How to Work with Segments
LIGO data is not always available. In addition, there are times when data
may be available, but of poor quality. For these reasons, most work with
LIGO data begins with a "segment list", specifying the times of data
to be analyzed.
Finding Segment Lists with Timeline
application can be used to find and download segment lists. For example, a segment list of times where H1 data passed the Burst Group's category 2 data quality
checks might look like this:
Burst CAT 2 segment list
Each line of the text file corresponds to 1 segment of data that passed
the data quality checks.
Shown are the GPS start time of the segment, the GPS end time, and the duration (in seconds) of the segment.
We can download both the data and the segment list for this period of time:
- Navigate to the S5 Timeline Query Form
- Enter a start time of 844605900 and a duration of 10000
- Choose the "Plot" option
- Choose H1_BURST_CAT2
- To download the segment list, click the link under "Download Segment List". Save it as "h1_burst_cat2.txt"
- To download the data, click the link that says "Get Strain Data"
Once you have the data and the segment lists, you can use the
GWOSC example API
to read in the segment list and the good data.
import readligo as rl
segs = rl.SegmentList('h1_burst_cat2.txt')
for (begin, end) in segs:
strain, meta, dq = rl.getstrain(begin, end, 'H1')
#-- Your analysis code goes here
Constructing Segment Lists from Data Files
Segment lists can also be constructed directly from GWOSC data files. For details, see
documentation of the GWOSC Example API
Introduction to LIGO Data Files
tutorial. If your prefer to
write your own segment API, you may want to see this
Matlab example script
or read about the structure of GWOSC data files
Types of segments
LIGO data files contain a variety of segment types. For GWOSC, the segments called DATA
indicate when strain data
is available. Then the segments CAT1, CAT2, CAT3 indicate problems with the data (CAT1 being worst).
There are also segments of times when hardware injections are not occuring. For more information,
see here for the S5 and S6 runs
and here for the O1 run