| Popis: |
This paper covers the operations and lessons learned for the MOVE-II and MOVE-IIb satellites. Both are 1U CubeSats, with their purpose being hands-on education for students of all technical fields related to aerospace. The hardware of the spacecraft consists of a commercial on-board computer and an electrical power system, while all other systems, including the software, were designed by the student team. The MOVE-II CubeSat was successfully launched on December 3rd, 2018 and remains active in orbit to this day with almost daily commanding. The operations were full of surprises that pre-launch simulations did not foresee. With on-orbit data, we were able to correlate thermal, electrical and attitude dynamics simulations, thus uncovering flaws in former assumptions. We present the evolution of key properties of the spacecraft over its lifetime, such as the internal battery resistance, temperature and hardware defects. Compared to the expected 23°C average temperature, the satellite is quite cold at 3°C average. Furthermore, it shows a tendency to spin up uncontrollably due to a current loop in the solar cell wiring. To replicate the real behavior with simulations, a thermal model and a solar cell wiring current loop were added to the model. We also corrected the internal resistance of the battery in the model from 0.42 Ω to 1.26 Ω and added a temperature dependency to the internal resistance. The tendency to spin up, combined with a tight power budget, has remained a problem since the beginning of on-orbit operations. Although the anomaly shows non-deterministic behavior, regular detumbling maneuvers keep the spacecraft at tumbling rates between 2.5°s−1 and 200°s−1. At low turn rates, we downloaded a significant amount of data from the attitude determination and control system, enabling us to calibrate the magnetometer on ground with data recorded and downlinked over a span of several months. Additionally, we were also able to conduct payload measurements. The MOVE-IIb CubeSat, which launched on July 5th 2019 from the Vostochny Cosmodrome, is a copy of MOVE-II with minor improvements to correct the flaws of its predecessor. Unfortunately, a signal strength of 15 dB less than MOVE-II hindered any practical operations but it has been confirmed as alive in space. As possible causes we analyzed our initial guesses of a faulty deployment of the solar panels and antennae but also a malfunction of the transmitter. With the lessons learned from the MOVE-II/IIb missions, critical mistakes can be avoided for future CubeSat missions. As part of these lessons learned, the most useful and most hindering features of the spacecraft and its ground infrastructure are discussed. Furthermore, the training routine for the Mission Control team and its changes over time are described. The impact of the COVID-19 pandemic on spacecraft operations is also discussed, including lessons learned for future missions. This paper takes a look at the evolution of this mission since 2018. It discusses new findings, degradation of the spacecraft, lessons-learned and operations of the CubeSats. |
