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 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

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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 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.

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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 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. Low frequency HELRAS claimed 14dB sensitivity improvement, Long pulses improve Doppler resolution L3 claims that the HELRAS DS-100 is the better beam-forming, longer pulses and an in shallow water with high levels of highest performing helicopter dipping FM mode. As a further option, L3 offers an reverberation, while its more refined FM sonar in the world, with its long-range upgrade to a low-frequency transducer to processing enables detection of targets detection and wide-area search prowess provide performance that the company with low Doppler shifts and boosts range demonstrated in deep and shallow water. says is comparable to that of a surface resolution. The display has a finer set of A high source level of 218dB and narrow ship sonar. range scale divisions of 1, 1.5, 2.5, 5, 7.5, vertical transmitted beams extending from 10, 12.5, 15 and 20nm, suggesting better -15 to +15°, along with high array gain Export variants range discrimination out to the claimed provide a high figure of merit, says the The AQS-18(V)-3 is the export version 20nm (37km) maximum. company. For reception, it forms 32 half- of the AQS-13F and uses an adaptable L3 emphasises that the system is beams and 16 full beams. reeling machine compatible with a wider compatible with 1553B databus The system is claimed to be able to range of helicopters and a different architectures to ease integration. track ten targets simultaneously, while the acoustic processor, dubbed the Adaptive Its 264kg weight is divided between the display offers selectable range scales of 1, Processor Sonar (APS), which improves 88.9kg transducer assembly and the 1.5, 2.5, 4, 6, 10, 16, 25, 40 and 60nm. the system’s ability to detect difficult 122.7kg dome control, reeling machine Active operation is based on targets in shallow water subject to severe and cable assembly, with the remaining transmissions centred on three reverberation, while minimising false 51.8kg accounted for by the sonar frequencies, namely 1.311, 1.38 and alarms. interface unit, cable interface power 1.449kHz. HELRAS can transmit CW It uses fast Fourier transform techniques supply and sonar control unit. It has the pulses with widths from 0.039 to 10s. FM to provide narrowband analysis of the same operating depth capabilities as the modes include linear period pulses with shaped CW pulse and to work better in AQS-18F and (V)-3 variants. widths from 0.156 to 5s, plus FM triplets non-reverberant conditions more typical Signal processing takes place in the with pulse widths from 0.625 to 1.25s. of deep water. sonar interface unit, which runs algorithms The latter include 50 or 100Hz down-

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sweeps at three centre frequencies and a transmitter rings, allowing full-power pings 300Hz down-sweep at a single centre even in shallow water, where many sonars It is no longer frequency of 1.380kHz. are limited by cavitation – the formation and practical for This last active centre frequency, collapse of water vapour bubbles on the combined with proprietary transducer and surface of the emitter caused by large, rapid a sonar operator technologies, allows multiple pressure changes. boundary interactions, says the company, Thales emphasises that FLASH’s low to sift through meaning that it can take account of frequency and large bandwidth enable it to reflections from the surface, the seabed match its detection range to the reach of detections one and even temperature changes that define modern lightweight torpedoes, such as the layers in the water, while reducing the effect Raytheon Mk 54 and the Eurotorp MU90. The by one to find the of reverberation caused by reflections from large bandwidth in particular helps minimise suspended particles in the water and even the difficulties caused by reverberation, target. fish. It also enables interoperability with particularly in shallow water. In active mode, it shipboard sonars and sonobuoys in bistatic transmits on three frequencies within its or multistatic operations. range of 3-5kHz, emitting FM, CW and High-resolution Doppler processing and combined FM/CW pulses. the USN announced its search for long, shaped pulses aid detection of faint, FLASH can operate at depths of between innovative solutions that would enable an slow-moving targets, says L3, adding that 15 and 750m, the long cable allowing the operator to detect and discriminate targets broadband FM pulses up to 5s long can be transducer to put sound into the whole water efficiently in an airborne ASW mission. used to detect near-zero Doppler targets. column, penetrating layers to maximise The service said that the growing The company claims a maximum operating probability of detection, although the number of sources and receivers and the depth of 500m and a figure of merit high lightweight Compact FLASH variant has a resulting higher transmission rates are enough to achieve second convergence shorter cable, limiting it to a maximum of creating a dramatic increase in the zone detections in deep water. 300m. Furthermore, the short dip cycle number of contact reports generated HELRAS’s folding array allows it to fit a enabled by the fast reeling machine automatically from the signal processing, large aperture essential for low-frequency maximises the area coverage rate. declaring that it is no longer practical for a operation into spaces aboard helicopters It can operate as a standalone system or sonar operator to sift through detections that previously only had to accommodate be integrated with the company’s SONICS one by one to find the target. relatively small medium-frequency processing suite, which includes a The USN wants techniques and tools transducers. COTS acoustic processor and a digital that consolidate information from the The projector comprises eight elements wideband VHF receiver. SONICS provides up contact reports to enable the operator to (seven sonar and one underwater telephone) to 32 channels and can process all NATO find and focus on target detections rapidly. in an array 5.2m from top to bottom when standard buoys. Active contact reports typically provide fully extended, below a 1.2m tall receive The system is capable of processing data time difference of arrival, bearing, signal- array, whose arms fold out under hydraulic from sonobuoys, and the dipping sonar can at to-noise ratio and Doppler information, pressure to a diameter of 2.6m. the same time participate in multistatic along with geographical areas in which the As a standalone system, HELRAS operations with sonobuoy fields laid by the target is probably located, while the USN is weighs 341.3kg, but by sharing display helicopter or by other aircraft, and with Thales interested in display innovations to reduce resources with other sensors and avionics shipboard variable-depth and hull-mounted operator workload by ranking detections in an integrated suite, it weighs in a little sonars. and automatically suppressing clutter. lighter at 326kg. FLASH operator aids include displays of raw Areas of particular focus include returns in passive and active modes, a information superiority through data FLASH rival panoramic display to improve situation fusion, continual monitoring of changes in HELRAS’s principal rival is Thales’ Folding awareness, the ability to track multiple targets the situation to assess potential impacts Light Acoustic System for Helicopters (FLASH), simultaneously, performance predictions on and limitations of the mission plan, which has a slightly smaller wet end with a calculated using regularly updated sound modifications to the plan to maximise the receive array that folds out to deploy above speed profiles, and tools to help find the probability of detection and, finally, tactical the transmitter. optimum depth for the transducer. decision aids. Thales’ heritage in lightweight dipping SONICS provides a similar range of As long as submarines remain a threat, sonars came from predecessor company operator aids, including a tool to optimise the helicopters are likely to be pitted against Thomson Sintra Activités Sous Marines, which placement of buoys. them, and low-frequency sensors, ever developed the DUAV-4 plus the HS12 and smarter algorithms and the promise of HS312 models. Multistatic complexity maturing multistatic capabilities look set Operating at higher frequencies between Evidence of the sheer complexity of modern to remain at the heart of their ability to do 3 and 5kHz, FLASH uses wideband multistatic ASW emerged last May when so effectively. ▪

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