A new method for accurate alignment and calibration of strapdown INS

A new method is proposed for the coarse and fine alignments of the strapdown inertial navigation system (INS) along with the estimation of the INS systematic errors. This is achieved by utilizing an accurate external positioning system such as the global positioning system (GPS) and external gravity disturbances obtained from the Earth gravity models (EGMs) during the INS initialization time. First, a new method for the transformation of the INS acceleration from the sensor frame to the inertial frame using gyro output data is disclosed. Then, using the new method of the transformation, new techniques are developed for the alignment and calibration of the INS using minimizing the variations of the residuals of the gravity disturbances during the back-and-forth rotational motions. The common and conventional initializations cannot be started from any initial orientation, and cannot yield the accurate and actual INS errors. In contrast, under the alignment condition, the accurate and actual estimation of all the INS systematic errors is possible using the developed fine alignment and calibration which can guarantee the accurate autonomous strapdown INS positioning for a long period of time even during high dynamic motions. This is while the estimated error values of the INS by the conventional Kalman filter is unrealistic. In the field of autonomous positioning, a horizontal position with an error less than 200 m and 500 m was achieved during low and high dynamic motions, respectively, after the fine alignment and calibration, for the two hours of the INS positioning for a flight distance about 880 km. The employed INS was a high-performance strapdown INS, the Honeywell LASEREF III.