Keywords: sic-gdfv2

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### Complex visibilities

The complex visibility values are always `real(4)`. The range of 32-bit columns for the complex visibility values is indicated by `h%gil%fcol` and `h%gil%lcol`. We thus have the following relations
`h%gil%fcol` = `h%gil%nlead+1`
`h%gil%lcol` = `h%gil%dim(visi_axis) - h%gil%ntrail`
`h%gil%nlead+h%gil%ntrail` = `sum (h%gil%column_size)`
where `visi_axis` is 1 for the ``natural'' (UVT) order, in which each visibility is contiguous, and 2 for the ``transposed'' order (TUV).

The UV data format is in fact a generic telescope table format, and can also handle tables from a Single dish telescope. As weights are often common to all values, the complex visibility can be made of only 2 ``atoms'' provided `h%gil%column_pointer(code_uvt_weig) 0`. It can also have only 1 atom, if the imaginary part is always 0. The ``atom'' types are explicitely described by the h%gil%atoms(1:4) array. Currently defined atom types are

    ! Visibility atom description
    integer(kind=4), parameter :: code_atom_real = 1
    integer(kind=4), parameter :: code_atom_imag = 2
    integer(kind=4), parameter :: code_atom_weig = 3

Other codes may be added if needed, for example integration time (as Tsys weighting is not necessarily the thing to do when handling different polarizations).