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Multistatic Sonar Technology has become key to Anti Submarine Warfare (ASW) operations

Three decades ago, only a handful of major powers had effective submarine capabilities but, today, fleets in operation around the world are growing rapidly. Emerging superpowers, like China, can add naval capacity equivalent to a European country’s in the space of a few years. The threat from submarines is serious and ever-more difficult to counter. They can block a key trade corridor, destroy a high-value asset, and deliver devastating land strikes. What’s more, streamlined designs and quieter propulsion encourage forays into previously impenetrable zones.

Defence and Freedom: Modern sonar technology and possible consequences

Anti- Sumarine warfare (ASW) is becoming increasingly important for national security. Anti-submarine warfare (ASW) is a branch of underwater warfare that uses surface warships, aircraft, submarines, or other platforms, to find, track, and deter, damage, and/or destroy enemy submarines. Such operations are typically carried out to protect friendly shipping and coastal facilities from submarine attacks and to overcome blockades.


Successful ASW operations typically involved a combination of sensor and weapon technologies, along with effective deployment strategies and sufficiently trained personnel. Sonars are one of the primary systems for detection and tracking of submarines in Anti Submarine warfare. Sensors are therefore a key element of ASW. Common weapons for attacking submarines include torpedoes and naval mines, which can both be launched from an array of air, surface, and underwater platforms. ASW capabilities are often considered of significant strategic importance, particularly following provocative instances of unrestricted submarine warfare and the introduction of submarine-launched ballistic missiles, which greatly increased the perceived lethality of submarines.


Typically, sophisticated sonar equipment is used for first detecting, then classifying, locating, and tracking a target submarine. However modern nuclear and diesel electric submarines have become very stealthy and quiet. Work done over decades has resulted in some remarkable advances in radiated noise management to make detection by passive sonar more difficult. Against active sonars, submarine stealth is achieved by anechoic tiles and clever design to reduce the strength of reflected signals. Today’s submarines are many decibels quieter than their predecessors and present lower target strengths to active sonars. But ultimately, large submarines are large, and there’s a limit to what can be done to reduce propeller and flow noise, and target strengths can’t be reduced below some physical limits—especially at low frequencies.


A modern diesel-electric boat operating on AIP is, quite simply, too dangerous to attempt to counter with a passive-sonar approach. Another challenge is diesel-electric submarines operating in littoral waters which generate high false alarm rates (FAR). The risks of operating large submarines in contested spaces will likely be higher in the future than is the case today.


The evolution of quieting technologies in submarines have spurred on the development of active sonar, especially lowfrequency systems that allow long-range detection.

1 Antisubmarine Warfare | Technology for the United States Navy and Marine Corps, 2000-2035: Becoming a 21st-Century Force: Volume 7: Undersea Warfare | The National Academies Press

One of the ways of overcoming such challenges and detecting stealthy and quiet submarines is Multi static sonar. These systems rely on Network of Active and Passive sonars collaboratively tracking the stealthy targets.

Multistatic sonar

A multistatic sonar system is composed by a combination of sonar sensors (either active or passive) placed at different locations. As the type of platforms and their numbers are variable, configurations are multiple: a multistatic sonar can include an active sonar (a hull-mounted or towed source in a frigate), a passive array towed by another ship or an autonomous underwater vehicle (AUV), an array of sonobuoys and another array of moored hydrophones, becoming a real sensor network.


There are several advantages of the multistatic over the monostatic solution: excellent triangulation of the target position and its tracking, increased covertness of the receiving platform(s), extended echo range (or rather flexible echo range by optimally positioning the transmitter and receiver position), speed denial through ping diversity, multiple-angle observations and other tactical advantages.


Some of the advantages reported for adopting multistatics compared with a field of sonars operated monostatically, the same field operated multistatically may  give longer detection ranges; increase the number of detection opportunities per ping; allow higher ping-repetition rates; complicate the tactical situation for the submarine. Where more than one receiver makes a detection simultaneously (or nearly so),
allow improved performance in localisation, classification and tracking. Provide a tactically significant application for passive sonobuoys that would otherwise be of only marginally utility.


The multistatic concept was already formulated in the 1950s, but it experienced a revival in the 1980’s with the advances in digital signal processing and communications (either under or above water), which greatly facilitated the implementation of underwater sensor networks. In addition to the operational necessity, a revival in multistatics is generated by: COTS processing technology, an increasing variety in sonar solutions (VDS, HMS, dipping sonar, buoys and AUVs) and modern communication techniques.


Moore’s Law is bringing Increased processing power, so that sorting even weak signals from noise becomes faster and more reliable. It enables Networked processing power means that signals can be integrated over a wide area. Increased bandwidth makes moving volumes of data between sensors and processors faster. As well, the current trends in the development of unmanned platforms will continue
apace, and Moore’s law will see them increase in their ability to collect and process information, and they’ll be able to forward the information collected to a central hub.


Using small and relatively uncomplicated platforms to do that would allow them to be deployed in large numbers, which could greatly
complicate the job of submarine commanders. And not all of the sensors need to be broadcasting their presence. Multistatic detection
means that some platforms can continue to operate passively while others provide the noise source.


The rapid expansion of computing power… ushers in new sensors and methods that will make stealth and its  advantages increasingly difficult to maintain above and below the water.said ADM Greenert, Chief of Naval Operations for the USN. U.S. forces can take advantage of those developments by employing longrange sensor, weapon, and unmanned-vehicle payloads instead of using only stealth platforms and shorter-range systems to reach targets.


The networked Multistatic system requires many components such as Free-Flooded Ring (FFR) sound projectors that offer a large bandwidth (typically one octave) and high source level. Low Frequency Active Sonar (LFAS) sensors have emerged as other major potential components of near-future networked multistatic systems. Hull mounted sonar has been around for decades and certainly is part of the sonar systems to be considered for near future use in multistatic / networked scenarios. The networked sonar solutions will also include Autonomous Underwater Vehicles (AUVs). Communication links are vital to the success of the networked system.


But, for all its potential, multistatism is no panacea. The challenges are immense. All the sonar in use must be fully interoperable; they must be able to seamlessly exchange information for synchronisation and processing; and environmental conditions (like salinity and temperature) must be known over large areas if data is to be interpreted accurately.


NATO Undersea Research Centre, NURC’s approach to fusion and tracking employs contact data, for which transfer rates of ~100 kbps are required. Experience during the CERBERUS sea-trial learned that fading occurs for (UHF-type) communication as the range between nodes increases. Hence the need for Delay Tolerant Networks arises Multistatic sonar has many challenges including Signal processing, communication, synchronization, Tracking and fusion. Robust and effective tracking & fusion is vital for networked multistatics. Positioning errors, contact redundancy and the high number of false alarms are the major challenges.


Key to Future Submarine Warfare

NATO  hosted the Dynamic Manta 2020 anti-submarine warfare exercise, bringing together nine nations to combine ASW capabilities from ships, maritime patrol aircraft, helicopters and submarines, and improve their ability to work together to keep these southern European waters safe.


CMRE Director Catherine Warner said the organization has been working with autonomous vehicles in the undersea warfare area for the past 20 years to understand how they can contribute to perhaps the most complex type of naval warfare. “The big idea in this whole realm of unmanned systems is figuring out the right systems with the right sensors and the right scenario that’s going to be cost and operationally effective,” she told USNI News after the kickoff of Dynamic Manta. She said ASW is “high-end asset-intensive” and that, while unmanned vessels can’t do everything a manned sub or plane can, they can perform some specific missions that would be cost-prohibitive to do with manned vehicles.


One prime example is the passive acoustic barrier. Noting that CMRE puts passive sensors on all the autonomous vehicles, buoys and seabed devices the organization puts in the water, Warner said CMRE used all its sensors to demonstrate a passive acoustic barrier off the coast of Sicily in the days leading up to the start of Dynamic Manta. While in this demonstration they tracked the flow of commercial ships across the “barrier,” the ultimate idea would be to track the movement of submarines at chokepoints such as the Greenland-Iceland-United Kingdom (GIUK) Gap. The specifics of the unmanned vehicle wouldn’t matter as much as the quality of the sensor and the ability to differentiate the clutter from the sounds of submarines.


On the more active side of sub-hunting, CMRE has been particularly focused on the idea of multi-nation multistatic ASW, where an active sonar source would create pings for dozens or hundreds of passive sensors listening for those sound waves to bounce off of enemy submarines. The more sensors that are in the water, the better they can detect pings and recognize what kind of submarine is moving through the water and in what direction.

NATO Centre for Maritime Research and Experimentation (CMRE) operates two Ocean Explorer autonomous unmanned vehicles named Harpo and Groucho. NATO CMRE photo.


During Dynamic Manta, CMRE operated alongside manned warships to join in the hunt for submarines, using its “network”: NATO research vessel NRV Alliance, two Ocean Explorer 21-inch diameter autonomous underwater vehicles named Harpo and Groucho, and a fleet of Liquid Robotics’ Wave Gliders that serve as communication nodes between the ship and the AUVs. Harpo and Groucho have a towed array to listen for pings, and more recently CMRE developed a towed array for the Wave Gliders as well to put more ears in the water.


Having that extra set of sensors makes a huge difference” in multistatic ASW, Warner said, because when an active sonar source like the variable depth sonar on Alliance or a warship like Italian frigate ITS Carabiniere (F 581) sends out energy, they want as many passive sensors in the water as possible to listen for pings. “When you do multistatic, there’s so many more advantages because of the geometry and the extra chances for reflections. So we can do it with ourselves, but if we could do it with all the nations – and that is something that we strive to do with our interaction with the nations … – then everybody, wherever they are, that has a sensor, being able to know the sound source and sync to it and coordinate on the reflections – it is very power to be able to do that.”

The key to multi-nation multistatic ASW is information-sharing: they’d all have to know where exactly the active sonar source is, so they could correctly calculate what the pings they pick up mean, and then they’d have to share what they’re hearing with all the other nations involved, too, so they could all adjust their positions as needed to get the best chance at hearing the target submarine and help track it through the water. Information-sharing can be a hurdle with something as sensitive as ASW, with nations often not wanting others to know the exact nature of their capabilities, but Warner said the scale to which NATO could track submarines under the water would be powerful if everyone could find a way to come together.


Today, Harpo and Groucho talk to each other while looking for subs, and if one picks up a sound they will coordinate amongst themselves to get into the best positions for the best geometries to hear sonar pings. The more AUVS in the water collaborating, the better. “We’ve done it. We’ve already shown that multistatic ASW works. That’s our system: we’ve been doing it since 2012 in Dynamic Manta, we’ve demonstrated it operationally, and we just keep adding things onto it. So it can be done. So, whether other nations want to do it with us, that’s up to them,” Warner said.


Warner said Harpo and Groucho are 21-inch diameter AUVs that were built by Florida Atlantic University. The vehicles themselves are 18 years old, but the batteries and sensors are constantly being upgraded, meaning the vehicle that originally had four hours of battery life can now operate for 72 hours without intervention.


CMRE’s Dan Hutt told USNI News that the next step would be to scale up these operations. To conduct multistatic ASW in the GIUK Gap, for example, would require hundreds of AUVs from participating NATO nations. The idea, though, would be to “flood the ocean with lots of cheap assets – they all have sensors, potentially different kinds of sensors, they can all talk to each other over a vast network – that’s a really powerful concept for ASW. We only have a handful of these, so we want to scale up and work with the nations to do a bigger demonstration.”


While several NATO countries are upgrading their fleets of “high-end submarines and frigates,” many cannot afford such exquisite systems, Warner said. “But they certainly can afford a fleet of unmanned vehicles with towed arrays. And if they were all using the same standard, they could all buy from their own countries’ industry – that’s what we’re about, we’re not competing with industry, we’re developing standards,” she continued. “Every nation’s industry would benefit from building these vehicles and the towed arrays, and then they could all operate together.”


CMRE has already done a machine learning effort to support the back end of this effort – researchers collected 52 days worth of sonar echoes from diesel-electric submarines (SSKs) and created algorithms to help the unmanned vehicles recognize SSK sounds and ignore the clutter. This could be shared with the NATO members who want to join in this effort. Warner said Norway, Belgium and the Netherlands are taking steps to incorporate AUVs into their ASW efforts, but she’s hoping to see more.


A final technology CMRE is showing off at Dynamic Manta is an undersea communication network. NATO nations had previously agreed to use the JANUS as the digital underwater communications standard, but CMRE is still hard at work developing waveforms that will be cyber-secure and low-probability of intercept, as well as developing concepts of operations for its usage.


Ahead of Dynamic Manta, CMRE demonstrated they could use JANUS to send submarines the surface picture with Automatic Identification System (AIS) tracks – so the submarines could know how to safely surface – by sending the message from a ship, through the Wave Gliders as comms nodes, and to the submarine underwater.


Warner said they call this setup “WetsApp” – a nod to the WhatsApp digital communication app on cellphones – and said it’s a vast improvement over the voice communication tools they previously used to send messages to submarines, which could easily get garbled or lost altogether.


“Before, when they were submerged, submarines could only use something called an underwater telephone, which is very difficult to use, it’s distorted, hard to understand,” she said. “But we can actually text them – we have a little program, we call it WetsApp, sort of WhatsApp, and we can send them for example the surface picture – if they were going to come to the surface, they would know where all the ships are on the surface. So that’s very important technology that we’ve already helped insert into the industrial base.”


Airborne Multistatic Anti-Submarine Warfare

Multistatic sonar technologies are emerging as an important strategy for ASW. Multistatic operations can potentially improve ASW effectiveness in challenging shallow-water environments while allowing critical assets to remain acoustically covert or at a safe standoff distance. The Navy uses multistatic acoustic sensor systems with aircraft and surface ships, in network-centric ASW surveillance missions.


But, for all its potential, multistatism is no panacea. The challenges are immense. All the sonar in use must be fully interoperable; they must be able to seamlessly exchange information for synchronisation and processing; and environmental conditions (like salinity and temperature) must be known over large areas if data is to be interpreted accurately.

anti-submarine warfare (ASW) sonobuoys multistatic | Military & Aerospace Electronics

In the face of this, sonobuoys can offer an advantage: fixed in position and able to transmit information to a single aircraft, they can, among other things, easily solve synchronisation issues. It’s perhaps no surprise that, with increasing submarine activity and advancing technology, navies are taking a keen interest in sonobuoy multistatism.


Multistatic sonobuoy fields  for air ASW search mission are becoming more complex. The ability to utilize more sources, more receivers, and the resulting higher transmission rates provide an influx of information. As a result, the detection capabilities and data rate for contact reports (automatic detections produced by the received signal processing) is increasing dramatically. It is no longer practical for a sonar operator to be able to sift through detections one-by-one to find the target.


Techniques and tools which consolidate information from the contact reports and provide the operator with the capability to rapidly find and focus on target detections are sought. Typical active contact reports provide time difference of arrival (TDOA), bearing, signal-to-noise ratio (SNR), and, Doppler , depending on the waveform type. Geographical locations based on these measurements and the positions of the sonobuoys are also typically displayed to the operator, along with a target probability surface  based on Bayesian inference from the observations. Innovations in graphical display of data that ease the operator’s workload, ranking of contacts to bring target detections to the forefront, and automatic suppression of clutter are potential topics of interest.


Focus Areas / Elements of Consideration:

Information Superiority; the ability to gather, process, integrate, disseminate, and display information together with a corresponding increase in the ability to use that information.

Data Fusion; combining track information from a variety of sources into a single best picture of the tactical operational area.
Situational Awareness / Assessment; continually monitoring the dynamic picture for impacts to the plan (recognizing potential limitations in the mission plan).

Mission Planning; recommending updates / changes to the operational plan to ensure the highest probability of detection is possible.
Execution Aides, assisting the crew in executing the operational plan / mission.

Automation, how the information is provided within the elements of the decision-making. The Tactical Decision Aides (TDA) should be prosthesis, adding additional capabilities to the operators, or simply as a tool available to the operators. The TDA shall assist, or replace, the operator when the situation causes an excessive workload that cannot be managed by the human capabilities. These approaches are not mutually exclusive, but complementary, depending on situation context, the specific nature of the TDA element and the operator’s role.


Coherent Multi-Static Active Sonar SBIR

Current incoherent systems suffer from high false alarm rates when operating in shallow water. A method of reducing false alarms and not unnecessarily alerting our adversaries is needed. For future multistatic systems, coherent active sonar techniques are being investigated, because of the potential to extract motion target classification clues. Exploiting Doppler in multistatic systems is challenging because of target scattering effects. Increasing the duty cycle results in many potential echoes from the target, and the task of combining these target echoes in the presence of is critical. This project will develop a signal, information and data fusion processing software prototype that will extract Doppler and motion features contained in coherent waveforms. We will use a Likelihood Ratio Tracking track-before-detect approach to optimally combine multiple echoes, in order to form consistent tracks over space and time that discriminate between targets and clutter.


Target Localization Using Multi-Static Sonar with Drifting Sonobuoys

Quick and accurate sonobuoy location is becoming more critical in the airborne ASW problem as multi-static search systems and data fusion techniques play more dominant roles. Sonobuoy locations must be determined quickly, and be constantly updated in order to estimate target locations as soon as target detection clues are available to the acoustic subsystem. Obsolete methods of buoy location such as “Mark-on-Top”, and systems based on the assumption that the target will be held in contact for significant periods, will not work.


In Phase I, RDA proposes two techniques that can be used to locate sonobuoys by using only acoustic data from the drifting active and passive buoys, without the use of other data. These techniques can be used to locate both source and receiver sonobuoys. One method relies on the received incoherent Direct Blast / coherent ping, and one method uses internal buoy measurements. These techniques can be combined for more comprehensive and smaller positional error measurements.


Develop improvements for active sonar search detection, classification, and localization performance by using or adding non-acoustic sensors to sonobuoys.

Air ASW multistatic active sonar detection, classification, and localization (DCL) performance relies on advanced processing algorithms to exploit transmitted and received sonobuoy signals. The uncertainty surrounding these signals place fundamental limits on system performance and mission success.


The Navy seeks to upgrade or add non-acoustic sensing hardware to sonobuoys which will measurably improve DCL or tracking performance for active sonar (threshold 10% improvement over a sonobuoy without the capability, objective 25% improvement), particularly for scenarios where GPS is not available. An ideal solution will be low cost (adding less than $50.00 to the cost of a production sonobuoy), fit within the existing sonobuoy size (i.e., cylinder of diameter 4 7/8 inches, length 36 inches), weight (i.e., not cause a sonobuoy to exceed a maximum of 39 lbs) and power (SWaP) constraints (ideally a sensor requiring less than 12 volts and 25 milliamps), and be capable of improving several performance metrics.


Proposed solutions should identify the sonobuoy(s) to be upgraded, the performance metrics expected to benefit from the proposed sensor hardware improvements, and quantify the expected improvement through simulation and/or experiments. Sonobuoy improvements may consider adding transducers and/or replacing existing ones. Examples of such include, but are not limited to, buoy localization performance could potentially be improved by adding/replacing sensors to increase accuracy of time-of-flight and/or bearing measurements. Temperature and/or salinity sensors could be added to provide a partial sound speed profile for individual buoys. Sensors such as inertial measurement units (IMU), gyroscopes, and accelerometers could be used for motion compensation.



Signal Systems to develop enabling technologies for continuous active sonar signal processing, multi-static sonar, and spread-spectrum techniques.

Officials of the The Naval Air Warfare Center Aircraft Division in Lakehurst, N.J., announced a $13.5 million order to Signal Systems this week to develop new enabling technologies for difficult ASW problems. Signal Systems experts will move forward with three Small Business Innovation Research (SBIR) projects called Continuous Active Sonar Signal Processing; Target Localization Using Multi-Static Sonar with Drifting Sonobuoys; and Spread Spectrum Techniques for Sonar Ping Technology. Continuous active sonar signal processing involves relatively low-power active sonar to detect enemy submarines, rather than a relatively high-power pulse-type sonar approach, which can be harmful to marine mammals like whales.


Instead, Signal Systems experts are investigating low-power continuous-wave sonar approaches with the same ASW detection and tracking capability as higher-power systems. Continuous-wave sonar also may enable continuous tracking for accurate localization of submarines. Continuous-wave sonar also has the potential to exploit Doppler effects more completely than pulse-type sonar, Navy officials say. Signal Systems’s approach emphasizes sonar signal processing for continuous active sonar to help the Navy address the risks of self-jamming, blind speed, and tracking issues.


The company also is developing waveforms, direct-blast cancellation, space-time adaptive, and track-before-detect algorithms for continuous-wave sonar operation. Airborne target localization using multi-static sonar with drifting sonobuoys seeks to develop technologies for quick and accurate sonobuoy location for future multi-static search systems and data fusion techniques. Obsolete methods of buoy location based assuming the target will be held in contact for significant periods, will not work, Navy researchers point out.


Signal Systems engineers are developing techniques to locate source and receiver sonobuoys by using only acoustic data from drifting active and passive buoys, without the use of other data. Spread spectrum techniques for sonar ping technology seeks to reduce peak power to improve detection performance while reducing the power requirements and the impact on marine life, as well as improve active sonar detection performance by improving time-bandwidth gains received signal to noise ratio.


This order calls for Signal Systems to move forward with developing acoustic ASW sensors and systems; telemetry and recording systems; signal and data processing; algorithm development; mathematical modeling; system and application prototyping; active and passive display enhancements; information assurance, anti-tampering and cyber security concepts; and techniques and analysis to predict the performance of ASW systems.



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