Strapdown inertial navigation systems (INS) need an alignment process to determine the initial attitude matrix between the body frame and the navigation frame. The conventional alignment process is to compute the initial attitude matrix using the gravity and Earth rotational rate measurements. However, under mooring conditions, the inertial measurement unit (IMU) employed in a ship's strapdown INS often suffers from both the intrinsic sensor noise components and the external disturbance components caused by the motions of the sea waves and wind waves, so a rapid and precise alignment of a ship's strapdown INS without any auxiliary information is hard to achieve. A robust solution is given in this paper to solve this problem. The inertial frame based alignment method is utilized to adapt the mooring condition, most of the periodical low-frequency external disturbance components could be removed by the mathematical integration and averaging characteristic of this method. A novel prefilter named hidden Markov model based Kalman filter (HMM-KF) is proposed to remove the relatively high-frequency error components. Different from the digital filters, the HMM-KF barely cause time-delay problem. The turntable, mooring and sea experiments favorably validate the rapidness and accuracy of the proposed self-alignment method and the good de-noising performance of HMM-KF.
Common mechanizations are not applicable for a marine strapdown Inertial Navigation System (INS) in Polar Region. Aiming at this problem, transversal strapdown INS mechanization and corresponding damping technology are proposed in this paper to replace common strapdown INS mechanizations. The transversal coordinate system is constructed for the mechanization of transversal strapdown INS, and then an error analysis for transversal strapdown INS is performed. The error analysis for transversal strapdown INS reveals the fact that the system also includes three kinds of periodic oscillating errors as common strapdown INS. To restrain the periodic oscillating errors, damping equalizers applicable to transversal strapdown INS are designed with reference velocity compensating the effect of ship motion. Finally simulation test is carried out to demonstrate the performance of transversal strapdown INS and damping technology in Polar Region.
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