5. Monitored using Geodetic Grids of length measurement interferometers Approx 800 Grid Line Interferometers

6. Interferometer design means returns Are only -70 dB to -90 dB!

8. Phase unwrapping

9. To measure GLI phase shift… (Reference phase)

10. Minimise Fit Quality Search For minima using MINUIT

13. Think modulo 2 

14. Signal degrading effects in ATLAS Low signals – 800 hungry interferometers to be fed Potential for reflections off many surfaces inside ATLAS Potential for mechanical vibrations (& turbulence)

15. Low signal power

16. Low signal power reduces depth of minimum in fit quality

18. In the presence of spurious reflections… …other minima appear

19. Measurement precision unaffected by spurious reflections 4 spurious Reflectors added

22. Vibration

23. More vibration….. Fit collapses!!

25. To Stop the Fit Collapsing Redefine the WEIGHT in the fit quality to penalise decrease in fringe amplitude New WEIGHT = Fringe amplitude 1/2

28. Is there a problem at 400 nm = 0.5 ? or was it just because two subscans are not enough? Increase peak to peak vibration amplitude to 1.8  m (>2 )and use 6 subscans At 70 Hz

30. Looks very promising…

31. CONCLUSIONS Initial investigations into the effects of low SNR, spurious reflections and vibrations show that… …FSI is a robust technique for precise distance measurements. Further work needed to improve linking and investigate data analysis alternatives and their potential benefits, eg FFT.