Introduction to Sonar INF-GEO4310 Roy Edgar Hansen Department of Informatics, University of Oslo September 2011 RH, INF-GEO4310 (Ifi/UiO) Sonar Sept. 2011 1 / 76 Outline Outline 1 Basics Introduction Basic Physics Underwater sound 2 Sonar Theory Sonar types Position Estimation Signal processing 3 Sonar Applications Fish finding HUGIN AUV Mapping Imaging 4 Summary RH, INF-GEO4310 (Ifi/UiO) Sonar Sept. 2011 2 / 76 Sonar Basics Outline 1 Basics Introduction Basic Physics Underwater sound 2 Sonar Theory Sonar types Position Estimation Signal processing 3 Sonar Applications Fish finding HUGIN AUV Mapping Imaging 4 Summary RH, INF-GEO4310 (Ifi/UiO) Sonar Sept. 2011 3 / 76 Sonar Basics Introduction History SOund Navigation And Ranging If you cause your ship to stop and place the head of a long tube in the water and place the outer extremity to your ear, you will hear ships at a great distance from you. Leonardo da Vinci, 1490 1914 Fessenden: first active sonar system (detect iceberg 2 miles) Images from wikipedia.org. RH, INF-GEO4310 (Ifi/UiO) Sonar Sept. 2011 4 / 76
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SOund Navigation And RangingIf you cause your ship to stop and placethe head of a long tube in the water andplace the outer extremity to your ear, youwill hear ships at a great distance fromyou.
Leonardo da Vinci, 14901914 Fessenden: first active sonar system(detect iceberg 2 miles)
Images from wikipedia.org.
RH, INF-GEO4310 (Ifi/UiO) Sonar Sept. 2011 4 / 76
Sonar Basics Introduction
The masters in sonar
From wikipedia.org.
RH, INF-GEO4310 (Ifi/UiO) Sonar Sept. 2011 5 / 76
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
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]
RH, INF-GEO4310 (Ifi/UiO) Sonar Sept. 2011 8 / 76
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
RH, INF-GEO4310 (Ifi/UiO) Sonar Sept. 2011 9 / 76
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
The acoustic wave expands asa spherical wave to thereflectorThe reflected field expands asa spherical wave back to thereceiverIn homogeneous media, thetwo way loss becomes
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
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
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
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.
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
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
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
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
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
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
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
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
Multibeam echosounders maps the seafloor by estimating therange in different directionThe map resolution is determined by the 2D beamwidth and therange resolution
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
Collect succesive pulses in a large synthetic array (aperture)Increase the azimuth (or along-track) resolutionRequires accurate navigation - within a fraction of a wavelength
Synthetic aperture sonar principleThe length of the synthetic aperture increases with rangeAlong-track resolution becomes independent of rangeAlong-track resolution becomes independent of frequency
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