Repeater systems in a geostationary orbit utilizing free space optical communication offer great potential to backup, process and archive large amounts of data collected or generated at remote locations. In contrast to existing or upcoming global satellite communication systems, such optical GEO relays are able to provide a huge return-channel data throughput with channel rates in the gigabit-per-second range. One of the most critical aspects of such data uplinks are atmospheric disturbances above the optical ground terminals used to connect to the space segment. In this study we analyse the design drivers of optical ground stations for land-based applications. In particular, the effects of atmospheric attenuation and index of refraction turbulence are investigated. Moreover, we present implementation ideas of the necessary ground infrastructure and exemplify our results in a case study on the applicability of free space optical satellite communication to the radio astronomy community. Our survey underpins pre-existing ventures to foster optical relay services like the Space-Data-Highway operating via the European Data Relay System. With well-designed, self-sufficient and small-sized ground terminals new user groups could be attracted, by offering alternatives to the emerging LEO mega-constellations and GEO-satellite communication systems, which operate at low return channel data rates across-the-board.

The microwave spectrum has become a highly limited resource in satellite communications owing to an ever increasing demand for bandwidth and capacity. Therefore, a shift to the exploitation of optical carrier frequencies is currently underway. Focusing on high-rate transmissions of payload data from remote sensing satellites, operational systems, like the well-known European Data Relay Satellite system, are based on optical inter-satellite links. Besides, direct-to-earth free-space optical communications from low Earth orbiting spacecraft hold high potential for upcoming space missions through lower complexity. In that regard, we study the viability of the ground-to-space beacon laser signal of optical ground stations to be additionally modulated with tele-command tokens. Such an optical return channel could be variously put into use, e.g. to trigger automatic repeat requests of payload data downlinks, for jamming-free control of the spacecraft or for high-rate software uploads to its on-board processor. A particular challenge is posed by the unequal fading behavior of the optical channel regarding the down-and uplinks, which cover asymmetric optical pathways through the atmosphere. We define the end-to-end architecture of the communication chain including the transmitter on ground and the spacebased receiver. Special attention is given to compatibility with established space data and system standards. Moreover, we examine the effects on the scheduling of satellite control, resulting from a constrained availability of the optical uplink due to cloud blockages. Our analysis aims at the employment of available space protocols for bidirectional optical communications with low earth orbiting spacecraft. Further on, we consider the adoption of upcoming standards to account for the optical fading channel. Certain applications like immediate automatic-repeatrequests for the downlink will require novel, optimized protocols.

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Different pointing errors from different sources cause an angular deviation in the uplink beam transmitted from an optical ground station (OGS) to a satellite. In optical link-budget calculations, the beam intensity loss due to pointing errors, "pointing loss", is usually given a constant value regardless of the satellite elevation. In this paper, elevation-dependent intensity losses are calculated, considering a transmitted uplink beam with a Gaussian profile. The elevation of the satellite and the divergence of the uplink beam are considered to assess the impact of the following sources of tracking and pointing errors: OGS static pointing misalignment, uncorrected or fixed-corrected point-ahead angle (PAA), satellite orbital data uncertainties (specifically the along-track error), and mechanical jitter at the OGS. Each source of error is first evaluated separately and then the combination of their effects on the intensity loss in the LEO uplink is determined. It is demonstrated that the elevation-dependent pointing errors analyzed in this work have a greater impact on the intensity loss for satellites at lower altitudes and higher elevations. Therefore, not considering that the value of the "pointing loss" varies with elevation -especially for LEO satellites-, would result in lower link performance. Values of intensity loss are provided for LEO satellites at different altitudes and elevations, and uplink beam divergences. The results provided can be used for link-budget calculations in the optical LEO uplink in the presence of elevation-dependent pointing errors, and for system improvements in the design of future ground and space optical terminals.