TOPICS COVERED:- How SONAR works Factors that affect the performance of a sonar unit Factors that affect underwater acoustic propagation in the ocean Principles of sonar Application of sonar. Significance of frequency Conclusion…

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Table of Contents
Introduction ……………………………………….2
How SONAR works……………………………. ..3
Factors that affect the performance of a sonar unit ..4
Factors that affect underwater acoustic propagation
in the ocean………………………………………...6
Principles of sonar………….……………………...14
Application of sonar..……………………………...21
Significance of frequency…………………………..31
Conclusion………………………………………… 32
Summary……………………………………………33

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Introduction
Sonar is a technology that was designed and developed as a means of tracking
enemy submarines during World War II. Although this was the main aim at that
period of time this technology has taken a different shape in today’s world. We
use modern sonar systems for many purposes such as detection, identification
and location of submarines, in acoustic homing torpedoes, in acoustic explosive
mines and in mine detection, in finding schools of fish, in depth sounding
applications, to map the seabed, for navigation purposes and in locating
submerged wrecks and also for echo detection to maximize the range at which
submarines can be detected and tracked and for many other applications.
This report highlights the basic concepts and principles of sonar, its modern-day
uses and possible future applications. It also discusses the way we see fish and
harmful effects of sonar systems while providing a brief description of the
factors that affect the performance of a good sonar system.

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How SONAR works
The word "sonar" is an abbreviation for "Sound, Navigation, and Ranging".
This is a technology that was developed as a means of tracking enemy
submarines during World War II. A sonar consists of a transmitter, transducer,
receiver, and display.
In the simplest terms, an electrical impulse from a transmitter (such as a very
short burst of electrical energy generated by an electronic “power pack”) is
converted into a sound wave (which is also a very short burst of high frequency
sound energy) by the transducer and sent into the water. When this wave strikes
an object, it rebounds.
This echo strikes the transducer, which converts it back into an electric signal,
which is amplified by the receiver and sent to the display. The time variation is
displayed on the read out of the sonar screen device by means of flashing lights,
Liquid Crystal Display (LCD) or
Cathode Ray Tube (CRT or TV screen). Since the speed of sound in water is
constant (approximately 1440
meters per second), the time lapse between the transmitted signal and the
received echo can be measured and the distance to the object determined.
Example: Let’s assume that the time variation is 3 seconds and that the speed of
sound in water is 1440 m/s the distance to the object would be:
(1440 m/s * 3 seconds)/2
= 2160m.
The sonar unit sends and receives signals, then “prints” the echo on the display.
Since this happens many times per second, a continuous line is drawn across the
display, showing the bottom signal. By knowing the speed of sound through
water and the time it takes for the echo to be received, the unit can show the
depth of the water and any fish in the water.

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Factors that affect the performance of a sonar unit
There are four facets to a good sonar unit:
High power transmitter
This enables the user to get a return echo under deep or poor water
conditions and also lets the user to see a fine detail, such as baitfish and
structures.
Efficient transducer
This is a device that converts the electronic energy from the transmitter to high
frequency sound, which is sent down through the water. When it strikes an
object it bounces back as shown in illustration 1. (i.e. it echoes). When these
echoes reach the transducer it converts them into electric signals once again,
which are then amplified by the receiver and passed onto the display. Therefore
this unit is quite often refereed to as the “antenna” of the sonar unit. The
transducer should not only be able to withstand the high transmitter power
impulses, converting as much of the impulses into sound energy as possible, but
it also has to convert them with little loss in signal strength and has to be able to
detect the smallest of echoes returning from deep water or tiny bait fish.

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There are two basic types of transducers
Magnetostrictive transducers:
These are used with the higher powered, low frequency units. The advantage
of this type is that they can take almost unlimited power and may be
overloaded without damage
Ceramic transducers:
These have the advantage of having a higher efficiency factor than the
magnetostrictive types. With ceramic transducers, the lower the frequency
the higher the cost and also if too much power is applied it can be damaged.
Some of the more common frequencies used are 38, 40, 50, 75, 107, 120,
150, 192, 200, 400 and 455 kHz. However it should be noted that transducer
must match the sonar unit's frequency. In other words, a 50 kHz transducer
or even a 200 kHz transducer cannot be used on a sonar unit designed for
192 kHz.
Sensitive receiver:-
This has an extremely wide range of signals it has to deal with. It should
dampen the extremely high transmit signal and amplify the small signals
returning from the transducer. During amplification the strength of the
signals is increase to the point that they can be used to light a neon bulb,
Light Emitting Diode, or to activate a pixel on an LCD. The location of the
flashes on a dial or the location of the pixels on the display can then be used
to indicate the range, or distance, from the transducer to the

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object (bottom) or objects (fish) which have bounced back the echoes. It
also has to separate targets that are close together into distinct and separate
impulses for the display. This process repeats itself many times per second.
High resolution/contrast display
This must have high resolution (vertical pixels) and good contrast to be able
to show all of the detail sharply and clearly. This allows fish arches and fine
details to be shown.
All these facets together are called “Total System Performance”. All of the
parts of this system must be designed to work together, under any weather
condition and extreme temperatures.
Factors that affect underwater acoustic propagation in the
ocean:-
Loss and attenuation, refraction, scattering and noise.
A. Spherical spread
The acoustic wave expands as a spherical wave in a homogeneous medium. The
acoustic intensity I decreases with range R in inverse proportion to the surface
of the sphere as in homogeneous media.
For two way propagation, the acoustic wave expands as a spherical wave to the
reactor. Then, the reflector sphreads the signal in all directions, and the
reflected field expands as a spherical wave back to the receiver

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B. Absorption:-
Seawater is a dissipative medium through viscosity and chemical processes.
Acoustic absorption in seawater is frequency dependent, such that lower
frequencies will reach longer than higher frequencies.
The frequency relation to absorption is such that the travelling distance
measured in wavelengths has a fixed absorption loss. This is summarized in
Table I.
This shows the absorption coefficient in dB per km for frequencies from 100 Hz
to 1 MHz for 4 different temperatures. The absorption coefficient is a function
of temperature, salinity, depth (pressure) and pH in addition to frequency.

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C. Refraction
Consider a plane interface between two di_erent media with di_erent sound
velocity c1 and c2. A plane incoming acoustic wave will partly reect where the
reection angle is equal to the incident angle, and partly refract into the other
medium. This is illustrated in Fig. 5. The angle of refraction is given by Snell's
law
FIG. 5 Snell's law.
The sound velocity in the ocean can be approximated with the following
empirical formula:
c = 1449.2 + 4.6T – 0.055T2
+ 0.00029T3
+ (1.34 – 0.010T)(S -35) + 0.016D
where T is temperature in degrees Celsius, S is salinity in parts per thousand,
and D is depth in meters.
Hence, sound velocity contains information about the ocean environment. As an
example: T = 12.5degree C; S = 35 ppt ; D = 100 m gives c = 1500 m/s.

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The deep water sound velocity can be
divided into four different regions:
 _ The surface layer
 _ The seasonal thermocline
 _ The permanent thermocline
 _ The deep isothermal layer
Since the sound velocity continuously changes with depth, an acoustic ray will
continuously refract into a
new direction. The rate at which the ray changes direction is directly
proportional to the gradient of the sound (according to snell’s law). Fig. 7
shows three example rays with different initial angles, in an upwards refracting
sound velocity profile. For constant gradient in the sound velocity, the rays
become parts of circles.
Fig:-6 Deep water sound velocity

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Fig:-7 Ray trace of sound in an upwards refracting environment.
Consider the interface between two different media, as shown in Fig. 5. A plane
incident acoustic wave will partly reflect and refract at the interface (given by
Snell's law). The amplitude of the reflected and refracted wave are determined
by the angle of incidence and the material properties given by the characteristic
impedance
At normal incidence, the refection coefficient is
V =Z - Z0/Z + Z0
and the transmission coefficient is
W =2Z0/Z + Z0 = 1 – V.
Thus, by operating the sonar in a known environment (with known Z0),
estimating the reflection coefficient from a plane interface to an unknown
medium, the characteristic impedance Z can be calculated, and
Thereby the material properties determined. Note that this description is not
valid when the second medium is absorbing or is elastic.
As an example, consider reflection from the sea surface seen from beneath (as
illustrated in the left panel
of Fig. 9). Using the characteristic impedance for air Z = 415 and seawater Z0 =
1:54 *106
, we get a reflection coefficient of
V =Z - Z0/Z + Z0~-1
which states that the sea surface is a perfect acoustic reflector.

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FIG. 9 Reflection in the sea-air interface
E. Scattering
Scattering of acoustic waves can be of two categories in the ocean:
1. Surface scattering from the sea surface or from the seafloor.
2. Volume scattering from ocean fluctuations, marine
life or objects.
Surface scattering from a smooth surface compared to the acoustic wavelength
(shown in the upper panel of Fig. 10) will mainly give specular reflection. If the
surface is rough, some part of the reradiated
Acoustic energy will be scattered diffusely in random directions, as shown in
the lower panel of Fig. 10. The more rough the surface is the more acoustic
energy will be scattered diffusely.
For non-normal incident waves, such that specular reflection cannot reach
theobserver, the surface has to be rough in order to facilitate any observed
scattered signals. The scattered field is dependent on the roughness of the
surface (relative to the wavelength) and the characteristic impedance

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FIG. 10 Upper: Sound scattering from a smooth surface.
Lower: Sound scattering from a rough surface.
F. Ocean fluctuations:-
In coastal areas and in the upper layer, random variability will affect acoustic
propagation. These effects (see Fig. 11) are ocean turbulence, currents, internal
waves (gravity waves in density variations below the sea surface), the sea
surface and micro bubbles. Ocean acoustics can be used to monitor and estimate
these variations. This is called acoustical oceanography.

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FIG. 11 Random variability in the ocean.

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III. PRINCIPLES OF SONAR
There are two different operational modes for sonar:
1. Passive sonar where an acoustic noise source is radiated by the target, and the
sonar only receives
the acoustic signals (see Fig. 12).
FIG. 12 Passive sonar.
2. Active sonar: where the sonar itself transmits an acoustic signal, which again
propagates to a reflector (or target), which again reflects the signal back to the
sonar receiver (see Fig. 13).
FIG. 13 Active sonar.

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In the following section, we will describe the basic principle of echo location
for active sonar. We also list the components involved in the signal processing
for active sonar.
A. Range estimation
Range defined as the radial distance between the sonar and the reflector, can be
estimated as follows (see Fig. 14)
 A short pulse of duration Tp is transmitted in the direction of the
reflector.
 The receiver records the signal until the echo from the reflector has
arrived
 The time delay _ is estimated from this time series
The range to the target is then given as
R=ct/2
The sound velocity c has to be known to be able to map delay into space.
FIG. 14 Estimation of range
The accuracy of which the range is estimated is related to the pulse length Tp
for traditional pings (or gated CW pulses)
dR =c Tp/2
This is equivalent to the range resolution defined as the minimum spacing two
echoes can be separated and still detected (see Fig. 15). A shorter pulse gives
better range resolution. However, shorter pulses has less energy in the pulse,
which again gives shorter propagation range. An alternative to this is to
modulate (or phase code) the pulse. For phase coded pulses, the resolution is dR
=c/ 2B

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where B is the bandwidth (or frequency spread) of the acoustic signal. actually
covers, since B =1/Tp for gated CW signals.
FIG. 15 Range resolution.
B. Bearing estimation:-
There are two key elements involved in the estimation of direction (or bearing)
in sonar
1. The electro-acoustic transducer and its size
2. The grouping of transducers into arrays
A transducer (or antenna or loudspeaker) is directive if the size of the antenna is
large compared to the wavelength. The directivity pattern generally contains a
main lobe, with a beam width (or field of view)
where D is the diameter (or length) of the antenna. This is shown in Fig. 16. We
note that the beam width is frequency dependent. Higher frequency gives
narrower beam for a given antenna size. Or, conversely, higher frequency gives
smaller antenna size for a given angular spread. This is the single most
important reason to choose high frequencies in sonar imaging.

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FIG. 16 Transducer directivity
C. Imaging sonar:-
The principle of imaging sonar is to estimate the reflectivity for all calculated
ranges and in all selected directions. This is illustrated in Fig. 19. The field of
views given by the angular width of each element. The angular (or azimuth)
resolution is given by the array length measured in wavelengths. The range
resolution is given by the bandwidth of the system.

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D. Signal model:-
The basic signal model for an active sonar contains three main components (see
Fig. 20):
FIG. 20 Signal model.
1. The signal which has propagated from the transmitter, through the medium to
the reflector, is backscattered, and then propagated back to the receiver. The
backscattered signal contains the information about the target (or reflector) of
interest. It depends on the physical structure of the target and its dimensions, as
well as the angle of arrival and acoustic frequency.
2. Reverberation is unwanted echoes and paths of the transmitted signal. This is
typically caused by surface and bottom scattering, and/or volume scattering.
3. Additive noise is acoustic signals from other sources than the sonar itself The
sonar equation is an equation for energy conservation for evaluation of the
sonar system performance. In its simplest form, the equation states the
following:

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Signal - Noise + Gain > Threshold
where Threshold is the value for which the signal after improvement (gain) is
above the noise level. A more detailed version for active sonar is:
SL - 2TL + TS - NL + DI + PG > RT
where SL is source level, TL is transmission loss, TS is target strength, NL is
noise level, DI is directivity index, PG is processing gain, and RT is reception
threshold. Note that the sonar equation describes logarithmic intensity in dB.
Fig. 21 shows the received time series (or range profile) for a single ping of
data with a target of interest.
.
FIG. 21 Example range profile.
E. Signal processing:-
Active sonar signal processing can be divided into a number of di_erent stages.
With reference to Fig. 22 these are
 Preprocessing: filtering and applying time variable gain (TVG)
 Pulse compression: matched filtering in range (convert the time spread
coded pulses to delta functions).

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 Beam forming: direction estimation (or matched _l-filtering in azimuth).
This is to convert element data in an array into directional beams (array
signal processing).
 Detection: Detection of potential targets (i.e. a fish, a submarine).
 Parameter estimation: Estimation of position and velocity of the detected
object.
 Classification: target recognition, pattern recognition.
FIG. 22 Active sonar signal processing chain

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IV. APPLICATIONS OF SONAR:-
• The military uses a large number of sonar systems to detect, identify and
locate submarines.
• In acoustic homing torpedoes, in acoustic explosive mines and in mine
detection
• In finding schools of fish
• For depth sounding applications
• Mapping of the seabed
• Navigation purposes
• Acoustic locating of submerged wrecks
• Special sonar systems are used for sound and echo detection to maximize
the range at which submarines can be detected and tracked.
• Dam Inspection
• Marine Archaeology
• Bridge Footing and Abutments Inspection
• Location of sunken logs from lumber operations
• Oil pipe line location and inspection
• Reef monitoring

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A. Fish finding
Modern vessels for fisheries are usually well equipped with sonar systems. The
basic usage of sonar is to detect and locate fish schools, as shown in Fig. 23.
The most basic sonar type is the downward looking echo sounder for range
positioning of the fish. By moving the ship forward, a 2D scan (or slice) of the
ocean is produced. Typical frequencies used are 20 kHz to 200 kHz, where the
different frequencies have different depth rating (see Fig. 24).

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FIG. 24 Fish detection range (vertical axis) for different sonars operating at
different frequencies.

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B. Seafloor mapping with multi beam echo sounders :-
.
The principle of mapping the seafloor with multi beam echo sounders is as
follows (see Fig. 26):
 The multi beam echo sounder forms a large number of beams for each
ping. The beams are in different direction, spanning a fan cross-track of
the vehicle.
 Along each beam (or direction), the range (calculated from the time
delay) to the seafloor is estimated.
The range estimate in each beam gives the relative height of the seafloor
relative to the vehicle. This again transformed into a map of the seafloor along
the fan.
 The vehicle is moving forward, and consecutive pings gives a continuous
map of the area surveyed.
 The map resolution is determined by the 2D beamwidth and the range
resolution.
FIG. 26 Multi beam echo sounder geometry.

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C. Sonar imaging with side scan sonar:-
Side scan sonar is used to produce acoustic images of the seafloor with high
resolution. The sonar geometry is side looking (hence the name) as illustrated in
Fig. 29.
FIG. 29 Sidescan sonar geometry.
The principle is based on forming an acoustic image by moving the sonar
forward and stacking the sonar response from successive pings (see Fig. 30).
The side scan sonar works best when operated fairly close to the seafloor,
typically mounted on a tow fish (a towed lightweight vehicle), or an
autonomous underwater vehicle (AUV) (as illustrated here). It can also be
mounted on a surface ship hull when operated in shallow waters. The
operational frequencies for sides can sonars are typically from 100 kHz to 1
MHz, with a operational range from 500 m down to a few tens of meters. A
common design in side scan sonar is to choose the highest possible frequency
for a given range (from calculations of absorption) to obtain the best possible
along-track resolution.
The area covered by a side scan sonar is shown in Fig. 30. The cross-track
coverage is given by the maximum range of the system, which again is related
to pulse repetition interval and the maximum range of the acoustic signals (from
absorption). The cross-track resolution is the range resolution, given by the
pulse length (or the bandwidth for coded pulses). The along-track coverage is
given by the pulse repetition interval. Finally, the along track resolution is given
by the directivity of the sonar antenna and the range.

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FIG. 30 Area coverage and resolution in sidescan sonar.
D. Synthetic aperture sonar
A fundamental limitation to traditional side scan sonar is the spatial resolution
along-track (or azimuth). At far ranges, it is usually much worse than the range
(or cross track) resolution. The angular resolution is given by the array length
measured in wavelengths. By decreasing the wavelength (or increasing the
frequency), the angular resolution is improved. This, however, limits the
practical range, due to the frequency dependent absorption is seawater. The
other approach is to increase the length of the array. This, however, requires
more hardware, more electronics and more space on the vehicle. Another
similar
approach is to synthesize a larger array by using consecutive pings from the
moving sonar. This is the principle of synthetic aperture sonar (SAS) as
illustrated in Fig. 31.

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FIG. 31 Principle of synthetic aperture sonar.
By choosing the synthetic array (or aperture) to th maximum length given by
the field of view for each physical element (see Fig. 32), the spatial along-track
resolution becomes independent of both range and frequency. Existing SAS
systems achieve more than one order of
magnitude improvement in along-track resolution compared to similar side scan
sonar systems at long ranges.

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Fig. 33 shows a SAS image compared with a side scan sonar image. The object
in the sonar images is a WWII submarine wreck at 200 m water depth outside
Horten ,Norway. Note the difference in resolution. Even though the SAS data is
collected at longer range, and at much lower frequency, the image contains
much more detail and fidelity, making it easier to determine the contents of the
image.
FIG. 33 Comparison of traditional sidescan sonar with synthetic aperture sonar.
Images collected by HUGIN AUV. Courtesy of Kongsberg Maritime / FFI.

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The properties and practical application of a sonar image, depicted in Fig. 34,
is related to a number of factors: the imaging geometry will cause acoustic
shadows from elevated objects on the seafloor; the image resolution and fidelity
(signal to noise) is related to the ability to detect and resolve small objects; the
water-seafloor interface and its relation to the acoustic frequency will affect the
reflectivity for different bottom types.
FIG. 34 Properties in a sonar image.

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Despite all these uses, there are several disadvantages in using sonar
systems too.
The following is an abstract from a newspaper in Honolulu-Hawaii on the 17
th
of March 2000.
“A new sonar system being tested by the U.S. Navy could pose a threat to
marine life, including whales, and to human swimmers and divers, critics say.
Environmental groups, the Hawaii’s County Green Party, and a member of the
Hawaii’s County Council have filed a lawsuit to halt the Navy's preparations to
deploy the low frequency active sonar system. Humpback whales are among the
marine species that could be harmed by the Navy's new sonar system. The suit
alleges that the Navy is violating environmental laws by spending hundreds of
millions of dollars on the sonar system before completing the analysis of the
system's environmental effects. Increased concern about the safety of the low
frequency sonar system emerged during testing off the Island of Hawaii’s in
March 1998. Whale watching vessel captains observed that humpback whales
were leaving the testing area, and reported this to National Marine Fisheries
Service (NMFS).
A snorkeler in the water during a 120-decibel broadcast emerged with
symptoms a doctor described as comparable to acute trauma.
Two whale calves and one dolphin calf were found abandoned in the area of the
Hawaiian testing. One abandoned calf would be extremely unusual, and three -
all of different species - is unprecedented. The plaintiffs say NMFS claimed that
they did not believe the reports of these abandoned calves, although two of the
sittings were made by an experienced research team from the Ocean Mammal
Institute (OMI), and the third abandoned calf was actually rescued and sent to
Honolulu. Environmentalists also fear the new sonar system could threaten all
marine species, including beluga whales
The NMFS permit under which the tests were being conducted stated that the
tests would be suspended if whales left the test area or if other acute behavioral
responses occurred, but NMFS did not suspend the tests.”
This shows what drastic changes sonar could do to the natural
environment and how important it is to do various tests and experiments
under proper supervision and standards.

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V.Significance of frequency
Specific frequencies are used depending on the particular task concerned. For
almost all the fresh water applications and most salt-water applications, 200 or
400 kHz is used because it gives the best detail on the sonar screen and works
best in shallow water and at speed. It also shows less disturbances and
undesired echoes.
A 32 or 50 kHz sonar (under the same conditions and power) can penetrate
water to deeper depths than higher frequencies due to the fact that water's
natural ability to absorb sound waves is greater for higher frequency sound than
it is for lower frequencies. This means that the strength of the bounced signals
would be comparatively stronger for lower frequencies than for higher
frequencies, which would give a better-detailed picture on the sonar screen.
Therefore, generally a 32 or 50 kHz unit is used in deeper salt-water
applications

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Conclusion
Sonar is of great importance to us in many different ways. It is being used
almost in all parts of the world today. Although most of the present day sonar
systems are not capable of giving a 360-degree view of the surroundings it
should be noted that this technology is also being tested and experimented
frequently. This would enable us to view a wider range of the surrounding
which would make a considerable change in many fields such as the fishing
industry, salvage and robotics in the near future. The fishermen would find it
easier to track where the fish are and the people involved in salvage would find
it easier to spot shipwrecks and to map the seabed with a lot of detail. Robots
are presently used in many different ways including for fire fighting. When a
360 degree view of the surrounding is made possible, the robots will be able to
get signals from all directions which would enable them to do a better service
than doing the same with the signals from only one direction. Also sonar
systems with higher resolution would enable the readers to identify the exact
objects that are displayed on the sonar screen. Though the present day sonar
screens can cater only 2 dimensions, the day we are going to see sonar screen
which are attached to computers capable of producing a 3 dimensional view of
the object is not far away.

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Summary:-
Underwater acoustics is the only information carrier that can transport
information over large distances in the ocean. The acoustic waves are affected
by sound velocity variations with depth which cause refraction of the acoustic
energy. The ocean is a lossy medium for acoustics. An acoustic wave can travel
a fixed number of wavelengths, such that lower frequencies have longer range.
The sonar principle is to locate an object by estimating the acoustic travel time
and direction of arrival between sensor and object. Sonar imaging is estimation
of backscattered acoustic energy in all directions and for all ranges. Typical
sonar applications are: fish finding, imaging and mapping of the seafloor,
military and navigation.