Development of a passive drifting seafloor imaging float with hybrid altitude and illumination control for increased survey endurance

Abstract

We describe the development of an underwater mapping system that gathers high-resolution seafloor imagery by drifting passively on near bottom currents while maintaining low altitudes using vertical thrusters. Since energy is not used for lateral propulsion, drifting platforms can achieve longer endurance than traditional Autonomous Underwater Vehicles. However, the precise altitude tracking needed to gather consistently lit images consumes a significant amount of energy when using conventional thruster control. We propose a novel hybrid altitude and illumination control method to reduce this energy cost and increases platform endurance. The hybrid control attempts to reduce the total energy use by reducing thruster use, overcoming the impact of deviations in altitude on image quality by adapting the illumination. The method takes into account the amount of lateral overlap between sequentially taken images, the wavelength dependent attenuation of light in water and blurring to maintain the quality of images that are obtained. We give an overview of the platform's hardware and the main design considerations. We describe a novel image quality metric that considers both the physics of light in water and imaging hardware limitations, and use this to assess the performance of our hybrid control method in numerical simulations. Our simulation results show that a significant reduction in dynamic thruster activity can be tolerated by introducing adaptive illumination control while maintaining image quality.

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