Optical Inter-Satellite Communication: the Alphasat and Sentinel-1A in-orbit experience

Since late 2013, the Inmarsat operated Alphasat GEO communication satellite has been providing a reliable platform for the experimental activities of its 4 hosted Technical Demonstration Payloads (TDPs), procured and operated by ESA. TDP1 was developed by TESAT Spacecom for DLR, with the objective to demonstrate and characterise a LEO to GEO optical communications link. TDP1 is able to receive observation data from a lower orbit (LEO) spacecraft thanks to its Laser Communication Terminal, and route the data to ground via a Ka-band RF link performed by its own Ka-band System. In the absence of a counter-terminal aloft, the activities concentrated initially on characterising and optimising the terminal performances using the ESA Optical Ground Station in Tenerife. With the launch in April 2014 of Sentinel-1A in its sun-synchronous orbit, an in-flight companion for the ASA terminal was made available. Operated by ESA on behalf of the European Commission as part of the Copernicus program, S1A performs its earth observation mission thanks to a Synthetic Aperture Radar, and carries on board an Optical Communication Payload, based on another TESAT Laser Communication Terminal. As a proof of concept for the upcoming European Data Relay Satellites system (EDRS), an Optical Inter-Satellite Link demonstration campaign started in September 2014. Using an incremental approach that led to the first data transmission from Sentinel 1A in November 2014, the campaign was completed in December 2014, with results well beyond expectation in terms of link performances. A subsequent “Experimentation Phase” aiming at the characterisation of the paired terminal performances over its operational envelope has been going on since the beginning of 2015. In the meanwhile, the launch of Sentinel 2A provided the second LEO counter-terminal for TDP1. Beyond the technological complexity of the laser communication terminal and the outstanding results obtained, the exercise presented some remarkable operational challenges. Planning, scheduling and executing a link implies coordination of the activities of the two missions engineering and operations systems and teams, with the necessary involvement of the laser terminal provider team expertise. In addition to providing the ultimate demonstration of the technology underlying the EDRS, the Alphasat and Sentinel 1A experiences have thus provided an important opportunity to test the system operations in a realistic scenario, and gain valuable experience to be put to fruition for the EDRS GS and operations segment.