RFM Flags: FOV Function Represents Refracted Tangent Heights
17FEB24
Enabled
Regard .fov file as containing
FOV-shape after refraction
(i.e. strictly only valid for a particular tangent height view)
Disabled
Regard .fov file as containing
FOV-shape as projected from satellite
(normal usage, valid for all tangent heights since distortion due to refraction
will be taken into account)
Although it represents an approximation, using this flag with multiple
tangent heights when the FOV is tabulated as some regular division of
the tangent height spacing
allows the same ray paths to be reused for simulating views
at adjacent tangent heights, so saving both time
and memory.
For example
Suppose a FOV is 2km high projected on to the limb,
and tabulated at 1km relative geometric spacing
(-1km, 0km +1km), defined by fixed
angles relative to the nominal central ray.
For a (refracted) tangent height of 10 km the RFM normally allows for
distortion of the FOV and so would convert the geometric ±1 km paths
to refracted tangent heights slightly less than 9km and 11km.
This means that for a simultaneous calculation of another FOV with
nominal tangent point at 12 km, the lower ray path path for the 12 km
view won't quite match the upper ray path for the 10 km view, hence
6 'pencil-beam' ray paths are required to synthesise the two views.
By setting the FVZ flag, the RFM treats the FOV representation
as relative refracted spacing, so the 10 km view will be
synthesised from refracted paths at exactly 9, 10 and 11 km, and
similarly the 12 km view will use 11, 12 and 13 km, ie re-using the
11 km view, hence 5 pencil beams rather than 6.