Listening to the ocean – the secretive enablers in the underwater battle
Critical in the ongoing battle to detect hostile submarines is a little-known network of ocean sensors that support the more visible deployment of frigates and maritime patrol aircraft. Here we examine the history and development of this network, a key to UK maritime power.
The Sound Surveillance System (SOSUS) codenamed Project Caesar, was began in 1954 as a classified US programme to use an extensive network of hydrophones laid on the seabed to track Soviet submarines. The technology was successfully refined and gave NATO a great advantage over their underwater adversaries throughout the Cold War. The UK has been fortunate to have involvement and access to this project since the early days. SOSUS had been built under the cover of civilian oceanographic research and was not made public until 1991. The Soviets were largely unaware of its importance until its existence and scale was revealed to them in the intelligence passed on to them by the Walker spy ring in the 1970 and 80s. Soviet submarines were notoriously noisy and easy to detect but, partly on learning of the passive detection capabilities of SOSUS, they began to build quieter submarines. In general US and RN submarines were considerably more stealthy but Russia has now closed that gap, its newest submarines are comparable to NATO designs in terms of stealth.
SOSUS comprised fixed, passive linear hydrophone arrays for long range detection of the noise radiated by submarines. In simple terms, the noises from the machinery and the cavitation effects of a submarine propellor can potentially be detected hundreds of miles away because seawater is a very good conductor of sound energy. Using hydrophones at dispersed locations it is possible to triangulate and locate the source of the noise to a precise point in the ocean. The arrays were laid at strategic points around the Atlantic and Pacific and relayed information to shore stations via undersea cables. The shore stations were linked by satellite and phone lines. At its Cold War peak, SOSUS employed around 4,000 personnel working at 20 shore stations. In 1974 a SOSUS station was constructed at RAF Brawdy in Wales and by 1980 over 300 personnel were stationed there, analysing acoustic data gathered from arrays laid around the British Isles.
In October 1995, NAVFAC Bawdy was closed and its functions moved to the Joint Maritime Facility at RAF St. Mawgan in Cornwall. In 2009 St Mawgan was closed and the combined USN and British operation was moved to Navy Operational Processing Facility (NOPF) at Dam Neck in Virginia. Data collected from ocean sensors across the Atlantic is now processed at this single facility before the intelligence is passed onto the frontline. (There is a parallel facility that serves the Pacific region at NOPF Whidbey Island in Washington State). It must be assumed that the analysis and submarine tracking information gathered here is passed on to the UK Joint Headquarters at Northwood to the RN Commander Maritime Operations (COMOPS) where it is used to cue submarines and warships to their targets.
By the late 1980s, SOSUS had evolved to become just a part of what is now known as Integrated Undersea Surveillance System (IUSS). Purpose built towed array sonar ships were integrated into the system in the form of the Surveillance Towed Array Sensor System (SURTASS) ships. Designed to be quiet, stable in all weathers and able to track targets at long range from the optimum location, their data is transmitted back to IUSS land stations by satellite. Unlike the SOSUS seabed arrays, they also incorporate Low-Frequency Active (LFA 100-500hz band) transducers that transmit energy into the water. If reflected back off the target, the sound is detected by the long passive arrays trailed behind the ship. The RN does not have the luxury of single-role dedicated towed array platforms but 8 of the 13 Type 23 frigates carry the renowned Type 2087 system. RN submarines also deploy towed arrays which must be attached to the submarine by a support vessel before leaving for a patrol. Following the “Asia pivot”, SURTASS vessels now operate almost exclusively around the Chinese coast and Western Pacific but the USN is in the process of fitting all its destroyers and cruisers with a new TB-37/U Multi-Function Towed Array (MFTA) sonar system.
During the Cold War, the Warsaw pact had deployed their own towed array platforms, although it is likely they were not as effective as western equivalents. During ‘Operation Barmaid‘ in August 1982, HMS Conqueror was fitted with special pincers and undetected, managed to cut and steal an array belonging to a Polish AGI to be taken to the US for analysis.
The behaviour of sound waves in water varies enormously depending on conditions such as depth, currents, salinity and temperature layers. These variables will affect if, when and where submarines may be detected. The Royal Navy’s hydrographic ocean survey ship HMS Scott does not just collect data for creating charts but contributes oceanographic information for both submarine operations and anti-submarine warfare. By understanding the composition of the water column it assists the deterrent submarines in knowing where they may be safest from detection. For the submarine hunter, understanding the composition of the ocean helps them predict how their sonars will perform. As submarines have become quieter, ASW has had to move back to a greater reliance on active sonar. Active sonar gives a more precise fix on the location of the target but has the disadvantage of immediately alerting the submarine that it is being tracked.
China and Russia spur renewed US ASW developments
The US is now in the process of making the biggest upgrade to the IUSS since the Cold War. The key component is the Deep Reliable Acoustic Path Exploitation System (DRAPES). This system will be far less reliant on potentially vulnerable seabed cables and utilises acoustic modems that pass data through the water, allowing the creation of something like an undersea wireless network. Wireless underwater communications have been available for some time but only at relatively low bandwidth and short range. Recent breakthroughs make it possible to scale this up and transmit much greater volumes of data further. Acoustic sensor data can be transmitted long distances through a series of nodes which may include other hydrophone arrays, Unmanned Underwater Vehicles (UUVs), Unmanned Surface Vehicles (USVs) or a surface buoy. Data is then either sent via satellite back to Dam Neck for evaluation or to nearby surface ships and MPAs. The USN is also experimenting with the ASW Continuous Trail Unmanned Vessel (ACTUV). This long endurance craft can deploy sonar specifically designed to detect and trail very quiet conventional submarines and will be another node on the network that feeds data via satellite to IUSS.
The USN has already proved this concept with its Seaweb system but designed for use in shallow littoral water less than 300 meters deep. DRAPES will be on a vastly bigger scale, and able to span the deep ocean. Reliable Acoustic Path Vertical Line Arrays (RAPVLAs), bottom-mounted, high-grain sensor systems, will be laid at significant depths in the open ocean where background noise levels are low. This gives them a very large field of view to detect submarines passing overhead. These are a maritime equivalent of a satellite and are known as subullites. The Reliable Acoustic Path (RAP) refers to the dense and quieter waters in the deeper parts of the ocean where sound transmission is more detectable and predictable.
Although submarines are getting even quieter and there is more man-made background noise in the shallower parts of the oceans than ever, IUSS has the advantage of the enormous computer processing power available today. Huge volumes of sensor data can be quickly analysed by computers to filter the background noise and amplify even the very faint telltale sound of the submarine.
The location of an explosion that points to the loss of the missing Argentine submarine ARA San Juan was established using ocean hydrophone arrays. The official story is that the source of the location was data gathered by from hydrophones belonging to the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). It is possible that this is a cover story for a more accurate fix provided by the more sophisticated and extensive IUSS network, although its coverage of the South Atlantic is less than that of the North.
Recent reports of increased Russian submarine activity in the waters around Scotland and the GIUK (Greenland-Iceland-UK) gap probably stem from initial detections by IUSS sensors. The media has suggested we “rely on Scottish fishermen reporting when they see a periscope” but of course the initial detections probably come from seabed hydrophone arrays or towed arrays trailed by frigates and submarines. NATO Maritime Patrol aircraft flying from RAF Lossiemouth or Keflavik Air Base in Iceland are unlikely to find a submarine by chance and must also rely heavily on this data to cue them to the approximate area before they can localise submarine contacts with sonobuoys.
The RN’s submarine force appears to have recovered slightly from the material defects that limited its operations in the last 18 months or so and the service is now described as “busy”. We can assume SSNs are much in demand to trail Russian submarines in the North Atlantic. Observing the recent increase in US Navy Virginia and Los Angeles class SSNs visiting Faslane also confirms this. IUSS, surface units and MPAs all contribute but a submarine is by far the best platform to detect another submarine and then keep on its tail.
This underwater battle of wits and technology has varied in intensity since submarine warfare began in earnest in World War I. Britain has twice been close to the brink of starvation and defeat as its lifeblood of merchant shipping was almost cut off by submarines. Today we are arguably more vulnerable to this threat than ever. The giant modern container ships that deliver goods to the UK may have cargoes valued in millions of pounds and transport the equivalent of a 50-ship World War II convoy. We are also reliant on a steady stream of tankers delivering LNG from the Middle East to keep many of our power stations going. Even one or two well-handled submarines could easily disrupt this shipping and quickly cause chaos and economic paralysis to the UK. For the RN to conduct carrier strike and other offensive naval operations, a prerequisite will be having the upper hand in the undersea battle. IUSS is a critical and little-known part of this fight, every penny invested in equipment, training and development of anti-submarine measures is money well spent.
- US Navy Invests in Subsea Threat Detection Array (Maritime Executive)
- Surveillance Towed-Array Sensor System (Federation of American Scientists)
- USNS Zeus – Cable Repair Ship (Federation of American Scientists)