jeffrey.kissel@LIGO.ORG - posted 15:57, Thursday 18 February 2016 - last comment - 15:39, Friday 19 February 2016(25624)
H1 Buried GND STS may not be the Silver Bullet; Internal and External STS Signals Comparable at 25-30 mph
J. Kissel, J. Warner Excited to use the buried STS (see LHO aLOG 25574), Jim had switched over to using the recently-moved-to-20 [m]-from-the-building STS for sensor correction (coupled with some other new configuration changes he's trying out; see LHO aLOG 25623). He was getting poor results when comparing internal vs external sensor correction use, so I've compared the times yesterday when we had great coherence at 10 [m] and 0-5 mph winds against today, when the STS is at 20 [m] away and there are consistent 25-30 mph winds. The message: both the internal and external ASDs, in X, Y, and Z, are comparable in amplitude and yet incoherent at this location and these 25-30 mph wind speeds. That means the buried STS is not so promising for sensor correction use at this level of wind. One can compare this to results originally presented by Robert at the 40 [m] location in ~15 mph winds; see LHO aLOG 19210. You'll notice that in both of these data sets, the contrast in amplitude difference between windy and not windy is "better" in the X DOF (perpendicular to the arm) than in the Y DOF (parallel with the arm; what we're trying to improve). We shall continue to take a smattering of data points to gather statistics at all wind speeds in this location. For the impatient and saddened -- recall that we have or will employ three different methods to attack wind at EY: (1) This buried, external STS [in the testing phase now] (2) A new BRS [scheduled for delivery in mid-March] (3) A wind screen system [working on getting funding] We're trying all three just in case one actually works. Let's hope one does!
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J. Kissel More data on comparing the external, buried seismometer: - 20 [m] from the building, at 15-20 [mph] - 20 [m] from the building, at 5-10 [mph] - 20 [m] from the building, at 0-5 [mph] all of which are compared against the 0-5 [mph] data while the STS was 10 [m] away from the building. Conclusion -- there's really no substantial difference between the signal in these STSs at any wind speed between 0 to 30 [mph]. Bummer. Also note that the coherence for the no wind data at both 10 [m] and 20 [m] locations shows that the external instrument is behaving just as well after Robert moved it from 10 to 20 [m]. Perhaps we should move back to the 40 [m] location? That's where we'd seen the most dramatic results. Recall, the distance from the building is a balance between decoupling from building tilt (better further away) and frequency band where coherent (better closer to the building). At 40 [m] we were worried that we didn't get coherence (under no-wind conditions) out past ~0.5 [Hz]. But we'd only need coherence out to that high a frequency if we stick with the current 0.5 [Hz] narrow-band sensor correction; we don't have to -- we can explore moving to a lower frequency, broad-band sensor correction and re-allocate the feedback blend filters if need be. The message -- we need to do more work if we want this external STS to do us any good.
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