To support the Netherlands Ministry of Defence and other customers, TNO carries out sonar research on topics such as anti submarine warfare, mine countermeasures, torpedo defence and harbour protection.

The TNO Low Frequency Active Sonar Source SOCRATES onboard HNLMS Van Nes

New technology and new threats call for new sonar systems. Our research includes the design of new sonar concepts, sonar signal processing (array processing, waveform design, false alarm reduction…) and assessment or improvement of existing systems. Very often, the results of our research is put to operational use, either in technology demonstrators (Flank Array Towed Array demonstrator) or the development of brand new systems (The MAPS operational Low Frequency Active Sonar). To support our research, we develop and use an array of models for target signatures and response, oceanographic phenomena, acoustic propagation and sonar performance.


Watch what the project SALSA, Smart Adaptive Long- and Short-range Acoustic network, entails

Role of mobile unmanned platforms in military scenarios

Smart underwater robots, such as Autonomous Underwater Vehicles (AUV), have the potential to take over lengthy and labour-intensive missions in dangerous areas from navy ship crews and special forces. In general, the role of mobile unmanned platforms in military scenarios (REA, MCM, ISR, SPECOPS) is becoming more important. However, crucial to their success is their seamless integration in the heterogeneous (mixed mobile/static) wireless network of surface ships, submarines, bottom/moored sensor nodes and surface gateway buoys.

This requires scalable underwater acoustic networks that allow ad-hoc joining, participating and leaving of allied mobile (sub/surface) platforms, as well as the capability to adapt autonomously to time-varying communication conditions, for example by switching between frequency bands and data rates, such that network assets may remain connected for extensive (battery-limited) operation times without recovery and redeployment.

Heavy-duty communication in the military

Flexible and self-configurable underwater acoustic networks as described above require a smart adaptive protocol stack, which has been developed in SALSA. At the physical layer, where bits are converted into sound and vice versa, the JANUS standard (STANAG 4748) is applied for first contact, after which the more robust and flexible FRSS modulation is employed to enable the required heavy-duty communication in the military scenario at hand.

Demonstration of a self-configurable underwater acoustic network

At the network layer, the versatile GUWMANET routing protocol is being employed with the accompanying application-layer protocol GUWAL. The decisions for adaptations, and their synchronization within the network to maintain interoperability, are controlled by an adaptivity module inside the network layer.

On 4 May 2022, the EDA-SALSA consortium performed a successful demonstration of a self-configurable underwater acoustic network in the Oslofjord, Norway. Demonstrated functionalities include ad-hoc network extension, data muling, multi-band/topology routing and adaptive modem parametrization:

  • Two AUVs joining the underwater acoustic network (“first-contact” functionality) using the JANUS protocol.
  • Data muling of an image and buffered messages by an AUV between two separated (clusters of) network nodes.
  • Exploitation of two distinct acoustic frequency bands (underwater) and a radio link (above water) for optimal (multi-topology) routing (GUWMANET protocol).
  • Measurement of acoustic communication conditions (input/output signal-to-noise ratio) to enable in-situ optimization of transmission power and equalizer settings (data rate of FRSS protocol).

These functionalities were demonstrated in the context of surveillance/barrier (ISR) and minehunting (MCM ) scenarios using up to 20 network nodes (6 ship nodes, 9 bottom nodes, 3 AUVs, 2 buoys). About 20 VIP guests from the five participating nations witnessed the demonstration from a ship and an on-shore operations centre, aided by a situational-awareness plot showing nodes from which (relayed) messages were received by a gateway buoy.

Detecting intruders and bottom mines

In the ISR scenario, bottom nodes were detecting an intruder – TNO’s RHIB – and in the MCM scenario, AUVs were detecting bottom mines. The RHIB was also used to disturb the network at several occasions during the day by crossing the area at high speed (approx. 30 knots), thus forcing automatic adaptations of the network communication (band usage, transmit power, data rates). The actual status of the network could be followed on a large screen showing a Situational Awareness plot that was relayed by a gateway buoy to the on-shore operations centre.

The navies of the five SALSA nations, supported by their national research establishments (TNO, Fraunhofer, WTD71, FFI, FOI), have the intention to submit a proposal for a NATO Standardization Agreement (STANAG). The technical implementation of this standard in acoustic modems has the commitment of at least the six industries participating in SALSA (ELAC, ATLAS Elektronik, Develogic, Kongsberg, Saab, Patria).

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