Radio Refractive Index over India and Their Climatic Classification for Microwave Propagation

Abstract

The climatology of refractivity has assumed great importance due to large increase in the use of many kinds of microwave radar and communication systems. The refractivity of the air on the surface (N_s) computed from the pressure, temperature and humidity over 230 stations in India and also from over neighbouring countries and adjacent seas has been studied in this paper. Since refractivity at the surface varies with height, and cannot therefore be used to draw isogonals, the most suitable factor to reduce these values to the sea level has been obtained from the mean radiosonde data of 12 stations over India.

The charts showing the surface refractivity (03 GMT) and their values reduced to sea level have been drawn for January and it is shown that with the former the isogonals followed the contour of the land, while it is very much simplified when reduced to sea level. Charts have been drawn for each month of the year showing the sea level refractivity, as also the annual mean, the highest and the lowest monthly values and annual range. The diurnal variation of refractivity over India has been examined from the charts for 03 GMT and 12 GMT observations in January and July. While the horizontal gradient of refractivity increases over the country in January from morning to evening, there is very little diurnal change In July.

Since the radio-refractive index combines three of the meteorological elements normally used to specify the state of the atmosphere on either a synoptic or a climatological basis, it has been shown how the refractivity patterns and the annual ranges facilitate climatic classification of India.

The coefficient of 0·2 dB per unit change in N_s,(surface refractivity) has been adopted by the International Radio Consultative Committee (CCIR) in the 30- 300 mc. tropospheric wave propagation curves to account for the geographic and seasonal variations of field strength. Assuming that the world average value of N_s is 330, climatic variation of field has been evaluated for some representative stations and also for some of the representative climatic regions in India.

Also, based on the present knowledge of radio-meteorology on radio propagation, the CCIR has defined arbitrarily eight ‘climates’, and have provided various charts and curves for the long term medium propagation loss (i.e. transmission loss between Isotropic antennae), due to forward scatter, the transmission loss variability and estimation of losses not exceeding definite percentages In the worst months of the year for the eight different types of climates. To use these curves in India, it has been shown how the climate of different regions of India can be classified.