TTYPE1  = 'START   '           / start time
TFORM1  = '1D      '           / data format of field
TUNIT1  = 's       '           / physical unit of field
TTYPE2  = 'STOP    '           / stop time
TFORM2  = '1D      '           / data format of field
TUNIT2  = 's       '           / physical unit of field
EXTNAME = 'STDGTI '            / Extension Name
TELESCOP= 'BURSTCUBE'          / Telescope mission Name
INSTRUME= 'string'             / Instrument name
DATAMODE= 'CBD     '           / Data mode
OBS_ID  = 'yymmdd'             / Observation ID
MJDREFI =                59215 / MJD reference day 01 Jan 2021 00:00:00
MJDREFF =  0.00080074074074074 / MJD reference (fraction of day)
TIMEREF = 'LOCAL   '           / Reference Frame
TASSIGN = 'SATELLITE'          / Time assigned
TIMESYS = 'TT      '           / Time system
TIMEUNIT= 's       '           / Time unit for timing header keywords
CLOCKAPP=                    F / If clock corrections are applied (T/F)
DATE-OBS= 'isodate'            / Start Date
DATE-END= 'isodate'            / Stop Date
TSTART  = 1.0                  / Start Time
TSTOP   = 1.0                  / Stop Time
HDUCLASS= 'OGIP    '           / format conforms to the OGIP standard
HDUCLAS1= 'GTI     '           / hduclass1
CREATOR = 'string'             / Software that create 1st the file
PROCVER = 'string'             / Processing version
CALDBVER= 'string'             / CALDB version
SEQPNUM = 1                    / Number of times the dataset has been processed
ORIGIN  = 'GSFC    '           / Origin of the FITS file
DATE    = 'isodate'            / File creation date
CHECKSUM= 'string'             / data unit checksum updated date
DATASUM = 'string'             / HDU checksum updated date