1. 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 , 2008, F. Jean

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

3. 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

4. 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

5. 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)

6. 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

7. 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

8. 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

9. Fusion

10. Results in shallow water
Portsmouth, spring 2008

11. 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

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

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

14. 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)

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

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

17. 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

18. Questions