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Session D3: Marine Vehicle Navigation

An Error Compensation Method of DVL Assisted by IMU and Differential GPS
Jianxiong Wei, Ya Zhang, Pan Jiang, Shiwei Fan, Fei Yu, Harbin Institute of Technology, China
Location: Galleria I/II
Date/Time: Wednesday, Apr. 22, 12:17 p.m.

As an acoustic velocity measuring instrument, Doppler velocity log (DVL) has a wide range of applications in marine engineering, especially with the development of marine navigation system, SINS / DVL integrated navigation system has been widely used in ships and underwater vehicles, and the research on DVL velocity measuring technology has attracted more and more attention. DVL is an underwater acoustic navigation instrument based on Doppler sonar technology, which can measure the absolute velocity of a ship or underwater vehicle to the bottom of the water. In the field of navigation, it can measure the velocity information of the vehicle in real-time, and the velocity error does not accumulate with time, so it has strong anti-interference ability. It is very important to reduce the error of DVL and improve the accuracy of DVL speed for improving the accuracy of DVL/INS integrated navigation system.
To reduce the speed measurement error of DVL, the common method is to calibrate the speed of ultrasonic by measuring the water temperature and salinity of the driving area before its actual application, select the appropriate acoustic emission angle according to the depth of the ocean, and adjust the frequency of the acoustic emission. These methods can improve the speed measurement accuracy of DVL itself. However, the errors caused by installation error angle and ship swing attitude can not be eliminated only by improving the speed measurement accuracy of DVL itself. As the main error source, the error caused by the installation error angle and the ship's swing attitude needs to be estimated and compensated by the external navigation measuring instrument. In this way, the DVL coordinate system coincides with the IMU coordinate system as much as possible, and outputs the information of the carrier system, so as to improve the accuracy of DVL speed measurement.
In order to reduce the influence of the ship's sway motion on the determination of speed, DVL uses the method of 2-beam velocity measurement. A pair of beams installed in the fore and aft direction of the ship can measure the speed in the longitudinal direction, and a pair of beams in the starboard and starboard directions can measure the speed in the transverse direction. However, the method of 2-beam velocity measurement is not strong enough to eliminate the speed measurement error when the ship has a large swing angle. In simulation , when the pitch has a 2° swing angle, the DVL longitudinal speed measurement error is only 0.06%, but when the pitch has a 10° swing angle, the DVL longitudinal speed measurement error is 1.52%. In this instance, the velocity error can not be ignored, and the method of 2-beam velocity measurement can not eliminate the error in the case of large swing angle, which seriously affects the velocity measurement accuracy of DVL. If the velocity error of each DVL measuring coordinate axis is calculated and compensated separately, the calculation is tedious. In order to solve this problem, the 4-beam projection method is proposed to obtain the speed of DVL. By projecting the coordinate system of DVL velocity measurement to the beam coordinate system, the projection of velocity vector on the beam coordinate system can be obtained. The velocity of IMU coordinate system without compensation is obtained by using the Doppler frequency shift in the direction of four acoustic beams, and the velocity of IMU coordinate system measured by DVL is corrected in time by using the installation error angle information between DVL and IMU. Then, according to the direction cosine method between coordinate systems, the velocity of DVL coordinate system is projected to navigation coordinate system. Due to the existence of installation error angle between DVL and IMU coordinate system, the conversion error will occur when the speed measured by DVL is converted to navigation coordinate system. it is necessary to measure the installation error angle between DVL and IMU coordinate system accurately before practical application. Differential GPS can measure high precision velocity information, so differential GPS is used to give velocity reference of IMU coordinate system. The difference between the velocity measured by differential GPS and the velocity projected by DVL into IMU coordinate system is used as the observation of Kalman filter to calibrate the installation error angle. After the filter is stable, the estimated installation error angle is fed back to DVL, so that the measurement information is more accurate and the estimation accuracy is higher.
According to the simulation and the navigation experiment on the lake, for the installation error of DVL instrument, taking differential GPS as the reference speed, using Kalman filter algorithm to estimate the installation error angle can effectively calibrate the installation error angle of DVL and reduce the speed measurement error. For the log speed measurement error caused by the ship attitude angle swing, the 4-beam projection method is used as the Doppler speed measurement method. Then the attitude strapdown matrix of the strapdown inertial navigation system is used to project the measured speed to the navigation coordinate system, which can effectively suppress the speed measurement error caused by the ship attitude angle swing. The validity of the design method is verified.



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