A model of the earth is required to calculate things like satellite altitude, and range from the observer.

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