Calculate modulus of quaternion
n = quatmod(q)
n = quatmod(q) calculates the modulus, n, for a given quaternion, q. Input q is an m-by-4 matrix containing m quaternions. n returns a column vector of m moduli. Each element of q must be a real number. Additionally, q has its scalar number as the first column.
The quaternion has the form of
The quaternion modulus has the form of
 Stevens, Brian L., Frank L. Lewis, Aircraft Control and Simulation, Wiley–Interscience, 2nd Edition.