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​Illustration of a HUGIN AUV utilizing UTP to aid its navigation.
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03.02.2011
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Precise navigation remains a substantial challenge to all underwater platforms, including AUVs.

​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:

  • Vehicle GPS: The HUGIN vehicle is equipped with its own GPS receiver, which can be used whenever the GPS antenna is above the sea surface. As GPS fixes often are time-consuming and revealing during underwater operations, GPS fixes are usually not frequently used.

 

  • DGPS/USBL aiding: Position updates are sent to the vehicle from a surface vessel, based on a combination of the ship position and acoustic USBL (Ultra Short Base Line) positioning of the vehicle relative to the surface vessel. In the HUGIN vehicles, Kongsberg Maritime’s HiPAP system is used for the acoustic positioning of the vehicle. This technique has high accuracy, but it requires that the surface vessel follow the AUV during the mission, which is unpractical and undesirable in many operations.

 

  • UTP (Underwater transponder positioning): A technique developed by FFI that uses range measurements from underwater transponders in order to obtain submerged position fixes. The transponders have to be deployed and positioned prior to the operation. The algorithms are able to utilize the range measurements from only one transponder in an optimal manner, as opposed to a standard LBL (Long base line) system, which requires at least three transponders within the range of the vehicle in order to obtain a distinct position fix (See main illustration on top of this page). UTP is commercialized as a product from Kongsberg Maritime.

 

  • Terrain Navigation: Most AUVs are equipped with sensors that are capable of measuring the depth variations of the sea floor. The HUGIN vehicles have advanced multi beam echo sounders and interferometric SAS (Synthetic Aperture Sonar) that provide detailed bathymetry. The measurements from these sensors can be compared with an existing map, to obtain submerged position updates. Terrain navigation has been a research topic for years at FFI, and a real-time terrain navigation system for the HUGIN vehicles is currently under development and testing.

 

  • DPCA micro navigation: SAS uses consecutive pings to synthesize a larger array. For this to work properly, the array position displacement between pings must be found with extremely high accuracy. This is called micro navigation or DPCA (displaced phase centre antenna). By using this position displacement as a velocity measurement, it can potentially be an order of magnitude better than even the most accurate DVL.

 

  • Macro navigation: As an extension of the idea of the micro delta-position aiding, we can consider sensor measurements where the same patch of the seafloor is seen with an arbitrarily large time interval between the measurements. One example is feature-based navigation, in which the position features measured with a sonar or camera are used as position measurements whenever the same feature is revisited during the operation. Such techniques may be labeled macro navigation, due to the potentially long time elapsed between consecutive measurements. Macro navigation techniques for the HUGIN vehicles are currently under development at FFI.

 

 

NavLab

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

  1. Navigation system research and development
  2. Analysis of navigation system
  3. Decision basis for sensor purchase and mission planning
  4. Post-processing of real navigation data (maps from more than 200 000 km of billed survey lines have been positioned by NavLab)
  5. Sensor evaluation 
  6. Tuning of navigation system and sensor calibration


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.

 

Real-time navigation

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)

 

Publications

A list of publications on HUGIN navigation technology can be found at www.navlab.net/Publications/.

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