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    Change or recompute the Doppler factor according to the CLASS convention
    (-V/c). With no argument, checks whether the header value is  consistent
    with the CLASS value computed according to the date and observatory. The
    checking precision corresponds to 0.6 m/s in V. A logical SIC  variable,
    DOPPLER_PB,  is  set  to  true if the values disagree. If an argument is

      SIGN   change the sign of the Doppler factor but do not recompute it,
      *      compute and update the Doppler factor according to the  observ-
             ing  date-time  and the observatory position (this position can
             be customized with SET OBSERVATORY). This is  suited  when  the
             frequencies in the Class header are described in the observato-
             ry frame. See also undefined Doppler section below.
      0      force the Doppler factor to 0,  i.e.  the  frequencies  in  the
             Class  header are already corrected for the Doppler factor (de-
             scribed in the LSR frame).
      Value  set the Doppler factor to <Value>

    Undefined Doppler factor:
        If the Doppler factor is -1 (no physical meaning), it is  considered
      "undefined"  by Class. In most of the cases, this happens when reading
      old Class data files where the Doppler factor was not available in the
      data  format.  Since then, telescopes are encouraged to use the newest
      data format and write directly a valid Doppler  factor  value  at  the
      time the Class files are created.

        At  read time (GET or other commands reading a spectrum), Class con-
      siders the value -1  as  a  request  to  recompute  automatically  the
      Doppler factor from the telescope position on Earth and from observing
      date-time, assuming the frequencies in the  spectrum  header  are  de-
      scribed in the observatory frame. In other words, Class tries to apply
      implicitly the command MODIFY DOPPLER * on the spectrum.

        The above automatic patch works if the observatory can be recognized
      by  Class from the telescope name (R%HEAD%GEN%TELES). If not, you will
      encounter this error:
        E-COMPUTE_DOPPLER,  Telescope not understood from XXX
      In this case, use the command SET OBSERVATORY beforehand.

        Beware that the automatic patch makes sense if the  frequencies  are
      described  in  the observatory frame. If they are described in the LSR
      frame, the Doppler factor must be forced to 0.

        Finally, if you have an old Class file without the  Doppler  factor,
      it  is advised to copy it to a newer Class Data Format. This will save
      the computed Doppler value directly in the new file instead of  recom-
      puting again and again this factor each time the spectrum is read from
      the old file. If MODIDY DOPPLER * is what you want (frequencies in the
      observatory frame), this will do the job:
        FILE IN old.30m
        FILE OUT new.30m SINGLE ! Uses newest data format
        COPY                    ! Implicit MODIFY DOPPLER * on each spectrum
      If you need a custom Doppler value (most probably 0), use:
        FILE IN old.30m
        FILE OUT new.30m SINGLE ! Uses newest data format
        FOR I 1 TO FOUND
          GET NEXT
          MODIFY DOPPLER 0

Gildas manager 2020-04-07