Hamza Eren Gunaltay

Spacecraft in Very Low Earth Orbit (VLEO) could play a significant role in improving daily life through enhanced communication, Earth observation and environmental monitoring. Their close proximity to Earth enables higher resolution imagery for improved weather forecasting and disaster monitoring, faster data transmission with lower latency, and faster natural deorbiting to ensure compliance with space debris mitigation policies. However, operating at such altitudes poses significant challenges, particularly due to atmospheric drag, and requires active propulsion systems to maintain operational orbits over extended periods. Additionally, Earth's gravity field variations exert greater influence on VLEO satellites. In this research, a high-fidelity orbital propagator is being developed including atmospheric drag effects calculated via simplified Gas Surface Interaction (GSI) models and perturbations from the Earth's non-spherical gravity field and third-body attractions from the Moon and the Sun. Aerodynamic forces acting on the satellite were computed using panellised satellite models, with relative velocity including horizontal neutral winds. The propagator's accuracy was verified using Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) data with GSI parameter's described in the literature, achieving close alignment with GOCE's ephemeris during its free-fall. An air-breathing satellite platform aligned with the orbital velocity vector was tested with the propagator. This alignment, caused deviations in the relative particle velocity vector, resulting in elevated drag. This rendered passive compression systems insufficient to the meet required number density thresholds, necessitating the need for active intake systems. A control methodology was derived from the rate of change of perigee using Gauss Variational Equations which demonstrated improved flexibility and robustness compared to previous methods available in the literature.