The Development of a FOG Based Tightly Coupled GNSS/INS Integrated System with Simple Temperature Compensation Method for Land Applications

J-K. Liao, T-T. Duong, K-W. Chiang, T-H. Kuo

Abstract:	The last two decades have shown an increasing trend in the use of positioning and navigation (POS/NAV) technologies in various applications in geomatics and navigation communities. GPS can provide continuous, accurate positioning with lines of sight to more than four satellites. However, the accuracy and availability of GPS-based vehicular navigation systems are subjected to the open-sky condition and degrade in the presence of signal blockage and reflected signals INS can fill the GPS gaps to provide continuous navigation solutions (position, velocity, and attitude). In principle, an IMU refers to a set of inertial sensors including three gyroscopes and accelerometers and it provides compensated raw measurements including velocities changes (delta-Vs) and orientation changes (delta-?s) along three directions of its body frame. On the other hand, an INS usually refers to an Inertial Measurement Unit (IMU) combining with an onboard computer thus it can provide navigation solutions in the chosen navigation frame directly in real time, in addition, it also provides compensated raw measurements. Although an integrated navigation system can work in GPS-denied environments, problems include the cost of inertial sensors and the length of time during the unavailability of GPS signals affects its applicability. Tactical-grade or better inertial systems can achieve good positioning accuracy and sustainability during long GPS signal blockages?The Fiber Optical Gyros (FOG) of tactical grade IMU has more high accuracy than Micro Electro Mechanical System (MEMS) gyros. Thus it is more suitable for more applications which need high accuracy. The calibration of tactical grade IMU should be more precise and complete because of those more sensitive sensors. Inertial sensor errors can be divided into two parts: random and deterministic or systematic. In order to integrate inertial sensors with GPS, and to provide a continuous and reliable navigation solution, the characteristics of different error sources and the understanding of the stochastic variation of these errors are of significant importance. The random errors include bias-drifts or scale factor drifts, and the rate at which these errors change with time. These random errors have to be modeled stochastically. The deterministic error sources include the bias and the scale factor errors which can be removed by specific calibration procedures in a laboratory environment. However, those inertial sensor errors are environmental dependent, especially temperature, which makes calibration a necessity. More explicitly, the actual values of the bias and the scale factor vary from those obtained through calibration process due to the difference between the operational and calibration temperatures. If the thermal variations for both accelerometer and gyroscope biases and scale factors are not properly modeled and compensated, the position accuracy will be degraded since these errors get accumulated with time. Hence there is a need for developing accurate, reliable and efficient thermal models. However, a standard thermal calibration procedure for FOG based IMU usually requires a dual axis rate table with thermal chamber but such rate table is very expansive. The process of characterizing the stochastic variation at different temperatures is one of the most important steps in developing a reliable low cost integrated navigation system. The actual values of the bias and the scale factor vary from those obtained through calibration process due to the difference between the operational and calibration temperatures. Unless an accurate temperature-dependent stochastic model is developed, the mechanization parameters will have larger errors and could potentially degrade system performance. Therefore, this study will implement the calibration based on multi-positions method with biaxial turntable and also calibrate the temperature effect based on the internal temperature provided by the built-in temperature sensor of the FOG IMU assembled in this study. The proposed tactical grade IMU is composed of three FOG gyroscopes with 1deg/hr bias stability as well as three MEMS accelerometers with 1mg bias stability. The twelve positions method with dual axes turntable simplify the the calibration procedure as the testing unit only needs to be installed once which can avoid the errors of different installation when the IMU is unleveled. In additional, twelve positions also can reduce the errors of asymmetry and unleveled cause by turntable and IMU. The study used the average and least square method to calculate the bias, scale factor and non-orthogonality. Furthermore, we will calculate those parameters with different internal temperature and determine the trend of variability.. The difference with the general procedure is the proposed procedure doesn’t require temperature chamber . The procedure of calibration is the same to the multi-positions method which used for general calibration at the constant temperature. In other words, we can get the bias and scale factor at different temperature to analyze the characteristics of the errors based on some regression method during the internal temperature variation and accurate information provided by the rate table. In addition, the FOG IMU assembled in this study will be integrated with GNSS receiver with tightly coupled scheme after applying thermal compensation model obtained with proposed method. The study will also analyzes the performance of proposed tightly coupled GNSS/INS integrated navigated system based on FOG IMU with various EKF design with different states with real field test in GNSS denied environment.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	2204 - 2212
Cite this article:	Liao, J-K., Duong, T-T., Chiang, K-W., Kuo, T-H., "The Development of a FOG Based Tightly Coupled GNSS/INS Integrated System with Simple Temperature Compensation Method for Land Applications," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 2204-2212.
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