But why is this critical?
- I may be missing the point of your question, Dick, and forgive me if I am being over-simplistic, but the Azimuth AND heading that the electronic wizardy spits out has to represent the attitude and azimuth of the aircraft, ie the 'pitch' must be referenced to whichever datum the designers decide is the pitch datum (presumably the roll axis), and roll likewise (to wings level). Heading
has to be aligned with the aircraft roll axis. If these are not set accurately then all derived acceleration vectors and attitude references will be out by 'whatever' which will degrade the overall nav performance of the kit.