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The New Enemy Below

Bryan Clark,
Senior Fellow,
Center for Strategic and Budgetary Assessments


Undersea threats to the homeland have concerned U.S. leaders since long-range submarines joined enemy navies about a century ago. In the First and Second World Wars, U-boats attacked shipping and frightened Americans along the U.S. East Coast, who watched helplessly as oil tankers burned offshore after being torpedoed. During the Cold War, Soviet diesel and nuclear submarines created the specter of unwarned, “decapitating” cruise or ballistic missile attacks on U.S. leaders and command centers.

But the undersea threat did not end with the Cold War. Since the Berlin Wall fell, dozens of countries fielded new submarine forces and existing submarine powers modernized their undersea fleets.

In response to the growing number and sophistication of submarines, many navies are pursuing improved anti-submarine warfare (ASW) capabilities. For its part, the U.S. Navy is fielding new sub-hunting P-8A maritime patrol aircraft, Littoral Combat Ships, and unmanned systems such as the DARPA-built Sea Hunter unmanned surface vehicle and the Transformational Reliable Acoustic Path passive sonar array.

Submarines, however, are being replaced as the most likely undersea threat for homeland defenders. Unmanned undersea vehicles (UUVs) are less expensive for America’s enemies to buy and easier for them to operate than submarines. They also don’t have to travel transoceanic distances to reach valuable U.S. undersea infrastructure. An enemy wishing to attack fiber-optic cables, piers, oil terminals or cruise ships can simply drop a few UUVs in the water from a civilian boat or a nearby pier. And unlike shore-side perimeters with fences, guards and intrusion detection systems, the waterside of a critical coastal facility is often only protected from divers and fast boat attack.

Protection from UUVs is the next big challenge for maritime homeland defense. Small, inexpensive UUVs such as the Hydroid REMUS 100 and 600 or the Riptide 3-ft.-long micro-UUV can carry several pounds of explosives and the sensors to place them precisely. As larger UUVs become commercially available and proliferate, the threat of catastrophic attacks against critical infrastructure will grow. In an extreme example, Russia claims to have a nuclear-powered UUV that can carry a nuclear warhead transoceanic distances, circumventing U.S. missile defenses and enabling a direct attack on America’s largest coastal cities.

The U.S. Navy’s improving ASW capabilities, however, will largely be ineffective against most UUVs. Whereas commercial UUVs are relatively noisy, military-grade versions are built to emit very little machinery sound for passive sonar to detect. The small size and carbon-fiber or fiberglass hulls off UUVs do not reflect sound well, making them only detectable at short range by high-frequency (HF) active sonar. And even if they are found, UUVs are too small and lack the magnetic signatures needed by most torpedoes for homing and fuse activation. Countering the UUV threat will require a new approach.

Efforts to defeat UUVs will need to exploit their inherent limitations. By design, UUVs are relatively small and inexpensive compared to submarines, resulting in them having less payload and endurance and less sophisticated inertial navigation or sensor systems. To counter them, defenders should employ a system of systems (SoS) around the most likely and valuable targets, such as pipeline or fiber-optic cable junctions, since UUVs won’t be able to cause wide area effects. The SoS should confuse or jam UUV sonars to keep them from finding targets; prevent UUVs from obtaining satellite navigation updates; decoy UUVs away from targets; and destroy UUVs at short range when they get close enough to cause damage.

A system-of-system approach would be easier and less expensive than trying to find and destroy every UUV. It could repurpose existing HF sonars as acoustic decoys and jammers, use current GPS jammers, and employ current mine neutralization systems to destroy UUVs at short range. It will be better to put these measures in place now to protect our most valuable undersea infrastructure than scramble to respond following the first successful UUV attack.

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