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GBM LLT Settings

Note: The GBM team now have other flight software settings that minimize triggering off the same transient event (and the Sun has not been as active), so LLT settings have not needed to be modified since 2012.

In order to support Fermi pointed observations without loss of data quality or excessive data volume, the GBM team is occasionally adjusting the level of the discriminators for the sun-facing NaI detectors (NaI 0 - 5) so that their energy threshold is higher than in nominal mode. During these periods of non-standard Low Level Threshold (LLT) operation, photons with energies below this threshold are not recorded. Data above this threshold energy are nominal, as are all data from the BGO detectors and from NaI detectors 6-11. The energy thresholds of the affected detectors are between 18 - 30 keV, depending on the nature of the pointing. The following table shows the time spans with non-standard LLT settings.

• Column 1 shows the start time of the LLT setting in YYYY-MM-DD hh:mm:ss (UTC).
• Column 2 shows the start time of the LLT setting in Fermi Mission Elapsed Time (MET).
• Column 3 shows the end time of the LLT setting in YYYY-MM-DD hh:mm:ss (UTC).
• Column 4 shows the end time of the LLT setting in Fermi Mission Elapsed Time (MET).
• Column 5 shows the affected NaI detectors.
• Column 6 shows the LLT setting (lowest usable channel out of 4096 raw channels, which are then grouped for our science data products).
• Column 7 tells the user of CSPEC or TTE data the lowest channel which is safe to use with these settings. This is a number between 1 and 128.
• Column 8 tells the user of CTIME data the lowest channel which is safe to use with these settings. This is a number between 1 and 8.
• Column 9 gives the approximate energy threshold of this LLT setting. This varies slightly over the orbit and from one detector to another.
• Column 10 gives the reason for the non-standard LLT setting.

A downloadable version of this table is available in glg_llt.csv and on the Fermi FTP site as /FTP/glast/data/gbm/llt/glg_llt.csv. This file also contains columns showing the start and end times of the LLT settings in YYYY-DOY (day of year) format.

This table will be promptly updated as changes occur. Outside these periods, thresholds are nominal. Please consult the Data Caveats page for using GBM data taken in nominal mode. Questions regarding this and other issues regarding GBM data should be addressed to fermihelp@milkyway.gsfc.nasa.gov.

• Curator: J.D. Myers
• NASA Official: Elizabeth Hays
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• › Page Last Updated: Mon, Mar 11, 2019