Communication underwater is severely limited when compared to communications in air because water is essentially opaque to electromagnetic radiation except in the visible band. Even in the visible band, light penetrates only a few hundred meters in the clearest waters and much less in waters made turbid by suspended sediment or high concentrations of marine life. Consequently, acoustic techniques have been developed for underwater communication systems and now represent a relatively mature and robust technology. Acoustic systems, however, offer limited data rates and significant latency (due to the speed of sound in water).
The PI?s request funding to complete the development of an optical communication system that complements existing acoustic systems resulting in an underwater communications capability offering high data rates and low latency in clear water combined with long range and robustness in the presence of high turbidity. This combination of capabilities will make it possible to operate self-powered ROVs from support vessels without requiring a physical connection to the ROV. This in turn will simplify operations, reduce costs and reduce the requirement for dynamically positioned vessels. In addition, it will be possible to transfer large data files from fixed sensors using AUVs (or ROVs) as data mules, to transfer real-time video from untethered vehicles for inspection, identification and other related operations, and to provide interconnectivity for dense arrays of underwater sensors without the need for expensive and difficult to install undersea cables.
Broader Impacts
High rate optical modems will be an important adjunct to cabled observatories and acoustic networks. New sensing applications for ROVs will be enabled with video streaming capability. Existing outreach programs to K-12 at WHOI will be leveraged to include the new optical modem development. Reliable, high speed wireless communication is an extremely important "missing link" in underwater research. A good solution to this problem will have wide-reaching impacts on the use of AUVs, sensor networks or wirelessly controlled ROVs in oceanography, will make the deployment of vehicles easier and lower risk, and open up new possibilities.
The intellectual merit of this project, as stated prior to its award, was that currently available methods for underwater communications provided limited bandwidth which, in turn, impeded the advancement of new wireless applications. The development of high speed optical communications that supplement existing acoustic technologies would make possible new methods for science exploration in the ocean. Underwater vehicle control requires a high speed, low latency communication link. High speed underwater optical communications removes this bottleneck and allows seafloor instruments to collect data at scientifically relevant data rates as well as enables real-time vehicle control without the need for a physical tether. Once proven, optical communications would provide a first step toward the development of untethered remotely operated vehicles and would promote the development of new underwater sensor networks. We have advanced previously funded NSF work on optical communications to bring the technology to a maturity level suitable for use where high reliability for underwater applications is required. An underwater free-space link is challenging partly because of the high variability in both attenuation and ambient noise. The link is subject ot faults and must be able to recover gracefully under many demanding situations such as high attenuation of the transmitted data or a complete loss of the signal. In near-surface cases, it needs to be immune to large amounts of ambient noise that can overwhelm the data signal. Some of the techniques used to overcome these pervasive issues included optical filtering for high ambient light environments, forward error correction for varying attenuation, and line coding for variable rates. The end goal of this project was to wirelessly control an underwater vehicle, including video transmission for visualization, and to demonstrate a "data mule" type application using underwater autonomous vehicles to retrieve accumulated data from a subsea observatory. This goal was achieved during several at-sea tests that are well documented in journals and conference proceedings. We have shown that a diffuse light source based on LED technology and an optical detector system based on photomultiplier tube technology can successfully be used to operated as an underwater optical communication system among a variety of platforms. We have successfully demonstrated seafloor data retrieval to a surface vessel and an AUV, wirelessly controlled an ROV, and performed data offload from an AUV to other vessels. Another important outcome was the training and development of young engineers by providing opportunities for them to work on cutting edge technology. A number of junior engineers were involved in this development project and in the field operations. These engineers were able to successfully translate their benchtop experience into real world applications. Underwater optical communications will likely have an important role to play in the development of autonomous underwater data networks that require periodic data downloads from vessels or autonomous robots. Free-water optical communications as demonstrated in this project is also likely to lead to tetherless vehicle communication modalities useful for a wide range of operations.
