File based operations, CFDP assembly and Euclid

Abstract

The first large scale European Space Agency (ESA) mission that will adopt a file based approach to operations is Euclid, which is expected to be launched in 2021 and will explore dark energy and dark matter in order to understand the evolution of the Universe since the Big Bang and, in particular, its present accelerating expansion. It will have an operational orbit around Sun-Earth-Liberation-Point 2 and will generate about 100 GB of science data per day. To transfer these data to ground a high telemetry rate via K-band is required. The weather dependent quality of a K-band link requires a failure detecting downlink protocol with automatic retransmissions of corrupted or missing data segments. For this purpose, the CCSDS File Delivery Protocol (Ref. [1]) (CFDP) class 2 (reliable) has been selected. The operations concept for Euclid is that it will be an “offline” mission, i.e. there is a high level of on-board autonomy for routine operations and failure discovery, isolation and recovery. Scientific data and housekeeping telemetry (instruments and service module) will be recorded in the Mass Memory Unit (MMU) and daily ground contact will be of the order of 4 hours. Data will be downlinked as files from the MMU and telecommands uplinked in files to the Mission Timeline. Due to the guaranteed available bandwidth between the Operational Control Centre (OCC) and the ground stations to be used for the normal operations phase of the mission (the ESA Deep Space Antennas at Malargüe, Argentina and Cebreros, Spain), it is not possible to rely on all the data being delivered to the OCC in real-time in a reliable way. In view of this, it has been necessary to make some modifications to the way in which the CFDP entity is deployed to cater for this. Essentially this requires the instantiation of a CFDP assembly at the ground station where the downlinked files are reconstituted, whilst CFDP control PDUs are routed to the control system at the OCC for uplink to the spacecraft. The routing of the CFDP PDUs for the uplink is largely a consequence of the use of the CLTU protocol for uplinking – CLTU only supports one command source. No changes to the underlying CFDP protocol are required.