MODIFY DOPPLER

        LAS\MODIFY DOPPLER [SIGN|*|Value]

    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
    present:

      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
        FIND
        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
        FIND
        FILE OUT new.30m SINGLE ! Uses newest data format
        FOR I 1 TO FOUND
          GET NEXT
          MODIFY DOPPLER 0
          WRITE
        NEXT