<title>Dynamic north-finding scheme based on a fiber optic gyroscope</title>

Fiber optic gyroscopes (FOGs) are vital components for manned, unmanned, and autonomous platforms, as well as applications requiring extremely accurate data for stabilization, pointing, orientation of payloads, and navigation and control. FOG-based inertial systems also provide highly accurate data for all types of navigation, positioning, and georeferencing systems. Substantial improvements in the performance of FOGs and FOG-based inertial systems at KVH are directly related to advancements in the design and manufacture of optical fiber, as well as in manufacturing operations and signal processing. Open-loop FOGs, such as those developed and manufactured by KVH Industries, offer tactical-grade performance in a robust, small package. The success of KVH FOGs and FOG-based inertial systems is due to a series of key innovations including the development of proprietary D-shaped fiber with an elliptical core, as well as KVH's ThinFiber. KVH continually improves its FOG manufacturing processes and its signal processing, with the goal of improving accuracies across its entire FOG product line. KVH acquired its FOG capabilities, including its patented E·Core® fiber, when the company purchased the assets of Andrew Corporation's Sensor Products Group in 1997. E·Core fiber is unique in that the light-guiding fiber core — critical to the FOG's performance — is elliptically-shaped. The elliptical core results in an optical path with low loss and high polarization-maintaining ability. In 2011, KVH developed its ThinFiber, a 170-micron diameter dual acrylate-coated fiber that retains the full performance characteristics of E·Core fiber. ThinFiber has enabled the development of compact, high-performance open-loop FOGs, which are also used in a line of FOG-based inertial measurement units (IMUs) and inertial navigation systems (INS).

With the development of laser and optical fiber, the north finding system (NFS) with fiber optic gyroscopes (FOGs) has become an active trend in inertial technology research. This paper demonstrates the error analysis of dynamic north-finding with a fiber optic gyroscope and a new way for the FOGs noise measurement in the dynamic implementation. Moreover, in order to suppress the north-finding error caused by FOG bias drift, especially for the small azimuth angle, we propose a modified dynamic north-finding scheme by taking advantage of the dynamic method. The experiments verify the validity of the proposed error analysis for the dynamic implementation. Experimental results show that for a FOG with bias instability of 0.005°/hr, the average (Avg) north-finding bias error is less than 0.03° (0.5 mrad) around 2° azimuth angle by the proposed modified scheme. And compared with the conventional dynamic north-finding, this scheme reduces the estimated error (1σ) of azimuth angle and the bias error (Avg) by 22% and 23.8%, respectively, at the small azimuth angle. For a poorer-performance FOG with 0.0262°/hr bias instability, the proposed method is more effective and reduces 32.4% estimated error (1σ) of azimuth angle.

The article presents a telemetry system for monitoring the bit trajectory during well drilling. Fiber-optic closed-loop gyroscopes are the main sensitive element in the telemetry system. The gyroscopes allow determining the azimuth and zenith angles of the well with a high accuracy exceeding most magnetometers. There is no need to use the expensive non-magnetic steels in the body of the telemetry system and non-magnetic drill pipes in the drill string. Fiber-optic closed-loop gyroscopes allow using the telemetry system when drilling near the casing of adjacent wells and magnetized rocks. An inertial navigation system is installed in the developed telemetry system as a measuring unit containing three angular velocity sensors and three accelerometers mounted rigidly. The sequence and results of simulation of a hydraulic link for data transmission from the bottom of the well to the top are described. Pilot tests of the telemetry system were carried out at well No. 450 of the Tatsuksinskoye field. The obtained values of the spatial intensity of the borehole curvature in depth are close in values to the directional survey data for the borehole. The trajectory obtained during the lift is close to the trajectory gathered from directional survey data.

Draper Laboratory's experience with Interferometer Fiber Optic Gyro (IFOG) technology started via an Internal Research and Development effort in 1978. This work led to developing fiber optic gyros in which significant advances in IFOG component technologies, assembly, integration, and test were achieved. In excess of 30 patents have been issued to Draper as a result of this pioneering effort. More recently, Draper collaborated with JPL to transition their fiber optic breadboard gyro to a space qualifiable instrument for interplanetary long duration missions. During the initial phase, brassboard gyros were designed and fabricated that demonstrated performance that bettered NASA's CRAF-Cassini spacecraft objectives. In the second phase, the Engineering Model gyro was developed to meet mission qualification tests. Concurrent with this later phase, analysis, tests and qualification activities were performed to validate that gyro components would realize the required 16 year life. Two major inventions (patent pending) were conceived; one provides for continuous adaptive scale factor stability and the other integrates the optical source and photodetector into a single component [Source Integrated Detector (SID)], thereby eliminating a coupler and several splices thus reducing loss by 6 db. Using the above technologies and the implementation of common mode rejection of laser noise, Draper has defined a low noise high performance gyro in which rate power spectral density (PSD) is projected to be < 7 X 10-7 (deg/hr)2/Hz. In those applications, where the PSD < 4 X 10-5 (deg/hr)2/Hz is sought, a small size three component (SID, integrated optics circuit and fiber coil) gyro is easily implemented.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The fiber optic gyroscope (FOG)based on Sagnac effect has became to one of the most important sensors in developing due to light in quality, high accuracy, compact in dimension and long life and has played a very important role in both military and civil use. It is the most difficult problem that the FOG has an obvious bias drift caused by temperature change and temperature grade, so its application is limited to a great extent. Fiber coil is one of the most critical components in FOG. Here, the characteristic of temperature error of the fiber optical coil was analyzed. At first, by studying the glass transition of coating adhesive in the fiber coil, the element model of the fiber coil with the glass transition of coating adhesive in FOG was built. Then the discrete mathematics model of SHUPE error with the glass transition of coating adhesive in FOG was built. Finally, based on the temperature models mentioned above, the effects caused by the glass transition of coating adhesive on temperature performance of fiber optic gyroscope were analyzed. Theoretical analysis and experimental results show that effect caused by the glass transition of coating adhesive had seriously affected the temperature performance of FOG. By optimizing the glass transition temperature of coating adhesive, the SHUPE error of fiber coils can be reduced. At the same time, the amplitude uniformity of the SHUPE error can be improved greatly to reduce the difficulty in temperature compensation.

At present the precision of fiber optic gyroscope (FOG) is so lower that it is necessary to analyze the performance of FOG and set up its drift error model to raise the precision. There are many noise source in FOG, these noise sources and environment intrusion cause many random error terms, such as bias instability, angular random walk,rate random walk and rate ramp. It is impossible to adopt general analytical method (such as calculating mean and covariance) to confirm these random errors. Now, the precision of FOG made in our country is low and to improve it cost high and is difficulty. So this paper improve the system precision by software in the error modeling and filtering of the FOG random drift. Now, the main method to minish the FOG random drift is Kalman filter. In this paper,the FOG drift data is processed by Kalman filter,and the effect of filtering is analyzed. The simulation result show that Kalman filter can minish the FOG random drift more simply and more efficiently. Because the Kalman filter is based on the steady time series model of FOG random drift, in this paper, FOG random drift data is validated to be a non-stationary time series, so the unsteady sample of FOG drift needs statistical test and corresponding pretreatment using stochastic signal processing methods, and then the mathematical model is establishing by time series analysis theory. It is proved that the random noise can be represented by a single equivalent ARMA(auto-regressive moving average) or ARIMA model that is simple to implement. The data pretreatment is made, the model is identified,the method of using long autoregression method to estimate the coefficients are studied. In the end, experimental modeling of the FOG random drift is carried out, the random noise of FOG is processed by using Kalman filter. Experimental results demonstrate that the performance of the filter is feasible and the model can reject the random noise of FOG.

The optic-bound effect on fibre optic gyro (FOG) precision is analysed theoretically, and formulas describing the change of FOG light power under the action of optic-bound effect are given. It is confirmed that optic-bound effect is a main instantaneous non-reciprocity effect of FOG in addition to the Sagnac effect and can lead to a dynamic error of FOG in practical environments. An approach to suppressing or eliminating the error due to the optic-bound effect is presented and verified experimentally. The result shows that the approach is valid and the precision of FOG is improved observably.

Shupe error, a kind of bias error over temperature-varying environments, still exists even with strict quadrupolar winding pattern, and it affects the environment adaptability of fiber optical gyroscope (FOG) and limits its application. A novel configuration to realize optical compensation for Shupe error in FOG is promoted, employing the opposite polarity of Shupe error in two FOGs working with polarization-maintained (PM) fiber’s fast and slow axis, respectively, in a single PM fiber coil. Analysis is also made for polarization non-reciprocity (PNR) in both FOGs, based on which the optimum coupling angles between integrated optic chip and PM fiber coil are obtained to guarantee that Shupe error arising from both FOGs can be counteracted to zero, and PNR error is suppressed to be minimum at the same time.

A Star Sensor based Fiber Optical Gyroscope Error Separation method (SSFES) was proposed, which can be applied in the Fiber Optical Gyroscope (FOG) performance evaluation and fault diagnosis for the space-borne Inertial Navigation System (INS) & Celestial Navigation System (CNS) combined navigation system. The method takes the satellite rotational angular rate and the attitude quaternion to separate the long-term drift component from the FOG output and provides the useful information for FOG error compensation and model construction. The output quaternion of the star sensor was utilized to reconstruct the satellite rotational angular rate information, which was then used to separate the error component from the FOG output. Simulations have been carried out to verify the method and have shown that the error component of FOG output can be extracted efficiently by SSFES and the algorithm is easy to be performed by the onboard computers.

North-finding system provides true north azimuth information to all kinds of ground-based equipment. A dynamic automatic north-finder based on the modulation output of fiber optic gyroscope (FOG) is presented in this paper. In this scheme, the sine output signal of the FOG was sampled and disposed, and then the real north of surveyed point on the surface of earth was calculated. The deviation caused by the platform base tilt was compensated by an accelerator, Allan standard deviation is used to analyze the random drift of FOG, and establish the random error mode of FOG. The FOG north-finding and laser collimation theodolite are assembled to achieve orientation and aiming integral, which lays foundation for switching to project application.

The goal of this work is to identify the main source of noise in the output signal of a fiber-optic gyroscope (FOG) and ways to reduce it. To achieve the stated goal, two mathematical models of the noise component of the output signal of the FOG were developed. These models are based on fundamental expressions defining shot and thermal noise, as well as an expression for the excess noise of an optical radiation source. The general expression of the analytical model takes into account the four fundamental sources of noise in FOG. Their influence on the output signal noise is expressed through the relation between the measured optical radiation power and the angular velocity acting on the FOG. The computer model was developed in the “Octave” environment. Model performs cyclic execution of the FOG operation algorithm with a closed feedback loop for measuring angular velocity. In the computer model, noise from four independent sources is added to a given useful signal. An array of model output data is generated. The noise power spectral density is calculating. The paper analyzes the influence of FOG circuit parameters on the noise level. The noise source having the greatest significance in the output signal was identified. To confirm the correctness of the models, a comparison was made between the simulation results and the noise of commercially produced FOG. The model error is no more than 10%.

A platform inertial platform system (IPS) is a kind of independent navigation system. A fiber optic gyro (FOG) based IPS can stabilize its inertial measurement unit (IMU) in inertial space using its FOG readouts, and calculate its carrier's real-time displacement and velocity using its accelerometer readouts. Alignment is the initialization process of the IPS. During the coarse alignment of the FOG based IPS, the instrument errors, random noise and base vibration can cause decline of the alignment accuracy. To improve the precision of coarse alignment, the IMU rotation modulation is used to suppress the errors of inertial instruments by utilizing the equivalent integration process contained in the averaging computation. On the stationary base, calculate the attitude matrix of the rotating IMU in each sampling period, and averaging computation can compensate the errors caused by biases of inertial instruments. When the carrier is in vibration condition, the alignment coordinate frame (CF) is introduced to assist the rotation modulation. Carry out the attitude updating within the alignment CF in each sampling period to correct the estimated attitude matrix, and the rotation modulation based method can still improve the precision of coarse alignment in vibration condition.

To improve the assembly reliability of Fiber Optic Gyroscope (FOG), a light leakage detection system and method is developed. First, an agile movement control platform is designed to implement the pose control of FOG optical path component in 6 Degrees of Freedom (DOF). Second, an infrared camera is employed to capture the working state images of corresponding fibers in optical path component after the manual assembly of FOG; therefore the entire light transmission process of key sections in light-path can be recorded. Third, an image quality evaluation based region segmentation method is developed for the light leakage images. In contrast to the traditional methods, the image quality metrics, including the region contrast, the edge blur, and the image noise level, are firstly considered to distinguish the image characters of infrared image; then the robust segmentation algorithms, including graph cut and flood fill, are all developed for region segmentation according to the specific image quality. Finally, after the image segmentation of light leakage region, the typical light-leaking type, such as the point defect, the wedge defect, and the surface defect can be identified. By using the image quality based method, the applicability of our proposed system can be improved dramatically. Many experiment results have proved the validity and effectiveness of this method.

Wavelet analysis technique is proposed in the filter processing to remove the noise from random drift signal of fiber optic gyroscope (FOG). Based on the component elements of random drift signal of FOG, mathematical model of random drift signal of FOG is built. According to the shortcomings of hard threshold and soft threshold, threshold function of forced denoising is used in signal filtering. It is shown from simulation and statistic analysis that the wavelet analysis technique is an effective method to eliminate noise from random drift signal of FOG, and that selection of wavelet base and decomposition scale also has an impact on filtering effect of wavelet analysis.

Fiber optic gyroscope (FOG) is a new type of angular velocity sensor in the field of inertia technology. Its non-moving parts and all-solid-state characteristics leads to many advantages compared with traditional electromechanical gyro and ring laser gyro. Scale factor is a key parameter of FOG, which affects the angle measurement accuracy. Closed-loop feedback phase modulation is an important method to enhance the scale factor stability in FOG. The multifunctional integrated optical circuit (MIOC) is the actuator of closed-loop feedback, and the mean wavelength stability of transmission spectrum at full temperature affects the scale factor stability of FOG. With the improvement of FOG's accuracy, the effect of the mean wavelength shift on FOG's performance are becoming more and more important, which has been an important factor limiting the scale factor stability enhancement. Based on the optical waveguide theory, we found out alignment deviation (axial gap, transverse dislocation and axial angle deviation) is the main factor of the mean wavelength stability, and verified by coupling experiments. Moreover, the mean wavelength is the most sensitive to the transverse dislocation. Take z=10 μm for example, the mean wavelength increases by 134 ppm relative to the wavelength of 1.31 μm when x increased from 0 to 2 μm. Secondly the axial gap, the mean wavelength increase by 19 ppm when z increased from 0 to 20 μm. Thirdly the angle deviation plays a minor role, and the mean wavelength decreases by 5 ppm when θ increased from 0° to 2°. This study is of great significance to optimize MIOC, and to provide device guarantee for the enhancement of scaling factor stability of FOG.