MCM forces aim for a clean sweep

Since its introduction in the 1970s, there has been considerable investment in minehunting as a complement to the traditional mechanical and influence sweeping methods of mine clearance. However, it remains a slow process, complicated by the introduction of low-signature mines which are difficult for minehunting sonars to detect. Once again the tide is moving in the direction of sweep systems.

Even so, for reasons of cost and longevity of mine stockpiles, it is years before any particular innovation has a global impact on the day-to-day business of clearing mines. Thus, the need for a full range of complementary mine countermeasures (MCM) is unchanged. However, in the near to medium term a greater proportion of MCM development budgets will be devoted to the introduction of new sweeps of one kind or another.

Another trend is the spread of computerised planning aids or mine tasking units for MCM. Compared to their counterparts on land, naval minefields tend not to be well defined. Consequently, naval operators place considerable emphasis on intelligence and threat assessments to deploy MCM vessels (MCMVs) in the maximum of safety. In addition to intelligence and accumulation of physical information of specific localities (through route surveys), clearance task planners have to take account of many other factors, including the type of ship intended to be used for clearing the route, the time available, and the acceptable remaining risk.

In wartime, other than in the most acute tactical situation, a 'passive' counter-measure such as re-routeing may well be the best expedient. However, in the immediate aftermath of war - or in a low-intensity conflict - authorities generally have no option but to get to grips with the mine. Arguably these two scenarios are the most challenging from the mine clearer's point of view: the mine threat is at its highest and in neither of them are governments likely to be disposed to concede that any level of risk to shipping is 'acceptable'. (Anything less than 100 per cent clearance requires merchant operators to continue paying punitive war rates for their insurance).

Threats and solutions

Around 70 per cent of the world's mine stockpile comprises tethered contact mines of a kind that has been in existence since the beginning of the century. The other 30 per cent comprise newer moored or ground mines that are remotely actuated by sensing the target's acoustic, magnetic, pressure, UEP (underwater electrical potential) or ELFE (extremely low-frequency electromagnetic) signatures, either singly or - more usually - in combination.

Countermeasures used to address these include conventional mechanical sweeps with explosive cutters for moored mines, and acoustic/magnetic signature generators for those tethered and ground mines that have influence fuzes.

The conventional approach has been for both mechanical and influence sweep categories to be towed behind a manned platform. No matter how stringent the signature management, this exposes both the platform and its crew to the risk of triggering a mine before the sweep does. So from the 1980s remotely controlled signature generators and towing platforms (such as the Chinese Type 312, German Troika, Danish Stanflex) that precede the parent vessel began to see service, and these are likely increasingly to be preferred.

Like the rest of the naval community, the mine warfare community is focussing its attention on operations in littoral rather than deep waters - including the sea/land interface or 'surf zone'. The only guarantee of crew safety is the use of remote countermeasures or stealthy platforms, particularly in those waters dominated by shore defences. However, the snag is that basic contact mines are not easily dealt with by the normal gamut of remotely operated vehicles (ROVs) or drones, as the latter are not powerful enough to sustain the drag loads of the requisite mechanical sweeps. In this case a safe, if expensive, option might be to convert the MCMV itself into an unmanned vehicle.

The advent of microprocessor-controlled 'intelligent' mines in the 1970s, which can distinguish decoys and be set to respond to the signatures of specific types of ship or submarine (or, theoretically, even of individual vessels) led in turn to a requirement for signature-emulation sweeps. Contemporary influence sweeps are therefore defined as functioning in either 'target-setting' or 'mine-setting' modes (or both). In the former case the sweep emulates the target (the protected vessel) and in the latter it emits 'brute force' signals within the response range of the mine sensor, to trigger or block it. Both sweep techniques are still needed, as it is unlikely those laying mines would find it effective to limit them to a very narrow signature combination. Also emulation sweeps give only a narrow cleared-path width, which is undesirable.

Due to their onboard sensor systems, warship signatures are usually well defined in terms of current magnetic or acoustic status, but those of merchant ships are not. Sweep suppliers therefore generally include portable signature measuring equipment as part of their systems packages, the data from these being used to program the remote signature emulator.

One difficulty in using ROVs in the target-setting as opposed to the mine-setting mode is that they are likely to be dimensionally much smaller than the ship being emulated. In order to generate gradients convincing to the modern mine sensor they must move only at very slow speeds (1-2kt), which in turn slows the clearance process. Provided the drone's towing capabilities are sufficient, these problems can be partly overcome by adding an offboard array to alter the linear aspect of the signature presented to the mine.

AMASS advances

ADILtd sells a complete minesweeping and support system under the designation AMASS. It comprises a mission planning support system, mechanical sweeps, a computerised minesweeping control system, a portable magnetic/acoustic range, and Dyad influence sweeps. The latter have been described as 'cheap and cheerful' devices, originally developed by the government Defence Science and Technology Organisation (DSTO) as a clip-on system for craft of opportunity, but they are nonetheless said to have been "surprising people with their success".

The Dyads incorporate shock-resistant permanent magnets, comprising mild-steel cylinders magnetised by two ferrite magnet discs, to produce a dipolar magnet with an extremely high magnetic moment independently of shipboard power supplies. Sweeps have been tested to a metric shock factor of 2.8, without damage to the sweep or the field strength of its signature.

The settings for the desired signature are determined by the laptop control system. Signature changes are effected manually, the factors being the moment, spacing, number and polarity of the Dyads. Two sizes of Dyad are employed, the 1.6 tonne Mini-Dyad being used to emulate warships and the 9.3 tonne Maxi-Dyad for large merchant ships. The smaller version is very portable, being suited to trucking or air transport.

Simple pipe noisemakers are attached to the Dyad array to provide an acoustic influence, but these have a limited operational life and an optimal tow speed of only 5-7kt. ADI has therefore teamed with Resonance Technology to develop the torpedo-like AAG (ADI Acoustic Generator) as a programmable replacement. Like the German GHA, it is water-turbine driven to render it independent of shipboard power supplies; the system design speed range is 6-12kt, though the turbine's power is sufficient for up to 15kt. AAG operational system trials are due to begin at the end of this year. When part of Dyad, the excitation frequency range is 10-250Hz, but investigations have shown that a larger AAG variant based on the same hydraulic vibrator design could be developed to function in the 1-30Hz range (output 180dB at 5Hz).

AMASS entered RAN service in 1992. The Dyad influence sweep has also become operational with the Royal Danish Navy, eight units having been bought for towing from drones controlled from an MCM-adapted variant of the Standard Flex 300. Contracts have also been placed by the navies of Indonesia, Japan and Thailand. The US Navy has acquired four Dyads for evaluation, and is understood to have successfully conducted trials against several different modern mines at speeds in excess of 20kt. The UK Royal Navy (RN) recently bought two Mini-Dyads as a calibration and training source for helicopter-borne MAD (magnetic anomaly detection) equipment.

Finnish industry is represented in the minesweeping field by Elesco and Finnyards. The former has developed the FIMS family of integrated minesweeping systems. The version fitted to the Pakistan Navy MCMV Muhafiz includes a three-electrode MRK-960 magnetic sweep, the MKR-400 pipe noisemaker, and a shipboard sweep control and positioning unit. Similar systems have been supplied for evaluation to the Royal Swedish Navy (RSwN) and the Finnish Navy.

Finnyards Electronics has developed its SONAC AMS acoustic minesweeper as a light and simple system suited to small-ship applications. It comprises a 210kg deck unit, plus a 460kg wet end towed by a buoyant cable and suspended beneath a 150kg buoy. The wet end has an electro-dynamic shaker to cover low frequencies, and several optional piezoceramic transducers to cover audible frequencies, full frequency range being 17Hz-25KHz. This arrangement is said to give better linear control of the lower-end frequencies than air-gun type systems, with a typical output of 190dB at 20Hz. Trials are continuing, a series production order being expected later this year from the Finnish Navy, which will integrate the SONAC AMS with an Elesco magnetic sweep.

Sterne for KMV

The Belgian Navy has adopted Thomson Marconi Sonar's private-venture Sterne combined influence minesweeping system for its KMV (Kustmijnveger) coastal minesweeper programme, after evaluating alternative systems including the British MMIMS. The first of four ships is expected to enter service in 1999. BAeSEMA will be supplying the associated range facility known as MERS (MCM Evaluation Range System), which utilises an adaptation of the body of the VEMS versatile exercise mine.

Sterne is a sophisticated system relying on target simulation (including UEP/ELFE as well as magnetic and acoustic signatures) for its effect. Its modular configuration allows the output to be tailored to include magnetic (AC or DC); magnetic (AC or DC) and acoustic; or magnetic and electric (AC or DC) plus acoustic signature emulations.

The baseline Sterne M incorporates a linear array with up to six towed magnetic bodies spaced 10-20m apart and powered from a shipboard source. The Sterne MA variant has a number of acoustic generators attached to some of the Sterne bodies, and these can in addition be equipped with electrodes to generate an AC/DC electric field component, the whole sweep being managed by a three-cabinet control system, including a dedicated computer which synthesises the required signature profile. For improved low-frequency response, the array can be supplemented with an air-gun or DCN AP5-type acoustic generator, the drag when the latter is incorporated not exceeding 11 tonnes at its 10kt design speed.

If manned, the towing platform for Sterne has to have minimal acoustic and magnetic signatures. However, Thomson also offers a remotely operated integrated system variant incorporating an unmanned tow platform based on a non-specialist vessel. The latter is fitted with degaussing coils to control its magnetic signature during transit and to simulate a specific section of the sweep magnetic signature during clearance operations, with a corresponding reduction in the number of towed bodies required.

Troika evolves

The operational use of drone sweeping systems was pioneered in Germany, the HFG-F1 Troika system having been introduced into German Navy service 14 years ago. Troika, supplied by Lurssen, is a combined influence system, up to three three drones being controlled by the platform simultaneously, over distances as great as 10nm. It is now being modernised under the umbrella MA2000 programme, which among other things will enable four drones to be controlled by a single operator.

The Royal Netherlands Navy (RNLN) is participating through its PAM programme. The Netherlands' TNO Physics and Electronics Laboratory is engaged in three principal activities connected with PAM. These include SWEEPOP, a study aimed at developing a measure of the effectiveness of Troika in both target-simulation and mine-setting modes. This four-part study is being carried out in co-operation with Norway (FFI) and SACLANTCEN. Phase 1 started in February last year and should conclude in February next year with a practical demonstration of simulation measurement and a calculation of mine-setting mode effectiveness. TNO is advising on training and education issues arising from PAM, and providing technical advice on development and acquisition of sweep-gear components.

Earlier, under its MARS project, TNO determined the specification of an onboard sensor-package for Troika, designed to detect mine explosions as part of the real-time evaluation of sweep effectiveness during operations. The laboratory also reviewed planning, evaluation and scheduling algorithms that could be used with Troika, and these planning and evaluation procedures are now being incorporated by Data Sciences in the design of the command and control system.

The German Navy is converting five of its Type 343 MCMVs to serve as Troika control ships, while the RNLN will be converting three Alkmaar-class minehunters for a similar purpose. The 90-tonne Troika drones have a maximum speed of 10kt, and incorporate an internal steel cylinder with magnetic coils fore and aft to generate a magnetic field, plus two medium-frequency acoustic signature generators. Improved target simulation techniques, including the use of the Norwegian-developed AGATE (air-gun and transducer equipment) acoustic sweep to give an enhanced low-frequency capability, are under consideration for the revised Troika.

The three-drone Troika system, also known as ROSS (Remotely Operated Solenoid Sweepdrone) is being offered as one of the fits for Abeking & Rasmussen's Modular Mine Warfare Platform (MMWP) concept. Alternatively, the MMWP can be fitted out with the German Navy's HFG-G1 towed system incorporating both solenoids and noise generators. A medium-frequency GBT-3 turbine-powered acoustic sweep can be towed in tandem behind the tubular solenoid, or a GBE electric acoustic sweep can be streamed separately, powered from an onboard generator via its towing cable. The complementary low-frequency system is the GHA programmable water-driven electrodynamic noise generator, manufac- tured by IBAK of Kiel.

The Norwegian Defence Research Establishment (NDRE) has been pursuing development of the AGATE acoustic sweep for use aboard the Royal Norwegian Navy's five Alta-class sidewall minesweeping craft. The industrial lead is GECO Defence, with CelsiusTech Electronics providing support on transducer production and the associated simulation programme. AGATE is controlled by a shipboard computer whose library contains a wide range of ship signatures. The outputs generated can include both noise and spectral lines, AGATE using GECO-produced air-guns for low-frequency emissions, Celsius-manufactured flex-tentional transducers made of a magnetostrictive material called Terfenol-D for medium frequencies, and piezoelectric transducers for high frequencies. For a given size, Terfenol-D transducers generate higher power levels than similarly sized piezoelectric units. The air-guns, which can have two different firing-chamber volumes, may be modulated to fire 1-6 shots per second. The complementary magnetic sweep is an electrode magnetic influence device known as EIMa, also developed by NDRE.

A wide range of sweeps is being offered by the state sales organisation Rosvoorouzhenie, which among others represents St Petersburg-based Gidropribor.

Those listed in an official catalogue include two forms of magnetic solenoid sweep, the SEMT-1 for shallow-water and harbour clearance, and the ST-2 for coastal operations. The former embraces two 18.2m-long magnets per set (up to four magnets can be linked together), weighing 12 tonnes each (minimum water depth 5m, towing speed 4-8kt) and the latter a single 29.1m-long magnet weighing 70 tonnes (minimum water depth 10m, towing speed 5-12kt). Both systems are powered and controlled from on board ship, the shipboard elements weighing 1 tonne.

Also available for coastal use is a magnetic loop sweep, the PEMT-4, which comprises a 424m three-core cable to which a 227.5m two-core feeder is connected. Maximum towing speed is 6kt, and effective sweep depth varies between 7m and 35m. The IU-2, described as a magnetic bottom mine detector-destructor , is a sophisticated system incorporating a detector towed 2-3m above the bottom. If its magnetic field is disturbed by a metallic mine casing, either a marker or a destructor is released from a container towed in tandem behind the detection element(s). Billed as 'in service with harbour minesweepers', the IU-2 has a destruction capability down to 60m, and a search zone width of 16m.

The listed mechanical sweep is the GKT-2, described as a 'deep' contact sweep (of unspecified depth-capability), in service with both 'coastal and sea minesweepers'. Weighing 1.95 tonnes, with a tow-speed up to 12kt, the GKT-2 can be used as a standalone device against moored mines, or in combination with a sound generator (in which case its cutters and cartridges are supplemented by corner reflectors) against acoustic mines.

At the 1994 Defence Services Asia exhibition, several other systems were publicised, including the GKT-3 deep contact sweep, a single-ship system incorporating integral sonar sensors to monitor the sweep position; the GKT-3MP team sweep with mechanical sensors for bottom-following; and three types of acoustic sweep (AT-2/3/6). The last two were both billed as wideband systems, the AT-3 appearing to have three transducers powered by a water turbine.

SAM developments

Sweden's main interest has historically been in 0-50m water depths and 'brown' waters, and is promoting its experience as germane to the conditions preoccupying major navies in littoral waters.

Programmes include further development of the SAM drone, which is being pursued unilaterally now that the US Navy has dropped out of the SAM 2 project. The latter was an air-cushion platform variant, on which Karlskronavarvet (part of Celsius) carried out a one-year study on behalf of the Swedish and US navies. The original focus of interest was on achieving a platform with improved shock resistance and a capability for high transit speeds, suiting it for use in archipelagos as a beach reconnaissance vehicle and as an emulator for the US Navy's LCAC air-cushion landing craft.

Celsius and FMV have been conducting their own studies on SAM enhancements. These include a stealth version with a pyramid-shaped superstructure and capable of working in minimum depths of 2.5-3m. The baseline SAM 1 catamaran drone (most recently sold to a Southeast Asian navy, believed to be Singapore) has a maximum speed of 8kt, and uses integrated coils for magnetic sweeping, and a towed noise-generator for acoustic sweeping. For current production Celsius has gone over to commercial components, with an integrated differential GPS (1m accuracy), and an automatic track-keeping facility, which is pre-programmable for autonomous operations, and can be supplemented by an automatic electronic chart table if needed.

All four of the new Styrso-class MCM vessels on order for the RSwN are expected to be in service by mid-1998. The third of class began trials in April. They have an Elesco open-loop magnetic sweep with Karlskronavarvet control electronics, and an AT205 acoustic sweep developed by sister-company Bofors SA Marine. (Acquisition of a new acoustic sweep is under consideration, one of the candidates being the Norwegian-designed AGATE.)

For conventional minesweeper applications, Bofors SA Marine is offering its own IMAIS integrated magnetic-acoustic influence sweep, in light, intermediate and heavy configurations. These weigh 3, 4 and 7 tonnes respectively, excluding generator sets. The magnetic sweep-element comprises three electrodes typically 15m long, 40-50mm in diameter, the foremost of which serves to shield the towing vessel from the field generated by the sweep.

Earlier systems of this kind were affected by seabed conditions and sea conductivity, but Bofors says that the advanced computer programs in IMAIS allow accurate predictions to be made of the sweep's magnetic output in different conditions. The magnetic performance of the electrodes is independent of salinity thanks to the type of current control technique employed and to optimisation of cable resistances. In its light configuration, IMAIS generates a magnetic flux density of 600+nT (1,200nT in its heavy version) at 30m depth over a 100m swept path. A 430kg AT205 acoustic transmitter, which incorporates an electrically driven hammer with an output of more than 185dB, is integrated in the magnetic sweep between the second and third electrodes.

Having deployed its first target-setting sweeps in anger 14 years ago, the RN now seems to be taking a back seat when it comes to current use of emulation systems. In 1982 the Admiralty Underwater Weapons Establishment (AUWE) within 10 days designed, developed and delivered both magnetic and acoustic sweeps. These were parachuted to the fleet in the South Atlantic by C-130 Hercules, and used to simulate the signature of logistics vessels (LSLs) making troop resupply runs along Teal Inlet in the Falklands Islands. The Magnetic Assault Sweep, which was towed in front of the LSL by a heavily padded (manned) LCVP (landing craft vehicle personnel), consisted of a bar magnet in a wooden box supported by air bags. The associated noise sweep was based on Kango Type 637 and 2500 commercial hydrosounders incorporated in cotton waste bins. (In the event no mines were found, as it transpired the mine-supply vessel had been sunk by naval gunfire before any could be deployed.)

The AUWE then developed its Variable Moment Magnet (VMM), which had a no-field state for air transportation. VMM was marketed by Vosper in the late 1980s as part of its Sea Serpent sweep, and later by the AUWE successor (Defence Research Agency [DRA]) and Marconi as part of the Modular Multi-Influence Mine Sweep (MMIMS). Neither was bought by the RN.

The RN is, however, able to derive a partial emulation capability from its in-service Combined Influence Sweep (CIS). Developed by BAeSEMA, the CIS combines the Towed Acoustic Generator from the MSSA1 minesweep system with the company's MS Mk14 magnetic sweep. A microprocessor-based control system synchronises the outputs of each to enable particular mine types or categories to be countered. This is achieved primarily in mine-setting mode, though the CIS also has some of the functionality required in target-setting mode. BAeSEMA, which also supplies mechanical sweeps for single-ship or team applications, is expected to add to its MCM range shortly.

Next year the RN plans to initiate a feasibility study into a new remote influence minesweep (RIMS) with which to update its 'Hunt' class MCMVs. It is understood that Germany's Troika is representative of the baseline performance level which the RN wishes to improve on. Among the additional capabilities required is control of up to four drones. It is likely that the study will note proposals put forward under the NATO NIMS and US ALISS programmes, the UK having lately been allowed to take part in the US DoD's Joint Countermine Advanced Concepts Technology Demonstration (ACTD). The RIMS staff requirement should be defined by 2000, enabling production systems to start delivery in 2005.

It is accepted that the system might not be fully capable of all the emulation modes by then, and would therefore need to be adaptable to technology improvements. It is suggested that from 2014 the 'Hunts' would be replaced by a 'new MCM capability', one that could embrace underwater vehicles and remote sweep systems, possibly including a further updated RIMS.

NIMS under study

Last year the US Navy awarded a US$2 million contract to EDO Defense and Space Division, well known for its work on the lightweight Mk105 airborne sweep, to lead a multinational study team working on the Next-generation Influence Minesweeping System (NIMS). The feasibility study, which is being carried out under the auspices of NATO's Programme Group (PG) 22, also involves Canada, France, Germany and Norway, and is expected to focus on remote-controlled systems.

As part of the Joint Countermine ACTD, the US Navy is exploiting novel technologies to develop a towed sweep called ALISS (Advanced Lightweight Influence Sweep System). This is to be demonstrated in fiscal year 1998, installed aboard a remote-control 17m planing- hull drone, but adaptations of the technology could also be applied to MH-53 helicopters, mine-clearing versions of LCAC (Multimission Craft, Air-Cushion, or MCAC), or Avenger and Osprey class MCM vessels. The acoustic subsystem has been contracted to Hughes (formerly Alliant Techsystems), working with Physics International (now part of Primex) and the US Naval Surface Warfare Center (NSWC), Dahlgren Division.

ALISS's NSWC-designed programmable spark-gap transducer arrays generate the desired acoustic tones, these being driven by Physics International pulse-power systems. Alliant is responsible for producing the transducer pod, the deployment system and system integration. The associated magnetic signature generator is being developed by the NSWC, Carderock Division, based on a General Atomics magnet module, which incorporates a conductively cooled closed-cycle solenoid magnet wound with a niobium-titanium superconductor. The test article will have a 5X10E6 amp-metre square dipole moment.

Recent reports indicate that even more radical thoughts are being nurtured with regard to the use of acoustics for MCM. In this instance high-power sound bursts would be used to disable or physically break up mines laid in shallow water (10m or less). The Defense Advanced Projects Agency (DARPA) has announced its intention to negotiate with Advanced Power Technologies as a possible supplier of a chemically generated acoustic source, which could provide the basis of a mine neutralisation system mounted in an unmanned vehicle. The DARPA statement indicates the platform would generate pressure pulses of 1,000psi per millisecond, yielding a peak pressure of 2,000psi against a 3m-diameter target at 20m.