Spacecraft Micro-vibrations Analysis for Optical Communication Payloads

Current navigation systems on satellites base their measurements on synchronized signals from MEO constellations. The employment of Free-Space Optical (FSO) Communication for next-generation KEPLER navigation constellation would bring several advantages, such as precise global synchronization, sub-millimetre ranging accuracy and high data-rate communication to allow a fast measurement exchange. The core of this technology is represented by optical terminals, which are characterised by extremely narrow laser beams, with a divergence in the order of micro-radians. Hence, pointing requirement is critical for the payload performance. Extreme levels of required pointing accuracy bring new challenges in the design of the spacecraft, such as micro-vibrations management. Micro-vibrations are low-level disturbances affecting the spacecraft during its nominal on-orbit operations, occurring in the 1 Hz - 1 kHz range. Engineering and research teams already demonstrated the possibility of solving this problem. Since the first experiments with ESA’s GEO optical data relay ARTEMIS spacecraft, progresses for a deeper knowledge about space micro-vibration environment have been made, which led to more and more reliable isolation systems and approaches. Nowadays, ESA’s EDRS spacecrafts represent a corner stone for FSO Communication systems. In the past two decades, several solutions have been investigated and proposed to overcome the micro-vibrations criticality. Although, this spread of different concepts to address the same problem highlighted the necessity of identifying a baseline when starting a design of a new spacecraft with a payload susceptible to this low-magnitude mechanical disturbances. Lack of a common ground in terms to approach this problem led to the analysis in this thesis. The thesis first provides an overview on statistical elements to investigate the stochastic nature of vibrations, along with reports from on-board measurements of spacecraft micro-vibrations. The so-called “ESA model” for micro-vibrations is analysed and reproduced through a simulation, that was used for an experiment between two optical terminals in the near-field range in order to investigate effects of the beam jitter at the receiving terminal. Afterwards, several isolation systems already proposed in literature are discussed, encompassing both isolation and compensation systems, addressing platform-to-payload or disturbance-source-to-platform mechanical configurations. In the final chapter, pointing requirements and optical terminal characteristics are entwined to assess the micro-vibration level the payload could withstand, to define a micro-vibration budget for the spacecraft. Hence, state of the art technologies for vibration rejection and models for micro-vibration budget are presented, with a focus on optical communication payloads and upcoming KEPLER constellation. Furthermore, first iteration of an experiment to investigate jitter effects through a simulated vibration signal is discussed.