Reconstructing the calibrated strain signal in the Advanced LIGO detectors
Advanced LIGO's raw detector output needs to be calibrated to compute dimensionless strain h(t). Calibrated strain data is produced in the time domain using both a low-latency, online procedure and a high-latency, offline procedure. The low-latency h(t) data stream is produced in two stages, the first of which is performed on the same computers that operate the detector's feedback control system. This stage, referred to as the front-end calibration, uses infinite impulse response (IIR) filtering and performs all operations at a 16 384 Hz digital sampling rate. Due to several limitations, this procedure currently introduces certain systematic errors in the calibrated strain data, motivating the second stage of the low-latency procedure, known as the low-latency gstlal calibration pipeline. The gstlal calibration pipeline uses finite impulse response (FIR) filtering to apply corrections to the output of the front-end calibration. It applies time-dependent correction factors to the sensing and actuation components of the calibrated strain to reduce systematic errors. The gstlal calibration pipeline is also used in high latency to recalibrate the data, which is necessary due mainly to online dropouts in the calibrated data and identified improvements to the calibration models or filters.
© 2018 IOP Publishing Ltd. Received 27 October 2017, revised 12 March 2018; Accepted for publication 13 March 2018; Published 4 April 2018. The authors would like to thank Chris Wipf, Evan Hall, Jolien Creighton, Patrick Brockill, John Zweizig, Kipp Cannon, Bruce Allen, Chris Pankow, Chad Hanna, and Les Wade for helpful discussions and feedback while developing the methods discussed here. The authors were supported by National Science Foundation grants PHY-1607178, PHY-1607585, and PHY-1506360. LIGO was constructed by the California Institute of Technology and Massachusetts Institute of Technology with funding from the National Science Foundation and operates under cooperative agreement PHY-0757058. This paper carries LIGO Document Number LIGO-P1700236.
Submitted - 1710.09973.pdf