LIGO Document P1800038-v1
- Modern gravitational-wave observatories require robust low-
frequency active seismic isolation in order to keep the interferometer operating stably at their ideal operating point at all times. To ensure good seismic isolation from microseismic motion, seismometers are used to measure ground or platform acceleration at frequencies below 0.1 Hz, which makes them susceptible to acceleration from ground tilt, particularly arising from wind-pressure acting on building
walls. Consequently, both LIGO observatories suffered signifcant downtime under wind-speeds above 7 m/s in the first observation run. We describe the use of ground rotation sensors at LIGO Hanford Observatory to correct nearby ground seismometers, thus producing a tilt-free ground displacement signal. Using these signals for feedforward isolation of the optical platforms enabled a robust low-frequency seismic isolation, which allowed gravitational wave observation time under wind speeds as high as 15-20 m/s during the second observation run.
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