Fig. 5. Plan view of the AUV position estimate during the plume tracking survey downstream from the outfall diffuser.

Although the position estimate, provided by the navigation system, is accurate enough for the guidance and control of the AUV, it is not well suited for spatially locating the data collected. In fact, the discrete nature of the correction mechanism used in real time gives rise to discontinuities in the evolution of the estimate, which is a major drawback for spatially locating the collected data (Matos et al., 2003).

An algorithm that processes the data collected by the vehicle navigation system during the execution of a mission and produces an estimate of the trajectory described by the vehicle more suited for spatially locating the oceanographic data gathered was applied (see the details in Matos et al. (2003)). The final result is shown in Fig. 5.

Little deviation from the line transects specified (in red) is observed, with an exception to the trajectory at 4 m depth, in sections 3 and 6, where the navigation system could not correct the vehicle position estimate, because no valuable information was being received at that time from the transponders.

A larger distance of ~20 m from the middle point of the diffuser to the middle point of section 1, instead of the 12 m considered in the mission specification, was observed. This offset of about 8 m between the predicted and performed trajectories was not result of a poor performance of the navigation algorithm, but a direct consequence of the relative strong water current to the south during the mission. In fact, the heading reference generated by the vehicle guidance system does not integrate the off track error, giving rise to large off track errors when there are large water currents perpendicular to the desired trajectory (Matos et al. 2003).

During the vehicle deployment process, a drift from the "home" position, where it was supposed to be launched, occurred, so a correction to the expected trajectory from Point 1 to Point 2 was efficiently performed by the navigation system at the beginning of the mission in the trajectory at 2 m depth.

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