Over the last two decades, global navigation satellite systems such as GPS have solved this issue for most surface, land and air based applications. No similar system exists for positioning below the sea surface. Autonomous operation in deep water or covert military operations requires the AUV to handle submerged operation for long periods of time. The philosophy behind the HUGIN navigation system is to employ the best possible inertial navigation system (INS) together with a large toolbox of aiding sensors and techniques. The range of available aiding techniques makes the system robust and adaptable to a wide range of applications, operating either in supervised or autonomous mode.
An Inertial Navigation System calculates position, velocity and attitude using high frequency data from an Inertial Measurement Unit (IMU). An IMU consists of three accelerometers measuring specific force and three gyros measuring angular rate. If left unaided the INS will, after a short period of time, have unacceptable position errors. The position error growth is determined by the class of the IMU. Currently the best available IMU with a feasible size for an AUV gives a position error growth in the order of 1 nmi/h (1 sigma), when integrated in an INS. To reduce this error the INS needs to be aided by redundant sensor measurements. These sensors are usually integrated with the INS through a Kalman filter, which performs this integration in a mathematically optimal manner. The HUGIN KF structure illustration shows a schematic view of the HUGIN integrated INS, where the Kalman filter is based on an error-state model and provides a much higher total navigation performance than is obtained from the independent navigation sensors. The solution for most modern AUVs is a low drift Doppler Velocity Log (DVL) aided inertial navigation system that can integrate various forms of position measurement updates. DVL accuracy is dependent on acoustic frequency. Higher frequency yields better accuracy at the cost of decreased range.
Research performed in the HUGIN navigation group at FFI is concerned with the problem of integrating different aiding sensors in the main navigation system in an optimal manner. The navigation system in the HUGIN vehicles today is the result of a close collaboration between FFI and Kongsberg maritime. The basis of the system is a navigation grade Honeywell HG9900 IMU aided by a DVL measuring the bottom relative (or, optionally, water relative) velocity of the vehicle. In addition to this, a number of aiding techniques are being used or are under development, including:
The common software tool used for the development of the various aiding techniques is called NavLab (Navigation Laboratory), and is developed by FFI. NavLab consists of a Simulator part and an Estimator part, as shown in the NavLab structure illustration.
Any vehicle trajectory and sensor configuration can be simulated, and the Estimator runs equally well on simulated measurements as real logged sensor measurements. This structure makes NavLab useful for a range of different applications, such as
NavLab is used by international research groups, universities, commercial companies and the Norwegian Navy. Various vehicles have been navigated by NavLab, including AUVs, ROVs, cars, ships and aircraft. For more information, see www.navlab.net.
The HUGIN real-time navigation system is running the navigation algorithms from NavLab in real-time. This navigation system is commercialized by Kongsberg Maritime under the product name HAIN (Hydroacoustic Aided Inertial Navigation, http://www.km.kongsberg.com/hain)
A list of publications on HUGIN navigation technology can be found at www.navlab.net/Publications/.