Atmospheric absorption and scattering impact on optical satellite‐ground links

Abstract

Free-space radio-frequency (RF) communication links for intersatellite or satelliteto-ground communications are getting increasingly constraint by the insufficient spectrum availability and limited data rate of RF technology. With the advent of large satellite mega-constellation networks for global communications coverage, this limitation of classical RF communication becomes even more critical. Therefore, the establishment of point-to-point free-space optical link technology (FSO) in space will become of paramount importance in future systems, where the application will be for data-relays links, or for mega-constellations inter-satellite links, as well as for direct data downlinks, or from deep-space probes to ground. Further advantages of optical FSO-besides spectrum availability-is its increased power efficiency, higher data rates, avoidance of interference, and inherent protection against interception. When, however, these optical communication links have to pass through Earth's atmosphere, attenuation and scattering effects do influence the signal transmission. In this publication, we investigate the effects of atmospheric attenuation, including the effects of molecular absorption as well as aerosol scattering and absorption, for typical wavelength regions used for FSO, dependent on the link geometries. Based on transmission-simulation databases, we show useful spectral ranges and their specific attenuation strength. Free spectral transmission windows dependent on atmospheric quality and elevation angle are identified for reliable and efficient use of the optical transmission technology in space applications.

Link budget calculation in optical LEO satellite downlinks with on/off‐keying and large signal divergence: A simplified methodology

“…Additional loss is introduced by atmospheric aerosol scattering and molecular absorption. By avoiding delicate spectral regions of molecular absorption lines (see figure 9 of Giggenbach and Shrestha 30 ), only aerosol absorption and scattering have to be regarded. 54 Its effect is amplified by low link elevations, because the atmospheric path becomes longer and the according attenuation is emphasized.…”

“…T A B L E 2 Typical atmospheric attenuation to zenith. The plots in Figure 6 do not regard local topographic effects like an elevated plane surrounding an OGS on a mountain top-such would resemble a typical location for an OGS (as scenario A in Giggenbach and Shrestha 30 ). The first (better) values in the next table serve as a reference for this scenario A, whereas the other values serve as a worse-case scenario B-from sea level and in humid tropical regions.…”

“…Figure 2 shows the elements influencing the OLEODL performance: The satellite transmitter (a rather small optical aperture based on a lens or mirror telescope) sends the laser‐based optical data signal toward the OGS, with a residual pointing error as small as technically possible. The distance reduces the signal intensity through beam widening, and the atmosphere perturbates, attenuates, or blocks (clouds) the laser beam and causes intensity scintillations through its different effects 30,31 . The OGS receiver telescope finally tracks and collects the optical signal and converts it into an electrical signal through the receiver front end (RFE), where the sky's background power can reduce the performance of such a detector due to additional shot‐noise current and other effects 32,33 .…”

“…The absorption is further influenced by the OGS location and air quality. In Table 2, we state exemplary attenuation values to zenith for a favorable (first value) and a more adverse (second value) OGS location and air quality (compare figure 4 of Giggenbach and Shrestha 30 for the nomenclature of atmospheric models). Derived with Equation (10) of Giggenbach and Shrestha, 30 assuming a flat‐Earth model, the zenith transmission value T z allows the calculation of the total attenuation under elevation angle ε : …”

“…The distance reduces the signal intensity through beam widening, and the atmosphere perturbates, attenuates, or blocks (clouds) the laser beam and causes intensity scintillations through its different effects. 30,31 The OGS receiver telescope finally tracks and collects the optical signal and converts it into an electrical signal through the receiver front end (RFE), where the sky's background power can reduce the performance of such a detector due to additional shot-noise current and other effects. 32,33 Although optical links from/to geostationary Earth orbit (GEO) are static by nature, 34 the OLEODL scenario is highly dynamic in elevation and range and thus imposes different and more severe challenges to maintain stable communication.…”