Differentiate between 'navigation frame' and 'body frame' in aircraft INS.

• A. Navigation frame is body-fixed; body frame is local tangent plane.
• B. Navigation frame is a local tangent plane aligned with north/east/down; body frame is fixed to the aircraft.
• C. Navigation frame is earth-centered; body frame is geocentric.
• D. Navigation frame is fixed to the aircraft; body frame is the local plane.

Answer: (B)

In an aircraft INS, two reference frames are used to describe motion: a navigation frame and a body frame. The navigation frame is a local tangent plane to the Earth at the current position, oriented with north, east, and down. It’s effectively a small, Earth-fixed frame that moves with your location but is aligned to the surface and gravity, so you can express position and velocity in meters north and meters east (with down for altitude).

The body frame, on the other hand, is fixed to the aircraft itself. Its axes move and rotate with the airplane—typically forward along the fuselage, to the right wing, and downward (depending on convention). The INS uses attitude information to rotate between these frames, converting measurements from the moving, body-fixed perspectives to the Earth-referenced navigation frame (and vice versa).

So the best description is that the navigation frame is a local tangent plane aligned with north/east/down, while the body frame is fixed to the aircraft.