OPS-SAT-1's Final Orbits and Reentry Analysis Amid Mission Extension Attempts

The OPS-SAT-1 spacecraft, an innovative CubeSat mission launched on December 18, 2019 by the European Space Agency (ESA), successfully demonstrated advanced in-orbit technology until its reentry on May 22, 2024. This paper offers a comprehensive overview of the satellite's final weeks of operations. It focuses on onboard telemetry analysis and mitigation against altitude loss. It also discusses operational experiences and lessons learned during the reentry phase to extend the lifetime of future small satellite missions nearing end-of-life. Two Line Element (TLE) data published by the US 18^th Space Defense Squadron revealed significant periods of altitude loss closely correlated with anomalies in the Attitude Determination and Control System (ADCS). To minimize atmospheric drag, the spacecraft primarily used the ‘horizontal pointing’ mode of its fine-pointing ADCS system, which orients the satellite so that the smallest surface area faces the flight direction. However, failures in reaction wheels and control algorithm anomalies degraded performance thus increasing susceptibility to atmospheric drag. With the risk of S-band communication loss due to increasing spin rates, the Ultra High Frequency (UHF) radio system on OPS-SAT-1 was used as an alternative. The amateur radio community was engaged through a public platform to collect and submit UHF packets. This campaign was critical during reentry, as UHF packets provided telemetry and ADCS data. Utilizing the SatNOGS network and a frames-collector portal resulted in extensive ground coverage and invaluable observability of the state of the spacecraft during its final hours. ESA mission operators faced several operational challenges in the weeks preceding reentry. Deteriorating components led to undesired reboots and spacecraft recoveries, along with limited operational windows. The operations team rapidly developed creative solutions, such as On-Board Control Procedure (OBCP) that injected newly defined Fault Detection, Isolation, and Recovery (FDIR) software to prevent impromptu and frequent shutdowns of the onboard computer. Ground infrastructure issues, including system-wide hard disk failures days before the reentry, compounded these challenges. Additionally, the May 2024 solar storms drastically decreased the spacecraft's altitude at an inopportune time.