Digital Communications with Emphasis on Data Modems

19ATMOSPHERIC PROPAGATION

19.1 INTRODUCTION

Communication links that propagate through the atmosphere encounter a number of effects that distort the received signal making detection and parameter estimation difficult and prone to errors. The signal distortion results because the atmosphere is an inhomogeneous medium with spatial and temporal variations that result in random behavior leading to: absorption or signal attenuation, inter‐ and intra‐symbol distortion due to symbol dispersion, variations in range‐delay, Doppler spreading, polarization rotation, and signal amplitude and phase fluctuations resulting from multipath interference. The principal regions of the atmosphere that impact electromagnetic wave propagation are the troposphere, and ionosphere, and the principal parameter that characterizes the performance in each region is the index of refraction.

The lower region of the atmosphere is the troposphere extending to about 30 km in altitude and the upper region is the ionosphere extending to several thousand kilometers. The troposphere is essentially an ion‐free region consisting of about 99% oxygen with nitrogen and water vapor at the lower altitudes. In this region electromagnetic wave propagation characteristics are determined by the refractive index which is a function of pressure, temperature, and water vapor; other natural phenomena like dust, rain, and clouds must also be considered. Propagation in the ionosphere is influenced primarily by free‐electrons and ...