Depth sounding systems
2.1 Introduction
Sonar (sound navigation and ranging) is the acronym identifying those systems that rely for their
operation on the transmission and reception of acoustic energy in water. The term is widely used to
identify all modern systems that propagate acoustic or electromagnetic energy into seawater to
determine a vessel’s speed or the depth of water under the keel. This book is not concerned with those
specialized sonar techniques that are used for locating submerged objects, either fish or submarines.
A navigator in the Merchant Navy is interested only in the depth of the water beneath the vessel, an
indication of the speed of his ship and the distance run. See Chapter 3 for a description of speed
logging equipment.
The first section of this chapter deals with the characteristics and problems that arise from the need
to propagate energy in seawater.
2.2 The characteristics of sound in seawater
Before considering the problems of transmitting and receiving acoustic energy in seawater, the effects of
the environment must be understood. Sonar systems rely on the accurate measurement of reflected
frequency or, in the case of depth sounders, a precise measurement of time and both these parameters are
affected by the often unpredictable ocean environment. These effects can be summarized as follows.
Attenuation. A variable factor related to the transmitted power, the frequency of transmission,
salinity of the seawater and the reflective consistency of the ocean floor.
Salinity of seawater. A variable factor affecting both the velocity of the acoustic wave and its
attenuation.
Velocity of sound in salt water. This is another variable parameter. Acoustic wave velocity is
precisely 1505 ms–1 at 15°C and atmospheric pressure, but most echo-sounding equipment is
calibrated at 1500 ms–1.
Reflective surface of the seabed. The amplitude of the reflected energy varies with the consistency
of the ocean floor.
Noise. Either inherent noise or that produced by one’s own transmission causes the signal-to-noise
ratio to degrade, and thus weak echo signals may be lost in noise.
Two additional factors should be considered.
Frequency of transmission. This will vary with the system, i.e. depth sounding or Doppler speed log.
Angle of incidence of the propagated beam. The closer the angle to vertical the greater will be the
energy reflected by the seabed.
Depth sounding systems 23
2.2.1 Attenuation and choice of frequency
The frequency of the acoustic energy transmitted in a sonar system is of prime importance. To achieve
a narrow directive beam of energy, the radiating transducer is normally large in relation to the
wavelength of the signal. Therefore, in order to produce a reasonably sized transducer emitting a
narrow beam, a high transmission frequency needs to be used. The high frequency will also improve
the signal-to-noise ratio in the system because ambient noise occurs at the lower end of the frequency
spectrum. Unfortunately the higher the frequency used the greater will be the attenuation as shown in
Figure 2.1.
The choice of transmission frequency is therefore a compromise between transducer size, freedom
from noise, and minimal attenuation. Frequencies between 15 and 60 kHz are typical for depth
sounders fitted in large vessels. A high power is transmitted from a large magnetostrictive transducer
to indicate great depths with low attenuation. Small light craft use depth sounders that transmit in the
band 200–400 kHz. This enables compact electrostrictive or ceramic transducers to be used on a boat
where space is limited. Speed logs use frequencies in the range 300 kHz to 1 MHz depending upon
their design and are not strictly sonar devices in the true definition of the sense.
Beam spreading
Transmission beam diverging or spreading is independent of fixed parameters, such as frequency, but
depends upon distance between the transducer and the seabed. The greater the depth, the more the
beam spreads, resulting in a drop in returned energy.
Temperature
Water temperature also affects absorption. As temperature decreases, attenuation decreases. The effect
of temperature change is small and in most cases can be ignored, although modern sonar equipment
is usually fitted with a temperature sensor to provide corrective data to the processor.
Consistency of the seabed
The reflective property of the seabed changes with its consistency. The main types of seabed and the
attenuation which they cause are listed in Table 2.1. The measurements were made with an echo
sounder transmitting 24 kHz from a magnetostrictive transducer.
Figure 2.1 A linear graph produced by plotting absorption loss against frequency. Salinity of the
seawater is 3.4% at 15°C.
10000
m '000 ~ •• '00 •• '0 .. c
0
"g 0.' • <~ 0.01
O.cX)1
0.' '0 '00 '000 '0000
Freauency in kHz
24 Electronic Navigation Systems
2.2.2 Salinity, pressure and the velocity of the acoustic wave
Since a depth sounder operates by precisely calculating the time taken for a pulse of energy to travel
to the ocean floor and return, any variation in the velocity of the acoustic wave from the accepted
calibrated speed of 1500 ms–1 will produce an error in the indicated depth. The speed of acoustic
waves in seawater varies with temperature, pressure and salinity. Figure 2.2 illustrates the speed
variation caused by changes in the salinity of seawater.
Ocean water salinity is approximately 3.4% but it does vary extensively throughout the world. As
salinity increases, sonar wave velocity increases producing a shallower depth indication, although in
practice errors due to salinity changes would not be greater than 0.5%. The error can be ignored except
when the vessel transfers from seawater to fresh water, when the indicated depth will be
approximately 3% greater than the actual depth. The variation of speed with pressure or depth is
indicated by the graph in Figure 2.3.
It can readily be seen that the change is slight, and is normally only compensated for in apparatus
fitted on survey vessels. Seasonal changes affect the level of the thermocline and thus there is a small
annual velocity variation. However, this can usually be ignored.
Table 2.1 Sea bed consistency and attenuation
Consistency Attenuation (dB)
Soft mud 15
Mud/sand 9
Sand/mud 6
Sand 3
Stone/rock 1
These figures are typical and are quoted as a guideline only.
In practice sufficient transmitted power will overcome these
losses.
Figure 2.2 Graph showing that the velocity of acoustic energy is affected by both the temperature
and the salinity of seawater.
Depth sounding systems 25
2.2.3 Noise
Noise present in the ocean adversely affects the performance of sonar equipment. Water noise has two
main causes.
The steady ambient noise caused by natural phenomena.
Variable noise caused by the movement of shipping and the scattering of one’s own transmitted
signal (reverberation).
Ambient noise
Figure 2.4 shows that the amplitude of the ambient noise remains constant as range increases, whereas
both the echo amplitude and the level of reverberation noise decrease linearly with range. Because of
beam spreading, scattering of the signal increases and reverberation noise amplitude falls more slowly
than the echo signal amplitude.
Figure 2.3 Variation of the velocity of acoustic waves with pressure.
26 Electronic Navigation Systems
Ambient noise possesses different characteristics at different frequencies and varies with natural
conditions such as rainstorms. Rain hitting the surface of the sea can cause a 10-fold increase in the
noise level at the low frequency (approx. 10 kHz) end of the spectrum. Low frequency noise is also
increased, particularly in shallow water, by storms or heavy surf. Biological sounds produced by some
forms of aquatic life are also detectable, but only by the more sensitive types of equipment.
The steady amplitude of ambient noise produced by these and other factors affects the signal-tonoise
ratio of the received signal and can in some cases lead to a loss of the returned echo. Signal-tonoise
ratio can be improved by transmitting more power. This may be done by increasing the pulse
repetition rate or increasing the amplitude or duration of the pulse. Unfortunately such an increase,
which improves signal-to-noise ratio, leads to an increase in the amplitude of reverberation noise.
Ambient noise is produced in the lower end of the frequency spectrum. By using a slightly higher
transmitter frequency and a limited bandwidth receiver it is possible to reduce significantly the effects
of ambient noise.
Reverberation noise
Reverberation noise is the term used to describe noise created and affected by one’s own transmission.
The noise is caused by a ‘back scattering’ of the transmitted signal. It differs from ambient noise in
the following ways.
Its amplitude is directly proportional to the transmitted signal.
Its amplitude is inversely proportional to the distance from the target.
Its frequency is the same as that of the transmitted signal.
The signal-to-noise ratio cannot be improved by increasing transmitter power because reverberation
noise is directly proportional to the power in the transmitted wave. Also it cannot be attenuated by
improving receiver selectivity because the noise is at the same frequency as the transmitted wave.
Furthermore reverberation noise increases with range because of increasing beamwidth. The area
covered by the wavefront progressively increases, causing a larger area from which back scattering
will occur. This means that reverberation noise does not decrease in amplitude as rapidly as the
transmitted signal. Ultimately, therefore, reverberation noise amplitude will exceed the signal noise
Figure 2.4 Comparison of steady-state noise, reverberation noise and signal amplitude.
Depth sounding systems 27
amplitude, as shown in Figure 2.4, and the echo will be lost. The amplitude of both the echo and
reverberation noise decreases linearly with range. However, because of beam spreading, back
scattering increases and reverberation noise amplitude falls more slowly than the echo signal
amplitude. Three totally different ‘scattering’ sources produce reverberation noise.
Surface reverberation. As the name suggests, this is caused by the surface of the ocean and is
particularly troublesome during rough weather conditions when the surface is turbulent.
Volume reverberation. This is the interference caused by beam scattering due to suspended matter
in the ocean. Marine life, prevalent at depths between 200 and 750 m, is the main cause of this type
of interference.
Bottom reverberation. This depends upon the nature of the seabed. Solid seabeds, such as hard rock,
will produce greater scattering of the beam than silt or sandy seabeds. Beam scattering caused by
a solid seabed is particularly troublesome in fish finding systems because targets close to the seabed
can be lost in the scatter.
2.3 Transducers
A transducer is a converter of energy. RF energy, when applied to a transducer assembly, will cause
the unit to oscillate at its natural resonant frequency. If the transmitting face of the unit is placed in
contact with, or close to, seawater the oscillations will cause acoustic waves to be transmitted in the
water. Any reflected acoustic energy will cause a reciprocal action at the transducer. If the reflected
energy comes into contact with the transducer face natural resonant oscillations will again be
produced. These oscillations will in turn cause a minute electromotive force (e.m.f.) to be created
which is then processed by the receiver to produce the necessary data for display.
Three types of transducer construction are available; electrostrictive, piezoelectric resonator, and
magnetostrictive. Both the electrostrictive and the piezoelectric resonator types are constructed from
piezoelectric ceramic materials and the two should not be confused.
2.3.1 Electrostrictive transducers
Certain materials, such as Rochelle salt and quartz, exhibit pressure electric effects when they are
subjected to mechanical stress. This phenomenon is particularly outstanding in the element lead
zirconate titanate, a material widely used for the construction of the sensitive element in modern
electrostrictive transducers. Such a material is termed ferro-electric because of its similarity to ferromagnetic
materials.
The ceramic material contains random electric domains which when subjected to mechanical stress
will line up to produce a potential difference (p.d.) across the two plate ends of the material section.
Alternatively, if a voltage is applied across the plate ends of the ceramic crystal section its length will
be varied. Figure 2.5 illustrates these phenomena.
The natural resonant frequency of the crystal slice is inversely proportional to its thickness. At high
frequencies therefore the crystal slice becomes brittle, making its use in areas subjected to great stress
forces impossible. This is a problem if the transducer is to be mounted in the forward section of a large
merchant vessel where pressure stress can be intolerable. The fragility of the crystal also imposes
limits on the transmitter power that may be applied because mechanical stress is directly related to
power. The power restraints thus established make the electrostrictive transducer unsuitable for use in
depth sounding apparatus where great depths need to be indicated. In addition, the low transmission
frequency requirement of an echo sounder means that such a transducer crystal slice would be
28 Electronic Navigation Systems
excessively thick and require massive transmitter peak power to cause it to oscillate. The crystal slice
is stressed by a voltage applied across its ends, thus the thicker the crystal slice, the greater is the
power needed to stress it.
The electrostrictive transducer is only fitted on large merchant vessels when the power transmitted
is low and the frequency is high, a combination of factors present in Doppler speed logging systems.
Such a transducer is manufactured by mounting two crystal slices in a sandwich of two stainless steel
cylinders. The whole unit is pre-stressed by inserting a stainless steel bolt through the centre of the
active unit as shown in Figure 2.6.
If a voltage is applied across the ends of the unit, it will be made to vary in length. The bolt is
insulated from the crystal slices by means of a PVC collar and the whole cylindrical section is made
waterproof by means of a flexible seal. The bolt tightens against a compression spring permitting the
crystal slices to vary in length, under the influence of the RF energy, whilst still remaining
mechanically stressed. This method of construction is widely found on the electrostrictive transducers
used in the Merchant Navy. For smaller vessels, where the external stresses are not so severe, the
simpler piezoelectric resonator is used.
Figure 2.5 (a) An output is produced when a piezoelectric ceramic cylinder is subjected to stress.
(b) A change of length occurs if a voltage is applied across the ends of a piezoelectric ceramic
cylinder.
Stress
Stress r·::1:-r, ------,
lal
Ibl
! ,-----
i~~-~- ___ n
,
,, , ,
It ------I-L
~ rL:----
Depth sounding systems 29
2.3.2 Piezoelectric resonator
This type of transducer makes use of the flexible qualities of a crystal slice. If the ceramic crystal slice
is mounted so that it is able to flex at its natural resonant frequency, acoustic oscillations can be
produced. The action is again reciprocal. If the ceramic crystal slice is mounted at its corners only, and
is caused to flex by an external force, a small p.d. will be developed across the ends of the element.
This phenomenon is widely used in industry for producing such things as electronic cigarette lighters
and fundamental crystal oscillator units for digital watches. However, a ceramic crystal slice used in
this way is subject to the same mechanical laws as have previously been stated. The higher the
frequency of oscillation, the thinner the slice needs to be and the greater the risk of fracture due to
external stress or overdriving. For these reasons, piezoelectric resonators are rarely used at sea.
2.3.3 Magnetostrictive transducers
Figure 2.7 shows a bar of ferromagnetic material around which is wound a coil. If the bar is held rigid
and a large current is passed through the coil, the resulting magnetic field produced will cause the bar
to change in length. This slight change may be an increase or a decrease depending upon the material
used for construction. For maximum change of length for a given input signal, annealed nickel has
been found to be the optimum material and consequently this is used extensively in the construction
of marine transducers.
As the a.c. through the coil increases to a maximum in one direction, the annealed nickel bar will
reach its maximum construction length (l+ l). With the a.c. at zero the bar returns to normal (l). The
current now increases in the opposite direction and the bar once again constricts (l– l). The frequency
Figure 2.6 Construction details of a ceramic electrostrictive transducer.
30 Electronic Navigation Systems
of resonance is therefore twice that of the applied a.c. This frequency doubling action is counteracted
by applying a permanent magnet bias field produced by an in-built permanent magnet.
The phenomenon that causes the bar to change in length under the influence of a magnetic field is
called ‘magnetostriction’, and in common with most mechanical laws possesses the reciprocal quality.
When acoustic vibrations cause the bar to constrict, at its natural resonant frequency, an alternating
magnetic field is produced around the coil. A minute alternating current is caused to flow in the coil
and a small e.m.f. is generated. This is then amplified and processed by the receiver as the returned
echo.
To limit the effects of magnetic hysteresis and eddy current losses common in low frequency
transformer construction, the annealed nickel bar is made of laminated strips bonded together with an
insulating material. Figure 2.8 illustrates the construction of a typical magnetostrictive transducer unit.
The transmitting face is at the base of the diagram.
Magnetostrictive transducers are extremely robust which makes them ideal for use in large vessels
where heavy sea pounding could destroy an unprotected electrostrictive type. They are extensively
used with depth sounding apparatus because at the low frequencies used they can be constructed to
an acceptable size and will handle the large power requirement of a deep sounding system. However,
Figure 2.7 (a) A bar of ferromagnetic material around which is wound a coil. (b) Relationship
between magnetic field strength and change of length.
~ ;,
,•"0
u
0;
~
u
.~
0
m :•:;
Annealed
nickel t::=11T ~Length
1.1
1- d/ Length 1+ '"
Annealed
nickel
Steel Cobalt
/bl
Depth sounding systems 31
magnetic losses increase with frequency, and above 100 kHz the efficiency of magnetostrictive
transducers falls to below the normal 40%. Above this frequency electrostrictive transducers are
normally used.
2.3.4 Transducer siting
The decision of where to mount the transducer must not be made in haste. It is vital that the active face of
the transducer is in contact with the water. The unit should also be mounted well away from areas close to
turbulence that will cause noise. Areas close to propellers or water outlets must be avoided.
Aeration is undoubtedly the biggest problem encountered when transducers are wrongly installed. Air
bubbles in the water, for whatever reason, will pass close to the transducer face and act as a reflector of
the acoustic energy.
As a vessel cuts through the water, severe turbulence is created. Water containing huge quantities of
air bubbles is forced under and along the hull. The bow wave is aerated as it is forced above the surface of
the sea, along the hull. The wave falls back into the sea at approximately one-third the distance along the
length of the vessel from the bow. A transducer mounted aft of the position where the bow wave re-enters
the sea, would suffer badly from the problems of aeration. Mounting the transducer ahead of this point,
even in the bulbous bow, would be ideal. It should be remembered, however, that at some stage
maintenance may be required and a position in the bulbous bow may be inaccessible.
A second source of aeration is that of cavitation. The hull of a vessel is seldom smooth and any
indentations or irregularities in it will cause air bubbles to be produced leading to aeration of the
transducer face. Hull irregularities are impossible to predict as they are not a feature of the vessel’s
design.
2.4 Depth sounding principles
In its simplest form, the depth sounder is purely a timing and display system that makes use of a
transmitter and a receiver to measure the depth of water beneath a vessel. Acoustic energy is
Figure 2.8 Cross-section of a magnetostrictive transducer. (Reproduced courtesy of Marconi
Marine.)
32 Electronic Navigation Systems
transmitted perpendicularly from the transducer to the seabed. Some of the transmitted energy is
reflected and will be received by the transducer as an echo. It has been previously stated that the
velocity of sound waves in seawater is accepted to be 1500 ms–1. Knowledge of this fact and the
ability to measure precisely the time delay between transmission and reception, provides an accurate
indication of the water depth.
Distance travelled =
velocity × time
2
where velocity = 1500 ms–1 in salt water; time = time taken for the return journey in seconds; and
distance = depth beneath the transducer in metres. Thus if the time taken for the return journey is 1 s,
the depth of water beneath the transducer is 750 m. If the time is 0.1 s the depth is 75 m, and so
on.
The transmitter and transducer, must be capable of delivering sufficient power and the receiver must
possess adequate sensitivity to overcome all of the losses in the transmission medium (seawater and
seabed). It is the likely attenuation of the signal, due to the losses described in the first part of this
chapter, which determines the specifications of the equipment to be fitted on a merchant vessel.
2.4.1 Continuous wave/pulse system
The transmission of acoustic energy for depth sounding, may take one of two forms.
A continuous wave system, where the acoustic energy is continuously transmitted from one
transducer. The returned echo signal is received by a second transducer and a phase difference
between the two is used to calculate the depth.
The pulse system, in which rapid short, high intensity pulses are transmitted and received by a
single transducer. The depth is calculated by measuring the time delay between transmission and
reception.
The latter system is preferred in the majority of applications. Both the pulse length (duration) and the
pulse repetition frequency (PRF) are important when considering the function of the echo sounding
apparatus.
Continuous wave system
This system is rarely used in commercial echo sounding applications. Because it requires independent
transmitters and receivers, and two transducer assemblies it is expensive. Also because the transmitter
is firing continually, noise is a particular problem. Civilian maritime echo sounders therefore use a
pulsed system.
Pulsed system
In this system the transmitter fires for a defined period of time and is then switched off. The pulse
travels to the ocean floor and is reflected back to be received by the same transducer which is now
switched to a receive mode. The duration of the transmitter pulse and the pulse repetition frequency
(PRF) are particularly important parameters in this system
The pulse duration effectively determines the resolution quality of the equipment. This, along with
the display method used, enables objects close together in the water, or close to the seabed, to be
Depth sounding systems 33
recorded separately. It is called target or echo discrimination. This factor is particularly important in
fish finding apparatus where very short duration pulses (typically 0.25 or 0.5 ms) are used.
Echo discrimination (D) is:
D = V × l (in metres)
where V = the velocity of acoustic waves, and l = pulse length.
For a 0.5 ms pulse length:
D = 1500 × 0.5 × 10–3 = 0.75m
For a 2 ms pulse length:
D = 1500 × 2 × 10–3 = 3m
Obviously a short pulse length is superior where objects to be displayed are close together in the water.
Short pulse lengths tend to be used in fish finding systems.
A short pulse length also improves the quality of the returned echo because reverberation noise will
be less. Reverberation noise is directly proportional to the signal strength, therefore reducing the pulse
length reduces signal strength which in turn reduces noise. Unfortunately, reducing the signal strength
in this way reduces the total energy transmitted, thereby limiting the maximum depth from which
satisfactory echoes can be received. Obviously, a compromise has to be made. Most depth sounders
are fitted with a means whereby the pulse length can be varied with range. For shallow ranges, and
for better definition, a short pulse length is used. On those occasions where great depths are to be
recorded a longer pulse is transmitted.
For a given pulse length, the PRF effectively determines the maximum range that can be indicated.
It is a measure of the time interval between pulses when transmission has ceased and the receiver is
awaiting the returned echo.
The maximum indicated range may be determined by using the following formula:
Maximum range indication (r) =
v × t
2
(in metres)
where v = velocity of sound in seawater (l500 ms–1) and t = time between pulses in seconds. If the
PRF is one per second (PRF = 60), the maximum depth recorded is 750 m. If the PRF is two per
second (PRF = 120) the maximum depth recorded is 375 m.
The maximum display range should not be confused with the maximum depth. For instance, if the
PRF is one per second the maximum display range is 750 m. If the water depth is 850 m, an echo will
be returned after a second pulse has been transmitted and the range display has been returned to zero.
The indicated depth would now be 100 m. A system of ‘phased’ ranges, where the display initiation
is delayed for a pre-determined period after transmission overcomes the problem of over-range
indication.
2.4.2 Transmission beamwidth
Acoustic energy is radiated vertically downwards from the transducer in the form of a beam of energy.
As Figure 2.9 shows the main beam is central to the transducer face and shorter sidelobes are also
produced. The beamwidth must not be excessively narrow otherwise echoes may be missed,
particularly in heavy weather when the vessel is rolling. A low PRF combined with a fast ship speed
34 Electronic Navigation Systems
can in some cases lead to the vessel ‘running away’ from an echo that could well be missed. In
general, beamwidths measured at the half-power points (–3 dB), used for depth sounding apparatus are
between 15° and 25°. To obtain this relatively narrow beamwidth, the transducer needs to be
constructed with a size equal to many wavelengths of the frequency in use. This fact dictates that the
transducer will be physically large for the lower acoustic frequencies used in depth sounding.
In order to reduce the transducer size, and keep a narrow beamwidth, it is possible to increase the
transmission frequency. However, the resulting signal attenuation negates this change and in practice
a compromise must once again be reached between frequency, transducer size and beamwidth. Figure
2.10 shows typical beamwidths for a low frequency (50 kHz) sounder and that of a frequency four
times greater.
Figure 2.9 Transmission beam showing the sidelobes.
Figure 2.10 Typical beamwidths for echo sounders transmitting low and high frequencies.
(Reproduced courtesy Furuno Electric Co. Ltd.)
Depth sounding systems 35
2.5 A generic echo sounding system
Compared with other systems, echo sounder circuitry is relatively simple. Most manufacturers of deep
sounding systems now opt for microprocessor control and digital displays, but it was not always so.
Many mariners preferred the paper-recording echo sounder because the display was clear, easy to read
and provided a history of soundings.
Marconi Marine’s ‘Seahorse’ echo sounder (Figure 2.11) was typical of the standard paperrecording
echo sounder. Built in the period before microprocessor control, it is used here to describe
the relatively simply circuitry needed to produce an accurate read-out of depth beneath the keel. From
the description it is easy to see that an echo sounding system is simply a timing device.
The system used a transmission frequency of 24 kHz and two ranges, either manually or
automatically selected, to allow depths down to 1000 m to be recorded. The shallow range was 100m
and operated with a short pulse length of 200 μs, whereas the 1000 m range uses a pulse length of 2 ms.
Display accuracy for the chart recorder is typically 0.5% producing indications with an accuracy of
±0.5 m on the 100 m range and ±5 m on the deepest range.
2.5.1 Description
Receiver and chart recorder
When chart recording has been selected, transmission is initiated by a pulse from a proximity detector
which triggers the chart pulse generator circuit introducing a slight delay, pre-set on each range, to
ensure that transmission occurs at the instant the stylus marks zero on the recording paper. This system
trigger pulse or that from the trigger pulse generator circuit when the chart is switched off, has three
functions:
to initiate the pulse timing circuit
to operate the blanking pulse generator
to synchronize the digital and processing circuits.
The transmit timing circuit sets the pulse length to trigger the 24 kHz oscillator (transmission
frequency). Pulse length is increased, when the deep range is changed, by a range switch (not shown).
Power contained in the transmitted signal is produced by the power amplifier stage, the output of
which is coupled to the magnetostrictive transducer with the neon indicating transmission.
When the transmitter fires, the receiver input is blanked to prevent the high-energy pulse from
causing damage to the input tuned circuits. The blanking pulse generator also initiates the swept gain
circuit and inhibits the data pulse generator. During transmission, the swept gain control circuit holds
the gain of the input tuned amplifier low. At cessation of transmission, the hold is removed permitting
the receiver gain to gradually increase at a rate governed by an inverse fourth power law. This type
of inverse gain control is necessary because echoes that are returned soon after transmission ceases are
of large amplitude and are likely to overload the receiver.
The echo amplitude gradually decreases as the returned echo delay period increases. Thus the swept
gain control circuit causes the average amplitude of the echoes displayed to be the same over the
whole period between transmission pulses. However, high intensity echoes returned from large
reflective objects will produce a rapid change in signal amplitude and will cause a larger signal to be
coupled to the logarithmic amplifier causing a more substantial indication to be made on the paper.
The logarithmic amplifier and detector stages produce a d.c. output, the amplitude of which is
logarithmically proportional to the strength of the echo signal.
Figure 2.11 A block schematic diagram of the Seahorse echo sounder. (Reproduced courtesy of Marconi Marine.)
Prox. de!. output pulse.
Transmitter
I
I
I
I
I
indicator I ---- __________ 1
System trigger
Triggering
ChartON
Chart OFF
From chart select switch
r---
(Pickup
reduction)
Blanking
pulse
generator
Blanking
Dig ital display
and processing
ci rcuits
Swept
gain
Receiver board
Data
Proximity
detector
-l
I
Det.O/P I
pulse
....-- ,-, --,
11 Echo
led
==-- ------
Paper drive
motor
Paper
chart
Alarms
Depth sounding systems 37
In the chart recorder display, electrosensitive paper is drawn horizontally beneath a sharp stylus.
The paper is tightly drawn over the grounded roller guides by a constant speed paper-drive motor.
Paper marking is achieved by applying a high voltage a.c. signal to the stylus which is drawn at 90°
to the paper movement, across the surface of the paper on top of the left-hand roller. The paper is
marked by burning the surface with a high voltage charge produced through the paper between the
stylus and ground. Depending upon the size of the returned echo, the marking voltage is between 440
and 1100 V and is produced from a print voltage oscillator running at 2 kHz. Oscillator amplifier
output is a constant amplitude signal, the threshold level of which is raised by the d.c. produced by
a detected echo signal. Thus a high-intensity echo signal causes the marking voltage to be raised above
the threshold level by a greater amount than would be caused by a detected small echo signal.
For accurate depth marking it is essential that the stylus tracking speed is absolutely precise. The
stylus is moved along the paper by a belt controlled by the stylus d.c. motor. Speed accuracy is
maintained by a complex feedback loop and tacho-generator circuit.
Digital circuits
The digital display section contains the necessary logic to drive the integral three-digit depth display,
the alarm circuit, and the remote indicators. Pulse repetition frequency (PRF) of the clock oscillator
is pre-set so that the time taken for the three-digit counter to count from 000 to 999 is exactly the same
as that taken by the paper stylus to travel from zero to the maximum reading for the range in use. The
counter output is therefore directly related to depth.
When the chart recorder is switched off, the digital processing section and the transmitter are
triggered from the processor trigger pulse generator circuit. Both the transmit and receive sections
work in the same way as previously described. A low logic pulse from the trigger pulse standardizing
circuit synchronizes the logic functions. The d.c. output from the receiver detector is coupled via a
data pulse generator circuit to the interface system. Unfortunately in any echo sounder it is likely that
unwanted echoes will be received due to ship noise, aeration or other factors.
False echoes would be displayed as false depth indications on the chart and would be easily
recognized. However, such echoes would produce instantaneous erroneous readings on the digital
counter display that would not be so easily recognized. To prevent this happening echoes are stored
in a data store on the processing board and only valid echoes will produce a reading on the display.
Valid echoes are those that have indicated the same depth for two consecutive sounding cycles. The
data store, therefore, consists of a two-stage counter which holds each echo for one sounding cycle and
compares it with the next echo before the depth is displayed on the digital display.
The display circuit consists of three digital counters that are clocked from the clock oscillator
circuit. Oscillator clock pulses are initiated by the system trigger at the instant of transmission. The
first nine pulses are counted by the lowest order decade counter which registers 1–9 on the display
least significant figure (LSF) element. The next clock pulse produces a 0 on the LSF display and
clocks the second decade counter by one, producing a 1 in the centre of the display. This action
continues, and if no echo is received, the full count of 999 is recorded when an output pulse from the
counting circuit is fed back to stop the clock.
Each time transmission takes place the counters are reset to zero before being enabled. This is not
evident on the display because the data output from the counters is taken via a latch that has to be
enabled before data transfer can take place. Thus the counters are continually changing but the display
data will only change when the latches have been enabled (when the depth changes). If an echo is
received during the counting process, the output is stopped, and the output latches enabled by a pulse
from the data store. The new depth is now displayed on the indicator and the counters are reset at the
start of the next transmission pulse.
38 Electronic Navigation Systems
With any echo sounder, it is necessary that the clock pulse rate be directly related to depth. When
the shallow (100 m) range is selected a high frequency is used which is reduced by a factor of 10 when
the deep range (1000 m) is selected.
Modern echo sounders rely for their operation on the ubiquitous microprocessor and digital
circuitry, but the system principles remain the same. It is the display of information that is the outward
sign of the advance in technology.
2.6 A digitized echo sounding system
The Furuno Electric Co. Ltd, one of the world’s big manufacturers of marine equipment, produces an
echo sounder, the FE606, in which many of the functions have been digitized. Transmission frequency
is either 50 or 200 kHz depending upon navigation requirements. A choice of 50 kHz provides greater
depth indication and a wider beamwidth reducing the chance that the vessel may ‘run away’ from an
echo (see Figure 2.10).
The pulse length increases with depth range from 0.4 ms, on the shallow ranges, to 2.0 ms on the
maximum range. This enables better target discrimination on the lower ranges and ensures that
sufficient pulse power is available on the higher ranges. Pulse repetition rate (sounding rate) is reduced
as range increases to ensure adequate time between pulses for echoes to be returned from greater
depths.
The system shown in Figure 2.12 is essentially a paper recorder and two LCD displays showing
start depth and seabed depth. As before, transmission is initiated at the instant the stylus marks the zero
line on the sensitive paper by a trigger sensor coupled to the control integrated circuits. Depending
upon the range selected, the pulse length modulates the output from the transmit oscillator, which is
power amplified and then coupled via a transmit/receive switch to the transducer.
A returned echo is processed in the receiver and applied to the logic circuitry. Here it is processed
to determine that it is a valid echo and then it is latched through to a digital-to-analogue converter to
produce the analogue voltage to drive the print oscillator. Thus the depth is marked on the sensitive
paper at some point determined by the time delay between transmission and reception, and the
distance the stylus has travelled over the paper.
2.7 A microcomputer echo sounding system
As you would expect, the use of computing technology has eliminated much of the basic circuitry and
in most cases the mechanical paper display system of modern echo sounders. Current systems are
much more versatile than their predecessors. The use of a computer enables precise control and
processing of the echo sounding signal. Circuitry has now reached the point where it is virtually all
contained on a few chips. However, the most obvious changes that users will be aware of in modern
systems are the display and user interface.
Once again there are many manufacturers and suppliers of echo sounders or, as they are often now
called, fish finders. The Furuno navigational echo sounder FE-700 is typical of many. Depending upon
requirements the system is able to operate with a 200 kHz transmission frequency giving highresolution
shallow depth performance, or 50 kHz for deep-water sounding.
Seabed and echo data is displayed on a 6.5 inch high-brightness TFT colour LCD display which
provides the navigator with a history of soundings over a period of 15 min, much as the older paper
recording systems did (see Figure 2.13).
Figure 2.12 Furuno FE-606 echo sounding system. (Reproduced courtesy of Furuno Electric Co.)
,r --:::~r / lransceiver
tt4...."'5kHz
I L
----LL..:!:tarrsdtKef
~
I ~- , fJ? ?"i =r I
L,D=-J 'Ira.
[)ala bus ~ --j ,,""
e~~'~~~'''''" , b<Jr. ~r. ' AA 'J ~EfEtL.. .. ~.-L. L" F...AM -
PU::!_.,~.r:M1 1I .td:-a:s .- ' =-.:!5
... -.. . , ~ "---- '
ControllC·s
-~I-II-... ~
11 Control I! EJ
I i ... il ~c:~ ..-I J'I=C'A. ~I1
Ralge"""j i
. I!
~ il
B Ell<! depOl
01
Seabed depth
I Osc 1800kHz m,, " " d i
~,
i
I _ n~~
--.-,, '~
~.,
-,
40 Electronic Navigation Systems
Figure 2.13 Furuno FE-700 LCD TFT data display (Navigation Mode.) (Reproduced courtesy of
Furuno Electric Co.)
Display mode
Gain setting
Range setting
Auto
mode
Alarm
setting
Depth alarm line
Depth
Explanation of depth
(Below transducer, or
below surface)
Screen
20 -
40 -
unit
Range scale
( ""'*') ( A~"')
( IlIM ) ( BRILL )
~ (COLO:;:)
( - )( + )
,~
', ~' .
~
2~'
~o
~
~
1*S~~~~1.
_~ MEN'U
( Mooe )
( ~)
Control panel
Depth sounding systems 41
Depths, associated time, and position are all stored in 24-h memory and can be played back at any
time. This is a useful function if there is any dispute following an accident.
The main depth display emulates a cross-sectional profile of the ocean over the past 15 min. At the
top of the display in Figure 2.13, the solid zero line marks the ocean surface or transducer level
whichever is selected. At 15 m down, a second line marks the depth at which the alarm has been set.
The undulating line showing the ocean floor depth is shown varying over 15 min from 58 to 44 m and
the instantaneous depth, also shown as a large numerical display, is 47.5 m. Other operation detail is
as shown in the diagram. What is not indicated on the display is the change of pulse length and period
as selected by range.
As shown in Table 2.2, the pulse length is increased with the depth range to effectively allow more
power to be contained in the transmitted pulse, whilst the pulse period frequency is reduced to permit
longer gaps in the transmission period allowing greater depths to be indicated
In addition to the standard navigation mode, Furuno FE-700 users are provided with a number of
options adequately demonstrating the capability of a modern echo sounder using a TFT LCD display
(see Figure 2.14). All the selected modes display data as a window insert on top of the echo sounder
NAV mode display.
There are four display-mode areas.
OS DATA mode. Indicates own ship position, GPS derived course, time and a digital display of
water depth.
DBS mode. Provides a draft-adjusted depth mode for referencing with maritime charts.
LOGBOOK mode. As the name suggests, provides a facility for manually logging depths over a
given period.
HISTORY mode. Provides a mixture of contour and strata displays. The contour display can be
shifted back over the past 24 h whilst the strata display (right-hand side of display) shows sounding
data over the last 5 min.
2.8 Glossary
The following lists abbreviations, acronyms and definitions of specific terms used in this chapter.
Aeration Aerated water bubbles clinging to the transducer face cause errors in the
system.
Ambient noise Noise that remains constant as range increases.
Table 2.2 Echo sounder range vs pulse length vs PRF
Depth (metres) Pulse length (ms) PRF (pulses per minute)
5, 10 and 20 0.25 750
40 0.38 375
100 1.00 150
200 2.00 75
400 and 800 3.60 42
42 Electronic Navigation Systems
Figure 2.14 Different display modes demonstrating the flexibility of a microcomputer-controlled echo
sounder. (Reproduced courtesy of Furuno Electric Co.)
OS DATA Mode DBS Mode
__ 0 _ - ___ ' _
--_Q_- --- '-
,
O.Okt
15:51 :52
28 .6111
LOGBOOK Mode HISTORY Mode
.". " " "
.. , " .. " •
" " " "
,
•.•. •• •• " • ---'-- .. .". o... n • .. " • " .. .. •
" .. .. .. n • .. " " " • .- .. .. •
Depth sounding systems 43
Beam spreading The transmitted pulse of energy spreads as it travels away from the transducer.
The use of a wide beam will cause noise problems in the receiver and a
narrow beam may lead to an echo being missed as the vessel steams away
from the area.
Chart recorder A sensitive paper recording system which, when the surface is scratched by a
stylus, marks the contour of the ocean floor.
Continuous wave
system
An echo sounding system that uses two transducers and transmits and receives
energy at the same time.
Electrostrictive
transducer
A transducer design based on piezoelectric technology. It is used when a
higher transmission frequency is needed such as in speed logging equipment
or fish-finding sounders.
Magnetostrictive
transducer
A design based on magnetic induction. A large heavy transducer capable of
transmitting high power. Used in deep sounding systems.
Pulse duration
(length)
The period of the transmitted pulse when the transmitter is active.
Pulse repetition
frequency (PRF)
The number of pulses transmitted per minute by the system. Similar to
RADAR
Pulse wave system A system that, like RADAR, transmits pulses of energy from a transducer
which is then switched off. The received energy returns to the same
transducer.
Reverberation noise Noise that decreases as range increases.
Sonar Sound navigation and ranging.
Velocity Speed of acoustic waves in seawater; 1505 ms–1 or approximated
to1500 ms–1.
2.9 Summary
Sonar stands for sound navigation and ranging.
Sound travels relatively slowly in seawater at 1505 ms–1. This is approximated to 1500 ms–1 for
convenience.
The velocity is not a constant, it varies with the salinity of seawater. Ocean salinity is approximately
3.4%.
Transmitted signal amplitude is attenuated by saltwater and the ocean floor from which it is
reflected.
Noise caused by sea creatures and ocean activity is a major problem affecting sonar equipment.
The temperature of the seawater affects the velocity of the acoustic wave and consequently affects
the accuracy of the displayed data. Temperature sensors are contained in the transducer housing to
produce corrective data.
Transducers are effectively the antennas of sonar systems. They transmit and receive the acoustic
energy.
There are two main types of transducer in use; magnetostrictive and electrostrictive. Magnetostrictive
transducers are large and heavy and tend to be used only on large vessels. Electrostrictive
transducers are lighter and often used in speed logging systems and on smaller craft.
Low frequencies are often used in deep sounding systems typically in the range 10–100 kHz.
The depth below the keel is related to the time taken for the acoustic wave to travel to the ocean
floor and return. Put simply if the delay is 1 s and the wave travels at 1500 ms–1 then the depth is
0.5 × 1500 = 750 m.
44 Electronic Navigation Systems
Pulsed systems, like those used in maritime RADAR, are used in an echo sounder. The pulse length
or duration determines the resolution of the equipment. A short pulse length will identify objects
close together in the water. If all other parameters remain constant, the pulse repetition frequency
(PRF), the number of pulses per minute, determines the maximum range that can be indicated.
The width of the transmitted beam becomes wider as it travels away from the transducer. It should
not be excessively narrow or the vessel may ‘run away’ from, or miss, the returned echo.
Modern echo sounding equipment is computer controlled and therefore is able to produce a host of
other data besides a depth indication.
2.1 Introduction
Sonar (sound navigation and ranging) is the acronym identifying those systems that rely for their
operation on the transmission and reception of acoustic energy in water. The term is widely used to
identify all modern systems that propagate acoustic or electromagnetic energy into seawater to
determine a vessel’s speed or the depth of water under the keel. This book is not concerned with those
specialized sonar techniques that are used for locating submerged objects, either fish or submarines.
A navigator in the Merchant Navy is interested only in the depth of the water beneath the vessel, an
indication of the speed of his ship and the distance run. See Chapter 3 for a description of speed
logging equipment.
The first section of this chapter deals with the characteristics and problems that arise from the need
to propagate energy in seawater.
2.2 The characteristics of sound in seawater
Before considering the problems of transmitting and receiving acoustic energy in seawater, the effects of
the environment must be understood. Sonar systems rely on the accurate measurement of reflected
frequency or, in the case of depth sounders, a precise measurement of time and both these parameters are
affected by the often unpredictable ocean environment. These effects can be summarized as follows.
Attenuation. A variable factor related to the transmitted power, the frequency of transmission,
salinity of the seawater and the reflective consistency of the ocean floor.
Salinity of seawater. A variable factor affecting both the velocity of the acoustic wave and its
attenuation.
Velocity of sound in salt water. This is another variable parameter. Acoustic wave velocity is
precisely 1505 ms–1 at 15°C and atmospheric pressure, but most echo-sounding equipment is
calibrated at 1500 ms–1.
Reflective surface of the seabed. The amplitude of the reflected energy varies with the consistency
of the ocean floor.
Noise. Either inherent noise or that produced by one’s own transmission causes the signal-to-noise
ratio to degrade, and thus weak echo signals may be lost in noise.
Two additional factors should be considered.
Frequency of transmission. This will vary with the system, i.e. depth sounding or Doppler speed log.
Angle of incidence of the propagated beam. The closer the angle to vertical the greater will be the
energy reflected by the seabed.
Depth sounding systems 23
2.2.1 Attenuation and choice of frequency
The frequency of the acoustic energy transmitted in a sonar system is of prime importance. To achieve
a narrow directive beam of energy, the radiating transducer is normally large in relation to the
wavelength of the signal. Therefore, in order to produce a reasonably sized transducer emitting a
narrow beam, a high transmission frequency needs to be used. The high frequency will also improve
the signal-to-noise ratio in the system because ambient noise occurs at the lower end of the frequency
spectrum. Unfortunately the higher the frequency used the greater will be the attenuation as shown in
Figure 2.1.
The choice of transmission frequency is therefore a compromise between transducer size, freedom
from noise, and minimal attenuation. Frequencies between 15 and 60 kHz are typical for depth
sounders fitted in large vessels. A high power is transmitted from a large magnetostrictive transducer
to indicate great depths with low attenuation. Small light craft use depth sounders that transmit in the
band 200–400 kHz. This enables compact electrostrictive or ceramic transducers to be used on a boat
where space is limited. Speed logs use frequencies in the range 300 kHz to 1 MHz depending upon
their design and are not strictly sonar devices in the true definition of the sense.
Beam spreading
Transmission beam diverging or spreading is independent of fixed parameters, such as frequency, but
depends upon distance between the transducer and the seabed. The greater the depth, the more the
beam spreads, resulting in a drop in returned energy.
Temperature
Water temperature also affects absorption. As temperature decreases, attenuation decreases. The effect
of temperature change is small and in most cases can be ignored, although modern sonar equipment
is usually fitted with a temperature sensor to provide corrective data to the processor.
Consistency of the seabed
The reflective property of the seabed changes with its consistency. The main types of seabed and the
attenuation which they cause are listed in Table 2.1. The measurements were made with an echo
sounder transmitting 24 kHz from a magnetostrictive transducer.
Figure 2.1 A linear graph produced by plotting absorption loss against frequency. Salinity of the
seawater is 3.4% at 15°C.
10000
m '000 ~ •• '00 •• '0 .. c
0
"g 0.' • <~ 0.01
O.cX)1
0.' '0 '00 '000 '0000
Freauency in kHz
24 Electronic Navigation Systems
2.2.2 Salinity, pressure and the velocity of the acoustic wave
Since a depth sounder operates by precisely calculating the time taken for a pulse of energy to travel
to the ocean floor and return, any variation in the velocity of the acoustic wave from the accepted
calibrated speed of 1500 ms–1 will produce an error in the indicated depth. The speed of acoustic
waves in seawater varies with temperature, pressure and salinity. Figure 2.2 illustrates the speed
variation caused by changes in the salinity of seawater.
Ocean water salinity is approximately 3.4% but it does vary extensively throughout the world. As
salinity increases, sonar wave velocity increases producing a shallower depth indication, although in
practice errors due to salinity changes would not be greater than 0.5%. The error can be ignored except
when the vessel transfers from seawater to fresh water, when the indicated depth will be
approximately 3% greater than the actual depth. The variation of speed with pressure or depth is
indicated by the graph in Figure 2.3.
It can readily be seen that the change is slight, and is normally only compensated for in apparatus
fitted on survey vessels. Seasonal changes affect the level of the thermocline and thus there is a small
annual velocity variation. However, this can usually be ignored.
Table 2.1 Sea bed consistency and attenuation
Consistency Attenuation (dB)
Soft mud 15
Mud/sand 9
Sand/mud 6
Sand 3
Stone/rock 1
These figures are typical and are quoted as a guideline only.
In practice sufficient transmitted power will overcome these
losses.
Figure 2.2 Graph showing that the velocity of acoustic energy is affected by both the temperature
and the salinity of seawater.
Depth sounding systems 25
2.2.3 Noise
Noise present in the ocean adversely affects the performance of sonar equipment. Water noise has two
main causes.
The steady ambient noise caused by natural phenomena.
Variable noise caused by the movement of shipping and the scattering of one’s own transmitted
signal (reverberation).
Ambient noise
Figure 2.4 shows that the amplitude of the ambient noise remains constant as range increases, whereas
both the echo amplitude and the level of reverberation noise decrease linearly with range. Because of
beam spreading, scattering of the signal increases and reverberation noise amplitude falls more slowly
than the echo signal amplitude.
Figure 2.3 Variation of the velocity of acoustic waves with pressure.
26 Electronic Navigation Systems
Ambient noise possesses different characteristics at different frequencies and varies with natural
conditions such as rainstorms. Rain hitting the surface of the sea can cause a 10-fold increase in the
noise level at the low frequency (approx. 10 kHz) end of the spectrum. Low frequency noise is also
increased, particularly in shallow water, by storms or heavy surf. Biological sounds produced by some
forms of aquatic life are also detectable, but only by the more sensitive types of equipment.
The steady amplitude of ambient noise produced by these and other factors affects the signal-tonoise
ratio of the received signal and can in some cases lead to a loss of the returned echo. Signal-tonoise
ratio can be improved by transmitting more power. This may be done by increasing the pulse
repetition rate or increasing the amplitude or duration of the pulse. Unfortunately such an increase,
which improves signal-to-noise ratio, leads to an increase in the amplitude of reverberation noise.
Ambient noise is produced in the lower end of the frequency spectrum. By using a slightly higher
transmitter frequency and a limited bandwidth receiver it is possible to reduce significantly the effects
of ambient noise.
Reverberation noise
Reverberation noise is the term used to describe noise created and affected by one’s own transmission.
The noise is caused by a ‘back scattering’ of the transmitted signal. It differs from ambient noise in
the following ways.
Its amplitude is directly proportional to the transmitted signal.
Its amplitude is inversely proportional to the distance from the target.
Its frequency is the same as that of the transmitted signal.
The signal-to-noise ratio cannot be improved by increasing transmitter power because reverberation
noise is directly proportional to the power in the transmitted wave. Also it cannot be attenuated by
improving receiver selectivity because the noise is at the same frequency as the transmitted wave.
Furthermore reverberation noise increases with range because of increasing beamwidth. The area
covered by the wavefront progressively increases, causing a larger area from which back scattering
will occur. This means that reverberation noise does not decrease in amplitude as rapidly as the
transmitted signal. Ultimately, therefore, reverberation noise amplitude will exceed the signal noise
Figure 2.4 Comparison of steady-state noise, reverberation noise and signal amplitude.
Depth sounding systems 27
amplitude, as shown in Figure 2.4, and the echo will be lost. The amplitude of both the echo and
reverberation noise decreases linearly with range. However, because of beam spreading, back
scattering increases and reverberation noise amplitude falls more slowly than the echo signal
amplitude. Three totally different ‘scattering’ sources produce reverberation noise.
Surface reverberation. As the name suggests, this is caused by the surface of the ocean and is
particularly troublesome during rough weather conditions when the surface is turbulent.
Volume reverberation. This is the interference caused by beam scattering due to suspended matter
in the ocean. Marine life, prevalent at depths between 200 and 750 m, is the main cause of this type
of interference.
Bottom reverberation. This depends upon the nature of the seabed. Solid seabeds, such as hard rock,
will produce greater scattering of the beam than silt or sandy seabeds. Beam scattering caused by
a solid seabed is particularly troublesome in fish finding systems because targets close to the seabed
can be lost in the scatter.
2.3 Transducers
A transducer is a converter of energy. RF energy, when applied to a transducer assembly, will cause
the unit to oscillate at its natural resonant frequency. If the transmitting face of the unit is placed in
contact with, or close to, seawater the oscillations will cause acoustic waves to be transmitted in the
water. Any reflected acoustic energy will cause a reciprocal action at the transducer. If the reflected
energy comes into contact with the transducer face natural resonant oscillations will again be
produced. These oscillations will in turn cause a minute electromotive force (e.m.f.) to be created
which is then processed by the receiver to produce the necessary data for display.
Three types of transducer construction are available; electrostrictive, piezoelectric resonator, and
magnetostrictive. Both the electrostrictive and the piezoelectric resonator types are constructed from
piezoelectric ceramic materials and the two should not be confused.
2.3.1 Electrostrictive transducers
Certain materials, such as Rochelle salt and quartz, exhibit pressure electric effects when they are
subjected to mechanical stress. This phenomenon is particularly outstanding in the element lead
zirconate titanate, a material widely used for the construction of the sensitive element in modern
electrostrictive transducers. Such a material is termed ferro-electric because of its similarity to ferromagnetic
materials.
The ceramic material contains random electric domains which when subjected to mechanical stress
will line up to produce a potential difference (p.d.) across the two plate ends of the material section.
Alternatively, if a voltage is applied across the plate ends of the ceramic crystal section its length will
be varied. Figure 2.5 illustrates these phenomena.
The natural resonant frequency of the crystal slice is inversely proportional to its thickness. At high
frequencies therefore the crystal slice becomes brittle, making its use in areas subjected to great stress
forces impossible. This is a problem if the transducer is to be mounted in the forward section of a large
merchant vessel where pressure stress can be intolerable. The fragility of the crystal also imposes
limits on the transmitter power that may be applied because mechanical stress is directly related to
power. The power restraints thus established make the electrostrictive transducer unsuitable for use in
depth sounding apparatus where great depths need to be indicated. In addition, the low transmission
frequency requirement of an echo sounder means that such a transducer crystal slice would be
28 Electronic Navigation Systems
excessively thick and require massive transmitter peak power to cause it to oscillate. The crystal slice
is stressed by a voltage applied across its ends, thus the thicker the crystal slice, the greater is the
power needed to stress it.
The electrostrictive transducer is only fitted on large merchant vessels when the power transmitted
is low and the frequency is high, a combination of factors present in Doppler speed logging systems.
Such a transducer is manufactured by mounting two crystal slices in a sandwich of two stainless steel
cylinders. The whole unit is pre-stressed by inserting a stainless steel bolt through the centre of the
active unit as shown in Figure 2.6.
If a voltage is applied across the ends of the unit, it will be made to vary in length. The bolt is
insulated from the crystal slices by means of a PVC collar and the whole cylindrical section is made
waterproof by means of a flexible seal. The bolt tightens against a compression spring permitting the
crystal slices to vary in length, under the influence of the RF energy, whilst still remaining
mechanically stressed. This method of construction is widely found on the electrostrictive transducers
used in the Merchant Navy. For smaller vessels, where the external stresses are not so severe, the
simpler piezoelectric resonator is used.
Figure 2.5 (a) An output is produced when a piezoelectric ceramic cylinder is subjected to stress.
(b) A change of length occurs if a voltage is applied across the ends of a piezoelectric ceramic
cylinder.
Stress
Stress r·::1:-r, ------,
lal
Ibl
! ,-----
i~~-~- ___ n
,
,, , ,
It ------I-L
~ rL:----
Depth sounding systems 29
2.3.2 Piezoelectric resonator
This type of transducer makes use of the flexible qualities of a crystal slice. If the ceramic crystal slice
is mounted so that it is able to flex at its natural resonant frequency, acoustic oscillations can be
produced. The action is again reciprocal. If the ceramic crystal slice is mounted at its corners only, and
is caused to flex by an external force, a small p.d. will be developed across the ends of the element.
This phenomenon is widely used in industry for producing such things as electronic cigarette lighters
and fundamental crystal oscillator units for digital watches. However, a ceramic crystal slice used in
this way is subject to the same mechanical laws as have previously been stated. The higher the
frequency of oscillation, the thinner the slice needs to be and the greater the risk of fracture due to
external stress or overdriving. For these reasons, piezoelectric resonators are rarely used at sea.
2.3.3 Magnetostrictive transducers
Figure 2.7 shows a bar of ferromagnetic material around which is wound a coil. If the bar is held rigid
and a large current is passed through the coil, the resulting magnetic field produced will cause the bar
to change in length. This slight change may be an increase or a decrease depending upon the material
used for construction. For maximum change of length for a given input signal, annealed nickel has
been found to be the optimum material and consequently this is used extensively in the construction
of marine transducers.
As the a.c. through the coil increases to a maximum in one direction, the annealed nickel bar will
reach its maximum construction length (l+ l). With the a.c. at zero the bar returns to normal (l). The
current now increases in the opposite direction and the bar once again constricts (l– l). The frequency
Figure 2.6 Construction details of a ceramic electrostrictive transducer.
30 Electronic Navigation Systems
of resonance is therefore twice that of the applied a.c. This frequency doubling action is counteracted
by applying a permanent magnet bias field produced by an in-built permanent magnet.
The phenomenon that causes the bar to change in length under the influence of a magnetic field is
called ‘magnetostriction’, and in common with most mechanical laws possesses the reciprocal quality.
When acoustic vibrations cause the bar to constrict, at its natural resonant frequency, an alternating
magnetic field is produced around the coil. A minute alternating current is caused to flow in the coil
and a small e.m.f. is generated. This is then amplified and processed by the receiver as the returned
echo.
To limit the effects of magnetic hysteresis and eddy current losses common in low frequency
transformer construction, the annealed nickel bar is made of laminated strips bonded together with an
insulating material. Figure 2.8 illustrates the construction of a typical magnetostrictive transducer unit.
The transmitting face is at the base of the diagram.
Magnetostrictive transducers are extremely robust which makes them ideal for use in large vessels
where heavy sea pounding could destroy an unprotected electrostrictive type. They are extensively
used with depth sounding apparatus because at the low frequencies used they can be constructed to
an acceptable size and will handle the large power requirement of a deep sounding system. However,
Figure 2.7 (a) A bar of ferromagnetic material around which is wound a coil. (b) Relationship
between magnetic field strength and change of length.
~ ;,
,•"0
u
0;
~
u
.~
0
m :•:;
Annealed
nickel t::=11T ~Length
1.1
1- d/ Length 1+ '"
Annealed
nickel
Steel Cobalt
/bl
Depth sounding systems 31
magnetic losses increase with frequency, and above 100 kHz the efficiency of magnetostrictive
transducers falls to below the normal 40%. Above this frequency electrostrictive transducers are
normally used.
2.3.4 Transducer siting
The decision of where to mount the transducer must not be made in haste. It is vital that the active face of
the transducer is in contact with the water. The unit should also be mounted well away from areas close to
turbulence that will cause noise. Areas close to propellers or water outlets must be avoided.
Aeration is undoubtedly the biggest problem encountered when transducers are wrongly installed. Air
bubbles in the water, for whatever reason, will pass close to the transducer face and act as a reflector of
the acoustic energy.
As a vessel cuts through the water, severe turbulence is created. Water containing huge quantities of
air bubbles is forced under and along the hull. The bow wave is aerated as it is forced above the surface of
the sea, along the hull. The wave falls back into the sea at approximately one-third the distance along the
length of the vessel from the bow. A transducer mounted aft of the position where the bow wave re-enters
the sea, would suffer badly from the problems of aeration. Mounting the transducer ahead of this point,
even in the bulbous bow, would be ideal. It should be remembered, however, that at some stage
maintenance may be required and a position in the bulbous bow may be inaccessible.
A second source of aeration is that of cavitation. The hull of a vessel is seldom smooth and any
indentations or irregularities in it will cause air bubbles to be produced leading to aeration of the
transducer face. Hull irregularities are impossible to predict as they are not a feature of the vessel’s
design.
2.4 Depth sounding principles
In its simplest form, the depth sounder is purely a timing and display system that makes use of a
transmitter and a receiver to measure the depth of water beneath a vessel. Acoustic energy is
Figure 2.8 Cross-section of a magnetostrictive transducer. (Reproduced courtesy of Marconi
Marine.)
32 Electronic Navigation Systems
transmitted perpendicularly from the transducer to the seabed. Some of the transmitted energy is
reflected and will be received by the transducer as an echo. It has been previously stated that the
velocity of sound waves in seawater is accepted to be 1500 ms–1. Knowledge of this fact and the
ability to measure precisely the time delay between transmission and reception, provides an accurate
indication of the water depth.
Distance travelled =
velocity × time
2
where velocity = 1500 ms–1 in salt water; time = time taken for the return journey in seconds; and
distance = depth beneath the transducer in metres. Thus if the time taken for the return journey is 1 s,
the depth of water beneath the transducer is 750 m. If the time is 0.1 s the depth is 75 m, and so
on.
The transmitter and transducer, must be capable of delivering sufficient power and the receiver must
possess adequate sensitivity to overcome all of the losses in the transmission medium (seawater and
seabed). It is the likely attenuation of the signal, due to the losses described in the first part of this
chapter, which determines the specifications of the equipment to be fitted on a merchant vessel.
2.4.1 Continuous wave/pulse system
The transmission of acoustic energy for depth sounding, may take one of two forms.
A continuous wave system, where the acoustic energy is continuously transmitted from one
transducer. The returned echo signal is received by a second transducer and a phase difference
between the two is used to calculate the depth.
The pulse system, in which rapid short, high intensity pulses are transmitted and received by a
single transducer. The depth is calculated by measuring the time delay between transmission and
reception.
The latter system is preferred in the majority of applications. Both the pulse length (duration) and the
pulse repetition frequency (PRF) are important when considering the function of the echo sounding
apparatus.
Continuous wave system
This system is rarely used in commercial echo sounding applications. Because it requires independent
transmitters and receivers, and two transducer assemblies it is expensive. Also because the transmitter
is firing continually, noise is a particular problem. Civilian maritime echo sounders therefore use a
pulsed system.
Pulsed system
In this system the transmitter fires for a defined period of time and is then switched off. The pulse
travels to the ocean floor and is reflected back to be received by the same transducer which is now
switched to a receive mode. The duration of the transmitter pulse and the pulse repetition frequency
(PRF) are particularly important parameters in this system
The pulse duration effectively determines the resolution quality of the equipment. This, along with
the display method used, enables objects close together in the water, or close to the seabed, to be
Depth sounding systems 33
recorded separately. It is called target or echo discrimination. This factor is particularly important in
fish finding apparatus where very short duration pulses (typically 0.25 or 0.5 ms) are used.
Echo discrimination (D) is:
D = V × l (in metres)
where V = the velocity of acoustic waves, and l = pulse length.
For a 0.5 ms pulse length:
D = 1500 × 0.5 × 10–3 = 0.75m
For a 2 ms pulse length:
D = 1500 × 2 × 10–3 = 3m
Obviously a short pulse length is superior where objects to be displayed are close together in the water.
Short pulse lengths tend to be used in fish finding systems.
A short pulse length also improves the quality of the returned echo because reverberation noise will
be less. Reverberation noise is directly proportional to the signal strength, therefore reducing the pulse
length reduces signal strength which in turn reduces noise. Unfortunately, reducing the signal strength
in this way reduces the total energy transmitted, thereby limiting the maximum depth from which
satisfactory echoes can be received. Obviously, a compromise has to be made. Most depth sounders
are fitted with a means whereby the pulse length can be varied with range. For shallow ranges, and
for better definition, a short pulse length is used. On those occasions where great depths are to be
recorded a longer pulse is transmitted.
For a given pulse length, the PRF effectively determines the maximum range that can be indicated.
It is a measure of the time interval between pulses when transmission has ceased and the receiver is
awaiting the returned echo.
The maximum indicated range may be determined by using the following formula:
Maximum range indication (r) =
v × t
2
(in metres)
where v = velocity of sound in seawater (l500 ms–1) and t = time between pulses in seconds. If the
PRF is one per second (PRF = 60), the maximum depth recorded is 750 m. If the PRF is two per
second (PRF = 120) the maximum depth recorded is 375 m.
The maximum display range should not be confused with the maximum depth. For instance, if the
PRF is one per second the maximum display range is 750 m. If the water depth is 850 m, an echo will
be returned after a second pulse has been transmitted and the range display has been returned to zero.
The indicated depth would now be 100 m. A system of ‘phased’ ranges, where the display initiation
is delayed for a pre-determined period after transmission overcomes the problem of over-range
indication.
2.4.2 Transmission beamwidth
Acoustic energy is radiated vertically downwards from the transducer in the form of a beam of energy.
As Figure 2.9 shows the main beam is central to the transducer face and shorter sidelobes are also
produced. The beamwidth must not be excessively narrow otherwise echoes may be missed,
particularly in heavy weather when the vessel is rolling. A low PRF combined with a fast ship speed
34 Electronic Navigation Systems
can in some cases lead to the vessel ‘running away’ from an echo that could well be missed. In
general, beamwidths measured at the half-power points (–3 dB), used for depth sounding apparatus are
between 15° and 25°. To obtain this relatively narrow beamwidth, the transducer needs to be
constructed with a size equal to many wavelengths of the frequency in use. This fact dictates that the
transducer will be physically large for the lower acoustic frequencies used in depth sounding.
In order to reduce the transducer size, and keep a narrow beamwidth, it is possible to increase the
transmission frequency. However, the resulting signal attenuation negates this change and in practice
a compromise must once again be reached between frequency, transducer size and beamwidth. Figure
2.10 shows typical beamwidths for a low frequency (50 kHz) sounder and that of a frequency four
times greater.
Figure 2.9 Transmission beam showing the sidelobes.
Figure 2.10 Typical beamwidths for echo sounders transmitting low and high frequencies.
(Reproduced courtesy Furuno Electric Co. Ltd.)
Depth sounding systems 35
2.5 A generic echo sounding system
Compared with other systems, echo sounder circuitry is relatively simple. Most manufacturers of deep
sounding systems now opt for microprocessor control and digital displays, but it was not always so.
Many mariners preferred the paper-recording echo sounder because the display was clear, easy to read
and provided a history of soundings.
Marconi Marine’s ‘Seahorse’ echo sounder (Figure 2.11) was typical of the standard paperrecording
echo sounder. Built in the period before microprocessor control, it is used here to describe
the relatively simply circuitry needed to produce an accurate read-out of depth beneath the keel. From
the description it is easy to see that an echo sounding system is simply a timing device.
The system used a transmission frequency of 24 kHz and two ranges, either manually or
automatically selected, to allow depths down to 1000 m to be recorded. The shallow range was 100m
and operated with a short pulse length of 200 μs, whereas the 1000 m range uses a pulse length of 2 ms.
Display accuracy for the chart recorder is typically 0.5% producing indications with an accuracy of
±0.5 m on the 100 m range and ±5 m on the deepest range.
2.5.1 Description
Receiver and chart recorder
When chart recording has been selected, transmission is initiated by a pulse from a proximity detector
which triggers the chart pulse generator circuit introducing a slight delay, pre-set on each range, to
ensure that transmission occurs at the instant the stylus marks zero on the recording paper. This system
trigger pulse or that from the trigger pulse generator circuit when the chart is switched off, has three
functions:
to initiate the pulse timing circuit
to operate the blanking pulse generator
to synchronize the digital and processing circuits.
The transmit timing circuit sets the pulse length to trigger the 24 kHz oscillator (transmission
frequency). Pulse length is increased, when the deep range is changed, by a range switch (not shown).
Power contained in the transmitted signal is produced by the power amplifier stage, the output of
which is coupled to the magnetostrictive transducer with the neon indicating transmission.
When the transmitter fires, the receiver input is blanked to prevent the high-energy pulse from
causing damage to the input tuned circuits. The blanking pulse generator also initiates the swept gain
circuit and inhibits the data pulse generator. During transmission, the swept gain control circuit holds
the gain of the input tuned amplifier low. At cessation of transmission, the hold is removed permitting
the receiver gain to gradually increase at a rate governed by an inverse fourth power law. This type
of inverse gain control is necessary because echoes that are returned soon after transmission ceases are
of large amplitude and are likely to overload the receiver.
The echo amplitude gradually decreases as the returned echo delay period increases. Thus the swept
gain control circuit causes the average amplitude of the echoes displayed to be the same over the
whole period between transmission pulses. However, high intensity echoes returned from large
reflective objects will produce a rapid change in signal amplitude and will cause a larger signal to be
coupled to the logarithmic amplifier causing a more substantial indication to be made on the paper.
The logarithmic amplifier and detector stages produce a d.c. output, the amplitude of which is
logarithmically proportional to the strength of the echo signal.
Figure 2.11 A block schematic diagram of the Seahorse echo sounder. (Reproduced courtesy of Marconi Marine.)
Prox. de!. output pulse.
Transmitter
I
I
I
I
I
indicator I ---- __________ 1
System trigger
Triggering
ChartON
Chart OFF
From chart select switch
r---
(Pickup
reduction)
Blanking
pulse
generator
Blanking
Dig ital display
and processing
ci rcuits
Swept
gain
Receiver board
Data
Proximity
detector
-l
I
Det.O/P I
pulse
....-- ,-, --,
11 Echo
led
==-- ------
Paper drive
motor
Paper
chart
Alarms
Depth sounding systems 37
In the chart recorder display, electrosensitive paper is drawn horizontally beneath a sharp stylus.
The paper is tightly drawn over the grounded roller guides by a constant speed paper-drive motor.
Paper marking is achieved by applying a high voltage a.c. signal to the stylus which is drawn at 90°
to the paper movement, across the surface of the paper on top of the left-hand roller. The paper is
marked by burning the surface with a high voltage charge produced through the paper between the
stylus and ground. Depending upon the size of the returned echo, the marking voltage is between 440
and 1100 V and is produced from a print voltage oscillator running at 2 kHz. Oscillator amplifier
output is a constant amplitude signal, the threshold level of which is raised by the d.c. produced by
a detected echo signal. Thus a high-intensity echo signal causes the marking voltage to be raised above
the threshold level by a greater amount than would be caused by a detected small echo signal.
For accurate depth marking it is essential that the stylus tracking speed is absolutely precise. The
stylus is moved along the paper by a belt controlled by the stylus d.c. motor. Speed accuracy is
maintained by a complex feedback loop and tacho-generator circuit.
Digital circuits
The digital display section contains the necessary logic to drive the integral three-digit depth display,
the alarm circuit, and the remote indicators. Pulse repetition frequency (PRF) of the clock oscillator
is pre-set so that the time taken for the three-digit counter to count from 000 to 999 is exactly the same
as that taken by the paper stylus to travel from zero to the maximum reading for the range in use. The
counter output is therefore directly related to depth.
When the chart recorder is switched off, the digital processing section and the transmitter are
triggered from the processor trigger pulse generator circuit. Both the transmit and receive sections
work in the same way as previously described. A low logic pulse from the trigger pulse standardizing
circuit synchronizes the logic functions. The d.c. output from the receiver detector is coupled via a
data pulse generator circuit to the interface system. Unfortunately in any echo sounder it is likely that
unwanted echoes will be received due to ship noise, aeration or other factors.
False echoes would be displayed as false depth indications on the chart and would be easily
recognized. However, such echoes would produce instantaneous erroneous readings on the digital
counter display that would not be so easily recognized. To prevent this happening echoes are stored
in a data store on the processing board and only valid echoes will produce a reading on the display.
Valid echoes are those that have indicated the same depth for two consecutive sounding cycles. The
data store, therefore, consists of a two-stage counter which holds each echo for one sounding cycle and
compares it with the next echo before the depth is displayed on the digital display.
The display circuit consists of three digital counters that are clocked from the clock oscillator
circuit. Oscillator clock pulses are initiated by the system trigger at the instant of transmission. The
first nine pulses are counted by the lowest order decade counter which registers 1–9 on the display
least significant figure (LSF) element. The next clock pulse produces a 0 on the LSF display and
clocks the second decade counter by one, producing a 1 in the centre of the display. This action
continues, and if no echo is received, the full count of 999 is recorded when an output pulse from the
counting circuit is fed back to stop the clock.
Each time transmission takes place the counters are reset to zero before being enabled. This is not
evident on the display because the data output from the counters is taken via a latch that has to be
enabled before data transfer can take place. Thus the counters are continually changing but the display
data will only change when the latches have been enabled (when the depth changes). If an echo is
received during the counting process, the output is stopped, and the output latches enabled by a pulse
from the data store. The new depth is now displayed on the indicator and the counters are reset at the
start of the next transmission pulse.
38 Electronic Navigation Systems
With any echo sounder, it is necessary that the clock pulse rate be directly related to depth. When
the shallow (100 m) range is selected a high frequency is used which is reduced by a factor of 10 when
the deep range (1000 m) is selected.
Modern echo sounders rely for their operation on the ubiquitous microprocessor and digital
circuitry, but the system principles remain the same. It is the display of information that is the outward
sign of the advance in technology.
2.6 A digitized echo sounding system
The Furuno Electric Co. Ltd, one of the world’s big manufacturers of marine equipment, produces an
echo sounder, the FE606, in which many of the functions have been digitized. Transmission frequency
is either 50 or 200 kHz depending upon navigation requirements. A choice of 50 kHz provides greater
depth indication and a wider beamwidth reducing the chance that the vessel may ‘run away’ from an
echo (see Figure 2.10).
The pulse length increases with depth range from 0.4 ms, on the shallow ranges, to 2.0 ms on the
maximum range. This enables better target discrimination on the lower ranges and ensures that
sufficient pulse power is available on the higher ranges. Pulse repetition rate (sounding rate) is reduced
as range increases to ensure adequate time between pulses for echoes to be returned from greater
depths.
The system shown in Figure 2.12 is essentially a paper recorder and two LCD displays showing
start depth and seabed depth. As before, transmission is initiated at the instant the stylus marks the zero
line on the sensitive paper by a trigger sensor coupled to the control integrated circuits. Depending
upon the range selected, the pulse length modulates the output from the transmit oscillator, which is
power amplified and then coupled via a transmit/receive switch to the transducer.
A returned echo is processed in the receiver and applied to the logic circuitry. Here it is processed
to determine that it is a valid echo and then it is latched through to a digital-to-analogue converter to
produce the analogue voltage to drive the print oscillator. Thus the depth is marked on the sensitive
paper at some point determined by the time delay between transmission and reception, and the
distance the stylus has travelled over the paper.
2.7 A microcomputer echo sounding system
As you would expect, the use of computing technology has eliminated much of the basic circuitry and
in most cases the mechanical paper display system of modern echo sounders. Current systems are
much more versatile than their predecessors. The use of a computer enables precise control and
processing of the echo sounding signal. Circuitry has now reached the point where it is virtually all
contained on a few chips. However, the most obvious changes that users will be aware of in modern
systems are the display and user interface.
Once again there are many manufacturers and suppliers of echo sounders or, as they are often now
called, fish finders. The Furuno navigational echo sounder FE-700 is typical of many. Depending upon
requirements the system is able to operate with a 200 kHz transmission frequency giving highresolution
shallow depth performance, or 50 kHz for deep-water sounding.
Seabed and echo data is displayed on a 6.5 inch high-brightness TFT colour LCD display which
provides the navigator with a history of soundings over a period of 15 min, much as the older paper
recording systems did (see Figure 2.13).
Figure 2.12 Furuno FE-606 echo sounding system. (Reproduced courtesy of Furuno Electric Co.)
,r --:::~r / lransceiver
tt4...."'5kHz
I L
----LL..:!:tarrsdtKef
~
I ~- , fJ? ?"i =r I
L,D=-J 'Ira.
[)ala bus ~ --j ,,""
e~~'~~~'''''" , b<Jr. ~r. ' AA 'J ~EfEtL.. .. ~.-L. L" F...AM -
PU::!_.,~.r:M1 1I .td:-a:s .- ' =-.:!5
... -.. . , ~ "---- '
ControllC·s
-~I-II-... ~
11 Control I! EJ
I i ... il ~c:~ ..-I J'I=C'A. ~I1
Ralge"""j i
. I!
~ il
B Ell<! depOl
01
Seabed depth
I Osc 1800kHz m,, " " d i
~,
i
I _ n~~
--.-,, '~
~.,
-,
40 Electronic Navigation Systems
Figure 2.13 Furuno FE-700 LCD TFT data display (Navigation Mode.) (Reproduced courtesy of
Furuno Electric Co.)
Display mode
Gain setting
Range setting
Auto
mode
Alarm
setting
Depth alarm line
Depth
Explanation of depth
(Below transducer, or
below surface)
Screen
20 -
40 -
unit
Range scale
( ""'*') ( A~"')
( IlIM ) ( BRILL )
~ (COLO:;:)
( - )( + )
,~
', ~' .
~
2~'
~o
~
~
1*S~~~~1.
_~ MEN'U
( Mooe )
( ~)
Control panel
Depth sounding systems 41
Depths, associated time, and position are all stored in 24-h memory and can be played back at any
time. This is a useful function if there is any dispute following an accident.
The main depth display emulates a cross-sectional profile of the ocean over the past 15 min. At the
top of the display in Figure 2.13, the solid zero line marks the ocean surface or transducer level
whichever is selected. At 15 m down, a second line marks the depth at which the alarm has been set.
The undulating line showing the ocean floor depth is shown varying over 15 min from 58 to 44 m and
the instantaneous depth, also shown as a large numerical display, is 47.5 m. Other operation detail is
as shown in the diagram. What is not indicated on the display is the change of pulse length and period
as selected by range.
As shown in Table 2.2, the pulse length is increased with the depth range to effectively allow more
power to be contained in the transmitted pulse, whilst the pulse period frequency is reduced to permit
longer gaps in the transmission period allowing greater depths to be indicated
In addition to the standard navigation mode, Furuno FE-700 users are provided with a number of
options adequately demonstrating the capability of a modern echo sounder using a TFT LCD display
(see Figure 2.14). All the selected modes display data as a window insert on top of the echo sounder
NAV mode display.
There are four display-mode areas.
OS DATA mode. Indicates own ship position, GPS derived course, time and a digital display of
water depth.
DBS mode. Provides a draft-adjusted depth mode for referencing with maritime charts.
LOGBOOK mode. As the name suggests, provides a facility for manually logging depths over a
given period.
HISTORY mode. Provides a mixture of contour and strata displays. The contour display can be
shifted back over the past 24 h whilst the strata display (right-hand side of display) shows sounding
data over the last 5 min.
2.8 Glossary
The following lists abbreviations, acronyms and definitions of specific terms used in this chapter.
Aeration Aerated water bubbles clinging to the transducer face cause errors in the
system.
Ambient noise Noise that remains constant as range increases.
Table 2.2 Echo sounder range vs pulse length vs PRF
Depth (metres) Pulse length (ms) PRF (pulses per minute)
5, 10 and 20 0.25 750
40 0.38 375
100 1.00 150
200 2.00 75
400 and 800 3.60 42
42 Electronic Navigation Systems
Figure 2.14 Different display modes demonstrating the flexibility of a microcomputer-controlled echo
sounder. (Reproduced courtesy of Furuno Electric Co.)
OS DATA Mode DBS Mode
__ 0 _ - ___ ' _
--_Q_- --- '-
,
O.Okt
15:51 :52
28 .6111
LOGBOOK Mode HISTORY Mode
.". " " "
.. , " .. " •
" " " "
,
•.•. •• •• " • ---'-- .. .". o... n • .. " • " .. .. •
" .. .. .. n • .. " " " • .- .. .. •
Depth sounding systems 43
Beam spreading The transmitted pulse of energy spreads as it travels away from the transducer.
The use of a wide beam will cause noise problems in the receiver and a
narrow beam may lead to an echo being missed as the vessel steams away
from the area.
Chart recorder A sensitive paper recording system which, when the surface is scratched by a
stylus, marks the contour of the ocean floor.
Continuous wave
system
An echo sounding system that uses two transducers and transmits and receives
energy at the same time.
Electrostrictive
transducer
A transducer design based on piezoelectric technology. It is used when a
higher transmission frequency is needed such as in speed logging equipment
or fish-finding sounders.
Magnetostrictive
transducer
A design based on magnetic induction. A large heavy transducer capable of
transmitting high power. Used in deep sounding systems.
Pulse duration
(length)
The period of the transmitted pulse when the transmitter is active.
Pulse repetition
frequency (PRF)
The number of pulses transmitted per minute by the system. Similar to
RADAR
Pulse wave system A system that, like RADAR, transmits pulses of energy from a transducer
which is then switched off. The received energy returns to the same
transducer.
Reverberation noise Noise that decreases as range increases.
Sonar Sound navigation and ranging.
Velocity Speed of acoustic waves in seawater; 1505 ms–1 or approximated
to1500 ms–1.
2.9 Summary
Sonar stands for sound navigation and ranging.
Sound travels relatively slowly in seawater at 1505 ms–1. This is approximated to 1500 ms–1 for
convenience.
The velocity is not a constant, it varies with the salinity of seawater. Ocean salinity is approximately
3.4%.
Transmitted signal amplitude is attenuated by saltwater and the ocean floor from which it is
reflected.
Noise caused by sea creatures and ocean activity is a major problem affecting sonar equipment.
The temperature of the seawater affects the velocity of the acoustic wave and consequently affects
the accuracy of the displayed data. Temperature sensors are contained in the transducer housing to
produce corrective data.
Transducers are effectively the antennas of sonar systems. They transmit and receive the acoustic
energy.
There are two main types of transducer in use; magnetostrictive and electrostrictive. Magnetostrictive
transducers are large and heavy and tend to be used only on large vessels. Electrostrictive
transducers are lighter and often used in speed logging systems and on smaller craft.
Low frequencies are often used in deep sounding systems typically in the range 10–100 kHz.
The depth below the keel is related to the time taken for the acoustic wave to travel to the ocean
floor and return. Put simply if the delay is 1 s and the wave travels at 1500 ms–1 then the depth is
0.5 × 1500 = 750 m.
44 Electronic Navigation Systems
Pulsed systems, like those used in maritime RADAR, are used in an echo sounder. The pulse length
or duration determines the resolution of the equipment. A short pulse length will identify objects
close together in the water. If all other parameters remain constant, the pulse repetition frequency
(PRF), the number of pulses per minute, determines the maximum range that can be indicated.
The width of the transmitted beam becomes wider as it travels away from the transducer. It should
not be excessively narrow or the vessel may ‘run away’ from, or miss, the returned echo.
Modern echo sounding equipment is computer controlled and therefore is able to produce a host of
other data besides a depth indication.
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