FVZ Flag

RFM Flags: FOV Function Represents Refracted Tangent Heights


Regard .fov file as containing FOV-shape after refraction (i.e. strictly only valid for a particular tangent height view)

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)

FOV Flag must also be enabled.

  1. 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.

  2. 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.

[none reported]