heavier payloads. This means the usable frequency range depends on the payload and varies on  a case by case basis. The upper value of the usable frequency range is lower still than the natural  frequency of the table with payload, as the upslope of this mode will skew results. For the RCF, it  means the usable frequency range will be typically up to 900 Hz.   •Inappropriate modal design of mechanical ground support equipment (MGSE) can compromise data. The MGSE should be designed so its main modes are outside the measurement range of interest where possible. This may be challenging in certain cases as microvibration facilities have a limited mass capability. The combined mass of stiff MGSE and the unit under test may have an impact on the modes of the test facility itself as discussed above. •Gravity offloading equipment may impact the microvibration measurement. For example, microvibration isolation systems can be designed to be very compliant. Therefore, they may require gravity offload systems to operate. These offloading devices themselves are usually also designed to be very compliant. Care needs to be taken not to influence the performance of the isolation system with the offload system. A second example of impact by gravity offload systems will be discussed in the case study below. •Electrical ground support equipment (EGSE) can create electrical noise affecting the data. The EGSE used for the test facility itself are usually designed to avoid these effects, but the EGSE needed to run the mechanism may cause interference. If it is not possible to take this into account in the design stage of the EGSE, care should be taken when interpreting the results. It is common to see spikes at 50 Hz (in Europe) and its higher harmonics related to mains electrical noise. •Careful selection of test durations and post-processing methods is required. Specifically, obtaining valid low frequency data leads to long test durations. •Appropriate run-in of mechanisms may be required in order to measure stable performance, rather than start-up performance. For example, fluid lubricated mechanisms may require some time to clear out excess oil from the bearing races after a period of standstill and to reach a stable temperature which impacts the viscosity of the lubricant. On the other hand, mechanisms which are used intermittently may actually require testing after a long stand still. •Plan for microvibration tests throughout the qualification campaign and after the life test prior to mechanism strip-down for sensitive applications. Where possible, it is even recommended to test in between different mechanical tests (sine vibration, random vibration, shock). In case a significant performance degradation is measured, it will be easier to trace it back to a specific test environment and potentially negotiate a relaxation of this requirement or design in specific protection for the mechanism. •Assess other environmental impacts such as temperature and vacuum where possible. Often the impact is smaller than the impact of mechanical environmental tests. However, it is recommended to assess these effects on at least one unit. •Test ALL units. Small build to build variability can lead to significant microvibration performance changes, as demonstrated in Figure 1. Case study: Verification by test for a large rotating scanner  In order to demonstrate some of the issues with verification by test, one specific case study is discussed  here. The force versus frequency requirement shown in Figure 5 is actually applicable on a large rotating  scanning instrument. Verification by test is specified for this requirement. The scanner mass is   200-300 kg, with a rotating mass of more than half of the total mass. The scanner is designed to rotate at a constant speed of <1 Hz. When comparing the requirement to the specification of state of the art test facilities (Figure 12), several  observations can be made. Firstly, the instrument is too large for the entire instrument to be tested. A  larger facility may need to be developed in order to test the complete instrument. However, the scan  mechanism in the instrument is compatible with these two facilities.   NASA/CP—2018-219887 201