Abstract
This article describes submarine sonar concepts for use in littoral waters. Included are sonars for use in. on. or with an innovative submarine sail. The new sail is envisioned for submarines beyond the new attack submarine (NSSN), although some of the sonar concepts could be backfit to NSSN, 688 or SEA WOLF Class. The new concepts are a result of a clean slate look at future submarine sonars for littoral waters. Certain of these concepts will undoubtedly be accepted and others altered or discarded as more formal, detailed cost and effectiveness studies are conducted.
“What we anticipate seldom occurs; what we least expect generally happens.”
– Benjamin Disraeli 2
Background
With the end of the Cold War, the U.S. Navy laid out a dramatic new strategy. The essence of this strategy was documented in late 1992 in … From the Sea. The strategy was finalized after extensive senior (military and civilian) naval staff participation during fiscal year (FY)3 1992. At this time the Navy was also completing its FY 1994 budget for submission to the Department of Defense (DoD) and Office of Management and Budget prior to submission by the President to the Congress in early 1993. Because the strategy was well thought out, well stated, and had broad support, it survived the transition from the Bush to Clinton administration and has been adopted by Secretary of the
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1 The views expressed here are those of the author.
2 Quoted by RADM Thomas Brooke in USNI Proceedings, March 1994.
3 A list of acronyms is provided as an appendix.
Navy John Dalton4 and Assistant Secretary of the Navy Nora Slatkin.5 While this strategy had been public for some two years, the significance of this paradigm shift became clear to the Undersea Warfare Research & Development community while executing the FY94 budget, defending the FY95 budget and preparing the FY96 budget.
Having contributed to the collapse of the former Soviet Union, the United States now enjoys the freedom of the open ocean. As … From the Sea states, “With the demise of the Soviet Union, the free nations of the world claim preeminent control of the seas …” Moreover, for the foreseeable future, the focus of the U.S. Navy is to project power ashore from the sea. The coastline areas are referred to as littoral areas. They are often, but not always, shallow water areas.7 In these areas of the world, the DoD is beginning to envision new submarine sail concepts for submarines beyond the Navy’s new attack submarine (NSSN). Naval organizations, universities, and contractors are conducting initial submarine sail studies and developing sensor suites for littoral warfare; some of these are located on a new innovative submarine sail. As will be described, these new sonar suites are needed.
4In the August 1994 issue of the USN I Proceedings, Secretary Dalton stated that “We embrace the concept of …From the Sea and applaud the direction that it takes the naval service.”
5N. Slatkin, Undersea Warfare: An Acquisition Strategy to Meet New Dangers, Sea Technology, January 1994, pp. 30-34″.
6What follows is an example of a paradigm shift. For instance, because accurate time keeping was key to accurate navigation (in particular, determining longitude), the revelation of how to keep accurate time was a capital offense in the British Navy. Many years later, digital quartz technology replaced mechanical devices, making accurate time available at low cost; indeed thousands of people lost their jobs making finely crafted watch main springs.
7Note shaDow water and very shaDow water mean different things to different readers. To acousticians, shallow water is often defined in terms of wavelengths or bottom interactions; for others, it simply means a particular depth. For example, divers without underwater breathing apparatus think of 20 feet as deep, but for a submarine with a nominal30 foot hull diameter, 20 feet is very shallow. At 100 Hz, 150 feet is only three wavelengths, so an acoustician might well consider 150 feet shallow.
because “mastery of the littoral should not be presumed. It does not derive from command of the high seas. It is an objective that requires our focused skills and resources.”8
This article discusses the author’s conjectures of possible submarine sonars for use in littoral waters, including their relationship to innovative submarine sail concepts. Sonars are vital to a submarine’s success, but the paradigm shift that requires submarines to be fully integrated participants in the battle force means that submarine sonars must become more than just devices located on submarines. While the main objective of a new submarine sail is to improve communications, sonar performance is dependent on sail design. A new sail has the potential to improve submarine sonar in two ways. First, sonar performance of sail sensors can be improved. Secondly, with high data rate (HDR), real-time communications, sonar can be linked to offboard sensors and assets with cuing for improved sonar and combat system performance.9
Submarine Operations in the Littoral Environment
The nuclear submarine offers stealth, agility, and endurance for joint littoral operations. It can maintain a forward presence and be first on scene. The submarine can be covert and nonconfrontational. It is an ideal naval platform for providing the National Command Authority {NCA) with indications of upcoming hostilities. It complements other national assets by providing warnings of such activities as ships leaving port, underwater mine laying, the presence of underwater minefields and intercepted naval and coastal message traffic. This is the indications and warnings, or I&W, mission. With proper communications suites (especially HDR antennas), the submarine can transmit intelligence
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1 “…. From the Sea”
9 To a lesser extent, sonar performance of non-sail acoustic senson can be changed (perhaps improved) by changing (improving) hydrodynamic flow around the submarine, thereby reducing flow noise and distortion. We speculate that changes would be mainly to hull arrays but could include both existing and future sensors, such as, sphere, towed arrays including future MultiLine Towed Arrays (MLTAs), the Wide Aperture Array (WAA), the Advanced Mine Detection System (AMDS), Noise Augmentation Units (NAUs), noise monitors and ice-penetration sonars.
back to the NCA and receive detailed tasking for the next phase of operations.
With speed and stealth, the submarine is well positioned to covertly insert commandos or so-called Special Operations Forces (SOF)from the sea. The SOF, headquartered in Tampa, Florida, operates as a fourth branch of the DoD: Army, Air Force, Navy, and SOF.10 The SOF is a truly joint command with components from the three services, including the Navy’s Sea Air Land commandos (SEALs). Navy SEALs can be inserted and extracted from submarines using surface launched combat rubber raiding craft, subsurface launched wet swimmer delivery vehicle, or by subsurface launched dry, long range advanced swimmer delivery system, now being developed. The SOF ashore can provide intelligence and laser designation of key defensive radars and command control targets. During insertion and extraction of Navy SEALs, the submarine could be close to the surface and to the shore. This proximity might place the submarine in shallow water, say, less than 20 fathoms11 at speeds of less than three knots for prolonged periods. During these near-stopped, shallow water operations involving stopping and maneuvering, it is the author’s opinion that towed arrays will not be deployed and the needed vertical aperture of multiline towed arrays (MLTAs) will not be available. Moreover, the near surface position of the hull and WAA sonars (above the layer) will limit sonar performance to acoustic sources above the layer and severely restrict performance against sound sources (such as adversarial submarines) below the layer. These environments and scenarios favor offboard sensors close to the source because of propagation loss; because of the multiple acoustic rays in the shallow water waveguide, they also favor vertical sonar arrays that deploy below the layer and form narrow beams that can capture vertically separated acoustic rays while simultaneously discriminating against noise. Of course.
10Sec John M. Collins, Special Operations Forces, CRS Report for Congress 93.697S, July 30, 1993.
11“For those who question whether the submarine will operate in shallow water, ADM (then VADM) William Owens staled on May 11, 1993: “Thirty or more days before the landing is scheduled, the submarine could already be there in the 70 or 80 feet of water.”
detailed studies and analysis must be conducted to assess the cost and effectiveness of innovative sonars designed to operate in such complex environments.
If operations require, the submarine will be ideally suited to (covertly) initiate a submarine launched cruise missile (SLCM) attack from the sea. Such a SLCM land attack can disrupt enemy shore based anti-air radar, as well as command, control, and communications centers, thereby clearing the way for joint carrier based air attacks, land based U.S. Air Force stealth air attacks, and landing Marines ashore. However, the complexities of such attacks require large volumes of data (called air tasking orders) that can take hours to be downloaded to the submarine shooters. To be an effective player in this mission, the submarine must be able to receive large volumes of message traffic, including last minute updates.12 As envisioned, this will force the U.S. submarine to keep a large sail-mounted receiving antenna exposed above the water, potentially decreasing submarine stealth. Further, the shooter may be constrained to a fixed geographical launch basket. Clearly, these key joint operations will place new loads on the submarine combat (or command and control) systems and may restrict the maneuverability of the submarine. Restricted maneuverability, in tum, limits depth excursions and lead/lag legs historically used for target motion analysis making rapid localization sonar extremely important. Such constraints will force changes in methods for layered ship self defense, anti-submarine warfare (ASW) and anti-surface warfare (ASUW) operations and restrict the sonar performance if only conventional, submarine based sonar sensors are relied upon.
Littoral operations will have profound implications on future submarine sonar systems. Sonar and combat systems must be enhanced to operate effectively in littoral waters, and this must be accomplished in an increasingly tight fiscal environment. In addition, deep water, open ocean dominance must be retained. This involves conventional and evolving sensors, processing,
12“As VADM George W. Emery, USN, COMSUBLANT, stated on May 10, 1994: “Our (referring to the submarine force) ability to strike targets ashore must also keep pace with the rest of the Navy …Communications is a critical area for integrated operations with submarines and other forces, joint and allied. The key problem here is achieving the higher data rates and compatibility with the rest of the fleets because of the limitations of submarine antennas.”
displays, training and command and control for ASW and ASUW.
Submarine Sonar Forcing Functions
Three forcing functions for submarine sonar when the submarine operates in the littoral include: 13 intelligence gathering, the environment, and submarine posture.
First, intelligence gathering will require that the submarine (1) sonar act as a sensor for the Joint Task Force QTF) Commander and that (2) sonar/combat system integrates and fuses received signals from: (a) offboard sensors such as unmanned underwater vehicles (UUVs), {b) bistatic active sonar, and (c) national (satellite) assets with downloaded {minefield) intelligence. Note, for intelligence gathering involving minefields, localization accuracy drives submarine sonar {size and location) requirements. For example, accurate determination of underwater mine depth nominally requires a sonar with vertical aperture. Furthermore, minefield intelligence requires {two-way) communications and connectivity with JTF and perhaps NCA, to identify Janes for Marine Corps fares to go ashore.
Secondly, environmental conditions in the littoral vary widely as a function of space {i.e, geographical location) and time {of day and season). For example, sound propagation is dominated by temperature versus depth profiles. Storms, typically in the winter, tend to mix up the top part of the water column causing an iso-thermal layer that profoundly affects sound propagation. On a daily basis, biological scatterers tend to feed at different times of day, moving about within the water column changing reverberation levels that impact active sonar performance. Poor environmental conditions limit acoustic signal reception (due to downward refracting acoustic rays, steep grazing angles and numerous bottom and surface interactions between the source and receiver). Moreover, poor environmental conditions drive the need for (1) offboard deployable sensors and (2) cueing. Stated differently, environmental conditions may be so poor in littoral waters that the
13of course, they include other forcing functions, too, beyond the scope of this article. For example, U.S. sonars must take into account counter detection ranee by a potential adversary. This, in tum, would include concern for our radiated signallevela and our target strenght, including sail shape and reflective· ity. Unlike cold-war, open-ocean operations, we will now also be concerned with the radar cross section of a still exposed in littoral waters.
Only way to sense objects of interest is to be cued by external controllers or offboard sensors closer to the objects of interest.14 Of course, poor environmental conditions are a double edged sword in that such conditions provide added acoustic stealth for our submarines.
Thirdly, submarine operational posture will limit the submarine maneuverability normally required for optimal sonar and target motion analysis performance. Moreover, this posture requires NAUs during training and selected operations. Submarine operational posture requires low target strength under water and also requires low radar cross section in air. Further, this posture requires good open ocean sonars to get to littoral waters; this requires well behaved flow around submarine sensors so that spherical and wide aperture arrays, as well as other sensors, perform well. To be investigated are the impact of operating near the surface in the open ocean (enroute to the littoral) to receive HDR communications with mission planning updates, air tasking orders, and tactical pictures common to the JTF.
Technical Requirements
At the technical level, we envision at least the following submarine sonar requirements.
1. A mine avoidance sonar with a large (vertical or horizontal or both) aperture for accurate mine position estimation,15
2. Offboard sensors (e.g. UUVs, the deployable acoustic sensor system (DASS), or advanced deployable system (ADS)) with connectivity/linkage to the submarine and high gain aperture
3. A permanent NAU in the sail, to avoid continuous cross decking cost.
14It is noteworthy that of board sensors is on the July 1994 COMSUB-LANT/COMSUBPAC list of high priority Command Technology Issues.
15Vertical arrays are useful for vertical mine localization accuracy. One current assessment is that a vertical array on the sail is workable, inside the bow for mine avoidance appears to be unworkable or very high risk; while a vertical array affixed to the bow bas some risk (due to a variety of factors including anticipated interference to the spherical array’s reception).
4. Noise monitor sensors in several locations, including in and on the sail for acoustic stealth 5. AN/WQC-2 on the sail for acoustic communications call up
6. AN/WLR-9 or AN/WLY-1 on the sail for 360 degree intercept receive capability
7. A sphere and W AA for detection and rapid localization
8. A chin mounted AMOS, below and aft of the bow for underwater mine detection, (horizontal) bearing estimation, and bottomed mine detection, classification and localization
9. TAs and MLTAs for slow and high speed low frequency passive (bistatic active) operation.
Submarine Sonar Advanced Concepts
We have grouped advanced concepts for submarine sonars operating in the littoral into three areas: (1) sonars on, in or deployed for a new submarine sail, (2) sonars exploiting a new innovative sail with HDR communications, and (3) other sonars not on, in, or from a new sail. Several of these shipboard sonars are sketched in Figure 1. While Figure 1 depicts only a single vertical line array above the sail, this could be a vertical multiline system. Note also that while Figure 1 depicts both a vertical array deployed from the sail and a cylindrical sonar system (CYSS) below the submarine, for some operations only one of these array systems would be deployed at a time. If the submarine were submerged and hovering, near the bottom, the
CYSS couldn’t be deployed below the boat, rather a (single or multiline) vertical acoustic array would be deployed upward from the submarine sail. If the submarine had its sail exposed for communications or SOF operations and were traversing at low speed, it would lower (one or more) CYSS volumetric array(s) below the submarine; this would provide the needed vertical (and some horizontal) aperture and position some sensors below the layer. Notionally, this array could be lowered about 100 feet below the keel, or even deeper with spacers or affordable very thin optical arrays. In water shallower than 100 feet (plus hull diameter), the cylindrical array would only be partially deployed. For ranging, three CYSS arrays might be used.
A. VERTICAL ARRAY
B. NOISE MONITOR
C. WQC SUBSAFE CALL
D. M.R-9 INTERCEPT
E. NAU
F. ICE PENETRATION
G. WAA
H. MILLS CROSS TRANST
L. AMDS
J. CYSS
K. SPHERE
L. TALHLTA
M. FLANK ARRAY
Sonars being considered in or from a new submarine sail include (1) a Mills cross or T array (with a full horizontal and vertical aperture) with an unimpeded vertical acoustic array in or on the sail (versus the riskier alternative of a high frequency array in or on the bow) and (2) acoustic sensors in a vertical line from a submerged submarine to a communications buoy. The T -shaped mine avoidance sensor with its vertical aperture would allow accurate depth determination when the submarine is submerged. Of course, when the sail is exposed in the air, the sail sonar would not be used to transmit or receive underwater signals, but it might receive in-air acoustic signals of air or patrol craft.
Sonars exploiting a new innovative sail with HDR communications include submarine sonar and combat systems linked by satellite, fiber optics, or RF communications to sonobuoys DASS and ADS arrays; ocean surveillance assets; national (imaging) assets Tactical Exploitation of National Capabilities (TEN CAP).
Finally, sonars for use during littoral operations include the CYSS, notionally this would consist of nine vertical arrays, each having 111 sensors deployed below the submarine. 16 However, the exact configuration requires additional detailed study. This sonar is illustrated below. A Navy patent is being prosecuted for the CYSS submersible sensor system invention.
16This is a notional array of either 999 sensors or the thin optical equivalent. An independent performance assessment and demonstration of array gain would be conducted before actilling on the final array configuration. The CYSS includes an active adjunct located on the depressor.
Submarine Sonar Advanced Demonstrations
We envision the need to demonstrate some of our advanced concepts (related to submarine sonar developments that are tied to a new innovative sail). For convenience, we group the submarine sensor research, development test and evaluation demonstrations into three areas: (1) sail specific sensors, (2) sensors that exploit new innovative sails, and (3) other littoral sensors.
First, sail specific acoustic demonstrations include the following: (1) Mills cross or T sail array beam patterns, (2) NAU transmission through new low radar cross section sail materials, and (3) a vertical acoustic line array from a submerged submarine sail to a floating radio frequency (RF) communications buoy. The demonstration treats both handling and acoustic performance.
Secondly, demonstrations to exploit new sail HDR communications include a submarine link from external sources to a prototype sail and then to land based sonar and combat system laboratories.
In these demonstrations, we would link the sail by satellite and RF communications to ocean surveillance (e.g., ADS) assets, sonobuoys, and NCA, and then line the prototype sail to land based test sites.
Third, other littoral and sail alternative demonstrations include: (1) T or Mills cross array, but now on the bow (instead of the sail), (2) CYSS with perhaps, for example, nine arrays of 111 sensors each, and (3) as a possible alternative to a sail mounted or T array, consideration of a vertical mine hunting transmit array that telescopes out of a vertical stowage tube.
Summary
Four key points must be summarized in discussing the new submarine sonars that will be needed to operate effectively in littoral waters. First, we need to continue to improve sonars so that we retain our ability to move ships, troops, and supplies from U.S. ports across the open ocean enroute to littoral waters. We note that sail shape will affect both our underwater target strength and flow characteristics around the sail. This, in tum, will impact both ship handling and sonar self-noise. Second, we need mine detection, mapping, and avoidance sonars (some sail mounted) to penetrate littoral waters. Third, we need offboard acoustic sensors and assets. Fourth and last, we need connectivity to offboard sensors and national cueing and command assets.
Environmental conditions are such that several submarine sonar challenges exist to meet the Navy’s new strategy focused on projecting power from the sea. Because of the profound shift in re-ordering the Navy’s missions, we have prepared recommendations to assess and demonstrate sonar performance in littoral waters.
Recommendations
Submarines operating in the littoral may require improved sensors and connectivity. It is recommended that the following effort be performed:
- Subject notional concepts here to careful, detailed analysis.
- Develop and validate models of environmental conditions critical to submarine sonar operation in key littoral areas.
- Conduct studies and analyses that assess the cost and effectiveness (including performance and contribution) of various submarine sonar sensors in achieving joint and combined missions.
- Continue to develop submarine based mine avoidance sonars, some of which will be forward looking chin arrays and some will be mounted on the submarine sail, such as a T array (with full vertical and horizontal aperture).
- Develop and demonstrate offboard acoustic sensors that are linked to the submarine sonar and combat control system, such as sonobuoys, the cylindrical DASS the ADS arrays, and ocean surveillance assets. Some offboard sensors will be on or connected to UUVs.
- Develop and demonstrate HDR real-time connectivity to offboard sensors, national (imaging) assets, and TENCAP for improved cuing
- Develop and demonstrate the performance of two CYSS-like sonars each consisting of, for example, nine vertical (thin optical) arrays, of 111 sensors or alternative design. Such a system would be deployed below the submarine for own-ship defense when the submarine operates at low speed, near the surface, in littoral waters.
- Develop and demonstrate a vertical sonar17 that reels into the sail or hull of the submarine. Such a nominally vertical.
17Such an array would serve a self-defence role in the littoral.
line array of acoustic sensors and would deploy from a submerged submarine to a floating communications buoy while the submarine loiters near the bottom.
- Continue to develop the deep water sonars necessary for (1) transiting to shallow water, (2) dominating deep water littoral undersea battlespace, and (3) (incidentally) retaining open ocean dominance.
APPENDIX A: LIST OF ACRONYMS
ADS: Advanced Deployed System
AMDS: Advanced Mine Detection System
ARPA: Advanced Research Project Agency
ASW: Anti-Submarine Warfare
ASUW: Anti-Surface Warfare
CYSS: Cylindrical Sonar System
DASS: Deployable Acoustic Sensor System
DoD: Department of Defense
FY: Fiscal Year
HDR: High Data Rate
I&W: Indication & Warning
JTF: Joint Task Force
LFA: Low Frequency Active
MLTA: Multi-Line Towed Array
NAU: Noise Augmentation Unit
NCA: National Command Authority
NSSN: New Attack Submarine
NUWC: Naval Undersea Warfare Center
RF: Radio Frequency
SBIR: Small Business Innovative Research
SEALs: Sea Air Land (SOF Commandos)
SOF: Special Operations Forces
TA: Towed Array
TEN CAP: Tactical Exploitation of National Capabilities
UUV: Unmanned Underwater Vehicles
WAA: Wide Aperture Array
