In this article, an improved in- motion coarse alignment method is proposed for the strapdown inertial navigation system (SINS) aided by the global positioning system (GPS). Traditional in- motion alignment methods suffer from complex noises contained in the outputs of inertial sensors and GPS. To solve this problem, this article proposes an in- motion coarse alignment method using the vector observation and truncated vectorized $K$ -matrix (VO-TVK) for autonomous underwater vehicles (AUVs). The contributions of this study are twofold. Firstly, a new simplified model can be applied to the in- motion alignment process by employing the zero-trace and symmetry of the $K$ -matrix. Secondly, the proposed VO-TVK algorithm can make up for the optimal-REQUEST algorithm’s drawbacks, where the optimal-REQUEST algorithm has the conservative covariance matrix and the scalar gain. The simulation, vehicle test, and lake trial results illustrate that the proposed VO-TVK algorithm can efficiently reduce the effects of noises contained in the vector observation and achieve better accuracy than the compared algorithms.