Fig. 3. Mission example with GPS surface fix using a HUGIN AUV, September 2003.
equipped with a GPS system, and tracks the AUV with an ultra short base line (USBL) positioning system, e.g. Kongsberg HiPAP. By combining GPS with USBL, a global position estimate can be obtained, which is then transmitted to the AUV using an acoustic link. The accuracy of the position estimate depends on both the GPS and USBL precision. The position accuracy of single measurements from the standalone HiPAP is in the order of 0.2% of the range (line of sight from the surface ship to the AUV).
While this positioning source will not be available in autonomous operations, it is very useful for providing accurate initialisation of the navigation system when operating in deep water. Transmitting a series of GPS-USBL position fixes to the AUV will yield a very low initial position estimate error (typically on the order of 1 m), and a very low heading estimate error - which in turn minimizes drift when no position aiding is available. The navigation precision obtained by combining DGPS-USBL and DVL with INS was demonstrated with the HUGIN 3000 AUV in an accuracy test carried out by C&C Technologies in the Gulf of Mexico in October 2000. The test was carried out at 1,300 m depth, and eleven different headings were used (five of the lines were mapped in opposite directions) when mapping a wellhead to maximize the visibility of any correlated errors following the AUV or ship. When post-processing the navigation data and wellhead observations in NavLab (see Section 2.11), a position accuracy of 1.2 m and 1.7 m (1o) north and east was obtained. The final map showing the wagon-wheel like trajectory and the experimentally validated navigation precision (the exact location of the wellhead was known in advance) are shown in Fig. 4. An absolute position accuracy of 4 m (1o) was later obtained during successive navigation accuracy tests carried out at 2,100 m depth.
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