The AWD sonar - fixed, or variable depth?

...Or both? As the RAN ponders its AWD equipment options, two sonar specialists describe an emerging Anti-Submarine warfare sonar capability.
Low Frequency Active Variable Depth Sonar (LFA VDS) has become the "sonar system of choice" for navies operating modern surface combatants with an Anti-Submarine Warfare (ASW) capability requirement. With longer-range detections, increased tactical flexibility and the ability to contribute meaningful capability into a broad Force ASW context, the LFA VDS is a far more potent system than the traditional, stand-alone ASW Hull Mounted Sonar (HMS).

However, the HMS is still relevant to the ship's overall ASW effectiveness, and despite ongoing debate, the complementary capability it provides in conjunction with LFA VDS is worthwhile. This article looks at the enhanced capability potential offered by combining the LFA VDS and HMS in a sonar suite concept.

In order to demonstrate the benefits of networking the Variable Depth Sonar (VDS) and HMS this article examines three challenging oceanographic situations not uncommon in Australia's strategic ASW environment. In each case the Sound Velocity Profile (SVP), which drives overall sonar coverage and is derived from the existing environmental conditions, is described in the diagrams that represent sonar system performance. The diagrams demonstrate how the acoustic energy is propagated in the prevailing conditions. In addition to examining the impact of the environmental conditions on the performance of the sonar system, this article also explores the tactical implications. Each situation reveals a powerful argument for complementing the VDS, as the primary sensor, with a HMS capability. Moreover, each one reveals that the HMS by itself is probably insufficient to counter the ASW threat.

The aim of ASW is to deny the enemy the effective use of his submarines. This does not mean that each and every enemy submarine has to be destroyed, although that is not a bad outcome! Although concepts such as deterrence and denial are much more difficult to quantify than detection they are, nevertheless, operationally effective.

All too often sonar systems are acquired on the basis of obsessive focus on maximising detection ranges. This is a gross over-simplification of the art of ASW. There are several other key determinants, arguably more important than "detection at the longest range", which need to be considered. These include:

* Denying submarines freedom of evasion within the water column

* Deterrence of the submarine threat by application of tactical pressure

* Contribution to long range, persistent ASW surveillance

* Cueing of ASW aircraft

* Distinction between hostile targets and NONSUB contacts (classification and detection).

Although it may be very difficult to accurately quantify and specify the above factors, this does not mean that they should not be considered when selecting appropriate systems.

Scenario 1 - Littoral Water: In this scenario the ship is operating in shallow water, near-isothermal conditions that creates a surface acoustic sound channel. Figure 1 shows that under these conditions, a surface combatant fitted with only a HMS cannot detect a submarine approaching it below the layer until it is within the tactical firing range at which it would engage the ship with its torpedoes.

As shown in Figure 2, the introduction of a VDS denies the submarine the freedom of manoeuvre below the layer.

Figure 2 highlights the fact that using both the VDS and the HMS to generate long-range ensonification of the entire water column out to the ranges at which a submarine is refining its attack solution and manoeuvering to maximise its chances of successfully engaging its chosen target. Therefore, the combination of HMS and VDS limits the tactical freedom to the attacking submarine and effectively prevents the submarine from closing to within its weapon release range.

Scenario 2 - Bottom Bounce Propagation: In this scenario the ship is operating in intermediate depth water with a shallow mixed surface layer, and with sea bed conditions that support long-range bottom-bounce propagation. Once again, as shown in fig. 3, the HMS provides good coverage of the surface duct but is unable to prevent an attacking submarine from closing to within weapon release range with relative impunity.

As shown in Figure 4, the introduction of a VDS allows the ship to exploit the bottom-bounce propagation path and create a barrier, an ensonified zone around the ship, through which an attacking submarine must penetrate.

This scenario shows that, again, both the VDS and HMS have different propagation path, each of which has tactical advantages. The HMS provides coverage of the surface channel to well beyond weapon release range but does not prevent the submarine from evading detection by manoeuvring beneath the layer. The VDS, on the other hand does provide long range coverage below the layer in a "barrier" created via bottom-bounce propagation at the range where the submarine is finalising his attack solution. At this range counter detection is critical.

Scenario 3 - Convergence Zone Propagation: In this scenario the ship is operating in deep water where there is a strong permanent thermocline which supports convergence zone propagation.

The HMS by itself provides very limited coverage of the water column out to the ranges at which the submarine will engage the ship with torpedoes, enabling virtually no self-defence let alone force protection. Therefore, by itself, the HMS is virtually useless except as a torpedo-defence sensor.

The poor short-to-medium range acoustic propagation conditions in this scenario highlight the benefit of the VDS. While the conditions also limit the VDS coverage, the CZ capability it provides can be exploited through creation of a long-range barrier which extends throughout the water column at extended ranges. Moreover, the inherent capability of the VDS to be deployed at the most advantageous depth allows the ship to maximise the width of the CZ annulus, making the management of maintaining contact in the annulus easier.

In these conditions the VDS is the only organic sensor that enables the surface combatant to detect an attacking submarine beyond its weapon release range. However, there is little that can be done to detect submarines in the "non-ensonified zone" between the CZ and the task group. If it suspected that a submarine may penetrate the CZ barrier, the VDS must be employed as a cueing sensor to detect and classify submarines in the CZ and cue ASW aircraft to hold down and prosecute the submarine to prevent it from threatening the force.

The scenarios above intuitively show the benefits of a sonar suite consisting of VDS and HMS. In conditions where an attacking submarine can "evade or approach below the layer" the VDS provides the surface combatant with the wherewithal to deny that course of action. Not only does the VDS increase the vulnerability of the submarine to detection, it also places the attacking submarine under greater tactical pressure. In conditions where the HMS is adversely affected by poor propagation conditions near the surface the VDS can still exploit any long-range propagation paths such as bottom-bounce and convergence zones. Additionally, although some HMS systems may exploit these conditions the VDS invariably has the advantage over these systems by being able to be deployed at the most effective depth.

The VDS is not a new system by any means, but it has made a comeback in recent times. This is because the very large, cumbersome towed acoustic transmitter bodies in older systems have been replaced by new more compact bodies with smaller transmitter arrays. The newer bodies are not only more acoustically efficient they are also more stable when being towed and, therefore, the VDS now has much less impact on towing ship manoeuvrability than was the case in the past. Several key navies seeking to greatly enhance the ASW capability of their surface combatants are introducing these new VDS systems into service.

Jim Manson is a former RAN ASW officer and Defence Adviser to the Department of the Prime Minister and Cabinet. He is now an ASW business development specialist with Thales Underwater Systems Pty Ltd.

Jeremy Ranicar retired from the DSTO as a Principal Research Scientist in 1995. Since then he has worked in industry as a business development manager and defence consultant.

By Jim Manson and Jeremy Ranicar, Sydney and Canberra