Author: Natalia Zotov
PT: Peak-Trough, otherwise known as peak-to-peak, values.
PTmon is a glitch monitor, designed to find glitches
onany of LIGO's auxiliary channels. It's primary purpose is to help
establish vetoes for inspiral candidates or unmodeled bursts appearing
in the 'gravity wave channel', AS_Q.
The raw time series is first downsampled to some frequency above the Nyquist frequency of the desired pass band. (This is done in PTmon using the DMT class DecimateBy2.) Besides speeding the filtering, decimation allows for steeper shoulders on the filters, with more cleanly defined pass bands. The decimated series is filtered using FIR filters, chosen partly for speed. Each type of channel needs to be studied, and typical resonances or glitching frequencies found to help determine what frequency bands should be monitored. There are a number of passbands of interest for the 16 kHz channels.
Dotted lines represent 3- and 4-sigma thresholds
(2) Alternatively, a glitch
must cross the 3 sigma threshold at least, say, four times within the same
time window. The choice of window will depend on the expected frequency
of the glitch.
Dotted line represents 3 sigma threshold
After it has been determined that a glitch has occurred, the setting of a trigger is delayed until a specified number of PT values has been accumulated. (The number depends on the downsampled channel rate and likely frequency.) The values are used to estimate the glitch frequency, which will only be aproximate, and to determine the peak amplitude, which is used to calculate the signal-to-noise ratio.
Glitches with snr < 5 are currently not reported.
This is partly to keep the number of triggers from becoming unreasonable
during online running, but also because it was found when running the monitor
on simulated data, that a 16KHz white noise background generated triggers
up to around 5 sigma just from random fluctuations, whereas all triggers
above 6 sigma corresponded to injections.
The following information is contained in PTmon triggers:
start_time, start_time_ns, size (which contains snr), and frequency.
A summary report is posted to the monitors' status page on 'blue'
at Hanford. This lists the channels monitored, the total number of
glitches, and the glitch rate for each channel. The glitch rate is
the only trend being written currently.
Currently all parameters are set internally, and the filters are set according to channel type. The author plans to move the selection of filters to the configuration file as soon as possible, to accomodate investigations on the fast channels using a variety of frequency bands.