Underwater navigation

This thesis describes an automated post-processing tool, designed for use on navigational data gathered by Autonomous Underwater Vehicles (AUVs), developed and operated by the Department of Ocean Engineering at Florida Atlantic University. The post-processing tool consists of a 9-state complementary Kalman filter in conjunction with a Rauch-Tung-Striebel (RTS) smoothing algorithm. The Kalman filter is run forward in time to merge navigational data from an Inertial Measurement Unit (IMU), a Doppler Velocity Log (DVL), a magnetic compass, a GPS/DGPS system and an Ultrashort Baseline (USBL) tracking system. Subsequently, the RTS smoothing algorithm is run backwards in time to find and compensate for drift errors in dead reckoned position and compass measurement error. The post-processing tool has been implemented as a graphical user interface, designed in MATLAB. Improved accuracy in post-processed position and heading has been verified by conducting sea trials and post-processing the collected data.

This report highlights important aspects of previous work with the Ocean Explorer (OEX) autonomous underwater vehicle (AUV) docking system as a background. This includes short baseline navigation, the Tracking Controller, Mechanical aspects of the dock, and results of testing of the docking system for the OEX Details of the Morpheus AUV are then given along with the major concerns faced in trying to adapt the OEX dock to the Morpheus. Using computer simulation, the reaction of the Morpheus when it impacts the dock is explored and the results of at sea testing (the collision of the vehicle and the dock) is discussed. A stinger strength analysis of the docking components is included and finally, suggestions for future work including modifications of the existing dock as well as another docking scheme are presented.

This thesis describes the implementation of a commercially available forward looking sonar (FLS) in an autonomous underwater vehicle (AUV) modified for the task of reactive obstacle detection. Any obstacle lying in the vehicle's path is a potential mission-terminating threat. Inclusion of a forward looking sensor would provide valuable information to the AUV. Threat assessment and navigation would use this information in order to avoid obstacles. The system used for this project is an 8-element transducer FLS at 200 kHz. The sonar control software is done in DOS on a dedicated personal computer in a PC/104 form factor. A variable cell-size grid occupancy search method is used to detect objects in the vehicle path. This thesis describes how this sonar is used for the obstacle detection task (software), how it is integrated (hardware and network) in the AUV and what are the results obtained with the system.

This thesis describes a new Inertial Navigation System (INS), designed for use on the latest generation of Autonomous Underwater Vehicle (AUV), the Morpheus AUVs, developed by the Department of Ocean Engineering. The INS makes use of a high precision Inertial Measurement Unit (IMU) along with a Doppler Velocity Log (DVL), a GPS/DGPS system and a flux gate magnetic compass to estimate the position of the vehicle during a mission. Extensive data processing methods are used in order to reduce the sources of error most critical to the navigation estimation. Accelerometers and gyroscopes are used to estimate the vehicle attitude in a complementary filter; a standard Kalman filter performs yaw rate bias estimation, whereas a second, extended Kalman filter, provides position estimation of the vehicle in real time, while also correcting for heading errors. Overall the INS is capable of providing the Morpheus with 1% navigation accuracy over an hour.

The design and integration of an unmanned surface vehicle (USV) control system is described. A survey of related work in both USV control, and unmanned vehicle operating software is presented. The hardware subsystem comprising a modular Guidance, Navigation, and Control (GNC) package is explained. A multi-threaded software architecture is presented, utilizing a decentralized, mutex-protected shared memory inter-process communication subsystem to provide interoperability with additional software modules. A generic GNC approach is presented, with particular elaboration on a virtual rudder abstraction of differential thrust platforms. A MATLAB Simulink simulation is presented as a tool for developing an appropriate controller structure, the result of which was implemented on the target platform. Software validation is presented via a series of sea trials. The USV was tested both in open- and closed-loop control configurations, the results of which are presented here. Lastly recommendations for future development of the GNC system are enumerated.

Standard GPS receivers are unable to provide the rate or precision required when used on a small vessel such as an Unmanned Surface Vehicles (USVs). To overcome this, the thesis presents a low cost high rate motion measurement system for an USV with underwater and oceanographic purposes. The work integrates an Inertial Measurement Unit (IMU), a GPS receiver, a flux-gate compass, a tilt sensor and develops a software package, using real time data fusion methods, for an USV to aid in the navigation and control as well as controlling an onboard Acoustic Doppler Current Profiler (ADCP).While ADCPs non-intrusively measure water flow, they suffer from the inability to discriminate between motions in the water column and self-motion. Thus, the vessel motion contamination needs to be removed to analyze the data and the system developed in this thesis provides the motion measurements and processing to accomplish this task.

The main contribution in this thesis is the design of a robust AUV docking guidance and navigation approach that can guide and home an AUV towards an acoustic source located on an oriented bottom-mounted underwater docking station, under presence of unknown current disturbances and in the absence of any form of onboard velocity sensor. A Complementary Filter and various forms of Kalman Filters were separately formulated to estimate the current and vehicle positions with strategic vehicle manoeuvres. A current compensator uses the estimated current to maintain the desired vehicle course while under current disturbance. Tagaki-Sugeno-Kang Fuzzy Inference System was designed to realize fuzzy docking guidance manoeuvres. Finally, Monte Carlo runs were performed on a designed AUV docking simulator to evaluate the docking robustness against various docking conditions. Simulation results demonstrated robustness in the designed docking guidance and navigation approach.