Part of the Assembly Validation testing of HAM-ISIs (that happens in the staging building) consists in taking power spectra with the ISI tilted. It allows us to check that the sensors, especially the seismometers, work in tilted position as well. To do so, spectra are taken with a 10kg mass set at each of the 6 corners of the optical table (Locations A to F in ISI_Tilted) 

This test revealed that GS13-H1 (Pod #94) was not functioning correctly. Symptoms are the same as observed on previous failing GS13s. The response of the instrument drops dramatically below 60Hz (ISI_Not_Tilted) which is characteristic of a stuck seismic mass.

This instrument was tested OK at reception from LLO. It is very likely that the tilting of the ISI induced the sticking of the seismic mass.

We have one last spare horizontal GS13 (Pod #18). It will be installed as replacement today. Testing of this unit will start over with sensor spectra, as soon as it is installed.