Environmental variability on acoustic prediction using cass grab
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Environmental Variability on Acoustic Prediction Using CASS/GRAB. Nick A. Vares June 2002. Purpose. Determine the impact of bottom type and wind variations due to limited data on bottom moored mine detection Determine the significance of transducer depth on bottom moored mine detection.

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Purpose l.jpg

  • Determine the impact of bottom type and wind variations due to limited data on bottom moored mine detection

  • Determine the significance of transducer depth on bottom moored mine detection

Relevance l.jpg
Relevance CASS/GRAB

  • Littoral engagement

  • Mine warfare

  • Diesel submarines

  • Unmanned Undersea Vehicles (UUVs)

Cass grab l.jpg

  • Comprehensive Acoustic Simulation System (CASS)

  • Gaussian Ray Bundle (GRAB) Eigenray model

  • Navy standard model for active and passive range dependent acoustic propagation, reverberation and signal excess

  • Frequency range 600Hz to 100 kHz

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CASS/GRAB Model Description CASS/GRAB

  • The CASS model is the range dependent improvement of the Generic Sonar Model (GSM). CASS performs signal excess calculations.

  • The GRAB model is a subset of the CASS model and its main function is to compute eigenrays and propagation loss as inputs in the CASS signal excess calculations.

Comprehensive acoustic simulation system guassian ray bundle cass grab l.jpg
Comprehensive Acoustic Simulation System/Guassian Ray Bundle (CASS/GRAB)

  • In the GRAB model, the travel time, source angle, target angle, and phase of the ray bundles are equal to those values for the classic ray path.

  • The main difference between the GRAB model and a classic ray path is that the amplitude of the Gaussian ray bundles is global, affecting all depths to some degree whereas classic ray path amplitudes are local. GRAB calculates amplitude globally by distributing the amplitudes according to the Gaussian equation

Mine hunting sonar l.jpg
Mine Hunting Sonar (CASS/GRAB)

  • Generic VHF forward looking

  • CASS/GRAB input file for MIW with signal excess output

  • Generic bottom moored mine

An sqq 32 mine hunting sonar system l.jpg
AN/SQQ-32 Mine Hunting Sonar System (CASS/GRAB)

  • The CASS/GRAB Acoustic model input file used in this study simulates a VHF forward looking sonar, similar to the Acoustic Performance of the AN/SQQ-32.

  • The AN/SQQ-32 is the key mine hunting component of the U.S. Navy’s Mine Hunting and Countermeasure ships.

Cass grab input parameters l.jpg

Bottom depth (CASS/GRAB)

Target depth

Transducer depth

Wind speed

Bottom type grain size index

Frequency min/max

Self noise

Source level

Pulse length

Target strength/depth

Transmitter tilt angle

Surface scattering /reflection model

Bottom scattering /reflection model

CASS/GRAB Input Parameters

Bottom type variability l.jpg
Bottom Type Variability (CASS/GRAB)

  • Muddy sand (3.0) and sandy silt (5.0)

  • Grain size index variation + 1.0 in 0.5 increments

  • 5.14 m/s, - 40, bottom 30 m, transducer 5.18 m

Yellow sea bottom sediment chart l.jpg
Yellow Sea Bottom Sediment Chart (CASS/GRAB)

  • Bottom Sediment types can vary greatly over a small area

    • Mud

    • Sand

    • Gravel

    • Rock

Wind variability l.jpg
Wind Variability (CASS/GRAB)

  • Muddy sand (3.0) and sandy silt (5.0)

  • Tilt angle - 40, bottom 30 m, transducer 5.18 m

  • 5.14 + 2.57 m/s wind

An sqq 32 employment l.jpg
AN/SQQ-32 Employment (CASS/GRAB)

  • Variable depth high frequency sonar system

    • Sonar can be place at various positions in the water column to optimize the detection of either moored or bottom mines.

Transducer 5 18 m vs 25 m l.jpg
Transducer 5.18 m vs 25 m (CASS/GRAB)

  • Tilt angles + 40 to – 120

  • Wind 2.57 to 12.86 m/s

  • Coarse sand to silt bottoms

  • 30 m water depth

Conclusions l.jpg
Conclusions (CASS/GRAB)

  • Bottom type and wind variability are important for sandy silt detections

  • Bottom type and wind data variability are on the order of a few decibels

  • Deep transducers provide higher signal excess for most detectable cases

Recommendations l.jpg
Recommendations (CASS/GRAB)

  • Sensor improvements of a few decibels are significant for detection

  • Money would be better spent on sensor development

  • Employment of sensors deeper aids bottom moored mine detection