What It Takes To Wage ASW In Shallow Waters

It is the threat from diesel-electric submarines (SSK) that drives the specific requirements for active/passive ultra low-frequency anti-submarine warfare (ASW) sonar. Because of the SSK threat capabilities, wide areas have to be searched and long standoff distances have to be cleared for the safety of high-value principal surface combatants. The most effective sensor to detect the SSK operating in shallow waters is the active/passive ultra low-frequency towed-array sonar (ATAS). A warship and its shipborne naval helicopter equipped with active long-range (hull-mounted, towed-array and dipping) sonars provide the necessary standoff safety, required mobility and technology for a successful ASW platform. Another equally important point to consider is that the shorter-range mid-frequency, hull-mounted panoramic sonar (HMS) like the Indian Navy’s existing HUMSA Mk1 and Mk2s has a much lower probability of detecting the SSK from simple geometric considerations. The SSK can out-run the area coverage of the short-range sonar between its pings. A SSK can hear the pings of the hunting HMS even before that sonar detects it, and the SSK can also figure out its likely detection range. A very good tactic for the SSK is to sprint at high speed between pings. If the HMS has a relatively short detection range the SSK has a good chance to escape. Such sprint tactics to avoid detection are far less likely to work against long-range, ultra low-frequency active/passive towed-array sonars. The conclusion is that a mid-frequency, HMS cannot reliably do the basic ASW mission in littoral waters, as its detection range will not exceed 5nm. In the case of short-range, mid-frequency ASW sonar the modern SSK therefore remains an undetected lethal threat to surface combatants.
The range of an active ASW sonar system is determined by environmental conditions, the operating characteristics of the sonar, such as power-level and beam-widths, and the operating frequency. The rule of thumb is, the lower the operating frequency, the longer the sonar’s range. However, a lower operating frequency in the past has required a larger and heavier hull-mounted sonar transducer. Since warships too have limitations in space and carrying capacity, traditionally a compromise would have to be made choosing as low as possible operating frequency while still maintaining a light and compact sonar to go in the hull. This compromise has generally ended up with an operating frequency around 4.5kHz. It is extremely important to note that AT ALL TIMES, if such a size/weight/frequency compromise was not necessary, the lower the frequency the better the sonar’s range performance. Lower frequency is particularly important for good range performance in shallow tropical waters as those found around India. The main reason is that the lower frequency suffers much less losses as the acoustic signal interacts with the shallow water surface and especially bottom boundaries.
Specialised sonar manufacturers from the US, France and Germany have since solved the size–performance compromise problem with the invention of new transducers that are much smaller and lighter, yet still transmit very high power at low frequencies. This new technology has been applied to the design of the new-generation sonars. The system comprises a towed body that integrates separate transmit and receive arrays operating at 1.38kHz, a compact winch along with its handling system, and shipboard electronics. Such an ATAS can reach an operational depth of 300 metres (985 feet), and are easily accommodated within an existing DDG and FFG of the Indian Navy and gives the ATAS, in shallow waters, a range that is, on the average, about two times the range of any competing mid-frequency sonar. In deeper waters the average range advantage increases to more than four times. Overall, this gives the low-frequency sonar (like the hull-mounted HUMSA-NG) an advantage of between four to 16 times the area coverage for all operating conditions that a navy might encounter. The beneficial impact of this performance advantage is staggering! It is something that should be of one of the primary considerations for any navy that is evaluating new towed-array sonars for eventual procurement.
The Nagan ATAS was specifically designed by the DRDO for warships operating in shallow waters against contemporary SSK threats. The Nagan, however, is a low-frequency ATAS, and not an ultra low-frequency system of the kind (from the US and France) that are now entering service worldwide. Traditionally, as one reduces the operating frequency in order to achieve the performance gains required in shallow waters, either the transducer size increases or the amount of power one is able to put into the water decreases. The Nagan’s patented transducer technology now allows a navy to put a very high level of power into the water, while maintaining a very small form factor. Typically, the Nagan is at least one quarter the size and weight of competitive sonar systems. Its small size and weight means that its integral winch and handling systems are also much smaller and lighter than competitive systems. This will give the Nagan a great advantage on smaller surface combatants (like the Project 28 ASW corvettes) operating in littoral waters, where a navy can embark the payload with minimal impact to the warship and its existing on-board equipment. The Nagan’s tow-body is uniquely configured for shallow water operations, which creates major competitive advantages over traditional variable depth sonars (VDS). Traditional VDS configurations have placed the tow-body a significant distance back from the warship to prevent acoustic interference from the warship’s own noise on the sonar. The Nagan’s adaptive noise cancellation, however, removes such own-ship interference and allows the short-scope tow of the system required for shallow water operations. The Nagan, incidentally, is designed to operate in waters as shallow as 15 metres. Its electronic nulling of own ship noise using adaptive techniques has already been demonstrated.
Traditional single-receive arrays on competitive ATAS systems are many hundreds of yards long. The Nagan’s tow-body has two short towed-arrays for active receive, which are less than 100 feet long. These dual short arrays facilitate shallow water operations without restricting the warship’s manoeuvres and also allow it to reliably and quickly resolve port and starboard bearing ambiguity, without the traditional need to wait while the array straightens out again after a course change. This also allows the warship to manoeuvre at will in shallow waters, without worrying about the towed-body and arrays grounding in turns and at slow speeds. The active projectors on the tow-body are a variable geometry. The projectors automatically retract at speeds greater than 23 Knots, allowing the warship to transit at sprint speeds up to 30 Knots without the need to recover the tow-body. When the warship slows down again to operating speed the system automatically resumes active operations without delay.
The Nagan’s low-frequency also minimises the effects of boundary (surface and bottom) reverberation and losses to maximise detection range in warm shallow waters of the type prevailing throughout South and Southeast Asia. Low-frequency operations have significantly lower volume absorption losses than high-/mid-frequency systems, thus enabling long-range detections. The Nagan’s broadband FM transmissions and processing will also provide significant reverberation reduction, thereby allowing the detection of slow-moving targets in shallow waters. In summary, the Nagan has the following operational advantages:
· It is extremely small and lightweight, allowing it to fit on the small vessels with ease.
· The same features allow minimal ship impact, in terms of installation, ship manoeuvre and operations. In addition, the Nagan has virtually no impact on ship speed, trim, fuel consumption, etc.
· Its small size and weight make it easy and safe to launch and recover the tow-body with a small crew.
· It puts 7 times the sound intensity into the water as the competing systems, in spite of its small size. This will be increased to 15 times once an ultra low-frequency variant of the Nagan is developed.
· It broadcasts in both omni and quadrant modes, allowing the tactical use in shallow waters by only putting power where the operator believes the target to be (away from the shore for example).
· Its high-gain, towed-array receiver allows target bearing ambiguity to be resolved instantly because of its twin receive arrays, without the need for time-consuming ship manoeuvres.
· It is safer to operate in shallow waters because of its short receive arrays, rather than a long towed-array, and/or long layback of the tow-body, which is vulnerable in turns and at slow speeds.
· Its short arrays stabilise quickly after manoeuvres--a significant advantage for operations in shallow or congested waters.
· It is equipped with a depth sensor allowing for the automatic adjustment of its cable scope and consequently depth control of the tow-body.
· It allows short-tow scope for shallow water operations with adaptive noise cancellation to remove own ship’s noise interference.
· It is designed for ASW work, allowing the warship to go to full speed without the need and time to recover the tow-body.
· Its low frequency maximises the detection range in warm, shallow waters of the type prevailing in South and Southeast Asia.
· Its broadband FM capability enables the detection of slow-moving targets in shallow waters.
· Its adaptive nulling techniques allow a high level of performance even in short-tow scopes in shallow waters.
· It is fully bistatically/multistatically compatible with the DRDO-developed Mihir low-frequency dipping sonar.
· It is frequency bistatically/multistatically compatible with sonobuoys and some towed-arrays.
· It is designed with sophisticated multi-purpose waveforms (CW and FM) and processing for the maximum figure of merit against fast and slow, or stationary, targets.
· It augments existing mid-frequency HMS with its ability to operate below the layer in both shallow and deep waters.—Prasun K. Sengupta