Free-Space Optical Communication (FSOC) links between Earth-based Optical Ground Stations (OGSs) and satellites offer immense potential to securely and efficiently exchange vast amounts of information with worldwide coverage. However, atmospheric turbulence inhibits this potential by distorting laser beams, as they propagate through the atmosphere. Adaptive Optics (AO) systems are typically employed at the OGS to correct for these adverse effects and can increase the efficiency of laser light being coupled into an optical fibre for a downlink laser beam. Concurrently, the same AO system can be used to increase the coupling of laser light into an orbiting satellite by pre-distorting the uplink laser beam. In such a scenario, the downlink laser beam is used to measure the distortions that are applied by the atmosphere, and the conjugate of these distortions can then be applied to the uplink laser beam. The atmosphere then corrects the pre-distorted beam, resulting in a flat wavefront at the top of the atmosphere, as well as stable and efficient coupling of light into the satellite. This work showcases the successful experimental ground-to-satellite links in the spring of 2023 between DLR’s recently commissioned OGS and TESAT’s laser communications terminal (LCT-135)—i.e., part of the Technology Demonstration Payload No. 1 (TDP-1) on the geostationary satellite, Alphasat. Pre-distortion was successfully applied via an AO system testbed within the OGS, which resulted in extremely power efficient bi-directional tracking links with Alphasat. The findings of this work show that the application of pre-distortion AO not only improves the coupling of laser light at the satellite, but also reduces the scintillation experienced at the satellite, thus improving the robustness of the link.
The Transportable Adaptive Optical Ground Station, which was located at Tenerife (Spain) from 2014 until 2019 was shipped for a refurbishment and several enhancements to Switzerland. Following the refurbishment, a measurement campaign at the Swiss Optical Ground Station and Geodynamics Observatory Zimmerwald (Switzerland) was performed, in July/August 2021. The optical counter terminal for the experiments was the TDP1-LCT at the geostationary Alphasat satellite. In the seven weeks of the campaign, measurements for optical uplink communication at a rural site in Europe at an altitude of 900 meters to a GEO spacecraft were gathered. We give an overview of the improvements of the T-AOGS and the actual status after the refurbishment. We present the results of the link activities, e.g. the uplink budget (ground to space) and the dynamic of the atmosphere (scintillations index, number of fades, fade duration).
An important step towards reliable optical communication between ground and space is the characterization and understanding of atmospheric conditions – even when being at one place and performing links into one direction, but at different times. Within this paper the analysis of five years of optical links from a laser terminal on a geostationary satellite (TDP1-LCT on Alphasat) to ground (T-AOGS at Tenerife) is shown. The data, in total 40.2 h of reliable measurements extracted from 516 performed links, are spread over several times of the year and almost all times of day. For one of the main parameters describing the atmosphere (the Fried Parameter r0) a clear characteristic over the day is validated. For the up-link the influence of different transmitted beam divergence angles on the link budget in terms of mean power and fade statistic is shown. A short outlook on upcoming changes within the TDP-1 software and the TAOGS functionality is given.
With the EDRS program, the European Data Relay System, a private public partnership program between the European Space Agency ESA and Airbus Defence and Space ADS, laser communication has entered the commercial service since November 2016 [1]. Currently four Earth Observation satellites named Sentinels equipped with TESAT Laser Communication Terminals from the Copernicus program of the European Union are served by 40 data relay links per day, already accumulating to more than 31000 links in total. We report on the performance of the systems in space and detail on other activities of Tesat.
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