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In recent years, the proliferation of quiet, conventionally and air independent propulsion submarines and technological developments in acoustics electronics have pushed manufacturers to develop Low-Frequency Towed Active Sonar (LFAS). This also applies to environmental tailored higher-frequency equipment as the sensor of choice for anti-submarine warfare (ASW), shrinking their weight and quarterdeck footprint of their associated launch-and-recovery systems to widen the portfolio of suitable platforms.


Turkish industry is working on a-low frequency active sonar under the DÜFAS programme. The programme was launched in August 2018, with the research and development (R&D) contract awarded by the Presidency of Defense Industries (SSB) to the industrial team led by Aselsan group, including Armelsan and Nanotech. Its aim is to develop piezoelectric mono-crystalline ceramic materials and transducers. The programme is reported to be centred on the development and integration of active elements into a towed sonar system based on the torpedo detection towed array element of the Aselsan HIZIR torpedo countermeasures system. The towed element includes a passive array and a decoy, the latter being deleted in favour of the active elements in order to provide a complete active/passive capability in a single towed system. Trials with prototypes are reported to begin in 2022 from a Turkish Navy platform. Aselsan has also developed and now promotes the HIZIR/LFAS long-range low-frequency towed active sonar suite including a transducer array with low-frequency active transmission capability which is deployed, recovered and stowed with a dedicated winch system, operating in combination with the separate HIZIR torpedo detection passive (receive) array in a bi-static mode.

Atlas Elektronik

Derived from the LFTAS system prototype extensively tested by the German MoD, the Atlas Elektronik Active Towed Array Sonar (ACTAS) system is a low-frequency ASW sonar system that operates simultaneously in active and passive modes and provides high-resolution target detection, according to the manufacturer. The ACTAS is centred on a towed body – housing the vertical transmitter (active) array (1.4-2.4 kHz) providing full 360° range coverage – with twin depressors for stabilisation, and associated passive reception array with triplet array to instantaneously resolve left/right bearing ambiguity. The launch and recovery equipment, associated power system and ancillaries have a total (inboard and outboard) 35 m2, while the complete system weighs 20 tonnes (or less, depending on the configuration).

Designed to detect, track and classify submarines, torpedoes and surface vessels, including speedboats, and currently also available with the SONIX sonobuoy processing, ACTAS features over-the-horizon surveillance capabilities at ranges considerably above 60 km (depending on propagation conditions), according to Atlas Elektronik. Capable of bi/multi-static operations with dipping sonar, sonobuoys or submarines, the ACTAS is already operational with the Indian and Royal Thai Navies, while also being considered by other undisclosed customers. Based on a supply contract awarded in 2013 by Daewoo Shipbuilding & Marine Engineering (DSME), the Royal Thai Navy BHUMIBOL ADULYADEJ class DW-3000F frigate features an ASW suite based on an Atlas Elektronik ASO series Hull Mounted Sonar (HMS) and ACTAS system. In 2014, the Indian MoD procured the latter equipment in an initial lot of six ship sets, which are to equip TALWAR class frigates and DELHI class destroyers. The system was also identified on at least one SHIVALIK class frigate, opening the way to potential additional procurement. The German Navy has declared a need for two containerised ASW mission modules equipped with both active and passive elements on a single towed array for installation on the new F126 frigates but no information has been released on the requirements and supplier.

DSIT Solutions

The Israeli company offers a cost-effective ASW suite centred on a Mission Control System (MCS), the BLACKFISH hull or bow mounted sonar and the SWORDFISH low-frequency towed array sonar. The latter is designed to operate in littoral and deep-water ASW operations with the medium-frequency BLACKFISH system, already sold to both NATO and non-NATO countries. Capable of underwater target search, detection, tracking and classification in passive, active and parallel modes, these sonar systems include machine learning technologies for automation of algorithms and reduction of operator workload. Suitable for surface vessels of various sizes, including small vessels such as fast patrol craft (FPC), the SWORDFISH comes in a modular configuration (MTAS) using medium-low frequencies, where the control system is housed in a container on board or on shore.

Elbit Systems

The company announced in September 2021 that it had been awarded different contracts for a total of approximately US$56M to supply ASW capabilities to a naval force of an undisclosed Asia-Pacific region customer. Under these contracts, Elbit Systems provides SEAGULL unmanned surface vessels configured to perform ASW missions equipped with L3Harris HELRAS sonar, together with the company’s autonomous combat and communications suite, alongside the Towed Reelable Active Passive Sonar (TRAPS) system to be installed on board the naval force’s corvettes. Developed by Elbit Systems’ Canadian subsidiary GeoSpectrum and designed to meet the requirement for a compact, lightweight and low-cost active/passive VDS that can be accommodated on smaller ships, the TRAPS’ unique advantage is that it employs an active sound source that uses a single-tow vertical projector array (VPA), which is stowed on a single winch drum together with the receiver array and tow cable.

With a Curtiss-Wright Defense Solutions-provided winch of 1.8×2.3×2 m footprint, which can accommodate a maximum 650 m towed cable for a total weight of 1,200 kg, in addition to 250 kg for the operator console and the power amplifier, the TRAPS has a maximum operating speed of 17+ knots and a survival speed of 25 knots, although there are plans to upgrade this to 22 knots and in excess of 30 knots. Minimum and maximum operating depths are 10 and 150 m respectively. GeoSpectrum also offers a lighter variant called TRAPS-USV whose overall system weight is reduced by approximately 40 per cent and can be installed on manned and unmanned platforms as small as 12 m long.


In September 2020, L3Harris announced it had been awarded a contract to deliver two Low-Frequency Active Towed Sonar (LFTAS) systems to an undisclosed NATO member. Using proven technology derived from the HELRAS helicopter DS-100 dipping sonar, the LFTAS VDS-100 system is a small, lightweight, air-transportable, ruggedised system which has been successfully deployed on ships as small as 100 tonnes. It features a unique extension/retraction mechanism which transforms a compact tow-body configuration to a large-aperture multidirectional transmitter, to which a two parallel receiving towed arrays package is attached. The latter configuration operates at 1.38 kHz in active mode, together with advanced signal processing, which achieves instantaneous, unambiguous left/right target discrimination, providing full 360° coverage.

With an overall weight of just over 6.5 tonnes, including winch & handling, towed subsystem and electronics, the system can operate at depths between 15 and 300 m, and thanks to a robust infrastructure, it is interoperable with the HELRAS helicopter dipping sonar and all key sonobuoys, thereby ensuring tactical bi-static and multi-static capability. L3Harris also offers the Model 980 ALOFTS (Advanced Low-Frequency Towed Sonar), which combines a high-powered active source in a variable depth towed body with a directional towed array in order to receive active and passive signals. The system is in service with the Republic of Singapore Navy’s FORMIDABLE class multi-mission frigates and the Israeli Navy on undisclosed platforms, reported to be the SA’AR 5 class corvettes.


Based on the Italian Navy’s requirements for an ASW suite to be installed on the THAON DI REVEL class PPA combatant patrol vessels, which cannot accommodate a hull mounted sonar, Leonardo developed the Active Towed Array System (ATAS). The latter is a low/medium-frequency active/passive VDS, characterised by reduced size, weight, and footprint. The ATAS can simultaneously perform panoramic surveillance and tracking in active and passive modes, manual/automatic classification of threats and interception of sonar pulse and torpedo acoustic head emissions, torpedo alarm, and training. With a maximum operating depth greater than 250 m at typical ASW patrolling speeds, according to Leonardo, the ATAS has a detection range, on “modern submarines” without specifying, beyond the first oceanic convergence zone (60 km in the Atlantic Ocean).

The ATAS is subdivided into modules of less than three tonnes for it to be easily transferable from one ship to another, although procured initially to equip the PPA ‘Full combatant’ platforms, the first of which is to be delivered in 2024. It has an overall weight of less than 9 tonnes, being composed of a Launch and Recovery System (LARS) less than 2 m high, towed body and array, power and processing electronics cabinets, local console.

Equipped with a single tow line for both transmitting element and receiving array, the towed body presents a main rear cruciform stabiliser allowing for enhanced stability. It weighs less than 600 kg and is less than 2 m long. It incorporates a vertical array transmitter consisting of free flooded rings, which provides, according to the Italian group, omnidirectional transmission with good vertical directivity and high source level. The connected receiving linear array consists of multiple hydrophone triplets allowing left-right ambiguity resolution. The towed body cable has a maximum length of 400 m, while the connected smaller diameter cable adds less than 150 m and features the trailing passive array which is 20 m long.


In January 2020, Patria was awarded a contract by Saab, the Squadron 2020 combat system contractor, for the design and delivery of a new ASW suite to equip the Finnish Navy’s new POHJANMAA class corvettes. Patria is developing a containerised solution called SONAC DTS, offering both active and passive modes of operation with two separate wet-end sub-systems: a VDS and a long TAS. The detection of targets can be performed through passive, monostatic and bistatic operations with these wet-ends set separately to desired depth levels.

This enables optimal target detection, location and tracking capabilities in the prevalent environmental conditions and operative situations. In this setup, the system also includes torpedo detection capabilities. The wet-ends are both accommodated in and launched from a Launch and Recovery System (LARS) that has dimensions of a 20-foot standard container. SONAC DTS and its frequency range (including the 7-11 kHz range in the active mode) are optimised for littoral ASW and designed for open water conditions. The lightweight system design is optimised for detection ranges up to 20 km. This dual-tow sonar system also includes real-time processing for the purpose of underwater situational pictures. The Graphical User Interface (GUI) for the operators is optimised for combined passive/active operation.


The Systems Engineering & Assessment (SEA) UK company, part of the Cohort Group, has developed the KraitArray family of low profile miniaturised acoustic arrays for low speed towed or static applications, which was deployed successfully from small manned and unmanned surface and underwater platforms. According to SEA, the latest array iteration extends the range and increases the bearing accuracy, thereby enabling a full ASW capability on small platforms and OPVs. In addition to the ability to link modules together with a length up to 150 m (5-to-20 kHz receive bandwidth), SEA has increased the acoustics sensors’ number up to 192, as well as the non-acoustic to 24, improved hydrophone performance, increased the strength to support tow speed of 30 knots and added smart power management. In October 2021, SEA conducted new at-sea trails on a further improved array which will support the next stage in product development.


The European group continues to enlarge the customers’ portfolio of its well-established and successful family of Combined Active Passive Towed Array Sonar (CAPTAS) low-frequency VDSs. During the Sea Air Space 2022 exhibition, The US Naval Sea System Command (NAVSEA) confirmed that the CAPTAS-4 VDS provided by the Advanced Acoustics Concepts (AAC), a joint-venture company between DRS and Thales, was chosen by the US Navy to equip the new CONSTELLATION class (FFG 62) frigates under a design and construction contract with Fincantieri Marinette Marine (FMM). Based on the free-flooded ring (FFR) transducers and triplet receiver array technology (to resolve left/right bearing ambiguity), the CAPTAS family was initially based on two products both including independent transmit and receiving arrays. These were the four-ring CAPTAS-4, specifically designed for ASW frigates of 3,500 tonnes and above and characterised by extended range surveillance capabilities, and the smaller CAPTAS-2 for installation on board ships down to 1,800 tonnes.

With the technological developments and an expanded smaller ASW platform market, Thales offers a CAPTAS-4 Compact version having already registered international success with its lower-end CAPTAS 1 model. With 900-2100 Hz active frequency coverage and wide bandwidth against reverberation effect, the CAPTAS-4 offers extended range simultaneous active and passive 360° surveillance with typical detection range up to 150 km (second oceanic convergence zone in the Atlantic Ocean) and permanent all-round torpedo alert, according to Thales.

Already operational in two variants on board UK and Chilean TYPE 23 frigates (Sonar 2087), and on French and Italian FREMMs (CAPTAS-4), including foreign customers (Royal Moroccan and Egyptian Navies), thanks to its recognised capabilities and technological upkeep, the CAPTAS-4 has been procured for the Royal Navy’s new TYPE 26 frigates, as well as its Australian HUNTER class counterparts. It is also being considered or/and is in the pipeline for new Italian (two new platforms) and the Indonesian Navy’s FREMM platforms. As anticipated, the latest CAPTAS-4 customer is the US Navy, but no further details have been released on the system procurement.

CAPTAS Evolution

Thanks to the reusing and rationalisation of CAPTAS family components and hardware, Thales developed the CAPTAS Compact version, achieving a 20 per cent weight reduction and a 50 per cent reduction in footprint compared to CAPTAS-4 (weighing 34 tonnes and featuring an 84 m2 footprint), while maintaining about 90 per cent of the latter’s performance characteristics. The CAPTAS-4 compact version comes in two configurations. The independent tow model, which retains a separate towed body and receiver array (25 tonnes in weight and circa 45 m2 footprint), and the dependent tow model with the receiver array attached to the towed body (20 tonnes and circa 43 m2 footprint), the latter being the same configuration used by smaller CAPTAS 2 and CAPTAS 1 systems.

The launch platform for the CAPTAS-4 Compact version in the independent tow configuration was the French Navy’s Frégates de Défense et d’Intervention (FDI), which was recently followed by the Hellenic Navy with the procurement of the same platform from Naval Group, while the Spanish Navy acquired the Compact version in the dependent tow configuration for its new F110 frigates being built by Navantia. Designed for both deep and shallow water operations, the smaller CAPTAS-2 offers long range simultaneous active and passive 360° surveillance (typically up to 60 km – first oceanic convergence zone in the Atlantic Ocean) and torpedo alert, with a <16 tonnes in weight (on the quarterdeck) and <39 m2 footprint (including maintenance area).

The CAPTAS 2 was procured for the Royal Norwegian Navy’s NANSEN class and Royal Saudi Navy’s F4000 frigates, UAE Navy’s ABU DHABI class corvette and the Mexican Navy’s POLA platform, while being provided for the Malaysian, Egyptian Navies and with undisclosed GOWIND corvette customers. The Thales CAPTAS VDS family and hull-mounted sonar have been indicated for the Polish Navy’s new MIECZNIK class frigates, without specifying the models. Working with the Thales BLUESCAN integrated multi-sensors acoustic system, CAPTAS VDS systems allow bi-static and multi-static operations with the Thales family of hull-mounted sonar, including the BLUEHUNTER, BLUEMASTER and BLUEWATCHER heliborne dipping sonar and sonobuoys. The systems also feature new user-friendly Human-Machine Interface (HMI), introducing 3D analysis and chart underlay to increase situation understanding. Thales also offers the CAPTAS-1 with a typical detection range up to 30 km, for installation on OPVs weighing about 800 tonnes, using a single dependent tow for both the active sonar source and a triplet receiver array with a <16 m2 deck footprint and weighing 7.3 tonnes.

Ultra Electronics

A key milestone in the development of new ASW capabilities by the Ultra Electronics group was reached in February 2021 when the company was contracted to commence work on the key Variable Depth Sonar (VDS) system for the Canadian Surface Combatant (CSC) – named the Towed Low-Frequency Active Sonar (TLFAS), marking the beginning of the full manufacture and delivery of the sonar suite to equip the new frigates for the Royal Canadian Navy (RCN). ‘Optimised for the detection and tracking of stealthy submarines in challenging ocean environments’, according to Ultra Electronics, ‘when delivered it [the TLFAS] will represent a step-change in Canada’s ASW assets and will provide a level of sonar capability never before enjoyed by the RCN’.

Fully designed and manufactured by Ultra in Nova Scotia, Canada, it is expected to leverage on the ongoing research and development activities funded by the Canadian Department of National Defence, as well as on the SEA SABRE VDS currently promoted worldwide by the company. The latter VDS consists of a tow body providing a high power acoustic source (1 to 2 kHz), combined with a QUAD directional passive receive array and its tow cable. With an installation mass (typical configuration) of 12,107 (active) and 8,815 kg (passive) respectively, and a deployment and recovery speed of 4-to-12 knots, the control of the towed receiver array is independent through use of separate winch and handling systems allowing the receiver array to be located further astern to minimise own-ship acoustic interference.

Ultra Towed Active Sonar Systems Characteristics

According to Ultra Electronics, its LFA sonar source is ideal where search speed is a critical factor. With the company’s Free Flooded Ring (FFR) effectors at its core, the SEA SABRE tow body emits acoustic energy in a toroidal pattern that is uniform in azimuth, each ping providing full 360° azimuth coverage. The SEA SABRE is available in a dedicated configuration for bistatic operation with the hull mount sonar, alongside optional sensors for mammal detection and active interception of torpedo sonars. Within the CSC programme, Ultra will also provide its next-generation all-digital SONAR 2150 hull-mounted sonar, already procured by the UK and Australia, and will lead the integration with sonobuoys and other capabilities for wide-area underwater battlespace surveillance to meet Canada’s future strategic needs.

The company also promotes the SEA LANCER (2nd generation) high performance, low-frequency active and passive sonar in a single tow together that is ideal for smaller platforms and multi-role ships according to the manufacturer. Available also in a containerised configuration, with an overall installation mass of 8,690 kg and a 1.5 to 3.5 kHz operating frequency (configuration dependent), the SEA LANCER (2nd generation) consists of a Horizontal Projector Array (HPA), which is a line array of independently driven projector arrays, combined with a receive array of QUAD sensors to provide an LFA sonar with instantaneous port-starboard ambiguity resolution.

As part of the UWSU (Underwater Warfare Suite Upgrade) programme for the RCN’s HALIFAX-class frigate, managed by General Dynamics Mission Systems – Canada, Ultra Electronics provides a 3rd generation SEA LANCER LFA VDS where a co-funded (Canadian DoD/Ultra) new thin-line HPA is introduced. In January 2022, Ultra announced that in partnership with Indian company Mahindra Defence Systems, it was awarded a contract for the Integrated Anti-Submarine Warfare Defence Suite (IADS) programme for selected frontline warships of the Indian Navy.

According to Ultra, IADS provides a powerful multi-sensor ASW capability using in-line active and passive towed LFA VDS, as well as Torpedo Defence with embedded detection, classification and localisation to defeat detected torpedo threats. Ultra (Australia) also provided the ASW suite for the Australian HOBART class guided-missile destroyers, based on a hull mounted sonar and an in-line active and passive towed LFA VDS, together with torpedo defence system capabilities, for which the company continues to provide in-service support.