Space-to-ground optical data links enable higher data rates, require less electrical power, and allow more compact system designs than their corresponding RF counterparts. They may be applied to, for example, downlink Earth observation data from low Earth orbit satellites, or as so-called feeder links for data transmission to geostationary telecommunication or multimedia satellites. However, optical space-to-ground links suffer from limited availability due to cloud blockage. The application of optical ground station (OGS) diversity and thus a network of OGS is required to overcome this limitation. In this paper, we report on OGS networks and the calculation of combined network availabilities. Five years of cloud data gathered by a Meteostat Second Generation satellite have been evaluated. Single- and joint-site statistics as well as correlation between OGS sites are introduced. In order to effectively reduce computational effort, a network optimization method, exploiting the correlation between sites and single-site availabilities, is proposed. Furthermore, the cloud data are used to find several optimal OGS networks and to simulate the networks’ availabilities and temporal behaviors. Optimal German, European, and intercontinental networks are identified. With the increasing number of stations, the German network converges to an availability of 84.7%, and the European network to around 99.9%. The intercontinental network even reached an availability of 100% for nine or more stations during the considered time span.

Recently, the availability of innovative and affordable COTS (Commercial Off-The-Shelf) technological solutions and the ever-improving results of microelectronics and microsystems technologies have enabled the design of ever smaller yet ever more powerful satellites. The emergence of very capable small satellites heralds an era of new opportunities in the commercial space market. Initially applied only to scientific missions, Earth observation and remote sensing, small satellites are now being deployed to support telecommunications services. This review paper examines the operational features of small satellites that contribute to their success. An overview of recent advances and development trends in the field of small satellites is provided, with a special focus on telecommunication aspects such as the use of higher frequency bands, optical communications, new protocols, and advanced architectures.

The high data rates enabled by free-space optical communication links and additional factors, such as the unregulated spectrum, make them an attractive solution in future geostationary feeder link applications with capacities beyond one Terabit per second. However, optical links through the atmosphere are heavily affected by atmospheric turbulence, leading to significant scintillation. Transmitter diversity is an effective way to improve the signal quality of optical links. Transmitter diversity with Phase-Division in Bit-Time is a novel scheme for mitgating interference and thus enabling the use of equal wavelengths for multiple transmitters. This paper will evalute the Phase-Division in Bit-Time transmitter diversity scheme and present measurement results from a lab testbed.

Within this chapter, high-speed optical satellite data downlinks have been reviewed and the key characteristics of this application scenario have been described. The excellent properties of optical links, especially the high data rate, license free operation and favourable SWaP (size, weight and power) provide a game-changing technological alternative to RF-links for Earth observation satellite operators. Despite some drawbacks of the technology, industries and research organisations around the world are now developing optical communication systems that are suitable for downlink applications, demonstrating the potential of the technology and underlining its future importance for various applications.

Future Very High Throughput Satellite Systems (VHTS) will perform at several Tbit/s throughput and thus face the challenge of limited feeder-link spectrum. Whereas with conventional RF feeder links several tens of ground gateway stations would be required, the total capacity can alternatively be linked through a single optical ground station using Dense Wavelength Division Multiplexing (DWDM) techniques as known from terrestrial fiber communications. While intermittent link blockage by clouds can be compensated by ground station diversity, the optical uplink signal is directly affected by scintillation and beam wander induced by the atmospheric index-of-refraction turbulence. The transmission system must be capable to mitigate these distortions by according high-speed tracking and fading compensation techniques. We report on the design of a near-ground long-range (10km) atmospheric transmission test-bed which is, with its relatively low elevation of 1.8 degrees, exemplary for a worst case GEO uplink scenario. The transmitting side of the test-bed consists of a single telescope with a a fine pointing assembly in order to track the atmospheric angle-ofarrival and precisely aim towards the beacon of the receiver. On the other side of the test-bed, the receiver telescope is also capable of fine pointing by tracking the transmitted signal. The GEO uplink scenario is modelled by a precise scaling of the beam divergence and the receiver’s field of view as well as by the beacon offset to model the point-ahead angle. In order to make the experimental test-bed correspond to an actual feeder link scenario, the link budget as well as the turbulence profile of the experimental scenario are modelled and compared to the GEO uplink. Several DWDM channels are multiplexed to reach the total link capacity of above one Tbit/s.