Detecting the stealthy submarine starts with maintaining a tool kit of different sensors. Each sensor has specific applications that counters different submarine operations. Many of these sensors complement and corroborate each other to enhance ASW effectiveness. Air ASW sensors are divided into two basic types; acoustic and non-acoustic. In some foreign services, these acoustic and non-acoustic sensors are commonly referred to as wet- and dry-end sensors,
Non-acoustic sensors augment the detection capability provided by acoustic sensors. These sensors use radar to detect exposed periscopes and hull surfaces, electro-magnetic systems to intercept the radar emissions from submarines, infra-red receivers to detect the heat signatures of surfaced submarines, or Magnetic Anomaly Detectors (MAD) to sense small changes in the Earth's magnetic field caused by the passage of a submarine. This sophisticated technology is further enhanced by vigilant lookouts who are carefully scanning the turbulent ocean surface for submarine periscopes and wakes.
Radar sensors have been used since World War II for the detection of surfaced or snorkeling submarines. Back then, submarines relied upon their batteries for submerged operations. Eventually their batteries would become drained to the point where they were forced to return to the surface and operate their diesel engines to re-charge the battery. While surfaced, the submarine was extremely vulnerable to detection by both radar and visual sensors. The addition of a snorkel enabled the submarine to operate its battery-charging diesel engines while minimizing its exposure to radar and visual sensors. Additionally, the background clutter of the surrounding ocean waves limited radar and visual detection. Also, the development of submarine-based electro-magnetic sensors provided the submarine commander with suffficient warning to dive if approaching radar emissions were detected.
Eventually, nuclear submarines where developed which eliminated the need to periodically recharge the batteries. Despite this significant advance, not all nations were able to build nuclear submarines due to financial and technological reasons. Those nations which remain committed to diesel power have pursued technology which limits the number of times the submarine has to recharge its batteries. However, many submarine commanders must still use their periscopes to provide final visual classification of targets prior to attack. Because of this requirement for target verification, radar systems are still used to detect submarine periscopes.
Today's airborne radar systems must be lightweight yet sufficiently capable for ASW operations, long-range detection and surveillance of surface vessels, airborne navigation, and weather avoidance. For that purpose, many Air ASW radar systems use different radar frequencies, scanning speeds, transmission characteristics, pulse lengths, and signal processing methods that reduce background sea clutter and enhance radar returns from exposed pericopes and submarine hulls. The hostile submarine using electro-magnetic sensors, however, can still detect ASW aircraft radar emissions at a much greater distance than the aircraft can detect the submarine by radar. Nevertheless, the threat of radar detection is sufficient to keep the submarine submerged. Radar systems now used aboard U.S. Navy ASW aircraft include the AN/APS-115 (P-3C), AN/APS-124 (SH-60B), and AN/APS-137 (S-3B, some P-3Cs).
Magnetic Anomaly Detection (MAD) Sensors
MAD sensors are used to detect the natural and manmade differences in the Earth's magnetic fields. Some of these differences are caused by the Earth's geological structures and sunspot activity. Other changes can be caused by the passing of large ferrous objects, such as ships, submarines or even aircraft through the Earth's magnetic field. MAD sensor operation is similar in principle to the metal detector used by a treasure hunter or the devices used by utility companies to find underground pipes.
For ASW purposes, the ASW aircraft must almost be essentially overhead or very near the submarine's position to detect the change or anomaly. The detection range is normally related to the distance between the aircraft sensor ("MAD head") and the submarine. Naturally, the size of the submarine and its hull material composition normally determines the strength of the anomaly. Additionally, the direction travelled by both the aircraft and the submarine relative to the Earth's magnetic field is also a factor. Nevertheless, the close proximity required for magnetic anomaly detection makes the MAD system an excellent sensor for pinpointing a submarine's position prior to an air-launched torpedo attack.
In order to detect an anomaly, the MAD head of the aircraft tries to align itself with the noise produced by the Earth's magnetic field. Through this alignment, the noise appears as a near-constant background noise value which enables the operator to recognize any contrasting submarine magnetic anomalies from the background noise. However, any rapid changes in aircraft direction or the operation of certain electronic equipment and electric motors can produce so much aircraft electro-magnetic noise that makes the detection of the submarine's magnetic signature virtually impossible. Special electronic circuitry is enabled to compensate and null out this aircraft magnetic noise. Additionally, the MAD head is placed the farthest distance away from all the interfering sources. That is why the P-3C Orion aircraft has its distinct tail stinger or "MAD boom". On the S-3B, a similar MAD boom is installed and is electrically extended away from the aircraft during MAD operations. Additionally, the SH-60B extends a towed device called a "MAD bird" to reduce aircraft magnetic noise. With continuing advances in both compensation and sensor technology, the detection ranges for MAD sensors may be enhanced for the search and localization phases of ASW missions. Currently all naval ASW aircraft use variations of the AN/ASQ-81 MAD system. A few P-3C aircraft use an advance MAD system, the AN/ASQ-208, that uses digital processing.
Electro-Magnetic (EM) Sensors
Electro-Magnetic (EM) sensors passively scan the radio frequency spectrum for intentional electronic transmissions from hostile forces. These electronic emissions originate from land sites, ships, and aircraft. They can also be detected from submarines. By comparison, Air ASW EM sensors are sophisticated versions of radar detectors used to sense police radar gun signals. The difference, of course, is that Air ASW EM sensors provide all the details necessary to classify and localize the type of electro-magnetic emission that has been detected.
Since the radio-frequency spectrum is extremely cluttered with both hostile, friendly, and neutral electronic emissions, ASW aircraft EM systems are designed to search mainly for radar signals. To further reduce the electronic clutter, signature libraries are used to selectively search for specific submarine radar signals while disregarding signals from friendly and neutral radar systems. Detection of electronic emissions, however, is dependent upon the submarine commander's gamble to operate the submarine radar. Although, EM systems are not normally one of the primary ASW sensors, its flexibility for detecting hostile aircraft and naval combatants at long ranges makes it an effective sensor for all air warfare missions. Its potential presence deters the operation of submarine radar systems forcing the submarine commander to rely on other less accurate sensors to find targets. EM systems installed on naval ASW aircraft include the AN/ALQ-78 and AN/ALR-66 series on the P-3C Orion, the AN/ALQ-142 on the SH-60B Seahawk, and the AN/ALR-76 on the S-3B Viking.
Infra-Red (IR) Sensors
Infra-Red (IR) sensors are used to detect the heat signatures that extend beyond the visible light spectrum. They are commonly called either FLIR (Forward Looking Infra-Red) or IRDS (Infra-Red Detection System). The major difference between FLIR and IRDS is that FLIR passively scans for IR sources forward of the aircraft whereas IRDS searches all around the aircraft. This passive sensor device must be cryogenically cooled in order to detect IR sources. The IR signature itself can be masked by warm waters and high humidity. When conditions permit, medium detection ranges can be obtained that are comparable or even better than normal visual search ranges. At night, the system works even better as long as there is a noticeable difference in temperature between the source and the background environment. IR systems for nighttime ASW operations have replaced the previous method of illuminating the ocean with either a searchlight or flares; active visual search methods. By using a passive system such as either FLIR or IRDS, the submarine commander has another dilemma to solve on whether to snorkel or surface during the night. Most ASW aircraft utilize the IR sensors not only for ASW, but also for maritime surveillance.
Many submarine contacts are still detected using visual scanning techniques. These techniques are sometimes augmented by sophisticated binocular and other electro-optical devices. Submarine commanders are still wary of being visually spotted and maintain a safe speed when their periscopes are exposed so that their telltale wake remains indistinct compared to the background sea clutter. The position of the Sun and the Moon as well as the direction of the ocean waves are all factors the submarine commander must consider in order to remain unobserved. In some regions of the world, phosphorescent marine organisms illuminate a submerged submarine allowing it to be visually spotted. Additionally, some aircrews may use night vision goggles to aid in visual detection at night.
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