TO DETERMINE THE REFRACTION OF RADIOWAVES

Among those problems connected with refraction of radiowaves in the atmosphere, a sufficiently accurate calculation of the refraction for low elevation angles is of significant interest. This work considers the refraction of microwaves (wavelength % ~ 5 cm), whose propagation is not influenced by the ionosphere. In the range of elevation angles i0 ~ <~- 8o ~< 90 ~ the refraction to an accuracy of ~i is determined, according to Laplaces's theorem, by the index of refraction near the earth's surface [I]. To calculate this refraction, various models of the dependence of the index of refraction on altitude have been used, such as the biexponential model of Kolosov [i], and the model of the dielectric permittivity developed in the Institute of Radio Electronics, Academy of Sciences of the USSR [3]. At lower elevation angles, significant deviations from Laplace's theorem are observed. These deviations are connected with the random character of the distributions of meteorological parameters along the ray path. It has been shown [2] that layered inhomogeneities near the earth's surface have a fundamental influence on the refraction. In this work the ordinary (not connected with fluctuations) astronomical and atmospheric refraction (corresponding to R and R H in Fig. i) in a spherically stratified atmosphere are considered. The geometry of this problem is presented in Fig. I. The index of refraction, whose variation along the ray path determines the refraction, is connected with the pressure, temperature, and humidity of the atmosphere by relationship (5). The brightness temperature of the characteristic thermal radiation from the atmosphere likewise depends upon the distribution of these meteorological parameters. This provides the possibility to extract from radiation measurements information about meteorological parameters and, consequently, also the refraction. This has made feasible the creation of an automatic system for real-time determination of refraction. In the present work a method is investigated whose application was shown to be promising in [4, 5]. This method is based on finding direct connections in the form of regression relationships between the magnitude of the refraction and the atmospheric brightness temperatures at a series of wavelengths and angles. These relationships also include the statistical connections between the refraction and near-earth meteorological parameters. In comparison with [4, 5], in this work actual seasonal meteorological statistics are used, the variations in pressure near the earth's surface are taken into account, a greater number of measurement combinations are investigated, and a method is presented that allows the computation of the refraction in the range of elevation angles 1 ~ ~ 8o ~ 5 ~ Also shown is the preferability of using the near-earth values of temperature and humidity instead of the near-earth index of refraction in the estimation scheme. All this has permitted a significant improvement in the accuracy of the refraction estimate. i. For these investigations, we took ensembles of sounding data, corresponding to summer and winter conditions for the Central partof the European Territory of theUSSR. These data