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More than thirty countries produce mines, and twenty countries export them. Iran has reportedly laid several thousand naval mines, North Korea’s 50,000, China 100,000 or so, and Russia estimated quarter-million. Since World War II, sea mines have damaged or sunk four times more U.S. Navy ships than all other means of attack combined, according to a Navy report on mine warfare.
Sea mines range from cheap, simple explosive devices which many fear may fall in the hands of terrorists, to sophisticated computerized systems equipped with sensors and designed to wait hidden on the sea bed for years until the right target presents itself. They are fitted with acoustic, magnetic, seismic, and pressure sensors, which can pinpoint the size and shape of a ship moving in water and detect ship’s approach. They have become stealthier by minimizing their sonar profiles, smarter in distinguishing targets from decoys and evolved into lethal systems that can fire torpedoes. There are also rumors of nuclear armed mines in the inventories of China and North Korea. They have potential to become surprise weapon in any future war. Indeed, sea mines are key to regional navies’ anti-access/area-denial (A2/AD) and sea-control strategies and operations.
The capability to detect, locate, classify and neutralise these weapons remains a key requirement for navies around the world.
Mine Countermeasure Challenges
The U.S. Navy uses five objectives in MCM efforts (JP3-15 2011): Exploratory – determine if mines are in a given area, Reconnaissance – assess mine threat in an area. Breakthrough – open a channel or lane in or out of a port, and Clearing – remove all mines from a given area.
The broad range of mine technology presents challenges unique to mine warfare. Mine warfare must be able to counter technology that is a century old, while still preparing for developing technologies.
The water depth and bottom conditions are important when considering the types of mines used, as well as the countermeasures to be used against them. The water depth, therefore, limits the types of mines that are expected to be encountered, whether bottom, moored, or floating mines. Of those, sensors would be expected in bottom and moored mines, as the floating or moored contact mines do not utilize influence sensors.
Major limiting factors in MCM, aside from cycle time, are the need to replenish power through return to a host platform, the distance a platform can travel from a host platform (if it is controlled via umbilical), or the need (in either case) for the platform to return to the host platform for PMA.
For MCM missions, data transfer from any surface craft back to the host ship is critical. Barring the ability to shut down the enemy’s jammers, new methods for data transfer will need to be developed to ensure mission success.
As machine learning progresses, it will become harder to “fool” and influence mine into detonating.
Challenges for Lockheed Martin Remote Multi-Mission Vehicle (RMMVs)
Remote Multi-Mission Vehicle also called a “semi-submersible” swims along with its upper surface just above the water. Being mostly submerged helps stabilize the small craft in choppy seas, enabling its sonar a much better picture. Being partly exposed to air allows it to burn diesel fuel, which gives it much longer endurance than batteries.
The Lockheed Martin-built system has three key problems to overcome:
Unreliable. The RMS was supposed to run for 75 hours between operational mission failures, but scored a dismal 19 hours in tests this summer. The Remote Multi-Mission Vehicle, the system’s centerpiece, fared slightly better with 25 hours between operational failures.
Losing contact. The RMMV is an unmanned, autonomous, semi-submersible system. It was to be a key component of the littoral combat ship’s mine countermeasures mission package, but has been unable to interface with LCS systems, according to a 2014 Pentagon report. The RMMV is billed as capable of line-of-sight and over-the-horizon operations, but control from the ship has not proven reliable, and communication is typically lost when the vehicle is out of sight
Missing mines: The self-propelled RMMV tows the AN/AQS-20A minehunting sonar that is supposed to detect and classify mine-like contacts, but a 2013 Government Accountability Office report noted that the towed sonar failed to detect certain mines, was slow in identifying others, and falsely identified some objects as mines
MCM Sensors
Currently, mine location and identification are typically accomplished using sonar. However, the medium in which
sonar must work is less than ideal. Underwater sound propagation is rarely a straight line because of non-uniformities in sea water as well as variations in temperature and salinity.
Reflections from the surface and bottom are common, which creates additional problems for sensors. Acoustic noise is ever present, from waves, ships, and undersea life and the “absorption of sound in water is a strong, increasing function of the acoustic frequency” (National Research Council 2000, 377). Long range requires low frequencies (≤ 3 kHz), but high angular resolution cannot be achieved due to the size of the aperture that is required and, although high angular resolution is possible at high frequencies (35 to 350 kHz), it is only achievable at short ranges of several hundred meters or less .
The sonars commonly in use today are side scan and sector scan sonars. During side scan sonar operation, sound energy is transmitted into the water column in the shape of a fan directly under and alongside the tow fish and the
“echo” (return energy) that bounces back is evaluated. The side scan sonar system records the strength of the echo and creates a “picture” of the sea floor and/or objects in the water column.
Conventional side scan sonar has limited and range-dependent resolution Additionally, the resolution depends on frequency. Higher frequencies provide a shorter wavelength. Shorter wavelengths directly improve angular resolution. This, however, limits the range. A larger aperture size improves resolution but it is not always practical to have a large aperture array.
Sector scan sonar is dropped into position at a fixed location and gathers data in a 360º environment . Aside from the disadvantage of requiring a fixed location, it has the same fundamental limitations that side scan sonar has: resolution degrades with range, resolution depends on receiver (aperture) size, and resolution depends on frequency.
Synthetic aperture sonar (SAS)
SAS uses the sonar platform’s movement to synthesize a long receiver, or aperture, by combining data from multiple “pings.” Image resolution is increased significantly compared to side scan sonar because SAS resolution is independent of range.
SAS is a form of side scanning sonar, which sends pings to the port and starboard sides of the AUV and records the echoes. Synthetic aperture sonar (SAS) uses advanced computing and signal processing to create fine-resolution images of the sea floor based on reflected sound waves. The principle is to combine successive pings coherently along a known track in order to increase the azimuth (along-track) resolution. The commonly accepted measure for fine resolution is a 1-inch by 1-inch pixel size, which can be achieved by SAS.
Whereas RAS sensors emit relatively high acoustic frequencies that are quickly absorbed by the seawater. SAS uses lower-frequency acoustics that can travel farther underwater, thus increasing the range at which fine-resolution pictures can be produced. SAS can provide same resolution at 300 m what RAS produces at 30 to 50m. SAS in AUVs have produced fine-resolution images of sunken ships, aircraft, and pipelines.
Raytheon and US Navy partner to improve AN / AQS-20A mine hunting sonar
Raytheon and the US Naval Undersea Warfare Center (NUWC) – Division Newport have successfully upgraded the US Navy’s AN/AQS-20A mine-hunting sonar to better identify and classify mines. The team evaluated the equipment’s synthetic aperture sonar to optimise its ability to capture high and low-resolution images of potential mine threats undersea. During testing, the equipment demonstrated better performance and reliability, and generated better quality imaging of objects undersea, thereby improving the system’s mine detection capability.
The AN/AQS-20A system is towed undersea to simultaneously scan the water column for anti-shipping mines forward of, to the sides, and beneath the vehicle. Sophisticated sonar, electro-optical sensors, and high-precision location information are used to provide high-resolution images of mines and mine-like objects. It offers higher-quality imaging of objects found deep undersea to aid in the identification and classification of mines. The system supports the US Navy’s critical minehunting missions that make sure safe access and passage for military and civilian vessels on the world’s oceans and waterways.It can be deployed for mine-clearing missions in both deep-ocean and littoral waters, where the AN/AQS-20A identifies bottom, close-tethered and volume mines.
Optical Sensing
To monitor for submarines that may be used to lay mines, sensors that use lasers or lights from light emitting diodes, carefully tuned to the frequencies that carry well underwater, could be used instead of standard sonar . The U.S. Navy’s ALMDS (Airborne Laser Mine Detection System), uses Light Detection and Ranging (LIDAR) technologies to detect, classify and localize naval mines near-surface moored sea mines. US Navy is seeking novel and innovative techniques to exploit the laser signal for fusion with image processing techniques to: better detect, recognize, and identify mine-like targets; reduce the false alarm rate; and to quantify results
How far one can see in the water optically is determined by a variety of factors, including attenuation length. The attenuation length is based on the quality of the water, how much particulate is present, as well as other factors.
The key to achieving greater distance will be to use lasers with the shortest possible pulse; these narrow pulses have a greater probability of passing through the particulates. Therefore, the smaller the pulse, the less effect particulates have on the light reaching the target (backscatter).
Large lasers, like those used for ALMDS, are based on diode-pumped solid-state technology and are already being replaced by smaller compact lasers (i.e., fiber lasers and single emitter laser diodes), which emit light at exactly the wavelength needed (in the blue-green spectrum for underwater imaging applications).
Another possibility is development of lasers with multiple colors that auto-select based on the type of water; for
example, use blue for deep ocean and green for littorals . Additionally, there is a system under development that is a
three-dimensional (3D) imaging sensor using a 532-nm laser (Imaki et al. 2017). The sensor consists of a dome lens with coaxial optics to achieve a wide-scanning angle of 120 degrees (horizontal) by 30 degrees (vertical) while being compact in size (25-cm diameter and 60-cm length) (Imaki et al. 2017).
C4I
Innovations in the C4I field could drastically affect MIW. One of the biggest issues with MCM is response time. The advancements in the realm of C4I will improve that greatly, creating both an advantage to the Navy but also a risk as adversaries’ C4I capabilities may improve as well. Some of the araes that may have the most impact on MCM
include cloud, edge, or fog computing; MCM with integrated C4I; and the common operational picture (COP).
Shallow Water Communications for Mine Warfare
US Navy has issues SBIR for development of an innovative secure communication capability for Navy mine warfare systems to enable two-way remote command and control of a minefield deployed across the hostile littoral environments. Reduction of power consumption of at least 15% and/or increased range of at least 25%, when compared to existing commercial-off-the-shelf (COTS) acoustic modems is desirable. Effective command and control of mine warfare systems requires two-way communication with reliable data transmission and reception over a range of 1000+ meters.
The objective of this topic is to develop an innovative, secure wireless communications technology for use on mine warfare systems to enable command and control in the hostile littoral environments. This technology will enable warfighters too remotely “turn on,” “turn off,” and/or reprogram the targeting logic of the minefield to respond to mission needs. Additionally, this could be used to remotely terminate a minefield after hostilities have ceased, saving significant cost and labor typically required to clear minefields.
Automatic Target Recognition
The current state of the art in mine detection has people scanning through large sonar images looking for the mines. However, when looking at an image of the sea floor from above, operators sometimes have difficulty discerning the identity of simple objects. On the surface the mines are easily confused with floating garbage in the sea. The bottom mines. Many objects look similar to mines, e.g., ripples in the sand. And it is even harder in some ways since the images are blurry and there is noise.
Automatic Target Recognition (ATR) software identifies potential threats directly from the data gathered by the AUV’s sonar, enabling decisions in-missions in real time. The ATR software allows the AUV to autonomously inspect possible mine targets, allowing both for real-time mine detections or simplified and consistent post-processing. Various ATR algorithms are now readily available to the MCM community. Many solutions have been proposed in technical literature.
A common solution is for ATR software to be tuned to recognize specific mine-like shapes. These contacts are then flagged and presented to the operator for review. Other ATR solutions model the characteristics of a mine and look for matches in the data.
ATR Challenges
Sonar performance is dependent on underwater currents; differences in pressure, temperature, and salt content (salinity); even how sound waves reflect off the bottom — which can change on hourly basis.
Regardless of the choice of algorithm, a common problem when using ATR software is the level of false-positive alerts. ATR in challenging in cluttered environments or in different seabeds from those used to tune the algorithms is likely to increase the level of false alarms. Future algorithms should understand the environment, then predict the expected performance of the ATR in different scenarios to provide information on the ‘huntability’ of the different regions within the survey area. This provides context to the detections, allowing the operator to make more informed decisions.
Shadows cast by mines are often easy to distinguish from ones cast by clutter objects, such as tires. New shadow contrast prediction technology can lead to improved imagery, power conservation, and better performance for automatic target recognition software.
Raytheon and US Navy partner to improve AN / AQS-20A mine hunting sonar
Raytheon and the US Naval Undersea Warfare Center (NUWC) – Division Newport have successfully upgraded the US Navy’s AN/AQS-20A mine-hunting sonar to better identify and classify mines. The team evaluated the equipment’s synthetic aperture sonar to optimise its ability to capture high and low-resolution images of potential mine threats undersea. During testing, the equipment demonstrated better performance and reliability, and generated better quality imaging of objects undersea, thereby improving the system’s mine detection capability.
The AN/AQS-20A system is towed undersea to simultaneously scan the water column for anti-shipping mines forward of, to the sides, and beneath the vehicle. Sophisticated sonar, electro-optical sensors, and high-precision location information are used to provide high-resolution images of mines and mine-like objects. It offers higher-quality imaging of objects found deep undersea to aid in the identification and classification of mines. The system supports the US Navy’s critical minehunting missions that make sure safe access and passage for military and civilian vessels on the world’s oceans and waterways.It can be deployed for mine-clearing missions in both deep-ocean and littoral waters, where the AN/AQS-20A identifies bottom, close-tethered and volume mines.
US and Germany partner to develop advance underwater mine hunting technologies
US Naval Surface Warfare Center Panama City Division (NSWC PCD) has partnered with Bundeswehr Technical Center for Ships and Naval Weapons, Maritime Technology and Research in Northern Germany on the Allied Munitions Detection Underwater (ALMOND) project.
The collaboration will aim to advance technologies and techniques for the detection, classification, and mapping of bottom and buried munitions. It will also benefit the warfighters by providing them with an increased ability to detect submerged and silent sea mines in complex environments.
NSWC PCD physicist Dr Jesse Angle said: “There is a significant worldwide capability shortfall in reliable techniques for mapping underwater munitions for unexploded ordnance remediation and hunting buried and stealthy sea mines in complex environments.
“We are seeking to bring together the best of our unexploded ordnance (UXO) detection capabilities and merge them with those of the Germans, so that both countries can learn and benefit from developments ongoing in the other country.”
References and Resources also include:
http://gtri.gatech.edu/casestudy/synthetic-aperture-sonar-help-navy-hunt-sea-mines
http://www.nap.edu/read/9773/chapter/4#13
