A model of the earth is required to calculate things like satellite
altitude, and range from the observer. WinOrbit has two such models:
The simplest approach is to treat the Earth as a sphere.
Some improvement in accuracy can be had by representing the earth as an
ellipsoid (decreases position errors at the poles and equator by up to about 20
km). One can include higher-order shape terms, but with rapidly diminishing
benefit (the ellipsoid is accurate to about +/- 100m at sea level).
The model, reflecting the mean sea-level surface, is sometimes called
the reference geoid. Local terrain altitude above mean sea level is
then added as required.
WinOrbit permits either the sphere or the simple
ellipsoid representation to be selected. These choices can be made in the
Observer Information Window
and in the
Print Ephemeris Window.