advantages of a combined sonar data acquisition system for auvs and asvs

Advantages of a combined sonar data acquisition system for AUVs and ASVs

Tom Hiller

[email protected]

Abstract:

• Autonomous underwater vehicles (AUVs) often operate close to the seabed (5m-15m) enabling higher resolution surveys using high frequency sonars. Compact Autonomous surface vessels (ASVs) are often deployed in shallow water environments where deeper-draft manned survey vessels are unable to operate. On such vehicles there is limited space to deploy separate imaging, mapping and sub-bottom sonars. This presentation describes the technology deployed in the EdgeTech 2205 sonar system, which enables combined data acquisition in one system on AUVs and ASVs. Examples of the data acquired are given, which can include dual- or triple- frequency side scan, Multiphase Echosounder (MPES) swath bathymetry, and sub-bottom profiler data.

There are 4 factors that make a successful Unmanned Vehicle:

The Vehicle

Positioning

Data Acquisition

Control

What does an autonomous vehicle do?

The AUV/ASV must:

• Be able to carry out a mission plan

• Be able to position itself accurately and reliably

• Collect useful data

A lot of development effort has gone into the vehicle and control

Positioning is a whole other presentation

The point of having the system in the field is to collect data

• Industry leader in Underwater Acoustics

• EdgeTech (formerly EG&G Marine Instruments)

– Started in 1966 by Doc Edgerton

• ORE Offshore

– Formed in 1963

• Manufacturing and Support - Massachusetts USA

• R&D – Florida USA• + Worldwide rep network and regional Authorised Service Centres

EdgeTech Background

Common Technologies

Underwater Acoustics

Digital Signal Processing

• Sonar Systems– Side Scan Sonar

– Sub-bottom Profilers

– Bathymetry

– Combined Systems

– AUV/ROV Systems

• Actuated Products– Acoustic Releases

– Pop-up Systems

– Acoustic Actuators

• Navigation & Positioning– USBL Acoustic Tracking Systems

– Motion Reference Unit (MRU)

EdgeTech Products & Solutions

• A Modular and Configurable Sonar System for Hosted Platforms

• For installation on Unmanned Underwater Vehicles (UUVs / AUV), Remotely Operated Vehicles (ROVs), Unmanned Surface Vehicles (USVs), and other hosted platforms.

• Modular units able to be configured, based on the application, to collect side scan sonar imagery, sub-bottom profiler data and wide swath bathymetry data, singly or in concert with one another.

Designing Data collection for AUVs and ASVs

• Stage 1 Introduced 1st Qtr 2013

– Dual frequency side scan• Option for Dynamic focused capability : 230 / 850 DF one of the first frequency sets

– Sub-bottom capability with linear amplifier card• Various transmit / receive array options

• Stage 2 Completed 2014

– Bathymetry capability• Needs an additional board (24 ch A/D)

• Needs bathymetry capable arrays

– Tri-Frequency side scan• Can run dual simultaneous side scan, where the operator can choose any two frequencies from

the three available

As of end 2015 over 50 types of unmanned vehicles have integrated EdgeTech sonars

2205 Development:

• Side Scan – Single or dual frequency Arrays

– Dynamic Aperture / Dynamic Focussing options

• Sub-bottom Profiler– Range of frequencies / Transducer Options

• Swath Bathymetry– Multiphase Echosounder wide swath bathymetry

Combinations of frequency options to suit the application

2205 Options

Configuration 1 – SSS / Bathymetry only

SSS / Bathy

Port Array

Starboard Array

AUV control/ ROV Mux

Sub-Bottom

Configuration 2 – SSS / Bathymetry / SBP

SSS / Bathy

Port Array

Starboard Array

AUV control/ ROV Mux

Hydrophones

Hydrophones

Tx Tx

2205 Board Set Building Blocks

PSU

CPU

SAIBU

Cap Banks

SIM(blue board)

HCCA(bathymetry only)

Board Stack Weight in Air with Cabling = ~3.2lbs

Size – Housing for SSS / SBP / Bathymetry

• Ethernet– Data out– Navigation data in– Command and Control

• Backward compatible with existing EdgeTech Interface

• RS232 – 2 low latency ports as standard– Navigation and motion data in

• Triggers– 3 configurable as in / out– Typically configured as trigger in, PPS in, and Trigger out

• Expansion capability– 2 x USB 2.0 ports– SATA interface for onboard storage

Standard Interfaces

JSF File Format

• May contain the following (depending on what sensors are configured/installed):

– Dual Frequency SS

– Bathymetry

– Sub Bottom

– Navigation (lat/lon)

– Attitude (roll/pitch/heave)

– Heading

– Pressure (or Depth)

– All with a common time stamp

A Common, Standardised File Format

Deployment Examples

ASV Deployment Examples :

ECA Inspector

• Side Scan / Bathymetry system

• Automated Bow mount

ASV deployment example: swath bathymetry +

dual frequency side scan

C&C / ASV 6300

• Side scan sonar

• Sub-bottom Profiler

• Hull mounted systems

Deployment examples :

ASV example :

Sea Robotics

• Side Scan Sonar

• Sub-Bottom Profiler

AUV / ROV Systems

2205 Example Configurations

Application Sensor Frequencies Survey Goals & Considerations

Military MCM

Side Scan only

540/1600 kHz High resolution small size target location and identification.

230/850DF kHz Highest resolution-to-range on a standard military COTS side scan sonar

Side Scan + Bathymetry540/1600 kHz

with Bathymetry at 540kHz

High resolution small size target location and identification that has

bathymetry for added 3D verification.

Commercial Survey

(small vehicle 4-9”)

Side Scan only

410/850 kHz Maximum range-to-resolution package for a small auv

540/1600 kHzHigher resolution survey solution for small auv with added Bathymetry

capability

Side Scan + Bathymetry 540/1600 kHz (bathy on 540 kHz) Higher resolution survey solution for small auv

Commercial Survey

(large vehicle 10”+)

Side Scan only

120/410 kHz Maximum range surveying on a large vehicle.

230/540 kHz Best mid-range compromise between range/resolution for surveying

230/850DF High res on a large auv. DF for extra focus on small targets

230/540/1600 kHz Tri-FrequencyAny two frequencies at the same time, with the selection optimized for

the mission

Side Scan + Bathymetry

230/540 kHz (bathy on 540 kHz) Best mid-range compromise with added Bathymetry

230/540/1600 kHz (bathy on 540 kHz)Tri-Frequency (any two side scan frequencies at the same time) , with

bathymetry also available if 540 kHz is selected as one of the frequencies

Sub-bottom Profiler

(Optional)

4-24 kHz Smallest size, highest resolution, but least penetration

2-16 kHz Good balance between size and performance (penetration & resolution)

1-8 kHz Best penetration, largest size and power consumption

Technology Details

• Side scan

• Sub bottom

• Bathymetry

• Sideways looking ; 2 sides

• Wide vertical beam

• Narrow horizontal beam

2205 : Side Scan Sonar

• The sonar sends out sound pulses or pings from each side of the tow fish and then receives or listens for the pulse to return.

• Each ping returns a line of imagery. These pings are combined together to create an image.

2205 : Side Scan Frequencies

Chirp CentreFrequency

NominalFrequency*

Range

120 kHz 100 kHz 250 to 500m

230 kHz 300 kHz 150 to 300m

410 kHz 400 kHz 130 to >200m

540 kHz 600 kHz 100 to 150m

850 kHz 900 kHz 50 to 75m

1600 kHz 1600 kHz 20 to 35m

You will sometimes see the ‘nominal’ frequency quotedin specifications and marketing material.

The selection of the best side scan sonar frequency for any

application is a compromise between range and

resolution. Lower frequency systems provide longer range (allowing a larger area to be covered in each pass), whilst

higher frequency systems provide a higher definition image

• Standard or Customised Arrays– Standard arrays from towed systems– Customised arrays for a particular

vehicle

• Rear radius to fit vehicle

• Different connection options

• Dual Frequency Combinations– Lower frequency provides a search

capability

– Higher frequency provides classification / identification

– Typical Combinations:• 120 / 410 kHz• 230 / 540 kHz• 230 / 850 kHz• 400 / 900 kHz• 600 / 1600 kHz

• Tri-Frequency– Operator selects two from three

– Selection depends on mission

– Typical Combinations:• 230 / 540 / 850 kHz• 230 / 540 / 1600 khz

Dual Frequency or Tri-Frequency Side Scan

Dynamic Focussing : Results

300 kHz 8 Element Dynamic Focusing300 kHz No Dynamic Focusing

• Various configurations have been built– Different Frequencies– Different number of segments– Variable length segments– Not necessarily symmetrical

• Range of SBP Transducer Options

2205: Sub-bottom Profiler

DW-424 DW-216 DW-106

Frequency Range 4 – 24 kHz 2 – 16 kHz 1 – 8 kHz

Pulse TypeFull Spectrum chirp frequency modulated pulse

with amplitude and phase weighting

Vertical Resolution (depends on Pulse selected)

4 – 8 cm 6 – 10 cm 15 – 25 cm

Penetration (typical)In coarse calcareous sandIn clay

2 m40 m

6 m80 m

15 m 150 m

Beam Width(depends on centre frequency)

15° - 25° 15° - 25° 28° - 36°

• EdgeTech “Full Spectrum” pulse characteristics– Bandwidth

• The wider the bandwidth, the better the resolution

– Length• Longer pulses provide more acoustic energy

– Waveform• For “FM” shaped pulses the Full Spectrum wavelet is weighted in the frequency domain to have a Gaussian like

shape (Blackman-Harris window) which provides a great rejection of the side lobes.

• Wideband (WB) pulses have a flat response over the entire pulse bandwidth, so more low frequency content.

• Sweep has a linear variation of frequency with time.

• Quadratic Pulses– Waveform

• Sweep rate varies with time, with greater proportion of pulse length spent at lower part of the frequency range.

• Designed to help improve penetration.

• Dual Frequency Mode– Two data streams

• Two data streams, each utilising a different pulse.

Sub-bottom Profiler Pulse Options

• Transmit and Receive on the Transducer– Reduced Hardware

• Adds a T/R (Transmit/Receive) switch, but removes the need for separate receive hydrophones.

– Limits Pulse Length• Limits the pulse length that can be used when close to the seabed – need to stop transmit pulse, and eliminate

ringing, before 1st receive.

• Not recommended for AUV’s.

• Separate PZT receive Hydrophone arrays– Pulse length independent of altitude

• Can receive on hydrophones whilst still transmitting.

– Linear arrays in parallel used to define receive beam pattern

• Separate PVDF receive Hydrophone panels– Pulse length independent of altitude

• Can receive on hydrophones whilst still transmitting.

– Standard panel sizes or Custom built • Custom sizes can be designed to fit vehicle

• Multiple receive options for different applications.

Sub-bottom Profiler Receive Options

Sub-bottom Profiler on an AUV

AUV Sub-Bottom data

DW-106 Transmit

Quadratic pulse

PVDF receive arrays

Applications : Pipeline and cable detection

2205 Bathymetry Technology

• Multi-Phase Echo Sounder (MPES) technology (same as EdgeTech 6205)

• Swath up to 10 x water depth, and IHO SO to up to 9 x water depth.

• Two Bathymetric frequency Options

– 230 kHz, longer range

– 540 kHz, higher resolution

• Integrated Dual Frequency Side Scan

• Better Spatial Resolution, but without the Nadir gap traditionally associated with Interferometers

Typical Multibeam Footprint

Typical 6205 Footprint

2205 Bathymetry: Frequency Options

• The 6205 is available in several standard frequency configurations:

– 230 kHz Bathymetry : to 215m depth below transducers

With 230 & 550 kHz Dual Frequency Side Scan

– 550 kHz Bathymetry :to 75m depth below transducers


– 550 kHz Bathymetry


The modular design of the 6205 also allows for multi-frequency bathymetry options with field exchangeable array design for survey operations in both shallow and deep water operations.

Data Examples

6205 Performance across swath-Data Example

Data Resolution

retained across swath

Cross Section

Shown

1600kHz SS Data Sample, 5m Altitude

1600kHz Data Sample, MLO

• Bathymetry & Simultaneous Dual Frequency Side Scan Sonar

• Precisely Co-Registered Bathymetry & Side Scan Imagery

Co-Registered Bathymetry & Side Scan

550kHz SS Data Sample, 12m Altitude

Matching 550kHz Bathymetry

High-Density Cloud

View of Processed

Bathymetry

Sand Waves

Swath Bathymetry with Side Scan

Co-Registered Bathymetry & Side Scan

Co-Registered Bathymetry & Side ScanNadir Coverage

Data: Examples

Swath Bathymetry : Pipeline Data Example

Eel Grass in Side Scan & Co-Registered Bathymetry Water Depth less than 1 meter at Chart Datum

Side Scan and Bathymetry : AUV Data

At this resolution, the

Bathymetry data quality is

limited by the quality of

the motion data.

AUV Swath Bathymetry with Co-registered Side Scan

The Great Lakes Research Center (GLRC) at Michigan

Technological University, deployed an OceanServer

Iver AUV with an EdgeTech 2205 combined Side Scan

Sonar and Bathymetry system with the goal of imaging

an underwater pipeline.

Side scan sonar and bathymetry images were used to

check the location and integrity of the pipeline over a

long and varied terrain. Using side scan sonar co-

registered with bathymetry the team was able to view

intricate details about the pipeline and the bottom

topography, helping accomplish their mission.

Conclusion

• Flexible and configurable sonar data collection via the EdgeTech 2205 sonar system now enables AUV and ASV technology to carry out a variety of offshore survey, search and inspection tasks.

Advantages of a combined sonar data acquisition system for AUVs and ASVs

Tom Hiller

[email protected]