Royal Navy submarines and non-acoustic sensor technology

The arrival of HMS Talent in Gibraltar in February fitted with additional sensors on her fin has raised the public awareness of non-acoustic submarine detection methods. Sonar remains the primary means of locating submarines but here we examine what is known about other technologies that may be used in the undersea battle to detect and trail adversary boats.

In ‘conventional’ anti-submarine warfare submarines are usually located acoustically, either via active sonar that requires the hunter to send out a sound wave, which hits the target and bounces back, or passive sonar that simply listens for noise radiated by the target. As sound waves can travel very long distances through water it is the primary means by which submerged submarines understand their environment and hunt their prey. Sonar has its limitations and other passive methods of detecting submarines would be extremely useful. Wake detection systems may rely on finding traces of either heat, radiation, chemicals or bubbles left by the submarine as it passes through water as well as disturbance to the water itself.  These faint signatures may be left behind for some hours after the submarine has passed by and offers a means to trail boats that may be used to supplement acoustic methods. With sophisticated analysis of the data generated by the sensors it may also be possible to determine the direction of travel, speed and distance travelled by the boat.

Any articles in the public domain on this subject must rely on non-classified sources and information is understandably limited. Wake-detection and similar technologies for civilian use able to sense exceptionally small amounts of impurities in water is a highly technical subject and the science that underpins it continues to advance.

The Red’s SOKS

The Soviet navy appears to have been the first to develop wake detection systems. The Systema Obnaruzhenya Kilvaternogo Sleda (SOKS) was developed in the late 1960s. During the later part of the Cold War it became known that Soviet submarines had managed to trail NATO boats undetected on a few occasions. The Russians confirmed this publicly in the 1980s, claiming that the Victor class boat, K-147 had followed an American SSBN (probably USS Simon Bolivar) for 6 days. Western sonars and submarine silencing capability was known to be considerably ahead of the Russians and NATO boats routinely tracked them for long periods using passive sonar only. Initially, analysts considered it almost impossible the Russians had managed this and were at a loss to understand how this might have been achieved.

An investigation by the CIA concluded by the early 1970s that wake-detection technology had been used. The Soviets had succeeded in creating a variety of sensors that could be used operationally, given the right circumstances. The USN had explored similar technology in the 1960s but had abandoned it, putting all its effort into improving sonar capabilities where it enjoyed technical superiority and the huge advantage provided by the SOSUS seabed array system.

Russia has continued an iterative development process for SOKS with systems code-named by NATO including Kaira (1978), Bullfinch-2 (1979), Toucan-1 (1981) and Ear (1982). The investment in this technology over many years suggests it has matured to be a reliable and useful system and its use is part of Russian operational doctrine. Their modern Akula and Yassen class SSNs are equipped with the latest versions.

Royal Navy not out-done

Surprisingly, there is more is in the public domain about Soviet and Russian systems than about UK wake-detection technology. What is certain is that RN submarines have received various Non-Acoustic Sensors (NAS) for trials and testing going back at least three decades. Whether used operationally across the fleet or on a temporary experimental basis is unclear. The UK has a reputation for excellence in sonar research and manufacture. The highly sophisticated Sonar 2076 suite fitted to the Trafalgar and Astute class is reputedly one of the best in the world but the RN still considered it prudent to pursue other detection technologies.

HMS Trafalgar was fitted with NAS sometime around 2004-5 and at least part of the equipment remained mounted until she decommissioned in 2009. Around the same time, HMS Torbay also received additional sensors mounted the port side of her fin (but this appears to have been removed by about 2010). During this period, Torbay also sported an experimental navy blue paint scheme intended to reduce the visibility of the boat at shallow depths when operating in the blue waters of the Mediterranean. Torbay was eventually repainted in the traditional pusser’s ‘North Atlantic dark grey’ but HMS Trenchant emerged from a very long refit in 2017 with the dark blue paint scheme.

Until the reappearance of the probes fitted to HMS Talent in 2019, this is all that can be said with certainty about modifications to the Trafalgar class boats through observing visible external additions. Other boats may have had less obvious equipment fits, either retractable probes or housed within the casing, the fin or mounted below the waterline. It is possible that the Astute-class boats were designed to carry NAS integrated from the outset and the sensors are hidden from view, although this is only speculation.

Hunting without sound

Details of how the non-acoustic sensors work are sketchy but in broad terms, infrared or laser light is passed through the water being sampled by the probes. The absorption or refraction of light by tiny particles present in the water can be measured. The science of spectroscopy enables the detection of very low concentrations that may only be a few parts per billion. Small changes in salinity, temperature and density may also be measured. The sensors may take thousands of samples per second which require computers to process the stream of data to distinguish between naturally occurring phenomenon and the distinct signatures left behind by a submarine.

The wake of a submerged submarine naturally spreads out over time which offers a sporting chance of locating it in the first place. The sensors on a single submarine are relatively close together and likely to distinguish only between actually being in or out of the wake. The needs for multiple sensors arranged it different places on the hull is to confirm the wake is being detected and it not just one probe getting a false reading. The different shaped probes are likely optimised to sense different properties of the wake. Just as sonar propagation is affected by environmental conditions in different parts of the ocean, non-acoustic sensors may also be hampered by factors such as salinity, pollutants, strong currents and water disturbed by other vessels, particularly in shallow and busy littoral waters.

Radiation. Nuclear submarines have many tactical advantages over conventional boats but emit a very faint but distinctive radioactive signature. Radionuclides are a by-product of the nuclear fission process and other radioactive elements will be left behind by an SSN or SSBN.

Heat. Nuclear submarines use large quantities of seawater to condense the superheated steam and cool the reactor. It may be possible to detect the trail of slightly warmer water pumped out by the submarine for some hours after it has passed by, although the effectiveness of this method could vary greatly depending on the ambient sea temperature.

Both radiation and heat detection methods are not applicable to conventional submarines. Russia’s newer Kilo and Lada class boats (Project 636, 636М and 677) are exceptionally quiet SSKs,  hard to detect with passive sonar and are known to be active in the Eastern Mediterranean. The RN’s non-acoustic sensor outfit would be particularly useful against these targets so radiation and heat detection is unlikely to be of prime importance.

Chemicals. A vessel passing through water will leave a distinct chemical trail behind it. This may include tiny particles of paint, rust and zinc from the sacrificial anodes fitted to reduce corrosion. Hydrogen is a by-product of the system used to make oxygen for the submarine’s crew to breathe and is detectable when dissolved in seawater. Some of this chemical trail may be reduced by careful design and use of materials but it is almost impossible to eliminate entirely.

Bubbles and waves. All moving submarines leave a trail of small bubbles behind them. Great efforts have been devoted to reducing the cavitation effects of submarine propellers through careful design, high precision manufacturing and polishing. The rapid changes of pressure in the water around the rotating propeller or propulsor lead to the formation of small vapour-filled bubbles at the blade tip which then collapse, creating unwanted noise. Shrouded propulsors with large, slow-turning rotors have helped reduce cavitation and radiated noise but bubbles will still be formed, especially at higher speeds. Tiny bubbles remain present in the water column for some time. It may also be possible to detect the movement of the water disturbed by the passage of the submarine. The waves and eddies may linger in the water column and the tiny variations in pressure may be detectable long after the submarine has gone. Theses signature are likely the most important method of non-acoustic submarine detection.

Talent gets toys

Sometime in 2019, HMS Talent was fitted with a wake detection system on the forward part of the casing. The visible mounting consists of 4 probes. This was further supplemented with another two identical mountings fitted on either side of the upper fin in 2020. The reappearance of NAS on a Trafalgar class boat suggests that the technology is continuing to improve, investment is ongoing and its tactical value is worth pursuing. Open-source information from the civilian industry indicates that similar particle-in-water sensors are becoming more accurate and compact. Perhaps of greater significance are the advances in artificial intelligence and machine learning. Computers can be ‘trained’ to search for very specific patterns buried in huge amounts of data that are the unique tell-tales indicating the presence of a submarine, while eliminating false positives. Not only is vastly more computing power available relatively cheaply, but these new processing techniques can quickly make sense of the data stream from the sensors, and provide the submarine commander with more reliable guidance for trailing a submerged submarine.

If a submarine cannot be detected by passive means, resort to active sonar is usually undesirable. Going active will immediately alert the prey that they have been detected. Active sonar provides bearing and range data but also reveals the same information to the target, potentially allowing its adversary to achieve a fire-control solution, should it wish to launch a torpedo. NAS are far from a panacea that will allow all submarines to be instantly detected but is another tool that may be integrated with the conventional sonar picture. NAS could be used to confirm fleeting contacts from passive sonar or localise a target when cued into the general area by intelligence from other sources. Above all, NAS appears to be ideal for trailing a very quiet SSK at medium or short range. Faced with a proliferating conventional submarine threat, the RN’s small SSN force needs every possible sensing advantage that can get, although well trained, experienced and canny submariners are still the most important element of success.

As the penultimate Trafalgar class boat, HMS Talent was originally scheduled to decommission this year but the lives of the three remaining T-boats have been extended by at least 2-3 years. HMS Trenchant was due to go in 2019 but is still going strong. Delays to the Astute class service entering service have left little choice but to invest further in maintaining and updating the venerable T-boats, that despite their age, are such an effective platform that they can still hold their own in the 2020s.

The NAS discussed in this article are carried by submarines to detect other submarines but there is a whole other branch of scientific investigation dedicated to finding submerged targets from the air or by satellite using non-acoustic means. Despite many decades of supposed scientific breakthroughs that claim to “make the seas transparent” using radar, magnetism, lasers or optical detection methods, the submariner community remains highly sceptical as none have been yet been fully proven to work operationally. Wake detection has been around for some decades and there are operational torpedoes that use this guidance method (The larger bubbles and more obvious wake of a surface ship is much easier to track than the discrete signature of a submarine). NAS clearly continue to have significant and improving utility in the undersea battle and it is interesting to speculate about how other nations besides Russia and the UK may also have developed this technology.

(Main image: Moses Anhory, via Flickr)