System-Level Aspects and Microvibration Requirements  How is microvibration handled at system level?  System-level microvibration assessments investigate the transmission of very low vibrations by the  spacecraft structure from the disturbance source to the receiver location which might be a sensitive  instrument or payload and, where necessary, to find adequate methods to attenuate the vibrations along  the transmission path. The system-level responsible engineer needs to identify whether the performance  requirements for the instrument or payload are fulfilled under the influence of the relevant disturbance  sources.  System-level microvibration verification is not straightforward. Directly measuring microvibration  susceptibility is difficult. The performances cannot be validated on ground in a representative on-orbit  environment (e.g. zero gravity, in-vacuum, perfectly unconstrained “free-free” condition). In addition, the  background noise, mechanical and/or electrical, may be too high to measure the actual performance.  Therefore, the verification of the microvibration performances is usually based on a combination of  analytical predictions and hardware tests to validate models. Different analysis methods, such as Finite  Element Modeling (FEM), Stochastic Energy Analysis, or a combination, may be appropriate.  Microvibration generally occurs in the mid-frequency range where FEM is inaccurate. Therefore, several  ESA R&D studies have been performed over the last years to further develop these methodologies [3] [4]  [5]. For one of these methodologies, the on-orbit performance of the mission has later been reported [7].  Several challenges occur at system level that can have an influence on the mechanism development. For  example, microvibration emissivity and susceptibility requirements need to be defined at an early stage in  the project while the system-level modelling is still at an immature stage. Microvibration budgeting is  notoriously difficult and over specification can drive the system-level design. In addition, contractual  interfaces can lead to over specification. It is up to the system-level authority to manage these  requirements between all parties as the design matures. However, as mechanism designers it is  imperative to be critical towards the initial requirements and highlight its impact on the design in case it is  a driving requirement as there may be scope for flexibility.  Another difficulty at system level is to appropriately combine the microvibration generated by different  noise sources simultaneously. Generally, a satellite has different sources of microvibration. They all  generate noise in 6 DOF and have varying modes of operation, which can lead to a large number of  cases to simulate. Alternatively, enveloping cases can be devised, although they can in turn lead to  additional, potentially unwanted, conservatism.  Lastly, coupling noise sources to the flexible spacecraft structure changes the dynamic behavior of both  source and structure. This is notable, as microvibration requirements are typically defined on a clamped  interface. Depending on the methodology used at system level, additional requirements can be imposed  on the mechanism designer to accurately model this coupling, such as an accurate model of the  mechanism or a dynamic mass measurement to obtain the accelerance matrix of the mechanism [3].  What does a mechanisms microvibration requirement look like?  Microvibration requirements will either be in force and torque (or PSD) versus frequency, or as a  maximum instantaneous disturbance versus time.   Figure 5 shows an example of a force versus frequency requirement. It is important to note this  requirement is applicable over the entire performance range of the mechanism, for example speed and  position range, as well as for the entire set of potential environments. This requirement is specified for a  clamped boundary condition. This particular requirement is challenging and may not be realistic, as it is  specified over a very large frequency range. Verification of this requirement may be problematic,  especially at the low and high end of the frequency range. The mechanism designer is encouraged to be  critical of such a requirement, and of its prescribed verification approach, as either one may not be  appropriate. This will be illustrated later in this paper when the verification approaches are discussed.  NASA/CP—2018-219887 195