ALTIUS Attitude and Orbit Control System Software and System-Level Test Results
Authors:   NGC Aerospace, Sherbrooke, Canada, St-Amour Amélie , Woo Pamela , A Sobiesiak Ludwik , Lobo-Fernandes Bernardo , Travis William , Langelier Marie-Kiki , de Lafontaine Jean , Debois Simon , Wenderski Piotr 
Published: January 2024

The Atmospheric Limb Tracker for Investigation of the Upcoming Stratosphere (ALTIUS) project is ESA’s ozone-monitoring mission developed as part of ESA’s Earth Watch programme and is planned for launch in 2025. The small satellite will use a high-resolution spectral imager and limb-sounding techniques to profile the ozone and other trace gases within the upper atmosphere, while supporting weather forecasting and monitoring long-term ozone trends. To carry out these mission objectives, the Attitude and Orbit Control System (AOCS) software must provide the spacecraft with high agility, high autonomy and fine pointing accuracy.
The ALTIUS satellite platform and its AOCS software are an evolution of the ESA Project for On-Board Autonomy (PROBA) series of satellites, which have near 45 combined years of successful in-orbit experience from PROBA-1 (launched in October 2001), PROBA-2 (launched in November 2009) and PROBA-V (launched in May 2013). Building upon its PROBA predecessors, ALTIUS ensures a high-level of on-board autonomy to the spacecraft, minimising the need of ground station commands.
To complete the mission objectives, ALTIUS requires several AOCS software innovations compared to the previous PROBA missions. The autonomous novelties implemented for ALTIUS include the following:
•Limb Looking Mode:
Limb Looking mode is an Earth atmosphere observation mode during which the AOCS software autonomously orients the spacecraft such that the payload line of sight points to a desired tangent point at a fixed altitude above the Earth surface. It includes nine submodes dictating the desired direction of the tangent point, including for instance Backwards Limb Looking (BLL) for anti-velocity limb pointing and Pole Limb Looking (PLL) for observation towards the North or South poles.
•Augmented Bdot Safe Mode:
The previous PROBA satellites offer two Safe modes: Bdot and 3-Axis Magnetic modes.
The Bdot mode uses the magnetic field to reduce angular rates and to roughly align, along the orbit normal, a momentum bias generated by maintaining the reaction wheels at a constant speed.
The 3-Axis Magnetic mode is based on the Bdot algorithm with an additional pointing of a third axis to nadir. It requires spacecraft position knowledge.
As a novelty on ALTIUS, an additional Safe mode is included: the Augmented Bdot mode. This mode is similar to the Bdot mode but additionally roughly aligns a user-specified spacecraft axis (perpendicular to the reaction wheel momentum bias axis) with the Earth’s magnetic field. This mode offers 3-axis pointing with respect to the magnetic field and the orbit normal and is more robust than the 3-Axis Magnetic mode because it does not require spacecraft position knowledge.
•Thruster Management with Off-Modulation:
The ALTIUS spacecraft is equipped with 4 thrusters aligned in the same direction. The AOCS software is designed to command the 4 thrusters of the spacecraft, falling back to a 2-thruster configuration in case of thruster failure, for ΔV manoeuvres while performing off-modulation of the thrusters to contribute to the attitude control and avoid wheel saturation due to disturbing torques.
•Occultation Event Prediction:
Time and orientation (in terms of azimuth and elevation angles) prediction of the next occultation entry and exit for up to 10 celestial targets and the Sun, where occultation is defined as the event during which a celestial target is visible from the spacecraft within lower (-10km) and upper (+100km) tangent altitude limits with respect to the Earth’s horizon.
•Occultation Event Monitoring:
Continuous monitoring of the elevation angle for up to 10 celestial targets and the Sun.
•Stellar Occultation Mode:
-Standby Submode: aligns the spacecraft with the anticipated orientation of the next occultation start and avoids blinding of the star trackers and payload.
-Tracking Submode: tracks a celestial target such as the Sun, a star, a planet, or the moon through the Earth’s atmosphere during occultation.
After an overview of the ALTIUS mission, its AOCS software and its novelties, this paper will focus primarily on three of the above-mentioned novelties: Limb Looking mode, Augmented Bdot Safe mode and thruster management with off-modulation. It will discuss challenges associated with their design and present their performance based on the AOCS Software System-Level Tests (SST) results. The SST campaign has been performed on a closed-loop MATLAB/Simulink simulator and has been completed in January 2023. The SST results for the other novelties, namely the occultation event prediction and monitoring and the Stellar Occultation mode, are not detailed in the present paper as they are the subject of another paper to be published and presented at the 45th Annual AAS Guidance & Control Conference in Breckenridge, Colorado, USA in February 2023.