Well, in thinking about it more I think I may have figured it out. Forgive me while I think "aloud". After converting the aircraft and target locations to ECEF coordinates, you subtract them to get the vector between them. BUT that vector is only the straight line distance between the two points. It doesn't take into account the orientation of the aircraft, which may very well be pitch up 25 degrees and banked 10 degrees. Therefore to get the new "coordinates" of the target, with respect to the planes' attitude, the transformation needs to be done to get them in the same "plane" (no pun intended ;P ). Any confirmation or correction of this thought is appreciated. thanks.
