Prediction of Line-of-Sight Microwave Signal Fading Through Meteorological Parameters

The variability of troposphere in certain cases pose serious problems in achieving microwave communication and there are situations when the line-of-sight (LOS) link may deviate from its expected path of propagation. LOS microwave links though found to be reliable and widely used, but the tropospheric medium through which the signal propagates is found to create problems in such links. Bending of LOS path from its desired trajectory along with deviation in angle-of-arrival of the signal causes much degradation in the energy levels of the signal in well-designed links. In this paper effort has been made to predict fade depth of a microwave signal of frequency less than 10 GHz. through a model associating with the radio refractive index gradient (RRI grad), which is a function of temperature, pressure and humidity. The RRI is calculated at the surface, at 100 m. height and 500 height levels. In cases where RRI increases or decreases uniformly then the slope is defined by a single parameter ´N1´. But for a non-uniform atmosphere, another parameter ´N2´ is introduced. Here ´N1´ is the RRI grad between the surface and 100 m. height and ´N2´ is the gradient for the upper atmosphere. Model is then developed correlating different situations of the atmosphere (whether N1 or N2 is in normal, sub-refractive or super-refractive state) with the fade depth of a 7 Ghz. 42 km. line-of-sight microwave link.