The Atmosphere and Its Effect on the Microwave Signal
Studying characteristics of the upper atmosphere is important in both weather prediction and climate studies. The atmosphere is made up of several layers (Brunner and Welsch 1993). Each layer is characterized by temperature, pressure and humidity. The closest layer to the earth is the troposphere, which begins at the earths surface and extends to between 9 and 16 km above earth surface. This roughly 7 km Region between the troposphere and the next layer, the stratosphere, is called the tropopause. The tropopause has some characteristics of both the troposphere and the stratosphere. The troposphere, tropopause, and stratosphere make up what is referred to as the neutral atmosphere, because it is electrically neutral. Above the stratosphere, the atmosphere is electrically charged and is called the ionosphere. Outside the mesosphere is the remainder of the ionosphere that extends from about 80 km above the surface to the space. These atmospheric regions adversely affect the microwave signal.
The propagation phenomena concerning earth-space links mainly originate in the troposphere and the ionosphere. Respectively, propagation effects are separated into two categories : ionospheric effects, influencing systems operating below 3GHz, and tropospheric effects, influencing systems operating above 3GHz. The most important tropospheric phenomena are as follows.
Melting layer attenuation
Rain and ice depolarization
Gaseous absorption, cloud attenuation, melting layer attenuation, and rain attenuation are absorptive effects producing both signal attenuation and a proportionate increase in the thermal noise received at the antenna port. Systems employing orthogonal polarization to implement frequency reuse suffer from interference produced by rain and ice depolarization. Tropospheric scintillation is nonabsorptive and produces signal attenuation as well as enhancements.
Moisture enters the air as water vapour, which comes mostly from bodies of water. As the vapour rises, it may change into drops of water and form clouds. There are three main kinds of clouds that are named according to their height above the ground. Cirrus clouds are high in the sky and made of ice crystals. Cumulus clouds are middle-height clouds and look like heaped up piles of cotton. Stratus clouds are low to the ground and look like a blanket above the ground.