How do you measure the sound of something if it makes no sound? Ask ORNL.
The Navy prefers to call its subs "quiet underwater weapons platforms." What little noise they do make must still be monitored in operational tests, both to improve designs and to ensure that the craft are operating to specifications. The testing requirements presented engineers with a challenge: how do you measure the sound of something if it makes almost no sound? Ask ORNL. The U.S. Navy has relied on ORNL's Instrumentation and Controls Division's expertise in electronics, real-time computer applications, and systems integration to develop state-of-the-art acoustic measurement systems.
ORNL has been working with the Navy on a special program for the past 6 years called AMFIP IIthe second phase of the Acoustic Measurement Facilities Improvement Program. According to Randall Wetherington, who heads the program in the I&C Division, ORNL has devised ways to "measure the undetectable" for its sponsor, the Naval Surface Warfare Center's Carderock Division.
The system uses several arrays of hydrophone sensors--laceworks of underwater microphones suspended from buoys--to gather sound from the craft as it goes by. Computers process the sound signatures to extract the signal from background noise. "Waves and wind cause an ambient noise level in the ocean," Wetherington explained. "This new technology can detect noises below this ambient level. It's like a TV satellite dish in that the array, like the big dish, focuses the energy, which amplifies the sensor signals. As you add sensors, with their placement based on sophisticated math and geometry, you get more signal gain over a large bandwidth."
The USNS Hayes is a specialized ship that is home to the instrumentation systems used in the Navy's acoustics tests.
Much of the signal processing hardware and software for I&C Division's AMFIP II system is currently aboard the Navy's laboratory research ship the USNS Hayes, which is currently ported at Cape Canaveral, Florida. When test runs for submarines a re scheduled, the Hayes sails to the test course set up in Exuma Sound, which lies in the middle of the Bahama Island chain. There the arrays, which have more than 1000 individual hydrophone sensors each, are placed to pick up sounds from the vessels as they pass through the course. Long umbilical cables connect the arrays to the rest of the system, which is located aboard the Hayes.
The Navy's test course is located in the Bahamas within a corner of the fabled and mysterious Bermuda Triangle. Researchers in the AMFIP II program like to point to this fact, although few express reser-vations about working after dark.
The radiated noise emanating from the Navy's submarines, called the acoustic signature, is normally low enough to be masked by the natural background noise of the ocean in a conventional hydrophone setup. The next generation of nuclear submarines the SSN 21 Seawolf operates below this natural background, requiring the world's most advanced underwater acoustic measuring system to ensure that its acoustic emissions remain below design limits and thus stay hidden beneath the vast ocean.
Like a number of ORNL researchers involved in the AMFIP II project, Randall Wetherington (left) and Andy Andrews have made a number of trips to the Bahamas' Eleuthera Island, which lies near the Navy's test course in Exuma Sound.
The initial problem, says Andy Andrews, deputy program manager in the I&C Division, is how to measure something that makes virtually no sound. "Staying hidden is part of the submarine's job," Andrews says." Acoustics is one of the few ways these ships can be found. The craft in the Trident class can be the length of two football fields and not make any noise as they go by. Our systems help characterize the normal acoustic signature of the vessel as it runs through a course of instrumentation. Our system is not classified, although the data it generates are. If you know what a vessel's acoustic signature is, you could develop ways to detect it."
As are many a sailor's, shipboard accommodations aboard the Hayes are spartan.
Wetherington likens the AMFIP II system's acoustic feats to a familiar scenario: "It's the Tennessee-Alabama game at Knoxville in the last minute of play. The score is 28-27, and Alabama, who has just scored to come within one point, is lined up to try for a go-ahead two-point conversion. Imagine the crowd noise from about 100,000 people. Now, sitting near the 50-yard line is an English professor with a heart condition who, in times of stress, calms himself by reciting aloud Edgar Allen Poe's Annabelle Lee.
"This acoustic technology could pick the professor's poem out of all of that crowd noise."
Electronics and Hot Rod Chips
The ORNL researchers' efforts have focused on three main areas--telemetry and underwater electronics; beamforming, or signal processing; and system integration, or, more simply, making it work. More generally, they've designed and built electronic instrumentation and developed a processing system that takes the data and generates information in a usable form.
AMFIP II's instrument arrays feature numerous hydrophones and sophisticated underwater electronics.
The telemetry system was a challenge to the I&C AMFIP II team because of the large number of signals from the sensor arrays that must be acquired, conditioned, and transmitted. These signals have a very wide bandwidth and must be transmitted over a long distance. The system required the identification and use of emerging technologies that arrived on the market at the same time that the design effort was initiated, making AMFIP II truly state of the art. The AMFIP II team had to clear technical hurdles presented by power distribution, heat dissipation, high-speed and high-resolution signal digitization, and very high-speed data multiplexing combining information cascading in on multiple lines into a single line suitable for fiber-optic transmission.
Resembling a huge, mechanical Portuguese man-o-war, the AMFIP II sensor array is hoisted over the water by technicians aboard the Hayes. The latticework of hydrophones more than 1000 for the system is strung above pressure vessels containing instrumentation.
"We used a 'hot rod' multiplexer chip developed by DARPA that combines information on 40 lines into a single line," Andrews said. "Our success in identifying new technologies produced significant cost savings for the Navy. For instance, we convert signals from the hydrophones to digital data almost immediately, which gives us very high-resolution signals, and then transmit the data by fiber-optic cable to the test ship. We identified a $20 digitization chip that will do tasks that used to cost $3000 per channel. We built three systems with more than 1000 channels eachone channel for each sensor."
Along with combing the marketplace for available electronics, the AMFIP II team also oversaw a large subcontract effort required by the construction of around 1300 multilayer printed circuit assemblies of 50 different designs, 22 equipment chassis, and countless cables and test fixtures. All of this, as well as automated test equipment to verify that they were all working properly as they were being made, had to be designed, fabricated, and integrated into the final system.
When something breaks, as is apt to happen in a network of thousands of hydrophones, AMFIP II features a built-in diagnostic system that enables the technicians to quickly pinpoint the trouble, right down to the dead hydrophone or defective integrated circuit. A major advantage of the system is that sensors can be monitored and diagnostics can be performed aboard ship, a luxury much appreciated by technicians who would otherwise have to hoist arrays of a multitude of sensors and instrument cylinders aboard to troubleshoot. "It takes an incredible amount of computer power just to do the testing," Andrews says, but it's obviously worth it. The system also allows operators to monitor system degradation in the circuitry that is immersed in the salty, high-pressure environment.
Signal Processing: On the Beam
The beamforming, or signal processing, capabilities of AMFIP II produces the high-quality measurements and acoustic images of the submarines. The image beams are computed in real time for up to 35 frequency bands simultaneously. The data from the hydrophones bobbing in Exuma Sound pours into the Hayes at a mind-boggling rate of 165 megabytes per second. I&C Division researchers Eva B. Freer and Bill Zuehzow have been instrumental in developing the algorithms and structure that bring all of the data into a usable configuration.
"To get a sense of how much information that is, a 3.5-in. floppy disk holds 1.4 megabytes," Freer says. "That's 115 full high-density floppy disks per second for each array!"
The Navy's nuclear submarines run through a course to be monitored by the AMFIP II system's sensors. The staggering amount of data generated by the sensors is manipulated by the system's signal processors and powerful computers to produce the acoustic signature of a craft that makes almost no noise.
"The custom electronic equipment aboard ship preconditions the data using more than 40 billion fixed-point instructions per second from embedded digital signal processing chips. This preconditioning is done even before the supercomputers process the data. The ability of this system to collect a multitude of signals and increase their gain with the computer algorithms enables the system to raise the submarines' image from the ocean's background noise."
As might be imagined, handling data from thousands of instruments takes a huge amount of computing power. Devising ways to handle and analyze the amount of sensor data coming in at this rate takes very specialized programming, and that is a specialty of I&C's Real-Time Systems Group. I&C Division was home to a 40-gigaflop supercomputer--for a time the biggest at the Laboratory to perform the signal processing algorithms. The result of all of this data crunching is the image of a ship, or at least its sound image.
Building and Debugging
A sizable portion of the electronics for the AMFIP II system is housed in thick-hulled pressure vessels built to stay watertight in deep water. The system's printed circuit assemblies, which are contained in the vessels, number over a thousand.
Because part of the system operates in seawater at considerable depth, it must be well built. The hydrophone arrays feature thick-hulled cylindrical instrument packages that are custom made by the Navy and pressurized with helium, which conducts heat away from the electronics. When something does malfunction, such as a sensor going out, it can be detected and compensated for aboard the Hayes. Calibration of the system can also be done remotely.
"Close to half of our effort is for testingproving that the system is working and debugging it when it doesn't," Wetherington said. "In all, the I&C Division and subcontractors have put about 30 years of conceptual development and prototyping, 20 work-years of software development and integration, and 10 years into electronics design, integration, and tests. We've also fabricated and performed quality assurance checks on 1300 multilayer printed-circuit assemblies."
The AMFIP II system consists of components and software from seven commercial suppliers, ORNL researchers, and three subcontractor teams. Putting together a product of the complexity of AMFIP II from these diverse sources required careful systems integration. "An important aspect of this project is the teamwork," Wetherington says. "ORNL had 147 staff members who worked on the effort. Our sponsor was very supportive and worked as a member of the team. We also had top-notch support from several subcontractors, including Planning Systems, Inc.; Cray Research, Inc.; Colonial Assembly and Design; and the University of Tennessee."
The I&C Division's AMFIP project began in 1986; the first acoustic processing system that ORNL developed was delivered in late 1989, and AMFIP II began soon after. The meticulous and ongoing attention to detail and project planning came to fruition again in October 1994 when ORNL successfully delivered and installed the first portion of the new AMFIP II measurement technology. The remaining components were installed in July 1995. The Navy sponsors have indicated that they intend to apply the technology to other tasks throughout the fleet. The acoustic signaling technology is also being considered for medical applications such as diagnosing a malfunctioning heart through its acoustic signature.
AMFIP II evolved from a project that played to the I&C Division's strengths in instrumentation, computing, and systems integration. The team of researchers also proved themselves in adapting to new projects and identifying when to go outside the Laboratory. They identified new technologies and coordinated a complex effort that involved six large organizations and a variety of engineering disciplines from electronics to high-speed computing. The astonishing stealth of the Navy's "quiet platforms," Wetherington said, made necessary the awesome amounts of data and the electronic and computing expertise and toil involved in harnessing that data. "All of that effort has been needed to lift one analog signal, a submarine's acoustic signature, up out of the ambient noise of the ocean."
I&C Division researchers, who considered customer satisfaction as
one of the most important goals of the program, count the Navy as a
happy customer and the product that they have delivered as a giant step forward for
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