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Anti-Submarine Warfare

The art of detecting, tracking, targeting, and destroying advanced submarines is extraordinarily complex, requiring a highly developed set of skills and robust training to integrate the efforts of the strike group. The complexities of the undersea environment -- water density, temperature, salinity, currents, above-surface weather conditions, and sea floor bathymetry -- all significantly affect propagation of sound. The development and honing of these skills requires intensive real-world sonar training to master the art and processes of identifying submarines in the complex subsurface environment. Moreover, proficiency in MFAsonar operations is highly perishable.

Repeated training, therefore, is required to achieve and maintain combat proficiency and effectiveness. That training must also facilitate the development of the skills needed to coordinate the anti-submarine efforts of Strike Group assets and overcome additional complexities that arise from simultaneous use of MFA sonar by multiple ships (including mutual interference). To that end, training in real-world conditions designed to replicate real-world scenarios with a live, subsurface adversary whose tactics will exploit the ocean's ever-changing complexity is essential. This training occurs as an important element of the coordinated efforts of a carrier or amphibious assault ship, its escort ships, and other assets to conduct simultaneously offensive and defensive air, sea, undersea, and amphibious operations in simulated warfare conditions where resources are limited and time is of the essence.

Anti-Submarine Warfare (ASW) Training ASW involves helicopter and sea control aircraft, ships, and submarines, operating alone or incombination, in operations to locate, track, and neutralize submarines. Controlling the underseabattlespace is a unique naval capability and a vital aspect of sea control. Undersea battle space dominance requires proficiency in ASW. Every deploying strike group and individual surface combatant must possess this capability.

The Navy's ASW training plan, including the use of active sonar in at-sea training scenarios, includes multiple levels of training. Individual-level ASW training addresses basic skills such asdetection and classification of contacts, distinguishing discrete acoustic signatures including thoseof ships, submarines, and marine life, and identifying the characteristics, functions, and effects ofcontrolled jamming and evasion devices. More advanced, integrated ASW training exercises involving active sonar is conducted in coordinated, at-sea operations during multi-dimensional training events involving submarines, ships, aircraft, and helicopters. This training integrates the full anti-submarine warfare continuum from detecting and tracking a submarine to attacking a target using either exercise torpedoes or simulated weapons. Training events include detection and tracking exercises (TRACKEX) against "enemy" submarine contacts; torpedo employment exercises (TORPEX) against thetarget; and exercising command and control tasks in a multi-dimensional battlespace.

Various types of active and passive sonars are used to determine water depth, locatemines, and identify, track, and target submarines. Passive sonar "listens" for sound waves byusing underwater microphones, called hydrophones, which receive, amplify and process underwater sounds. No sound is introduced into the water when using passive sonar. Passivesonar can indicate the presence, character and movement of submarines. However, passive sonar provides only a bearing (direction) to a sound-emitting source; it does not provide an accuraterange (distance) to the source.

Passive sensors, for example, are effective at short range but have become more limited in their capability since the development of quieter submarines. Even though recent improvements to passive systems have extended their range, submarine quieting measures have lowered submarine noise levels to nearly the level of the ambient noise of natural sounds in the ocean. As a result, the US Navy is concerned that an enemy submarine could get within effective weapons' range of U.S. forces before passive systems could make contact with an enemy submarine. Passive systems by the nature of howthey operate are environmentally benign because they do not transmit sound.

Active sonar is needed to locate objects because active sonar provides both bearing and range to the detected contact (such as an enemy submarine). Active sonar transmits pulses of sound that travel through the water, reflect off objects and returnto a receiver. By knowing the speed of sound in water and the time taken for the sound wave totravel to the object and back, active sonar systems can quickly calculate direction and distancefrom the sonar platform to the underwater object. There are three types of active sonar.

  • High-frequency active sonar, which operates at frequencies greater than 10 kilohertz (kHz).At higher acoustic frequencies, sound rapidly dissipates in the ocean environment, resulting in short detection ranges, typically less than five nm. High-frequency sonar is used primarilyfor determining water depth, hunting mines and guiding torpedoes.
  • Mid-frequency active sonar operates between 1 and 10 kHz, providing an optimal balance ofdetection range and resolution. Typical mid-frequency sonar detection ranges are up to 10nautical miles making it the primary tool for conducting anti-submarine warfare.
  • Low-frequency sonar operates below 1 kHz and is designed to detect extremely quiet diesel-electric submarines at ranges far beyond the capabilities of mid-frequency active sonars. There are only two ships in use by the U.S. Navy that are equipped with low frequency sonar; both are ocean surveillance vessels operated by Military Sealift Command.

Active sensors systems that can be used from aircraft provide extended ranges and large area coverage, but large area coverage requires a high number of assets of both aircraft and sensors to be deployed. Antisubmarine warfare aircraft are expensive to operate, and they require shore-based facilities, which are limited because of continued decreases tothe number of these installations. A shipboard system, such asSURTASS/LFA, provides the advantage of extended range and duration of searches, but when it is used in a continuous search mode, it has the drawback of revealing the ship's position.

Anti-submarine warfare has historically used two methods for detecting the presence of submarines. The first method incorporates a plurality of buoys or "sonobuoys" which are adapted to descry the sound or "acoustic signature" of a submarine. The second method utilizes an airborne magnetic field sensor, which is towed behind an aircraft, for detecting disturbances in the earth's magnetic field. These magnetic disturbances may be caused by large, metallic, underwater objects such as a submarines. Both prior methods are outlined below with each having respective advantages and disadvantages.

The first method or "sonobuoy" method incorporates a plurality of buoys which are adapted to be positioned in a two dimensional, geographical relation, at or near the surface of the ocean. Each sonobuoy continuously monitors sonic vibrations received from within the ocean and continuously transmits information corresponding to the sonic vibrations as radio signals. Sophisticated sound detection equipment or "hydrophones" detect the acoustical vibrations. Once the radio signals are received onboard a nearby aircraft, they are analyzed by a technician. The sonic vibrations as received and analyzed may potentially correspond to the acoustic signature of a modern nuclear or diesel submarine.

The acoustic signature of a submarine is produced by the engine, propeller, and internal mechanics of the submarine as it passes through the water. Each sonobuoy, once deployed, transmits all of the acoustical information received. This acoustical information may include the acoustical signature of a submarine. However, the sonobuoy method suffers from a disadvantage in that the acoustical information must be continually monitored by technical operators onboard a nearby aircraft. This necessitates that each sonobuoy be assigned to its own respective radio frequency for transmitting its signal to the aircraft.

Further, the technical operators onboard the aircraft must undertake extensive training to recognize the acoustical signature of a submarine apart from the surrounding background noise. Unfortunately, the sonobuoy system suffers from the added disadvantage that the number of sonobuoys which may be deployed depends upon the number of radio frequency channels available for transmission. For example, the equipment onboard the aircraft may only support a limited number of frequency channels, such as 50 to 100, thus limiting the number of channels and therefore the number of buoys available for use. Thus, the sonobuoy method affords coverage of a relatively predetermined geographical area of the ocean but is limited by the number of buoys available.

A number of systems have been developed which utilize the sonobuoy detection method for detecting the presence of an underwater vehicle such as a submarine. A directional hydrophone buoy system which produces an electrical signal in response to the sound pressure emitted from an underwater sound source, such as the propeller of a submarine. The buoy is given appropriate ballast such that the buoy is submersed below the surface of the water. A mass including a coil winding is suspended by springs within a local magnetic field, which is produced by permanent magnets. Sonic vibrations from the surrounding ocean produce a corresponding movement of the coil within the magnetic field and therefore a voltage output corresponding to the rate at which the coil moves, i.e. the amount of sound detected.

A small radio telemetering buoy is dropped into the ocean from an aircraft. When the buoy falls into the water, a bag disposed on a distal end thereof is inflated. A radio antenna is suspended within the inflatable bag. An echo sounding system is employed by first electrically detonating an explosive charge by way of a delayed sea water switch. A hydrophone disposed within the telemetering buoy then receives an impulse from the detonated charge and relays the information to a nearby aircraft via radio signals.

It thus appears that the sonobuoy method offers the advantage of placement over a relatively predetermined ocean area for submarine detection. However, the sonobuoy method suffers from the disadvantage of size limitation i.e., the acoustic detection equipment takes up a large physical area. The sonobuoy method also suffers from the requirement of a separate radio frequency channel for each sonobuoy, and the requirement that highly trained operators must continually monitor all received signals from the sonobuoys. Additionally, the limited number of radio channels available onboard the aircraft effectively limits the number of sonobuoys deployed and thus the geographical area of coverage.

The second method for detecting submarines utilizes an airborne magnetic field sensor which is suspended behind an aircraft such as an airplane or helicopter. The airborne magnetic field sensor detects an anomaly in the earth's magnetic field and then communicates this information to the aircraft for analysis. Airborne magnetic field sensors have also been employed for detecting geological noise effects produced by the varied shape of the sea floor. The airborne sensor method generally requires a large power source for detecting the presence of a submarine far below the ocean surface.

The airborne sensor method provides the advantages of maneuverability and that the course of search may be readily changed. Further, this method is not limited by the number of buoys that may be housed by an aircraft. However, this method suffers from the disadvantage of limited swath width and thus cannot readily and simultaneously cover a wide two dimensional area. A body or bird is towed behind an aircraft by way of a cable or the like. A magnetic field sensor having three mutually perpendicular axes is enclosed within the bird. One of the sensors or coils is selected as a detector coil and is adapted to be maintained in alignment with the lines of force of the earth's magnetic field, for example by servo motors. The other axes are placed mutually perpendicular thereto.

Each of the airborne magnetic field sensors must be supported by a dedicated aircraft. Thus the magnetic field sensor method offers the advantage of portability, however it provides a relatively narrow area of coverage when compared with the sonobuoy method.

Heretofore, the range of sound detection has been far superior to that of magnetic anomaly detection, given the same power requirements. However, with the advent of quieter and quieter submarines, the effective range of sonobuoy detection has become considerably weakened. These technological advances in the art of quieting submarines have not been economically addressed by the advances in the art of acoustical search sensing. Thus, modern submarine detection has become an increasingly difficult exercise.

The US Navy determined that non-acoustic technologies, such as radar, laser, magnetic, infrared, electronic, optical, hydrodynamic, and biological sensors, have demonstrated some utility in detecting submarines. Their usefulness, however, is limited by range of detection, unique operating requirements, meteorological / oceanographic disturbances, and/or a requirement that the submarine be at or near the surface for detection. Today, nuclear submarines can remain submerged at considerable depths indefinitely, and new battery technology and air-independent propulsion have increased the time that diesel submarines can remain at depth.

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Page last modified: 16-11-2011 13:18:05 ZULU