The RFM assumes no spectral dependence of the FOV so
only one FOV can be specified for each RFM run.
The FOV response can be expressed either as a function of altitude
or as a function of elevation angle. Elevation angles are required
if used in conjunction with the OBS Flag
since tangent points (and tangent heights of the paths required for the FOV
convolution) can, in principle, vary by large amounts, e.g. right beside the
instrument for a horizontal view.