Gyrocompass Alignment with Base Motions Results for a 1-Nmi/hr INS/GPS System

Cuong Tu Hua

Abstract:	The accuracy characteristics of inertial sensors in an integrated INS/GPS system are frequently specified by its performance during a gyrocompassing operation. This fine alignment procedure is usually done in the pre-flight phase when the system is at rest and is on the ground. In practice, the base of an INS/GPS system is constantly subjected to random disturbances and/or vibrations induced by the host vehicle’s unwanted motions. An aircraft on a runway is constantly buffeted by wind gusts, crew motions, fuel/stores loading, etc. Tactical missiles are affected by launcher motions such as those set up by ripple fires. The calculation of alignment accuracy statistics for a 1-nmi/hr class INS/GPS system undergoing base motion disturbances is formulated as a 144-state Kalman filtering problem with zero integrated velocity updates. Its base motions are modeled as second-order Markov stochastic processes: white acceleration noises are used as inputs to a second-order system characterized by two time constants (velocity and position feedbacks). Platform velocities are further filtered by structural smoothing, modeled as a first-order system. Kalman filter observables are formed from the integration of smoothed velocities. Results demonstrate that the alignment accuracy is extremely sensitive to base motions. In many cases, the performance of inertial sensors is completely masked by such disturbances. In general, north and east leveling errors converge very quickly and their final values are independent of initial estimates. On the other hand, azimuth alignment errors converge at a much more gradual rate. Error time histories are presented for a number of cases to show the interrelationship of important factors such as latitude, inertial sensor errors (gyro & accelerometer biases, angular & velocity random walks, misalignment between gyro and accel triads, etc.), measurement noises, and disturbance characteristics. Finally, this paper is intended to demonstrate that a precise definition of base motions is required for a gyrocompass alignment procedure.
Published in:	Proceedings of the 1999 National Technical Meeting of The Institute of Navigation January 25 - 27, 1999 Catamaran Resort Hotel San Diego, CA
Pages:	717 - 725
Cite this article:	Hua, Cuong Tu, "Gyrocompass Alignment with Base Motions Results for a 1-Nmi/hr INS/GPS System," Proceedings of the 1999 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 1999, pp. 717-725.
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