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Feature Article

Low-Altitude Terrain Navigation For Underwater Vehicles
Integration of an Interferometric Side Scan Sonar Improves Terrain Navigation in Low-Altitude Scenarios

Feature Author
Ove Kent Hagen
Senior Scientist
Feature Author
Dr. Kjetil Bergh Ånonsen
Feature Author
Dr. Torstein Olsmo Sæbø
Senior Scientist
Norwegian Defence Research Establishment
Kjeller, Norway

HISAS 1030 interferometry with a blind zone beneath the AUV (left) and EM 3000 bathymetry (center). The fusion between the sensors (right) gives full coverage.
Challenges still remain for the navigation systems of AUVs in sustained submerged missions. If not supported by some kind of external positioning system, the position error of a state-of-the-art aided inertial navigation system (INS) for AUVs grows about 0.1 percent of traveled distance along a straight line. New aiding techniques are under development utilizing suitable payload sensors for improving navigation. Terrain navigation is one of these techniques that enable submerged position updates.

By correlating bathymetric measurements made by an AUV with an a priori digital terrain model (DTM), the terrain navigation algorithms find the best matching position. Several real-time systems for AUVs already exist. In a sea trial last year, a DTM was constructed in real-time onboard Kongsberg Maritime's (Kongsberg, Norway) HUGIN AUV and later used for terrain navigation within the same dive. This enables relative position updates when no DTM is available before the mission.

At low altitude, the footprints of the sensors used so far, e.g., the multibeam echosounder and Doppler velocity log, become so small that it is difficult for the terrain navigation algorithms to converge, and they become less accurate.

In mine countermeasure (MCM) missions, HUGIN operates at about 20 meters altitude in the sonar detection phase and about 5 meters altitude in the optical identification phase. The swath width of a Kongsberg EM 3000 multibeam echosounder mounted on the HUGIN AUV then reduces from 200 meters in typical surveys at 50 meters altitude to 80 meters during MCM detection and 20 meters during MCM identification.

The HUGIN AUV can also be equipped with side-looking interferometric sonar on both sides of the vehicle, which has a wide swath even at low altitudes. By combining a down-looking multibeam echosounder and side-looking interferometric sonar, a large footprint for the terrain navigation algorithms can be maintained at low altitude.

HISAS 1030
The HISAS 1030, manufactured by Kongsberg Maritime, is a side-looking interferometric synthetic aperture sonar (SAS) that can produce two different bathymetric products: bathymetry based on SAS images and bathymetry based on side-scan lines. Only side-scan bathymetry is considered in this article.

The HISAS 1030 transmits broad chirp signals around 100 kilohertz with a pulse repetition frequency of around 4 hertz. For each transmitted pulse, the echo is received at two vertically separated receivers, and the differences in two-way travel time are estimated by running short cross-correlations along the beams. Finally, the time differences are converted to relative depth measurements.

As with the EM 3000, the HISAS 1030 depth measurements are compensated for roll and pitch, and for effects induced by the sound speed profile (SSP). The depth accuracy is dependent on range and the signal-to-noise ratio, but typically it is better than 5 centimeters if the SSP is known. But, since the HISAS 1030 observes more horizontally than the EM 3000, it is more sensitive to SSP errors.

Since the HISAS 1030 is a side-looking sonar, it has a blind zone directly below the vehicle. On the HUGIN, one HISAS 1030 is mounted on each side. Each HISAS 1030 has a swath 45 degrees away from the nadir out to 200 meters range. For a typical AUV altitude of 20 meters, this means that the swath is 20 to 200 meters on each side. Since the EM 3000 covers the blind zone of the HISAS 1030, the two sensors are complimentary and their measurements well-suited for fusion.

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Ove Kent Hagen is a scientist in the navigation group at the Norwegian Defence Research Establishment. Hagen holds a master's in fluid mechanics and a bachelor's in mathematics from the University of Oslo. Since 1999, he has worked on underwater navigation of the HUGIN AUV, with special focus on terrain navigation.

Dr. Kjetil Bergh Ånonsen is a scientist in the navigation group at the Norwegian Defence Research Establishment, which he joined in 2007. He received his master's in applied mathematics in 2003 and Ph.D. in engineering cybernetics in 2010 from the Norwegian University of Science and Technology. His Ph.D. thesis focused on terrain navigation.

Dr. Torstein Olsmo Sæbø is a scientist in the synthetic aperture sonar group at the Norwegian Defence Research Establishment. He received a master's in astrophysics in 2002 from the University of Oslo and a Ph.D. in physics in 2010 from the University of Tromsø. His expertise is in interferometry on synthetic aperture sonar.

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