intro to sonar – noaa hydro training 2009 introduction to sonars

Intro to Sonar – NOAA Hydro Training 2009

Introduction to Sonars


Learning Objectives• How do sonars work? • Materials used to make transducers• Resolution

– Pulse length and range resolution– Beamwidth and angular resolution

• Sonar beam patterns– Effect of array diameter to wavelength ratio– Effect of array length to wavelength ratio

• Frequency review– High frequency: high resolution; poor range– Low frequency: low resolution; good range

• Common sonar settings and what they do– range, power, gain, pulse length/width.


Brief History of Sonar

• Initially a tool for detecting icebergs (shortly following Titanic 1912)

• Quickly developed by navies in response to submarines.

• SOund Navigation And Ranging

• Active Sonar (transmit and receive)

• Passive Sonar (receive only)


Basic Active Sonar Principle

• Transmit Sound• Measure round trip

travel time.• Use sound speed to

get distance

2.

speedtimeDist


Transducer Materials

• Transducers are typically composed of Piezoelectric (crystals) or electrostrictive (ceramic) materials– Quartz crystals– Barium titanate– Lead zirconate titanate (PZT)– Polyvinyldene flouride (PVDF)

• But explosives, air guns, and other sound sources are used in some applications.


Transducer Materials

PZT Ceramic shapes (EDO Corp)

http://www.edoceramic.com/Materials_Shapes/

1-3 Composite (Materials Systems Inc.)

http://www.matsysinc.com/im.html

PVDF polyvinyldene fluoride (Airmar Corp)

http://www.airmar.com/


Pulse length/width

• Sound is transmitted in pulses, not continuously

• Pulse length and width often used interchangeably

• Length of time of pulse• Width of pulse in the water

• Pw = c x Pl

Naval Echo-ranging (WWII)

Killer Whale Feeding


Pulse Length

Pulse Length


Pulse Length and Resolution

Objects separated by less than ½ the pulse width cannot be resolved as independent objects.

d

incident sound pulse

2d


Pulse Length-Example

• A typical launch sonar (8101) setting for pulse width is 93 micro seconds (93µs)

• Take sound speed to be 1500 m/s.

Pulse Length = speed * time

PL = 1.5x103 m/s * 93x10-6 s

PL = 140x10-3 m

PL = 14 cm

but resolution is ½ pulse lengthso:Range Resolution = 7 cm


Pulse Length and Resolution

Long Pulse Short PulseGood signal to noise ratio OK signal to noise ratio

High Energy in the water Less energy in the water

High maximum range Shorter maximum range

Low range resolution High range resolution

Good for deep, flat areas Good for shallow, feature rich areas


Beamwidth and Angular Resolution

Yellow: Wide beamwidth

Blue: Narrow Beamwidth

Objects separated by less than the beamwidth cannot be resolved as individual objects.


Beamwidth and Resolution

Advantages of Wide Beam

•Better Coverage

•Typically smaller and cheaper transducers

Advantage of Narrow Beam

•Good spatial resolution

•Higher effective range


Interference and Beam Formation

The central region of constructive interference is the main lobe

The off center regions of constructive interference are called side lobes

The regions of destructive interference are called nulls.


Beamwidth- Fix Analogy

Closely spaced sources gives a wide beam

Just like closely spaced radar targets give a imprecise fix


Beamwidth- Fix Analogy

Widely spaced sources gives a narrow beam

Just like widely spaced radar targets give a more precise fix


Beam Patterns

Effect of increasing radius / wavelength ratio

SE 3353 Imaging and Mapping II: Submarine Acoustic Methods

© J.E. Hughes Clarke, OMG/UNB


Beam Patterns

Anatomy of a beam pattern from a circular plane array


Non-Symmetrical Beam Patterns

Effect of increasing length / wavelength ratio

SE 3353 Imaging and Mapping II: Submarine Acoustic Methods

© J.E. Hughes Clarke, OMG/UNB


Beamwidth, wavelength and transducer size

• The shape of the beam is governed by the dimensions of the transducer.

• Big and small are relative to the wavelength of the transmitted sound.

• For a desired beamwidth, if we double the wavelength (halve the frequency), the transducer needs to double in size.



Consider the difference between a 12kHz deep water system and an 455kHz shallow water system.

To achieve the same angular resolution as the 455kHz system, the 12kHz transducer needs to be about 40 times the size.



12kHz deep water system, 1º beamPhoto: NAVO

455kHz shallow water system, 0.5º beamPhoto: Reson


Sonar Frequency Review

• High Frequency– High resolution

• Shorter pings• Smaller scatter size

– Small transducers– Poor range

• Low Frequency– Good range – Big Transducers– Generally poorer resolution


Sweep Sonar Systems


Side Scan Sonars

• Rather than just looking at the time of first return, we can look at the entire time series return, and image the bottom acoustically.

• Sidescan sonars typically use a towed sensor, although they can be hull mounted

• Sidescan sonars transmit and receive signals on a linear array of transducers.

• Sidescan sonars are used for locating and identifying targets (i.e. Dangers to Navigation) based on the strength of the returned signal

http://woodshole.er.usgs.gov/operations/sfmapping/sonar.htm


Multibeam Sonar

•Simultaneous beam formation from one transducer array.


Phase Differencing Sonars

• Bathymetric Sidescan Sonars

• Able to give depths as well as imagery

•Benthos C3D

•Klein 5410

•GeoSwath


Sonar Settings

• Range

• Gain– Time Varied Gain (TVG)– Fixed

• Power

• Pulse length (pulse width)


Knobs Shared by All Sonars

Range

• How long the sonar listens for a return

• Determines how frequently to ping– Pulse repetition rate (PRR)

• At a given speed, determines along track ping spacing.


Range

• Incorrect range setting increases noise– Set too short – the outer beams aren’t long enough to

reach the seafloor– Set too long – ping rate is reduced and the sonar

‘listens’ for too long, increasing the noise in the return signal

Good Too Low – Bad Outer Beams are Lost

1 2 3


Power

Power

• How loud to project the sound– volume on the

speaker


Power

• This setting should be set as low as possible

• When power is too high, fliers increase and there is a halo around each ping on the display

• There is the possibility of a double return

• May need different power settings for different bottom types


Gain

• The gain function controls how much the returned sonar signal is amplified

• In manual mode, gain settings between 4 and 12 yield the best results

• In auto mode, settings Auto 2 through Auto 4 are typical

• Most NOAA vessels use manual gain


Time Varied Gain

• Compensates for spreading and attenuation by increasing gain for more distant signals

• Basically listening harder to the signal that comes later in the return.

Receiver gain = (2 α R) + Sp logR + G

α = Absorption loss (dB/km)R = Range (m)Sp = Spreading loss coefficientG = Receiver gain


Transmit Pulse Width

• Variable depending on sonar• Measured in microseconds or milliseconds• The smaller the number, the shorter the pulse

width – typical setting between 70 and 120• Lower frequency system have longer pulses• High frequency, high resolution systems have

short pulse lengths• The shorter the pulse, the better the resolution,

the longer the pulse the better the range performance (more energy in the water) but the poorer the resolution


Common Issues

Q: What’s wrong with this data? What could be done to improve it?

A: It is overpowered. The transducer power needs to be turned down


Common Issues


A: The range is too low. Noise is apparent in the data.


Common Issues


A: The range is too high. The outer beams are lost. The range should be lowered.

intro to sonar – noaa hydro training 2009 introduction to sonars

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