Addendum to Study on Coastal Atmospheric Effects on Line-of-Sight Links Located in Visakhapatnam

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1 Addendum to Study on Coastal Atmospheric Effects on Line-of-Sight Links Located in Visakhapatnam G.S.V. Radha Krishna Rao and M. Purnachandra Rao * Faculty of Information Technology, Multimedia University Cyberjaya, Selangor, Malaysia Abstract Based on the average values of the pressure, temperature and relative humidity near the ground, corresponding values of radio-refractivity have been computed for Visakhapatnam. Mean monthly values and seasonal mean values of surface refractivity at Visakhapatnam are presented. Keywords: Radio refractivity, microwaves, LOS links, fading, RRI, radiosonde. Introduction Line-of-sight (LOS) microwave links are subject to severe fading, owing to refraction of the transmitted beam along the propagation path. Refractive fading can significantly impair service on terrestrial LOS microwave transmission systems. Refractivity gradient is responsible for the bending of radio ray and this controls the height, angle of scattering, reflection etc. Radio refractive index (RRI) variation in the troposphere has been subject of sustained interest because of its important role in microwave radio communication. The quality of signal reception and probability of the link failure in microwave communication systems are largely controlled by RRI gradient. It is a function of tropospheric parameters such as temperature, pressure and humidity. Radioclimatological studies have been made for different regions of the world (Sarkar 1978; Judd 1985). The presence of the marine boundary layer in coastal regions is known to significantly affect microwave propagation (Gerstoft, et al. 2000). The relatively high specific humidity of the underlying marine layer creates elevated trapping layers in the radio refractivity structure. In the coastal regions, the radio refractivity changes are closely associated with * Department of Physics, Andhra University, Visakhapatnam, Andhra Pradesh, India. the sea and land breezes, and super-refraction and ducting over the sea along the coast is most-marked when dry warm air from the adjacent land mass extends over a comparatively cooler sea. For microwave frequencies, the radio refractive index of air is a function of pressure, temperature and relative humidity. In the atmospheric radio refractive index, all the three parameters can be combined into one single parameter that facilitates the analysis and study of the effects of meteorological factors on radio wave propagation of the higher frequencies like microwaves. The computation of the mean values of radio refractive index is useful for the preparation of the radio-climate charts depicting the normal distribution of radio refractive index near the surface (Kulshreshta and Chatterjee 1966). The characterization of the seasonal variation in fading and its dependence on meteorological parameters provides the way to improve transmission performance by better tailoring of performance by better tailoring of transmission equipment design and usage to the amount of fading expected at a given location and time of year. Realizing the importance of such a study in the coastal atmospheric boundary layer, an attempt has been made in this direction to deduce the radioclimatology of the Port City Visakhapatnam (Lat N; Long E). 276

2 Radio Refractivity and Methods of Its Measurement Radio waves travel through vacuum with a speed equal to the speed of light. In any other medium, the speed of the radio waves will be nearly c/n where c is the speed of light in vacuum and n is the radio refractive index of the medium. The value of n for dry air is almost the same for radio waves and the light waves. But the n of water vapor, which is always present in some quantity in the lower troposphere is different for the light waves and radio waves. This arises from the fact that water vapor molecule has a permanent dipole moment which has different responses to the electric forces of different frequencies and at microwave frequencies water vapor molecules are subjected to electronic polarization. Hence, for these frequencies, the dielectric constant and thus the refractive index of water vapor is greater than that of dry air. The radio refractive index n for moist air near the surface has the value of the order of and the variation in n is only of the order of few parts in Hence the customary practice is to define a modified radio refractive index, N, called radio refractivity, as N = (n - 1) (1) This enables the easy manipulation of N which is of the order of 300 rather than n which is an inconvenient number. A theory based on the molecular polarisation of the gases of the atmosphere enables the refractivity to be related to the temperature T in K, to atmospheric pressure p in millibars and to water vapor pressure e in millibars. The refractivity N is expressed by the well known relation: N = 77.6 p + 4,810 T e T. (2) At the surface of the earth, P, e and T have values of the order of 1,000 mb, 10 mb and 300 K and the value of N is around 300. N can also be expressed as: 77.6 e N = P + 4,810 T s RH T, (3) where e s is the saturation vapor pressure in millibars and RH is the relative humidity in percent. The conditions of the atmosphere for electromagnetic wave propagation purposes can be assessed by examining the vertical profile of refractivity. The basic values of temperature, pressure and relative humidity for calculating the profile can be derived from radiosonde measurements. The ducts can be located by examining refractivity profiles derived from atmospheric measurements. A previous radio-meteorological experiment conducted along the coast of southern California showed a high correlation between microwave signals and the base of the elevated temperature inversion (Rogers 1994). A number of mesoscale processes characteristic of the coastal environment are known to modify atmospheric refractivity (Thompson, et al. 1994). Another study presented an analytical approach to the modeling of refractivity profiles, together with ray-tracing considerations, with a view to provide relevant information regarding path parameters in a multipath situation (Webster 1982). The expression for the refractivity N may be regarded to consist of a dry term (which does not depend on humidity) p/t, proportional to the density of the air, plus a wet term (77.6 4,810 e/t 2 ), which is responsible for most of the temporal and spatial variability of N in the troposphere. As the temperature increases, the dry term decreases for a given atmospheric pressure; but the wet term increases for a given relative humidity. This is because of the relation between saturation vapor pressure and air temperature. The saturation vapor pressure over water in the range -20 C to +50 C can be derived as (Kulshreshta and Chatterjee 1966) ln e s T 273 = T T 5.31 ln. (4) 273 Direct measurement of n for microwave frequencies, makes use of resonance of tuned circuits. The resonant frequency of a microwave cavity is a function of its 277

3 dimensions and the index of refraction of its contents. If a cavity is open to the atmosphere, the resonant frequency changes with the n of the air inside the cavity as Δ f Δ =, (5) f nn where f is the resonant frequency of the cavity when filled with air of refractive index n, Δf is the consequent change in f as a result of the change of n by Δn. Since n for air is of the order of unity, Δ f = Δn, (6) f the value of n at surface or near the surface is measured indirectly by measuring temperature, pressure and humidity which are standard observations recorded by the meteorological services. Even though the refractometers may be capable of superior accuracy but the high cost and requirements of competent personnel for running a network of such observing stations seem to outweigh this advantage. Also, for long-term studies, the use of refractometers may not be essential. Hence generally for all average conditions, the data on radio refractive index computed from meteorological parameters is sufficient. Because the present study deals with the radio refractive index near the surface of the earth, this can be denoted by N s, refractivity near the surface of the earth. An attempt has been made to evaluate the monthly, annual maximum, annual minimum and annual ranges of N s in Visakhapatnam, based on the pressure, temperature and relative humidity data near the ground (surface data), obtained from India Meteorological Department (IMD) Center at Visakhapatnam. Towards the analysis of the available data, a typical year is divided in different seasons. As per the established traditions of IMD, the period of a year in India is divided into four seasons. They are June to September (monsoon), October to November (postmonsoon), December to February (winter) and March to May (pre-monsoon or summer). The average amount of multipath fading, and the microwave radio transmission outage it causes, varies from month to month and from year to year. Seasonal mean values of N s have been worked out by averaging the values of N s for the months representing each of the four seasons. The values of N s were recalculated by following a specific (modified) season pattern as applicable to Visakhapatnam and presented in Table 1. Maximum and minimum values, annual range and annual mean values of N s for the Visakhapatnam have also been worked out and are given in the Table 2. The monthly variation of N s at the Visakhapatnam is graphically shown in Fig. 1. Table 2 and Fig. 1 have been published previously (Radha Krishna Rao and Purnachandra Rao 2005). Table 1. Recalculated seasonal mean values of refractivity at Visakhapatnam Season Winter Summer Monsoon Post Monsoon Mean N (N units) Radha Krishna Rao and Purnachandra Rao (2005) Recalculated Values Table 2. Mean monthly values of refractivity at Visakhapatnam (Radha Krishna Rao and Purnachandra Rao 2005) Results and Discussion 278 Name Annual maximum Annual minimum Annual range Annual mean Mean N (N units)

4 Fig. 1. Mean Monthly Variations of Surface Refractivity (N s ) at Visakhapatnam (Radha Krishna Rao and Purnachandra Rao 2005) The monthly distribution patterns of N s over Visakhapatnam reveals that the seasonal mean N s increases from winter to summer and then to monsoon when it reaches a peak. There is a decrease in the seasonal mean N s after the monsoon season, which reaches a minimum in the winter season. The surface refractivity is around 400 N units in the summer months over the coastal stations while it is of the order of 350 N units over the inland stations (Narayana Rao, et al. 1985). Another study reported high refractivity in premonsoon months and low in winter months (Rama Rao, et al. 1999). Two important and variable atmospheric features in the coastal region are the surfacelayer evaporation duct and the elevated refractive layers at the top of the marine boundary layer. Both are caused by vertical gradients of temperature (increase) and humidity (decrease). When the surface temperature falls rapidly than the air temperature, temperature inversions may occur near the ground at heights of up to 300 m. Such temperature inversions may give rise to suitable conditions for ducting. Ducting conditions can be caused by an advection process in which a body of warm dry air blows from the land over a cooler sea. Evaporation then occurs increasing the humidity near the surface and decreasing the temperature. Under anticyclonic conditions the air mass at the center of the anticyclone often sinks or subsides causing an outflow of air over the surface. The air mass in sinking has its temperature raised by adiabatic compression. Near the surface, especially over the sea, the humidity rises. The evaporation duct is surfacebased and is persistent over ocean areas because of the rapid decrease of moisture immediately above the surface. Evaporation ducts are small but have a substantial effect on the propagation of radio waves. Although temperature inversions are often referred to as the characterizing feature of elevated ducts, it is the humidity gradient that dominates the radio refractivity. Ducts occur when a stable atmospheric condition results in a temperature inversion and a sufficient amount of water vapor is trapped below that inversion. This condition causes a rapid decrease in the refractive index with increasing height leading to waveguide-like trapping. Low-angle and low-altitude microwave EM transmissions will generally encounter an atmospheric refractive index that will give rise to anomalous propagation which results from the presence of a ducting refractive index profile caused by evaporation. The formation and effect of ducts on microwave signals will be important for maritime applications such as communication links in the marine environment. The acoustic sounder data are analysed (Radha Krishna Rao, et al. 2003; Radha Krishna Rao & Purnachandra Rao 2004a) to estimate the monthly mean percentage time of occurrence of sea breezes, thermal plumes, and nocturnal inversions. Increase in sea breeze activity from 11.5% in January to 54.5% in May is observed which is due to increasing solar radiation, increasing surface temperatures over land. A sudden fall in the percentage occurrence of sea breezes from May to June is observed which is due to the onset of the southwest monsoon and its associated dominant offshore flow. As the monsoonal circulation intensifies, the sea breeze fronts become weaker although there is a tendency for marine air to arrive late in the afternoon when the winds are light and variable. The sea breeze maintains moderate activity in the first two months of the southwest monsoon (June and July). In August and September the monsoon is dominant with increased cloud and rain amount while subsidence persists over the sea, resulting in a further decrease in the frequency of occurrence of sea breezes. The decrease in surface temperatures over land and the increase in sea surface temperature due to subsidence off the coast reduce sea breeze 279

5 activity in the post monsoon period. The variation of percentage occurrence of thermal plumes and nocturnal radiative inversions is mostly out of phase to that of the sea breeze. The lesser frequent the sea breeze occurrence, the greater will be the persistence of convective plumes and the development of radiative inversions. The inflow of stable air from the sea quenches the convective activity and introduces a ground-based or elevated inversion. Further, the presence of cool air aloft during the prolonged period of sea breeze activity until well into the night retards the formation of nocutural inversions. All these factors (Radha Krishna Rao & Purnachandra Rao 2004b; Radha Krishna Rao & Purnachandra Rao 2004c) support that the sea breeze circulations over Visakhapatnam region have significant effect on surface radio refractive values. Conclusions The current short study on radio refractivity over Visakhapatnam is based on the radiosonde data obtained from IMD Visakhapatnam. Values of surface radiorefractivity have been computed for Visakhapatnam. Mean monthly values and seasonal mean values of surface refractivity at Visakhapatnam are presented. The monthly distribution patterns of N s over Visakhapatnam reveals that the seasonal mean N s increases from winter to summer and then to monsoon when it reaches a peak. There is a decrease in the seasonal mean N s after the monsoon season, which reaches a minimum in the winter season. Acknowledgements The authors extend sincere appreciation to Dr. J.V.M. Naidu, Director, Cyclone Warning Center, India Meteorological Department, Visakhapatnam, for providing the meterological data. Also the authors express sincere gratitude to Dr. Murthy, Meteorologist, Cyclone Warning Centre, Visakhapatnam, for many illuminating discussions at various stages of the work. References Gerstoft, P.; Gingras, D.F.; Rogers, L.T.; and Hodgkiss, W.S Estimation of radio refractivity structure using matched-field array processing. IEEE Trans. Anten. Prop. 48: Judd, F.C Radio wave propagation I. Pract. Wireless 61: Kulshreshta, S.M.; and Chatterjee, K Radio-climatology of India: Radio refractive index near the ground surface. India Meteorol. Dept., pp Narayana Rao, D.; Kesava Murthy, M.J.; Sarkar, S.K.; Dutta, H.N.; and Reddy, B.M Radioclimatology over southern India. Indian J. Radio and Space Phys.14: Radha Krishna Rao, G.S.V.; Purnachandra Rao, M.; and Murthy Radioclimatology over Visakhapatnam. Proc. 3 rd Int. Symp. Communic. Info. Tech. [ISCIT], Thailand, 3-5 Sept. 2003, pp Radha Krishna Rao, G.S.V.; and Purnachandra Rao, M. 2004a. On-line recording of the microwave signal data: A short study. IEEE Int. Conf. Software in Telecom. Computer Networks [SoftCOM], pp Radha Krishna Rao, G.S.V.; Purnachandra Rao, M. 2004b: Coastal Atmospheric effects on line-of-sight links located in Visakhapatnam A Study, IEEE International Symposium on Intelligent Signal Processing and Communications Systems [ISPACS], Seoul, pp Radha Krishna Rao, G.S.V; and Purnachandra Rao, M. 2004c. A study on the influence of sea breeze on line-of-sight links located in Visakhapatnam. IEEE Int. Multitopic Conf. [INMIC], Lahore, Pakistan, pp Radha Krishna Rao, G.S.V.; and Purnachandra Rao, M A study on coastal atmospheric effects on line-of-sight links located in Visakhapatnam. Assumption University Journal of Technology 9(1): 21-26, July. Rama Rao, T.; Vijaya Bhaskara Rao, S.; Prasad, M.V.S.N.; Sarkar, S.K.; and Lakshmi, D.R Effect of sea breeze on propagation charactristics over a LOS 280

6 microwave link located in Indian south-east coast. Indian J. Radio and Space Phys. 28: Rogers, L.T Statistical assessment of the variability of atmospheric propagation effects in the southern California coastal area. IGARSS 94 Int. Geosci. Remote Sensing Symp. Surface and Atm. Remote Sensing: Technologies, Data Analysis and Interpretation (Cat. No. 94CH3378-7), Pasadena, CA, 8-12 Aug. 1994, 1: Sarkar, S.K Radioclimatological effects on tropospheric radiowave propagation over the Indian subcontinent. Ph.D thesis, Delhi University, Delhi. India. Thompson, W.T.; Burk, S.D.; Cook, J. and Love G.G Variations in coastal atmospheric refractivity induced by mesoscale processes. IGARSS 94 (Int. Geosci. Remote Sensing Symp. Surface and Atm. Remote Sensing: Technologies, Data Analysis and Interpretation (Cat. No. 94CH3378-7), Pasadena, CA, USA, 1: Webster, A.R Raypath parameters in tropospheric multipath propagation. IEEE Trans. Antennas and Propag., AP-30: