NAVAL AVIATION SOUNDING Several operators of the NH90 NFH employ the aircraft in the ASW role. (Photo: Airbus Helicopters) While other technologies are increasingly playing a part, dipping sonar remains at the heart of the contemporary ASW helicopter’s detection capabilities. By Peter Donaldson odern submarines are the most difficult threats against M which naval forces must or significant milestones in ongoing ASW Automatic Radar Periscope Detection and protect themselves, thanks to their helicopter programmes. Discrimination (ARPDD) capabilities, with fits stealth, constant evolution in quieting and While the sensor most closely associated with to the Romeo fleet expected to be complete by the growing potency of their torpedoes ASW helicopters is the dipping sonar, it is part of September 2020, the US DoD announced last and missiles. an integrated suite required for the prosecution July. The ARPDD technology has been The proliferation of submarines around of submarine targets, in which even radar plays developed by Lockheed Martin Mission the world is reviving an interest in ASW that a role. It can provide a short cut by detecting Systems and Training. subsided after the Cold War, with periscopes and other masts, all of which have numerous nations – including Bangladesh, low radar cross-sections. Waves in the waves Canada, India, the Philippines, Poland, The USN is in the process of upgrading its Sensors that exploit sound, however, still South Korea, the US and Vietnam – MH-60R fleet with up to 103 kits to provide retain the top spot, and the complexity of the announcing new requirements, upgrades the Telephonics AN/APS-153(V)1 radar with way sound waves move through water plays a 24 DEFENCE HELICOPTER MAY/JUNE 2017 VOLUME 36 NUMBER 3 WWW.ROTORHUB.COM DH-03-17_p24-27_ASW.indd 24 4/21/2017 1:16:40 PM NAVAL AVIATION crucial part in making submarines such challenging targets. Sound travels roughly four times faster in water than in air, but its speed varies with water conditions and its path bends towards regions of lower sound speed. Increases in depth and pressure cause sound to travel faster because the molecules are closer together and collide more often. Sound also travels faster in warmer water than in cold, because the molecules have more energy, which also increases the frequency with which they collide. US Navy technicians unload sonobuoys from an MH-60R Sea Hawk on the flight deck of the guided- These effects mostly cancel out because missile destroyer, USS Mason (DDG 87). (Photo: US Navy) deeper water is generally colder, but mixing by wind and waves produces a surface layer in which the pressure effect dominates. takes for the echo to return and dividing by resolution – the ability to measure range Sound speed also increases with two reveals the range. Measuring changes precisely – while physics dictates that lower salinity, because dissolved salt makes in frequency between the transmitted pulse frequencies demand larger arrays. water less compressible and better able and its echo due the Doppler effect then Increasingly, dipping sonars are capable to transmit energy. Salinity can be highly reveals the target’s speed. of multistatic operation in which one or variable near the sea’s surface and in Many sonars can transmit both more transmitters work with multiple shallow water near estuaries. continuous-wave (CW) and frequency- receivers in different locations. These interactions produce layers and modulated (FM) pulses. CW pulses Coordination over data link networks channels that facilitate long detection maintain frequency from the beginning to enable simultaneous triangulation on ranges, but also create shadow zones into the end, while FM pulses change frequency detected targets, making the submarine which little sound will penetrate and as they proceed. FM signal processing commander’s life much more difficult. convergence zones where rays come provides significantly greater accuracy in together in annular regions of high intensity. range and range resolution. AQS-13 and friends Convergence zones enable detection at Sonars can also act as passive sensors The Western medium-frequency market is long ranges, but in bands interspersed with simply listening for the beat of propeller dominated by L3’s AQS-13 and AQS-18 others that hinder that detection. In shallow blades, flow noise, internal machinery family. The AQS-13F built a reputation as water, the seabed can reflect sound and frequencies or transients, such as the USN’s primary ‘inner zone’ ASW sensor enable ‘bottom bounce’ detections at long cavitation or hull popping, that might reveal when deployed by Sikorsky SH-60F ranges or absorb sound and reduce range. the presence of a submarine. Seahawks from aircraft carriers. In this mode, they can determine range It transmits on 9.23, 10 and 10.77kHz, Going for a dip by triangulation, using successive target- because this allows filters tuned to these While Russia and China make dipping bearing measurements, and measure discrete frequencies to reject most sounds sonar systems, little is known about them. speed by, for example, comparing the that are not echoes from the target. Even NATO countries and others inclined to propeller beats with target databases. All tactical sonars have to be loud, and buy Western equipment can only effectively Dipping sonars also carry instruments L3 quotes a source level of 215dB choose between two manufacturers, that measure water properties such as measured at the standard distance of 1m namely L3 Ocean Systems and Thales, conductivity (which is affected by salinity), from the AQS-13F’s transducer. Such power whose products are divided into medium- temperature and depth to update the sound is essential for long ranges in active mode, frequency sonars that operate at around speed profiles that are crucial to accurate because spherical spreading losses reduce 10kHz and low-frequency systems working performance prediction. Additionally, they can the power by 6dB for every doubling of the at 3-5kHz and below about 1.5kHz. usually function as underwater telephones. distance from the transducer. Sonars operating around 10kHz are Advances in array design and signal The AQS-13F can transmit rectangular called medium-frequency sets because that processing enable modern systems to form pulses lasting 3.5 or 35ms to aid Doppler figure is in the middle of the human ear’s multiple beams in azimuth and elevation, moving target indication (MTI), or shaped frequency response, which extends improving bearing accuracy, target pulses lasting 200 or 700ms to put large approximately 10Hz to 20kHz. discrimination and depth estimation. amounts of energy in the water to maximise All can operate as active sensors, Lower frequencies mean longer the probability of detection. transmitting pings and listening for echoes. wavelengths, which are less susceptible In passive mode, it receives sound in Knowing when the ping was transmitted and to absorption losses and so offer longer channels with 500Hz bandwidth between 9 multiplying the speed of sound by the time it detection ranges at the price of some range and 11kHz. WWW.ROTORHUB.COM VOLUME 36 NUMBER 3 MAY/JUNE 2017 DEFENCE HELICOPTER 25 DH-03-17_p24-27_ASW.indd 25 4/21/2017 1:16:43 PM NAVAL AVIATION The sonar data computer can run shallow-water algorithms for use in areas of high reverberation. Like all acoustic processors, it does not deal with sound directly but with digitised electrical signals, as the conversion between sound and electricity happens in the transducer. It can also control, process and display sonobuoys in a single integrated system. An MH-60R Sea Hawk assigned to the Raptors of Operators can select range scales with Helicopter Maritime Strike Squadron 71 lowers a maxima of one, three, five, eight, 12 or 20 dipping sonar into the water next to USS John C. kiloyards – ie half a nautical mile to 10nm Stennis (CVN 74) during an air power demonstration. (900m-18.5km) plus numerical readouts (Photo: US Navy) of target range, rate of change of range, and target bearing. The AQS-18(V)-5 is similar to the (V)-3, specifically designed for increased pulse Audio options include separate left and sharing most of its performance lengths and has spare processing capacity right headphone channels and automatic parameters and the APS, but is lighter for additional features such as computer- gain control, with a summed constant level at 260kg thanks to a smaller reeling aided detection and classification, multi- as an alternative in which both the machine – the price paid for this is a sensor target fusion, embedded training operator’s ears get the same sound. lesser operating depth of 290m. Its or performance prediction, with the latter L3 quotes a nominal operating depth of electronics suite is divided into larger based on environmental data. 440m and says that the system’s reeling number of smaller modules, making L3 says that the AQS-18A has 80% machine can get the transducer there and it more adaptable to different cabin commonality with the Helicopter Long back inside three and a half minutes. shapes and sizes. Range Active Sonar (HELRAS) DS-100, The AQS-13F weighs 284kg and The AQS-18A is L3’s latest medium the main difference lying in the ‘wet end’ consists of a number of subsystems frequency dipping sonar system, offering transducer systems. This commonality, including the transducer, reeling machine, both CW and FM transmissions. says the company, enables navies with transducer control module, multiplexer, L3 quotes CW frequencies of 9.23, both large and small helicopters to azimuth/range indicator, receiver and 10.003 and 10.774kHz, plus FM combine the two sonar types in bistatic sonar data computer. frequencies of 9.485 and 10.520kHz. or multistatic operations. Upgrade options include new receiving, It is also significantly louder at 217dB processing and display units that provide a to improve detection ranges.