First attachment shows the ISS 1st loop sensors and second loop sensor when 1st loop was engaged but 2nd loop open.
Solid lines are now, and dashed lines are from Friday (the same reference time used in alog 29778).
Even though the ISS state is the same between solid and dashed, 1st loop out of loop sensor (PDA) is much larger than it was on Friday.
It's not like the loop was crazy, as the in-loop sensor (PDB) didn't change, and the second loop sensor didn't change either.
The second attachment is a one-day trend of pointing into the ISS 1st loop QPD (ch1 and 2), various environment signals (ch3 to 11) and PMC trans.
Pointing changed a bit, mostly in DX (this is probably YAW in PSL speak) though PMC transmission increased.
ISS QPD spectrum didn't change much (third attachment).
It's as if the jitter coupling for PDA changed.
Confession of two who remain unnamed.
Two gentlemen fessed up that they did something and forgot to alog in the heat of commissioning.
Because the jitter coupling of the EOM mon and the second loop sensor are almost the same OR very small, placing the 1st loop sensors in the same path as the EOM/2nd loop would make it better in that all relevant sensors see the same thing.
So they T-ed the EOM mon cable at the PSL rack, made an ad-hoc whitening that mimics the PDA whitening (using two SR560?), and plugged that into the 1st loop interface instead of PDA. In the heat of commissioning (and the possibility to lose lock to switch the cable back), the setup was left in place.
Attached is the 50W spectrum with 2nd loop ON at about 6kHz or so UGF as of now.
As expected, "PDA" signal, which is actually the EOM mon, comes down much smaller than PDB which is the 1st loop in-loop sensor, so the theory holds true. It's not clear if the difference between the second loop sensor and "PDA" is due to whitening mismatch or something else.
This test setup (collection of 3 SR560s) is still in place, but the output is no longer connected. The PDA cable is back in its usual place.