these were very quiet diesel-electric propelled vessels but they are more likely to be nuclear-powered these days. The development of these was strongly influenced by the duel between and . A significant detection aid that has continued in service is the Magnetic Anomaly Detector (MAD), a passive device. First used during the Second World War, MAD uses the Earth's magnetosphere as a standard, detecting anomalies caused by large metallic vessels, such as submarines. Modern MAD arrays are usually contained in a long tail boom (fixed-wing aircraft) or an aerodynamic housing carried on a deployable tow line (helicopters). Keeping the sensor away from the plane's engines and avionics helps eliminate interference from the carrying platform. At one time, reliance was placed on electronic warfare detection devices exploiting the submarine's need to perform radar sweeps and transmit responses to radio messages from home port. As frequency surveillance and direction finding became more sophisticated, these devices enjoyed some success. However, submariners soon learned not to rely on such transmitters in dangerous waters. Home bases can then use extremely low frequency radio signals, able to penetrate the ocean's surface, to reach submarines wherever they might be. Modern warfare The military submarine is still a threat, so ASW remains a key to obtaining sea control. Neutralizing the SSBN has been a key driver and this still remains. However, non-nuclear-powered submarines have become increasingly important. Though the diesel-electric submarine continues to dominate in numbers, several alternative technologies now exist to enhance the endurance of small submarines. Previously the emphasis had been largely on deep water operation but this has now switched to littoral operation where ASW is generally more difficult. Anti-submarine warfare technologies There are a large number of technologies used in modern anti-submarine warfare: Sensors Acoustics particularly in active and passive sonar, sonobuoys, and fixed hydrophones aid in the detection of radiated noise. Sonar can be mounted on the hull or in a towed array. Pyrotechnics in the use of markers, flares and explosive devices Searchlights Radar, for surfaced parts High frequency radio direction finding (HF/DF or huff duff) to determine the bearings of U-boats. Hydrodynamic pressure wave (wake) detection Submerged submarines may produce a Kelvin wake pattern depending on their speed and depth. Kelvins wakes are difficult to detect for submarines below the depth of 100m, however, submarines below 100m may still generate "internal wakes" that may be detectable from the surface. Laser detection