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Navy Plans System To Counter Shallow Water Submarine Threat In Regional Areas

The Soviet global submarine threat has declined dramatically, and is no longer the primary threat to the United States. Instead, the focus is on a still evolving threat of regional conflict in shallow water. The Navy has not yet developed the performance requirements for an undersea surveillance system to counter the shallow water threat. The Navy's Surveillance Towed Array Sensor System (SURTASS) was built to counter the Soviet submarine threat in deep water, but the Navy now plans to use it against the regional—shallow water—submarine threat. Navy officials said that preliminary test results show SURTASS has potential for shallow water detection; however, the Navy has not yet demonstrated SURTAS S' shallow water capability through developmental and operational tests. At the same time, the Navy and other Department of Defense (DOD) organizations are exploring alternative ways to detect submarines in shallow water. - Currently, the Navy has 19 SURTASS ships —18 monohull and onetwinhull Small Waterplane Area Twin Hull (SWATH). The Navy expects to replace its 18 monohull SURTASS ships with nine twin hull SWATH ships by 1998. SURTASS program officials believe they need the new SWATH ships to develop tactics, define Low Frequency Active (LFA) sensor capabilities and limitations in shallow water, and develop environmental data bases for regions of potential conflict. The Navy already uses a modified monohull ship to test LFA capabilities, and it will have four small SWATH ships with a capability to receive LFA signals by 1993, and one large SWATH ship with active LFA capabilities by 1994.

Between fiscal years 1992 and 1998, the Navy plans to spend about $1.2 billion on its SURTASS upgrades and procurement of new SURTASS SWATH surveillance ships.

Before the collapse of the Soviet Union, the Navy's primary antisubmarine warfare target was the open ocean, deep water Soviet nuclear attack submarine. Diesel submarines in coastal, shallow water areas were largely disregarded as threats. To detect and track Soviet nuclear submarines in deep ocean basins and at long ranges, the Navy deployed its integrated undersea surveillance systems—the fixed Sound Surveillance System and the m o b i le SURTASS. The Navy also designed a shorter range, fixed surveillance system, the Fixed Distributed System, and an active acoustic detection system, Low Frequency Active, to be used in conjunction with SURTASS, to counter the emerging threat from quiet Soviet nuclear submarines.

SURTASS surveillance ships, equipped with a 6,000-foot towed sensor array, augment the fixed Sound Surveillance System by collecting acoustic data in areas where there is no fixed system coverage. The data SURTASS collects are initially analyzed onboard the ship, then relayed by satellite to shore processing facilities for display, further analysis, integration with other surveillance data, and dissemination to operational users. Figure 1 shows the SURTASS SWATH ship. Until 1990, the Navy regarded the shallow water regional threat as a low priority. The former Soviet Union was the primary submarine threat to the United States. This deep, o p en ocean threat d r o v e t he Navy's development of surveillance requirements for undersea antis u b m a r i n e warfare. The Navy is now in the process of defining and developing the threat from submarines that could be used in regional conflicts.

Although the actual events of a potential conflict remain uncertain, the Navy anticipates that antisubmarine engagements are likely to occur in shallow water— an acoustically difficult environment that has not been analyzed as extensively as deep water. The Navy is currently modeling scenarios for regional conflicts and conducting war gaming, sea tests, and fleet exercised to better define how to counter the shallow water threat. SURTASS' acoustic sensors are designedprimarily to operate in deep open oceans against the Soviet threat. The Navy plans to equip the larger SWATH ships with active sensors— LFA—to enhance deep water submarine detection and location capability and to provide shallow water detection. The small SWATH ships will be equipped with a capability to receive LFA signals. Although the Navy is planning to use SURTASS to detect submarines in shallow water coastal regions, it has not completed the operational testing needed to demonstrate SURTASS sensor proficiency in shallow water operations. Shallow water has unique characteristics that make acoustic detection difficult, and shallow water environmental data have not been quantified. Shallow water is generally defined as between 600 and 1,000 feet deep; it includes the continental shelf and mid-latitudes. In addition to depth, other factors such as the topography of the ocean floor and the temperature of the water can affect the travel of sound signals and acoustical sensors' ability to receive them.

Deep water promotes sound wave propagation (spreading out) enabling acoustical sensors — like SURTASS sensors—to detect enemy submarines; however, according to Navy officials, shallow water limits sound propagation. Further, the temperature, depth, and underwater terrain, as well as noise from commercial ships and boats, interfere with SURTASS sensors' ability to detect enemy submarines in shallow water.

The Navy believes that, with the addition of active LFA sensors that emit strong acoustic pulses, SURTASS will provide near-term shallow water detection of submarine targets. However, for SURTASS to detect submarines in shallow water, the Navy requires not only LFA sensors on a SWATH ship but also another ship, aircraft, or other platform to receive the echoes from the LFA pulses.

Because of its focus until 1990 on the deep, open ocean threat, the Navy gathered and quantified large amounts of data on the deep water acoustic environment but did little to develop threat data on shallow water acoustics.

The Navy's existing analytical models, which are designed for deep water, are not effective for shallow water testing of LFA performance. Navy officials are in the process of collecting data on the shallow water environment but have not yet collected or quantified sufficient data to develop new analytical models. Despite the dramatic changes in the submarine threat, the Navy is continuing to build new SWATH designed for the Soviet submarine thrcst The nine SWATH ships are designed to be more seaworthy and, therefore, more capable of tracking Soviet submarines in the open ocean in winter, a threat that has declined dramatically and is no longer the primary threat to the United States.

The Department of Defense pointed out that, in view of the dramatic change in the Soviet submarine threat, it has reduced the planned procurement of SURTASS ships from 39 to nine. Navy officials noted, however, that, upon satisfactory completion of ship design and initial LFA systems testing, they support future planned procurement of SURTASS ships to meet other anti-submarine requirements for deep water.

The other requirements supporting future SURTASS ship procurement include the requirement to (1) respond to defense planning guidance that calls for keeping sea lanes of communication open, (2) maintain a watch over the significant number of Russian and third world submarines that remain at sea, and (3) be prepared to deal with future uncertainties should the current state of affairs - Russian goodwill and intentions - change dramatically. The Navy originally planned to keep its 18 monohull ships and procure an additional 21 SWATH ships to give it a total of 39 ships for conducting undersea surveillance against submarines. With the reduced submarine threat and because of budgetary constraints, the Navy now plans to maintain a fleet of nine active ships for conducting undersea surveillance. According to Navy officials, annual operating costs are about $4.5 million per ship for monohull ships compared to an estimated $4.7 million for small SWATH ships and $5.3 million for large SWATH ships. The planned procurement of nine new SWATH ships is estimated to cost about $1.2 billion. To date, the Navy has contracted for five new SWATH ships - $487 million- and plans to spend $674 million more to build four larger ships through 1998.

Another $47 million will be spent to upgrade the small SWATH ships with a capability to process and exploit target submarines' echoes from sound signals transmitted from LFA -the small SWATH ships will have the receive capability, but not the LFA active capability.

Table 2 shows the current procurement and delivery schedules for SWATH ships, as well as their costs, according to the fiscal year 1993 President's budget submission. The Navy has already built one small (3,400-ton) SWATH ship and is building three additional small and one large (5,300-ton) SWATH ships, and according to the President's fiscal year 1993 budget, it plans to build four more large SWATH ships.

The five large SWATH ships will accommodate equipment needed to generate power for the active LFA sensors. The first of the nine SWATH ships (T-AGOS 19) has been delivered to the Navy and is undergoing operational testing. Navy officials have said that the ship has experienced minor problems and that sea testing has not been completed. The Navy's Operational Test and Evaluation Force expected to report on the results of the testing in January 1993. The three other small SWATH ships are being built and are planned for delivery some sometime in 1993. The fifth ship (T-AGOS 23) - first of the larger SWATH ships - is under construction, with an expected delivery date of 1994. The contract for the ship contains options for the four remaining large SWATH ships.

Navy program officials said they need more SWATH ships in the near term to develop effective tactics, define the capabilities and limitations of LFA in shallow water, and create environmental data bases for regions of potential conflict.

Officials believe that SURTASS - with passive and active capabilities - will meet future system requirements for regional conflict. They said that SURTASS procurement should continue, despite the fact that performance requirements have not been developed.

 
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