Minesweeping + Mine Hunting = Success

By Commodore Hector J. Donohue, Royal Australian Navy (Retired)

A large range of mines suitable for different environments and scenarios, both offensive and defensive, is available today. Developments in our knowledge of the underwater environment, in computer processing and sonar, and in advanced autonomous torpedoes are having profound effects on the design and effectiveness of ground and independent mines. Modern mine sensors are integrated systems comprised of basic sensor elements and the associated signal processing function. Solid-state circuitry and microminiaturization have enabled complex processing, and the low power requirement has resulted in a major increase in the mine's active life. Modern technology also allows for easy mass production of mine sensors.

These developments increase the mine's range and depth of operation, as well as its target selectivity and discrimination, which results in greater resistance to traditional minesweeping techniques. Sensor developments, for example, enable the mine to detect minehunting operations and to target the mine disposal vehicle. Given the low cost of a mine relative to the cost of a mine disposal vehicle, the exchange rate is in the miner's favor. An attrition rate of even one vehicle per five mines could not be sustained for long by a mine countermeasures force.

Advances in munitions technology involving insensitive explosives have made mines less susceptible to countermining. Other developments aim to prevent detection by minehunting sensors. These include self-burying, low target strength, and unusually shaped mines that quickly become encrusted with camouflaging marine growth. Cheap, concrete-filled dummy mines or decoy mines laid in large numbers, interspersed with a few genuine weapons, increase the mine clearance task significantly. Some mines are designed with a degree of self-mobility. The Swedish Rockan mine, for example, travels horizontally as it sinks through the water. This means that such mines do not end up in a neat "mine line" that could be identified easily by mine hunters.

In mine warfare, the minefield is the weapon, not the individual mine. In fact, minefields often contain a range of mines, to complicate the countermeasures effort. All these factors make mine clearance operations even more protracted and difficult, and they emphasize the need for a balanced and versatile mine countermeasures force structure.

A New Minesweep

The most effective mine countermeasures (MCM) force structures include both mine hunting and minesweeping, as these two techniques complement each other. The effectiveness of the mine hunter is influenced directly by its sonar capabilities, its ability to use historical seabed survey data, and the skills of its personnel. Adverse environmental conditions also can degrade the effectiveness of minehunting techniques. In such conditions, minesweeping is an essential complement and may be the preferred technique. Even in the most favorable conditions, the mine hunter can only be expected to achieve a clearance of some 90%.

An analysis done by the Australian Defence Science and Technology Organization (DSTO) has summarized the effectiveness of combined mine hunting and minesweeping undertaken by the Royal Australian Navy (RAN), using the RAN Inshore Mine Hunter and the Dyad Influence Sweeps. (See Figure 1.) This combination results in an overall probability of clearance, in all minehunting conditions, approaching 100%.

Further analysis of the use of Dyad Influence Sweeps in a short-term MCM operation is shown in Figure 2. As can be seen, the intelligent, modern mines set to a low ship count were all swept to around 99%, while the older technology mines with a higher ship count were swept to around 80%. The lower clearance for the older mines reflects the relatively high ship count and the limited time available to sweep. With more time, clearance could have been on the order of 99%.

New Sweeping Technology

Up to the 1980s, influence mine design was constrained by sensor limitations and comparatively rudimentary logic, with actuation taking place when the magnetic and acoustic signatures stimulation reached a certain magnitude or rate of change. Traditional sweeps such as the closed loop and electrode were developed to defeat this rather simple mine logic. These sweeps provided a strong magnetic signature and were effective because there was no requirement for the influence characteristics to have any resemblance to those of the target. Similarly, acoustic sweeps were designed to generate an acoustic output over the frequency band where the mine being swept had its maximum sensitivity.

Advances in sensor capability and the introduction of microprocessor technology have increased the flexibility of modern mine logic, which can permit analysis of magnetic and acoustic signatures. Modern mine logic can assess the magnetic anomaly in three axes, to ensure that it meets the spatial and temporal characteristics of the target. When this is combined, for example, with the simultaneous detection of underwater sounds that are frequency analyzed for structure, traditional sweeping techniques tend to be ineffective.

Today's mine threat will include such "smart" mines as well as older types. Consequently, a sweep will have to work effectively against both threats. The solution is the emulation sweep.

The Dyad technology, developed in Australia by DSTO, has improved the effectiveness of influence minesweeping dramatically. DSTO developed a mathematical model of steel-ferrite magnets, in which the steel in three tubes acts as a magnetic amplifier for two ferrite magnet discs. The model provided the theoretical maximum value of magnet efficiency (magnetic moment per unit mass of magnet) that could be used as a goal when designing practical magnets. The model also provided optimum design-parameters values, including tube wall thickness, separation distance of the ferrite discs, and the mass of ferrite in each disc.

By the early 1980s, DSTO was confident that the technology had the potential to be developed into an operational minesweep. Following extensive developmental trials, the system was accepted into RAN service in 1992. A model developed by the Australian company ADI can predict the magnetic field resulting from any array of Dyads. Ranging of Dyad Influence Sweeps by DSTO has confirmed the accuracy of this model, which means that they can be configured easily to produce an emulation sweep.

Target Emulation Mode Sweeps

An emulation sweep is one designed to produce magnetic and acoustic signatures that closely resemble those of a particular class of ship, and so enable the sweep to be accepted as a valid target by modern mine logic set to fine ship-catching tolerances. Using a sweep that emulates a ship signature is known as sweeping in target emulation mode (TEM).

The prerequisite for designing an emulation sweep is knowing the magnetic signature of the vessel or vessels to be emulated. These can be obtained through magnetic ranging and can be presented as magnetic models of the vessel or in the form of a profile plot at specific depths and offsets. Magnetic profile plot data also can be obtained through the use of exercise mines. The most accurate results are achieved when the signatures of vessels are obtained within the area of operations, immediately prior to the start of minesweeping operations.

Based on the nature of ship influence signatures, it is possible to derive some simple guidelines for the design of emulation sweeps:

  • The length and speed of the sweep must be similar to those of the emulated vessel class, to satisfy magnetic gradient and temporal logic during the encounter.
  • Acoustic sources must be distributed along the sweep, to provide temporal fusion of magnetic and acoustic influences. Signatures must be stable and not modulated; for example, magnetic sweep must not be pulsed.
  • To provide structural emulation, the sweep magnetic signature should have a tri-axial vector component structure and the vector components should exhibit the field ratios, gradients, and polarity changes appropriate to the emulated ship class. For example, the ratios between the vertical and horizontal field components for a particular sensor orientation should reflect the fluctuating ratios expected from a ship.
  • To emulate the intensity, the strength of the sweep magnetic field should be of similar magnitude to that of a ship. To achieve the widest possible swept width, the intensity of the field should exceed that of the target ship class—but not by too large a margin, or emulation will be compromised.
  • The sweep must be maneuverable and able to withstand high levels of shock without disruption to operations. This is important because modern mine logic is designed to detonate within the damage radius of a target, and the sweep will be subjected to severe shock loadings.

Emulation sweeps, by definition, are deterministic—that is, the actuation probability during a sweep/mine encounter approaches 100%. If the sweep signature falls within the targeting criteria of the mine, then the mine must actuate. If the sweep signature is outside the mine's targeting criteria, the mine will not actuate, but that means that the signature of the vessel being emulated also will be outside the mine logic, and that vessel will be able to transit the danger area safely.

Emulation sweeps designs are effective against all mine types, but the ability to manipulate an emulation sweep signature also provides the capability to configure a sweep for optimal performance against a particular mine sensitivity and logic setting—to operate in a mine setting mode (MSM). Modern microprocessor-controlled mines probably will apply a number of logics and sensitivity settings simultaneously to the various magnetic-field vector components; therefore, MSM would be used only when the mine settings are known, for example, against a protective minefield laid by own forces. MSM is a valid tactic for older mine types if the mine logic is available.

The essential difference between the two sweep design modes is that TEM is designed to clear those mines that would be a threat to the emulated vessel class, regardless of mine sensitivity, settings, or logic. MSM, on the other hand, is designed to have improved performance against mines for which logic and sensitivities are known, and the actuation range for the sweep is greater. Because TEM is effective against all mine types, in the absence of intelligence, it is the preferred technique.

Modern Mine Constraints

The design of influence mines always has included a compromise between ship catching capability and sweep-rejection capability. If a mine's tolerance for the magnetic and acoustic signatures required for actuation is too narrow, targets will be missed. If a tolerance is too wide, then the mine can be swept too easily. With the introduction of microprocessors into mine mechanisms, the miner has been able to reduce this problem.

During an encounter, the mine assesses the influence signatures from the seabed as the target passes. The sensor will "see" a profile of the magnetic field at a particular depth and unknown (to the mine) offset distance, and must assess the signature changes as a function of time. The orientation of the mine sensors relative to the target is unknown to the mine, and temporal considerations such as gradient and signature duration must be addressed without accurate knowledge of the geometry of the encounter or target course and speed.

Microprocessor technology gives the miner the hardware needed for sophisticated analysis and targeting logic. Although, theoretically, the tolerances could be set to catch one particular ship, the miner is subject to some real-world limitations in applying mine logic. In addition, a miner is unlikely to have precise data on target signatures. Development of the software algorithms for mine logic is constrained by the following factors:

  • The requirement exists to threaten a range of possible targets.
  • The induced magnetic signature of a ship varies with heading and geographic position, while the permanent magnetic signature will change over time. There are variations in signatures of vessels of the same class.
  • The magnetic signature of a degaussed warship will vary with depth and offset distance.
  • A warship's magnetic signature will depend on the status of its degaussing system and magnetic treatment history.
  • Magnetic and acoustic signatures can be varied deliberately.
  • The magnetic signature of a merchant vessel will vary with the type of cargo. Mechanical defects, machinery operating status, and transit speed affect a ship's acoustic signature.
  • The acoustic influence is subject to variable propagation loss, ambient noise, multipath transmission, reflections, temperature effects, and possible interference from other noise sources in the vicinity.

In programming magnetic and acoustic logic the miner is constrained by the wide variation in valid signatures as well as by the fact that the geometry of a target/mine encounter normally will be unknown. As a result, the mine logic must include tolerances for the acceptance of magnetic and acoustic signatures required for actuation, and these tolerances must be wide enough to ensure an acceptable ship-catching capability.

These tolerances provide the key to the practical use of TEM sweeps, because the sweep does not have to simulate a particular ship signature in minute detail. Rather, it must provide a valid ship signature representing a particular class of ship.

Magnetic and Acoustic Emulation

One method of modeling a ship signature from magnetic ranging data is to see the ship as consisting of a large number of dipolar magnets, each of which makes a contribution to the resultant magnetic field. The Dyad Influence Sweep uses this modeling technique to emulate a ship signature using dipolar magnets.

A Dyad acts as a large dipolar magnet with a magnetic monopole near each end. The magnetic field for any array of Dyads can be accurately predicted using ADI software. When a number of dipolar magnets are placed in a linear array, they produce a quite complex combined field, the structure of which will depend on the magnetic moment of the Dyads, the number used, their polarity (the direction of Dyad north and south poles in the array), and the distance between them. This ability to modify the magnetic signature of a number of magnets in an array is the key to the Dyad Influence Sweep design.

Dyad Influence Sweeps are self-contained, clip-on arrays, and the Dyads themselves are high-strength permanent magnets that have the magnetic moment fixed on manufacture. There are two types of Dyads: Mini Dyads, which have a magnetic moment tailored to emulation of degaussed warships; and Maxi Dyads, which have a much greater magnetic moment, to provide the field intensity needed to emulate large merchant vessels. Variation of the physical configuration of an array provides the required flexibility in signature manipulation, negating any need for variable magnetic moments.

For emulation purposes, the acoustic output of the sweep should be broadband, covering the infrasonic, audio, and ultrasonic bands with ship-like spectral levels, and it should include multiple, constant line structure. The Dyad Influence Sweep can be used with a range of acoustic noise makers, but ADI—in collaboration with Resonance Technology and with the assistance of the RAN and DSTO—is developing a new generator, the Australian Acoustic Generator (AAG). Currently at the prototype stage, the AAG will complement the Dyads in producing a realistic emulation capability. It is a hydraulically driven, programmable acoustic module, powered by a water-driven turbine. Independent of external sources for power and control, the AAG will be easily deployable from vessels of all types, including remote control drones.

The module has completed both acoustic tank testing and deep-water acoustic trials successfully. The goal of the project was to optimize the performance of the acoustic module, mate the acoustic module to the water-turbine power source, and shock-harden the design. This was completed in mid1997, and the AAG was trialed successfully by the RAN and DSTO in October 1997.

The sensor and integrated logic capabilities of the modern mine have reduced the effectiveness of traditional minesweeping systems. Today, a system must be able to provide a ship-like signature—to operate in target emulation mode. The Dyad Influence Sweep is, to date, the only operational emulation sweep available. In service in the Royal Australian Navy for some five years, it is extremely effective against a wide range of influence mines. The U.S. Navy and Royal Danish Navy both trialed the sweep successfully in 1996, using remote-controlled drone boats. The system also is entering service in some Asia-Pacific navies, with interest being shown by a number of other navies worldwide.

Minesweeping is a necessary complement to mine hunting. The significant advances in mine technology have required a corresponding development in minesweeping technology. The ADI Dyad Influence Sweep combined with the AAG is a demonstrated solution in this important area of warfare.

Commodore Donohue is general manager for mine countermeasures with ADI Limited. Prior to his retirement from the Royal Australian Navy, he held a range of sea appointments, including command of the destroyer Yarra and the guided-missile frigate Darwin . He was both a mine warfare and clearing, diving, and torpedo antisubmarine specialist.

 

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