A 3 mm lateral shift of the NE mirror is expected to induce a change of the near NCal lines of 0.58%. The ratio of the south and north NCal lines should therefore change by 1.16%. The plot is showing the NSN and NNN NCal lines amplitude (averaged over 1000 seconds) and their ratio. Although, it’s a bit noisy, we can see a slight increase of the NSN line and decrease of the NNN line between 13 and 18pm. Their ratio is going down at the same time by about 1% as expected.

The fact that NSN, located on the east side of the mirror, is going up means that the NE mirror was shifted toward east.