Precise tracking and positioning of the towed vehicle is an important part of doing ocean bottom surveys. This is usually accomplished passively by slowly moving the ship from one position to another and assuming that, after the transients have died out, the towed vehicle will have moved the exact same distance and direction as the ship. The combination of delays and the slow rate at which the system returns to steady state makes this method slow and time consuming. To quantify this, we simulated a maneuver of a vehicle being moved to a location 400 m away, holding position within 20 m of the new location for 10 minutes, and then returning to within 1 m of the original position. Using passive ship movement as described above, the maneuver took almost 160 minutes to accomplish. To improve performance, we examined two alternate strategies. One strategy involved the ship overshooting its estimated end point on the sea surface before settling back just as the vehicle was approaching its actual end point on the ocean floor. Without doing any optimization, we were able to use this ship-overshoot strategy to reduce the time of the maneuver to about 100 minutes. Finally, we simulated the action of a thruster on the vehicle that is used in conjunction with ship movement. This control strategy reduced the time of the maneuver to about 65 minutes, again with no optimization. These results indicate that active control through a combination of actuation at the top and bottom of a tow cable could greatly improve the performance of towed oceanographic survey systems.
This paper examines, through numerical simulations, the use of active control for towed oceanographic vehicles that are used in deep water research.