Torpedo defence in the FFGs

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Sydney-based Thales Underwater Systems has developed the Sea Defender torpedo defence system for the Navy's FFGs and is now looking at enhancing it for larger RAN ships.
Surface combatants are vulnerable to torpedo attack. The problem is well known and, at least in terms of tactical doctrine, is one that demands considerable effort in operational research and analysis. Breaking down the problem into its constituent parts reveals its true nature, that is, detecting and classifying the emerging threat in sufficient time to undertake appropriate and effective tactical reactions.

Traditionally, the focus in torpedo defence systems (TDS) has been on reaction rather than detection simply because the means of achieving timely, accurate detection and classification have not been available to the ship's command. Consequently, the reactions have been complex and radical and success has been highly dependent upon instantaneous decision-making, stringent execution and a fair degree of good luck. Clearly, a less than satisfactory situation.

It is against this background that the RAN is acquiring the Thales Underwater Systems (TUS) Sea Defender TDS as a major component of the Underwater Warfare (UWW) self-defence system through its FFG Upgrade Project.

The best way to understand the Sea Defender system and its underpinning functionality it is to first encapsulate the torpedo defence problem.

Detection. The central issue in detection is the need to generate maximum warning time, to enable appropriate reactions in a timely and efficient manner. However, while detection at longer range translates into greater warning time it is also inextricably linked to the false alarm rate problem, which unfortunately also becomes less manageable with increased range. This is due to the fact that longer-range acoustical detections are inextricably linked to lower frequency, higher power signature components that show up in all sorts of sonar contacts other than torpedoes.

Classification. Accurate classification of a sonar contact is based on the collection of sufficient target acoustic data to support minimisation of the false alarm rate and to avoid recurrent reactions to non-torpedoes. Because of the difficulty in obtaining a library of the exact acoustic signatures of all possible threat torpedoes, the classification problem is best managed by avoiding attempts to classify one type of torpedo against another. This classification philosophy greatly reduces the need for acoustic intelligence data that may not be readily available or difficult to acquire.

Threat Analysis and Reaction. Threat analysis involves assessing the effectiveness of a planned reaction in the time available. The process informs tactical decision making which must be simplified in order for it to be robust. The reaction component involves the successful employment of the full range of torpedo countermeasures available, including tactical manoeuvres and both hard-kill and soft-kill decoys and countermeasures.

Sea Defender is a torpedo self defence system designed to detect and classify threat torpedoes and provide guidance for the execution of appropriate tactical reactions such as ship-manoeuvre and employment of acoustic decoys. The system is based on the principle of false alarm rate management to ensure that the Command Team reacts only to torpedo threats and not to the myriad of acoustic contacts which can easily be mistaken for torpedoes.

This is achieved though the use of an advanced and complex processing network designed to classify accurately torpedo and non-torpedo tracks. The sonar processing network receives, in turn, data from the ship's Spherion MFS hull mounted sonar (HMS) and the Albatross towed array (TA). The latter employs unique features such as automatic resolution of port/starboard bearing ambiguity. In turn, the system displays fused sonar information and track data on a sonar operator's console and then interacts with the ship's command system.

Sea Defender is managed from the sonar operator's console (SOC) installed in the FFG. In addition to providing the human-machine interface for the FFG's UWW capability, the SOC enables the operator to manage the sonar
Within the Sea Defender system, detection of threats is derived from sonar data from the HMS and the TA sensor sub-systems. The TA is dedicated to torpedo defence, but it does provide limited performance in more traditional anti-submarine warfare (ASW) sonar surveillance applications. The HMS array provides significant ASW functionality to the ship in addition to its torpedo defence role.

Having detected water-borne acoustic signals, the sonar processing sub-system then classifies them. In order to manage the false alarm rate, the system classifies received contacts as either "torpedo" or "non torpedo". The system does not distinguish between one type of torpedo and another. Torpedo contact data is then passed through a complex processing chain, which determines the correct reactions to be taken and countermeasures to be deployed. The reaction system is populated with RAN torpedo countermeasures (TCM) data determined by RAN operational and tactical doctrine, and tailored for the specific operational area and conditions.

TUS is now focussing on upgrading the Sea Defender system, particularly by extending the area of coverage provided by the sonar sensor. This can be achieved by introducing a new towed array sensor, one developed as the receiver array for the Royal Navy's Sonar 2087 program and now widely used in a umber of Low Frequency Active, Variable Depth Sonar (LFA VDS) programs around the world.

The new array uses the basic Albatross hydrophone technology but in a configuration that extends the acoustic coverage - making it suitable for deployment in larger, less manoeuvrable ships. The new variant of Sea Defender will be developed in time for it to be offered to the RAN for new ships such as the amphibious ships being acquired under JP 2048. In addition to enhanced acoustic capability, it has all the advantages of commonality with the existing Sea Defender and also with the multi-function integrated sonar suites being offered for future surface combatants such as the Air Warfare Destroyers being acquired under SEA 4000.

Looking ahead, the benefits of increasing detection range without compromising false alarm rate management will be useful in concepts like Task Force torpedo defence where the combined coverage generated by individual platforms can be integrated into a force ASW scenario.

In conclusion, the Sea Defender TDS provides the RAN with a high performance, highly reliable torpedo defence capability. This, combined with the ASW capability offered by the Spherion MFS hull mounted sonar not only provides the FFG with one of the most advanced and capable UWW self defence suites at sea today but it is also the foundation of the next generation of TDS for Task Forces and for individual, high value platforms.

Jim Manson is the Manager of Defence Strategy for Thales Underwater Systems. He served over 24 years as a specialist ASW officer in the RAN, serving aboard a variety of Australian and UK surface combatants. Jim also held several strategic-level appointments in ADHQ and the Department of Prime Minister and Cabinet.

By Jim Manson, Canberra
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