The Navy's surface warfare community is hoping to capitalize on a so-called transduction revolution which if pursued for backfits to the Oliver Hazard Perry (FFG-7)-class frigates and the Zumwalt (DD-21)-class land-attack destroyer, could achieve dramatic improvements in submarine detection capabilities. An at-sea test of the system, fitted in a towed body, is set for the summer of 2001.
The effort already is under way for DD-21 through the integrated undersea warfare initiative managed by the program executive office for mines and undersea warfare. The program executive office is managing the evolution of sonar array transducer technology that, it hopes, will generate enough acoustic power to punch through ocean layers that typically distort or refract sonar to capture high-resolution, highly precise acoustic images of detected submarines.
The program aims to develop a lightweight, broadband variable-depth sonar (LBVDS) in a towed-body configuration that could be selected for incorporation into the DD-21 design. Two industry teams—one led by Bath Iron Works and Lockheed Martin Naval Electronics and Sensor Systems-Moorestown (Blue), and another led by Litton Ingalls and Raytheon Naval and Maritime Integrated Systems (Gold) —are competing for the DD-21 program. The program executive office for surface strike plans a selection for March 2001. In keeping with the DD-21 acquisition strategy, which gives the teams extensive flexibility in designing the ship, the winning team will be free to use or bypass the LBVDS work.
Lockheed Martin's Naval Electronics and Sensor Systems-Syracuse business unit is acting as prime contractor for the LBVDS effort. The company's missiles and space group in California is working as a subcontractor to Syracuse for assembly of the array components.
The LBVDS effort, funded by the Office of Naval Research, is based on the use of a new ceramic-type material for the fabrication of transducers (called lead magnesium niobiate and dubbed PMN) that when stimulated by an electric current, emits a considerably stronger acoustic signal and is considerably lighter than the conventional material used in Navy transducers (called PZT). Frank Herr, director of the Office of Naval Research's sensing and systems group, says that PMN "promises to change the form factors associated with Navy sonars." He points out that PMN provides the capability for generating a high-power acoustic signal that would permit broadband transmission.
The use of high-power broadband, Navy and industry officials say, will enable surface-ship sonars to overcome the distortions to acoustic signals caused by environmental variations in ocean layers, such as temperature, salinity, and current flow. Broadband transmission also overcomes the cacophony of acoustic interference typical of shallow waters caused by reverberation from the bottom and surface contacts.
Herr says that the LBVDS effort, which was conceived in the early 1990s, has carried out some testing off the East Coast. He also says though that a key hurdle had been the ability to produce PMN material on a scale needed to build transducers. The concept has been demonstrated in small batches. Early production processes of larger quantities led to flaws in the material that permitted it to retain heat as it flexes during the generation of the acoustic signal, causing it to overheat and fail.
Meanwhile, the Office of Naval Research expects to receive funding through a $3 million plus-up to the 2001 Defense Authorization Bill to award a development contract to L-3 Communications' ocean systems group for work already under way on a second broadband program. The company has invested research funds into the effort with the goal of building a high-power, broadband surface-ship sonar that it calls Tsunami. Company official say they are aiming at a potential backfit on board the 37 Oliver Hazard Perry-class frigates still in the fleet, which are fitted with the SQS-56 keel-mounted sonar.
The Tsunami concept, they say, is a long, cylindrical design mounted on the keel that will eliminate the need for the bulbous non-hydrodynamic dome required for the SQS-53C sonar built for other surface combatants. They also hope to win funds to develop a receiver array. L-3 engineers say that the Tsunami development is based on a "high-temperature variant" of the "room temperature" PMN used for the LBVDS effort. The material represents a different mix of ceramic elements that achieves optimum performance at 85'C and, they add, permits a greater degree of control of the temperature that the material will reach during normal transducer operations.
The high-power transducers, they say, will overcome the distortions caused by water layers and environmental reverberation and thereby provide a similar level of capability to the towed-- array LBVDS. Variable-depth towed systems, operating beneath the ship, typically are more effective than hull-mounted systems—though towed systems require more topside equipment.