Abstract: In this paper, a ptactical open-loop alignment scheme for strapdown inertial navigation systems is discussed in detail. The alignment process is separated into two steps, i.e.,coarse alignment, or analytical alignment, and open-loop fine alignment. Gram-Schmidt method, which can result in an ideal orthogonal matrix, is used to orthogonalize coarse alignment matrix. Least Squares estimators are used to estimate instantaneous misalignment angle between the true geographic frame and the computed one, which is determined by the coarse alignment matrix. To eliminate efficiently the affections of environment disturbance on alignment process the misalignment angle is divided into two components, i. e., instantaneous one and average one. All measurement equations used to estimate the two components are in the same form. This feature will be helpful in implementing the scheme in a real strapdown sys-tem, since it can reduce the burden on on-line computer. Lowpass digital filters are introduced into the open-loop alignment process to enhance the accuracy and shorten the time needed.Digital simulations are employed to verify the effectiveness of such an alignment scheme. The simulation results reveal that this scheme can reach the accuracy superior to 1 arc-minute in azimuth misalignment and less than 6 minutes is needed to complete the alignment process under the simulation conditions listed in Table 1 . Other results also indicate that this scheme is efficient and satisfactory.