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TRANSCRIPT
Introduction to SonarINF-GEO4310
Roy Edgar Hansen
Department of Informatics, University of Oslo
September 2012
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Outline
Outline1 Basics
IntroductionBasic PhysicsUnderwater sound
2 Sonar TheorySonar typesPosition EstimationSignal processing
3 Sonar ApplicationsFish findingHUGIN AUVMappingImaging
4 Summary
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Sonar Basics
Outline1 Basics
IntroductionBasic PhysicsUnderwater sound
2 Sonar TheorySonar typesPosition EstimationSignal processing
3 Sonar ApplicationsFish findingHUGIN AUVMappingImaging
4 Summary
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Sonar Basics Introduction
History of Underwater Acoustics
If you cause your ship to stop and place thehead of a long tube in the water and place theouter extremity to your ear, you will hear ships ata great distance from you.
Leonardo da Vinci, 1490In 1687 Isaac Newton wrote his MathematicalPrinciples of Natural Philosophy which includedthe first mathematical treatment of soundIn 1877 Lord Rayleigh wrote the Theory ofSound and established modern acoustic theory
Image from wikipedia.org.
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Sonar Basics Introduction
History of SONARSOund Navigation And RangingA lot of activity after the loss of Britishpassenger liner RMS Titanic in 1912English meteorologist Lewis Richardson, April/May 1912: Patenton Iceberg detection using acoustic echolocation in air and waterGerman physicist Alexander Behm 1913: patent on echo sounderCanadian engineer Reginald Fessenden, 1914: Demonstrateddepth sounding, underwater communications (Morse Code) andecho ranging detecting an iceberg at two miles rangeFrench physicist Paul Langevin and Russian immigrant electricalengineer, Constantin Chilowski 1916/17: US patents on ultrasonicsubmarine detector using an electrostatic method
Image from wikipedia.org.
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Sonar Basics Introduction
The masters in sonar
From wikipedia.org.
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Sonar Basics Introduction
Similar technologies
SONAR = Sound Navigation And RangingRADAR = Radio Detection And RangingMedical ultrasound, higher frequencies, shorter range and morecomplex mediumSeismic exploration, lower frequencies, more complex medium
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Sonar Basics Introduction
Literature
Course text: sonar_introduction_2012.pdfCourse presentation: sonar_presentation_2012.pdfXavier Lurton, An introduction to underwater acousticsSpringer Praxis, First edition 2002, Second edition 2010www.wikipedia.org
I sonarI underwater acousticsI side-scan sonarI biosonar, animal echolocationI beamforming
Ocean Acoustics Library http://oalib.hlsresearch.com/
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Sonar Basics Physics
Basic Physics
Sound is waves travelling in pressure perturbationsOr: compressional wave, longitudal wave, mechanical waveThe acoustic vibrations can be characterized by
I Wave period T [s]I Frequency f = 1/T [Hz]I Wavelength λ = c/f [m]I Sound speed c [m/s]
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Sonar Basics Underwater sound
Underwater sound
Acoustics is the only long range information carrier under waterThe pressure perturbations are very smallObtainable range is determined by
I free space loss and absorptionI the sensitivity to the receiver
The ocean environment affects sound propagation:I sea surfaceI seafloorI temperature and salinityI currents and turbulence
Underwater sound propagation is frequency dependent
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Sonar Basics Underwater sound
Geometrical spreading loss - one way
The acoustic wave expands asa spherical waveThe acoustic intensitydecreases with range ininverse proportion to thesurface of the sphereThe acoustic wave amplitudeA decreases with range RThe intensity I = A2
In homogeneous media
I ∼ 1R2
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Sonar Basics Underwater sound
Geometrical spreading loss - two way
The acoustic wave expands asa spherical wave to thereflectorThe reflected field expands asa spherical wave back to thereceiverIn homogeneous media, thetwo way loss becomes
I ∼ 1R2
1R2 =
1R4
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Sonar Basics Underwater sound
Absorption
Seawater is a dissipative mediumthrough viscosity and chemicalprocessesAcoustic absorption in seawater isfrequency dependentLower frequencies will reach longerthan higher frequencies
f [kHz] R [km] λ [m]0.1 1000 151 100 1.510 10 0.15100 1 0.0151000 0.1 0.0015
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Sonar Basics Underwater sound
Transmission lossTransmission loss is geometrical spread + absorptionLogarithmic (dB) scale: IdB = 10 log10(I)A certain frequency will have a certain maximum rangeFrequency is a critical design parameter
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Sonar Basics Underwater sound
The Ocean as Acoustic Medium
The sound velocity - environmental dependencyLayering and refraction - waveguidesThe sea floor and the sea surface - scatteringNoise sources
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Sonar Basics Underwater sound
Reflection and Refraction in Acoustics
Recall from first lecture on opticalimagingThe reflection angle is equal to theincident angleThe angle of refraction is given bySnell’s law
sin θ1
c1=
sin θ2
c2
The index of refraction n = c2/c1
Snells law can be derived fromFermats principle or from thegeneral boundary conditions From Wikipedia
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Sonar Basics Underwater sound
Refraction and the underwater sound velocity
Medvins formula:
c = 1449.2 + 4.6T − 0.055T 2 + 0.00029T 3
+ (1.34 − 0.010T )(S − 35) + 0.016D
The sound velocity depends on 3 major parts:Temperature T in degrees CelsiusSalinity S in parts per thousandDepth D in meters
The sound velocity contains information about the ocean environment.Example: T = 12.5 ◦C,S = 35 ppt,D = 100 m gives c = 1500 m/s
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Sonar Basics Underwater sound
Deep sound velocity variation
The surface layerThe seasonal thermoclineThe permanent thermoclineThe deep isothermal layer
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Sonar Basics Underwater sound
Sound refraction
The sound will refract towards areas of slower speedSOUND IS LAZY
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Sonar Basics Underwater sound
Underwater sound channelwaves are trapped in a guideThe energy spreads in one dimension instead of two I ∼ 1/RMuch longer rangeAcoustical Oceanography: Map the effect of the medium onunderwater acoustics
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Sonar Basics Underwater sound
Coastal variability
Factors that affect soundpropagation:
The sound velocity profileThe sea surfaceInternal wavesTurbulenceOcean current
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Sonar Basics Underwater sound
Coastal variability
The sound is trapped in awaveguideThe boundaries of thewaveguide changes theproperties of the soundwave
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Sonar Basics Underwater sound
Reflection: basic physics
Characteristic impedance Z0 = ρcI ρ is the density [kg/m3]I c is the sound speed [m/s]
Reflection coefficient (normalincidence)
R(f ) =Z − Z0
Z + Z0
Transmission coefficient (normalincidence)
T (f ) =2Z0
Z + Z0
The characteristic impedance isa material propertyMaterial ImpedanceAir 415Seawater 1.54 × 106
Clay 5.3 × 106
Sand 5.5 × 106
Sandstone 7.7 × 106
Granite 16 × 106
Steel 47 × 106
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Sonar Basics Underwater sound
Reflection: the sea surface
The sea surface (sea-air interface)Air: Z = 415Seawater: Z0 = 1.54 × 106
Reflection coefficient
R =Z − Z0
Z + Z0≈ −1
The sea surface is a perfect reflector
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Sonar Basics Underwater sound
Reflection: the sea floor (or bottom)
The sea floor (sea-bottom interface)Sand: Z = 5.5 × 106
Seawater: Z0 = 1.54 × 106
Reflection coefficient
R =Z − Z0
Z + Z0≈ 0.56
Sandy seafloors partially reflects,partially transmitsEstimated reflection coefficient canbe used in classification of bottomtype
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Sonar Basics Underwater sound
Scattering - smooth surfaces
Scattering from rough surfacesThe sea surfaceThe seafloor
Other scattering sourcesVolume scattering fromfluctuationsScattering from marine life
A smooth surface gives mainly specular reflection
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Sonar Basics Underwater sound
Scattering - rough surfaces
Scattering from rough surfacesThe sea surfaceThe seafloor
Other scattering sourcesVolume scattering fromfluctuationsScattering from marine life
A rough surface gives specular reflection and diffuse scattering
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Sonar Basics Underwater sound
Ambient Noise
The ocean is a noisy environmentHydrodynamic
I Tides, ocean current, storms,wind, surface waves, rain
SeismicI Movement of the earth
(earthquakes)Biological
I Produced by marine lifeMan made
I Shipping, industry
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Sonar Basics Underwater sound
Marine Life and Acoustics
Dolphins and whales use acousticsfor echolocation and communication.Whale songs are in the frequencybetween 12 Hz and a few kHz.Dolphins use a series of highfrequency clicks in the range from50 to 200 kHz for echolocation.
From wikipedia.org. Courtesy of NASA.
From wikipedia.org. Author Zorankovacevic.
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Theory
Outline1 Basics
IntroductionBasic PhysicsUnderwater sound
2 Sonar TheorySonar typesPosition EstimationSignal processing
3 Sonar ApplicationsFish findingHUGIN AUVMappingImaging
4 Summary
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Theory Sonar types
Active sonar
Transmits a signalThe signal propagates towards theobject of interestThe signal is reflected by the targetThe signal is recorded by a receiver
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Theory Sonar types
Active sonar
Transmits a signalThe signal propagates towards theobject of interestThe signal is reflected by the targetThe signal is recorded by a receiver
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Theory Sonar types
Passive sonar
Passive sonar only records signals
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Theory Positioning
Range Estimation
Estimation of time delay (or two way travel time) τRelate time delay to range
R =cτ2
Sound velocity must be known
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Theory Positioning
Range ResolutionThe minimum distance two echoes can be seperatedRelated to the pulse length Tp for non-coded pulses
δR =cTp
2Related to bandwidth B for coded pulses
δR =c
2B
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Theory Positioning
Pulse forms 1 - active sonarDifferent pulse forms for different applications
Gated Continuous Wave (CW)Simple and good Doppler sensitivity but does not have high BTLinear Frequency Modulated (LFM) (or chirp)Long range and high resolution but cannot handle Doppler
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Theory Positioning
Pulse forms 2 - active sonarDifferent pulse forms for different applications
Hyperbolic Frequency Modulated (HFM) pulsesLong range and high resolution and Doppler resistivePseudo Random Noise (PRN) BPSK Coded CWHigh resolution and good Doppler sensitivity but low efficiency
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Theory Positioning
Directivity
Transducers (or antennas or loudspeakers) are directiveThe beamwidth (or field of view) of a disc of size D is
β ≈ λ
D
The beamwidth is frequency dependent. Higher frequency givesnarrower beam.
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Theory Positioning
Bearing estimation - arbitrary Rx positions
Direction of arrival can be calculated from the time difference ofarrival
θ = sin−1{
cδtL
}
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Theory Positioning
Bearing estimation - array sensor
By delaying the data from each element in an array, the array canbe steered (electronically)
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Theory Positioning
Bearing estimation - array sensor
Direction of arrival from several reflectors can be estimated byusing several receivers.
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Theory Positioning
Imaging sonar / beamforming
Echo location is estimation of range and bearing of an echo (ortarget)Imaging sonar is to produce an image by estimating the echostrength (target strength) in every direction and range
Algorithmfor all directions
for all rangesestimate echo strenght in each pixel
endend
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Theory Positioning
Beamforming defined
BeamformingProcessing algorithm that focus the array’s signal capturing ability in aparticular direction
Beamforming is spatio-temporal filteringBeamforming turns recorded time series into images(from time to space)Beamforming can be applied to all types of multi-receiver sonars:active, passive, towed array, bistatic, multistatic, synthetic aperture
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Theory Positioning
Beamforming algorithm in time domain
Algorithmfor all directions
for all rangesfor all receivers
Calculate the time delayInterpolate the received time seriesApply appropriate amplitude factor
endsum over receivers and store in result(x,y)
endend
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Theory Positioning
Imaging sonar resolution
Range resolution given bypulse length (actuallybandwidth)Azimuth resolution given byarray length measured inwavelengthsField of view is given byelement length measuredin wavelengths
Array signal processing in imaging is the primary topic in INF 5410
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Theory Positioning
Imaging: Performance measures
Detail resolutionGeometrical resolution - minimum resolvable distanceContrast resolutionValue resolution, echogenicity, accuracyTemporal resolutionNumber of independent images per unit timeDynamic rangeResolvability of small targets in the presence of large targetsSensitivityDetection ability of low level targets
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Theory Signal processing
Sonar signal model
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Theory Signal processing
Active sonar processing
The basic active sonar processingconsists of
PreprocessingPulse compression (range)Beamforming (azimuth)DetectionParameter estimation - positionClassification
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Theory Signal processing
The sonar equationThe sonar equation is an equation for energy conservation forevaluation of the sonar system performace.
In its simplest form: Signal − Noise + Gain > ThresholdMore detailed (for active sonar):
SL − 2TL + TS − NL + DI + PG > RT
SL is source levelTL is transmission lossTS is target strenghtNL is noise levelDI is directivity indexPG is processing gainRT is reception threshold
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Applications
Outline1 Basics
IntroductionBasic PhysicsUnderwater sound
2 Sonar TheorySonar typesPosition EstimationSignal processing
3 Sonar ApplicationsFish findingHUGIN AUVMappingImaging
4 Summary
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Applications Fish finding
Echosounders
The echosounder is orientedverticallyThe target strength is estimated inevery range (depth)The ship moves forward to make a2D map of fish densityThe target strength is related to fishsize (biomass)Different frequencies can be usedfor species characterisation
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Applications Fish finding
Stock abundance and species characterisation
From www.simrad.com. Courtesy of Kongsberg Maritime
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Applications Fish finding
Fish detection range
Modern echosounderscan detect a single fishat 1000 m range.Some fish have aswimbladder (air filled)which gives extra largetarget strength
From www.simrad.com.
Courtesy of Kongsberg Maritime
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Applications HUGIN AUV
The HUGIN autonomous underwater vehicle
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Applications HUGIN AUV
The HUGIN autonomous underwater vehicleFree swimming underwater vehiclePreprogrammed (semi-autonomous)Used primarily to map and image the seafloorRuns up to 60 hours, typically in 4 knots (2 m/s)Maximum depth: 1000, 3000, 4500 m
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Applications HUGIN AUV
Acoustic sensors on HUGIN
Multibeam echosounderImaging sonarAltimeterAnti collision sonarDoppler velocity loggerSubbottom profilerAcousticcommunications
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Applications Mapping
Multibeam echosounders
Multibeam echosounders maps the seafloor by estimating therange in different directionThe map resolution is determined by the 2D beamwidth and therange resolution
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Applications Mapping
MBE Example 1
Data collected by HUGIN AUVMaps from the Ormen Lange fieldThe peaks are 50 m high
Courtesy of Geoconsult / Norsk Hydro.
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Applications Mapping
MBE Example 2Data collected by HUGIN AUVMaps from the Ormen Lange fieldThe ridge is 900 m long and 50 m high.
Courtesy of Geoconsult / Norsk Hydro.
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Applications Mapping
MBE Example 3Data collected by HUGIN AUVExample area with large sand ripples
Courtesy of Kongsberg Maritime / FFI.
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Applications Mapping
MBE Example 3
Hull mounted MBE70 - 100 kHzMagic T (Mills cross) layout
From www.simrad.com. Courtesy of Kongsberg Maritime
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Applications Mapping
MBE Example 4Hull mounted MBE70 - 100 kHzColour coded seafloor depth
From www.simrad.com. Courtesy of Kongsberg Maritime
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Applications Imaging
Sidescan sonar
Sidescan sonar: sidelooking sonar to image the seafloorTypical platform: towfish, hull mounted, AUVAn image is created by moving and stacking range linesTypically frequency 100 kHz - 500 kHzTypical range 100 - 500 m
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Applications Imaging
Sidescan sonar area coverageRange resolution is given by the pulse length (or bandwidth)Along-track resolution is range dependent
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Applications Imaging
Sidescan sonar example
Image collected by HUGIN AUV. Courtesy of Kongsberg Maritime / FFI
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Applications Imaging
Synthetic aperture sonar principle
Collect succesive pulses in a large synthetic array (aperture)Increase the azimuth (or along-track) resolutionRequires accurate navigation - within a fraction of a wavelengthVery similar to Synthetic Aperture Radar (SAR)
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Applications Imaging
Synthetic aperture sonar principleThe length of the synthetic aperture increases with rangeAlong-track resolution becomes independent of rangeAlong-track resolution becomes independent of frequency
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Applications Imaging
Sidelooking Example - very high resolution (SAS)
Image collected by HUGIN AUV. Courtesy of Kongsberg Maritime / FFI
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Applications Imaging
Resolution matters
Image collected by HUGIN AUV. Courtesy of Kongsberg Maritime / FFI
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Applications Imaging
Example: Fishing boat
Image collected by HUGIN AUV. Courtesy of Kongsberg Maritime / FFI
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Applications Imaging
Properties in a sonar image
Geometry: Rangeand elevationResolutionRandom variability -speckleSignal to noiseObject highlight andshadow
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Applications Imaging
Example large scene with small objects
Image collected by HUGIN AUV. Courtesy of Kongsberg Maritime / FFI
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Applications Imaging
Comparison of sonar image with optical image
Sonar range: 112 mOptical range: 4.5 m
Image collected by HUGIN AUV. Courtesy of Kongsberg Maritime / FFI
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Applications Imaging
Comparison of sonar image with optical image
Sonar range: 73 mOptical range: 5 m
Image collected by HUGIN AUV. Courtesy of Kongsberg Maritime / FFI
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Summary
Outline1 Basics
IntroductionBasic PhysicsUnderwater sound
2 Sonar TheorySonar typesPosition EstimationSignal processing
3 Sonar ApplicationsFish findingHUGIN AUVMappingImaging
4 Summary
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Summary
Summary
Acoustics is the only long ranging information carrier under waterSound velocity variations cause refraction of acoustic wavesThe ocean is lossy: higher frequencies have shorter rangeSONAR is used for
I positioningI velocity estimationI characterisation
Applications:I Fish findingI Imaging of the seafloorI Mapping of the seafloorI Military
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Summary
Pa Norsk
Engelsk Norskbeam stralebeamwidth stralebredderange avstandbearing retningechosounder ekkoloddsidescan sonar sidesøkende sonarmultibeam echosounder multistrale ekkolodd
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