Autonomous systems and new types of sonar seek to detect, neutralize ship killers before they strike.
The U.S. Navy is automating the mine clearance mission with new technologies such as the battlespace preparation autonomous underwater vehicle (BPAUV). The BPAUV is designed to remotely locate and map enemy minefields with high-definition sonar.
The U.S. Navy is working hard to keep humans out of minefields. The service is developing a host of autonomous and air-deployed capabilities to detect and neutralize mines at sea and in littoral zones. These systems, which are now entering service, will reduce and ultimately eliminate the need for divers to disarm and destroy mines in person.
The new technologies are part of an ongoing realignment of the Navy’s minehunting capabilities, says Thomas Swean, a team leader with the Office of Naval Research’s (ONR’s) mine countermeasures program in
Traditionally, Navy minesweepers were assigned to Navy task forces on an as-needed basis, but the service is shifting its requirements and installing systems directly on warships such as destroyers and frigates, Swean explains. Navy acquisitions programs currently underway include an initial group of organic MCM capabilities now beginning to enter service, such as helicopter and unmanned robotic vehicle-based capabilities.
“Mine hunting is a very dangerous job,” says Swean. He notes that mine disposal traditionally involved Navy divers manually disarming or attaching demolition charges to mines. The thrust of the Navy’s research and development effort is to remove humans from this hazardous environment. Swean states that another goal is to accelerate the process of detecting, locating and neutralizing mines.
Unmanned systems are already beginning to assist with accelerated mine detection and clearance. The ONR oversees the Navy’s autonomous underwater vehicle (AUV) research and development programs, several of which are almost ready for deployment, Swean says.
The battlespace preparation AUV (BPAUV), a 21-inch diameter AUV currently in the acquisition phase, is designed for launch from a 21-inch torpedo tube. This AUV is equipped with a fleet-class, high-definition sonar and has a 12- to 13-hour operational endurance, making it capable of surveying large areas of ocean.
Another capability entering service is the autonomous unmanned surface vehicle sweep. This robot boat is equipped with a magnetic and acoustic detection/sweeping system. Swean adds that the automated sweep will be installed on littoral combat ships beginning this November.
The Navy also is actively studying new classes of sonar to detect mines. “It’s the best sensor you can put in the water,” he says of sonar. The ONR has developed a synthetic aperture sonar that is being installed on a new class of 12-inch-diameter AUVs. Swean maintains that synthetic aperture sonar is groundbreaking because of its increased resolution and range, both of which are critical features for detecting mines and other undersea weapons.
According to Swean, the greatest sonar development is low frequency broadband sonar. This sensor system transmits and receives sound signals at very low bandwidths. It can provide warfighters with precise estimates of a target ship’s size and shape, greatly enhancing detection and classification. This technology is what he refers to as a “game changer.” For mine detection, the sonar makes it possible for Navy units to acquire, detect and classify mines in a single pass. The new sonar is scheduled to be installed on an AUV platform and ready for Navy acquisition by 2013.
The Navy’s AUV technology was developed to search, classify and neutralize mines. The software and sonar systems necessary for reacquisition are now mature enough to begin entering service. Swean explains that traditionally, a search will detect an object on the first sweep and then classify it on subsequent sweeps. The ONR is now focusing on developing an autonomous capability that will return to an identified mine and destroy it. This autonomous detection and neutralization program is in its early stages. It is scheduled to move to an acquisition phase by fiscal year 2011.
ONR scientists also are developing techniques that will permit different autonomous systems such as AUVs to operate together, rather than operating sequentially. Swean shares that autonomous systems will be able to cue each other and function in unison, greatly accelerating minefield clearance.
Successful mine detection depends on the environment, water clarity and bottom clutter. Mines tethered on or near the bottom are difficult to detect in areas with coral reefs, rocks or other mine-sized debris on the bottom. Swean notes that detecting mines under such conditions can be time consuming. Although the specific durations are classified, he shares that clearance time is tied to bottom conditions, the size of the minefield and available resources. The Navy has several technologies in its research and development pipeline designed to decrease detection and clearance times by an order of magnitude, he says.
Using trained dolphins is the only reliable method currently capable of detecting mines buried under sediment or sand in shallow water. Their natural sonar is very sensitive for detecting metallic objects under a variety of conditions. But dolphins must be flown into an area, acclimated, fed and cared for. Swean adds that growing political pressure exists to end using dolphins and other marine mammals, such as seals, for mine detection and clearance. Because low- frequency broadband sonar is approaching the capability of dolphins’ sonar, the Navy also is seeking to replace the mammals with AUVs.
Additionally, the Navy is focusing on mine detection in littoral areas as military operations increasingly shift from blue to green, or coastal, waters. Littoral environments are complex areas with abundant sonar noise and bottom clutter from garbage and other debris. Sensors such as sonar often are compromised and have less range in this environment than in deep water.
Autonomous vehicles are a major part of Navy plans to stealthily detect and map minefields prior to military operations. But obstacles such as fishing nets can snag or capture an AUV, reducing the element of surprise and compromising an operation. The presence of people, whether fishermen or pleasure boaters, greatly complicates operations in coastal waters, Swean says.
Another system being deployed to the fleet is the Airborne Laser Mine Detection System (ALMDS), which consists of a blue-green laser system designed to sweep and locate mines. It is engineered to detect mines up to 40 feet under the surface. Swean notes that much of the program’s early research was conducted by the ONR and adds that ALMDS is now transitioning to the Navy.
ALMDS is intended to work in conjunction with the Rapid Airborne Mine Clearance System (RAMICS), a helicopter-mounted 30-millimeter cannon firing special supercavitating ammunition designed to reach targets up to 40 feet under water. Once the ALMDS detects and classifies a mine, it passes the data to RAMICS, which then uses its own dedicated light detection and ranging (LIDAR) sensor to lock onto the mine and destroy it with the cannon.
The Navy has made significant improvements in its ability to detect mines under water and buried on beaches and at the sea/shore interface. Swean notes that a new detection system will be a payload for the Fire Scout UAV system that will be deployed on the littoral combat ship.
Another technology that will be deployed is an airborne neutralization system designed to clear mines buried on beaches. Swean explains that in the 1990s, the Navy had no means to breach mines rapidly in the surf zone. The Joint Assault Breaching System (JABS) is a modified Mark 84 bomb with a Joint Direct Attack Munitions System kit designed to destroy mines from the air. The weapon can be dropped up to 2 miles away from its targets. He notes that the service is moving JABS to full operational capability and modifying it by replacing the high explosive warhead with 6,000 darts. Released before impact, the darts can penetrate water, seabed and land up to 10 feet, peppering and destroying mines.
Mines, an Enduring Threat
For more than a century, mines have threatened warships and commercial vessels. Two general types of marine mines exist: contact and influence devices. Mines can be tethered at varying depths, set on the bottom or set to drift in shipping lanes. Contact mines have been in use since the 19th century. Detonating on impact, contact mines have four to six “horns.” These are hollow lead protrusions containing a glass vial of sulfuric acid. When the horns are struck, the acid is released into a lead acid battery without any acid electrolyte. The addition of the acid activates the battery and detonates the mine’s explosives. International treaties require nations to announce when they mine an area to allow civilian vessels to avoid the zone, explains Thomas Swean, team leader for mine countermeasures programs at the Office of Naval Research in
Mines can be moored to the bottom at varying depths. These tethered mines can be traditional horned mines or devices detonated by a ship’s acoustic or magnetic signature. More sophisticated mines are essentially tethered torpedoes or rocket-propelled munitions that sit on the bottom. These weapons can be activated by and home in on a combination of a ship’s acoustic signature from the sound of its propellers and bilge pumps and its magnetic signature. Modern mines can be set to attack specific-sized ships or they can “count” passing ships, only attacking after a certain number of ships have passed. Such mines can be programmed to be random in selecting a target, Swean adds.
Another type of mine that is hard to counter is the pressure mine. This device is activated by the pressure wave created by a passing object. Like other types of modern mines, it can be set to activate and attack a specific type or size of ship, or it can combine acoustic and magnetic capabilities as well.
Swean explains that minesweeping attempts to mimic the acoustic and magnetic signatures of a ship through towed arrays containing magnetic fields and noisemakers. Contact mines can be trawled in nets or by cables that cut their mooring lines, causing them to float to the surface where they can be destroyed with gunfire. Traditionally, minesweepers have been wooden-hulled vessels specifically to reduce their magnetic signatures for sweeping operations. However, because of the physics behind them, pressure mines are hard to counter, he says. Rocket-propelled bottom mines also are difficult to defeat once they activate because they race toward their targets at speeds approaching 80 miles per hour.