As observed a few times, in certain environmental condition (strong wind or earthquake) the lock is lost due to the saturation of NE/WE MAR_Z_CORR. Having the availability of NI/WI MAR actuator, with range a factor of 4 larger, the best thing to do in that condition is to move the MAR actuation from NE/WE to NI/WI.

Today a short test has been performed, doing that in LN3 (in quiet environmental condition). The aim was to check the impact on ITF stability and on hrec. In Fig 1 we see that when the correction is moved to NI/WI, apparently nothing change on NE/WE. This is due to a modification of NE/WE probes: even if the correction is actually switched to zero, it is still visible because now the signal sent to the probe is the one before the switch (if the switch is 1, the probe is taken in the same place as before, after the switch). So for hrec everything looks unchanged. In fact a small difference is there: the calibration lines are not anymore in the probes, so the line subctraction does not work anymore (fig 2). A second, more relevant, difference is that the calibration of NI/WI actuators cannot be exactly the same of NE/WE. In particular, there is a relevant difference in WI reallocation filter (a notch at 6.96 Hz, added in order to avoid an oscillation during the lock acquisition). This fact has for sure an impact on the reconstruction, at least at low frequency. An effect, maybe due to this, is visible in DARM at 20 Hz (fig 3). This is just to say that, in case the INPUT reallocation is required to avoid an unlock, it is correcto to put the ITF in adjusting mode.

The test has been performed also to check if NI/WI actuation balancing is good enough. According to DIFFp_TX/TY accuracy (fig 4, fig 5), it looks like that.

Considering the test successful, a procedure to move to NI/WI MAR can be released.

In case of strong earthquake (typical risky magnitude is 6.5, but even lower if the epicenter is not very far), when all the SUSP flags start to be red and BNS Range is going down, the operator can decide to do the following actions:

1) set Adjusting Mode (but a more appropriate flag should be defined);

2) set gNAME: SAT_NI_MAR_ENBL = 1

3) set gNAME: SAT_NE_LOWN_ENBL = 1 and SAT_WE_LOWN_ENBL = 1 with a ramp (5 s)

Typically in half an hour, if we are lucky and the ITF is still locked, the situation should get back to normality; the full recovery of the range could be a good indicator. At this point, the operator can set the standard configuration:

1) set gNAME: SAT_NE_LOWN_ENBL = 2 and SAT_WE_LOWN_ENBL = 2 with a ramp (5 s) (please pay attention to the correct value: it is not 0)

2) set gNAME: SAT_NI_MAR_ENBL = 0

3) set Science.