Performance achievement and verification of unprecedented stability AOCS for EUCLID
Authors:   SENER Aeroespacial, Tres Cantos (Madrid), Spain, Llorente-Martinez Salvador , Agenjo Alfredo , Sanchez-Maestro Raul , Meijer Lex , Saponara Massimiliano , Rosso Carole , Cacciatore Francesco 
Published: January 2024

Euclid is an ESA medium class cosmology mission dedicated to the investigation of the properties of Dark Energy and Dark Matter. The mission will operate in a large Quasi-Halo orbit around the Sun-Earth Lagrangian point L2 about 1.5 million km away from the Earth. The scientific objectives of this mission impose challenging requirements on the system performances, arriving to levels of pointing stability of 75 milli-arcsec (99.7% confidence level) within periods of 700 seconds.
The AOCS for this mission requires a very specific and advanced design, with incorporation of dedicated state-of-the-art attitude sensors and actuators. The achievement of those levels of stability mandate a quite special combination of units and dedicated sequence of operation, resulting in a delicate architectural organisation, and specific determination and control process, including several configurations of Attitude Determination functions, organised in an AOCS architecture which includes three organisation levels: modes, submodes and states. The resulting AOCS Application SW(AASW) is split into one part that is manually coded and one part that is auto-coded from the AOCS Guidance, Navigation, and Control (GNC) design models. The AASW interfaces only with the service framework and the math library of the Central ASW (CASW). Model Based Design (MBD) and Autocoding has been used, allowing an agile development and adjustment when needed.
A dedicated process has been followed for the AOCS verification, involving a staggered verification from the design environment, up to the qualification in the Avionics Model (AVM) and acceptance in the Proto-Flight Model (PFM). In terms of performance verification, the verification starts with an extensive usage of the Engineering Simulator Environment (ESE). ESE is a High Fidelity Simulator including all the performance-relevant effects in the AOCS chain, and in particular, properly validated models for the AOCS units and the environment, including the GNC SW (Model-in-the-Loop) to be applied later in the construction of the OBSW. AOCS verification in ESE includes a combination of worst cases and Monte Carlo approaches, allowing a complete confirmation that the design and implementation of the GNC SW complies with the demanding performance requirements. From this solid starting point, the AOCS requirements are later verified with the complete On-Board SW (OBSW) and incorporating the operational environment, in the so-called SW Verification Facility (SVF). In this SVF the results obtained in the ESE environment are confirmed to be fully equivalent with the ones obtained in SVF. SVF is later complemented and confirmed for relevant aspects in a HW Computer in the Loop facility (HILF). Here all the electrical interfaces to/from the computer are generated/acquired by the AOCS-SCOE (Special Check-Out Equipment). A similar AOCS-SCOE is also applied by the spacecraft prime in the SC AVM and spacecraft PFM.
The proposed paper will provide the logic, organisation and description of the verification process, including the verification of the performance in ESE and the confirmation within SW and HW environments. The application of the MBD and autocoding techniques will be addressed, with consideration of the measures taken to ensure equivalence of results after AASW integration into the complete OBSW and moving to the HW environments.
SENER is the overall responsible and prime contractor of the Euclid AOCS, with Airbus Defence and Space Netherlands as main partner, while more than 7 additional direct subcontractors are contributing to different components of the subsystem. Thales Alenia Space Italy is Euclid prime contractor and AOCS customer.