Earth and Atmosphere. While the earth and sky are pretty fascinating objects, to a navigation system, they are dreadfully boring. The main reason for modeling the earth is to make sure that the aircraft gets to where it is going, which is normally expressed as a relative position with respect to the earth. Earth and atmosphere models incorporate large amounts of static fixed knowledge about geometry and atmospherics. Models of the earth, especially, should be highly pre-wired with facts about the earth geometry, magnetic variation, etc.
Updates mainly provide fine tuning.
In most other ways, these models play the role of intermediate Stages , with data that are used as inputs along with others to ultimately estimate useful aircraft properties. Some objects, such as Radio Bases, are statically known to exist, but are not always detectable because they enter and leave detection range. Updates and information from such objects usually form parts of earth models.