Abstract: A linear aircraft model for a rigid aircraft of constant mass flying over a flat, nonrotating earth is derived and defined. The derivation makes no assumptions of reference trajectory or vehicle symmetry. The linear system equations are derived and evaluated along a general trajectory and include both aircraft dynamics and observation variables.”
This paper is talking about a mathematical, engineering model for an aircraft. Have you read page 32 of the document? It talks about the assumptions used to simplify the model which are suitable for most aircraft. The very next paragraph states “For trajectory propagation, SINCE WE CANNOT ASSUME A FLAT EARTH [emphasis added], the original inertial reference frame, denoted with an ‘i’ subscript, is modified for the elliptic Earth.” So to claim that the paper supports FE is false, and, dare I say, dishonest.
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I did a search of the document itself and cannot find the language you reference in it. ?? Please clarify.
The word assume is not in the document at all. The sentence you mention even less. the words since we cannot are not even found. NASA must of removed your sentence. I guess it didn’t fit in!