%Id
        $Id: offstop.doc,v 1.11 1999/10/28 06:15:00 sanjay Exp sanjay $
       
 offstop
		
	Program for offline fringe stopping.

        A new antenna table consisting of co-coordinates and fixed
        delays can be provided.  The differential phase due to
        geometric phase + fixed delay between the antenna table in
        the LTA header and the user supplied table is applied to the
        data.

        This differential phase Phase_i for i th. frequency channel of
        width dNu Hz, for a baseline composed of antenna number A1 and
        A2 is calculated as follows:

           Tau_g_old = (w_old[A2] - w_old[A1])/SPEEDOFLIGHT;

           Tau_g_new = (w_new[A2] - w_new[A1])/SPEEDOFLIGHT;

           Tfix_old  = (AntTab_old.FixedDelay[A2] -	
                        AntTab_old.FixedDelay[A1])/SPEEDOFLIGHT;

           Tfix_new  = (AntTab_new.FixedDelay[A2] -	
                        AntTab_new.FixedDelay[A1])/SPEEDOFLIGHT;
        
           Ramp_old  = 2*PI*dNu*(Tau_g_old + Tfix_old);

           Ramp_new  = 2*PI*dNu*(Tau_g_new + Tfix_new);

           Phi0_old  = 2*PI*[Tau_g_old*(LO1_old + LO4_old) + Tfix_old*LO4_old];

           Phi0_new  = 2*PI*[Tau_g_old*(LO1_new + LO4_new) + Tfix_new*LO4_new];

           Phase_i   = -[(Ramp_new - Ramp_old)*i + (Phi_0_new - Phi_0_old)]


       where Tau_g is the geometric delay, Tfix is the fixed delay,
       Ramp is rate of change of phase as a function of frequency and
       Phi_0 is the phase rotation required for the zeroth frequency
       channel.  This will be the additive constant phase for all
       channels (though the zeroth channel is really auto-correlation
       and therefore real!).


 Author
 
        Sanjay Bhatnagar (sanjay@ncra.tifr.res.in)


 in (default=stdin)

        Name of the input LTA file.


 out (default=stdout)

        Name of the fringe stopped output LTA file.


 antfile (default=No table supplied)
        
        Name of the file with antenna co-ordinates to be used for
        fringe stopping.  By default, this co-ordinate table will
        replace the one present in the input LTA file.
      
        If no antfile is supplied, the new antenna table is
        initialized to the old antenna table (from the LTA heder).


 skip (default=0)

        Number of initial scans to skip.


 lo1 (default=Value in the SCAN header)

        The GMRT first LO value in Hz.  If not set, this value is read
        from the FIRST_LO keyword in the scan header.  If set, this
        value replaces the FIRST_LO keyword value in the scan header.
        As a result, if this value is supplied, the program can handle
        only those LTA files which have the same first LO setting for
        all scans.

        The value of the RF keyword in the scan headers will be replaced 
        with the value lo1+lo2.


 lo4 (default=Value in the SCAN header)

        The GMRT forth LO value in Hz.  If not set, this value is read
        from the BB_LO keyword in the scan header.  If set, this value
        replaces the BB_LO keyword value in the scan header.  As a
        result, if this value is supplied, the program can handle only
        those LTA files which have the same fourth LO setting for all
        scans.

        The value of the RF keyword in the scan headers will be replaced 
        with the value lo1+lo2.


 mode (default="New")     ["dbg" class keyword]

        Mode in which the antenna co-ordinate table given by
        "antfile" is to be used.

          mode="New" implies that the new table will replaced old table
               in the output LTA file.

          mode="Add" implies that the user supplied table will be
               added to the table in the LTA header to produce the new
               table.  This new table will replace the table in the
               header of the output file.
         
        In all cases, the delta phase computed from New-Old values
        will be applied to the data.


 toff (default=0)     ["dbg" class keyword]

        Time offset in hours to be added to the time-stamp in the LTA
        data records before computing fringe parameters.