Offboard sonar charts a fresh course for ASW

The Anti-Submarine Warfare sonar system of the next century could be a network of expendable arrays that use wide band communications to alert weapon platforms and direct them to a target, as David Foxwelland Richard Scott explain.

The post-war history of the development of ASW sonar has been a story of the development of active, and then of passive sonar. In the last decade, as submarines have become quieter and ambient noise levels higher as the focus of interest in ASW has moved inshore, so the effectiveness of passive towed array sonar has diminished, and attention has focused once again on the use of active sonar.

This has taken the form of the development of a new generation of active low frequency towed sonar systems, examples of which are now under development in Europe and in the US. Already however, the concept of operations originally envisioned for this new generation of ship-borne low frequency sonar has been superseded, and on both sides of the Atlantic, research and development (R&D) into the ASW sonar system of the next century is focusing on quite a different approach.

In a 'network centric' ASW sonar system, transmitters and receivers aboard individual surface ships and submarines will be but a part of a much wider network of sonar dispersed across a wide geographical area, expanding the 'ASW battlespace' beyond the range of conventional ASW sonar systems. The next-generation ASW sonar could thus be described not so much as an ASW sonar system, but a 'system of sonar', spread across an entire task force.

Under this concept of operations, a group of widely distributed offboard sensors would act as a single sonar system which was organic to a group of ships, rather than an individual platform. In such a distributed or 'multi-platform' sonar, detection would be automatic, beam-forming would be adaptive, the sonar would 'probe' the environment with adaptive signals, and the sonar system would 'map' the ocean automatically. Such a system would also adapt by selecting its own operating envelope, without recourse to an operator.

Achieving this goal will require the development of advanced 'in sensor' processing; a new generation of underwater communications; and above-water, high data rate communications. It will also rely heavily on the development of affordable arrays - probably fibre optic - and on advanced broadband processing techniques.

Remote sensors: going offboard

Typifying this renewed interest in off-board sonar is the Advanced Deployable System (ADS), and a number of recent Broad Agency Announcements (BAAs) from the Office of Naval Research (ONR) in the US seeking proposals for applied research in a range of autonomous sensors. ADS is a transportable acoustic surveillance system designed to be deployed prior to the arrival of a battlegroup in-theatre' (see JNI June 1997, p21). The Mission Needs Statement for ADS, which could be deployed from a variety of platforms, dates from 1992. Currently, Low Rate Initial Production of ADS is scheduled for early in the next century.

As Nancy Harned, team leader for the ONR's Undersea Surveillance Signal Processing Program, explains, the BAAs are a response to the growing requirement in the US Navy for low cost fixed or drifting sensors. Capable of operating simultaneously with minimal operator intervention, autonomous arrays such as these would extend the ASW battlespace significantly compared to that obtainable with ship- or submarine-mounted sensors. The benefits they would provide would thus be twofold: making up for any shortage in the number of ASW platforms available; and spreading sensors across a wider area, making the most of the useful signals available in shallow water.

Capable of providing continuous surveillance over wide geographic areas, the sensors would communicate detection alerts to post-processing sites - or directly to ASW units - using acoustic communications, RF links, and satellite communications (SATCOMs).

Harned says that the ONR is seeking proposals for a range of off-board and deployable sensors, which would be capable of being deployed, monitored and commanded by a variety of platforms including aircraft, unmanned aerial vehicles (UAVs), surface ships, and submarines.

Major Challenges

Key topics of interest to be addressed in the proposals include both acoustic and non-acoustic sensing; active and passive 'in-sensor' signal processing; lightweight, low cost, low power expendable drifting and fixed sensors and arrays; and low cost sensor and array manufacturing techniques. Proposals for sensor deployment systems for use from UAVs, unmanned underwater vehicles (UUVs), surface ships and submarines are also being sought.

Also of interest to the ONR are proposals that address the other major technical challenges involved in developing offboard, autonomous sensors, such as remote acoustic and radio frequency communications for data and command relay with airborne platforms and undersea vehicles. Data fusion techniques and technology capable of disseminating sensor data and cueing other ASW assets, and supporting the re-detection and engagement of targets, is also required.

An Advanced Technology Demonstration at the ONR is being used to examine techniques for a sophisticated underwater communications capability between surface ships and submarines. Other applied research projects are exploring techniques that would enable autonomous arrays to communicate with one another. 'Pop-up' buoys could be used to transmit sensor data to post-processing centres, says Harned.

The sensor concepts that the ONR hopes could be supported by the successful development on in-sensor passive signal processing techniques include:

- an Autonomous Drifting Line Array (ADLA), comprised of a very low frequency array of 30-100 hydrophones forming a near linear horizontal aperture or a vertically tilted and ocean current curved, multi-dimensional aperture. This continuous or split-aperture array would be beamformed as a function of its changing geometry (which it would itself sense), and VLF broadband and narrow-band acoustic energy would be analysed to develop a 'detection message'. The ONR anticipates that detection/classification information would be relayed to a post-processing site - or directly to a tactical unit - using a cellular-like (1.2-0.6kbps) RF data link. The same two-way link would also support processor/controller remote mode selection, so that in-sensor processing could be modified to suit specific operational or environmental situations. High priority events - such as detection of a target - could also trigger the use of larger data packages and communication band-widths (64-1,024kbps), but at the expense of reduced battery lifetime;
- an Autonomous Moored Line Array designed for use in shallower waters (200-1,000m), in which the array is fixed to the seabed. The geometries of this type of array might include near-straight horizontal line arrays on the bottom; a 'V' array, comprised of two or more line arrays on the bottom with different deployment azimuths, using a common anchor; and a weakly buoyed slack line array with considerable vertical and horizontal aperture. The ONR says in-buoy processing for these notional arrays might include matched field processing for target depth determination, but would have to support low bandwidth contact data communications by way of combined acoustic and RF techniques;

- a Fixed Shallow Water Surveillance Barrier Line, with processing capable of automated detection, classification, and localisation, for use in very shallow water (50-300m). Arrays of sensors of this type would form 'tripwires' or barriers. The barriers would be spaced - typically in the order of hundreds of metres apart - according to prevailing ambient noise levels and signal fields, and the type of acoustic propagation encountered in the area. According to the ONR, barrier technology is being sought which would be amenable to deployment from surface, sub-surface, and airborne platforms, such as the ONR/Naval Air Warfare Centre X-Glider;
- field nodes - for use in shallow water (150m-500m) - would be deployed in distributed fields for area surveillance, and use acoustic communication networks and 'gateway' RF surface buoys. Each node might have three to five directional acoustic sensors moored vertically off the bottom and have a detection/classification footprint with a greater area than a barrier line. The nodes, which would have to be capable of automatic detection, classification and localisation, could also be configured with magnetic and electric field sensors. Bearing in mind the longer ranges and propagation complexities involved, the ONR says classification for such a field node could be a much more difficult issue.

To complement the passive detection capability, the same sensors would also be capable of in-sensor active acoustic signal processing, and thus allow them to be used in a bi-static or multi-static arrangement. Coherent acoustic sources that could be used with the ADLA could thus include the Low Frequency Active (LFA) Surveillance Towed Array Sensor System (SURTASS), Long Endurance Low Frequency Active Surveillance (LELFAS) system, an air-dropped, lightweight version of LELFAS, or an Air Deployed Low Frequency Projector.

The US Navy's SSQ-110A series sonobuoy, or a similar broadband VLF device could provide an incoherent or broadband 'echo ranging' capability. A typical coherent multi-static scenario could make use of several sources and as many as 50 receivers distributed over as wide an area as possible.

Shallow water environment

Using low frequency sources in shallow water is more challenging still, because all sorts of features - such as sub-bottom trenches, schools of fish, and deep draught surface ships - can produce false targets (see JNI June 1996, pp10-15). To overcome these problems, broad-based research in ocean acoustics is being used to obtain a better understanding of the effects of the ocean environment on underwater acoustics, and to assess and predict how the underwater environment impacts upon the performance of ASW sonar systems.

The 'ocean environment' includes three-dimensional, evolving features, such as the air-sea interface, sub-surface bubbles and plumes, so-called 'volume effect' (such as internal waves, fluctuating media, pollutants, fronts and eddies), the interface with the seabed, and ocean bottom and sub-bottom regions.

In the field of active sonar signal processing, much effort is being directed towards the development of more sophisticated signal processing techniques for target detection and classification in bottom-limited, shallow water environments. Research and development is focused on low frequency (below 1kHz) and mid-frequency (1-5kHz) signals, on classification of low-Doppler targets, and clutter suppression techniques based on waveform design or adaptive signal processing.

Low frequency sources

As Dr David Randalls, a technology manager in the Underwater Sensors & Oceanography division of the UK's Defence Evaluation & Research Agency (DERA) explains, several types of low frequency projectors are in development around the world which could form the basis of the next generation of low frequency sound source. The key, says Dr Randalls, is obtaining the required source level and bandwidth in a manageable volume. New transduction materials, such as magnetostrictive and electrostrictive materials, and transducers that make use of a range of rare earth metals could be suitable says Dr Randalls, but none is seen as perfect for job.

Nancy Harned at ONR agrees that magnetostrictive and electrostrictive materials offer promise, as do low frequency transmitters that make use of a combination of the two technologies.

In the UK, free flooded ring technology is being proposed for the UK Royal Navy's Sonar 2087, and another approach that has been addressed by ONR - the Barrel Stave Projector - is being studied in Canada as part of that country's Towed Integrated Active & Passive (TIAP) programme. Working with Sparton of Canada, the Defence Research Establishment Atlantic has been examining a number of low frequency projector arrays suitable for use in TIAP, and has elected to pursue development of a Horizontal Projector Array based on BSPs.

The LELFAS mentioned above is an active offboard source that is due to be demonstrated under the US Department of Defense's Fiscal Year 1999 Advanced Technology Demonstration programme. This low-cost, affordable, rapidly deployable, long-endurance, low frequency acoustic source is designed to provide undersea surveillance in littoral waters and in the open ocean, pending the arrival in-theatre of US Navy SURTASS LFA ships with their AN/UQQ-2 sonar. With dimensions around half that of a Mk48 heavyweight torpedo, LELFAS could be deployed from a range of platforms, including submarines, providing the same kind of capability that was sought in the 'Pilotfish' programme, an active source for use by a submarine in a bi-static system.

Getting more from broadband

Broadband signatures have been exploited for a number of years, but are growing in importance with the increased emphasis on detecting submarines in acoustically challenging shallow water conditions. As Nancy Harned explains, in the shallow water environment, acoustic propagation is highly variable and it is necessary to look at a much wider range of frequencies in order to successfully detect and classify a target. Much effort is therefore being directed towards the development of broadband sound sources and into broadband signal processing techniques.

Likewise, broadband sensors and processing are being developed to guide and control ASW torpedoes in shallow water and to overcome the effects of reverberation, single and multiple-boundary scattering, false targets, and false alarms caused by biological, geological, and boundary features. For this application, robust and highly capable combined broadband sensors and processing systems are required that are robust and highly capable in order to provide efficient target detection, a high level of resistance to countermeasures, and a high level of target resolution and classification.

According to the ONR, a spin-off of the development of broadband sensors and processing technology of this type, could be its use in the design of diagnostic tools to detect and classify features in the ambient environment, and new types of guidance and control systems for UUVs.

Torpedo defence

Defence against torpedoes has become a major pre-occupation for most navies and a number of national and international torpedo defence programmes are already under way, such as the US Navy Surface Ship Torpedo Defence (SSTD) programme, the multi-national NATO SSTD project, the Franco-Italian SLAT (Systeme de Lutte Anti-Torpille) programme, and the new UK National SSTD programme.

In the longer term, long range science and technology programmes are being used to provide the means to develop more effective defence for surface ship and submarines. Technologies of interest include those capable of performing detection, classification and localisation of incoming torpedoes, and detection and classification of targets with low signal-to-noise ratios. Technology is also being sought by the major navies that could defeat salvoes and air dropped torpedoes, as are technologies capable of providing a quick reaction capability against incoming - possibly very high speed - torpedoes at ranges of less than 2,000m of a capital asset.

Fibre optics

Work on an all-optical towed array, and on fibre optic flank array sonar has been under way on both sides of the Atlantic for many years, including a fibre-optic version of the US Navy's Wide Aperture Array (WAA), which is significantly lighter than the original version that uses conventional transducers. More recently, the Naval Research Laboratory (NRL) in the US has said it is interested in receiving proposals for the development of new types of sophisticated fibre optic technology. The technology includes analogue communication systems and optical microwave links, digital communications links, generic fibre optic sensors (both interferometric and intensity), and the signal processing of fibre optic signals.

The NRL is also seeking innovative concepts for component and sub-system development that can improve the performance of existing systems, or generate new applications of fibre optics. It says that major areas of interest are large bandwidth laser diodes (or external optical modulators); high speed detectors; local area networks; and acoustic, magnetic, chemical and strain sensors. Given that the fibre optic cables in question in some remote sensor systems may only be 0.2-0.05 inches in diameter, survivability of the array is seen as the greatest potential risk to their use. Consequently, R&D is under way to develop means of protecting them.

In the US, funding is being made available to develop and test the capability to protect fibre optic micro-cables by burying them beneath the seabed. Future undersea surveillance systems employing fibre optic micro-cables and acoustic arrays based on them could therefore be deployed from a towed deployment vehicle operating close to the seabed. Another advantage of burying acoustic sensors based on fibre optic micro-cable is that doing so will produce a straight, linear array which will enhance the ability of the system to beam form, and detect targets.

As DERA's Dr Randalls explains, fibre optics are receiving renewed attention because they could form the basis of a new generation of ultra-thin line towed array. Like the US, the UK has its own optical array programme. As targets become quieter, says Randalls, longer arrays with a larger aperture are required, but cannot easily be accommodated within a submarine, and the amount of room available to stow an array effectively limits its length. Early types of submarine towed arrays could be 'clipped on' to a submarine, but submarines operating close to the shore in shallow water will need to be able to reel in their arrays.

Aboard submarines and surface ships alike, there is also pressure to accommodate a new type of multi-dimensional or 'volumetric' arrays - multi-line arrays with more than one line of transducers. Although shorter than conventional arrays, volumetric arrays would also consume valuable space aboard a submarine unless produced using fibre optics, but are important because they are more effective than single line arrays in shallow water.

Volumetric arrays of this type have been under development for a number of years. A volumetric array with five lines of transducers - which was intended for use from US Navy submarines - completed development four to five years ago, and although it is not yet in service, it was said to provide a dramatic improvement in detection and classification capability in shallow water.

Another advantage of fibre optic technology is that it is well proven, and mass production to meet commercial requirements in the telecommunications industry could make the next generation of fibre optic sonar comparatively inexpensive to produce. Currently, says Nancy Harned, conventional towed arrays are, effectively, hand-built, making them too expensive to consider using in an expendable array.

Adapting to the environment

The US Navy has already embarked on the early stages of a research and development programme, the aim of which is to develop the capability to provide the next generation of ASW sonar with the ability to adapt to their environment. Similar R&D is included in the DERA science programme in the UK.

According to Nancy Harned at ONR, the requirement for environmentally adaptive sonar reflects the fact that there are few historical databases of the shallow water environment, and the fact that the shallow water environment changes so quickly in time and space. In developing sonar that can sense and adapt to their environment it is hoped that sonar performance and effectiveness will be enhanced, and that operator workload will reduce significantly.

Existing sonar have become complex and difficult to use with hundreds of modes and waveforms for the operator to select from, although typically, operators might only use a handful of them. In deep water, sonar operators are used to using convergence zones to detect submarines and provide an 'ASW screen', but in shallow water there is no convergence zone, conditions change rapidly, and detection ranges are shorter.

In the US Navy, it is acknowledged that the effects of shallow water will also limit the effectiveness of the next-generation sonar, although the Lightweight Broadband Variable Depth Sonar (LBVDS) - which is being developed for the DD-21 land attack destroyer - will have some environmentally adaptive features. The LBVDS is due to be tested aboard a surface combatant in 2002, and in the meantime, existing sonar are expected to be upgraded with enhanced signal processing and more computing power to give them an environmentally adaptive capability.

For ships such as the DD-21, the ASW problem is compounded by the requirement that surface combatants conduct many other operations - such as theatre ballistic missile defence - that do not provide them with the option of using speed or manoeuvrability to avoid detection by a submarine.

In these circumstances, short detection ranges can leave a surface combatant vulnerable to attack. Given that in the shallow water environment the sonar operator cannot rely on the effectiveness of existing sonar, environmentally adaptive sonar are required that will 'sense' conditions and automatically adjust without operator intervention. Such a 'smart sonar' would build up a model of its local environ-ment through repeated observations and use this model to optimize signal selection.

National programmes

Many other countries are also developing low frequency active sonar. British Aerospace is continuing to market the Active Towed Array Sonar (ATAS) which was previously jointly marketed with Thomson Sintra - now part of Thomson Marconi Sonar Limited (TMSL) - and sold to the navies of Taiwan (the V(2) version with a Lamproie towed array), and Pakistan (the basic V(1) variant). The passive-only V(0) variant and the top-of-the-range V(3) system (with facilities for simultaneous active and passive operation) are also available.

Weighing 8 tonnes overall, ATAS V(1) comprises a low frequency (3kHz) trans-mitter based on 'flextensional' transducer technology and an in-line receive array. The latter features a cardioid processing technique to resolve left/right-bearing ambiguity. A range of product enhance-ments has been incorporated in the improved ATAS II which is now being marketed. These centre on the incorp-oration of a new processing architecture and a flat-panel operator console, using the latest commercial off-the-shelf processing technology.

STN Atlas Elektronik has also developed a compact, lightweight Active Towed Array Sonar, which operates at around 2kHz, TMSL has its Combined Active/Passive Low Frequency Sonar 20, while L3 Communications has its Low Frequency Active Towed Sonar. France and Germany have agreed to collaborate on the develop-ment of their next generation sonar. The Royal Australian Navy also has a requirement for low frequency active towed sonar, trials of which are due to start in two years time. The Royal Netherlands Navy, although not yet committed to an acquisition programme, has conducted extensive trials with its experimental LFA system.