by Barbara Honegger
In his 2009 guidance on executing the Maritime Strategy, Chief of Naval Operations Adm. Gary Roughead emphasized the importance of what he called "decision superiority."
"We must ensure Navy forces have decision superiority, particularly in intelligence, surveillance and reconnaissance (ISR), command, control, communications and computers (C4), information operations (IO), and cyber warfare."
In today's operational environment, achieving decision superiority is not so much a matter of enhancing any given sensor system, but rather of integrating autonomous sensor systems into coherent networks to provide timely and relevant information for any level of decision-making the situation requires. The Navy's concept for achieving this goal is called FORCEnet, which it defines as:
"The operational construct and architectural framework for naval warfare in the information age, to integrate warriors, sensors, networks, command and control, platforms, and weapons into a networked, distributed combat force, scalable across the spectrum of conflict from seabed to space and sea to land."
The Naval Postgraduate School (NPS) is on the cutting edge of the effort to bring the FORCEnet concept to the undersea environment. Its Seaweb research, development, test and evaluation program focuses on the use of underwater acoustic communications to integrate distributed autonomous ocean sensors into wireless, wide-area underwater networks. The mission and composition of the resulting distributed system can vary widely—it may even include submarines—but because the underlying principles and technologies remain the same, NPS uses the generic term "Seaweb" for any such system.
Every Seaweb system includes the three basic building blocks for an infrastructure capable of performing persistent, distributed undersea sensing: autonomous underwater sensor nodes, which can be either fixed or mobile; repeater nodes, which employ underwater acoustic modems; and radio-acoustic communication (Racom) gateway nodes. (The illustration above shows the compact electronics of an acoustic modem (above) and a Racom gateway (below) compared to a 6-inch ruler.)
But Seaweb is more than a scalable sensor net. Through a decade of engineering experiments and sea trials in diverse maritime environments, NPS, in collaboration with SPAWAR Systems Center Pacific and other research partners, has advanced Seaweb to the point where it not only routinely demonstrates maritime surveillance, but also permits remote-control of instrumentation, oceanographic sampling, underwater navigation, anti-submarine warfare (ASW) and even submarine communications at speed and depth.
"Seaweb is a realization of FORCEnet in the undersea battlespace," said NPS Research Professor Joseph Rice, the program's principal investigator. "Seaweb is the product of interdisciplinary R&D [research and development] involving underwater acoustic propagation, sonar systems engineering, transducer design, digital communications, signal processing, computer networking, and operations research. Our original goal was to create a network of distributed sensors for detecting quiet submerged submarines in littoral waters, where traditional ASW surveillance is challenged by complex sound propagation and high noise. But as Seaweb technology developed, its broader overarching value became evident."
For example, in a 2001 Fleet Battle Experiment, a U.S. attack submarine serving as a cooperative target for Seaweb ASW sensors was itself equipped as a Seaweb node. Thus instrumented, the submarine was able to access the deployed autonomous nodes as off-board sensors. While transiting at speed and depth, the submarine was also able to communicate through Seaweb with the command center and a collaborating maritime patrol aircraft.
"In effect, the Seaweb network served as a cellular communications and sensor infrastructure for the submarine," Rice said.
A major advantage of an undersea wireless network is the flexibility it affords mission planners and theater commanders to appropriately match resources to the environment and mission at hand. For example, a number of Seaweb experiments have demonstrated the ability to combine fixed sensor nodes with unmanned underwater vehicles (UUVs). In addition to serving as a mobile sensor node, a UUV can perform a number of other useful functions within the network.
"The UUV can serve the fixed nodes as their deployment platform, their gateway node, or as a mule for delivering and recovering large volumes of data," Rice explained. "In turn, the fixed network can support UUV command, control, communications and navigation."
Another example of the flexibility of Seaweb networks is the networking of surveillance sensors with meteorological and oceanographic (METOC) sensors to improve the performance and relevance of both. The ready availability of local METOC data enhances the effectiveness of the underwater surveillance assets, and networking with other assets also helps the METOC sensors operate more effectively.
Seaweb's wireless architecture allows ASW sensors to be distributed sparsely, to cover a wide area, or deployed more densely, to monitor a chokepoint or to achieve a level of resolution that will permit them to serve as a tripwire for engaging potential targets. It can also interconnect the undersea sensors deployed by different government agencies or even different countries. For example, in a current international project known as "Next-Generation Autonomous Systems," Seaweb is interconnecting ASW sensors from several NATO nations to form a single integrated network.
"In short," Rice points out, "Seaweb integrates undersea warfare systems across missions, platforms, systems and nations."
Major attributes of Seaweb's architecture are its low cost, its suitability for rapid deployment from a variety of platforms, and its ability to autonomously self-configure into an optimal network. Through a "build-test-build" spiral engineering process and through rigorous sea testing of diverse configurations of underwater sensors and Seaweb modems, the effort is honing the blueprint for a multipurpose, two-way undersea communications architecture that can cover wide areas and is environmentally adaptive, energy efficient, cost-effective and expendable.
"Seaweb has now been exercised in over 50 sea trials," Rice noted. "The system has proven to be effective in very shallow water, such as the Intracoastal Waterway, and in water up to 300 meters deep off the coasts of Nova Scotia, San Diego, Long Island and Florida. It has been demonstrated in the Pacific and Atlantic Oceans, in the Mediterranean and Baltic Seas, in Norwegian fjords and under the Arctic ice shelf."
Multi-agency trials in a maritime domain awareness environment demonstrated Seaweb's ability to provide useful front-end input for decision-makers. They showed that a distributed network of in situ sensors in the area being monitored can complement remote sensors and enhance commanders' situational awareness. This makes commanders more effective by helping them complete the classic decision-making sequence known as the OODA loop—which stands for "observe, orient, decide, act"—more rapidly and more in tune with the developing situation.
The year before last, Rice and his students completed a two-part "Bayweb 2009" experiment to test the use of Seaweb's undersea communication technologies in San Francisco Bay. They collaborated with the U.S. Coast Guard to install a Seaweb Racom gateway on an operational navigation buoy in the center of the Bay. Bayweb 2009 used a cellular telephone modem as the radio portion of the gateway module and connected it to a Seaweb acoustic modem mounted to the bottom of the buoy.
In addition to demonstrating the network architecture and testing system performance in the Bay environment, Bayweb 2009 used networked current sensors placed near the seabed to measure the strong currents around Angel Island and shared the resulting data with oceanographers. The Naval Postgraduate School's partners in this effort were the University of California, Berkeley; University of California, Davis; San Francisco State University; Monterey Bay Aquarium Research Institute; the Space and Naval Warfare Command's Systems Center, Pacific; the Office of Naval Research; and the U.S. Coast Guard.