IJCRR - vol 05 issue 05, March, 2013
Date of Publication: 22-Mar-2013
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WATER VAPOR ATTENUATION STUDIES FOR KA AND V BAND FREQUENCIES OVER A TROPICAL REGION
Author: G.Venkata Chalapathi, S. Eswaraiah, S. Vijaya Bhaskara Rao, N. Prabhakara Rao
Category: General Sciences
Abstract:The recent communication satellite systems tend to employ higher frequency (18-60 GHz) bands to satisfy the growing capacity requirements. Such wide bandwidths are valuable in supporting applications such as high speed data transmission and video distribution. The attenuation of Satellite signals due to water vapor absorption is very essential for high frequency (>10GHz) satellite communication. Using the formulae referred in Recommendations ITU-R, P.676-5, predictions of specific attenuation and path attenuation due to water vapor absorption are calculated and presented and also compared with the water vapor attenuation estimated from Radiosonde data collected from the India Meteorological Department (IMD), for the first time over tropics. The important findings of the current study includes the observation of slant path attenuation value, which is high for frequencies like 20 GHz, 24 GHz, 26 GHz, 47 GHz and () low for frequencies 18 GHz, 16 GHz, 14 GHz, and 12 GHz etc. It is also observed that for low elevation angles the slant path attenuation is maximum and its value decreases with the increase of elevation angle. For the 1200 GMT similar results are obtained indicating that there is significant diurnal change in the observed attenuation.
Keywords: Satellite communications, Water Vapor, Attenuation, Communication Satellites, High Frequency bands.
In the last decade, new and challenging satellite applications evolved, leading to spectral blocking of the conventional frequency bands allocated for satellite services, they are L, S and C bands operated 1/2 GHz, 2/4 GHz, and 4/6 GHz respectively. At present the recent developments in the space technology, intended for production of VSAT/USAT (Very/Ultra Small Aperture Terminals) systems primarily for data applications, the direct-to-home (DTH) services by Direct Broadcast Satellite (DBS) systems and also to non- geo-stationary (NGSO) orbit constellations. These are all belonging to the Fixed Satellite Service (FSS), and gradually tend to employ higher frequency bands to satisfy the increasing capacity requirements. Hence the recent satellite systems are operating at Ku (12/14 GHz), Ka (20/30 GHz) and V (40/50 GHz) band frequencies. The wave spectrum at 12-60 GHz is of increasing interest to both service providers and systems designers due to because of the wide bandwidths available for carrying communications at this frequency range. Such wide bandwidths are highly valuable in high speed data transmission and video distribution. In the absence of rain, cloud and atmospheric gases can play an important role in signal attenuation; hence their effects must be assessed in order to determine their impact on satellite communications (César Amaya, 2002). But, higher frequencies (Beyond 10 GHz) giving rise to signal fading due to physical phenomena related to the propagation of radio waves through the atmosphere (Crane, 2003) and hence the higher frequencies at Ka and V band are more vulnerable to propagation effects such as oxygen and water vapor absorption for both fixed and mobile satellite communications. Rain is typically the main cause of attenuation on most propagation paths at frequencies above about 10 GHz and it increases significantly as frequency increases from Ka to the EHF band (30-300 GHz). However, attenuation by clouds and atmospheric gases (oxygen and water vapor) also assumes generally greater significance with increasing frequency, and their persistent effects influence satellite communication systems. Throughout the globe the slant-path attenuation due to atmosphere gases and water vapor has been investigated (Althusher et al, (1978); Asoka Dissanayake et al, (2001); César Amaya (2002); Carlos Catalán et al, (2002); Kifah Al-Ansari and Awadallah Salama (2005); Mandeep and Hassan (2008)) by using different instruments and methods. Studies on attenuation over tropical region are sparse. Uppalet al. (1979) reported water vapor attenuation with passive radiometer operating on 22.235 GHz , Karmakar et al. (2001) using dual frequency microwave radiometers operating at 22.235 and 31.4 GHz and Sarkar et al.(1998;2005) for a frequency range of 1-350GHz at elevation angles 3 and 10 degrees. The studies reported are very limited especially in tropics. There is a need to take up large number of measurements to validate the existing ITU-R models. In the present paper an attempt is made to develop a systematic procedure for the estimation of water vapor attenuation and estimate probability of occurrence of attenuation for different elevation angles and for different frequencies over a tropical region. The procedure involved is very accurate line by line estimation of water vapor attenuation using the radiosonde data. Satellite observations were used for validation.The procedure of estimating water vapor attenuation, the results and discussions are presented in following sub sections.
DATA BASE AND METHODOLOGY
Absorption is a function of temperature, pressure and humidity. Therefore, it is necessary that these meteorological parameters be determined along the propagation path in order to calculate the gaseous attenuation for any geographical location. Radiosonde data, over Chennai, were obtained twice daily (at 0500 and 1700 h IST) from the IMD for 5 years (2000-2004). From these available data we computed the attenuation of Satellite signals due to water vapor absorption, using the formulas referred in Recommendations ITU-R, P.676-5(2001). The detailed procedure is given in Appendix-A.
RESULTS AND DISCUSSIONS
Variability in slant path attenuation using radiosonde data For the present work we analyzed the radiosonde data collected from the India Meteorological Department (IMD), over a period of 5 years (2000-2004). We individually analyzed both the data sets of morning radiosonde ascent starting at 00:00GMT and the evening radiosonde ascent at 12:00 GMT for each day. The maximum altitude considered in this study is 5 km as the water vapor data may not be available after 5 km as the zero degree isotherm is about 5 km over India. The slant path attenuation of water vapor attenuation statistics was calculated for frequencies between 12 GHz to 47 GHz at different elevation angles starting from 50 to 750 . We have analyzed and plotted Cumulative Distributive Frequency (CDF) plots for both morning and evening radiosonde data and found that no significant difference between the morning and the evening water vapor absorption. Hence we combined both the data and presented CDF graphs for various elevation angles and are shown in figures 1 to 8. The above observations clearly revealed that, slant path attenuation of water vapor is high for the frequencies 24 GHz, 47 GHz, 20 GHz, and 28 GHz etc. The corresponding value is less for the lower range frequencies like 18 GHz, 16 GHz, 14 GHz, and 12 GHz etc. It is also clear that for low elevation angles like 5?,15? etc, the slant path attenuation is maximum and about 10 dB. Its value decreases with the increase of elevation angle. For higher elevation angles like 65?,75? etc, the slant path attenuation is about 0.1dB. Comparison of slant path attenuation obtained from radiosonde data and ITU-R reference values: The slant path attenuation values of water vapor computed using radiosonde data for different frequency values and for different elevation angles are compared with ITU– R standard reference values. The maximum value, minimum value and also the difference between the maximum and minimum values is also tabulated. They are shown in Tables 1 to 4. In addition to above comparison we also made comparison for higher frequencies like 28 GHz, 30 GHz and 47 GHz and clearly noticed that; (i) The slant path attenuation values which are calculated by the radiosonde data are in good agreement with the standard ITU-R values, (ii) The slant path attenuation values are decreasing with increase of elevation angles and (iii) With the increase of frequency from 16 GHz to 47 GHz the slant path attenuation value is increased corresponding to each elevation angle. It can also be concluded that the difference value also decreases with the increase of elevation angles. There is a significant difference in the minima and maxima values at low elevation angles which is for serious concern for the operation of VSAT and HAPS.
The attenuation of Satellite signals due to water vapor absorption is calculated using the formulas referred in Recommendations ITU-R, P.676- 5(2001). Predictions of specific attenuation and path attenuation due to water vapor absorption are also presented and compared with the water vapor attenuation calculated using Radiosonde data collected from the India Meteorological Department (IMD), Chennai station. The important findings of the present work are summarized as follows. (1) The slant path attenuation value is high for frequencies like 20 GHz, 24 GHz, 26 GHz, and 47 GHz. The corresponding value is low for frequencies 18 GHz, 16 GHz, 14 GHz, and 12 GHz etc. (2) For low elevation angles like 50 , 150 etc, the slant path attenuation is maximum (10dB) for morning radiosonde observations. Its value decreases with the increase of elevation angle and for higher elevation angles like 650 , 750 etc, the value is about 0.1dB. For the evening data is also similar results are obtained indicating that there is significant diurnal change in the observed attenuation. (3) The estimated slant path attenuation values from radiosonde data are in good agreement with the standard ITU-R reference values except in few cases. It can be due to the fact that the ITU-R values are averaged over a longer time compared to the present data (5 years), and hence the present observed values may be slightly varying from the ITU-R reference values. (4) The slant path attenuation value gradually decreases with the increase of elevation angle. It can also be concluded that the difference value also decreases with the increase of elevation angles. There is a significant difference in the minima and maximum values at low elevation angles which are of serious concern for the operation of VSAT and HAPS (High Altitude Platform Stations).
We deeply appreciate the Advanced Center for Atmospheric Science (ACAS) funded by Department of Space (DOS) under RESPOND to S. V. University, Tirupati and University Grants Commission (UGC) and other necessary facilities to carry out this work.
The atmosphere is a layer of gases covering the surface of the earth. There are many gases in the atmosphere. The two that cause almost all of the radio wave attenuation are Oxygen (O2) and Water Vapor (H2O). The two components (O2 and H2O) should be added together for the total specific attenuation. Hence, the gaseous absorption is calculated as the sum of water vapor absorption and oxygen absorption. Recommendation: ITU-R P.676-5 (2001) can be used to estimate the attenuation by atmospheric gases on terrestrial and slant paths, which are based on curve fitting to the line-by-line calculation. The input parameters required for the calculation include frequency, path elevation angle, and height above mean sea level and water vapor density. The oxygen attenuation is considered a background effect with very little temporal variation; variations in gaseous absorption arise from changes in the amount of water vapor in the atmosphere. Oxygen absorption is not considered in the present study, as the FSS (Fixed Satellite Services) do not employ the frequencies close to the oxygen absorption band (~60 GHz) at present in India. The water vapor absorption at Ku- and Ka-band satellite communication links is only considered in this study. The attenuation of Satellite signals due to water vapor absorption is calculated using the formulas referred in Recommendations ITU-R, P.676-5(2001). Predictions of specific attenuation and path attenuation due to water vapor absorption are presented in this chapter.
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