Atmospheric Absorption Loss
In satellite communications, atmospheric absorption loss results from the absorption of the Earth-satellite signals as they pass through the Earth’s atmosphere. The value of the atmospheric loss is strongly dependent on frequency and is treated quite separately to attenuation of the signal due to hydrometeors (weather-related loss).
While the atmosphere consists mainly of 78% nitrogen, 21% oxygen, 0.9% argon and 0.1% carbon dioxide, it is only oxygen that has an appreciable effect on satellite communications frequencies. Oxygen has negligible effect up until 10 GHz, beyond which attenuation increases until a major oxygen-absorption band centered around 60 GHz. Although frequencies around 60 GHz are therefore of little use for Earth-satellite links, the band is of interest for inter-satellite links since the frequencies cannot pass through the atmosphere and interfere with terrestrial systems.
Precise calculation of the attenuation requires knowledge of the air density, which decreases roughly exponentially with height. An approximate calculation can be made, however, by assuming that the density of oxygen is the same from 0–6 km and zero after that. Consequently, an estimate of attenuation due to absorption by oxygen can be made by estimating the attenuation per kilometer and then multiplying by 6 km.
In addition to oxygen absorption, attenuation peaks result from absorption of the signal by water vapor (about 1.7% of the atmosphere at 100% humidity). The first major absorption band is centered around 22.3 GHz, after which attenuation continues to rise markedly with increase in frequency. The degree of absorption by water vapor is a function of frequency, temperature, pressure, and humidity of the atmosphere. In addition, absorption is a function of the elevation since the attenuation increases as the path length increases (as elevation decreases). Since the attenuation by water vapor is not very large for the frequencies up to 30 GHz, it is usual to assume a standard atmosphere of density 7.5 g m–3, temperature 15° and pressure 1,013 mb. Although the density of water vapor changes with height, an approximation can be made by assuming that the water vapor has constant density for 2.2 km and is zero beyond that height.
The total atmospheric losses due to atmospheric absorption by oxygen and water vapor are quite small at low frequencies but can be quite high at Ka-band.