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Captain Gray is a retired submariner who commanded USS TEXAS (SSN 775) – the second Virginia-class submarine.

The seabed’s growing importance to a maritime nation’s defense and economic prosperity, and the ability of U.S. nuclear submarines to operate clandestinely in littoral areas, argue for improving the submarine’s intelligence collection capability by installing synthetic aperture and side looking sonar systems.

Maritime nations are increasingly reliant on the oceans for economic and defense needs, including energy and mineral recovery, a source of protein, the transmission of government and commercial data on seabed fiber optic cables, and the deployment of sensors and weapons. These undersea trends highlight the importance of understanding the nature of an adversary’s seabed infrastructure so U.S. forces can avoid, exploit or hold them at risk during conflict. Obviously, the U.S. must conduct these surveys as part of its Phase Zero intelligence operations so it can develop operational plans, tactics and countermeasures in advance of conflict.

The U.S. currently conducts bathymetric and other surveys with purpose-built oceanographic and surveillance ships operated by the Military Sealift Command. However, this approach has limitations. Specifically, there are a small number of these ships and the area requiring surveillance is increasing as the U.S. shifts its security focus to the Pacific. Second, and of greater concern, are extraterritorial claims by some nations who oppose foreign ship operations inside their economic exclusion zones. China, for example, has been particularly aggressive in attempting to counter overt U.S. intelligence operations. Incidents involving Chinese forces and U.S. airborne and seaborne surveillance platforms have increased tensions between the two nations on several occasions. The near loss of a U.S. EP-3 surveillance aircraft in 2001 and the harassment of a U.S. surveillance vessel off Hainan Island in 2009 are stark reminders. In addition, China is not alone in asserting exaggerated territorial claims or interfering with other nations’ intelligence collection operations. North Korea and Iran have acted similarly in the past.

Submarines can bypass nearly all of these concerns. U.S. attack submarines outnumber purpose-built oceanographic ships, allowing submarines to survey more locations while deployed for other missions. In addition, submarines represent a non-provocative intelligence collection capability. Their stealth allows them to operate undetected, which avoids interactions with foreign forces and the consequent antagonism of diplomatic demarche. Moreover, unlike their surface counterparts, weather does not limit submarine operations; nor does the presence of ice. Arctic energy exploration and recovery operations, and the possible introduction of surveillance or defensive systems by claimants, may become important intelligence targets in the future. For all these reasons, adding synthetic aperture and side looking sonar (SA-SLS) to submarines would significantly enhance U.S. intelligence collection capability, especially against seabed infrastructure.

Synthetic aperture sonar is an offshoot of synthetic aperture radar. While the phenomenology is different, the concept is similar. Both make multiple target observations from a moving source-in this case high frequency sound emitted by the submarine- and combine the information with advanced computer processing systems to produce resolutions that are an order of magnitude greater than traditional sonar systems can provide. The higher resolutions help operators discriminate between natural seabed topography and manmade objects like energy recovery systems, fiber optic cables, sensors or mines.

The U.S. is currently building Virginia-class attack submarines, which will eventually become the mainstay of its submarine fleet. These ships are the stealthiest, most combat capable submarines in the world and have features that enhance their ability to operate in littoral environments. With each successive block, the Navy is adding capability through spiral development, while reducing construction time and cost through design and process improvements. Adding SA-SLS is consistent with this spiral development philosophy and can leverage the significant amount of research and development that has been accomplished to field mine hunting sonars. It is also consistent with the recently released Submarine Force Integrated Undersea Strategy that seeks to satisfy current defense needs with mature technology, and provide commanders with affordable capabilities that are operationally practical. Submarine-mounted SA-SLS systems would meet these criteria.

Virginia-class submarines are large relative to past attack submarines classes. It is likely they could accommodate a SA-SLS array in the bow, somewhere aft of the High Frequency Chin Array (HFCA), which only Virginia-class submarines possess.1 Its forward location (providing an unobstructed view) and its ability to develop very narrow precise beams, would allow the HFCA to cue the SA-SLS to potential objects of interest in the water column or on the seabed. A future sonar advanced processor build and technology insertion could add the necessary processing and displays to provide real time seabed images and bathymetry information. Further, integration with the precision underwater mapping (PUMA) processing and display software would enable automatic target recognition and seabed feature change detection. These capabilities would be especially useful for counter-mine operations and help submarines penetrate anti-access I area denial shields.

One major goal of the Integrated Undersea Strategy is the addition of payload modules to future Virginia-class ships. This Virginia Payload Modules (VPM) would increase the ship’s hull

While Virginia is the only attack submarine class with chin-mounted sonar, the Navy installed a chin-array on USS Asheville (SSN-758) in 1995, as an operational test platform for the High Frequency Sonar Program (HFSP).

length and could provide an unintended benefit by accommodating longer SA-SLS arrays. Longer arrays support higher search speeds and sweep rates, while also creating opportunities for even more efficient array installation processes.

Operationally, some might question the wisdom of using active sonar, especially in an adversary’s littoral area. That is a legitimate concern, because active sonar and other noise-producing operations provide detection opportunities for antisubmarine warfare (ASW) forces. However, the high frequencies these systems use attenuate quickly with distance and are typically above the threshold of sonar intercept receivers. Therefore, operating SA-SLS or the HFCA represents a small detection risk. Of course, that risk is not static- it could increase if adversaries invest in improved detection capabilities.

Undersea platforms have experience with SA-SLS systems. For example, Submarine NR-1 employed side-looking sonar for surveillance and navigation. Lessons learned from that experience could help design and operate new systems. However, equally important in developing new capabilities is experimentation. A demonstration and development project to add SA-SLS capability to a Virginia-class submarine during a scheduled availability would allow near-term experimentation. The ship could test the system to assess its capability and provide feedback for design improvements. Testing could also assess potential submarine vulnerabilities while using high frequency active systems and countermeasures adversaries might adopt.

Adding SA-SLS to U.S. submarines would not offset the need for purpose-built oceanographic ships that can cover significant portions of the world’s oceans and provide needed bathymetric information. However, their vulnerability to harassment or attack when operating in foreign littorals, or even open ocean areas during conflict makes Virginia-class submarine SA-SLS installation a vital adjunct capability.

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