source material is in the Seismic svn at
SeismicSVN/seismic/Common/Documents/T1500610_IFO_Basis_ISI_Motion
The SUSpoint calibration rolls off at low frequency with 4 real poles at 10 mHz - see https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=4553