In early 1981, a ready-alert P-3C Orion launched from Naval Air Station Bermuda with a crew of 12 officers and enlisted men. An hour earlier the crew had been comfortably asleep as their 24-hour-alert shift was winding down. Now they pulled themselves together as their Orion clawed for altitude, arcing northeast toward the vastness of the North Atlantic Ocean. During the scramble to get airborne there had only been time for a hasty briefing, but they understood the sudden urgency: A Soviet submarine was nearby.
The mission commander, Lieutenant Frederick “Buzz” Lineburg, faced a challenging task. He knew the submarine was transiting from the north, and he knew its last estimated position. As Buzz studied his chart, he realized that his quarry could be hiding anywhere within several thousand square miles of ocean. An area of that size was too large for “direct-path” search tactics, which exploit one of several acoustic-ray paths that sound can follow through water. Instead, by capitalizing on the target’s relatively loud acoustic signature, Buzz elected to employ search tactics optimized for “convergence-zone” (CZ) ray paths, which can propagate much farther through water. Thus, with only 1 aircraft and 16 passive sonobuoys, Buzz and his crew were able to search several thousand square miles of ocean with an acceptable probability of detection—a capability known in the air antisubmarine warfare (ASW) community as wide-area ASW search.
Wide-area ASW search is a critical warfighting capability.1 During the Cold War, NATO forces leveraged their technological edge to develop sensors, operators, and tactics, techniques, and procedures (TTPs) capable of detecting and tracking Soviet submarines. Many of these sensors, contained in towed arrays and sonobuoys, were passive systems that listened for the telltale sounds of a submarine without betraying the presence of their host platforms. NATO’s ASW forces effectively parlayed these sensors and TTPs into an ability to find Soviet submarines at sea, providing their commanders with important situational knowledge during the heightened military tensions between East and West.
For over a quarter century, however, the sharp decline in submarine acoustic-source levels has been well documented (see Figure 1). When combined with a shift in focus away from ASW, the Navy’s ability to effectively search a large area of ocean for a submarine has eroded. Make no mistake: The U.S. Navy leads the world in undersea warfare. But its ASW forces have not kept pace with the threat and are facing a potential warfighting gap just as the nation turns its attention to the challenging Asia-Pacific region.
Links in the ASW Kill Chain
The Soviet submarine threat may be gone, but a wide-area ASW search capability remains a requirement.2 Into the void left by the Soviets steps China, with an increasing number of diesel-electric and air-independent-propulsion submarines; North Korea, whose midget-submarine program claimed the South Korean corvette Cheonan as a victim in 2010; Iran, whose fledgling submarine force enjoys home-field advantage in the Strait of Hormuz; and Russia, recommitted to a first-rate submarine fleet to patrol the Arctic Ocean and occasionally visit the western Atlantic.
The time has come to restore the U.S. Navy’s wide-area ASW search capability to the levels achieved during the Cold War. Fortunately, P-3 and P-8 maritime-patrol aircraft (MPA) are poised to employ a new sensor that can provide exactly that. To do so, warfare commanders and the air ASW community must be willing to approach the “search” phase of ASW differently.
For the uninitiated, understanding why a wide-area search capability is important first requires knowledge of the ASW kill chain. Kill chains may be defined differently; a generic ASW kill chain consists of four separate links: “search,” “localize,” “track,” and “attack”:
• Search: Employ ASW sensors in an area derived from cueing information
• Localize: Initial detection and determination of submarine position, course, and speed
• Track: Maintain constant knowledge of position, course, and speed
• Attack: Deliver an ASW weapon.
Of these four links, or phases, “search” is generally considered the most difficult. An ASW asset like an MPA aircrew often starts its search without any of its sensors in contact, knowing only that a submarine is out there somewhere. The longer the submarine remains undetected, the larger the size of that somewhere becomes. This translates to a broad swath of ocean that the submarine may hide in, ranging from a few dozen to several thousand square miles, and drives the need for a wide-area search capability. The optimum wide-area ASW search platform should combine speed and range (limiting the amount of time a submarine remains undetected) with a capable acoustic sensor that maximizes submarine-detection range.
Buzz and his Cold War peers took advantage of “noisy” Soviet nuclear submarines. They didn’t always succeed, but when tactical factors like oceanographic conditions and target source levels were favorable, they were able to search a broad area of ocean using passive sonobuoys deployed in widely spaced rows known as a sonobuoy field. As Soviet submarines became quieter, Buzz and his crew adapted by deploying more narrowly spaced sonobuoy fields to provide sufficient probability of detection, but at the cost of area coverage. Eventually, some adversary submarines became so quiet that search areas became a fraction of the size of the areas that were covered against earlier-generation submarines. The passive sonobuoy fields could no longer adequately search a large area of ocean against modern targets.
Active vs. Passive
Viewers of World War II movies are familiar with active sonar. They probably remember tense scenes where the German or American submarine crews nervously glance upward upon hearing the distinctive sonar “pings” of an approaching enemy warship. Those submariners had every right to be apprehensive: It can be difficult to avoid detection from an active sonar system. There is one downside to active sonar, however: It alerts the quarry that a hunter is near while also revealing the hunter’s position.
Passive sonar provides an opportunity to hunt a submarine without necessarily alerting it. During the Cold War, NATO navies often used passive acoustic systems for ASW. They took advantage of both superior technology and “noisy” first- and second-generation Soviet nuclear submarines. By only listening passively, NATO ships, submarines, and aircraft were often able to find and track unalerted Soviet submarines. When the acoustic environment was conducive to convergence-zone ray paths, as it often was in the northern Atlantic and Pacific oceans, a single ASW asset was able to passively search thousands of square miles of ocean. Passive ASW TTPs became the norm, and even today they remain the primary search method for P-3 and P-8 aircraft. There are (and will continue to be) situations where passive-search TTPs are appropriate. But soon they should be the exception, not the norm.
With respect to submarine stealth, a modern, quiet submarine holds most of the advantages. As Figure 1 shows, foreign nations have invested resources into submarine-quieting technology with effective results. The high source levels that NATO navies used to exploit are simply not there anymore. ASW assets like the P-3 have seen their effectiveness decline as they are forced to reduce their search-area size in order to have a reasonable probability of detection.
Modern submarines are ultra-quiet and make detection difficult for ASW forces, yet the MPA community still predominantly employs passive TTPs. The moment has come to break this paradigm. Sound energy allows for submarine detection. If submarines are no longer putting enough sound energy into the ocean environment, then submarine hunters must adapt and induce the required sound energy themselves. For a wide-area search, multistatic active sonar is the best option for MPA.
Enter Multistatic Active Technology
In the late 1980s, the dramatic decline in Soviet-submarine source levels was apparent. Resources were committed to establish a program of record for multistatic active-sensor technology to be employed by ASW aircraft. Active sonobuoys employed by MPA were useful, but only at the relatively short ranges that are effective for submarine localization and tracking. A multistatic active system would provide the needed capability to search large areas of ocean for submarines.
Multistatic active sonar is different from other active sonars used by ships, submarines, helicopters, and MPA. Most active sonar systems are monostatic, meaning that transmitter and receiver are co-located. With monostatic active sonar, sound energy travels from the transmitter to the target and is reflected back to the co-located receiver. Multistatic active sonar has a single transmitter but multiple receivers that are dispersed in a manner that maximizes probability of detection (see Figure 2). In general terms, when compared to monostatic sonar, a multistatic active sonar system increases probability of detection because more receivers are employed, and if proper oceanographic conditions exist, it greatly enlarges the amount of ocean area that is searched effectively.
While both sides experimented with multistatic acoustics during the Cold War, in the mid-1990s the United States introduced a multistatic active capability called the Extended Echo Ranging (EER) sonobuoy system. This first-generation multistatic sensor used two types of sonobuoys, a source sonobuoy (to provide a non-coherent, broadband acoustic pulse) and a receiver sonobuoy (to listen for the return echoes). The SSQ-110 source sonobuoy used an explosive to generate an acoustic pulse at a sufficient source level that the pulse could propagate the appropriate distance necessary for a wide-area search.
The introduction of EER into the P-3 fleet should have been the catalyst to force a change in MPA search TTPs. For EER, however, the end of the Cold War meant that it was already obsolete, five years before its introduction. As a first-generation system, EER was intended for use in deep water. Regardless of performance, with the Soviet threat diminished, there was little reason to persist with EER.
The next-generation multistatic active sensor, Improved EER (IEER), arrived in 2008. New source and receiver sonobuoys, combined with refined processing algorithms and digital radio-frequency links, initially showed promise. IEER continued the use of a non-coherent source buoy, but unlike its predecessor, was intended for employment in shallow-water regions against smaller, diesel-electric submarines.3 Commander, Operational Test and Evaluation Force (COMOPTEVFOR) rated IEER as “operationally suitable and operationally effective” in its initial operational test and evaluation (IOT&E) report, yet passive acoustic-search tactics, not multistatic tactics, remained the norm for the P-3 community.4
Despite the conclusions of the IOT&E report, there was a lack of confidence in IEER among Fleet stakeholders. While IEER performed satisfactorily during operational testing, Fleet aircrews encountered training difficulties. The explosive charges used by the source buoys were largely to blame: The marine-mammal mitigation procedures required during IEER exercises precluded many operational areas, especially during whale-calving and migration seasons. Additionally, submarine officers were understandably wary about operating in close proximity to underwater explosives and therefore levied stringent safety procedures during IEER exercises (if they agreed to participate in them at all). IEER is a manual system that dictates the operator be able to discern a positive contact from a false one, a difficult task when using a non-coherent acoustic source that can generate confusing acoustic-clutter returns. Reliable target-recognition ability requires intensive training and proficiency. Therefore, the lack of Fleet aircrew proficiency in IEER resulted in skepticism about a capable system, both inside and outside the MPA community.
To date, the multistatic active-sensor family has not enabled the MPA community to abandon its reliance on outdated passive acoustic-search TTPs. Fortunately, the corporate knowledge gained during the development, testing, and use of the first two generations of multistatic active systems has been retained and advanced. Combined with improvements in processing power and battery technology, the third-generation system, known as “MAC,” is about to debut.5
Third Time’s a Charm: Multistatic Active Coherent
Multistatic active coherent (MAC) is an ASW search sensor whose improvements have overcome IEER’s deficiencies and yielded a more operationally usable system. Advancements in processing power, algorithms, battery capacity, automation, and operator/machine-interface technology are incorporated into MAC. A significant difference between MAC and its older siblings is the SSQ-125 source buoy. The SSQ-125 source is electronic, not explosive, and emits a coherent, narrowband pulse that will reduce acoustic clutter for easier target recognition while largely removing the restrictions related to the use of non-coherent sources (explosives) in the ocean.6
MAC provides a compelling cost-benefit to the Navy and, together with MPA, fits well into the payload-centric model called for in Chief of Naval Operations Admiral Jonathan Greenert’s “Payloads over Platforms” approach.7 Currently, only MPA employ MAC because the radio-frequency line-of-sight coverage required for a MAC field impedes integration on ships, submarines, and low-flying helicopters. However, potential concepts of operations (CONOPS) with ships, helicopters, and unmanned air systems are being evaluated. MAC is an Acquisition Category IV (Navy and Marine Corps only) program that cost $183 million to reach initial operating capability. In the world of defense acquisition, MAC is a relatively affordable program that enables the primary mission of MPA by providing a core warfighting capability.
COMOPTEVFOR recently determined that MAC is “operationally suitable and operationally effective;” P-3 squadrons have already begun to train with MAC and new P-8 crews will follow suit in mid-2014. During operational testing MAC met its threshold requirements for search-area size and probability of detection. The search-area size is classified, but is sufficient for current operational requirements.8 Furthermore, MAC is an evolutionary acquisition program with planned follow-on increments to improve its capabilities. Successive versions will double the search area by increasing source level and providing that robust CZ detection capability that Buzz so adeptly exploited. Should MAC continue to meet its performance gates, it has the potential to revolutionize how the MPA community prosecutes submarines.
It’s understandable that some ASW stakeholders will initially greet MAC with skepticism. The first two generations of multistatic sensors, EER and IEER, never matured into the “go-to” acoustic search systems envisioned by the Navy. MAC is a large step forward from its predecessors. The MPA community is looking to MAC as the catalyst to drive a change in wide-area search TTPs.
Overcoming the Barriers
A submarine is stealthy because it is shielded from non-acoustic sensors by the ocean surface that it hides under. Unfortunately for ASW forces, modern technology allows a submarine to remain completely submerged for long periods of time. Non-acoustic sensors like radar and electronic-surveillance measures will always play a role in ASW, but an acoustic sensor capable of extended detection ranges is the appropriate solution to search for submarines over a large area.
The return of a wide-area ASW search capability to MPA should hopefully generate interest and perhaps even excitement. During the Cold War, commanders looked to aircraft like the P-3 as a rapid-reaction, cue-to-kill asset that could quickly transit to an area and find a submarine underneath a broad swath of ocean. MAC is the sensor to restore this capability; operational commanders should be looking forward to the opportunity to employ it.
The Navy must take steps to understand and embrace MAC. Experiences with both EER and IEER may make the noses of some operational-planning staff members wrinkle. This lingering bias threatens the introduction of MAC, a capability solution that is too important to languish. Such a bias is one of two significant barriers to the successful incorporation of MAC into wide-area-search CONOPS and TTPs. The other is resistance among some professionals to the idea of consistently using active sonar as a search sensor.
First, to overcome the bias barrier, the Fleet obviously needs to see MAC succeed. Education and training are necessary. For starters, the submarine force should understand that MAC uses an electronic source, not an explosive source, and agree to participate as much as possible in MAC training events. Operational commanders should know that MPA would not be surrounding submarines with explosives. Next, all stakeholders, especially operational staffs and exercise planners, must grasp the capabilities and limitations of the system, and look for opportunities to employ MPA in a wide-area ASW search role. Of particular importance is understanding that MAC is a scalable system, meaning that it can be deployed in a small area for unit-level training as well as a broad area for joint exercises or operational use.
Second, all ASW stakeholders must be willing to consider using MPA-deployed MAC sonobuoys, instead of passive sonobuoys, on both U.S. and foreign submarines. There will be times, due to electronic silence or other restrictions, that active sensors should not be used. Therefore, the Navy should invest in air-deployed passive acoustic-sensor technology in order to comply with these situational requirements. The Office of Naval Research is seeking to develop such a capability, known as the Next-Generation Airborne Passive System (NGAPS), which could provide a passive wide-area ASW search capability.
Planners must consider prevention of mutual interference. They can mitigate the risk with TTPs. Similar TTPs accommodate other coherent signal sensors, such as the airborne low-frequency sonar, SQS-53C hull-mounted sonar, TB-37/U multifunction towed array, and compact low-frequency active.
Like all military weapon systems, MAC is a sensor designed for wartime use. It is unreasonable to expect MPA aircrews to “break glass” on D-Day and be able to effectively fight with MAC if they are not practicing with it in peacetime. It would be wise to include MAC in Phase 0 ASW operations in overseas environments as well as back home in training environments. Besides, sonobuoys (including MAC sonobuoys) have a shelf life of five years; the Navy might as well use them.
Wide-area ASW search was a core capability of the MPA force during the Cold War. MPA crews, like the one led by Buzz Lineburg, held Soviet submarines at risk and played an important role in the Navy’s ASW efforts. Since then, the submarine threat has incorporated advanced quieting technology, creating a capability gap whose solution has remained elusive. The MAC system is a significant improvement in the multistatic active family of sensors and provides an opportunity to recapture a capability not enjoyed since the Cold War. The Navy should embrace MAC and seize this chance to restore its ASW search capabilities.
1. Frederick “Buzz” Lineburg, November 2013 interview with author. Task Force ASW, Anti-Submarine Warfare Concept of Operations for the 21st Century (Washington, DC: Department of the Navy, 2004), 5.
2. Ibid., 5.
3. COMOPTEVFOR, Improved Extended Echo Ranging (IEER) Operational Test Agency Evaluation Report, (Washington, DC: Department of the Navy, 2008), 1.
4. Ibid., 4.
5. CDR Brian Toner, USN, Capability Production Document for (U) Multi-static Active Coherent (MAC) System Upgrades for Navy Maritime Patrol and Reconnaissance Aircraft (Washington, DC: Department of the Navy, 2012), 1.
6. LT James Evans, USN, et al., Developmental Test and Evaluation Phase B of the Multistatic Active Coherent System of Systems for the P-3C Aircraft (Patuxent River, MD: Naval Air Warfare Center Aircraft Division Patuxent River, 2012), 5. Toner, Capability Production Document, 2.
7. ADM Jonathan Greenert, USN, “Payloads over Platforms: Charting a New Course,” U.S. Naval Institute Proceedings, vol. 138, no. 7 (July 2012), 16–23.
8. COMOPTEVFOR, Multistatic Active Coherent (MAC) Operational Test Agency Evaluation Report (Washington, DC: Department of the Navy, 2014), 3. Toner, Capability Production Document, 2.