An eight-position self-calibration method for a dual-axis rotational Inertial Navigation System (INS) is provided in this paper. By experiencing two more positions with tilt attitudes than those experienced in a conventional six-position method, not only constant biases, scale factor errors, and misalignment errors, but also g-dependent biases can be calibrated. Field tests indicate that, after the calibration and compensation of the g-dependent biases, both a latitude error and a longitude error remain within a small range over time. In contrast, by using the conventional six-position method, a latitude error is several times larger and a longitude error diverges rapidly over time. Compared with the six-position method, accuracy of the dual-axis rotational INS is significantly improved more than 50% by the eight-position self-calibration method. The self-calibration method is feasible both in static and over a ship at the dockside.

An eight-position self-calibration method for a dual-axis rotational Inertial Navigation System

Navigation accuracy of the inertial navigation system (INS) could be greatly improved by rotating the inertial measurement unit around gimbals. However, the attitude output accuracy of rotational INS (RINS) would be affected by the non-orthogonal angles of gimbals, which should be accurately calibrated and compensated. In this paper, a novel self-calibration method for non-orthogonal angles of gimbals is proposed in tri-axis RINS. The three non-orthogonal angles could be calibrated using attitude errors and velocity errors as measurements without external equipment. The self-calibration scheme in this method could reduce the coupling of non-orthogonal angles with other errors, and the observability is clear to demonstrate during calibration. The proposed method is verified by both simulations and experiments. The experimental results show that the calibration accuracy of non-orthogonal angles could be less than 2”, and after compensation, the attitude output accuracy is improved from 200” to less than 10”. Therefore, the proposed calibration method could greatly improve the attitude output accuracy and make RINS more effective in some task systems where high attitude output accuracy is urgently required.

A Self-Calibration Method for Non-Orthogonal Angles of Gimbals in Tri-Axis Rotational Inertial Navigation System

Rotation modulation is an effective method to enhance the accuracy of an inertial navigation system (INS) by modulating the gyroscope drifts and accelerometer bias errors into periodically varying components. The typical RINS drives the inertial measurement unit (IMU) rotation along the vertical axis and the horizontal sensors’ errors are modulated, however, the azimuth angle error is closely related to vertical gyro drift, and the vertical gyro drift also should be modulated effectively. In this paper, a new rotation strategy in a dual-axis rotational INS (RINS) is proposed and the drifts of three gyros could be modulated, respectively. Experimental results from a real dual-axis RINS demonstrate that the maximum azimuth angle error is decreased from 0.04° to less than 0.01° during 1 h. Most importantly, the changing of rotation strategy leads to some additional errors in the velocity which is unacceptable in a high-precision INS. Then the paper studies the basic reason underlying horizontal velocity errors in detail and a relevant new calibration method is designed. Experimental results show that after calibration and compensation, the fluctuation and stages in the velocity curve disappear and velocity precision is improved.

https://www.mdpi.com/1424-8220/15/8/18443/pdf

An Accurate Calibration Method Based on Velocity in a Rotational Inertial Navigation System.

In the rotational inertial navigation system (RINS), the rotation scheme affects the system’s performance directly. Improper rotation scheme may excite devices’ errors instead of reducing the effects caused by them. RINS should choose a suitable rotation scheme based on the error characteristics of its own. The rotation modulation theory for kinds of errors is analyzed, and two rotation schemes, called scheme A and scheme B in this paper, are proposed first. After the merits and demerits of the two schemes are discussed, an improved rotation scheme, called scheme C, is put forward. Scheme C can compensate scale factor errors, biases and installation errors of inertial sensors by rotating inertial measurement unit along the yaw axis and horizontal axis alternately. The simulations based on MATLAB are conducted in specific conditions; the results indicate that the performances of the three schemes are great better than that of general strap-down INS, called scheme D in this paper. Among the three schemes proposed, the performance of scheme C is the best. Experimental results in a real dual-axis rotational inertial navigation system based on fiber optic gyro show that the radical direction position error of the system is about 0.7 n mile in 12 h if scheme C is adopted, which verify the practicability of the rotation scheme proposed in this paper.

An Improved Rotation Scheme for Dual-Axis Rotational Inertial Navigation System

For rotational inertial navigation system(RINS), the system error can be compensated by rotating the inertial measurement unit (IMU) with one or more axis regularly, and the error compensation effect is determined by the rotation scheme of IMU.A reasonable rotation scheme should remove the system errors caused by inertial sensors errors and meanwhile, should not introduce other additional errors by the rotation. The error compensation demerits of two common rotation schemes are discussed. In these two schemes, the attitude errors caused by installation error are not modulated completely in a whole rotation period, which will cause accumulative velocity errors by integration. The accumulative velocity errors will sequentially increase the amplitude of position oscillation error. Based on the aforementioned analysis, a comprehensive error compensation scheme for dual-axis RINS is proposed. The proposed scheme can not only modulate the constant bias of inertial sensors, but also compensate the scale factor errors and installation errors coupled with IMU rotation. Moreover, the coupling errors are modulated into a zero-mean form with half amplitude of those two traditional schemes. As a result, the system errors of RINS with this rotation scheme can be modulated to a maximum extent. The superiority of the proposed scheme is verified through simulations with 4 different error conditions. Under the same synthesized error condition, the position error of the proposed scheme decreases to 0.37nmile/72hours from 0.98nmile/72hours in the other two schemes.

Comprehensive Error Compensation for Dual-Axis Rotational Inertial Navigation System

The errors of an inertial navigation system (INS) in response to gyros' errors can be effectively reduced by the rotation technique, which is a commonly used method to improve an INS's accuracy. A gyro's error consists of a deterministic contribution and a stochastic contribution. The compensation effects of gyros' deterministic errors are clear now, but the compensation effects of gyros' stochastic errors are as yet unknown. However, the compensation effects are always needed in a rotational inertial navigation system's (RINS) error analysis and optimization study. In this paper, the compensation effects of gyros' stochastic errors, which are modelled as a Gaussian white (GW) noise plus a first-order Markov process, are analysed and the specific formulae are derived. During the research, the responses of an INS's and a RINS's position error equations to gyros' stochastic errors are first analysed. Then the compensation effects of gyros' stochastic errors brought by the rotation technique are discussed by comparing the error propagation characteristics in an INS and a RINS. In order to verify the theory, a large number of simulations are carried out. The simulation results show a good consistency with the derived formulae, which can indicate the correctness of the theory.

Mengxin Nie

Papers

The Compensation Effects of Gyros' Stochastic Errors in a Rotational Inertial Navigation System