PHASE


PHASE replaces the  phases  in  a  VLB  MERGE-format  file  with  ones
consistent  with the closure phases and a model.  Both MERGE-formats 1
and 2 are accepted.  The input file must contain closure phases.   The
algorithm  used  is that of Readhead and Wilkinson (1978:  Ap.J.  223,
25-36):  each baseline visibility phase is replaced by either a  model
phase  or  a  value computed from model phases and the closure phases.
If there are N antennas, then (N-1) visibility phases are  taken  from
the  model  and  the  remainder  of the N(N-1)/2 visibility phases are
computed from the closure phases.  The user  must  supply  a  list  of
baselines  which gives the "preference order" for use of model phases;
because some data may be missing, the program does not necessarily use
the  same  set of (N-1) baselines at each time.  Visibility amplitudes
and closure phases are not altered.

     The phases calculated by PHASE can be examined by use of LIST  on
the output file (specify "PHASE" in the control parameters).  They can
also be plotted, using VISPLOT (specify "PHASE"); the running time  of
PHASE  is  proportional  to  the  number  of  baselines, the number of
records in the data file and the number of components  in  the  model;
the time can be reduced by averaging the data before running PHASE.

     Usually AMPHI should be used rather than PHASE.  One  case  where
PHASE  must  be  used,  however,  is  when  the  visibility phases are
corrupted, e.g., by an incoherent average.


Example

$ PHASE
  Input =  [TJP]MOD5.VLB
  Output = PHAZED.TMP
  Modfile = [TJP]MOD5.DAT
  base = 12,13,23,14,24,34  
  brief    
  /


Parameters

Parameters are normally specified immediately after the command in the
command-file.   Parameter  names  can  be  abbreviated to uniqeness (2
letters).  End the list of parameters with slash  (/)  or  end-of-file
(control-Z on VMS, control-D on Unix).

INPUT name of input merge-file, up to 40 characters (no default;  must
          be specified)

OUTPUT name of output  file  for  INVERT,  up  to  40  characters  (no
          default; must be specified)

MODFILE name of input model-file, up to  40  characters  (no  default;
          must be specified)

PHASE                                                             Page 2


BASE = n1,n2,...  :  the n(n-1)/2 baseline labels (eg 12,24,910) in an
          order  of  preference,  giving  the order in which the model
          phases should be used.  At each UT, the program computes the
          N  visibility phases on N baselines either from the model or
          by solving the closure-phase relations.   The  model  phases
          will  be  calculated  for the first two baselines specified,
          and then  the  program  will  try  to  solve  any  available
          closure-phase relations.  If it cannot, it will use the next
          model phase and try again.  This  continues  until  all  the
          phases  have  been  determined.   If  no  closure-phases are
          available, the model phase will be used at every (u,v) point
          for which an amplitude has been measured.  The default value
          of BASE is a list of all  possible  baselines  in  canonical
          order, i.e., 12, 13, 23, 14, 24, 34, 15, ....

CGLIM, XCG and YCG (default XCG=YCG=0):  If CGLIM  is  specified,  the
          centroid  of  all  the components of the model whose flux is
          greater than CGLIM times the flux of the first component  is
          found, and the model is shifted so that the centroid lies at
          (XCG,YCG) milliarcsec.  Note that x increases  to  the  East
          (left)  and y increases to the North (up), so increasing XCG
          moves the source to the East and increasing YCG moves it  to
          the North relative to the map-centre.

NCPRINT = n:  the maximum number of model components to list  (default
          is to list the complete model)

BRIEF (no value):  if specified suppresses the listing of  the  "Phase
          Solution" which tells you what the program did.


History

Version 1.0:  1979 Jul  5 - [TJP].
Version 1.1:  1979 Oct 17:  change model calculation to x,y; :   allow
                            case NT=0 [TJP].
Version 2.0:  1981 Mar  5 - Change to 15  station;  Fortran-77  syntax
                            [TJP].
Version 2.1:   1981  Dec  22  -  Remove  check  that  data  have  been
                            calibrated.
Version 2.2:  1981 Dec  29  -  Introduce  OLDERRORS  option:   do  not
                            recalculate phase errors [SCU].
Version 2.3:  1982 Aug  4 -  Make  OLDERRORS  unnecessary  (disabled);
                            program now never recalculate phase errors
                            [SCU].
Version 2.4:  1982 Dec 19 - Fix 15 stations; put  MAXS  in  NAMES  etc
                            arrays.  [RCW].
Version 3.0:  1983 Jul 21 - Extend to include MERGE format  version  2
                            files [DLM].
Version 3.1:  1983 Aug  4 - Add W to version 2 files [DLM].
Version 3.2:  1988 Apr  4 - Changes for Convex [TJP].
Version 3.3:  1988 Dec 30 - new parameter names:  INPUT etc.  [TJP].
Version 3.4:  1989 Jul 17 - supply default baseline list [TJP].