Why is quaternion normalization important in an EKF used for attitude estimation?

• A. To preserve magnetic field alignment.
• B. To maintain unit-norm constraint of rotation representation and prevent drift due to numerical errors.
• C. To simplify the gravity model.
• D. To increase processing speed.

Answer: (B)

Quaternions used for attitude must have unit length to correctly represent a rotation. In an EKF, updates and propagation are done with finite-precision arithmetic, so tiny numerical errors accumulate and can push the quaternion’s norm away from one. Normalizing re-scales the quaternion back to unit length, effectively projecting it onto the unit sphere. This preserves the estimated rotation direction while enforcing the valid unit-norm constraint, keeping subsequent computations (like converting to gravity or magnetic references) consistent and physically meaningful. It doesn’t change the gravity model, nor does it inherently speed up processing; its purpose is numerical stability and a valid rotation representation.