Summary:
Calibration measurements utilizing Pcal to DARM transfer functions can be impacted by non-unity gain when making corrections for the modeled frequency response of the AA filtering. At LHO, the ER10/O2 model had an analog AA transfer function with non-unity gain based on the LTI measurements from ER8 (at LLO, the LTI model for ER10/O2 was normalized to 1). The analog AA model at LHO has a gain of ~0.99. Below we detail the impact on sensing function and actuation coefficients. In summary, the sensing function gain is ~1% larger than originally modeled, and the actuation coefficients are ~1% smaller. This would imply that the inspiral range is ~1% higher than currently predicted.

This new understanding means that the analysis code needs to be re-run for the optical response parameters and actuation coefficients. The front-end calibration will need to be updated, and, finally, the GDS pipeline needs new filters generated and installed.

Details:
The calibration of the Pcal channels (ex: H1:CAL-PCALX_RX_PD_DQ) determines the watts reflecting from the ETM per count of the channel at DC. Whatever gain of the analog AA, this is already accounted for in this calibration procedure. This gain is implicitly accounted for when the value of the calibration is installed in the front-end filter module.

A Pcal to DARM transfer function was previously understood as follows (note that PD calib., susnorm, m/N coeff are taken care of in the front end and (1+G)*(1/f^2) are taken care of in analyzing the measurements):

DARM   (IFO opt. resp.) (OMC DCPD TF) (AA(a) freq. resp.) (AA(a) gain) (AA(d) TF)
---- = ----------------------------------------------------------------------------------------------
PCAL   (PD calib.) (AA(a) freq. resp.) (AA(a) gain) (AA(d) TF) (susnorm) (m/N coeff.) (1 + G) (1/f^2)

where AA(a) is the analog AA, AA(d) is the digital AA, and "freq. resp." means the normalized transfer function, and G is the open loop gain. In the above (incorrect) understanding, the AA(a) and AA(d) terms cancel. The reason this is incorrect is that the AA(a) gain has an inverse in the PD calibration factor. So the real (correct) Pcal to DARM transfer function is:

DARM   (IFO opt. resp.) (OMC DCPD TF) (AA(a) freq. resp.) (AA(a) gain) (AA(d) TF)
---- = --------------------------------------------------------------------------------- .
PCAL   (PD calib.) (AA(a) freq. resp.) (AA(d) TF) (susnorm) (m/N coeff.) (1 + G) (1/f^2)

Thus, to isolate the IFO optical response, we need to divide out the modeled AA(a) gain. Since the gain is ~0.99, then the gain of the optical response should go up by ~1%.

The above equations are laid out in a graphical subway map schematic in G1501518-v14.

The actuation coefficients will also be impacted by this, although the coefficients will be multiplied by the AA(a) gain so that the overall DARM OLG remains unchanged.

I have pushed the changes to the DARM model code and scripts that account for this. Specifically:
computeSensing.m (r4025)
create_partial_td_filters.m (r4026)
create_full_td_filters.m (r4027)
fitDataToC_20161116.m (r4028).

Re-running analysis of optical response parameters requires re-running the fitDataToC_20161116.m script first (with printing data to file), then running the fitCTF_mcmc.m script. Re-running analysis of actuator coefficients requires re-running actuatorCoefficients_Npct.m (with printing data to file), then re-running fitActCoefs_Npct.m.

Hopefully this takes care of everything. Unfortunately, I cannot verify these changes with Matlab because I have no way of running Matlab offsite (need a network license). :(