Reconstructing the calibrated strain signal in the Advanced LIGO detectors

TL;DR

This paper discusses the methods used to calibrate the raw data from Advanced LIGO detectors into accurate strain signals, highlighting the two-stage process involving low-latency and offline calibration to correct systematic errors.

Contribution

It introduces the detailed calibration pipeline used in Advanced LIGO, including the front-end IIR filtering and the gstlal FIR correction stages, improving the accuracy of strain data.

Findings

The calibration pipeline effectively reduces systematic errors in strain data.

The two-stage calibration process improves the accuracy of gravitational wave signals.

Recalibration addresses online dropouts and model improvements.

Abstract

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 16384 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.