Overview

Knowledge of signal propagation characteristics is of fundamental importance to the design of communication satellites and their associated user-terminal networks [102, 103]. The relative importance of the various propagation factors depends to a large extent on the radio frequency of the transmission, the local climate, and the elevation angle to the satellite. Generally, the effects become more significant as the radio frequency increases.

In the design of Ka-band communication satellite systems, a critical requirement is achieving satisfactory service availability. Propagation effects, which can seriously degrade the reliability and performance of a communication link, strongly influence the choice of suitable satellite system parameters.

Propagation of Radio Signals Through the Atmosphere

Rain is the primary factor affecting satellite links. At the 20 and 30 GHz frequencies of the ACTS system, rain attenuation is a more serious problem than at the lower C- (4 and 6 GHz) and Ku- (12 and 14 GHz) band frequencies. For example, for a rain rate of 22 millimeters per hour in Washington, D.C., the rain attenuation for C-band is 0.1 dB/km, Ku-band is 1 dB/km, and Ka-band is 5 dB/km.

Physically, rain attenuation is due to scattering and absorption of the microwave signal energy by the raindrops. As the radio frequency increases, the wavelength decreases and the signal attenuation in rain increases. For instance, at 30 GHz, the wavelength is 1 cm compared to a typical raindrop, which is 0.15 cm in diameter. The droplets act as lenses, bending the waves and distorting the signals.