SDSN

The SDSN (Sonar Defense System Network) is a new, second-generation of the IDSN harbor defense system.  SDSN builds on the knowledge and experience gained during the development and deployment of IDSN, and will deliver significant improvements to the IDSN hardware and software.        

SDSN will produce a sonar node which will be about 30% the size of the IDSN sonar node, and which will be small and light enough to be carried by two people.  The SDSN sonar node will also be significantly lower cost than the IDSN sonar node.

The software signal processing will be enhanced to further improve detection, localization, tracking, and classification.  One of the techniques being used is multistatic signal processinng, processing a target’s acoustic data from multiple sonar nodes simultaneously, allowing multiple angles of view of a target.  SDSN is also doing experimental integration of oceanographic information (such as water currents) with the signal processing to further improve system performance.

SDSN will further provide for more flexible distributed system configuration and improved built-in test reporting.

The new SDSN system design is being tested in an in-harbor research installation at the Portsmouth Naval Base.

HFM3

The High Frequency Marine Mammal Monitoring (HFM3) sonar system is a novel sonar system that was specifically designed to meet the requirements for marine mammal monitoring as described by the Final Overseas Environmental Impact Statement and Environmental Impact Statement for Surveillance Towed Array Sensor System Low Frequency Active (SURTASS LFA) Sonar.  The HFM3 system is integral with the SURTASS LFA, residing at the top of the vertical transmit array.

The HFM3 sonar is a mechanically-steered active sonar operating in the 30 – 40 kHz frequency range. The system utilizes four independent transponders mounted on a rotating carousel.  Each transponder consists of an omni-directional hydrophone located at the focal point of a 12” x 18” air-backed, parabolic reflector.  The reflector provides for an on-axis transmit and receive directivity factor of 20-25 dB to depths of 1000 ft.  The HFM3 system will continuously monitor the area surrounding the source platform for the presence of marine mammals to a range of 2000 yards.

Modal Inversion

In shallow water, the acoustic field can be represented as a sum of contributions from a set of propagating modes. It is well known that modal dispersion data contain information about the characteristics of the shallow water wave guide including the acoustic characteristics of the sediment. Inversion scheme that use the modal dispersion data for estimating sediment acoustic properties in a range independent environment had been proposed earlier by Subramaniam Rajan This approach has now been modified by Subramaniam Rajan to determine the sediment properties in a range-dependent environment. Simulation studies show that range-dependent sediment properties can be extracted if modal dispersion data are obtained for multiple source/receiver locations. During the Shallow Water 2006 experiment data were collected while transmitting a linear frequency modulated signal with a bandwidth of 250 Hz. The data were obtained for multiple source/receiver locations. This data are being analyzed to estimate range dependent sediment properties from modal dispersion information.

DSTA Seisemic R&D

This project involves the detection of the impact of 400kg to 600kg objects on the seabed in approximately 20 meters of water.  Using sensor nodes consisting of geophones, tilt sensors, and hydrophones, SSI plans to measure seismic and acoustic signals from the impact of objects of varying weight and shape as a function of distance from the sensors.  Such an impact should produce seismic interface waves with a signature that is distinct from ship traffic and other ambient noise.  SSI recently completed an initial deployment using a single node to capture ambient noise data and test possible techniques for coupling the sensors to the seabed.