This flag is a convenient way for performing spectral averaging (for
example to reduce the net file size) without losing contributions from
atmospheric fine spectral structure (which might happen if the output resolution
is set too coarse).
With this flag, the output spectra represent a convolution of spectra
calculated on the RFM fine grid (see also FIN
flag) with a triangular function of base = 2 x output resolution. Thus
each fine grid point contributes to either 1 or 2 output points
(1 if the fine grid point coincides with an output grid point, otherwise 2).
The output spectral resolution is, as usual, specified in the
*SPC section (RESSPC)
The spectral averaging is performed by creating an internal
'instrument line shape' which is a triangle with a base width equal to twice
the output resolution. This is equivalent
to explicitly using the ILS
option and supplying such a triangular
ILS as the .ils file.