Antisubmarine Warfare after the Cold War

MH-60R Strikehawk - Lockheed Martin Systems Integration - Owego, Owego, N.Y., is being awarded a $144,041,340 modification to definitize a previously awarded cost-plus-fixed-fee contract (N00019-08-C-0005) to a cost-plus-incentive-fee contract. This modification provides for the system design and development of the MH-60R Advanced Radar Periscope Detection and Discrimination System, to include design, development, integration and test. Work will be performed in Owego, N.Y., (51 percent) and Farmingdale, N.Y., (49 percent), and is expected to be completed in Sep. 2013. Contract funds will not expire at the end of the current fiscal year. The Naval Air Systems Command, Patuxent River, Md., is the contracting activity.

Three specific aspects of this future environment create problems for the Navy’s ASW posture: the capabilities and relatively wide availability of modern non-nuclear submarines; the United States’ extreme aversion to casualties in post-Cold War conflicts over less than vital interests; and the U.S. Navy’s doctrinal focus on power projection from the sea at the expense of sea control.

First, a technical challenge to the Navy’s ASW posture analogous to that resulting from the first Soviet deployments of the Akula in the mid 1980s may recur in today’s security environment with the increasingly wide proliferation of modern non-nuclear submarines. Deployed relatively close to their homes, in or near littoral waters through which the United States may need to project power from the sea, these submarines pose a potentially formidable threat. With a competent crew and the kind of advanced weapons that are now widely available in global arms markets, a modern non-nuclear submarine deployed in its own backyard might become a poor man’s Akula. Of even more concern is the fact that modern weapons—wake homing torpedoes, for example— tend to reduce the demands on submarine crews, making even less competent crews too dangerous to ignore.

Modern non-nuclear submarines are both better than those deployed by the Soviets during the Cold War and more widely available as defense industries that served their home markets during the Cold War now struggle to use exports to stay alive. One reason that the submarines are better is because many decades of continual investment by countries like Germany and Sweden have finally paid off in the form of non-nuclear submarines with air independent propulsion (AIP) systems that make them true submarines rather than mere submersibles. These submarines still do not provide the mobility and endurance of a nuclear submarine, but they greatly reduce the “indiscretion rate” of a traditional diesel-electric submarine, which must expose a snorkeling mast to recharge its batteries every few days at a minimum, and much more frequently if forced to operate at high speed.

Modern submarines are also armed with better weapons and fire control systems. One particularly alarming development is the marriage made possible by the end of the Cold War of the air independent, non-nuclear submarine with the submarine-launched antiship missile. Armed with sophisticated antiship weapons available from several Western and Russian suppliers, these platforms can launch fire-and-forget missiles from over the radar horizon without the need for the noisy and battery-draining approach run necessary for a traditional, torpedo-armed, diesel-electric boat. This threat circumvents the time-honored ASW approach to dealing with very quiet diesel-electrics, i.e., to flood the ocean surface with radar and use speed to force the submarine to either run down its battery and expose itself in an attack run or stay quiet and defensive.

The primary ASW challenge has always been wide-area surveillance, and the main challenge initially posed by the new security environment in this mission area is a wide area search problem. Sound propagates better in deep water than in shallow water, and non-nuclear submarines can remain silent for extended periods when allowed to patrol small areas near their home ports at low speed. Using passive acoustics to search for such submarines is much more difficult than it was to search for relatively loud Soviet submarines operating in deep water during the Cold War. On the other hand, active sonars encounter serious problems with clutter in shallow water, much as early radars did when forced to look down at targets flying over land. And even in shallow water, the water column still remains relatively opaque to non-acoustic energy, limiting the role of RF and laser radars as long-range sensors.

Two new systems stand out as first steps toward gaining a wide area search capability in the littorals. The first is called the Advanced Deployable System (ADS), and the second is called Distant Thunder. ADS is a passive ocean bottom array that can be deployed by a surface ship and whose output is currently collected and processed ashore via fiber-optic cable. Distant Thunder is primarily a signal processing adjunct to existing ASW combat systems, combined with legacy, air-droppable, active sound sources and a relatively simple data link that uses existing UHF radios on participating platforms.

Unlike the Cold War Sound Surveillance System (SOSUS) arrays, which listened for low-frequency, narrowband tonals propagating outward horizontally along the deep-sound channel, nodes in an ADS array look upward along what is called the reliable acoustic path. ADS is a derivative of the Cold War Fixed Distributed System (FDS) program, which was an attempt to repair the ASW barrier strategy by using many simple passive sensors in an upward-looking array that used the reliable acoustic path rather than the deep-sound channel. Each sensor in the ADS would cover a small cone of the ocean column.

Distant Thunder adds commercial off-the-shelf (COTS) processing to existing towed arrays on ships (and potentially, submarines) and air-deployed sonobuoys, and links the processors together using legacy radios with modems to form a network that can do bistatic or multistatic processing of the echoes from the air-dropped sound source. The essence of Distant Thunder is that it uses both spatial and temporal processing to extract a submarine’s echo from the clutter and reverberation of a specific explosion deliberately introduced in the water. Long wavelength towed arrays allow spatial processing that can eliminate clutter and reverberation entering the array’s sidelobes, and temporal processing allows reverberating echoes from the same object to be compared over time, thereby exploiting the fact that a submarine’s echo loses less of its higher frequency spectrum in that time than do objects sitting on the bottom or floating on the surface.

One of the original concerns about Distant Thunder was that variations in bottom topography and content would interfere with its temporal processing capability, but worldwide experiments have demonstrated excellent performance over a wide range of environments. Like all acoustic sensors, performance will vary in practice, depending on many circumstances, yet Distant Thunder promises to return a substantial portion of the detection ranges initially lost when the Navy first shifted its focus to shallow water ASW. Another benefit of Distant Thunder is that it demonstrates long-range performance under a wide variety of acoustic conditions, including the very common case in the littoral where sound is refracted away from the surface, a condition which drastically reduces the performance of a traditional, hull-mounted sonar.

Distant Thunder is also a great example of the incredible power of networked sensors, and the relative ease of backfitting such a capability onto legacy platforms once the substantial initial challenge of developing the necessary signal processing algorithms is completed. Distant Thunder can be backfitted onto any towed-array ship or submarine and onto LAMPs helos and P-3s. For example, on surface ships with the SQQ-89 ASW system, the physical footprint of a Distant Thunder backfit consists of one server and two laptops.

Specialized periscope or mast detection radars can also play an important role in the ASW search problem. Even during the Cold War, Soviet nuclear submarines regularly exposed a periscope when seeking a torpedo fire control solution against the fast ships of a battle group. And, of course, radar has an important role to play in preventing diesel submarines from snorkeling to recharge their batteries. Thus, a combination of speed and radar deployed to search within the limiting lines of approach created by that speed have always been an important ASW tactic against all submarines. Likewise, radar flooding—in which a large area is continuously illuminated with RF energy so as to set off a submarine’s radar warning alarm whenever it exposes a periscope with a radar warning receiver on an ECM mast designed to detect and warn of an enemy’s search radars—is also a traditional tactic against diesel submarines. But specialized mast detection radars like the APS-137 used on the S-3 and the P-3 experience tremendous false alarm rates caused by both sea state and other floating objects and debris when their detection threshold is set low to maximize range or sensitivity.

The Automatic Radar Periscope Detection and Discrimination (ARPDD) program is developing the capability to process the S-3B’s APS-137 returns in such a way as to allow very low detection thresholds (i.e., long range and high sensitivity) and very low false alarm rates. Very impressive results have already been demonstrated in shipboard experiments, but unlike Distant Thunder, ARPDD needs further development time to simplify the massive processing capability it now requires before it can be backfitted onto legacy P-3 and LAMPs platforms.

Second, consideration must be given to a political challenge associated with conflicts in which the United States is fighting over less than all-out stakes. In such conflicts, there will be a very low tolerance for shipping losses, but the presence of even a small opposing submarine force will make it extremely difficult for the Navy to quickly eliminate the possibility of such losses.

Regarding casualties, even in a major regional contingency the stakes for the United States are limited, while those of its opponents are very high indeed. The opponent may be willing to run great risks and sustain high losses, while the United States will not. Faced with the possibility or the reality of losses at sea, the Navy will be forced to stop and eliminate that threat before proceeding, and when that threat is submarine-based, its elimination will not be immediate and may take weeks.

A good analogy is to the great Scud hunt of Desert Storm. Thousands of sorties were diverted over several weeks from the air war during Desert Storm to hunt for Scud missiles to little or no effect. From an ASW perspective, this experience is illuminating for both operational and political reasons.

Operationally, Scud hunting was like ASW against a quiet target. A large area needed to be searched for objects that easily blended into the background and only intermittently exposed themselves. Thus radar was used to flood Scud operating areas, unattended field sensors were also deployed, and aircraft were used to pounce on potential contacts. This was a protracted, extremely asset-intensive endeavor, characterized by false alarms, high weapon expenditures, and low success rates. In short, a Scud launcher was most likely to reveal itself by successfully launching its weapon, just as sinking ships are often the only reliable indication that there is a submarine in the neighborhood.

The political lessons of the Scud hunt also apply to ASW. Before the war, the Scud had rightly been dismissed as a serious military threat, but once they began landing in Israel, the political imperative to allocate scarce resources to at least appear to counter this threat rapidly overwhelmed these narrow military calculations. The same political pressures would be brought to bear on ASW forces facing active enemy submarines, but unlike Scud missiles, which remain terror weapons without much military utility, submarines are a deadly serious military threat as well a political one. Therefore, it will not do to simply appear to be addressing the ASW problem with a major allocation of resources. Real results will have to be forthcoming before political and military leaders will be willing to risk valuable seaborne assets, be they Navy aircraft carriers, amphibious force ships carrying Marines, or Army sealift ships.

A delay of several weeks during the halting phase of a major regional conflict might not be a war stopper all by itself, but it is important to understand the consequences for current time-phased force deployment list timelines, which assume the arrival of millions of square feet of pre-positioned sealift within the first two weeks of the commencement of hostilities. This would transform a rapid deployment into a slow one, throw the deployment timelines of all the services askew, and open a window of indeterminate size at the outset of a conflict in which the enemy can operate unmolested except by those opposing forces already in theater, assuming they do not need an open sea line of communication to sustain themselves.

Third, the Navy confronts a doctrinal challenge as it attempts to increase its ability to project power from the sea. The Navy faces a new operating environment in which it is increasingly relevant and therefore in demand. Unlike in the post World War II era when the Navy was searching for a mission, the Navy has been inundated with new missions in the post-Cold War era, and these new missions compete with ASW for resources.

This has serious consequences for ASW because, as noted above, ASW is a multiplatform mission area performed by multi-mission platforms. As the Navy’s strike warfare, anti-air warfare, missile defense, and amphibious warfare capabilities have grown in importance in the nation’s military strategy, the Navy has shifted its focus away from an emphasis on blue water sea control toward power projection and land control in the littorals. Yet these missions must be performed by the same platforms that perform ASW—the air, surface, and submarine communities, all supported by the ocean surveillance community. It is natural that the Navy’s platform communities should shift their focus; but in a time of declining resources, this shift inevitably comes at the expense of other missions performed by those platforms.

This “multi-mission pull” increasingly makes ASW compete with strike warfare and theater air and missile defense for the same resources and training opportunities. The other mission areas are winning these battles and pulling the Navy’s major platform communities away from ASW, particularly in the aviation and surface warfare branches.

This shift in orientation is occurring at a time when technology increasingly demands that ASW be a coordinated, “combined arms” exercise if it is to succeed. All elements of the Navy’s ASW posture must be maintained to succeed in the fight against quiet submarines, but all three of the Navy’s major platform communities perceive that their survival in the new security environment depends to some extent on their success in performing other missions.