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Navigation System Design Of a Deep-Sea AUV


Yanhui Wang

Shuai Shao

Shuxin Wang

The main sensors and their locations in the AUV: 1) Evologics S2C modem, 2) iXBlue PHINS, 3) GPS, 4) iXBlue GAPS USBL MT8 transponder, 5) NavQuest 300 DVL, 6) Paroscientific 8CDP depth gauge, 7) EdgeTech 2200-M side scan sonar, 8) Kongsberg EM 2040 AUV multibeam sonar and 9) Kongsberg 1007 altimeter.
AUVs have a broad range of applications in ocean exploration and offshore petroleum resources mining. Already in use are the Bluefin Robotics (Quincy, Massachusetts) AUV for deepwater target search with side scan sonar, the Kongsberg (Kongsberg, Norway) Hugin AUV for offshore survey and the Monterey Bay Aquarium Research Institute's Dorado AUV with intelligent algorithms for plankton research.

Navigation and positioning precision are fundamental technologies when the AUV cruises in deep water with a long range. Most of the AUVs with high positioning accuracy use integrated navigation systems. The systems use data fusion of multisensors, such as inertial element, Doppler velocity log (DVL) and depth gauge. The integrated navigation system increases the accuracy of navigation as well as reliability when some of the sensors are disabled.

Tianjin University cooperated with Beijing, China-based China Oilfield Services Ltd. (COSL) to develop a 3,000-meter-depth-class AUV in 2010 named TUCOS-I (Tianjin University - China Oilfield Services Ltd.). It is equipped with an EdgeTech (West Wareham, Massachusetts) 2200-M side scan sonar operating at 410 and 120 kilohertz. It also has a Kongsberg EM 2040 AUV multibeam sonar. When TUCOS-I dives underwater, the iXBlue (Marly-le-Roi, France) ultrashort baseline (USBL) system GAPS (global acoustic positioning system), along with the mother ship, locates the AUV. The position information is then sent to the AUV for position calibration over acoustic modems at a certain interval time.

Based on the offshore oil engineering demand of China, in order to get a high-quality map of the seafloor, a 9.5-meter positioning accuracy is requested by COSL for surveys. Because TUCOS-I works at different depths, the interval time for position calibration will also be changed according to the position accuracy requirement and cruise speed. The bandwidth of the acoustic channel is limited, hence the longer interval for location information. The considerable time gap is better for the communication system, but will lead to a loss of positioning accuracy.

An integrated navigation system was developed for TUCOS-I, and the positioning error was analyzed. The relationships between the position error and operation parameters such as cruise speed, working depth and interval time for position calibration were derived. This gave the reference for TUCOS-I parameters regulation when working at different depths. Sea trials of TUCOS-I were carried out in the Bohai Sea in May 2012, and clear seabed images of the deepwater port were obtained.

Integrated Navigation and Positioning System Design
iXBlue's PHINS (photonic inertial navigation system) is the fundamental inertial navigation element of TUCOS-I. Other sensors used for navigation are GPS, LinkQuest's (San Diego, California) NavQuest 300 DVL (Doppler velocity log), Kongsberg 1007 altimeter and Paroscientific Inc.'s (Redmond, Washington) 8CDP depth gauge. Position information from GPS, the relative velocity to the seabottom from DVL, and the height and depth from altimeter and depth gauge are filtered and then fused into PHINS.

An iXBlue GAPS USBL system was used to locate the position of TUCOS-I underwater. A pair of EvoLogics (Berlain, Germany) S2C acoustic modems was utilized to transmit the position data to calibrate PHINS at a preset time interval. The position drifting of PHINS during the time interval was then updated using this method. When TUCOS-I was on the surface, the GPS position was used for the position calibration. The output data of PHINS, including position, attitude and velocity information, were used for navigation of TUCOS-I.

Position Accuracy Analysis of the Navigation System
The sources of position errors of TUCOS-I include velocity error from the DVL, depth error from depth gauge and position error from GPS, GAPS and PHINS. When the AUV works underwater, the slant range is relatively large and the time delay of the acoustic modems is considered.

The differential GPS is equipped with the system, and the position accuracy is relatively high. Also, the accuracy of the depth gauge is 0.001 percent. Hence, the errors of GPS and depth gauge can be ignored in this article. Error sources mainly focus on position error of GAPS, position drift of PHINS and the DVL integration system, and time delay because of the sound speed in water.

Position Error of GAPS
GAPS has a relatively high location accuracy with the original iXBlue beacon. The range error is less than 0.2 meters. The location angle accuracy is 0.12 degrees. The major source of error is the angle-measuring error. Thus, the location error of GAPS can be written approximately as:

Where E is the location error of GAPS, L is the slant range and is the angle-measuring error. To continue this article please click here.

Yanhui Wang received bachelor's, master's and Ph.D. degrees in mechanical engineering from Tianjin University, where he is an associate professor at the school of mechanical engineering. He has been involved in the research of various underwater vehicles.

Shuai Shao received a bachelor's degree in mechanical engineering from the Hebei University of Science and Technology in 2011. He joined the school of mechanical engineering at Tianjin University in 2011. He has been engaged in developing AUVs.

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