The MetOp-SG Satellite Attitude Control System - Challenges and Solutions
Authors:   Airbus Defence and Space, Immenstaad, Germany, Wiedermann Georg , Rudolph Tobias , Vitelli Marianna , Winkler Stefan , Luzi Razvan 
Published: January 2024

Mission and Program Overview
MetOp-SG is a European polar satellite system consisting of a constellation of two complementary satellites (SAT-A and SAT-B). The purpose of the mission is to provide observations and measurements for numerical weather prediction and climate monitoring, based on a total of ten instruments flown across the two satellites. MetOp-SG is the follow-on system to the first generation series of MetOp satellites, which currently provide operational meteorological observations from polar orbit.
MetOp-SG is a collaborative programme between ESA and EUMETSAT with a total of six satellites being built by Airbus Defence and Space in a cross-national team. The program is currently in phase D with launch of the first two satellites scheduled for 2025. Three flight models of each type are built and will be launched in series to cover in total at least 21 years of in-orbit operations.
The MetOp-SG satellites, each with a launch mass of over 4 tons, operate in a sun-synchronous orbit with 817 km mean altitude and Earth-oriented attitude in all operational modes. After the end of the operational mission, a controlled re-entry into the South Pacific is performed using a dedicated 400 N mono-propellant engine.
Common Platform Design
SAT-A and SAT-B satellites share a common platform design for efficient design and verification. The satellite avionics system is based on the AstroBus product for risk mitigation and cost efficiency. It re-uses the overall architecture, modes, algorithms, and software modules with specific evolutions to to comply with the MetOp-SG requirements.
The MetOp-SG AOCS functionality is implemented in three main modes that are split into sub-modes.
Initial Acquisition and Safe Mode
For initial acquisition after launcher separation and solar array deployment, and in case of severe satellite failures, an initial acquisition and safe mode is used. A canted four-thruster configuration provides three-axis control torques for fast damping of the initial satellite rates deduced from the change of the magnetic field. A bias momentum in the pitch axis provides gyroscopic stiffness around the yaw axis. In steady state actuation is performed by magneto-torquers for control without the use of consumables. The solar rotation is steered using a coarse sun sensor on the solar array, which allows reliable power generation even for a rotating spacecraft.
Mission Pointing
During nominal operation, the AOCS provides local normal pointing including yaw steering as required for instrument operation. Attitude knowledge is provided by fusion of the three star tracker measurements in a close-to-orthogonal configuration. The required attitude knowledge allows for a cost-efficient and simple gyro-less solution.
Knowledge of satellite position and velocity is provided by a Global Navigation Satellite System (GNSS) receiver compatible with GPS and Galileo constellations, which feeds an orbit propagator to ensure position and velocity knowledge also in the case of GNSS outage.
The control torque is applied by an array of reaction wheels (5 wheels on SAT-A, 6 wheels on SAT-B) with enhanced wheel management via multidimensional null-space control and L-infinity commanding. The accumulated momentum due to disturbance torques is dumped using autonomous, continuous magnetic de-saturation.
To compensate the Microwave Imager (MWI) and Ice Cloud Imager (ICI) exported torque and momentum on SAT-B, an additional reaction wheel is implemented, while rate and acceleration telemetry is provided by the instruments for feed-forward to the reaction wheel array.
Instrument operation is supported by the detection and prediction of orbital events like eclipse transition, solar zenith angle transition of the instrument field of view or sub-satellite point on ground, and sun entry in the instrument calibration field of view.
Furthermore, the AOCS provides the guidance for the Ka-band antenna pointing mechanism to ensure correct pointing of the antenna to the ground station and the solar array steering.
Orbit Control
Orbit control is performed with three thruster configurations for in-plane and out-of-plane manoeuvres, for anti-velocity manoeuvres without the need for satellite slews, and for controlled re-entry manoeuvres with high thrust using a 400 N central thruster. The need for controlled re-entry results in a high propellant mass of 760 kg (68% of it for controlled re-entry) driving the controller design.
The large delta-V manoeuvres for controlled re-entry are performed with a central 400 N thruster with yaw slews before and after each manoeuvre. After initial orbit lowering to 750 km perigee, the AOCS is switched to thruster-based control of the Earth pointing attitude between boosts to compensate the high aerodynamic disturbance torques.
Conclusion
The design solutions for all modes are presented covering hardware as well as algorithmic solutions implemented to meet the mission requirements. Specific design challenges and solutions, e.g. the design of the safe mode, impact of flexible modes, sloshing, solar array drive disturbances, and controlled de-orbit are discussed in detail..