International Symposium onHYDROACOUSTICS AND ULTRASONICSEAA Symposium (formerly 13th FASE Symposium)Gdańsk-Jurata, 12-16 May 1997

Advanced Underwater Technologywith Emphasis on Acoustic ModelJing and Systems

Klaus Kremer, Managing DirectorIBH INGENIEURBURO FUR HYDROAKUSTIK, Germany

Abstract

The Sonar Performance Prediction System (SPPS) will be introduced. The SPPS is an integrated ocean-acousticpropagation model coupled with a sonar performance prediction model. A stationary sea surveillance sonarconsisting of a horizontally nested hydrophone array and a vertical transmitting/ receiving array may be used lodctcct, localisc and track surface and subsurfacc contacts within its coveragc arca.

The performance ofthe sea surveillance sonar has been investigated with the SPPS. Results ofthat performanceprediction will be presented.

Introduction

The Sonar Performance Prediction System is amathematical/physical description of the acousticalbehaviour of the sea and their influence to a givenactive ar passive sonar system mountcd on arbitraryplatforms. The goal is to simulate the sonar-signalwith the aim of measurable realistic oceanographicparameters and to give a prediction of the acousticalbehaviour of the sea-area of interes!

On the other hand the SPPS enables the sonar systemdesigner to optirnise li given sonar system on a Pc.This fact rcduccs the costs (ship-timc) for an optima!design of a sonar system tremendously.

The sea surveillance sonar is able to detect and trackall surface and subsurface contacts (like submarines,divers and diver delivery vehicles) within the as-signed covcrage area in the active and/or passivemode of operation.

The sea surveillance sonar is extremely beneficial incountering illegal activities that can strain a nation'seconomy. These activities include pirating, terrorism,smuggling, illegaI immigration.

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The SONAR Performance Prediction System(SP PS)

SPPS is a user-friendly software package, runningon a PC under WINDOWS 95.

AppIications of the SPPS:

Performance evaluation of difTerent sonar systems- Evaluation of naval warfarc scenarios

Sonar system designOceanographic studies

The Sonar Performance Prediction System (SPPS)[see fig. 1] consists of a Sonar Performance Predic-tion Model (SPPM), an Integrated Ocean-AcousticPropagatiun Model, several data bases and a noise-and reverbcration model.

The SONAR Performance Prediction Model(SPPM)

The SPPM has the following features

It handles active and passive sonar detection prob-lems, broadband and narrowband signals.

AII types of sonar antennas are included like

Towed array (nested)- Bottom moored !ine array (nested)

Flank array (Iine and planar)Conformal array

- Hull-mounted arrays (active, passive)Passive ranging arraysVariable depth arraysActive towed arrayVertieal arrayBistatical transrniuing/receiving arraysROV~arrays

The directivity pattems of the antennas are variahle(including side lobe level, horizontaJ beamwidth,horizontal direction, vertical beamwidth and tilt an-gle).

Coherent and noncoherent signal processing algo-rithms. ThreshoJd system.

Examination of environmcntal effects on the sonarsystem and the naval warfare scenario.

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Integrated Ocean-Acoustic Propagation Model

A PE, a ('M and a Ray propagation model are inte-gratcd undcr one umbreli a, the expert system.[n this way, the expert system will help and guide theoperator in selecting and using the propagationmodels:

The objective ofthis hybrid model system is to pro-vide accurate, valid and user-friendly 2 D, N x 2 Dand 3 D model capable of realistically predicting thepropagation of sound in the sca.

The frequency ranges ofthe hyhrid model systemcovers the ranges frorn 10Hz to l MHz. The hybridmodel system will be able to incorporate

shallow water propagationcornpressional and shear wavescoupling between wave types at interfaces andboundariesdepth dependence in the elastic propertieswide propagation ang!es

Data Bases

Geographic Information System (GIS)

The large size and the diversity of spatial data in en-vironmental problems makes it difficult to managethem with the traditional data based tools This istrue for both land and sea based spatial data. DBMShave advanced drarnatically in the last years with theintroduction of the relational systems, but still carmothandle easily information related to spacc location.

To address this problem the technology of Geo-graphic Information Systems (GIS) has been devel-oped in the last few years A GIS is a system forstorage, display. management and analysis of spatialdata

In a GIS, each piece of spatial informaticn has an as-sociated co-ordinate that explicitly specifies where itis located. For exarnple, when storing sea tempera-ture and/or salinity data, there are x, y, z-co-ordi-nates that specify the location and depth of each par-ticular measurement.

A GIS, therefore, consists of different layers andcoverages of information that are interreJatedthrough the co-ordinate system. Because of'theseinterrelationships, data on different layers (i.e. tern-perature, salinity etc.) can be analysed and potentialrelationships explored

Finally, a very important characteristic of a GIS isthat it perrnits visualisation of spati ał data.

A GIS system that stores data rclated to the seacould include layers related to:bathyrnetryhydrographic measurements (e.g. CTD,XBT data)bottom stratification

and could be "lied" tu interpolation routines that es-timatc temperaturo, salinity etc. betwccn measuro-ment points and display thematic maps.

In such an integrated system, the potential user maydefine any section ofinterest. The GIS will then ex-tract all data associated to this section, which in tumcan be either displayed or used as input for the SPPSThe result uf the acoustic calculations can then be fet!back to the GIS for storage, display and reference.For such system the geographic size of the units willbe a grid.

AlI other data bases will be due to custorner' s de-mands.

Reverberation Model

The reverberation modeł s will predict the single re-verberation components:

Volume (wat er colurnn, sediment)SurfaceBottom (for range dependent and range inde-pendent environments)

Noise Model

This modeł will predict :

Ambient noiseFJow noiseThermal noise of the seaSelfnoise (sonar platform and sonar system)

Visualisation of the Results of the SPPS, theVarious Sub-Modelsand the Contents of tbe Data Bases

The results ofthe SPPS and the tactical situation willhe presented in 3 D, the results of'the sub-rnodelsand the contents ofthe data bases in 2 D.

The Sea Surveillance Sonar

The sea surveillance sonar [set! fig. 2J is able lu Je-tect and track all surface and subsurface contacts(like submarines, divers and diver delivery vehicles)within its assigned coverage area in the active and/orpassive mode of operation. The underwater part (wetend) consists of a nested steerable holizontal hydro-phone array (256 hydrophones) and a verticalsteerable transducer array (64 transducers).

Thc system is tailored to thc local needs and re-quirements. IlIegal immigration might be conductedwith small freighters, For smuggling activities andpirating a frequcnt1y used craft is a high speed boatTerrorism might be conducted with submerged vehi-cles.

To detect these different targets passively, the ernit-ted propeller noise spectrum ofthe targets will bemeasured. That ofthe freighter in the low frequencyand that ofthc high speed boat in the high frequencvspectrum

The sonar system consists of the arrays, the transmit -ter, receiver, wet end control unit and the surfaceeontrel and processing unit.

a) Transmitters

64 transducers are wórking in a frequency rangefrorn 2 to 4 kHz. Each transducer will be used asa receiver and will possess its own power ampli-fier (few hundred watts) and pre-amplifier.

b) Receivers

A fixed nested horizontal receiving array will beused (in addition to the 64 channels vertical ar-ray). It consists of an array of about 300 metres(256 hydrophoncs) that will be suitably arrangcdin order to cover a very wide frequency range.

In addition, a vertical array offew elements willbe moored together with a pinger at a short dis-tance from the arrays in order to continuouslymonitor the data quality.

c) Wet End Control Unit

The wel end comroi unit will be in charge of ailthe dialoguc with thc surface contra! unit. It willgenerate pre-computed signals (or any new signalloaded frorn the surface) and transmus acousticdata to the surface. It also controls the power

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ampłifiers. The wet end control unit is connectedto the surface control unit by a speciał purposecable including power as well as several oplicalfibrc links.

d) Surface Control and Processing Unit

The surface unit constitutes the interface betwe enthe user and the underwater part. It will be in-stalled in a łand based unit and will be in chargeof data collcction conditioning, proccssing andstorage.

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Sonar Peńormance Prediction System

(SPPS)

Visualization ot Results

fh Hl

("-

...• ...•...• T_gal NoiseModel '"'~ l ) .J...•.. Sonar!'"

Perfonnance .,( } ...•.. Prediction Model..• PIatfonn r+ Reverberation...•l SPPM •• ..

t Model ..•

..• ( Sonar Sya.n }- Integrated~

ocean-acoustlc r+propagation model

l Expert System jI NavafWart.. \..~~~

....• , Sc:enartoe J r-.

....• J TactlcaI I 2D, Nx2D,3D....•

\ Constralnts ) II

~~

.. ( Geographlc "')....• \...InfonnatIon System )

(Data basa)Fig.1: Blockdiagram of the Sonar Performance Prediction System

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