Design of Vibration Isolation System for Laser Strapdown IMU

The US Army has a requirement for a highly accurate inertial/TACAN hybrid navigation system. This paper describes the navigation system architecture developed to meet this requirement, the specific hardware selected and the development status. At the beginning of the Integrated Inertial Navigation System (IINS) development program it was anticipated that significant technological advances would take place in its life cycle. In addition, mission requirements were expected to change as the system evolved. Particular attention was therefore given to the IINS system architecture, specifically the partitioning of hardware and software functions as well as the means of data transmittal between the subsystem elements. The paper describes in detail the design trade-offs made as well as hardware selections. The system is configured using several different manufacturers' equipment making the data interface and its configuration control of great importance. The above considerations lead to the conclusion that the appropriate data bus interface standard would be MIL-STD-1553. This interface standard not only facilitates configuration control during system development, but will allow rapid integration with future aircraft systems that will use it as a primary means of data transmittal. The paper describes in detail the signal interface between the subsystem element, onboard mission equipment and flight instruments. The most significant change anticipated in the course of the IINS life cycle is the substitution of Global Positioning System User Equipment for the external update function currently provided by TACAN. Another area where technology insertion may be deemed appropriate is the substitution of a strapdown inertial navigation unit (INU) employing ring laser gyros (RLG) for the currently employed conventional gimballed INU. The INU employed in the IINS is built to a form, fit and function specification, which will allow substitution of a RLG INU when such a unit is qualified and in production. The system hardware and software is structured so that these changes can be accomplished with a minimum of effort. The paper also describes the federated processing structure which allows this flexibility with particular attention to the inertial/TACAN update algorithm and data bus control functions. The paper concludes with a brief summary of the development status of the IINS, currently in full scale engineering development.

Suppression of the G-sensitive drift of laser gyro in dual-axis rotational inertial navigation system

In modern aerospace technology, strapdown inertial navigation systems (SINS) are widely used, using fiber-optic (FOG) or ring laser (CLG) gyroscopes. During the operation of such systems, the sensitivity axes are rotated relative to the basic coordinate system. The resulting angles between the axes of the base coordinate system and the axes of sensitivity of the navigation system (non-orthogonality) are one of the factors leading to an increase in the measurement errors of the device, which affects the measurement accuracy. During operation, the system is affected by vibrations of various nature, the impact of which can contribute to the appearance of non-orthogonality. The purpose of this work is to determine the maximum permissible vibration amplitudes affecting the SINS body according to the permissible values ​​of the deviation of the FOG sensitivity axes for two variants of the SINS layout. An approach to determining the permissible amplitudes of an external harmonic impact on the unit of a strapdown inertial navigation system based on fiber-optic or ring laser gyroscopes is considered. A design scheme, mathematical and finite element models for calculating natural frequencies and forced oscillations of a strapdown inertial navigation system unit have been developed. In various frequency ranges, numerical calculations have determined the boundary values ​​of the amplitudes of the external harmonic impact on the base of specific configurations of the SINS assembly. It has been established that dangerous states take place in the region of the 1st natural frequency of the system, as well as near higher frequencies. Comparison of the results for design options 1 and 2 allows us to conclude that in order to weaken the effect of vibrations on the accuracy of the SINS unit, it is advisable that the lowest natural vibration frequencies for the SINS assembly be as high as possible (more than 1000 Hz).
 Key words: vibration; fiber optic gyroscope; strapdown inertial navigation system; finite element method; natural frequencies and modes of vibration.

A 5 inch diameter high accuracy borehole surveying instrument has been developed using a full 3 axis inertial navigation system (INS). It has the following features: 3 Ring Laser GyrosNo mechanical gimbalsReal-time surface readoutObtains position data directly without the need for intermediate depths stops.Requires no stopping for drift checksUses wireline velocities up to 600 ft/minReduces INS's typical drift error to less than 6 ft/hr. This tool achieves a survey accuracy that is close to that of the 10" INS tool, while at the same time offering a smaller diameter and continuous survey capability.

The medium accuracy standard ring laser gyro (RLG) inertial navigation unit (INU) exemplified by Litton's LN-93 system has benefited from continual improvements during the production process such that the present navigational accuracy and reliability are better by a factor of two or more over the original requirements as defined by SNU 84-1. Performance enhancements are now available which further improve navigation performance to the high-accuracy category while maintaining the reliability of the production system. Two enhancement mechanizations are described-one that requires software-only changes to improve inertial performance, and one that employs a self-contained (embedded) GPS to augment the inertial performance. These enhancements may be applied in combination to provide uniquely effective immunity to GPS spoofing countermeasures. These enhancements retain the present standard RLG INU form factor, mounting, and electrical interface, and are therefore particularly advantageous for retrofit application. Potential retrofit applications with various aircraft avionics architectures are discussed. >

A new dynamic calibration method for IMU deterministic errors of the INS on the Hypersonic Cruise Vehicles

Inertial navigation provides a unique ability to know where one has been, where one is currently, and where one is going, given only a starting position. The laws of physics permit the sensing of dynamic motion without external information, making inertial systems impervious to jamming, masking, or spoofing. Measurements of six degrees of freedom are required - three linear accelerations, and three angular rates - to fully propagate the velocity, position, and orientation of the system. The first inertial sensors are traced to the early 19th century and specialized inertial guidance systems appeared in the 1940s, yet inertial navigation systems did not become commonplace until the 1960s. This is largely due to the fact that requirements for navigation accuracy inertial sensors - accelerometers and gyroscopes - are very challenging. In the past fifty years, significant evolutionary and revolutionary changes have taken place in the designs of inertial sensors and systems. These include the progression from fluid-filled to dry instruments and the transition from mechanically complex stabilized inertial platforms to computationally intensive strapdown systems. Gyroscopes have evolved from large mechanical devices to highly refined precision mechanical sensors. Optical rotation sensors such as the ring laser gyro and the fiber optic gyro have enabled new system designs and capabilities. Coriolis vibratory gyroscopes such as the hemispherical resonator gyro are capable of extreme accuracy and reliability; new opportunities for miniaturizing these types of sensors will lead to new classes of accuracy for inertial navigation systems. Advanced gyroscope technologies such as the nuclear magnetic resonance gyroscope which uses atomic spin to detect rotation have already been demonstrated to achieve navigation accuracy requirements. Cold atom technologies may also provide the opportunity for very high accuracy accelerometers and gyroscopes in the future. Inertial navigation technologies and applications of the past, present, and future are discussed.

In this paper, we present the requirements specification to develop the small ring laser gyroscope(RLG) which is applicable to the medium-grade inertial navigation system(INS) widely used as a main navigation system. To this end, first we analyse the performance specifications of RLG which is needed to configure the medium-grade INS and then we present the design results of RLG to meet the performance specifications, based on the overseas technology survey and the theoretical analysis. It is also shown in this paper what technology is required to develop a small RLC.

The integration of GPS, PL and INS sensors can be implemented at three different levels. Compared with loose and tight integration, ultra-tight integration offers numerous advantages including increased robustness under high dynamics, and improved antijamming performance. In current ultra-tight integration scenarios, a centralised Kalman filter is commonly employed to fuse either In-phase (I) and Quadrature (Q) data from the tracking loop or the pseudorange measurement and Position, Velocity, Attitude (P, V, A) measurements from the Inertial Navigation System (INS). Though relatively simple, this centralised filter structure has some disadvantages. Firstly, to reduce the computational load, the filter only makes coarse estimates of Inertial Measurement Unit (IMU) random errors, which significantly degrades the system performance. Secondly, for more accurate estimates, the filter becomes much more complicated, resulting in a large increase in the computation time. All of these hinder the performance of ultra-tight integration considerably. This paper proposes a federated filter structure for the ultra-tight integration of GPS, PL and INS sensors. The new filter structure distributes the computing tasks to different Kalman filters, leading to reduced filter complexities and improved system performance. IMU random errors are estimated separately by the pre-filter at a high data rate, whilst the main filter has a simplified structure, i.e. no estimation of the IMU random errors, and operates at a relatively slow rate. This paper will discuss the dynamic modelling method based on the Walsh function transform for implementing the pre-filter and the simplification of the main filter. Simulation tests were performed to compare the performance of the federated filter with that of the usual centralised Kalman filter in the estimation of the IMU random errors. The results show that with the simplification of the Kalman filter structure, the federated filter design can achieve the almost equally precise estimates as the centralised Kalman filter does but with less computational burden. Hence the federated design is more suitable for implementing the ultra-tight integration for real-time applications. Finally, the simulated high dynamic flight test results of ultra-tight integration based on the federated Kalman filter are presented.

Generally, an autonomous under vehicle (AUV) is equipped with some navigation devices to know its own position and motion. One of them is an inertial navigation system (INS). The INS calculates its own absolute position and attitude using just internal sensors in real time. And the position accuracy depends strongly on precision of inertial sensors. In particular, the precision of optical gyroscopes strongly influences the position and attitude accuracy. And moreover, the precision depends on the size of them. In fact, it is necessary that the size of INS becomes large in order to improve the position accuracy. Based on that, Japan Agency for Marine-Earth Science and Technology (JAMSTEC) developed a compact and high-performance INS which can be installed easily into a small AUV and output accurate position and attitude. It was developed with homegrown technology which is caused by JAMSTEC's knowledge and the technical capabilities of a domestic manufactures. In this paper, its schema, specifications and functions are described.

Longwall shearer positioning with an inertial navigation system (INS) aided by an odometer is the key technology for automation of the longwall coal mining process. In order to improve the shearer positioning accuracy without using external sensors, the closing path motion characteristics of a shearer working in a longwall mining face was summarized by analyzing the longwall mining method. A closing path optimal estimation model was proposed based on the motion characteristics of the shearer and a Kalman filter (KF). Through simulation tests, it was verified that the model reduced the influence of heading angle drift of an INS on the shearer positioning accuracy without using external sensors. The shearer positioning accuracy based on the circular error probable was improved by approximately an order of one under the effect of the optimal estimation model. This was beneficial for reducing the large positioning error of the shearer in a longwall mining face during each cutting cycle for approximately 1 h. Note to Practitioners —This paper was motivated by the problem of longwall coal mining automation, especially positioning of longwall shearer, but it also applies to other mobile mining equipment. Longwall shearer positioning with an INS aided by an odometer has been regarded as an effective technological approach to longwall mining automation by coal mining workers and researchers. However, the positioning accuracy is largely subjected to the performance of the INS. This paper proposed a closing path optimal estimation model based on the motion characteristics of the shearer and a KF. Through simulation tests, it was verified that the model improved the positioning accuracy of the shearer by approximately an order of 1. This was beneficial for astricting the large positioning error of the shearer in a longwall mining face during each cutting cycle for approximately 1 h. In the future research, we will address the field tests of the closing path optimal estimation model and extending other mining applications.

Gyroscopes and accelerometers are unique devices, the critical input sensors for inertial navigation and surveying systems. Among the many types of gyroscopes only a few have proved adequate for precise inertial applications. These are the ring laser gyroscope (RLG), electrostatic suspended gyroscopes (ESG), air-bearing floated-gimbal spinning mass gyroscopes, and dynamically tuned rotor gyroscopes (DTG). All have been developed to a high art among precision instruments, with supremely low noise and bias errors. Extensive acceptance of inertial navigation systems in commercial flight service, particularly using RLGs, has also required and stimulated development of practical and competitve reliability, cost, and lifetime.A few currently emerging gyroscope technologies might play a role in future inertial systems, but it is not yet clear whether they can advance to be competitive in performance and cost with current gyroscopes. Most notable is the fiber-optic gyroscope (FOG), another form of optical gyroscope, which in the last several years has advanced to a state suitable for engineering prototypes in aircraft attitude heading and reference systems. Another is the hemispherical resonator gyroscope (HRG), which uses the precession of vibrations on the rim of a “wine glass” shaped quartz resonator.

Rotation modulation technology could effectively improve the accuracy of the inertial navigation system (INS) by compensating for the biases of the inertial sensors automatically. However, the carrier angular motion and rotation control error could reduce the rotation modulation effect and then decrease the navigation accuracy. To address this problem, for the single-axis rotation INS, a novel rotation control scheme is presented. The control scheme employs the fiber optic gyros to control the inertial measurement unit (IMU) rotation angular velocity so that the INS with both rotation modulation and azimuth motion insulation functions. Furthermore, in order to reduce the control error, this study adopts two ways: optimizing the control strategy and shortening the delay time. The former way is to control the IMU rotating about the z-axis of the platform frame with respect to the navigation frame, rather than the up-axis of the navigation frame. The latter way is to apply interrupt mode rather than inquiry mode to complete the data transfer between the navigation and the control processors. The simulation and experimental results demonstrate that: the proposed method would not only realize the rotation modulation of the biases of the inertial sensors, but also achieve the insulation of the azimuth motion. The steady-state control error of the control system is less than 10" and the overshoot control error is less than 50". Compared to the traditional SRINS, the navigation position error in the single-axis rotation/azimuth-motion insulation INS could reduce 50% in some navigation application.

Inertial navigation system has been the core component of both military and civil navigation systems. Before the INS is put into application, it is supposed to be calibrated in the laboratory in order to compensate repeatability error caused by manufacturing. Discrete calibration method cannot fulfill requirements of high-accurate calibration of the mechanically dithered ring laser gyroscope navigation system with shock absorbers. This paper has analyzed theories of error inspiration and separation in detail and presented a new systematic calibration method for ring laser gyroscope inertial navigation system. Error models and equations of calibrated Inertial Measurement Unit are given. Then proper rotation arrangement orders are depicted in order to establish the linear relationships between the change of velocity errors and calibrated parameter errors. Experiments have been set up to compare the systematic errors calculated by filtering calibration result with those obtained by discrete calibration result. The largest position error and velocity error of filtering calibration result are only 0.18 miles and 0.26m/s compared with 2 miles and 1.46m/s of discrete calibration result. These results have validated the new systematic calibration method and proved its importance for optimal design and accuracy improvement of calibration of mechanically dithered ring laser gyroscope inertial navigation system.

This paper presents a method to extract heading information from the sky light polarization pattern and successfully implements the polarized light compass (PL-compass) under foliage environment with heavy occlusions for the first time. A classifier based on the primal support vector machine is trained to detect the sky area, and the random sample consensus algorithm is used to choose the inliers among the sky area. The sun vector is built upon the valid measurements. The attitude angles (Roll and Pitch) from the inertial navigation system (INS) are used to establish the Zenith direction when the platform tilts during traveling. The results show that the PL-compass can provide accurate heading angle (RMSE = 1.58°) by seeing only a tiny part of the skylight (minimum of 0.028%) in the dynamic vehicle test. The PL-compass is also integrated with the visual INS based on the indirect extended Kalman filter to improve the navigation accuracy. Each time a new measurement becomes available, the states estimated by the INS are used to check the consistency of the innovation and reject the outliers. The results show that the position errors of the integrated navigation system are greatly reduced with the help of the PL-compass. The estimated trajectory tracks the ground truth very well. The position error is 25.9 m (1.03% of the distance traveled) with heading errors less than 1° in the experiment.