Attitude Control on TET-1 and BIROS - Experiences from the FireBird Mission and End of Life Operations

Kurzfassung

The satellite TET-1 (Technologie-Erprobungsträger 1) was launched in 2012 as part of the DLR On-Orbit Verification program (OOV). Together with the second satellite BIROS (Bi-spectral InfraRed Optical System), which was launched in 2016, they represented the space segment of the scientific FireBird mission. The primary goal of the mission was to perform Earth observations in the infrared spectrum for high-energy events such as forest fires and to contribute valuable data to the scientific community and the International Charter on Space and Major Disasters. After seven years of successful operations, the FireBird project ended in December 2020. Both satellites are now monitored and operated by a significantly reduced operations team until their re-entry into the Earth’s atmosphere. The AOCS subsystem, like the satellite as a whole in project Phase E, has only two physical interfaces to the ground segment: telemetry and telecommands. Safety, reliability and efficiency in ongoing mission operations depend on these two interfaces. It must be assured at all times that the AOCS subsystem can be safely transitioned from one defined state to another. This paper shows on selected examples of TET-1 and BIROS, different aspects of the performance of the attitude control system as well as the operational challenges over the years in Phase E. The long-term impact of a permanent reaction wheel failure which occurred at an early stage of the TET-1 mission is discussed. In order to estimate the general degradation of the AOCS units, the amount and distribution of autonomous unit power cycles as part of the FDIR system with respect to the geographical location, space weather and operational as well as non-operational uptime is analyzed. Following this, the interdependency between the orbit and the operational temperature of the star tracker heads with respect to the amount of valid measurements is outlined. The operational effects and limitations due to the loss of the main Inertial Measurement Unit in 2019 as well as the degradation of the backup unit is described and the resulting consequences are discussed. Finally, the performance of the GPS receivers and the long-term degradation is shown based on the ability to provide a valid solution. In a subsequent section, the flyby and communication experiments performed in 2019 and 2020 between BIROS and the pico-satellite BEESAT-4 are described and the commanding approach for achieving the pointing accuracy and results are presented. In the final section, a brief conclusion and outlook for the remaining years are given.