Abstract: For the low-cost SINS/GPS integrated navigation system used in small unmanned helicopters,the Ψ angle model of SINS is not competent for SINS error estimation due to the neglected error terms when describing low precision inertial sensor errors.To overcome the disadvantages of Ψ angle model,a nonlinear error model of SINS based on quaternion is presented in this paper.Small misalignment angle assumption is not required in the error model.To estimate SINS errors of integrated navigation system,a sequential Unscented Kalman filtering approach is proposed.Computational complexity of the algorithm is reduced by sequentially processing measurement vectors.Simulation results reveal that the filtering approach can improve the performance of low-cost SINS/GPS integrated navigation system effectively.