  <span class="comment">% According to Crassidis, qua_skew should be</span>
                                        <span class="comment">% positive, but if positive NaveGo diverges.</span>
        <span class="comment">% Crassidis, Eq. A.174a</span>
        Xi = [qua(4)*eye(3) + qua_skew; -qua(1:3)'];

        <span class="comment">% Crassidis, Eq. 7.34</span>
        qua = qua + 0.5 .* Xi * kf.xp(1:3);
        qua = qua / norm(qua);          <span class="comment">% Brute-force normalization</span>

      