short wave lengths (centimetre and millimetre) in terrestrial and satellite links. Rain has been established to be the principal cause for attenuation in the microwave radio links. In recent years, for frequencies above 10 GHz in single

Fig. 1 — Cumulative distribution of attenuation prediction errors for all observations in the CCIR data bank from USA (after Crane, 1996)

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and dual polarization techniques, especially in satellite communications, various aspects of precipitation is being studied in a great detail31. 2.2.1 Study of rainfall rate over Indian locations

In tropical countries communication for higher frequencies are disturbed due to high rain rate. Rain strongly attenuates the radio waves above 10 GHz, which is a main impediment to satellite link performance32. Studies of the variations of rainfall pattern are helpful for predicting propagation conditions. The drop size distribution varied widely in the tropics. Distribution of rain attenuation at 11.7 GHz has been compared with prediction models33 by utilizing INSAT-2C satellite signals. Theoretically estimated attenuation of radio waves using the concept of decay of rain path profile at 11 GHz and 13.4 GHz for 56° elevation angle over Delhi27 showed that CCIR model underestimates the attenuation over India. Average altitude of 0°C isotherm varies from 4.7 km at 40° latitude to 3.1 km at 60° latitude34. The vertical structure of attenuation assumes a constant value up to 0°C isotherm based on the drop size distribution analysis35,36. It is found that excess attenuation due to rainfall above a certain threshold frequency limits the LOS links37. 2.3 Study of rain structure using X-band radar reflectivity

To assess the interference between terrestrial and earth-space satellite links, it is important to study the effect of scattered radio signals from transmitters. Advent of wind profiler38 and MST radar39 has generated a considerable interest in the study of radar bright band. A study of rain structure for both horizontal and vertical extensions has been made with X-band radar40 at 9.375 GHz. Reflected radiowaves from rain cells are used to find horizontal and vertical extension of rain for which measurements are made from radar plan position indicator (PPI) and range height indicator (RHI). Results of such extension are shown to depend upon radar reflectivity factor (dBz), which is essentially a measure of strength of scattering cross-section (which causes interference in radio signals) of raincells41. 2.4 Vegetation biomass and microwave emissivity

For better understanding and protection of our environment and good management of natural resources we need to keep vigil on soil and plant biomass. Presently, remote sensing technology is not much in use; rather, use of active and passive

microwave sensors are of great demand. They are used to find moisture content of open and vegetation covered soil to have a knowledge of the physical principle involved and to find observation parameters together with its accuracy. Study of plant biomass and its water content has been done by many researchers42,43 . Biomass is one of the most important parameters for evaluation of crop type and its yield, as it contains water, which attenuates microwave. Vegetation biomass changes with growth of crop and its dielectric is found to be related with microwave emissivity, which is used in remote sensing. From the study by Singh and Sharan44, microwave emissivity at X-band is found to increase with biomass. 3 Surface point rain rate prediction In absence of any theory, estimations of surface point rain rate are done empirically from available long term rain accumulation data of two types – one is long duration hourly, daily and annual accumulation data for a large number of geographical locations and the other is the excessive precipitation data available from a limited number of locations3. Instantaneous data are taken together with attenuation values to compare evaluation of model; but such data are not suitable to give expected rain rate distribution. In order to estimate attenuation along a path, the specific attenuation at each point of the path is needed. For prediction of attenuation, instantaneous rain rate distribution is regarded as very useful, but its measurements fluctuate due to atmospheric turbulence. To remove turbulence, rain rate averages are taken. Amount of averaging required for estimation of rain rate distribution depends upon its application. There is equivalence between temporal and spatial averaging for translating rain cells and Bussey25 recommended the use of hourly averages for attenuation estimation on 50 km terrestrial paths. Radar study shows that rain cells present in the high rain rate region cause significant attenuation. Rice and Holmberg2 and Lee3 devised models for instantaneous rain rate estimation. The Rice-Holmberg model provides a good estimate of rain rate distribution in 0.1-1 % range. 3.1 Vertical variation of specific attenuation and its prediction

The point-to-path model predicts rain rate variation along a horizontal path and is used in the design of terrestrial communication systems. But, for slant paths, vertical variation of specific attenuation is also


required. Although ice, snow, etc. do not produce attenuation, but raindrops, raindrop sizes and melting ice particles give significant attenuation. Radar studies show that rain can be represented, on the average, by a constant reflectivity from surface up to the height of 0οC isotherm21. Depending upon the raindrop size distribution, a constant reflectivity value means a constancy in the value of specific attenuation. This fact leads to model a vertical profile of specific attenuation, which gives a constant value from ground up to the height of 0○C isotherm. To estimate the vertical attenuation the required measurement is the rain height. Some investigations on the variation of rain height at different locations are reported by Mondal et al.26 taking the 0°C isotherm as the rain height. 3.2 Variation of attenuation about its estimated value

Specific attenuation is calculated from point rain rate value using a power law and employing Laws and Parsons raindrop size distribution35,45,46. Differences in specific attenuation obtained from above distribution as well as from a large number of observed drop size distributions are statistically very small for frequencies up to 50 GHz. But at higher frequencies, errors in the measurement of drop size distribution for small drop size range affect the accuracy of specific attenuation and rain rate relationship. Additional uncertainties linked with sampling of drop size distribution are small and ignorable16. The physical limit of attenuating zone is defined without any uncertainty for a terrestrial path, but it depends upon the vertical extent of rain region for a slant path. Thus, the several steps of attenuation prediction model use the median values of statistically variable quantities. The statistical processes linked with each step are assumed to be independent of each other and the variances combined on that basis. A new model for easy application for prediction of rain attenuation in case of terrestrial or slant path propagation has been prepared by Crane. This model was extended globally on the basis of zonal averages. It was successfully applied for stations in low mountains near the coast, but not considered for modification of vertical temperature profile due to large regions of high terrain. Further studies on understanding of the local variations in rain rate climate are essential. The model can also be

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extended to accommodate seasonal and monthly predictions. 3.3 Prediction of attenuation on earth-space links

Modification of refractive condition of atmosphere depending upon the moisture content has been investigated in India by many workers. It is found that LOS links are affected around the coastal regions up to a few kilometres due to land-sea breeze. The change of refractive profile, in turn, reduces the microwave signal47. Deterioration of a communication link at 13 GHz in monsoon month revealed that the communication link did not serve the purpose 5% of time48. Full communication link can be achieved during monsoon months if an extra gain of 12-15 dB is provided to the transmitting system. The attenuation is found to be 0.5 dB/km at 22.235 GHz during monsoon months (Fig. 2). Ground-based zenith looking radiometer has a window49 and the signal suffers no change with respect to the total attenuation within 1% height limit. Radar reflectivity is an important tool to estimate the microstructure of rain cell40. Rain rate is calculated using Z-R relation. A detail of precipitation structure is also calculated by Gairola et al.50 using combined microwave and IR rain algorithm during a Bay cyclone. The simultaneous measurements of precipitation by passive radiometry and radar reflectivity can be utilized to calculate rain attenuation for earth-space paths. According to ITU-R (formerly CCIR) rain climatic zones have been designed following the characteristics of precipitation for propagation modelling51. Prediction of path attenuation by ITU-R underestimates the radiometrically derived cumulative distributions (CDs) of path attenuation, in general. Furthermore, the ITU-R procedure may not match well with the rainfall rate characteristics. The model only predicts rain induced attenuation. Figure 3 shows total attenuation statistics for measured annual, worst month and predicted (ITU-R method) cumulative statistics. The ITU-R prediction model52 needs knowledge of the rain rate exceeded 0.01% of the time as measured using a gauge with one minute integration time. This factor contributes to the overestimation of attenuation by the ITU-R model. The standard ITU-R “H” zone rain profile has a rainfall rate exceeding 32 mm/hr for 0.01% of the time. Other factors may also be responsible for the

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difference between the measured and predicted attenuations. 3.4 Equiprobable point-to-path average models for different types of rain

Here one is to rely on dual polarization radar data. The Chilbolton dual polarization radar is, perhaps, the best instrument in Europe and USA53, which offers the most accurate calibration of dual polarization facilities with high volumetric resolution. Dual

polarization technique is able to characterize drop size distributions (for a given form) for every radar range gate. Hence, it enables us to retain systematic drop size rain-structure dependence in the model. A study of horizontal variation in rain intensity is done for characterization of spatial rain intensity. Widespread (stratiform) and showery (convective) are differentiated by the presence of bright band or its absence. For this purpose, an automatic edge detection technique is successfully applied to obtain

Fig 2 — Electromagnetic wave attenuation profile due to water vapour and oxygen as a function of frequency in the millimetre wave band (after Karmakar et al. 2002)

Fig 3 — Cumulative attenuation statistics (after ITUR Recommendation, 1992)


precise rain height in widespread rain. Leitao and Watson54 analyzed 40 distinct rain events, out of which 17 were in showery rain category. They employed edge detection technique very successfully and placed all the stormy convective structures in the widespread rain category. Such categorization was essential for development of the model, which could well be applied over UK and beyond. Showery (convective) rain data can also be applied to almost all regions in UK in order to make prediction of attenuation in 11-14 GHz band. However, a knowledge of relative occurrence of widespread-toshowery rain is essential for making prediction of attenuation over the European region. The spatial distribution data of rainfall intensity are, thus, related to the type of rainfall in this point-to-path average model. This strongly supports the approach of the said model. 4 Slant path attenuation statistics Terrestrial and satellite communication systems mainly operate at centimetre and millimetre wavelengths. Signal degradations in such links occur mainly due to rain. This poses a major problem in electromagnetic wave propagation through raindrops. Studies on microwave precipitation are based mainly on three steps: (i) to calculate scattered amplitude when a plane electromagnetic wave is incident on a rain drop, (ii) to evaluate the far-field intensities of a plane wave which penetrates through a thin layer of rain cloud and (iii) to assess the overall degradation of signal in presence of thick precipitation. On the basis of simple assumptions, rigorous mathematical analyses have been done. However, meteorological uncertainties of rainfall are very much complex and they greatly limit the necessity of the results obtained from theoretical studies for practical applications. 4.1 Drop size distribution for microwave link modelling

Radiowave propagation depends on the physical characteristics of rain. Rain attenuation characteristics at 11 GHz reveals that it has significant contribution within 10° elevation angle28 and cause degradation of the communication link. The drop size also plays an important role. It is found that the link suffers attenuation in April, May and June when the rain rate is not so high. High attenuation not always coincides with high rainfall. Study of drop size reveals that attenuation increased from 1.4 dB/mm to 3.2 dB/mm when the drop size increased from 3.4 mm to 4.5 mm

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diameter55. The rain drop size distribution depends upon the climatic conditions and varies from region to region, and also on rain types such as drizzle, widespread, shower, thunderstorm and convective, within the same region. Many experiments carried out using distrometer for developing rain drop size distribution reveals that the drop size distribution (DSD) would be log-normal for tropical region compared to exponential distribution for temperate region. The DSD experiments carried out in Nigeria, Brazil, Malaysia and India confirm the log-normal drop size distribution model to be suitable for tropical region. The coefficients of distribution would change with the region. Rain drop size distribution experiment conducted in India also shows the distinct difference in the presence of rain drop sizes during shower and thunderstorm rain at the same rain rate in different rain events, which is very important for evaluating the microwave and millimetre wave attenuation model for radio link engineering. The presence of small raindrops gives the absorption of radio signals at millimetre waves compared to the scattering effect, which is prevalent for larger drops56. 4.2 Power law

For engineering applications, the specific attenuation and phase shift are expressed by a power law relationship35 given by aRb (values of parameters a and b are available for widespread rain35,36 for a wide range of frequencies). Mie calculations of spherical raindrops were applied to get the values of a and b and hence do not differentiate between vertical and horizontal polarizations. Harden et al.57 and Fedi58 used log-normal drop size distribution to obtain the values of a and b for spherical raindrops. The specific rain attenuation has been evaluated based on rain drop size data collected from India during 1989 by Verma and Jha59. All such calculations give the value of specific attenuation only. In case of depolarization effect, values of parameters a and b for both specific attenuation and specific phase-shift are required. The relationship between the rain-rates for two different integration times can be obtained by using power law35, Rt = aRTb

…(1)

where R is the rain intensity in mm/h, t the integration time at which rain rate is calculated, T the integration time at which rain rate is available and a, b are

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parameters which depend on the frequency and microphysical properties of rain. Using 10s and 15 min integration times Sarkar et al.60 empirically tested the power low which comes out as: 0.852 R10 s = 2.567 R 15 min

…(2)

Simple power law and two segment power law fit for a particular phase-shift. In case of shower and thunderstorms, such phase-shift is significant for communication studies over a limited range of rain and hence a simple power law shift is enough. Olsen et al.35 calculated specific attenuation using Laws and Parsons distributions for spherical drops at 0оC and 10оC. They observed the horizontal and vertical polarization results averaged at 10оC. Moreover, with the increase in frequency, parameters a and b may become insensitive to the temperature. 4.3 Prediction of slant path rain attenuation statistics

Prediction of rain attenuation along a slant path can be done by measuring radar reflectivity data. Uses of a dual polarized data make our prediction of rain parameters more precise and accurate. To predict slant path attenuation successfully from radar reflectivity data, one needs to use the radar to find out slant path attenuation for any path. This necessitates for satellite beacon or radiometric studies. Radars have not been widely used so far to collect attenuation statistics data. Reasons are two fold: (a) one needs to handle a large number of data to be collected and analyzed and (b) a reliable dual polarized radar is to be used. However, a radar can be used to scan an area in space during rain to give us a database. Effects of elevation angle, frequency of operation, rain height and site diversity can be studied from such database. 4.4 Prediction of attenuation from reflectivity (Z) and differential reflectivity (ZDR)

Rain can be described by a gamma drop size distribution (DSD) as given below: N(D) = Nµ Dµ exp(–ΩµD)

…(3)

Here, we encounter three parameters (µ, N and Ω), but in radar measurements only two variables are required. Stutzman et al.61 made an extensive study to match predicted attenuation with the measured value on slant path and noticed that a value of µ = 2 gave the best matching and it was used in their subsequent work. In respect of other rain parameters, viz., drop

shape and its distribution, canting angle and its distribution, they used Morrison-Cross drop shapes assuming all drops to be oblate; also canting angle for all drops was taken to be zero as it does not affect very much the attenuation prediction for a circularly polarized wave. The DSD was estimated in a radar range cell by generating a large number of Tables relating Z and ZDR to a wide range of DSDs with µ = 2 together with a range of Ω and N. Although the process is rigorous, it is more accurate compared to empirical relationships for reflectivity and attenuation. For a known value of DSD, forward attenuation can be predicted at any frequency. Here, the transmission matrix for each rain cell is calculated and then summed over the entire path to get total path attenuation. The RHI data were used to eliminate melting layer effect and to obtain the layer height, if it was present. It also enabled not to carry out attenuation calculation at the lower edge. In case of low ZDR, radar signals undergo random fluctuations and lead to attenuation prediction unreliable. When attenuation is calculated from Z and ZDR, the approximate relationship gives attenuation to be proportional to {1/(ZDR)1.5}. From this, it is evident that attenuation goes to infinity when ZDR approaches zero and the finite value of ZDR is obtained with appreciable reflectivity. To avoid fluctuation of ZDR due to raindrop reflection, all range cells, for which reflectivity Z22 dBz and ZDR less than 1dB, Stutzman et al.61 used an approximate relationship to get the value of attenuation. The relationship is: Ω = aRb with R = (10z/10/200)0.625

…(4) 35

Values of a and b are given by Olsen et al . Attenuation prediction for many events from radar data shows good agreement with beacon attenuation measurements for every slant path. Over and underprediction of attenuation occurred in other events. The predicted attenuation was in very good agreement with the measured one. 4.5 Rain attenuation in the tropics

In tropical latitudes where rain rate is high, communication link at higher frequencies is a major problem. Severe attenuation and depolarization32 of radio waves occur above 10 GHz. For setting up of links above 10 GHz at places where rain plays havoc, study of meteorological variations of humidity and


rainfall is essential for reliable communication. Most of the attenuation studies so far for earth-space links were confined mostly to the temperate zones of the world. However, very few measurements have been carried out in tropical region like India, Singapore, Brazil, where rain characteristics (both horizontal and vertical rain structure) are different from those of temperate climate. In India, experiment were conducted at 20/30 GHz for LOS link distance of 6 km for prediction of horizontal characteristics of rain attenuation62. Similarly radiometer were used at the same frequency for vertical characteristics of rain and rain height for prediction of slant path attenuation. Tremendous use of satellites in telecommunication demands further study of attenuation in the tropics where rain plays major role compared to other hydrometeors63. Since the nature of precipitation in the temperate regions varies widely from that at the tropics, the system design of one place will not be suitable for the other. The causes which make system design different at the two places are: (i) rain drop size distribution which is usually larger in the tropics, (ii) 0○C isotherm height at the tropics is greater than that at temperate regions and (iii) the lack of reliable rain study at the tropics; even the recent ITU-R recommendations for rain attenuation measurement at the tropics failed to give satisfactory result64. For a reliable prediction of rain attenuation, rainfall characteristics of a particular region is to be investigated properly for system design. Two constraints are there for such investigation – one is the lack of availability of geographical rain rate probability in the tropics, and the other is the lack of a valid model for estimation of rain attenuation in the tropics. Hence, attenuation is directly measured from satellites and margins for future satellite systems are estimated. Prediction models of rain attenuation are available, but those are based upon the results obtained mainly in temperate climates. The prediction model of rain attenuation developed by Verma and Jha56 based on the experiment conducted in India are available for communication system designer for providing appropriate fade margin for reliable communication above 10 GHz; otherwise the communication system designed, based on rain attenuation data of temperate climate, can provide outage of radio signals leading to link failure for higher rain rate as experienced in Singapore by Singapore Telecommunications, which lead to the

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detailed study of rain attenuation. Raina and Uppal65 have also collected some data from radiometric studies over New Delhi, although these are not enough for a good understanding of propagation mechanism in the tropics33. 5 Attenuation and brightness temperature Gaseous absorption and emission of communication waves by the atmosphere were calculated by CCIR (International Radio Consultative Committee) employing radioactive transfer programme as described by Waters66. The role of atmospheric emission in communications and the relationship between absorption and emission have been given by Smith67. The atmospheric emission (i.e. brightness temperature) and attenuation model were discussed and comparisons with respect to measurements and other models were made67. At radio frequencies, scattering is insignificant and hence attenuation in the gaseous atmosphere will be due to absorption only. For frequencies up to 340 GHz, the gaseous atmospheric emission is a major source of external noise in the earth-space communication system68. Since NASA was interested in low noise receivers, attention was given on noise sources also. Earth’s atmosphere (dry state) consisting of oxygen (20.946%), nitrogen (78.084%) and argon (0.934%) all by volume and totaling to 99.964% is well mixed at nearly 80 km height where dissociation of O2 molecules to atomic oxygen takes place. Carbon dioxide accounts for the rest and mixes well in the atmosphere. However, water vapour is the main variable component in the atmosphere. The saturation vapour pressure is directly proportional to temperature. Centimetre and millimetre waves are absorbed chiefly by oxygen and water vapour. In fact, water vapour lines occur at 22.235 GHz, 183.31 GHz and 325. 152 GHz. Tails at higher frequencies need also to be considered; informations on these lines are given by McClatchey et al.69, Rothman and McClatchey70 and Rothman71,72. Oxygen and water vapour absorption in the window region has a remarkable effect. Ozone has spectral lines in the microwave region, which are neglected to a first approximation. Waters66 has discussed about stronger O3 lines and those of other minor constituents. The absorption coefficient which is a function of pressure, temperature and frequency can also be a


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function of the earth’s magnetic field in the upper atmosphere. Total absorption for a path through the atmosphere is given by: ∞

A = ∫ ∑ γ i dl 0

…(5)

i

where γi is the absorption co-efficient for the i-th gaseous constituent. In almost all communication purposes, two gaseous absorbers are taken — one for water vapour and the other for molecular oxygen. Thus, A is rewritten as, ∞

∞

0

0

A = ∫ (γ 0 +γ ω )dl = ∫ γ 0,ω dl

…(6)

where γ0 and γω are the absorption coefficients for oxygen and water vapour, respectively, and γo,ω is their joint absorption coefficient. According to Kirchoff’s law, while in local thermodynamic equilibrium (LTE), the emission from a gas must be equal to its absorption at each frequency. This leads to the earth’s atmosphere to be LTE. Rayleigh-Jeans law states that brightness is proportional to temperature at radio frequencies. The brightness temperature, TB, is used here to refer to the temperature of sky in a particular direction as seen by an antenna of very narrow beam width. 5.1 Model of absorption and brightness temperature

The JPL (Jet Propulsion Laboratory) model uses expression given by Waters66, but Rosenkranz73 expression is used for oxygen absorption coefficient. Oxygen (O2) spectrum between 48 and 67 GHz shows about 28 discrete lines, which are broadened to a single broad maximum at one atmosphere pressure. Above 30 km, these lines are well separated and exhibit marked Zeeman splitting. Since the earth’s magnetic field changes from 0.6 G at poles to 0.3 G at the equator, the zenith absorption between 48 GHz and 67 GHz depends on the magnetic latitude change. 5.2 Wet antenna attenuation

Besides attenuation of radio waves by rain, moisture, clouds, etc. along its propagation path, another major cause of attenuation occurs when the receiving antenna surface is wet74-78. Investigations on the type and amount of attenuation by several researchers78-80 led to the following results:

(i)

Wet surface of antenna can attenuate signals of the order of 10 dB at Ka-band. (ii) Deep and rapid fluctuations in antenna gain may occur due to the nature of rain type and other meteorological factors. (iii) Compared to wet reflectors, attenuation due to wet radome surface is higher. (iv) Whatever be the rain rate, the attenuation caused by a wet antenna is mainly due to quantity of water deposited on the antenna surface at that instant. Moreover, the antenna attenuation depends on several other factors, viz., type of antenna and its elevation angle, type of rain and drop size, wind speed and direction and nature of antenna surface. If all these factors and wetness of antenna are not considered, we cannot get actual attenuation. Signal degradation for a wet antenna surface is two-fold: (a) Fall in antenna gain gives rise to signal loss, and (b) Deep and rapid fluctuations in antenna gain at any instant due to water accumulated on the antenna surface at that time has an adverse effect on the signal strength. The design and cost of a link at Ka-band are severely affected for above two reasons. Since signal degradation depends upon the type of antenna used in a link81, the measured attenuation data are of very limited use. However, Kharadly and Ross82 estimated total path attenuation on a statistical basis considering all the factors on the Vancouver-ACTS path, and suitable models are developed for proper assessment of attenuation. 5.3 Microwave and infrared (IR) study of cyclones

Studies of visible and IR band from geostationary satellites provide us information of immense value to monitor cyclonic storms. There is a relation between the radiation at the cloud top and rain received by land and ocean; rain measurements from IR is done from this relation. Microwave radiation and brightness temperature have got a direct relation with precipitation; hence microwave radiation can be considered to obtain the amount of latent heat released in a tropical cyclone’s rain regions. Charles and Doswell83 obtained microwave brightness temperature and mapped rainfall regions in the Hurricane and tropical disturbances. Retrieval of rain rate and warm core structure during tropical cyclone


were done by Brueske and Velden84 using Advanced Microwave Sounding Unit (AMSU) radiometer. Some satellite-derived parameters are: precipitable water, liquid water, rainfall rate, sea surface temperature and wind speed which were considered by Elsberry and Velden85. They developed a linear regression relationship from the estimation of surface intensity and cloud cover associated with the tropical cyclones based upon hydrostatic assumptions. Gairola and Krishnamurti86 used Special Sensor Microwave/Imager (SSM/I), Nimbus and Sea-sat SMMR data to locate the position of deep depression, cyclones, etc. Microwave data were used successfully by Rao and MacArthur87, and Rodgers and Pierce88 to determine the intensity of a tropical cyclone and its change. Stress was also given on hybrid techniques by several workers who used geostationary satellite and polar passive microwave radiometer in order to cover a vast area to prepare a global map of tropical rainfall. The algorithm developed by Gairola and Krishnamurti86 is well tested for entire global tropics to predict medium range cyclones. Gairola et al.50 have used this algorithm to get precipitation regionally during a very heavy cyclonic storm over Bay of Bengal. Knowledge of rainfall at a place can help planning and design of water resources project. Although the algorithm was prepared initially for global scale, it is capable of predicting rain properly in the regional scale phenomena like cyclone. 6 Water vapour attenuation In the study of earth-space communication links, role of oxygen and water vapour need special treatment as they play a dominant role in such links. They attenuate signals to a large extent leading to unreliability of the links. The attenuation due to oxygen molecule remains almost constant over the entire world, whereas water vapour density changes to a large extent from place to place. As a result, attenuation due to water vapour will be of main interest89-92. 6.1 Measurement of attenuation

In clear air, both water vapour and oxygen attenuate signals to an extent depending upon its frequency; they also cause delay in propagation, bending of ray and produce noise in receivers. Most important gaseous element in the lower atmosphere is water vapour, which absorbs ultra short waves at 22.235, 183.3 and 325.152 GHz. The liquid water in

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the atmosphere absorbs strongly above radio waves due to heavy displacement current. As temperature is strongly connected with saturation water vapour pressure, so the attenuation of signals due to water vapour will vary as its concentration always change with place and time. Distribution of water vapour is obtained by using radiosonde data. The change in distribution of water vapour content over the Indian subcontinent is largely due to its geographical and seasonal pattern. Along the south-west coast the moisture content is maximum whilst the minimum value is obtained over Srinagar. The attenuation due to water vapour for small angle of elevation becomes high due to dominance of the tropospheric effects. At radio frequencies, scattering effect can be neglected in a gaseous atmosphere; hence attenuation measurement due to absorption is of major concern. Total gaseous absorption in the atmosphere for a path of length γg (in km) is: γ0

Aa (in dB) = ∫ γ a (r )dr

…(7)

0

where, γa(r) = γ0(r) + γω(r) γa = Specific attenuation (dB/km) γo, γω = Oxygen and water vapour attenuations. To facilitate computer calculations use of Van Vleck-Weisskoff line approximation was done. Hence, γω can be written as: 

γ ω = 0.067 + 

+

24 7.33 + 2 ( f − 22.3) + 6.6 ( f − 183.5) 2 + 5

 2 −4 4.4  ( f ρ10 ) 2 ( f − 323.8) + 10 

…(8)

here, f is the signal frequency and ρ the water vapour density (g/m3). This Eq. (8) is found to be valid up to 350 GHz. Upon integration of Eq. (8), one gets total attenuation over earth-space link through the atmosphere. The troposphere is largely responsible for attenuation in the slant path communication for elevation angle less than 10o. Introducing the concept of equivalent path by replacing actual length in the atmosphere we get the total path attenuation as:


336

A=

4γ ω 4 (sin 2 θ + + sin θ)1/2 R

…(9)

where, R = Effective radius of the earth. Sarkar et al.93 calculated attenuation for different frequencies between 5 and 350 GHz for different elevation angle (θ) by using water vapour concentration (ρ) and observations over the Indian subcontinent. 7 Comparison of rain attenuation prediction methods Terrestrial and slant communication systems, which operate above 7 GHz are affected by rain, snow and hail (also called hydrometeors). Above threshold value (7 GHz), it depends chiefly on the rain rate intensity and its drop size distribution. Among the above hydrometeors, rain is of primary concern whilst others occur very rare in tropical countries. Hence, rain attenuation, particularly in tropical countries, affects performance and faithfulness of communication systems. At WAR conference (World Administrative Radio conference), India was allotted two geosynchronous satellite positions at 56οE and 68οE longitudes at 12 GHz broadcasting frequency. This prompted Indian workers to study rain attenuation at 12 GHz. Prasad et al.94 have compared several prediction methods of rain attenuation. They observed attenuation by using radiometers operating at 11 and 13.4 GHz in order to find a suitable prediction technique. They used Dicke type radiometers, both consisting of horn-fed paraboloidal reflector type antenna one of whose diameter is 1.2 m with a gain of 17 dB (for 11 GHz) and the other was of diameter 1.5 m with a noise figure of 5 dB and brightness temperature 10-300 K (for 13.4 GHz). 7.1 Rain height

In order to calculate rain height and specific attenuation (which is needed to obtain effective path length), all prediction techniques of slant path rain attenuation need point rainfall statistics. The physical rain height up to which rain occurs (hFR) is taken to be equal to 0οC isotherm height. This fact is very important in slant path attenuation. Data from temperate zone countries (tropical) are mainly used to obtain rain height. Hence, prediction techniques overestimate attenuation in equatorial and tropical

zones. As a result, one is to consider a small effective rain height than the actual physical height in the new CCIR prediction method. Here we take into account the non-uniform vertical rain profile at low latitudes mainly in tropical and equatorial regions. By analyzing equiprobable measured attenuation and also the point rainfall intensity data, one can get effective rain height (hR). Rain height of 0οC isotherm is valid for widespread rain, but fails for warm convective type rain and thunderstorms. Prasad et al.94 deduced effective rain heights using 11 and 13.4 GHz zenith looking radiometers. Rain rates were measured simultaneously using raingauge over Delhi. In order to deduce effective rain height, vertical attenuation and corresponding rain rates were considered. The 11 GHz data were taken for 1988-89 period, whereas 13.4 GHz data correspond to the period 1984-86. Some deviations were observed in two sets of plots for height above 60 mm/h rain rate. Principal cause of such variation might be due to year-to-year variations in the microstructure of rain, as the measurements were made at different times. Also slight deviations in the two radiometers specifications can cause above variations. 7.2 Methods of prediction of rain attenuation

For any communication system, statistical estimation of rain attenuation is necessary. A prediction technique which can be applied widely should have the following criteria: (i) it is easy to apply and should agree well with experimental values for different rain zones, (ii) it should be very less dependent on methods of finding rainfall intensity and (iii) the technique must bear a physical meaning. Proper prediction method leads to right site location through the analysis of climatic variation of attenuation95,96. Some prediction methods of wide use are: (i) CCIR97, (ii) Garcia-Lopez’s method, (iii) Moupfouma’s method and (iv) Crane’s method. 7.2.1 Garcia-Lopez’s method

This is a simple method which has been developed by using a good data base. Here, values of the coefficients that occur during calculation of attenuation are supplied separately for tropical countries. The method gives attenuation formula as: A = kR a Ls

  Ls (bR + CLs + d )    a +  e   

…(10)


Values of the constants k and a depend on frequency, temperature and drop-size distribution.The CCIR Tables prepared on the basis of vertical and horizontal polarizations (between 1 and 400 GHz) provide values of the constants. Prasad et al.94 considered only vertical polarization. Here, R = rain rate in mm/h and Ls = Equivalent path length (in km), where, Ls = (hFR − hS)/sinθ

…(11)

Here, hS = ground elevation in km; θ = elevation angle; hFR = 0οC isotherm height and a, b, c, d, e are constants and their values are given by Garcia-Lopez et al.98. Two sets of values are given–one is for worldwide use, while the other is for Australia. Second set of values can be used in tropical countries like India as Australia gives tropical climatic variations. For tropical climates: a = 0.72, b = 76, c =–4.75, d = 2408 and e = 10,000.

A0.01 = γLs r0.01 (in dB)

…(16)

whereas for other percentage of time the predicted attenuation value is derived from: Ap /A0.01 = 0.12p–(0.546 + 0.043 log p)

…(17)

where p is the percentage. It should be noted that in recent ITU-R methods, the rain heights are given by a worldwide map rather than a function of a latitude (φ). This is an important modification done in expressing the rain height in rain attenuation prediction models. 7.2.3 Moupfouma’s method

The method has been developed99 by using mainly tropical data. The method needs elevation angle of the slant path, site height from sea level, etc. Here, the rain rate for 0.01% of time and also for other percentage of time is needed to introduce the concept of correction factor. Predicted attenuation is given by

7.2.2 CCIR method

kR a u ( p)( R0.01 / R pu )0.38/ Ls − 0.25

In CCIR method (now known as ITU-R), the rain height hFR is given by

A(dB) =

hFR = 3 + 0.028φ, 0○ < φ < 36○

where, m = 1 + 1.4 × 10-4f 1.76loge(Ls)

○

= 4 – 0.075 (φ – 36), φ > 36

…(12)

where φ = latitude angle. Equation (11) provides slant path length. Horizontal projection (LG) of slant path length is given by LG = LS cos θ

…(12a)

The reduction factor r0.01 for 0.01% of the time and for rain rate R0.01 < 100 mm/h is given by: r0.01 =

1 1 + ( LG / LO )

…(13)

where, L0 = 35 exp (– 0.015 R0.01)

…(14)

When, R0.01 > 100 mm/h, the accepted value of R0.01 in Eq. (13) is 100 mm/h. Considering the frequency dependent coefficients the value of specific attenuation (γ) is obtained from: a

γ = kR

…(15)

The predicted value of attenuation which exceeded 0.01% of time on an annual average is given by

337

1 + η( p / 0.01) −0.36 Ls m

× Ls

…(18)

f = Frequency in GHz Ls = Slant path length (km) η

= 0, if l < 5 km and f < 25 GHz and η = 0.03 for all other cases. k,a = Those constants mentioned in Garcia-Lopez’s method95. p = Percentage of time R0.01= Rain rate for 0.01% of time Rp = Rain rate for p % of time Moupfouma99 has provided the values of a for different locations throughout the world. Slant path formula is given by, Ls =

HR − H0

, for θ ≥ 50

1/ 2

 2  HR − H0  sin θ + 2  8500     

+ sin θ

…(19) Here, θ is the angle of elevation; H0 is the location height from mean sea level; HR is 0οC isotherm height.

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It should be pointed out that Crane100 has proposed a method using revised two-component model. However, the method is valid for slant paths only. 7.3 Prediction of microwave rain attenuation in the tropics

Attenuation of microwave signal by rain is a major problem to designers and service providers all over the world. Since 1940, researchers in Europe and USA are conducting work on microwave attenuation due to moisture, rain, clouds and other factors. It is now well known that different models on rain attenuation and drop size distribution are valid for different regions, i.e., a model which is valid in one region can give much different result in another region. So people gave serious thought on developing attenuation and rain drop size distribution models for different climatic regions. In Singapore, work on microwave attenuation started in 1988 with a 21 GHz link and a link path of 7 km. With the success achieved in the result, the link path was reduced to 1.1 km, which is within the average rain cell diameter (~ 2 km). Yeo et al.101,102 and Li et al.103 worked on rain attenuation and rain drop size models which were successfully employed in the climate of Singapore. Following earlier success, Yeo et al.104 extended their work on attenuation for horizontally and vertically polarized microwaves in 10-40 GHz band. Their model of attenuation and drop size distribution is well valid for use in tropical climates. In India, work on the rain drop size distribution data started during 1989, and 1100 rain events of 1 min duration’s DSD data were used to develop rain drop size distribution model and compared with MP (Moupfouma) and LP (Lopez) DSD model of attenuation coefficients. Specific attenuation shows a significant difference in attenuation in 60 and 100 GHz due to presence of different types of rain drop sizes above a certain higher rain rate105. 7.4 Fading of radio signals

Besides water vapour, cloud and rainfall, various other atmospheric constituents are responsible for fading of radio signals in millimetre waveband. Amount of these fading components in the atmosphere always vary with time. Basic problem in radio propagation is the calibration of background signal fluctuations from surrounding atmospheric gases. A renewed interest has grown to find out causes of such fluctuations and to develop separate models for each cause, which, in turn, might be

frequency dependent. A signal will fade when rain and cloud as well as water vapour and oxygen are present. Davies and Watson15 have devised a method, which shows that phase delay from clouds and rain drops are negligible. Tranquilla and Alrizzo106 have shown that clouds accompanied by heavy rain introduce measurable phase delay. Davies et al.107 have shown that, in a non-rain event, cloud effect can be separated from water vapour effect which makes one possible to develop a theoretical model for cloud attenuation. Attenuation due to water vapour and oxygen (gases) is eliminated from total attenuation measured from radiometer, which ultimately gives us attenuation due to rain and cloud only. Physical models for prediction of radiowave propagation should combine all rain and non-rain time events (cloud, rain and scintillation effects); the technique of Davies and Watson15 has become acceptable from this standpoint. 8 Discussion and conclusions Sometimes, measurement and model development run simultaneously. Any departure from model prediction lead to a new thought about the process and consequent refinement of the experiment. When several propagation mechanisms are present in the iterative model, each one need to be iterated separately. After a model matures, the development process must continue to explore the estimation of variability and maintenance of data quality. Prasad et al.94 observed radiometric attenuation at 10○ and 90○ angles of elevation and compared their results with the attenuation predicted by other methods. They deduced rain heights using the empirical relation of CCIR97 which is latitude dependent. They tested four cdfs and of these only one showed good agreement with the CCIR technique. In case of 10○ elevation angle, CCIR method gave more deviation. GarciaLopez’s method was in well agreement for all the four cdfs followed by Moupfouma’s technique. Moreover, Garcia-Lopez’s method showed minimum r.m.s. variation (0.3834). Although the cdfs’ tested were few in number, but still the study reveals that GarciaLopez’s method is very much useful in predicting rain attenuation over the northern India. Calculation of attenuation by this method follows the pattern of rain rate. This method is very much useful for calculation of attenuation with a great accuracy provided the cdfs of rain rate are available. The plus points of this method are that, it is simple, easily computable and


with large number of data the coefficients can be suitably changed according to regional needs. For calculation of attenuation at other percentage of time (0.01% and onwards), the scaling equation of CCIR97 model is re-adjusted depending upon the attenuation values, which are observed experimentally. From the above study, one gets a choice for picking up rain attenuation prediction technique over northern part of India and can carry out analysis with whatever database is available. Smith67 has presented a unified set of absorption and brightness curves from measurements and other models. Agreement was within 15% for absorption co-efficient (7.5 g/m3 water vapour at 290 K) and for total zenithal attenuation (for 7.5 g/m3 surface water vapour and for standard U.S atmosphere of 1976). Above unified set can be safely used for obtaining a consistent set of attenuation and emission curves. Water vapour attenuation of short radiowaves is found to decrease with the increase in angle of elevation. It is revealed from the studies that satellite communications with limited availability of transmitted power can be very much unreliable when water vapour concentration becomes high. Performance of a dual polarized satellite system degrades above 10 GHz due to rain attenuation and ice depolarization. Although rain attenuation is common to both single and dual polarized systems by reducing the received signal power, but icedepolarization is unique to the latter. Depolarization is defined as the change in the state of polarization of transmitted signal108,109. Performance prediction of a DPSC system at and above Ku-band including digital systems has been done quite successfully by Vasseur110. Link availability degradation by a dual polarized system compared to single polarized system has also been calculated. In a digital system, prediction of link availability is connected with transmission quality. Link availability prediction method has been applied to Ku-and Ka-band satellite links and performances of single and dual polarized systems are compared. In the tropical regions, however, link availability of both single and dual polarized systems become remarkably low. Needless to mention here that if a dual polarized system is hampered by depolarization then one can consider depolarization compensation method given by Stutzman108. Demand for satellite communication has grown remarkably in recent times thereby increasing the

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need for higher and higher operating frequencies. Use of digital systems at higher frequencies makes our technology more sophisticated. At frequencies above 5 GHz, particularly in the micro-and millimeterwave range, performance degradation in the earth-satellite link occurs due to attenuation of signal by atmospheric hydrometeors111,112 (viz., rain, hail, fog, snow, ice crystal, etc). Lin and Chen113 have considered attenuation (which include both absorption and scattering) due to rain as primary cause; attenuation due to other hydrometeors are considered as secondary which affect RF propagation. Many rain attenuation models have been developed for last few decades. Shape of raindrop is considered either as sphere or an oblate spheroid to obtain rain attenuation. In this respect experimental results were provided by Medhurst23, Yeo et al.102 and Crane and Sheih114. Out of all available models, Pruppacher and Pitter (P-P) model115 is widely accepted which has been further simplified by Li et al.116. Lin and Chen113 used this modified P-P model to investigate extinction crosssections of raindrops with mean radii between 0.25 and 3.5 mm and in the frequency band 0.6 GHz-100 GHz. In India 20/30 GHz radiometric measurements were done by Jassal et al117. Results of extinction cross-sections for horizontal and vertical polarizations can very well the predict rain attenuation in wireless communications113. In the optical region, water vapour, aerosols, ozone and, in particular, clouds impede monitoring of crops during monsoon. This necessitates the use of microwave remote sensing or temporal composting of optical data117,118. Passive microwave radiometer can globally assess surface temperature, its wetness and snow cover119. Moisture content of soil and evapo-transpiration fluxes were estimated by Lakshmi et al.120 by using special sensor microwave imager (SSMI) data. So far combined use of optical and microwave data for crop study has been less. Singh and Dadhwal121 used SSMI radiometer data for regional assessment of crop growth and developed temporal profiles of MPDI for different class of land cover which are very useful for crop growth monitoring. 9 Scopes for future investigations Capability of the wireless communication systems has reached its zenith today. It has brought the whole world at the fingertips of the mankind, yet the communication systems have many shortcomings to overcome. There are lot of impediments which

340


radiowaves suffer in its journey and thereby hampering reliable communication. Researchers have solved several problems to obtain a faithful link but much more work need to be done. The lines for further investigation are indicated below: (i) It has been observed that fog attenuation dominates in the infrared and optical bands, whereas rain attenuation plays havoc at millimetre wavebands; hence a combined dual waveband link or use of a compromise propagation frequency can be thought of. Absorption bands at 60 GHz and 183 GHz have many civil and defence applications for a short path-length of a few kilometres and even less since absorption at these bands beyond a few kilometres is very high. Amount of attenuation may vary from place to place; in cities or industrial belts pollution may play a big role; in forests vegetation may have a different role, whereas in villages or deserts attenuation will be different. A similar study may, therefore, be carried out for obtaining a proper path-length. (ii) For propagation along vertical and horizontal paths at 22.235 GHz and 94 GHz, absorption values are often found to be not correlated. It is suggested to look for correlation of attenuation, if any, for 22.235 GHz along horizontal and 94 GHz along vertical paths, since attenuation will be less at 22.235 GHz. At lower rain rates, attenuation becomes large and increases with frequency above a critical value. Due to growing need of millimetrewave frequencies, it is proposed to look afresh at the low rain rate attenuation statistics at higher frequencies. (iii) Post rain attenuation modelling has been done at 22.235 GHz (λ=1.35 cm); the same modelling can be considered at other window frequencies in the millimetre waveband. Again the amount of residual post-rain attenuation was found to be higher at lower rain rates, which indicates that such rain rates are associated with smaller raindrops and low velocity of fall. A study of post-rain attenuation with rain rate or a relation between rain drop size and velocity of fall may be carried out. (iv) Scattering of radio waves is another major cause of concern to the communication people.

Electromagnetic waves are scattered mainly by raindrops, water vapour and oxygen molecules. Scattered waves cause interference which degrades signal. Hence a thorough study of radioscatter in the microwave and millimeter wavebands will be vital for planning and designing of radiosystems. A major cause of attenuation is the effect of wet antenna during rain. Studies of antenna attenuation and development of antenna design which does not attenuate signal during rain can be undertaken. (v)

Multipath fading is an important cause of signal deterioration in the clear sky condition and during rain events in the microwave and millimetrewave ranges. Further researches can be carried out on multipath fading. With the ever increasing demand for radio wave communications, more frequencies above 10 GHz need to be considered for terrestrial and satellite communications. But signal attenuation beyond 10 GHz due to rain, water vapour and other atmospheric constituents is of major concern. Accurate prediction of attenuation is a great challenge to the communication people. Usually, rain data are collected for intervals of 1 h or more; but correct prediction of attenuation generally requires 1 min or less interval of rain rate data. Statistical conversion of 1h rain rate data into 1 min or less rain rate data for proper prediction of attenuation over tropical and nontropical zones may be explored globally.

(vi) Rain drop size distribution models and attenuation models are established to be highly regionalized. Studies can be undertaken on such models at different propagation frequencies and a global map for prediction of attenuation can be thought of. Microwave and millimetrewave remote sensing of various land, soil and environment at various frequencies are now in great demand because such a study can provide a lot of inputs to the agriculture and meteorology in association with modern satellites. Hence, a vivid investigation of such remote sensing technique needs to be probed further. Effects of sea breeze and breeze in hilly terrains on propagation factors over a LOS link in India and elsewhere can be investigated at different frequencies.


(vii) Remote sensing of microwave and milimetrewave radiometry is of paramount importance to monitor soil and vegetation status on a regional and global scale. It can also help us to study the moisture content of soil and its fertility to investigate vegetation biomass for estimation of crop yield and its type and many such agricultural needs may be explored in a great detail. Radio refractivity largely controls the quality and reliability of radars and communication systems. Hence, a thorough study of radio refractivity in the coastal regions, hilly places, forests and deserts can be undertaken at various frequencies in different seasons. Such a study can be of immense help for prediction of radar and other communication system performances. There are many questions still unanswered, hopefully, awaits for the improvement of the existing instruments and techniques and development of the new ones. Acknowledgements The authors are extremely grateful to the Reviewers of this paper for the valuable comments and suggestions. We are also thankful to the research workers whose work published in different journals/proceedings, etc. have been used in this paper. References 1

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