  In this paper, the optimization-based alignment (OBA) methods are
investigated with main focus on the vector observations construction procedures
for the strapdown inertial navigation system (SINS). The contributions of this
study are twofold. First the OBA method is extended to be able to estimate the
gyroscopes biases coupled with the attitude based on the construction process
of the existing OBA methods. This extension transforms the initial alignment
into an attitude estimation problem which can be solved using the nonlinear
filtering algorithms. The second contribution is the comprehensive evaluation
of the OBA methods and their extensions with different vector observations
construction procedures in terms of convergent speed and steady-state estimate
using field test data collected from different grades of SINS. This study is
expected to facilitate the selection of appropriate OBA methods for different
grade SINS.
