Conor, Jeff, Jim

Short summary:
- BRS results in a factor of three reduction in arm-length RMS velocity with moderate wind.
- BRS does no measurable harm at low frequencies.
- In moderate wind, all test-mass sensor corrections show the same RMS in the beam axis.
- Identical broadband sensor correction results in rejection of the primary microseism in arm-length


Moving one step further down the ISI chain, I looked at the sensor correction of the Stage 1 CPSs from the ground STS2s.

To do this, I fetched Ground and BRS data along the beamlines (ITMY_X, ITMY_Y, ETMX_X, ETMY_Y, ETMX_RY, and ETMY_RX) and
filtered the BRS using fetched zpk from Foton to tilt-correct the End-station STS2s (one could also grab the corrected channel now).

The STS2 outputs were then filtered for broadband sensor correction (using a filter very similar to the installed version). This is 
now the signal sent directly to the CPSs and at low frequency (f<0.1 Hz) is a good estimate of the relative motion of the optics.

Attachment 1 shows the CPS sensor correction inputs (in m/s/rtHz) for each test mass in the beam axis. End test masses are shown with and without 
BRS tilt correction. The RMS for each curve from 0.2Hz down is dashed. The RMS velocity is the thing I think we want to minimise at low-f.

What can be seen is that after correction and with moderate wind (~17mph), all channels except ETMX_X have identical RMS. With lower tilt and BRS correction, EX performs better.
Also note that even though there is a spectral 'bump' of BRS noise around and below 10mHz, it makes no difference at all to the RMS. BRS does no 
measurable harm after the Ground-to-CPS sensor correction filters.

Attachments 2 and 3 show the ARM DoF sensor correction, by taking the subtractions: ITMX_X - ETMX_X, and ITMY_Y - ETMY_Y. What this shows is that 
by using identical sensor correction filters, we get some (not insignificant) common-mode rejection of the primary microseism, which is *amplified*
by the sensor correction filter. This rejection will improve as the microseism increases. Additionally, in the important degree of freedom for the
interferometer, there is a factor of 3 improvement in RMS using the BRS.

Attachment 4 is the script used to produce these plots, it should be self-contained apart from an NDS fetch, Foton fetching, and these path entries:
addpath /ligo/svncommon/NbSVN/aligonoisebudget/trunk/Externals/SimulinkNb/Utils/
addpath /ligo/svncommon/SeiSVN/seismic/Common/MatlabTools/