# Atmospheric Effects on Satellite-Ground Free Space Uplink and Downlink Optical Transmissions
## 2 Uplink and Downlink Channels in Satellite-Ground FSO Communications
### Absorption and Scattering Loss  
Laser beams traveling through the Earth's atmosphere may interact with molecules in the atmosphere along their paths. As a result of the interaction, light photons are absorbed when the quantum state of a molecule is excited to a higher energy level, extinguishing some of the photons and converting their energy into heat [18]. At visible and infrared wavelengths, water, carbon dioxide, and ozone molecules act as the primary atmospheric absorbers. The biggest concentration of these atmospheric absorbers is often found between one and two kilometers above the Earth's surface [18]. Figure 1: Atmospheric effects on satellite\(-\)ground FSO communications for both uplink and downlink channels. Figure 2: Beam divergence in satellite\(-\)ground FSO communications for both uplink and downlink channels. The phenomena of absorption due to atmospheric absorbers depend on the wavelength [18]. Based on this, the FSO communications system's wavelength range is chosen for the least amount of absorption. The range of wavelengths having the least amount of absorption is known as the atmospheric transmission window. Several transmission windows exist in the near-infrared wavelength range of 700 to 1600 nanometers for FSO communications systems [2]. Transmitter and detector components in this wavelength range are easily available on the market since they are also used in fiber optic communications. Hence, this paper focuses on the 700-1600-nanometer near-infrared wavelength range to observe channel characteristics of satellite\(-\)ground FSO communications. Infrared and visible electromagnetic waves scatter when they travel through air molecules and other airborne particles. Scattering of light also results in the deterioration of FSO system performance. Scattering introduces noise into the FSO communication system by scattering sky radiance into the receiver. Like absorption, light scattering is wavelength-dependent. Rayleigh scattering is caused by air molecules and haze that are minuscule in comparison to the radiation's wavelength. Particles with the same size as the radiation's wavelength cause Mie scattering [2]. Rayleigh scattering is greatly influenced by air molecules and haze. Mie scattering is caused by fog and haze [2]. The combined effect of absorption and scattering is frequently calculated using extinction, which is defined as the reduction or attenuation of the amount of energy passing through the atmosphere. On the other hand, atmospheric transmittance refers to the ability of the atmosphere to transmit radiation. It is the ratio of transmitted radiation to total incident radiation in the atmosphere. The relation between atmospheric attenuation and atmospheric transmittance (T\({}_{\text{atm}}\)) is described by [PERSON]'s law [19], which is given by  
\[\text{T}_{\text{atm}}\left(\lambda\right)=\exp\left[-\sec(\theta)\int\limits_{ 0}^{\text{H}}\gamma\left(\lambda,\text{ h}\right)\text{dh}\right] \tag{1}\]  
where \(\gamma\) is the atmospheric attenuation coefficient, H is the vertical height of the atmospheric channel, \(\lambda\) is the operating wavelength, and \(\theta\) is the zenith angle. The atmospheric attenuation coefficient \(\gamma\) depends on the integrated distribution of atmospheric constituents along the propagating path [18] in addition to the operating wavelength. Fog is one of the most frequent types of atmospheric attenuation because the constituent particle sizes are comparable in size to the laser's wavelength inducing absorption and scattering [19]. To predict attenuation, it is essential to understand the size and water content of the fog particles. However, it is challenging to find this information because it is frequently not accessible at the FSO link installation site. Researchers, as a result, have produced empirical models based on visibility data gathered from city meteorological stations. Using common empirical models, visibility information can be used to calculate attenuation due to fog.