Info

Relative East position [m]

Fig. 4. A wellhead mapped repeatedly with different headings to evaluate the position accuracy of the final map, and hence the navigation system. The true position of the wellhead was located at coordinates (0,0) relative north and east.

2.7 LBL (Long Base Line)

Underwater transponders may be used to provide submerged position measurements to the AUV. If multiple transponders are arranged in a network for this purpose, it is called a long base line (LBL) system. The AUV interrogates the LBL network, and uses the reply from each transponder to calculate the range from the AUV to the transponders. If the 3D position of each transponder is known, the AUV can compute a unique position by triangulation if replies are received from three or more transponders. If only two transponders are within range (sparse LBL system), two possible position solutions exist. These are mirrored about the base line between the transponders.

The LBL network is deployed and calibrated by a surface vessel, and is therefore not a strictly autonomous solution. However, once the network is established, the AUV may navigate autonomously with bounded error drift by visiting the network occasionally.

2.8 UTP (Underwater Transponder Positioning)

Range measurements from underwater transponders may also be coupled tighter with the AUV's aided INS. This concept, called underwater transponder positioning (UTP) or synthetic baseline, has the advantage that it will work with even a single transponder. An example illustrating the deployment of a transponder is shown in Fig. 5. The transponder location can be determined within 1 m relative global datum (e.g. WGS-84) at 1000 m depth if applying a Kongsberg HiPAP USBL system during the box in process. As the AUV moves around or between transponders, new ranges from different directions will be obtained. Combined with INS, this can provide a navigation accuracy comparable to using USBL aiding from the support vessel. Fig. 6 shows the trajectory of a HUGIN AUV and UTP transponder locations. HUGIN navigated with UTP as the only source for position updates. A 30 kHz UTP system with approximately 1.4 km range was used. The AUV ran two straight lines of 7.5 km with two transponders 4 km apart. With NavLab navigation post-processing, the navigation data was compared to independent USBL data stored on the survey vessel. The difference between the UTP post-processed navigation solution and the independent USBL post-processed navigation solution is shown in Fig. 7. Since the deployment of transponders is time consuming, an important feature of UTP aided navigation is that only one transponder is required to bind the INS position error drift. While the system works with only one transponder, any number of transponders may be incorporated in an optimal way. Compared to a traditional LBL system (Section 2.7), UTP has improved accuracy due to tight coupling with the INS, increased operating area and significantly lower deployment costs. Further discussion on the use of UTP and risk reduction in autonomous under ice AUV surveys can be found in (Jalving et al., 2008).

Fig. 5. A Kongsberg Maritime release transponder

Log time: 10-Jul-2008 11:59:28 to 10-Jul-2008 14:07:58

Fig. 5. A Kongsberg Maritime release transponder

Log time: 10-Jul-2008 11:59:28 to 10-Jul-2008 14:07:58

Fig. 6. North (x) and east (y) relative coordinates of 2D AUV trajectory during sea trial. Two UTP transponders were deployed 4 km apart.

Log time: 10-Jul-2008 11:59:28 to 10-Jul-2008 14:07:58

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