I measured the MC cavity lock by adjusting the ifr frequency and measuring the transfer functions using an SR785. The source was injected into Exc A of MC common mode board and teed to ch1, meanwhile ch2 was connected to either IMON of REFL9 (1*FSR) and REFL45 (5*FSR). Data.txt is the data collected from REFL9 and Data_2.txt is that collected from REFL45. ArmCavityLength.m is the script that determines the length given the zero crossing of the projection. The attached plots show the results with a linear regression included.

The cavity length is determined to  be

• From REFL9 data:   L = 16.473612m ± 2um assuming 1Hz accuracy.
• From REFL45 data: L = 16.4736111m ± .4um assuming 1Hz accuracy.
• Averaging: L = 16.473612m ± 1um