The objective of this research is the creation of a coastal observing system that enables dense, in situ, 4D sensing through networked, sensor-equipped underwater drifters. The approach is to develop the technologies required to deploy a swarm of autonomous buoyancy controlled drifters, which are vehicles that can control their depth, but are otherwise carried entirely by the ocean currents. Such Lagrangian sampling promises to deliver a wealth of new data, ranging from applications in physical oceanography (mapping 3D currents), biology (observing the dispersion of larvae and nutrients), environmental science (tracking coastal pollutants and effluents from storm drains), and security (monitoring harbors and ports).
This observing system fundamentally requires accurate positions of the drifters (to interpret the spatial correlations of data samples), swarm control algorithms (to achieve desired sampling topologies), and wireless communication (to coordinate between the individual drifters). This research will create distributed techniques to self-localize the drifter swarm, novel swarm control algorithms that enable topology manipulation while purely leveraging the stratified flow environment, and efficient wireless underwater communication for information sharing.
This project has significant societal impact and educational elements. Underwater drifter swarms will enable novel insights into a wide array of scientific questions, including understanding plankton transport, accumulation and dispersion as well as monitoring harmful algal blooms. Undergraduates will play an active role in many aspects of this project, thereby offering them a uniquely interdisciplinary experience. Finally, outreach to high school students will occur through the UCSD COSMOS summer program.
Coastal waters play a crucial part in both the ecosystem and economy. Efficient monitoring and supervision are therefore essential. However, current oceanographic sensing technologies are fundamentally limited in space and time, constrained often to batch data processing and standalone operation. While they have yielded useful insights, many important coastal phenomena would be much better understood if observations at higher spatial and temporal resolution were available. This multi-institutional project created a novel technology, where a large number of sensor-equipped underwater vehicles organize themselves as a swarm, forming a dense four-dimensional spatio-temporal sampling system. The system’s unique strength is that it consists of autonomous buoyancy-controlled drifters. These drifters are underwater vehicles that can control their depth through buoyancy changes but are otherwise carried by the ocean currents and can therefore monitor ocean phenomena in their own moving frame of reference. Also, this setup allows for revolutionary sampling densities due to relatively low vehicle cost. This project developed, optimized, analyzed, and tested the entire cyber backbone of this system using real-world deployments, culminating in a demonstration of a complete prototype drifter swarm. To act as an intelligent collective, communication between the vehicles and/or surface buoys is essential. A cost-effective acoustic modem prototype was developed, suitable for swarms of vehicles, by leveraging cheap ceramic automotive transducers and adaptive modulation. Several novel underwater communication protocols were designed, optimizing how the wireless channel is shared effectively, how data reliability is achieved, and how information is best routed throughout the network. In addition, to operate as a moving sampling system, it is imperative to know both where and when a sample is taken. However, with GPS not penetrating underwater, vehicles quickly lose track of their exact position and time. Time synchronization algorithms were created to keep tight synchronization with minimal overhead, enabling distance estimation based on time-of-arrival of acoustic signals. These distance estimates, combined with information from inertial sensors, was fed into a suite of novel tracking algorithms that were developed, based on the powerful estimation framework of factor graphs. This also enabled the development of swarm control algorithms. As the vehicles only have the ability to be controlled to move vertically in the water column, the ocean currents may end up dispersing them. However, under-actuated control algorithms were designed to take advantage of different prevailing currents at different depths and allow the drifter swarm to self-organize in advantageous configurations. Full sea-trials of a drifter swarm were carried out using the mini-AUE (Autonomous Underwater Explorer) drifter, which had been developed under separate funding. The mini-AUE is equipped with buoyancy control, computation and storage, satellite and GPS communications for when on the surface, and has a hydrophone to receive underwater communications. Two large scale sea trials were conducted, one in February 2013 with 6 AUEs and one in October 2013 with 16 AUEs. Both tests were highly successful, demonstrating the ability of the drifter swarm to follow depth profiles for several hours, validating the deployment and retrieval procedures, and evaluating the algorithms to time-synchronize, manage communication and track all the vehicle paths. Initial analysis of the data generated in these sea-trials was also able to show a number of oceanographic ideas that had theretofore only existed in theory, such as the accumulation of depth-keeping organisms in internal waves. The dense spatio-temporal sampling enabled by the system further promises to deliver a wealth of new data, ranging from applications in physical oceanography (3D currents), biology (dispersion of larvae and nutrients), environmental science (coastal pollutants and effluents from storm drains), and security (harbors and ports). The resources of the project were also leveraged in various outreach activities. An annual one-week workshop on the use of the drifters was created for high school and junior high school teachers who took part in various activities related to the project, including at-sea experiments. Outreach to high school students was incorporated through the UCSD COSMOS summer school; its goal is to expose and motivate young and creative high-school students to various STEM disciplines. We participated in the 'Envision' event organized by the Society of Women Engineers at UCSD. The project also had an international dimension: a collaborator from Italy provided strong and active tie-ins with European efforts in this area. The results of this work were disseminated using scientific publications and project websites. In addition, the novel technologies formed the building blocks for a new research initiative, funded through the NSF INSPIRE program, to explore the underwater soundscape, i.e., the collection of sounds present in the immersive underwater environment. The ability to record soundscapes in a Lagrangian manner will be invaluable as it provides a novel sensing technology to study the effects of sound on marine ecosystems (e.g., from increasing anthropogenic activity) and the role that sound plays for species development (e.g., larval fish recruitment and settlement).
