Upload
others
View
2
Download
0
Embed Size (px)
Citation preview
Ongoing Developments in Side Scan Sonar
The pursuit of better Range, Resolution and Speed
Nick Lawrence
EdgeTech
Advances in Seafloor-mapping Sonar Conference
30th November 2009
Company Profile
• EdgeTech (formerly EG&G Marine Instruments)
• Started in 1966 by Doc Edgerton
• Offices in Massachusetts and Florida
• Sister division to ORE Offshore
• 3 main product lines:
– Side Scan Sonars (SSS)
– Sub-Bottom Profilers (SBP)
– Combined SSS/SBP Sonar Systems
Ongoing Developments ?
• Technologies that have been around long enough that we just accept them as normal.
• Technologies that are currently available
– Not all of which are widely adopted
• Where next ?
Range
• Definition of maximum Range?
– See an echo from a target (and how large?)
– Delineate shadows behind the target, so as to aid identification.
• Either way, it‟s a matter of signal strength and noise.
What affects the Range ?
• Signal Strength
– Absorption (depends on frequency, water temperature and salinity).
– Altitude (grazing angle – reflected energy)
• Noise
– Water Depth (surface noise /backscatter)
– Ambient Noise
– System Noise
Absorption Regional Differences
Absorption
Improving Signal Strength
• Wideband, Low amplitude, linear FM chirp pulses
– Provide high energy signals, with superior resolution
– CW: Tx power typ. 1kW, Pulse length ~ 50us
• Energy = 1e3 * 50e-6 = 0.05 Joules
• Resolution = 3.75cm (depends on pulse length)
– CHIRP: Tx 200W, pulse length ~3ms, BW = 40Khz
• Energy = 200*3e-3 = 0.6 Joules
• Resolution = 1.8cm (depends on bandwidth)
Reducing Noise
• Digital Towfish – no cable noise
• Linear FM chirp pulses (again!)– Implementation of matched filter processing
• out of band rejection > 70dB
– Wideband front end analog electronics
• excellent linearity and no phase distortion
More Signal…
Less Noise….
Better Range
Resolution
• Across Track Resolution is really not the issue
– Frequency dependant
– Typically side scan sonars have plenty of across track resolution
• Along Track Resolution is the real issue!
Resolution Along Track:The „simplified‟ case
• Beam width is determined by length of the sonar array
• Longer arrays with narrower beams provide finer spatial resolution
• Definition of beamwidth– See App note on
www.edgetech.com
Resolution Along Track:A better model
Resolution Along Track:Even that is not everything !
The classic dilemma: Near Field vrs Far Field
• Best resolution is limited by the length of the array
• But remember what happens with a short array!
Resolution: Multi-beam SSS
d
Transmit beamReceive Beams• Use highest
possible frequency
• Longest possible arrays
• Beam forming techniques
Resolution: Synthetic Aperture
• SAS overcomes the array length limitation by synthesizing an extremely long array– Coherently combines data from
many successive pings– Resolution is no longer a
function of physical array length
• Because array length is no longer an issue, neither is frequency– Can use lower frequencies to
increase range
“Synthetic”Aperture
“Physical”Aperture
Sub-Element
The EdgeTech solution:Dynamic Focussing
• What if we segment the array…
• … and stay in the near field as long as possible...
• …so we will get the best resolution possible at a given range…
The EdgeTech solution:Dynamic Focussing
Take Advantage of this Multi-Element Array in Another Way
• What if we curve the segmented the array?
• Why do we want to do this?
– Because sound does not come back to the array in a flat plane…
…it‟s curved slightly!
Application of Dynamic Focusing
• Various configurations have been built
– Different Frequencies
– Different number of segments
– Variable length segments
– Not necessarily symmetrical
300Khz, 8 Elements
Non Focused
Focused
Speed
• Why do we care ?
– Better imaging with more „hits‟ on a target
– Specifications
• IHOManual on Hydrography M-13 Ch 4
– the minimum number of returns to make a discernible mark on the trace is taken as five
• NOAA– The hydrographer shall tow the side scan sonar at a speed such that an object 1 m on a side
on the sea floor would be independently ensonified a minimum of three times per pass.
The general rule is 3 „hits‟ on a 1m target per pass
Speed – Conventional Sonar
• Conventional side scan sonar tow speeds typically limited to 4 - 5 knots or less...
• Low probability of target detection or recognition even at moderate (4 knot) speeds…
T
x
T = (2*Rmax)
C
T
x
Range Scale = Rmax
Rx Period
Speed – Conventional SonarAim is for 3 hits on a 1m target
• Based on travelling a distance d between pings
– Typical: d = 0.33 m, Range R = 125 m
– V1p <= 2 m/s (3.9 knots)
• At longer ranges, you have to go even slower….
Multipulse Transmit Sequence
Multipulse Timing
T
x
A
T = (2*Rmax)/C
T
x
A
T
x
B
T
x
Btp
T
x
C
T
x
D
T
x
C
T
x
C
T
x
D
Range Scale = Rmax
Rx A Period
Rx B Period
Rx C Period
Rx D Period
tp
tp
tp
Speed – MultipulseAim is for 3 hits on a 1m target
• Based on travelling a distance d between pings
– Typical: d = 0.33 m, Range R = 125 m
– V1p <= 2 m/s (3.9 knots)
– V2p <= 4 m/s (7.8 knots)
– V3p <= 6 m/s (11.7 knots)
– V4p <= 8 m/s (15.6 knots)
• Or more hits on the same target size, at the same speed….
Multipulse – Technical Issues
• The sonar system must be able to:– Process receive data whilst transmitting
– Handle returns from all ranges simultaneously (so no analogue TVG can be applied)
– Provide separate output data streams for each transmit pulse
• Once again it is coded pulses and matched filter processing that is required (Chirp !)
Receive while Transmitting
Pulse Separation
Receive data Processing
• The receiver
– uses digital (DSP) replica correlation techniques to reject out of band echo energy.
– correlation processing provides high resolution, pulse compressed, high signal to noise, output data.
– provides an output data stream for each pulse.
Pulse Separation Processing
Multipulse
Data Examples
Results – Comparison 1
Image of a shipwreck,
Conventional sonar
Across
Track
Range
(meters)
Fwd
Multipulse Image of a shipwreck, 3 Pulses
Results – Comparison 1
Image of a 2m x 2m target, 1 Pulse
Fwd
Multipulse image of a 2m x 2m target, 4 Pulses
3 pulse versus 1 pulse
Four pulse image of sand waves and rocks
Four pulse image of sand waves and rocks Zoomed
One pulse image of sand waves and rocks Zoomed
4700 Data Examples
4700 , 600Khz , 1 pulse, Speed 3.7 knots
4700 , 600Khz , 1 pulse, Speed 7.5 knots
4700HF, 2 pulses, same area as 1 pulse, speed 7.0kts
4700, HF 3 pulses, same area as 1 pulse, speed 7.3 kts, first screen,
4700HF, 3 pulses, same area as 1 pulse, second screen, 7.3 kts
Products – technology table
Chirp Dynamic Focusing
Multipulse
4125 CW SP
4125 SP
4200-SP SP
4200-MP 2
4700 DFX 3
AUV / ROV Systems
Option Up to 3
4200 Series
4200-SP (Single Pulse)
Digital dual simultaneous frequency side
scan sonar system
4200-MP (Multi-Pulse)
Digital dual simultaneous frequency side
scan sonar system
with added High Speed Mode allowing tow
speeds of up to 10 knots while meeting 3
hits on a 1 m target.
100/400, 300/600 or 300/900 kHz
Choice of rack mount, portable or 3rd party interface for topside processor
Choice of stainless steel (2000m rated) or lightweight aluminum (500m rated) towfish
Pressure, temperature, magnetometer, acoustic responder,
depressor and custom sensors available as options
4200 Series
4200 Series
What next ?