Unless an observer altitude is specified
LEV Flags) the observer is assumed to be
at the surface (defined as the lowest profile level rather than 0 altitude)
The plane parallel atmosphere allows several simplifications:
no field-of-view effects, analytical Curtis-Godson path parameters,
no requirement for altitude coordinate (assuming hydrostatic equilibrium
- see HYD Flag to turn this off).
For upward-viewing calculations with a
spherical atmosphere it is still possible to
use the normal 'limb-viewing' geometry but the viewing direction
has to be specified by elevation angle
section label with *ELE and specifying positive elevation angles).