Improvements in strain calibration for the third LIGO science run

Amplitude calibration procedures have been developed by the LIGO Scientific Collaboration (LSC) for use in determining the strain sensitivity of the three LIGO interferometers. These frequency-domain procedures rely on a fiducial calibration taken at a reference time t 0 . The calibration is then propagated to all other times during the science run via calibration factors (denoted by α and β), which are derived from sinusoidal length excitations in interferometer cavity lengths. We briefly review the standard calibration methods that were employed in the first two LIGO science runs (S1 and S2), and then describe improvements in calibration procedures implemented during the third science run S3.