The Euclid AOCS and FGS verification tasks on the Proto-Flight Model
Authors:   Thales Alenia Space, Turin, Italy, Saponara Massimiliano , Bosco Andrea , Llorente Salvador , Meijer Lex , Rosso Carole 
Published: January 2024

Thales Alenia Space in Italy (TAS-I) is prime contractor of the Euclid Medium Class mission that, belonging to the ESA 2015-2025 Cosmic Vision plan, is close to the start of the E1 phase. Euclid will be launched in 2023 and will operate for more than 6 years in large amplitude orbit around L2. The objective of Euclid is to understand the origin of the Universe's accelerating expansion, by mapping large-scale structure over a cosmic time covering the last 10 billion years. To investigate the nature of dark energy, dark matter and gravity, the mission will target two independent techniques, weak lensing and galaxy clustering. Both techniques require the ability to survey a large fraction of the extra-galactic sky over the mission lifetime, with very high system stability (telescope, focal plane, spacecraft pointing).

The extremely accurate pointing performance is achieved through the use of the Fine Guidance Sensor (FGS), which provide very precise attitude measurement to the Attitude and Orbit Control System (AOCS) control loop during the scientific mode. The FGS is accommodated inside the Payload Module (PLM) in order to limit the contribution of deformation between FGS and instruments field of view.

TAS-I has also driven the design of the Hybrid solution used in the scientific mode. It consists of the management of the AOCS actuators (reaction wheels (RWL) and cold-gas micro-propulsion subsystem(MPS)) in order to cope with spacecraft agility and fine torque commanding for high precision pointing. In this approach, the RWL are used only for the slews, bringing them to rest before each observation commences, where the MPS-based control loop is restored.

The program is organized allocating all Sub-Systems to different European industries, and in particular, Sener and ADSNL are in charge of the AOCS design, implementation and verification. Furthermore, the FGS is developed by Leonardo Firenze, while the Astronomic Observatory of Turin (OATO is in charge of the star catalogue.

Nevertheless, significant verification tasks at both AOCS and FGS level are performed by TAS-I. In particular, TAS-I has finalized the AOCS verification on the Euclid Proto-Flight Model (PFM), where AOCS closed loop simulation campaign with the complete set of real sensors/actuators hardware can be performed. The test environment setup implements also a FGS stimulation chain through a dedicated Electrical Ground Support Equipment (EGSE) connected with the AOCS Special Check-Out Equipment (SCOE). The testing configuration has also allowed closed-loop simulation during the Thermal Vacuum – Thermal Balance (TVTB) spacecraft test campaign.

The paper will present the final EUCLID AOCS architecture implemented in the PFM, with particular focus on the sensors, actuators and support equipment. It will also describe the simulation environment setup, and finally it will report the results of closed-loop simulation of all the AOCS modes during the TVTB test campaign.