Gravitational Wave Data¶
Archival gravitational wave data is stored in frame files: containers with some number of separate channels which each store some instrumental, environmental or astrophysical observation (typically, but not limited to, some number of seconds of time series observations). A technical document describing the frame file specification can be found here.
Frame files come in different types at increasing levels of data reduction. Raw frame files contain all instrumental channels. As further calibration measurements are made, these raw data sets are refined to produce frames which contain downsampled and smaller numbers of auxiliary channels. A final level of data reduction is then applied to produce HOFT (as in h(t); “h-of-t”) frames whose channels simply report the instrument state and the gravitational wave strain itself and which comprise the input files to most offline gravitational wave data analysis.
LIGO data sets then, are defined by:
Instrument: e.g., H1, L1 or V1
Frame type: raw, reduced-data-set (RDS) or HOFT which itself comes in 3 levels of calibration refinement: C00, C01 and C02.
Observing epoch: ERX (engineering runs), OX (observing runs) and post OX (inter-run data)
The LSC & Virgo also produce Short Fourier transform files (SFTs) used for continuous wave analyses. The present discussion will refer explicitly to frame files but SFTs follow similar conventions.