Portsmouth, Shallow Survey 2008, oct. 21-24, 2008, F. Jean Shadows, combination of synthetic aperture sonar and forward looking gap-filler. Results in shallow water in Portsmouth during spring 2008. Portsmouth, Shallow Survey 2008, oct. 21-24, 2008, F. Jean

Contents Introduction Presentation of the Shadows system Side Scan SAS Processing Forward looking sonar Processing Results in shallow water Complex Image Informations Further works

Introduction Side scan Sonar : Settings for Shadows : Grazing angle Echoes & shadows Gap Settings for Shadows : Maximal range 300m ( 600m swath ) Resolution 15cm Filling the gap with comparable resolution

Shadows system Shadows architecture 2 m long, 24 receivers Sides 2 m long, 24 receivers 3 transmitters F = 100kHz B = 30 kHz Front 70cm wide, 48 receivers 4 transmitters F = 300kHz B = 40kHz Real time georeferenced images PC algorithm PC acquisition ins

Synthetic Aperture Sonar : Side Scan SAS Synthetic Aperture Sonar : Sum of successive pings Length (~number of pings) adapted to range constant resolution along the swath Algorithm Steps INS Motion Compensation DPCA Motion Compensation Fusion and Real time beam forming (FFBP)

INS motion compensation Side Scan SAS INS motion compensation Highly accurate and synchronised INS Aided by a DVL -> limited linear drift Realtime georeferencing DPCA motion compensation Using correlation between signals Real time georeferenced mosaic

Forward looking sonar Sectorized Emission Principle: Real Time: 4 different footprints Ambiguous reception Real Time: Incoherent Integration of several pings Post Processing: movie of a target. SAS data SAS data

Fusion straight line on the sea-bottom Geo-referenced map Keeping Continuous textures No deformation straight line on the sea-bottom -> straight line on the image

Fusion

Results in shallow water Portsmouth, spring 2008

Results in shallow water 50m x 50m image 30 meter water depth Range : 20m to 100m Results in shallow water 20m x 20m image 30 meter water depth Range : approx. 70m

Complex Image Informations Spatial Fourier transform on amplitude 2D 1D Space (m) Time (s) Frequency (Hz) Spatial frequency (m-1)

Complex Image Informations Spatial Fourier transform on complex data Amplitude phase Frequency Amplitude (constant) phase Frequency -> All information is in the Phase

Complex Image Informations Real Data : 1mx1m structure FFT of amplitude Amplitude FFT of complex Phase 8cm : along track resolution (SAS) 3cm : across track resolution (30kHz Bandwidth)

Complex Image Informations Low frequency pixels Complex Image Informations Partial Spectrum Images High frequency pixels

Complex Image Informations Filtering using phase of image Hermitian filter : Enhancing the ‘white pixels’

Side scan SAS Forward looking Sonar Further Works Despeckling images Work on different frequencies (higher or lower) Interferometry Forward looking Sonar Increase fusion quality between sectors Coherent integration

Questions