An example of this is the Type 2082 fitted on the Vanguard class submarines by 3UYFPmEM

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									                UNIVERSITY OF SANTO TOMAS
                 FACULTY OF ENGINEERING
DEPARTMENT OF ELECTRONICS AND COMMUNICATIONS ENGINEERING




                          ECE 213

                          SONAR
               (Sound Navigation and Ranging)

                       Submitted by:

                        Jason Dee Uy
                   Kristan Bryan Simbulan
                      Geraldine Relavo
                      Genesis Maglaya
                       Jerome Belleza
                        Ciela Valencia
                        Kelvin Espiritu
                         Laurice Onia
                          Leo Conde

                        Submitted to:

                    Engr. Irineo P. Quinto




                                                           1
                          TABLE OF CONTENTS

SONAR
SOund NAvigation and Ranging

  I.      Introduction:
          a. What is SONAR
          b. Origin of SONAR
  II.     Types of SONAR
          a. Active SONAR
          b. Passive SONAR
          c. Other types
          d. SONAR based on Frequencies
                  i. High Frequency SONAR
                 ii. Mid Frequency SONAR
                iii. Low Frequency SONAR
  III.    Compositions of SONAR
          a. Transducer
          b. Transmitter
          c. Transfer Switch
          d. Receiver
          e. Display
  IV.     Operation of SONAR Systems
  V.      Performance Factors
  VI.     SONAR Designs
          a. Single-Beam Echo Sounder
          b. Dual-Beam Echo Sounder
          c. Multibeam SONAR
          d. Sidescan SONAR
  VII.    SONAR Configuration
          a. Hull-mounted
          b. Towed array
          c. Dipping SONAR
          d. Sonobuoys
  VIII.   Application
          a. Uses of SONAR




                                              2
Introduction:
What is SONAR?
      SONAR is an acronym for Sound Navigation and Ranging. It connotes an
   Acoustic system that can be used as an aid to navigation that can determine
   the range, or distance to objects, also called targets, that are usually found
   underwater. The targets might include obstructions and the sea floor.

Origin of SONAR
Although some animals (dolphins and bats) have used sound for communication
and object detection for millions of years, use by humans in the water is initially
recorded by Leonardo Da Vinci in 1490: a tube inserted into the water was said
to be used to detect vessels by placing an ear to the tube.

In the 19th century an underwater bell was used as an ancillary to lighthouses to
provide warning of hazards.

The use of sound to 'echo locate' underwater in the same way as bats use sound
for aerial navigation seems to have been prompted by the Titanic disaster of
1912. The world's first patent for an underwater echo ranging device was filed at
the British Patent Office by English meteorologist Lewis Richardson a month
after the sinking of the Titanic, and a German physicist Alexander Behm obtained
a patent for an echo sounder in 1913. Canadian Reginald Fessenden, while
working for the Submarine Signal Company in Boston, built an experimental
system beginning in 1912, a system later tested in Boston Harbor, and finally in
1914 from the U.S. Revenue (now Coast Guard) Cutter Miami on the Grand
Banks off Newfoundland Canada. In that test, Fessenden demonstrated depth
sounding, underwater communications (Morse Code) and echo ranging
(detecting an iceberg at two miles (3 km) range). The so-called Fessenden
oscillator, at ca. 500 Hz frequency, was unable to determine the bearing of the
berg due to the 3 meter wavelength and the small dimension of the transducer's
radiating face (less than 1 meter in diameter). The ten Montreal-built British H
class submarines launched in 1915 were equipped with a Fessenden oscillator.

During World War I the need to detect submarines prompted more research into
the use of sound. The British made early use of underwater hydrophones, while
the French physicist Paul Langevin, working with a Russian émigré electrical
engineer, Constantin Chilowski, worked on the development of active sound
devices for detecting submarines in 1915 using quartz. Although piezoelectric
and magnetostrictive transducers later superseded the electrostatic transducers
they used, this work influenced future designs. Lightweight sound-sensitive
plastic film and fibre optics have been used for hydrophones (acousto-electric




                                                                                      3
transducers for in-water use), while Terfenol-D and PMN (lead magnesium
niobate) have been developed for projectors.

ASDIC

In 1916, under the British Board of Invention and Research, Canadian physicist
Robert William Boyle took on the active sound detection project with A B Wood,
producing a prototype for testing in mid 1917. This work, for the Anti-Submarine
Division, was undertaken in utmost secrecy, and used quartz piezoelectric
crystals to produce the world's first practical underwater active sound detection
apparatus. To maintain secrecy no mention of sound experimentation or quartz
was made - the word used to describe the early work ('supersonics') was
changed to 'ASD'ics, and the quartz material to 'ASD'ivite: hence the British
acronym ASDIC. In 1939, in response to a question from the Oxford English
Dictionary, the Admiralty made up the story that it stood for 'Allied Submarine
Detection Investigation Committee', and this is still widely believed, though no
committee bearing this name has been found in the Admiralty archives. [2]

By 1918, both the US and Britain had built active systems, though the British
were well in advance of the US. They tested their ASDIC on HMS Antrim in 1920,
and started production in 1922. The 6th Destroyer Flotilla had ASDIC-equipped
vessels in 1923. An anti-submarine school, HMS Osprey, and a training flotilla of
four vessels were established on Portland in 1924. The US Sonar QB set arrived
in 1931.

By the outbreak of World War II, the Royal Navy had five sets for different
surface ship classes, and others for submarines, incorporated into a complete
anti-submarine attack system. The effectiveness of early ASDIC was hamstrung
by the use of the depth charge as an anti-submarine weapon. This required an
attacking vessel to pass over a submerged contact before dropping charges over
the stern, resulting in a loss of ASDIC contact in the moments leading up to
attack. The hunter was effectively firing blind, during which time a submarine
commander could take evasive action. This situation was remedied by using
several ships cooperating and by the adoption of "ahead throwing weapons",
such as Hedgehog and later Squid, which projected warheads at a target ahead
of the attacker and thus still in ASDIC contact. Developments during the war
resulted in British ASDIC sets which used several different shapes of beam,
continuously covering blind spots. Later, acoustic torpedoes were used.

At the start of World War II, British ASDIC technology was transferred for free to
the United States. Research on ASDIC and underwater sound was expanded in
the UK and in the US. Many new types of military sound detection were
developed. These included sonobuoys, first developed by the British in 1944,
dipping/dunking sonar and mine detection sonar. This work formed the basis for
post war developments related to countering the nuclear submarine. Work on
sonar had also been carried out in the Axis countries, notably in Germany, which



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included countermeasures. At the end of WWII this German work was
assimilated by Britain and the US. Sonars have continued to be developed by
many countries, including Russia, for both military and civil uses. In recent years
the major military development has been the increasing interest in low frequency
active systems.

SONAR

In World War II, the Americans used the term SONAR for their systems, coined
as the equivalent of RADAR. RADAR was considered very glamorous and
effective, and they wanted to cash in on the name. To be literal they should have
named it SODAR (Sound Detection and Ranging) to be the equivalent of,
standardization of signals led to the dropping of ASDIC in favor of SONAR for all
NATO countries.




                                                                                      5
Types of SONAR:
Active SONAR
           Active sonar uses a sound transmitter and a receiver.
           When a sound signal is sent into the water, part of it will be
            reflected back if it strikes an object or "target". The distance to the
            object can be calculated by measuring the time between when the
            signal is sent out and when the reflected sound, or echo, is
            received.
           A functional diagram of an active sonar system looks like this:




Figure 1. Active
sonar System.

The functional components are described below:

   1) Transmitter. The transmitter generates the outgoing pulse. It determines
      pulse width, PRF, modulation (optional), and carrier frequency. The output
      power can be controlled by the operator. The source level may be limited
      for several reasons. If the transducers are driven with too much power,
      they can cavitate (drop the pressure so low that the water boils). This is
      called quenching, and it can destroy the transducer since the normal
      backpressure is removed when bubbles form on its surface. Since the
      normal restoring force is gone, the surface of the transducer can travel too
      far (over-range) and damage itself. The quenching power limit increases
      with depth due to the increased ambient pressure.

Another common phenomenon that limits the maximum source level is
reverberation, which is an echo from the immediate surrounding volume of water.
The reverberation level (RL) increases with the source level (SL). At some point
the reverberation exceeds the noise level (NL) and will dominate the return signal.


                                                                                      6
Since reverberation always comes back from the same direction you are
projecting, the reduction in background noise, quantified by the directivity index
(DI) does not apply. When

RL > NL - DI,

the system is said to be reverberation-limited. The figure of merit equation must
be modified to reflect this:


FOMactive (reverberation-limited) = SL + TS - RL - DT


When the system becomes reverberation-limited, the display will begin to be
dominated by noise near own ship in the direction the active sonar is projecting.
The solution is to reduce power to just below the level at which reverberation-
limiting occurred.


2) Transducer array. The individual transducers are simple elements with little or
no directionality. They are arranged in an array to improve the directivity index,
which improves the figure-of-merit by noise reduction. The array of transducers
reduces the beamwidth in the horizontal (or azimuthal) direction, and is usually
circular in order to give more or less complete coverage, with the exception of the
region directly behind the array (where the ship is). The array is protected from
noise by own ship by discontinuing the array in the after regions, and also by
putting in sound attenuating material. This region aft of a hull-mounted array,
from which the sonar system cannot detect is called the baffles.

The array is also configured to reduce the beamwidth in the vertical direction.
Normally a hull-mounted array should only receive sound from the downward
direction, not directly ahead, since the noise from the ocean's surface would
destroy the sonar's performance.




Figure 2. Vertical beam of typical cylindrical transducer array.




                                                                                      7
3) Beamforming processor. The input/output of each transducer is put through a
beamforming processor, which applies time delays or phase shifts to each of the
signals in such a way as to create a narrow beam in a particular direction.




                                       Figure 3. Active beamforming.


The width of the beam formed by the beamforming processor will determine the
bearing accuracy of the system when searching. In an identical manner to dual-
beam tracking systems, sonar tracking systems can improve on this accuracy
tremendously, at the expense of the signal-to-noise ratio.

4.) Duplexer. The duplexer performs the same function in an active sonar as in a
radar system, namely to protect the receiver from the full transmitter power while
the pulse is going out. It can be thought of a switch that toggles between the
transmitter and receiver.

5.) Synchronizer. Performs same role as the synchronizer in radar. Provides
overall coordination and timing for the system. Reset the display for each new
pulse in order to make range measurements.

6.) Receiver. Collects the received energy. The receiver compares the power
level to noise with a threshold SNR (DT) in order to determine if the signal will be
displayed in a particular beam. If the DT is set too low, there will many false
alarms. If it is too high, some detection capability will be lost.
The receiver may also demodulate the return if frequency modulation is used on
transmission. Sonar systems often use pulse compression techniques to improve
range resolution.

7.) Display. Puts all of the detection information into a visual format. There are
several types:




                                                                                       8
A-scan: the signal along a single beam for a portion of the listening cycle. A
target appears as a raised section if it is in the beam.




                                   Figure 4. A-scan display.

PPI: plan position indicator. A top-down (geographic view). The sonar system
must sequentially search individual beams which are displayed in their true or
relative form. The strength of the return is represented by the intensity on the
display.




                                           Figure 5. PPI display.




                                                                                   9
Passive SONAR
            Passive sonar listens without transmitting.
            Ships, submarines, marine mammals, and fish all make noise, and
             this noise can be used by passive sonar systems to locate them, in
             much the same way humans use their ears to locate someone
             speaking in a room.
            The passive sonar system differs from the active system in many
             regards. Below is a typical functional diagram:




Figure 6. Passive sonar
system.



   1) Hydrophone array. These are the sensitive elements which detect the
      acoustic energy emitted from the target. Again, they are arranged into an
      array to improve the beamwidth. Common configurations are cylindrical or
      spherical. The cylindrical array operates at a fixed vertical angle, usually
      downward. The spherical array, which is common on submarines, has a
      much wider vertical field-of-view. Since the submarine may be below what
      it is tracking, the array must be able to look upwards to some extent. The
      large downward angles are only used for bottom bounce detection. Using
      a beamforming processor (described below) the field-of-view is broken
      down into individual beams in the vertical and azimuthal directions.




                                                                                     10
Figure 7. Spherical array showing multiple vertical beams.

   2) Beamforming processor. Unlike active systems which transmit and receive
      in a set direction, the passive system must listen to all angles at all times.
      This requires a very wide beamwidth. At the same time, a narrow
      beamwidth is required for locating the source and rejecting ambient noise.
      These two objectives are achieved simultaneously by the passive
      beamforming processor. The idea is very similar to the active system.




Figure 8. Passive hydrophone array.




                                                                                       11
The passive beamforming processor applies a unique set of time delays/phase
shifts to the signal to create a particular beam. The difference in a passive
system is that this process is repeated several times, each with a different set of
time delays/phase shifts, in order to listen to many narrow beams nearly
simultaneously. The result is a set of beams that cover the field-of-view of the
array.




                                                 Figure 9. Passive beamforming.


The beams should not be thought of as coming from the individual hydrophones.
In fact, each of the beams so created has a narrow beamwidth that comes from
the full aperture of the array, not the individual hydrophones.

   3) Broadband display. The output of the beamforming processor is displayed
      as a bearing time history (BTH):




Figure 10. Bearing time history (BTH) display.




                                                                                      12
The newest information is at the top of the display. The beamwidth of the system
determines how accurately the bearing can be measured by such a display. A
common beamwidth is about 5o. The total amount of time displayed from top to
bottom can be controlled (to some extent). A quickly updating display that only
kept information for a few minutes would be useful for close contacts whose
bearings are changing rapidly. On the other hand, a long tie history is more
useful for detecting long range contacts, whose bearings are only changing
slowly.

4)    Frequency Analyzer. The frequency analyzer breaks the signal into
separate frequencies. This is the spectrum of the signal. For processing
purposes, the frequencies are divided into small bands known as frequency bins.
The width of each bin is called the analysis bandwidth.




Figure 11. Frequency analysis.


Sonar systems can gain considerable signal-to-noise improvements by matching
the analysis bandwidth to the bandwidth of narrowband sources. The way to
illustrate this is by two counter examples. If the signal processing bandwidth is
too wide, then noise from the part of the spectrum beyond the signal is let in and
the SNR is degraded. If the bandwidth is too narrow, then part of the signal is
excluded, also reducing the SNR. It should be obvious now that the best situation
occurs when the bandwidth exactly matches the signal. This is possible when the
characteristics of the signal are well known, which they are for most targets.




                                                                                     13
The frequency analyzer separates (filters) the signal into discrete bins, inside of
which the SNR is maximized. The frequency content of the signals from a target
information provides vital information about its identity and operation. These
frequencies are also subject to the Doppler shift, just like radar, are therefore can
provide information about the range rate. This requires that the original frequency
be known exactly, which is generally not the case. However, many important
facts can be inferred by the changes in the received frequency over time.

       5) Narrowband Display. For a particular beam, the time history of the
          frequency is called a waterfall display.




                                                     Figure 12. Waterfall display.

This can be used to gain additional information from a contact which is already
being tracked by another system. In order to search for contacts on the basis of
narrowband information alone requires a different type of display. One possibility
is to simultaneously display several different beams, each showing a mini-
waterfall display, which are called grams.




                                                                                        14
Figure 13. Narrowband grams.

These are quite useful, but require great concentration on the part of the operator
because there is more information displayed at any one time. Many systems
require the operator to systematically search the entire field-of-view, looking at
only a few beams at a time.

Other Types of SONARS

Variable Depth Sonar (VDS)

Variable depth sonars use large transducers that are towed from the ship on a
cable with an adjustable scope. The combination of the buoyancy, ship speed
and cable scope determine at the depth that the transducer will be at. VDS is
used for two main reasons. At increased depth, the source level (SL) can be
increased greatly, since the quenching limit is higher. This is due to increased
backpressure on the surface of the transducer. Secondly, the VDS can be
operated below the layer.

Recall that the combination of positive over negative sound velocity profiles
created a layer at the interface. The layer makes it difficult to propagate sound
across it. Therefore, ships using hull-mounted sonar systems will be unable to
detect submarines operating below the layer, except possibly at short range.
However, if the VDS can be place below layer, the ship can take advantage of
the deep sound channel while being in the shadow zone of the submarine's
sonar.




                                                                                      15
Towed Array Sonar Systems (TASS)

A towed array is a linear array of hydrophones. The array is towed behind the
ship on a cable of variable scope like a VDS. However, it is strictly a passive
system.




Figure 14. Towed array.

The signal from the array is led to a beamforming processor which creates
several narrow beams. Because the array is linear, there is no vertical
directionality. This causes two problems. The first problem occurs when there is
bottom bounce propagation. In this case, the direction of the source is not known
without further analysis.

The second problem is an ambiguity in relative bearing. The linear array cannot
distinguish signals on the left from those on the right.




Figure 15. Bearing ambiguity with towed array.



                                                                                    16
The problem of bearing ambiguity can be resolved by maneuvering the ship.
When contact is regained, there will again be two ambiguous bearings, but only
one of which will match the previous case (assuming that the target hasn't moved
much between legs).

Example: a towed array contact is gained at relative bearing 030/330 R, while the
ship is on course 045 T. The ship changes course to 135 T and regains the
contact at 060/300 R. Find the true bearing to the actual contact.

First leg: the contact is at either 015 T or 075 T (045 030).

Second leg: the contact is at either 075 T or 195 T (135 060)

Therefore the actual contact is at 075 T.

Because the array is not constrained by the size of the ship, towed arrays can be
made very long. Therefore, they have very narrow beamwidths, or alternatively,
can operate at much lower frequencies. Low frequency capability is particularly
advantageous because there are many sources at low frequency, with large
source levels, and there is very little loss from absorption.

Sonobuoys

Sonobuoys are small, self-contained sonar systems. They are dropped into the
water by aircraft at which point they deploy themselves. The information from the
sonobuoy is transmitted to the aircraft by VHF radio link. The information can
also be relayed to ships. Signal processing and analysis is performed by
equipment in the aircraft, or ship. After some period of operation, the sonobuoy
will scuttle itself.




                                                                                    17
                                                         Figure 16. Sonobouy
deployment.

There are many types of sonobuoys, depending on their capabilties:

DICASS: directional command activated sonobuoy system. Active system which
transmits pulses when commanded by the aircraft.

DIFAR: direction finding acoustic receiver. This is a passive system, with some
directionality achieved by a small hydrophone array.

VLAD: vertical thin-line array. A linear, vertical array of DIFAR hydrophones. Has
improved directionality in vertical direction. Reduces noise from surface.

Most sonobuoys can operate at several preset depth settings. The shallow
setting is used for surface duct propagation, and the deep for sound channel
propagation.

Sonobuoys have poor values of directivity index, mostly because of their limited
size. Figure-of-merit is not always low, however, because sonobuoys also have
very low self-noise. Depending on the environment, and whether or not self-noise
is dominant, sonobuoys can actually outperform some larger hull-mounted
systems.




                                                                                     18
Bi-Static Sonar

This is a combination of an active system for transmission at one location with a
passive system for reception at another location.




Figure 17. Bi-static sonar.

The passive system does not suffer from the same reverberation limitations as
the standard active system. The source level can be very high (up to the
quenching limit). Some system use explosive driven projectors with incredible
source levels. The source may be well outside the weapons range of the target.
Furthermore, the transmission loss is less than the full two-way loss of normal
active. Sometimes the projector is a sonobuoy.

Non-Acoustic Detection

Visual

Submarines near the surface are very vulnerable to visual detection. Anything
that protrudes above the surface like a periscope, antenna or mast will leave a
significant wake if the submarine is moving at any speed over a few knots. Since
depth control and steerage is very difficult at low speeds, it is not uncommon for
submarines to be at 4 or 5 knots just below the surface. The mast will create a
wake, called feather, which is quite visible, and also leave a remnant of its
passage, called a scar. The scar is a long streak of foam or bubbles left behind
after the object passes. The feather may be a few meters, and the scar tens of
meters long. Either may be visible for up to 10 miles, and are easily spotted by
low flying aircraft in the vicinity. If the water is especially clear, the hull may be




                                                                                         19
visible for a few hundred feet under water, but is usually not distinguishable
unless the water is shallow with a light colored bottom (like white sand).

Radar

Exposed periscopes and masts can be detected by specially designed radars.
The radar cross section is very small and is generally not detectable by ordinary
radar systems. Furthermore, the sea clutter near the target will generally obscure
it. To be effective against a periscope or mast, the radar should have very small
range and bearing resolution and must be vertically polarized to match the
structure of the target. ISAR has proven to be very effective against submarine
periscopes and masts.

Some special radar systems have demonstrated the capability to detect the
presence of a submarine by the change in the surface water height as it passes,
known as the bernoulli hump. This effect is greatest when the submarine is
shallow and moving very fast. These are not real time assets however, since the
signal processing takes several hours to complete.

Infrared Detection

Submarines are vulnerable to infrared detection when they are snorkeling, since
the diesel exhaust is released close to the surface (as it must be because of the
backpressure limitation). The exhaust gases give off a sufficiently strong infrared
signature as to be detectable. However, this is only useful if the submarine is
snorkeling, which it very infrequent for nuclear powered submarines, and only a
few hours a day for diesel-electric submarines.

Magnetic Anomaly Detection (MAD)

Magnetic anomaly detection systems, measure the change in the earth's
magnetic field due to the presence of a large amount of ferrous material found in
most submarines. The effect can only be detected if the submarine is relatively
shallow, and therefore is not a great long range detection system. It can however,
provide a precise location of the submarine of sufficient accuracy to permit
weapons delivery, which is its main use.




                                                                                      20
SONAR based on Frequencies
High Frequency SONAR

        Greater than 10 kHz
        Primary uses
             Determination of water depth (fathometers)
             Hunting mines
             Guiding torpedoes
        Short range
             At higher frequencies, sound energy is greatly weakened
                due to absorption and scattering.


Mid Frequency SONAR

        1 kHz to 10 kHz
        Primary tool for identifying and prosecuting submarines
        Typical ranges: 1-10 nautical miles

Low Frequency SONAR

        Less than 1 kHz
        Suffers the least attenuation, therefore providing the greatest range.
        Primarily used for long-range search and surveillance of
         submarines.
        Typical range: up to 100 nautical miles




                                                                                  21
Composition of a SONAR:
Transducer
          Dual-purpose
          Transmit & receive
          Piezoelectric crystal
          Crystals that change shape when voltage is applied to them, or
           generates voltage when their shape is changed by an applied force
          Magnetostrictive element
          Nickel bar with wire coil around it
          Increased coil current = bar contracts
          Decreased coil current = bar expands
          Diaphragm
          Connected to the crystal or nickel bar
          Responsible for the reception of sound
          Responsible for the reception of sound

Transmitter

          High-powered ultrasonic pulse generator
          Typical average power: 8,000 watts
          Peak pulse power: 160,000 watts
          Frequency: 20,000 Hertz
          Pulse duration: 0.005 to 0.1 second
          Pulse repetition rate: single unrepeated pulse to 60 pulses per
           minute (depending on the maximum range to be searched)

Transfer Switch

          High-powered ultrasonic pulse generator
          Typical average power: 8,000 watts
          Peak pulse power: 160,000 watts
          Frequency: 20,000 Hertz
          Pulse duration: 0.005 to 0.1 second
          Pulse repetition rate: single unrepeated pulse to 60 pulses per
           minute (depending on the maximum range to be searched)

Receiver

          Listens to underwater sounds produced either by reflection of the
           transmitted pulse (echo) in the case of an active SONAR or simply
           by generated sound of a distant target in the case of a passive
           SONAR



                                                                               22
Display

             Echo signals are represented as a variation of voltage amplitude in
              time.
             Types of Displays:
                  Echogram – if the display is derived from an echo sounder
                  Echo image – if derived from a multibeam SONAR


Operation of SONAR Systems:
             Transmit regular pulses of sound energy that travel through the
              water and are reflected by the target.
                   SONAR ping.
             The echo is received, amplified and then displayed.
             The distance of the target is the time elapsed from transmission
              and reception.
             The display mechanism transforms the received sound waves into
              varying electrical pulses which causes the stylus to mark a strip of
              paper or the electronic display to produce records of different
              intensities represented by color.
             Sonar resolution (detail) improves as the transmitted pulse is made
              shorter. If the sound beams are narrowed, the angular resolution
              (ability to distinguish two targets at different angles) improves.

      A typical sonar device has a motor (1)
      that drives a recording arm (2) and -
      through a system of gears, not shown -
      a recording chart (3). A rotating contact
      (4) causes a transmitter (5) to emit a
      sound impulse once every revolution.
      No further impulse is sent until an echo
      arrives in the receiver (6) from the
      seafloor or from some intervening
      object. The echo is passed through an
      amplifier (7) and then activates the
      recording arm; a pen at the tip of the
      arm marks on the chart the time taken
      for the impulse to return. This time is
      given in terms of depth by a calibrated
      scale (8). A series of such readings is
      seen at the center of the chart; the
      steadier line at the right is the echo
      picked up almost immediately at the
      surface as the signal is transmitted.




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Performance factors:
The detection, classification and localisation performance of a sonar depends on the
environment and the receiving equipment, as well as the transmitting equipment in
an active sonar or the target radiated noise in a passive sonar.

Sound propagation

Sonar operation is affected by variations in sound speed, particularly in the vertical
plane. Sound travels more slowly in fresh water than in sea water, though the
difference is small. The speed is determined by the water's bulk modulus and mass
density. The bulk modulus is affected by temperature, dissolved impurities (usually
salinity), and pressure. The density effect is small. The speed of sound (in feet per
second) is approximately:

Speed of sound = 4388 + (11.25 × temperature (in °F)) + (0.0182 × depth (in
feet))
                 + salinity (in parts-per-thousand ).

This empirically derived approximation equation is reasonably accurate for normal
temperatures, concentrations of salinity and the range of most ocean depths. Ocean
temperature varies with depth, but at between 30 and 100 meters there is often a
marked change, called the thermocline, dividing the warmer surface water from the
cold, still waters that make up the rest of the ocean. This can frustrate sonar,
because a sound originating on one side of the thermocline tends to be bent, or
refracted, through the thermocline. The thermocline may be present in shallower
coastal waters. However, wave action will often mix the water column and eliminate
the thermocline. Water pressure also affects sound propagation: higher pressure
increases the sound speed, which causes the sound waves to refract away from the
area of higher sound speed. The mathematical model of refraction is called Snell's
law.

Sound waves that are radiated down into the deep ocean bend back up to the
surface in great arcs due to the increasing pressure (and hence sound speed) with
depth. The ocean must be at least 6000 feet (1850 m) deep, or the sound waves will
echo off the bottom instead of refracting back upwards, and the reflection loss at the
bottom reduces performance. Under the right conditions these sound waves will
then be focused near the surface and refracted back down and repeat another arc.
Each focus at the surface is called a convergence zone (CZ). This CZ forms an
annulus about the sonar. The distance and width of the CZ depends on the
temperature and salinity of the water. In the North Atlantic, for example, CZs are
found approximately every 33 nautical miles (61 km), depending on the season.
Sounds that can be heard from only a few miles in a direct line can therefore also be
detected hundreds of miles away. With powerful sonars the first, second and third



                                                                                         24
CZ are fairly useful; further out than that the signal is too weak, and thermal
conditions are too unstable, reducing the reliability of the signals. The signal is
naturally attenuated by distance, but modern sonar systems are very sensitive, i.e.
can detect despite a low signal-to-noise ratio.

If the sound source is deep and the conditions are right, propagation may occur in
the 'deep sound channel'. This provides extremely low propagation loss to a
receiver in the channel. This is because of sound trapping in the channel with no
losses at the boundaries. Similar propagation can occur in the 'surface duct' under
suitable conditions. However in this case there are reflection losses at the surface.

In shallow water propagation is generally by repeated reflection at the surface and
bottom, where considerable losses can occur.

Sound propagation is also affected by absorption in the water itself as well as at the
surface and bottom. This absorption is frequency dependent, with several different
mechanisms in sea water. Thus sonars required to operate over long ranges tend to
utilise low frequencies to minimise absorption effects.

The sea contains many sources of noise that interfere with the desired target echo
or signature. The main noise sources are waves and shipping. The motion of the
receiver through the water can also cause low frequency noise, which is speed
dependent.

Reverberation

When active sonar is used, scattering occurs from small objects in the sea as well as
from the bottom and surface. This can be a major source of interference but does
not occur with passive sonar. This scattering effect is different from that in room
reverberation which is a reflection phenomenon. An analogy for reverberation is the
scattering of a car's headlights in fog or mist. A high-intensity pencil beam will
penetrate the fog; main headlights are less directional and result in "white-out"
where the returned reverberation dominates. Similarly, to overcome reverberation,
an active sonar needs to transmit in a narrow beam.

Target characteristics

The target of a sonar, such as a submarine, has two main characteristics that
influence the performance of the sonar. For active sonar it is its sound reflection
characteristics, known as its target strength. For passive sonar the target's radiated
noise characteristics are critical. The radiated spectrum in general will consist of an
unresolved continuum of noise with spectral lines in it, the lines being used for
classification.




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Echoes are also obtained from other objects in the sea such as whales, wakes,
schools of fish and rocks.

Countermeasures

Active (powered) countermeasures may be launched by a submarine under attack
to raise the noise level and/or provide a large false target. Passive (i.e., non-powered)
countermeasures include mounting noise generating devices on isolating devices
and coating the hull of submarines.




SONAR Designs:
Single-Beam Echo Sounder
            Earliest, most basic, and still most widely used echo sounding
             device.
                 Used usually by fishermen in detecting schools of fish.
            Makes one-at-a-time measurements of the ocean depth at many
             locations.
            Set to make measurements from a moving vessel while it is in
             motion (scanning).
            Four basic components:
                 Transmitter
                 Transducer
                 Receiver
                 Control and Display System
            Performs a continuous cycle called the ping cycle
            Operation
                 In a single cycle, the Control and Display system signals the
                    Transmitter system to produce a sound pulse (ping).
                 The transmitter generates an oscillating electrical signal w/
                    frequency characteristics that can be uniquely distinguished.
                 The transducer converts the electrical energy into sound
                    waves.
                 Upon its return as an echo from the sea floor, the sound
                    pulse is received and converted back into electrical signals
                    by the Transducer.
                 Then passes to the Receiver System where it is amplified
                    and passed through a detection scheme to determine when
                    the echo arrives.
                 The time between the transmission and reception is used by
                    the Receiver System to compute the range or depth.




                                                                                            26
              The depth is reported and recorded by the Control and
               Display system then triggers the next ping.

Dual-Beam Echo Sounder

         Has a multi-element transducer from which two concentric beams
          of the same frequency but different beam widths are formed.
         Just like a single beam echo sounder but with two concentric
          beams instead of one narrow beam only.
         Consists of the following components:
           Transmitter
           Receiver
           Transducer - produces the broad beam and the narrow beam
                   Narrow beam – for the depth/vertical measurement
                   Broad beam – for the horizontal measurement
         Control and Display system
         The combination of the horizontal and vertical beams displays the
          target from above and from the side at the same time.
         Therefore, it is not necessary to go over the target to see the
          vertical distribution on the echo sounder.
         Applications:
           Hydrographic surveying
           Fish finders

Multibeam SONAR

         Uses a sonar device that emits fan-shaped pulses (multiple
          ultrasonic beams) down toward the seafloor across a wide angle
          perpendicular to the path of the sensor through the water
         The intensity or time of the acoustic reflections from the seafloor of
          this fan-shaped beam is recorded in a series of cross-track slices.
         When stitched together along the direction of motion, these slices
          form an image of the sea bottom within the swath (coverage width)
          of the beam.
         Generated sound frequency: 100-500 kHz
         Measures and records the travel time of the acoustic signal from
          the transmitter (transducer) to the seafloor (or object) and back to
          the receiver.
         Output data is in the form of depths rather than images

Sidescan SONAR

               The strength of the return echo is continuously recorded
         Output data is a picture of the ocean bottom.
              Dark image – strong return echo



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              Light image – little or no return echo
          Cannot provide depth information



SONAR Configuration:
Hull-mounted

          Hull-mounted – attached to the bottom of the vessel




Towed array

          Towed array – attached to a moving ship by a cable
            Example: sidescan SONARs and cabled hydrophones




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Dipping SONAR

         Dipping SONAR – attached to aircrafts (helicopters) via cables and
          dipped into the water




Sonobuoys

         Sonobuoys – attached to free-floatng buoys, released in the air to
          the sea




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Application:
Uses of SONAR.

Warfare
Modern naval warfare makes extensive use of sonar. The two types described before are
both used, from various platforms, i.e. water-borne vessels, aircraft and fixed installations.
The usefulness of active versus passive sonar depends on the radiated noise
characteristics of the target, generally a submarine. Although in WWII active sonar was
mainly used, except by submarines, with the advent of modern signal processing passive
sonar was preferred for initial detection. As the submarines have become quieter, active
operation is now more used. In 1987 a division of Toshiba sold the machinery to Russia
that allowed them to mill the submarine propeller blades so that they became radically
quieter, creating a huge security issue with their newer generation of submarines.

Active sonar is extremely useful, since it gives the exact bearing to a target (and
sometimes the range). Active sonar works the same way as radar: a signal is emitted. The
sound wave then travels in many directions from the emitting object. When it hits an
object, the sound wave is then reflected in many other directions. Some of the energy will
travel back to the emitting source. The echo will enable the sonar system or technician to
calculate, with many factors such as the frequency, the energy of the received signal, the
depth, the water temperature, etc., the position of the reflecting object. Active sonar is
used when the platform commander determines that it is more important to determine the
position of a possible threat submarine than it is to reveal his own position. With surface
ships it might be assumed that the threat is already tracking the ship with satellite date.
Any vessel around the emitting sonar will detect the emission. Having heard the signal, it
is easy to identify the type of sonar (usually with its frequency) and its position (with the
sound wave's energy). Moreover, active sonar, similar to radar, allows the user to detect
objects at a certain range but also enables other platforms to detect the active sonar at a
far greater range.

Since active sonar does not allow exact classification and is very noisy, this type of
detection is used by fast platforms (planes, helicopters) and by noisy platforms (most
surface ships) but very rarely by submarines. Ballistic missile submarines do not even
have active sonar, since they never want to risk detection. When active sonar is used by
surface ships or submarines, it is typically activated very briefly at intermittent periods, to
reduce the risk of detection by an enemy's passive sonar. As such, active sonar is
normally considered a backup to passive sonar. In aircraft, active sonar is used in the
form of disposable sonobuoys that are dropped in the aircraft's patrol area or in the
vicinity of possible enemy sonar contacts.

Passive sonar has several advantages. Most importantly, it is silent. If the target radiated
noise level is high enough, it can have a greater range than active sonar, and allows an



                                                                                                  30
identification of the target. Since any motorized object makes some noise, it may be
detected eventually. It simply depends on the amount of noise emitted and the amount of
noise in the area, as well as the technology used. To simplify, passive sonar "sees" around
the ship using it. On a submarine, the nose mounted passive sonar detects in directions of
about 270°, centered on the ship's alignment, the hull-mounted array of about 160° on
each side, and the towed array of a full 360°. The no-see areas are due to the ship's own
interference. Once a signal is detected in a certain direction (which means that something
makes sound in that direction, this is called broadband detection) it is possible to zoom in
and analyze the signal received (narrowband analysis). This is generally done using a
Fourier transform to show the different frequencies making up the sound. Since every
engine makes a specific noise, it is straightforward to identify the object. The classified
databases of unique noises are part of what is known as acoustic intelligence or ACINT.

Another use of the passive sonar is to determine the target's trajectory. This process is
called Target Motion Analysis (TMA), and the resultant "solution" is the target's range,
course, and speed. TMA is done by marking from which direction the sound comes at
different times, and comparing the motion with that of the operator's own ship. Changes
in relative motion are analyzed using standard geometrical techniques along with some
assumptions about limiting cases.

Passive sonar is stealthy and very useful. However, it requires high-tech components
(band-pass filters, receivers) and is costly. It is generally deployed on expensive ships in
the form of arrays to enhance the detection. Surface ships use it to good effect; it is even
better used by submarines, and it is also used by airplanes and helicopters, mostly to a
"surprise effect", since submarines can hide under thermal layers. If a submarine captain
believes he is alone, he may bring his boat closer to the surface and be easier to detect, or
go deeper and faster, and thus make more sound.

Examples of sonar applications in military use are given below. Many of the civil uses
given in the following section may also be applicable to naval use.

Anti-submarine warfare




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Variable Depth Sonar and its winch

Until recently, ship sonars were usually with hull mounted arrays, either amidships or at
the bow. It was soon found after their initial use that a means of reducing flow noise was
required. The first were made of canvas on a framework, then steel ones were used. Now
domes are usually made of reinforced plastic or pressurised rubber. Such sonars are
primarily active in operation. An example of a conventional hull mounted sonar is the
SQS-56.

Because of the problems of ship noise, towed sonars are also used. These also have the
advantage of being able to be placed deeper in the water. However, there are limitations
on their use in shallow water. These are called towed arrays (linear) or variable depth
sonars (VDS) with 2/3D arrays. A problem is that the winches required to deploy/recover
these are large and expensive. VDS sets are primarily active in operation while towed
arrays are passive.

An example of a modern active/passive ship towed sonar is Sonar 2087 made by Thales
Underwater Systems.

Torpedoes

Modern torpedoes are generally fitted with an active/passive sonar. This may be used to
home directly on the target, but wake following torpedoes are also used. An early
example of an acoustic homer was the Mark 37 torpedo.

Torpedo countermeasures can be towed or free. An early example was the German
Sieglinde device while the Pillenwerfer was a chemical device. A widely used US device



                                                                                             32
was the towed Nixie while MOSS submarine simulator was a free device. A modern
alternative to the Nixie system is the UK Royal Navy S2170 Surface Ship Torpedo
Defence system.

Mines

Mines may be fitted with a sonar to detect, localise and recognise the required target.
Further information is given in acoustic mine and an example is the CAPTOR mine.

Mine countermeasures

Mine Countermeasure (MCM) Sonar, sometimes called "Mine and Obstacle Avoidance
Sonar (MOAS)", is a specialised type of sonar used for detecting small objects. Most
MCM sonars are hull mounted but a few types are VDS design. An example of a hull
mounted MCM sonar is the Type 2193 while the SQQ-32 Mine-hunting sonar and Type
2093 systems are VDS designs.



Submarines

Submarines rely on sonar to a greater extent than surface ships as they cannot use radar at
depth. The sonar arrays may be hull mounted or towed. Information fitted on typical fits
is given in Oyashio class submarine and Swiftsure class submarine.

Aircraft

Helicopters can be used for antisubmarine warfare by deploying fields of active/passive
sonobuoys or can operate dipping sonar, such as the AQS-13. Fixed wing aircraft can
also deploy sonobuoys and have greater endurance and capacity to deploy them.
Processing from the sonobuoys or dipping sonar can be on the aircraft or on ship.
Helicopters have also been used for mine countermeasure missions using towed sonars
such as the AQS-20A




                                                                                              33
AN/AQS-13 Dipping sonar deployed from an H-3 Sea King.

Underwater communications

Dedicated sonars can be fitted to ships and submarines for underwater communication.
See also the section on the underwater acoustics page.

Ocean surveillance

For many years, the United States operated a large set of passive sonar arrays at various
points in the world's oceans, collectively called Sound Surveillance System (SOSUS) and
later Integrated Undersea Surveillance System (IUSS). A similar system is believed to
have been operated by the Soviet Union. As permanently mounted arrays in the deep
ocean were utilised, they were in very quiet conditions so long ranges could be achieved.
Signal processing was carried out using powerful computers ashore. With the ending of
the Cold War a SOSUS array has been turned over to scientific use.

In the United States Navy, a special badge known as the Integrated Undersea
Surveillance System Badge is awarded to those who have been trained and qualified in its
operation.

Underwater security

Sonar can be used to detect frogmen and other scuba divers. This can be applicable
around ships or at entrances to ports. Active sonar can also be used as a deterrent and/or
disablement mechanism. One such device is the Cerebus system.

See Underwater Port Security System and Anti-frogman techniques Ultrasound weapon.

Hand-held sonar

Limpet Mine Imaging Sonar (LIMIS) is a hand-held or ROV-mounted imaging sonar
designed for patrol divers (combat frogmen or clearance divers) to look for limpet mines
in low visibility water.

The LUIS is another imaging sonar for use by a diver.

Integrated Navigation Sonar System (INSS) is a small flashlight-shaped handheld sonar
for divers that displays range.

Intercept sonar

This is a sonar designed to detect and locate the transmissions from hostile active sonars.
An example of this is the Type 2082 fitted on the Vanguard class submarines.




                                                                                              34
Civil applications
Fisheries

Fishing is an important industry that is seeing growing demand, but world catch tonnage
is falling as a result of serious resource problems. The industry faces a future of
continuing worldwide consolidation until a point of sustainability can be reached.
However, the consolidation of the fishing fleets are driving increased demands for
sophisticated fish finding electronics such as sensors, sounders and sonars. Historically,
fishermen have used many different techniques to find and harvest fish. However,
acoustic technology has been one of the most important driving forces behind the
development of the modern commercial fisheries.

Sound waves travel differently through fish than through water because a fish's air-filled
swim bladder has a different density than seawater. This density difference allows the
detection of schools of fish by using reflected sound. Acoustic technology is especially
well suited for underwater applications since sound travels farther and faster underwater
than in air. Today, commercial fishing vessels rely almost completely on acoustic sonar
and sounders to detect fish. Fishermen also use active sonar and echo sounder technology
to determine water depth, bottom contour, and bottom composition.




Cabin display of a fish finder sonar

Companies such as Raymarine UK, Marport Canada, Wesmar, Furuno, Krupp, and
Simrad make a variety of sonar and acoustic instruments for the deep sea commercial
fishing industry. For example, net sensors take various underwater measurements and
transmit the information back to a receiver onboard a vessel. Each sensor is equipped
with one or more acoustic transducers depending on its specific function. Data is
transmitted from the sensors using wireless acoustic telemetry and is received by a hull



                                                                                             35
mounted hydrophone. The analog signals are decoded and converted by a digital acoustic
receiver into data which is transmitted to a bridge computer for graphical display on a
high resolution monitor.

Echo sounding

An echo-sounder sends an acoustic pulse directly downwards to the seabed and records
the returned echo. The sound pulse is generated by a transducer that emits an acoustic
pulse and then “listens” for the return signal. The time for the signal to return is recorded
and converted to a depth measurement by calculating the speed of sound in water. As the
speed of sound in water is around 1,500 metres per second, the time interval, measured in
milliseconds, between the pulse being transmitted and the echo being received, allows
bottom depth and targets to be measured.

The value of underwater acoustics to the fishing industry has led to the development of
other acoustic instruments that operate in a similar fashion to echo-sounders but, because
their function is slightly different from the initial model of the echo-sounder, have been
given different terms.

Net location

The net sounder is an echo sounder with a transducer mounted on the headline of the net
rather than on the bottom of the vessel. Nevertheless, to accommodate the distance from
the transducer to the display unit, which is much greater than in a normal echo-sounder,
several refinements have to be made. Two main types are available. The first is the cable
type in which the signals are sent along a cable. In this case there has to be the provision
of a cable drum on which to haul, shoot and stow the cable during the different phases of
the operation. The second type is the cable less net-sounder – such as Marport’s Trawl
Explorer - in which the signals are sent acoustically between the net and hull mounted
receiver/hydrophone on the vessel. In this case no cable drum is required but
sophisticated electronics are needed at the transducer and receiver.

The display on a net sounder shows the distance of the net from the bottom (or the
surface), rather than the depth of water as with the echo-sounder's hull-mounted
transducer. Fixed to the headline of the net, the footrope can usually be seen which gives
an indication of the net performance. Any fish passing into the net can also be seen,
allowing fine adjustments to be made to catch the most fish possible. In other fisheries,
where the amount of fish in the net is important, catch sensor transducers are mounted at
various positions on the cod-end of the net. As the cod-end fills up these catch sensor
transducers are triggered one by one and this information is transmitted acoustically to
display monitors on the bridge of the vessel. The skipper can then decide when to haul
the net.

Modern versions of the net sounder, using multiple element transducers, function more
like a sonar than an echo sounder and show slices of the area in front of the net and not
merely the vertical view that the initial net sounders used.



                                                                                                36
The sonar is an echo-sounder with a directional capability that can show fish or other
objects around the vessel.

Ship velocity measurement

Sonars have been developed for measuring a ship's velocity either relative to the water or
to the bottom.

ROV and UUV

Small sonars have been fitted to Remotely Operated Vehicles (ROV) and Unmanned
Underwater Vehicles (UUV) to allow their operation in murky conditions. These sonars
are used for looking ahead of the vehicle. The Long-Term Mine Reconnaissance System
is an UUV for MCM purposes.

Vehicle location

Sonars which act as beacons are fitted to aircraft to allow their location in the event of a
crash in the sea. Short and Long Baseline sonars may be used for caring out the location,
such as LBL.


Scientific applications
Biomass estimation

       Main article: Bioacoustics

Detection of fish, and other marine and aquatic life, and estimation their individual sizes
or total biomass using active sonar techniques. As the sound pulse travels through water
it encounters objects that are of different density or acoustic characteristics than the
surrounding medium, such as fish, that reflect sound back toward the sound source. These
echoes provide information on fish size, location, abundance and behavior. See Also:
Hydroacoustics

Wave measurement

An upward looking echo sounder mounted on the bottom or on a platform may be used to
make measurements of wave height and period. From this statistics of the surface
conditions at a location can be derived.

Water velocity measurement

Special short range sonars have been developed to allow measurements of water velocity.




                                                                                               37
Bottom type assessment

Sonars have been developed that can be used to characterize the sea bottom into, for
example, mud, sand, and gravel. Relatively simple sonars such as echo sounders can be
promoted to seafloor classification systems via add-on modules, converting echo
parameters into sediment type. Different algorithms exist, but they are all based on
changes in the energy or shape of the reflected sounder pings. Advanced substrate
classification analysis can be achieved using calibrated (scientific) echosounders and
parametric or fuzzy-logic analysis of the acoustic data (See: Acoustic Seabed
Classification)

Bottom topography measurement

Side-scan sonars can be used to derive maps of the topography of an area by moving the
sonar across it just above the bottom. Low frequency sonars such as GLORIA have been
used for continental shelf wide surveys while high frequency sonars are used for more
detailed surveys of smaller areas.

Sub-bottom profiling

Powerful low frequency echo-sounders have been developed for providing profiles of the
upper layers of the ocean bottom.

Synthetic aperture sonar

Various synthetic aperture sonars have been built in the laboratory and some have entered
use in mine-hunting and search systems. An explanation of their operation is given in
synthetic aperture sonar.

Parametric sonar

Parametric sources use the non-linearity of water to generate the difference frequency
between two high frequencies. A virtual end-fire array is formed. Such a projector has
advantages of broad bandwidth, narrow beamwidth, and when fully developed and
carefully measured it has no obvious sidelobes: see Parametric array. Its major
disadvantage is very low efficiency of only a few percent. P.J. Westervelt's seminal 1963
JASA paper summarizes the trends involved.




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