The Spectrometer FITS keywords for the STATE table conforms to the definition for common FITS headers as described in GBT Software Project Note 4.0 ``Device and Log FITS Files for the GBT.'' To reiterate from that document, value 0 indicates signal and non-zero indicates some reference; and value 1 indicates CAL signal was on and value 0 indicates CAL signal off. This table is used to record the commanded switching signal setup, but will not normally be needed for calibration of spectrometer data, as all the information needed for calibration will be measured and placed in the DATA table. An example STATE header, plus ASCII listing of typical table data, is given below:
STATE Binary Tables Extension: Header
-------------------------------------------------------------------------------
XTENSION= 'BINTABLE' / binary table extension
BITPIX = 8 / 8-bit bytes
NAXIS = 2 / 2-dimensional binary table
NAXIS1 = 24 / width of table in bytes
NAXIS2 = 4 / number of rows in table
PCOUNT = 0 / size of special data area
GCOUNT = 1 / one data group (required keyword)
TFIELDS = 4 / number of fields in each row
TTYPE1 = 'BLANKTIM' / label for field 1
TFORM1 = '1D ' / data format of field: 8-byte DOUBLE
TUNIT1 = 'SECONDS ' / physical unit of field
TTYPE2 = 'PHSESTRT' / label for field 2
TFORM2 = '1D ' / data format of field: 8-byte DOUBLE
TUNIT2 = 'NONE ' / physical unit of field
TTYPE3 = 'SIGREF ' / label for field 3
TFORM3 = '1J ' / data format of field: 4-byte INTEGER
TUNIT3 = 'T/F ' / physical unit of field
TTYPE4 = 'CAL ' / label for field 4
TFORM4 = '1J ' / data format of field: 4-byte INTEGER
TUNIT4 = 'T/F ' / physical unit of field
EXTNAME = 'STATE ' / name of this binary table extension
NUMPHASE= 4 / Number of Phases if only Internal Switching Sig
SWPERIOD= 4.000E+00 / Switching period
MASTER = 'Spectrometer' / Switching Signals Master
END
-------------------------------------------------------------------------------
STATE Binary Tables Extension: Data (complete)
-------------------------------------------------------------------------------
BLANKTIM PHSESTRT SIGREF CAL
1D 1D 1J 1J
SECONDS NONE T/F T/F
1 9.961472000000E-02 0.000000000000E+00 0 0
2 9.961472000000E-02 2.500000000000E-01 0 1
3 9.961472000000E-02 5.000000000000E-01 1 0
4 9.961472000000E-02 7.500000000000E-01 1 1
-------------------------------------------------------------------------------
Note that this table describes the Spectrometer's internal configuration of switching signals, it is name STATE in accordance with common M&C usage and convention in the other GBT backend FITS files. It is possible that the Spectrometer is being driven by external signals (or a combination of both!), and so the actual switching signals in use are given in the ACT_STATE table described below.