Optical Communications, 2nd Edition

9.1 THE ATMOSPHERIC CHANNEL

The atmosphere is composed of collections of gases, atoms, water vapor, pollutants, and other chemical particulates that are trapped by the Earth's gravity field; it extends to approximately 400 miles in altitude. The heaviest concentration of these particles is near Earth in the troposphere level (Fig. 9.1), with particle density decreasing with altitude up through the ionosphere. Actual particle distributions depend on the atmospheric conditions. The upper levels of the ionosphere contain ionized electrons that form radiation belts that surround the Earth. These atmospheric particles interact with all radiation fields that propagate through the radiation belts, with the primary effects being power losses and wavefront distortion.

Figure 9.1. Earth's atmosphere.

Power losses and distortion are caused by the absorption and scattering of the radiation fields by the particulates. These effects become most severe as the radiation wavelengths begin to approach the cross-sectional dimension of the particulates. Because particle sizes in the atmosphere range from centimeters down to micrometers, the atmosphere is especially deleterious to optical transmissions.

The communication effect also depends on the type of link. Figure 9.2 shows two of the basic space link types. Figure 9.2a shows a vertical link, with either up-link or down-link transmission, characterizing ...