Hardware and software architecture on board solar orbiter/METIS: an update

METIS, is one of the ten instruments selected to be part of the Solar Orbiter payload; it is a coronagraph that will investigate the inner part of the heliosphere performing imaging in the visible band and in the hydrogen Lyman α line @ 121.6 nm. METIS has recently undergone throughout a revision to simplify the instrument design. This paper will provide an overview of the updated hardware and software design of the coronagraph as presented at the Instrument Delta-Preliminary Design Review occurred in April 2014. The current configuration foresees two detectors, an Intensified APS for the UV channel and an APS for the visible light equipped with a Liquid Crystal Variable Retarder (LCVR) plate to perform broadband visible polarimetry. Each detector has a proximity electronics generating the control and readout signals for the sensor but the operations of the two devices are in charge of a centralized unit, the METIS Processing and Power Unit (MPPU). The MPPU operates the remaining electrical subsystems supplying them with power and providing on board storage and processing capabilities. Its design foresees the redundancy of the most critical parts, thus mitigating the effects of possible failures of the electronics subsystems. The central monitoring unit is also in charge of providing the communication with the S/C, handling the telemetry and telecommand exchange with the platform. The data acquired by the detectors shall undergo through a preliminary on-board processing to maximize the scientific return and to provide the necessary information to validate the results on ground. Operations as images summing, compression and cosmic rays monitoring and removal will be fundamental not only to mitigate the effects of the main sources of noise on the acquired data, but also to maximize the data volume to be transferred to the spacecraft in order to fully exploit the limited bandwidth telemetry downlink. Finally, being Solar Orbiter a deep-space mission, some METIS procedures have been designed to provide the instrument an efficient autonomous behavior in case of an immediate reaction is required as for the arising of transient events or the occurrence of safety hazards conditions.