; VBMRG
;---------------------------------------------------------------
;! Merge VLBI data, eliminate duplicate correlations
;# TASK VLBI
;-----------------------------------------------------------------------
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;-----------------------------------------------------------------------
;---------------------------------------------------------------
VBMRG     LLLLLLLLLLLLUUUUUUUUUUUU CCCCCCCCCCCCCCCCCCCCCCCCCCCCC
VBMRG     Task to merge 'BT' sorted VLBI data.
INNAME                             Input UV file name (name)
INCLASS                            Input UV file name (class)
INSEQ             0.0     9999.0   Input UV file name (seq. #)
INDISK            0.0        9.0   Input UV file disk unit #
OUTNAME                            Output UV file name (name)
OUTCLASS                           Output UV file name (class)
OUTSEQ           -1.0     9999.0   Output UV file name (seq. #)
OUTDISK           0.0        9.0   Output UV file disk unit #.
YINC              0.0     99999.   Integ. time of data (sec)
----------------------------------------------------------------
VBMRG
Task:  This task will consolidate a 'BT' sorted VLBI uv dataset.
       The last visibility with the heighest weight for each
       correlator will be passed.  The purpose of this task is
       to remove redundant processings of data and to collect
       all polarization data into the same visibility record.
Adverbs:
  INNAME.....Input UV file name (name).      Standard defaults.
  INCLASS....Input UV file name (class).     Standard defaults.
  INSEQ......Input UV file name (seq. #).    0 => highest.
  INDISK.....Disk drive # of input UV file.  0 => any.
             Input UV sort must be 'BT'.
  OUTNAME....Output UV file name (name).     Standard defaults.
  OUTCLASS...Output UV file name (class).    Standard defaults.
  OUTSEQ.....Output UV file name (seq. #).   0 => highest unique
  OUTDISK....Disk drive # of output UV file. 0 => highest w room
  YINC.......The integration time of the data.  Times within a
             window of this width will be considered identical.
             Value is in sec. Default = minimum = 0.2 sec.
----------------------------------------------------------------
VBMRG:  Task to eliminate duplicate correlations of VLBI data.
DOCUMENTOR: Jim Ulvestad NRAO/CV
RELATED PROGRAMS: TOVLB, UVSRT,VBFIT

                            PURPOSE

     VBMRG consolidates a VLBI data base which has been sorted
in baseline-time ('BT') order.  The task is necessary because
the multiple processings on a 3+ station processor that are
required in a many-antenna experiment will cause some baselines
to be correlated more than once. VBMRG searches the old data
base picking out only one record for each baseline at each
time; it writes a new data base specified by OUTNAME.  This new
data base will contain only a single record for each baseline
within the time window specified by YINC.
     VBMRG cannot be run on processed data sorted in
time-baseline ('TB') order.  That data must first be run
through UVSRT with the sorted order 'BT' specified.  See
comments below.

                            COMMENTS

INNAME:
     The input file must be sorted in 'BT' order.

OUTDISK:
     Although VBMRG will create a new data set that is somewhat
smaller than the old set, the user must be sure there will be
enough space on the OUTDISK go accommodate the new data base.
Running several UVSRTs and VBMRG on a large data set will
quickly eat up the available disk space, so old versions of the
data base should be deleted as soon as they're no longer
needed.  OUTDISK = 0 will cause the task to try all disks.

YINC:
     The default value of this parameter should be used in most
cases.  If YINC is specified, it must not be set to be longer
than the integration time for a single record, which will
result in the discarding of good, unduplicated visibility
records.

INTERACTION WITH OTHER PROGRAMS:
     As mentioned above, VBMRG can be run only on data sorted
in 'BT' order.  Before VBMRG can be run, UVSRT must be run to
sort the data into this order.  Then VBMRG will run through a
single baseline at a time, selecting only a single visibility
record from each time.  The selected record will be the one
given the highest weight (or last-written, for equal weights).
The output file specified by OUTNAME will contain only
unduplicated data from a single correlation of each baseline.
Before the global fringe-fitting task (VBFIT) can be performed,
the data base must be re-sorted by use of UVSRT with sort order
'TB' (time-baseline).  Thus UVSRT must be run twice, once
before and once after VBMRG.

EXECUTION TIMES:
     I have no idea.

DISK SPACE:
     I don't know the amount of scratch disk space.  The disk
space used by the output file will be somewhat less than the
space used by the input file, dpending on the fraction of
redundant correlations performed in the processor.  For
example, optimum processing of a 5-station experiment will
require 4 passes on a 3-station processor, giving 10 unique
correlations and 2 redundant ones.  In this case, the output of
VBMRG should occupy about 16% less disk space than the input
file.
