%N uvchkshadow %F $MIR/src/subs/uvio.c
%D Check if the record comes from shadowed antennas
%P pkgw
%: uv-i/o
%B
FORTRAN call sequence:
        logical function uvchkshadow(tno,diameter_meters)
        integer tno
        double precision diameter_meters

  Returns whether the most recently-read UV record comes from a
  baseline involving shadowed antennas. The antenna diameter used in
  the shadow computation is passed in as an argument. The test
  performed in this function is identical to the one performed when
  using the "shadow()" selection keyword. There is no way to obtain
  the results of the keyword test, however, without filtering out data
  records. This function makes it possible to check whether a given
  record was shadowed without filtering records.

  In order for this function to operate, you must apply a selection of
  the form "shadow(1e9)" when reading the data. This is because UVW
  recomputation must be performed as the data are read in, which is
  only reliably triggered by applying a "shadow()" selection. The
  extremely large argument ensures that no data are filtered out by
  the selection.  Invoking this function without applying the
  necessary selection will result in a fatal bug being signaled.

  Another routine, SHADOWED, is provided with MIRIAD and provides
  similar functionality. It is unclear, however, whether SHADOWED is
  correct. It and the "shadow()" select keyword do their work
  differently and yield different results. As of 2011 Feb 17, no tasks
  in MIRIAD use SHADOWED.

  Input:
    tno             Handle of the uv data file.
    diameter_meters The assumed antenna diameter in meters.
  Output:
    uvchkshadow     Whether the last-read baseline was shadowed.        
