Developments in Sonar Arrays.

Although the anti-submarine warfare "community' hankers after non-acoustic methods of detecting submarines, for the foreseeable future acoustics will continue to be the main path to success. Within that broad spectrum, however, there are many variations and special applications.

Many navies still use medium-frequency (MF) active sonars, but all can act in a passive mode, if needed. Major ASW operators have shifted to low-frequency (LF) passive systems to exploit the huge distances travelled by sound, as against the high power-output needed to extend the range of active systems. There is also considerable difference between surface ship and submarine sonars (although they share a common technological base) because of the way in which they are used.

As always, there is no true dominant sonar solution. The revolution in long-range passive detection triggered off a response from submarine and surface ship designers, leading to reduction of noise by a significant factor. Now the major ASW operators are looking at the need for active pinging to locate targets making little or no noise. Towed arrays have proved very valuable in blue water ASW, but the unpredicted shift of emphasis from mid-ocean to littoral warfare has resulted in a major weakness in shallow-water ASW, an environment which can be guaranteed to be hostile to the hunter.

Surface Ship Systems

The US Navy's standard integrated sonar and fire control is SQQ-89. Conceived under the ASW Combat System Integration (ASW-CSI) project, it was intended to handle large numbers of passive contacts provided by the SQS-53B/C bow sonar, the SQR-19 towed array and the SQQ-28 processor in the SH-60 Seahawk helicopter. As it developed, the fire control function became fully integrated, but the main advantage over older systems is the way in which sensor outputs can be displayed side-by-side. As many as nine sonar specialists can work together on solving the ASW problem. In the latest version, SQQ-89(V)4 onwards, an acoustic video processor (AVP) has been included into the lower drawer of the UYQ-21 control console. The major improvement is the transition to open commercial off-the-shelf (cots) processing, designated (V)X. SQQ89(V)10 is the (V)6 redesigned for the Arleigh Burke (DDG-51) Flight IIA Aegis destroyers, without the SQR-19 but with additional processors.

Until the DD-21 technologies mature (see below), an interim solution is the Multi-function Towed Array (MFTA), which should be available as a retrofit to the Arleigh Burke class by 2005. It will be incorporated into the SQQ-89(V)15 version, allowing a wide range of frequencies to be covered (HF, MF and LF). It will use a dual-density towing cable and telemetry enhancements towed from the existing SQR-19 winch. A prototype Lightweight Broad-band Variable-depth Sonar (LBVDS) for the DD-21 will undergo sea trials this year and next year, using the SQQ-89(V) 15A towed array as its receiver.

The British equivalent of SQQ-89 is the 2050 sonar system in the Duke class (Type 23) frigates, which is integrated with the DNA command system, SSCS Mk 7, the 2031Z towed array and the 2069 dipping sonar in the Lynx, Sea King or Merlin helicopter. Its successor is the 2087 suite, which has suffered delays in development and will not be fitted initially to the New Type 45 destroyers, according to a recent official announcement. Along the way it has absorbed the projected 2057 towed array and the 2080 hull-mounted system. Two of the consortia were invited in June 1999 to re-submit proposals against an amended technical specification placing less emphasis on passive detection. The projected in-service date has accordingly slipped from mid-2003 to late 2005, with Fleet Weapon Acceptance planned for May 2007. In support of its bid, Babcock Defence Systems is emphasising its skill in ship-sonar integration, whereas Thomson Marconi Sonar (TMS) is relying on its broad experience in Low Frequency Active Sonar (Lfas). This includes the very large Slasm system developed for the French Navy's big destroyers Tourville and de Grasse. The Slasm combines an active LF variable-depth sonar (VDS) with a Very Low Frequency (VLF) towed receiving array and an MF/LF hull sonar.

Lfas systems operate below the frequency at which submarines' anechoic tiling is effective. But they do have limitations, notably an excessive false alarm rate from reverberation. Environmental groups are increasingly vocal in claiming that whales and dolphins suffer from Lfas transmissions. Trials of the US Navy's Surveillance Towed Array Sonar System (Surtass) Lfas system off Hawaii in 1998 attracted protests, and similar trials in the Nato ASW research vessel Alliance were claimed to be linked to the beaching of a dozen whales in Greece.

The US Navy's projected Zumwalt (DJD-21) class of land-attack destroyers promises to take the concept of an integrated ASW suite further than ever. A risk-reduction and technology demonstration initiative known as Integrated Undersea Warfare for the 21st Century (IUSW-21) has been started by the Program Executive Office for Undersea Warfare, the PEO (USW), and the PEO DD-21. Together with the Naval Undersea Warfare Center, the two PEOs are working to eliminate technological risk and envelope advanced technologies (frequently incompatible objectives), before transferring the project to the competing Blue and Gold industry teams to develop.

Last year Raytheon won an $ 8.9 million contract to develop and test an IUSW-21 advanced development model. Before selecting the winning DD-21 industrial team, the IUSW-21 programme will continue to develop a notional design, from which the two DD-21 teams can choose selected components. The three major aspects of IUSW-21 include:

* the multi-function hull array, providing mine-avoidance and shallow-water ASW capability, using advanced transduction technology

* an integrated stern, providing stealthy handling of a broad-band variable-depth sonar, an advanced bi-static towed array, and towed torpedo countermeasures

* advanced processing and displays, supporting initiatives for automation of sensors and reduced manning.

To achieve these broad aims, IUSW-21 investments are examining critical technologies and techniques in the following areas:

* improved shallow-water performance, using a lightweight variable-depth transmitter and broad-band signal processing

* reduced manning by developing broad-band signal processing, automated detection, classification and localisation of contacts, data-fusion and environmental adaptation

* the provision for in-stride mine-detection and -avoidance by using onboard sensors

* minimising the impact on the preliminary DD-21 hull-design by studies focused on variable-depth sonar, multiple towed bodies and conformal arrays.

Another big technical challenge is to engineer undersea warfare functions into the DD-21 total-ship system architecture, recognising the fact that low manning and life-cycle cost objectives cannot be achieved unless the undersea warfare system is fully integrated into the overall combat management system of the ship. Integration in that sense includes operators and maintainers, as well as hardware and software.

Even the design of bow sonar domes is complex. The first of four domes was delivered by W & J Tod to Empresa Nacional Bazan to house the Raytheon DEll60LF sonar (a commercial variant of SQS-56) in the new Aegis frigate Alvaro de Bazan, currently fitting out at Ferrol. Each dome assembly weighs 10.9 tonnes, is 11.2 x 4.7 x 2.3 metres and built in two sections. The first, containing the sonar array, includes support plinths to allow the dome, transducer and underwater telephones to be installed as a unit. It incorporates transverse acoustic baffles and acoustic treatment of the ship's sole plate, to reduced external radiated noise and internal reflection. The dome is also filled with water from a header tank. The after section of the dome is filled with glass-reinforced plastic syntactic foam to reduce its weight, but carefully configured to withstand underwater shock.

Although many crackpot schemes have been marketed to allow small combatants such as fast attack craft to hunt submarines, there are many valid lightweight systems on the market, notably the Thales (ex-Thomson-CSF) Active Towed Array System (Atas) and STN Atlas' Actas active array. The fact that they are not widely sold reflects the dwindling interest in ASW in smaller navies, who find the game too expensive for them.

Minehunting Systems

Minehunting sonars differ from ASW sonars in having high definition at the cost of range, and most systems rely on flat-plate transducers, but there are signs of a change.

After four years of British Ministry of Defence delays the decision on the upgrade of the Royal Navy's eight remaining Hunt class mine countermeasures vessels (MCMVs) is about to be made. The so-called Hunt Mid Life Upgrade had previously been postponed for lack of funding, but now a contract in the region of 50 to 60 million [pounds sterling] is expected.

The competitors for prime contractorship are Thomson Marconi Systems and Lockheed Martin's Naval Electronics & Surveillance Systems' Syracuse division. Lockheed Martin is offering a radical solution, a broad-band circular array with 360 [degrees] scan, using electronically-steered beams. The main problem encountered by the Hunts is their difficulty in holding a steady line while keeping a mine-like object in the narrow sonar beam. This is inevitable with conventional planar-array sonars, but the high freeboard of the Hunts also creates windage, and in such circumstances the MCMV can all too easily drift over another (undetected) mine. By adopting 360 [degrees] electronic scanning the risk to life is reduced, and the minehunter's command and control system shows a much more complete picture of the underwater environment. The value of broad-band technology has been known for some time, but now cots processors can provide the power needed to make the concept work.

The Lockheed Martin array will fit into the trunk currently occupied by the Type 193M sonar, reducing the amount of structural work needed, and ship-fitting would be done by Vosper Thornycroft, the original designer and builder of all but two of the class.

If the Lockheed Martin (LM) offer is accepted the company will also offer a variant of the system as a cost-effective upgrade for the Sandown class minehunters, which are equipped with an early version of Nautis and the TMS 2093 variable-depth sonar, so upgrading would be cheaper. Both competitors have good track records in underwater detection, but LM has the added benefit of having developed and produced a wide range of advanced mine countermeasures for the US Navy.

The aim of modern mine countermeasures is to reduce risk to life and limb by putting the sensors into the minefield and using the minehunter as a control vessel. This has not been feasible before, but the technology has now advanced to the point where two-way data-links make such systems feasible. TMS and STN Atlas have both developed remotely operated vehicles capable of deploying a high-frequency sonar.

The cost of electronics has come down so sharply in recent years that expendable mine-destructors are now available. A good example of the genre is the German Navy's Seefuchs, which has been adapted by STN Atlas and Lockheed Martin as the Seafox element of its Airborne Mine Naturalisation System (AMNS). It has its own acquisition, homing and classification sonar system, capable of horizontal mechanical scanning, with resolution down to 0.9 degrees.

Submarine Sonars

The US Navy funded the BQG-5 Wide Aperture Array (WAA) for all successors to the Los Angeles class attack submarines. Using three large conformal arrays on either flank, WAA can operate at lower frequencies and also provide very accurate bearing and target-motion analysis (TMA) data, while avoiding the need for the submarine to change direction. It is integrated with the BSY-2 spherical bow sonar and the TB-23 and TB-16D towed arrays. The new Virginia (SSN-774) class will have the BQQ-10 bow sonar and WAA.

The UK Royal Navy's SSNs do not use the big spherical bow arrays seen in their US Navy counterparts, as their designers prefer conformal arrays or circular chin sonars. The 2076 suite is intended to provide a new integrated suite for not only the upgraded Swift-sure and Trafalgar classes, but also to be the baseline of the new Astute class SSNs. It integrates the bow, flank, towed, obstacle-avoidance and intercept arrays, but has, however, encountered developmental problems, and is currently running a bit late. The large flank arrays, analogous to the WAA in US Navy submarines, has just been installed in HMS Torbay during her latest refit. but other elements have not yet been tested at sea.

The first use of submarine towed arrays was in the early 1980s. when a variety of systems were retrofitted to existing SSNs. SSBNs and SSKs. They were normally carried in a trough on the casing and only streamed when clear of land: recovery required the assistance of an auxiliary vessel. This produced a relatively inflexible mode of deployment, countered by keeping arrays relatively short and thereby reducing their effectiveness. The current generation are. however, reelable, taking advantage of increasingly large hulls. Some navies, notably the Royal Australian Navy, use towed arrays for surveillance rather than ASW, exploiting their phenomenal detection-ranges.

In the decade since the end of the Cold War much more information has come to light about Russian undersea warfare. Much of the excitement over Lfas has been generated by intelligence source rumours of extremely silent Project 877 and Project 636 Kilo type diesel-electric boats. Now, however, customers such as the Indian Navy have denied claims that the Kilo is a `hole in the sea', and say that it is very noisy. The improved Project 636 boats are said to be quieter than the 877 series.

Conclusions

The main problem of undersea warfare is physics, the problems of propagating sound in a hostile and unpredictable environment. As we have seen, promising technologies are all too easily out-flanked, leaving the business in a constant state of flux. What can be changed is the integration and presentation of data, which can boil down to simple display technology to improve the operator's performance. Signal processing is the other area in which great strides are being made, for the difficulty lies in extracting the specific submarine-noise from the background clutter of marine life, sea-noise, self-noise etc.

In short

* "The sonar operator's task is still extremely difficult, consisting of long hours scanning the ocean"

* "The human brain is very efficient, but vulnerable to data overload"

* "Automating as many of the repetitive tasks as possible is the solution to reliving the operator's overload."