The increasing interest in Very Low Earth Orbit (VLEO) missions, typically defined as orbits below 450 km, is driven by
their unique advantages, such as improved spatial resolution for Earth observation, reduced latency for communications, and lower launch costs [1]. However, operating in VLEO introduces significant challenges, due to the residual atmosphere and the related aerodynamic efforts/perturbations acting on the satellite. These perturbations, combined with limited actuation capabilities, make traditional control strategies inadequate in VLEO, posing critical challenges for maintaining pointing accuracy and orbital stability [2]. Recent studies on disruptive technologies for very low earth orbit (H2020 DISCOVERER) have highlighted the need for robust control and advanced management of actuator saturation [3]. This paper addresses the preliminary design of a VLEO satellite platform, with a specific focus on developing a control-driven approach to ensure stability and pointing accuracy under these demanding conditions.

The proposed approach adopts an iterative methodology in which a numerical mock-up of the satellite, including a tentative sensors and actuators layout, undergoes aerodynamic analysis, enabling control synthesis and subsequent evaluation of attitude and orbit control performance. The resulting metrics are then used to validate or update the satellite configuration before proceeding to the next iteration. The process integrates a high-fidelity simulation environment with a simplified, control-oriented model to facilitate the design and validation of an advanced attitude control system. The high-fidelity simulator, developed in Simulink, incorporates a detailed orbit propagator, Earth’s magnetic field, and gravity gradient torques, with a particular emphasis on accurately modeling aerodynamic forces using Monte Carlo Simulation data. This enables a comprehensive characterization of the aerodynamic moment coefficients, critical for the control task at hand. The control system itself is structured around several key components:

1. an advanced control allocation strategy to manage the redundant reaction wheel configuration and magnetorquer-based desaturation,
2. model-based estimators for aerodynamic and gravity gradient torque rejection,
3. a nonlinear compensation block to address gyroscopic torques and kinematic nonlinearities,
4. H-infinity-tuned PID controllers for each axis.

The control allocation strategy is designed to prevent reaction wheel saturation, a common issue in VLEO due to the high torque demands, while the H-infinity optimization ensures robustness against measurement noise and external perturbations. The simplified model, derived from polynomial approximations of the aerodynamic coefficients, enables efficient preliminary validation and robustness analysis, bridging the gap between design and high-fidelity testing.

The main outcomes of this work validate the proposed procedure demonstrating the ability of the final satellite configuration
in maintaining pointing accuracy and stability over extended validation campaigns. Results obtained with two-weeks-long
time-horizon simulations, encompassing multiple orbits with varying initial conditions (e.g., high attitude errors or angular
rates), confirm the controller’s ability to handle aerodynamic and gravity gradient perturbations without actuator saturation.
The high-fidelity simulator’s efficiency allows for a reduced execution time of few hours per mission. The satisfactory results
validate the control-driven design approach and highlights its potential for real-world VLEO missions.

In the full paper, we will present a detailed description of the high-fidelity simulator and the simplified control-oriented
model, followed by an in-depth analysis of the control system design and validation process. Future work will focus on
integrating more realistic sensor models and dedicated fusion algorithms to enhance attitude and angular rate estimation,
further improving the controller’s performance. Additionally, we plan to explore the implementation of the controller on
onboard computers (OBCs) and conduct quantitative analyses via controller emulation in a C-coded format. This research
contributes to the broader effort of enabling sustainable and high-performance VLEO missions, paving the way for next generation of satellite platforms.

References

1. Nicholas H Crisp, Peter CE Roberts, Sabrina Livadiotti, Vitor Toshiyuki Abrao Oiko, Steve Edmondson, SJ Haigh, Claire Huyton, LA Sinpetru, KL Smith, SD Worrall, et al. The benefits of very low earth orbit for earth observation missions. Progress in Aerospace Sciences, 117:100619, 2020.
2. Josep Virgili-Llop, Halis C Polat, and Marcello Romano. Attitude stabilization of spacecraft in very low earth orbit by center-of-mass shifting. Frontiers in Robotics and AI, 6:7, 2019.
3. H2020/DISCOVERER. https://cordis.europa.eu/project/id/737183, 2022.