David Nicholls retired in 2001 after 31 years as a submariner whose tours included CO of HMA Submarines OXLEY and OT AMA. His final tour of duty was 3 years exchange posting on the staff of COMSUBPAC. Chris Donald retired in 1999 after 30 years as a naval aviator serving in VS and VP squadrons in his early life and working with and in submarines for the past 15 years, specializing in fast fits. He currently heads the Sonar and Ranges section of the Australian Defence Material Organisation. This paper represents the views of the authors and not necessarily those of the Australian Department of Defence.
On 10 Sep 2001 a Statement of Principles, for enhanced cooperation between the USN and RAN, in matters relating to submarines, was signed by Admiral Vern Clark, USN (CNO) and Vice Admiral David Shackleton, RAN (CN) in Washington, DC.
Amongst other issues, this SoP undertook to cooperate in research, development, and engineering projects as follows:
- Projects to improve the acoustic characteristics of submarines
- Projects to improve submarine combat systems
- Projects to enable submarines to achieve their full operational potential
- Projects to develop improvements jointly for software updates for a common combat system.
There are a number of areas of emerging technology in which Australia has demonstrated an ability to contribute in such a cooperative vein, perhaps via the ARCI/APB programs. These include:
- Data Fusion in tactical data handling, using Multi-Hypothesis algorithms
- Covert under water communications
- Ping Intercept Passive Ranging Sonar (PIPRS)
- Self Defence against ASW Aircraft attack.
The rapid acceleration in the development of modem technology has led to the advent of the COTS concept and the demise of the old legacy system design. The Plug and Play capability of COTS systems means that smart, new ideas can be incorporated into open architecture systems.
Millions of dollars can be saved in development and legacy system integration costs. It doesn’t matter if it wasn’t invented here-just get on and use it!
The Impetus for Self Help in Australian Submarine Sensor and Combat System Development
The current Australian Submarine Squadron was commissioned in 1967. The original six Oberon class boats were built in UK and fitted out with RN systems. Australian requirements for diesel submarine operations in the Pacific region led to a replacement digital combat system integrated with the U.S. Mk 48 MOD 4 torpedo and the sub-Harpoon missile in the late ’70s/early ’80s.
These requirements also led to the indigenous development of specialized sonar/combat system processing and the recent building of the Collins class submarines. Allied developments in the US and UK legacy submarine systems were focused on nuclear subma-rines-many were unsuitable for diesel submarine application and most developments were not releasable.
Diesel boat experience in shallow, tropical, littoral waters-with a high density of sensor contacts-has led to the development of specialist sonars and data handling/data fusion processors. The minimal levels of self noise in a diesel boat have led to the focused development of bull mounted sonar arrays (flank and distributed) and the associated ability to process passive ranging on active transmissions and transients using wavefront curvature techniques.
Other initiatives included the development of a covert Spread Spectrum underwater communications system (Hydro Acoustic Information Link (HAIL)), now used in both submarine forces for the annual joint USN/RAN PCO Training exercises.
The lack of sustained power and speed has emphasised the vulnerability of diesel boats to the expanding capabilities of regional ASW airborne threats. This led to the work by B. Ferguson and G. Speechley at Australia’s DST01 and later published in the U.S. Journal of Underwater Acoustics.
The following article addresses some personal Australian thoughts, on self-defense against airborne ASW.
Some Australian Ideas on a Defensive Submarine Anti-Air Capability
Background
In the January 1994 issue of THE SUBMARINE REVIEW, an article on Defensive Anti-Air Warfare for SSNs was published by Captain James H Patton, Jr., USN(Ret.). A Comment on that article by Ambassador Linton F Brooks (U.S. Chief START Negotiator) was published in a subsequent issue of THE SUBMARINE REVIEW.
The thrust of Captain Patton’s article addressed a scenario in which a submarine was deployed on an independent mission, in a littoral area in which U.S. I Allied forces did not have control of the air space. In such a scenario, if a submarine was unfortunate enough to be detected and localised by a hostile ASW aircraft, the submarine had no capability with which to defend itself against attack. Captain Patton then broadly addressed the likely tactics of both protagonists in such a scenario, together with the broad characteristics of a submarine launched air defence capability.
The Comment by Ambassador Brooks countered Captain Patton’s case for an AAW defensive capability, asserting that “In littoral warfare, the first and most important characteristic is stealth”. He next opined that “Fortunately, prospective targets for littoral warfare are not likely to be able to detect a submarine that wants to remain undetected”. Later in his Comment he qualified that opinion with the observation that “The fact that there is no current need for submarine based AA W does not, however, mean that there never will be.
This article addresses some Australian developments that have taken place over the eight years since those earlier articles were published.
Introduction
The Stealth Factor
The ability of a submarine to survive in a hostile environment relies predominantly on stealth to counter the efficiency of hostile Anti-Submarine Warfare (ASW) sensors by:
- reducing radiated acoustic signature to a minimum, to counter passive sonar systems
- reducing magnetic signature to a minimum, to counter Magnetic Anomaly Detection (MAD) systems
- reducing target strength to a minimum, to counter active sonar systems
- reducing any form of exposure above the sea surface to a minimum, to counter radar, infra-red and optical systems.
The aggressive pursuit of submarine missions, particularly in a littoral water environment, involves a calculated increase in the risk of detection through these factors.
Stealth and the Impact of Multi-Static Sonar
The best form of self-defence for a submarine in the face of an adversary, to date, has been to maintain a covert posture. Amongst other advances in battlespace ASW capability, the development of active multi-static sonar systems (MSS), in littoral areas, where national or allied coalition forces do not have control of the air space, makes the potential for long range active prosecution of our submarines a real possibility.
The ability of a hostile MSS system controller to maintain the localised position of the submarine (even when only a limited number of aircraft are available) may mean that prosecution of the submarine can be maintained for extended periods of time. This would allow hostile aircraft the option of returning to base to re-arm and conduct multiple re-attacks. Prior to the advent of MSS, the initiative in the underwater battlespace lay with the submarine. The ability to gain, and maintain, localised contact with the submarine, via MSS, potentia]]y shifts that initiative to the hostile ASW aircraft.
Self-Defence versus AA W Offence Against Hostile Aircraft
Sonar processing technology now provides the capability for submarines to detect, track and localise hostile ASW aircraft. 2 This provides the submarine with a heightened level of threat awareness and the trigger to reduce any risk of detection. However, the threat of MSS is difficult to counter-and the fact remains that a submarine localised/attacked by an ASW aircraft still has no intrinsic self-defence weapon.
In examining an option for providing submarines with an active deterrent against hostile ASW aircraft, it is recognised that sophisticated, high cost solutions involving homing/guided missiles are being developed. The capital costs of such systems, together with integration costs and the long time-scale for development, are aU likely to be significant. Future justification for such systems may well involve extending the concept of operations for submarines-specifically where the submarine is undetected and holds the tactical initiative to include aspects of offensive Anti-Air Warfare (AAW). Such a capability might have changed the role of UK SSNs in the Falklands campaign, from EW Picket, to AA W picket, with a consequent significant reduction in RN losses to land based Argentinian air attack!
Of major significance in the concept of a Submarine Launched Aircraft Countermeasure (SLAC) is the focus on the self defence aspects of submarine operations at a relatively cheap cost-it has no place in extending the overall submarine concept of operations.
This paper, therefore, focuses on a SLAC design derived from proven levels of engineering design and existing Submerged Signal Ejector (SSE) discharge systems (3 inches) to provide a low cost, fast-track development solution.
In one tactical concept for the use of SLAC, a parallel might be drawn with that of a defensive minefield used for area denial. Once the submarine CO has determined that there is a high risk of being localised, airborne ASW prosecution might be deterred by sowing the immediate submarine operating area with potentially lethal ordnance {multiple SLAC) munitions.
The Submarine Launched Aircraft Countermeasure (SL.AC) Concept Very briefly, the SLAC concept proposed here is a munition store that is ejected from the submarine signal ejectors (SSE) and buoyantly ascends to the sea surface. Using organic sensors, any aircraft within close proximity to the SLAC munition is detected. Once the tracking sensor detects the aircraft approaching Closest Point of Approach (CPA), ie overhead or very close to overhead, the encapsulated munition is fired.
The primary aim of the SLAC capability is to cause the crew of a hostile ASW aircraft to recognise the potential threat and change their tactical focus from unrelenting pursuit of an attack on the target submarine, to that of survival from counter-attack. The individual (or combined) effect of such a change in tactical execution is likely to be:
- delaying the launch of the weapon and/or
- avoiding an attack profile for weapon launch, which passes over the top of the submarine target and/or
- aborting the mission
Tactical Issues
The Decision to Deploy SLAC
A submarine being prosecuted by a hostile ASW aircraft will be very aware, from sensor analysis, that it has been localised by proximate sensors eg. by the fact that the aircraft has commenced executing Magnetic Anomaly Detection (MAD) runs prior to the commencement of an attack. The decision by the submarine CO to deploy SLAC would be made only in the certain knowledge that the submarine is at grave risk of being, or had already been, localised. Therefore from the submarine’s point of view there was nothing to lose, but perhaps much to gain, by firing a pattern of SLAC munitions. The SLACs would deploy to the surface whilst the submarine commenced evasive manoeuvring.
Multi-Static Sonar -MSS
The development of MSS, as a proximate sensor, will change the manner in which the submarine CO determines the chain of events via which he predicts his vulnerability to attack. As tactical experience in countering MSS increases, he will recognise a level of threat from his intercept of one (of perhaps a number) of MSS transmissions, e.g. certain angles on the hull, indicating that there is a high risk of detection and that an attack is likely to shortly ensue. At this point the CO should be shifting tactical posture to one of clearing the datum, employing evasion tactics and, as a deterring/diversionary tactic, consider deploying a SLAC munitions defensive minefield.
Recognition of MAD ‘Runs’ to Establish Target Datum
In the current use of MAD as a localising sensor, three close passes are normally conducted. Activation of a SLAC during one of these MAD runs would almost certainly force the aircraft to increase altitude and/or move away from the datum for weapon drop, if not inflict damage and cause the aircraft to abort the attack entirely. Even if the SLAC munition detonated behind the aircraft, the crew is likely to spend some time determining if the aircraft had sustained damage, offering an opportunity for the submarine to clear the datum and aggressively evade.
Weapon Launch-Significance of Datum Accuracy
Current tactics dictate that, once a level of confidence is reached, the aircraft drops to a low ( < 250 feet) altitude, flies directly along the line of the established course of the target submarine, and drops the weapon at the on-top position over the target. Subsequent weapon ‘splash’ is a short distance ahead of the position of the submarine. The use of SLAC to deter the aircraft away from the datum will cause errors in accuracy of the weapon drop position in relation to the actual position of the submarine. This would result in a commensurate reduction in the likely success of the weapon in detecting and acquiring the target submarine.
Genesis and Development of the SLAC Concept
In the mid 80s,an Australian operational requirement arose to improve the capability of submarine launched flares and markers. Australia’s Defence Science and Technology Organisation (DSTO) were tasked to design and demonstrate a new flare.
This device, having been launched by the submarine, ascends to the sea surface and explosively discharges an aerial flare to an initial design height of 600 feet. It is fitted with a parachute to provide extended time in the air. DSTO also designed a surface floating marker variant. The two variants are designated Signal Illuminating-Submarine Launched (SISL-parachute); and Signal Illuminating-Submarine Launched (SISL-surface); respectively.
The designs of the concept demonstrators were passed to industry and the two variants were re-designated Submarine Launched Flare (SLF) and Submarine Launched Marker (SLM). Subsequent development of the SLM is not pertinent to this article.
The SLF had an initial design bunt height of 600 feet. This generated air safety concerns from the airborne ASW operators and this became the genesis of a concept, developed by the co-author of this article, Chris Donald, to re-design the SLF into a self-defence Submarine Launched Aircraft Countermeasure (SLAC). This was based on his experiences with submarine sonars and observed submarine CO reaction to the close proximity of ASW aircraft. Many of the initial design features of the SLF have remained pertinent to the SLAC concept. However, a significant technical risk in the development of SLAC has been an acoustic targeting and triggering system that works for different types of ASW aircraft, in combination with an appropriate payload.
System Design Challenges and Technical Risk
The challenge was largely one of analysing a number of proposed payloads and sensor devices. Such analyses were conducted to establish:
- the issues associated with these two critical components
- which combination might most effectively threaten the attack profile of an ASW aircraft,
- which options could be integrated into the physical design limitations of a 3 inch SSE launched store.
The fundamental tactical concept is one of self-defense for the submarine, to deter the aircraft from executing the preferred attack profile (flying over the top) and to reduce the probability of success of an air launched ASW weapon.
SLAC System Sensors
Currently there are two options available as devices to initiate the SLAC munition propellant and discharge the payload. The first consists of an autonomous sensor. This might be one of a number of devices: perhaps an infra-red laser diode or a frequency Doppler trigger using an in-air acoustic sensor (SLAC-1).
The second option (SLAC-2) incorporates the Australian developed HAIL covert underwater acoustic telemetry link offering a controlled trigger.
SLAC-2: ‘In-Water’ Acoustic Telemetry System
This system would use the submarine’s sonars to track and localize the hostile aircraft. Accurate positions on all SLAC-2 munitions are maintained via time synchronization (at launch) and subsequent display on the command tactical plot. A two way acoustic telemetry link determines the relative position of each munition and proximity of the aircraft within the field of munitions. Once the SLAC-2 trigger system has been armed, the timing precision to initiate SLAC-2 firing at the coincidence of aircraft CPA may require TMA software automation. Command initiation should also be an option.
The SLAC-2 design has an in-water acoustic transmitter/ receiver and incorporates a microprocessor to send a time-stamped signal of its relative position to the submarine and to receive a trigger acoustic telemetry signal from the submarine. This system must employ complex wide band acoustic encoding sequences to avoid detonation or jamming by acoustic countermeasures. The Australian HAIL system has a range of20nm, is currently used for covert acoustic communications during joint USN/RAN PCO Operations.
The SLAC-2 system features the following capabilities:
- accurate aircraft tracking from organic submarine sonar provides triggering accuracy
- narrowband and broadband acoustic acoustic processing of airborne signatures encompasses all types of ASW aircraft
- evolution in ASW aircraft tactics and engine types can be accommodated via the development of submarine sonar processor algorithms and TMA algorithms.
- SLAC-2 is a controlled mine (can conform with Rules of Engagement)
- Controlled launch sequence from SSEs-pre-planned deployment to the surface in time to be effective
- Offers an option to create tactical confusion: for example, a submarine might seed its local operating area with SLAC-2 munitions that can be triggered up to 20 miles away from the datum, thus creating diversions when submarine-alerted ASW aircraft are close to localisation.
Probability of Hit Versus Deterrence
From the perspective of initiation, accurate timing of the trigger improves the chances of a hit on a MPA.
From the perspective of payload, the selection of the fragment dispersion angle is a trade-off with the horizontal and vertical CPA distances that can assure at least one hit. The optimum dispersion angle is also sensitive to assumptions about the minimum height at which MPA can make an attack. Further, the effectiveness of a fragment hit on a MPA depends on the number of fragments in a SLAC payload. This, in turn, is traded off against the spread of fragments and hence the exposure volume for which at least one hit is assured. Given the limited hit exposure volume of a single SLAC, as many SLAC devices as possible should be deployed to:
- Increase the probability of a hit
- Create the strategic effect of a minefield, and
- Accentuate the psychological impact and tactical effect of deterrence.
- With the proposed payload, the effective SLAC fragment impact horizontal radius is about 200 feet, therefore the hit probability depends on how closely the ASW aircraft follows current tactics. If, however, attack tactics are changed from the optimum because of a SLAC threat, the value of SLAC as a deterrent would already have been realised.
SLAC Munition Store-Ship/Safety and Top Level Operational Requirements
SLAC is required:
- to bestow able in a magazine locker, compliant with national ordnance safety requirements, adjacent to the location of each Submerged Signal Ejector (SSE)
- to be in compliance with national ordnance safety requirements for all explosive components
- to be fully compatible with SSE discharge systems
- in conjunction with the SSE discharge system, to have a firing interval capability of NMT 60 seconds
- to be positively buoyant on discharge from the submarine, at the maximum operational depth and maximum speed of the submarine
- to be operational in sea states up to, and including, sea state 6 and be fitted with a stabilising system and an attitude sensor
- to be fitted with a suitable power source adequate for reliable system operation
- to be fitted with a sensor designed to trigger the fuze, which actuates discharge of the payload
- to carry a payload
- to be fitted with a scuttling device
Conclusion
The development of MSS requires a revision of the dependency placed on submarine stealth in considering the risk of detection by a hostile airborne ASW unit. As a consequence, it resurrects the issue of self defence against ASW aircraft.
The development of a SLAC capability uses a large proportion of engineering design techniques that have already been tested. It offers a relatively cheap option to force a change in airborne ASW tactics by offering a credible deterrence against airborne ASW tactics currently in place.
