Echolocation Methods – Addendum
Roman Salamon & Henryk Lasota
(IX) X 2015 – I 2016
Part I
Doppler effect
[2016_01_25_EchoMeth_Add_script_HL]Doppler effect [H. Lasota]Henryk Lasota
XI 2015
1
Waves and motion
1.1
Doppler effect
Moving sound source [3]
If the transmitter and receiver move relatively to
each other or at least one of the propagation paths
is associated with the reflection from a moving object, the Doppler effect can be observed revealing
by a change in the carrier frequency or widening of
the signal spectrum. In fact, this is the time-scale of
waves and signals that changes, being compressed
when the object approaches and expanded when it
moves away.
Doppler shift:
fD = fT − fR ≈ fT
v
cos αr
c
http://en.wikipedia.org/wiki/Doppler_effect
http://Doppler-Ballot experiment
http://The Doppler Effect and Sonic Booms
Musical tones [4]
√
Changing the pitch of the tone up, is equivalent to multiplying its frequency in hertz
by 6 2 ≈ 1.122462.
√
Changing by a semitone corresponds to the frequency multiplied by a factor 12 2 ≈ 1.059463.
Changing by one octave corresponds to a frequency multiplying or dividing by 2, for example: A1 is
440 Hz, and A2 corresponds to the frequency 880 Hz.
Question:
If during the 1845 Ballot experiment musicians on the moving train (40 mph) were playing the A1 note
(440 Hz) what was the incoming/outcoming pitch?
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Doppler Effect
Moving receiver (subjective effect) [5]
Receiver: vr 6= 0,
source: vs = 0
Observer moving toward the source observes a subjective effect.
Fs 6= 0,
fr 6= fs
vr0 = 0
fr =
fs =
c
λ
1
= ⇔T =
T
λ
c
c + vr0
c + vr0
=
· fs = αr · fs
λ
c
vr0 = vr · cos β,
vr – radial speed
vr0 6= 0
Moving source (objective effect) [6]
Receiver: vr = 0,
source: vs 6= 0
λr = (c − vs0 ) T
c
1. vs0 = 0,
fr = = fs ,
λ=c·T
λ
c
c
2. vs0 6= 0, fr =
= fs ·
= fs · αs ,
λr
c − vs0
αs =
1
1−
2
vs0
≈1+
vs0
c
vr0
1
c
c
vs0 = vs · cos γ
R. Salamon & H. Lasota
2016-01-27
Doppler Effect
General remarks [7]
vs , vr > 0 object closing in,
The most important is:
vs , vr < 0 object moving away.
v
fD = |fs − fr | ∼
= fs
c
fD – Doppler frequency
1.2
→ communication effect
v
fD ∼
= 2fs → echolocation
c
Applications
Doppler log [8]
Backscatter effect on the seabed.
v 0 = v·cos α. When ship is moving, the return angle
is slightly different (transmitting angle α, receiving
angle β).
The frequency shift occurs mainly at the vicinity of
the transmitting / receiving transducer (antenna):
1. ship moves relative to water, fD = fs ·
v 0 (cos α + cos β) (v 0 – speed relative to water
stream),
2. on the wave travel to bottom and back, additional infinitesimal Doppler shifts appear on each
passage between subsequent stream layers. fD =
fs · v.
3. finally, the cumulative shift is fD h fs · 2v cos α
with v – speed relative to bottom.
In case of water stream there is a problem of change of wave speed. Doppler effect: speed relative to
water current vp , after all speed is linear relative to seabed/bottom.
fD
2
In hydroacoustics: I
= Hz/knot kHz.
v
3
1 knot = 1 nautical mile/1 h. 1 NM=10 (minute of arc) =1852 m.
Ultrasonography - blood flow monitoring [9]
Doppler ultrasonography works with obliquely incident rays. It uses hydrodynamic nature of the
flow inside blood vessels. Special sounding signals are used susceptible to time compression giving higher
range resolution. The effects are shown on display screen in red and blue; pixels are red when the flow
occurs in one direction, and blue when the flow is in the opposite direction. One color at an area means
laminar flow, two colors - that the flow is turbulent. The latter indication is extremely important for the
physician - cardiologist.
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Doppler Effect
Ultrasonic alarm [10]
Ultrasonic transducers (mainly 40 kHz buzzers) emit and receive acoustic wave inside a
closed area with no air movement (closed room, car with closed windows). When any
movement occurs in the area - an intruder or just slight air flow, a part of the received
signal is Doppler-shifted. The frequency change is immediately, very easily detected at
the receiver (even slight nonlinearity at the receiver gives rise to a mixing effect, creating
signal component of the Doppler difference frequency.
Car velocity measurement [11]
Required parameters:
δf = 1 Hz – dokładność pomiarowa
δv = 1 km/h
1
m
1000 ∼
km
=
= 0, 3
h
3600
s
fD = fs
2v
;
c
c = 3 · 108
δf = 2fs
m
s
δv
c
1
fs =
2
cδf
3 · 108 · 1
1
=
= · 109 = 0.5 GHz
δv
2 · 3 · 10−1
2
Continuous wave radars and sonars
CW FM radars and sonars [12]
Echolocation systems with a continuous wave (CW) and frequency modulation (FM) are used as “silent
radars (sonars)” and without FM (CW only) as Doppler radar to measure the speed of moving objects
(famous police “hair dryers”).
Working principle of a CW FM radar
Frequency of transmitted signal
t
f (t) = f0 − B/2 + B
T
Frequency of the received signal
t−τ
fe (t) = f0 − B/2 + B
T
2R
τ=
Delay
c
Frequency difference
F (t) = f (t) − fe (t)
τ
F =B
T
Distance measurement [13]
Difference frequency signal is obtained at the output of a mixer, by multiplying the transmitted signal
with the echo signal.
The values of the frequency difference are determined by performing the Fourier transform of the
“difference” signal.
1
2v - “to and fro”
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Doppler Effect
The distance to the target is calculated by changing the scale of the spectrum (only to the half of
maximum frequency).
cT
R=
F
2B
Spectrum of a CW FM sonar difference signal
Moving target [14]
Motion of the target makes the echo frequency changing due to the Doppler effect. The frequency change
is
2v
fd ∼
f0
=
c
As a result, the difference frequency changes:
2v
τ
F = F0 + fd = B + f0
T
c
This causes an error in the evaluation/estimation of
the target distance
T f0
v
R = R0 +
B
∆R
T f0
f0 v
=
v=2
Rz
BRz
Bc
Furthermore, it reduces the magnitude of spectrum
fringes, making their detection more difficult.
These errors are important mainly in sonar and aerolocation systems, where v/c is high; in the
radiolocation this quotient is very small.
Velocity-induced distance errors [15]
Example 1 – radar
f0 /B ∼
= 200, range Rz = 30 km,
target velocity v= 300 m/s (1080 km/h), v/c = 10−6 ,
∆R/Rz = 4 · 10−4 = 0.400/00.
Distance estimation error is ∆R =12 m – negligible.
In sonars, the error of distance estimation is much larger, which makes more difficult the use of this
type of systems. The reason is the relatively low speed of acoustic wave propagation (in water – 200 000
times smaller than the speed of electromagnetic waves).
Example 2 – sonar
f0 /B ∼
= 20, range Rz = 3 km,
target velocity v= 5 m/s (18 km/h = 10 kn) v/c = 3 · 10−3 ,
∆R/Rz = 1, 2 · 10−1 = 120/0.
Distance estimation error is ∆R= 360 m – unacceptable.
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Natural Echolocation
Part II
Echolocation in nature
[2016_01_25_EchoMeth_Add_script_HL]Nature’s Echolocation [R. Salamon, H. Lasota]Roman Salamon & Henryk Lasota X 2015
3
Echolocation in nature
Echolocation in nature [17]
The method of echolocation is beeing used by some animals for navigation and hunting. They use acoustic
sounding signals. Best known is the mechanism of echolocation of bats and dolphins.
https://en.wikipedia.org/wiki/Animal_echolocation
3.1
Bats
[18]
Echolocation of bats
The order of mammals (ca. 1100 species, in Poland – 25) who can fly. Generally nocturnal, so
eyesight is of litle use. They emit acoustic signals which, depending on the species, are located in the
frequency range from approx. 10 kHz to approx. 200 kHz.
Spectrogram of a bat echosounding signal.
http://.../richarddevine/bat-recordings
Sonogram of the recording
[19]
Bats emit a broadband, pulse and continuous, probing signals with frequency modulation (FM). Signal
level can exceed 130 dB. This is the highest level of sound produced by the animals. The signals are
generated by the vocal cords.
Signals parameters adapt perfectly to the propagation conditions and method of use. In the brain
correlation reception is performed, whereby it is possible to precisely measure the distance to the target
and determine the delay between the signals received by the left and right ear, and thus the precise
bearing.
This also provides an unprecedented opportunity to distinguish between two or more close targets
(resolution), even at a distance of 0.5 mm.
Thanks to the extraordinary complexity of the emitted signals bats are able to identify targets.
Bats are able to compensate the Doppler effect resulting from its own movement and use this effect to
highlight the movement of insects on which they pray.
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Natural Echolocation
[20]
Interesting fact
Some Tiger Moths from Arctiidae family in response to the echolocation calls of bats emit ultrasonic
signals that warn the bats that the moths have toxic compounds.
Tiger Moth Bertholdia trigona emits acoustic pulses at a frequency of 4.5 kHz, which disrupt the
process of echolocation of bats, reducing tenfold the effectiveness of hunting.
A. Corcoran "Tiger Moth Jams Bat Sonar" [Science (325) 2009]
Video depicting moth jamming the sonar of the bat
3.2
Dolphins & Whales
Dolphins and Whales [21]
Marine mammals of the order Cetacea, suborder Odontoceti (toothed) use acoustic signals in the water
for echolocation.
They emit complex broadband signals with frequency modulation of carring frequency up to 100 kHz
and a very complex structure, adapting to the conditions of propagation.
Spectrogram of a dolphin echosounding signal.
[22]
Dolphins have no ears, and the generation and reception of acoustic signals is carried out in parts of the
anatomy illustrated. The mechanism of generation and reception device is adapted to to a large acoustic
impedance of water.
Dolphin brain performs correlation processing of the received signals, providing wide coverage and high
resolution.
whale_echolocation.gif
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3.3
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Natural Echolocation
Other spieces
Shrews [23]
Terrestial echolocation
The shrews of two genera (Sorex and Blarina) emit series of ultrasonic squeaks.The nature of shrew
sounds, unlike those of bats, are low amplitude, broadband, multiharmonic and frequency modulated.
They contain no “echolocation clicks” with reverberations and would seem to be used for simple,
close-range spatial orientation. In contrast to bats, shrews use echolocation only to investigate their
habitats rather than additionally to pinpoint food.
Except for large and thus strongly reflecting objects, such as a big stone or tree trunk, they will
probably not be able to disentangle echo scenes, but rather derive information on habitat type from the
overall call reverberations.
Humans [24]
Some blind people use this abilty for acoustic wayfinding, or navigating within their environment using
auditory rather than visual cues. It is similar in principle to active sonar and to animal echolocation.
By actively creating sounds – tapping canes, stomping foot, snapping fingers, or making clicking noises
with their mouths – people trained to orient by echolocation can interpret the sound waves reflected by
nearby objects, accurately identifying their location and size.
http://.../ultrasonic-helmet.../ + movie
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Principles of Underwater Acoustics
Part III
Principles of underwater acoustics
[2016_01_25_EchoMeth_Add_script_HL]Principles of underwater acoustics [H.
Lasota]Henryk Lasota 2005 – 2016
4
Underwater environment
Operating environment of hydroacoustic systems [26]
• type of reservoir
– inland
∗ lake
∗ river
– sea
∗ offshore
∗ continental shelf (depth up to 200 m)
∗ deep ocean
4.1
Propagation conditions
Propagation conditions [27]
• refraction
– „curvilinear” propagation:
∗ shadow zones
∗ propagation channels
• rebound (reflection/scattering) from the bottom and water surface
– multiple paths of wave/signal propagation water surface motion (waves, ripples) causing fast signal fluctuations:
∗ deep changes in signal level – destructive interference
∗ change of signal – the Doppler effect by reflection
– daily volatility of propagation properties – extremely low frequency fluctuations
– internal waves – relating to weather
• absorbtion
• scattering (reverberation)
• high level of noise
– natural
– of civilization origin
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4.2
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Principles of Underwater Acoustics
(Hydro)acoustic waveguides
Water reservoirs as (hydro)acoustic waveguides [28]
The reservoir, as a medium of acoustic wave propagation in infrasound, sound, and ultrasound range, can be treated as a waveguide with a very heterogeneous “filler”. The main
phenomena affecting the wave wandering in it are:
• reflection/scattering – at medium borders
• refraction – deflection on the heterogeneity of distribution of sound velocity – in the
sense of changing the direction of the wave front of plane waves
• attenuation – the effect of shear and volume viscosity of water and the relaxation
of magnesium ions contained in MgSO4 (frm = 59.2 kHz) and boron ions contained
in boron acids (frb = 0.9 kHz)
• dispersion – on small heterogeneity of the medium, in terms of different acoustic
characteristic impedance, suspended in the depths
5
Sound modes
Sound modes [29]
• Shallow reservoir (relatively!) as a waveguide:
– wave equation for steady states (Helmholtz equation),
– harmonic sollutions are assumed, with separable dependence on r and z,
– boundary conditions are introduced (surface, bottom)
The solutions are waves (propagation modes) with „periodic" amplitude distributions
between boundaries and different phase and group velocities!
Modes are also called specific values of the problem (eigenvalues).
Mode propagation concerns low frequencies (depth comparable to the wavelength).
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Principles of Underwater Acoustics
[30]
6
Refraction
Sound velocity distribution [31]
• The speed of sound in water depends on:
– temperature T
– salinity S
– pressure/depth Ph /z
• These parameters are different in different places:
– the type of water reservoir (lake, river, sea, ocean)
– climate zone
• In given waters the distribution of T and S it is heterogeneous and varies in long,
medium and short-terms (eg. internal waves):
– season of the year (seasonal changes),
– time of the day (diel – 24 h) [diurnal, nocturnal],
– phase of tides (tidal – 12.5 h) https://en.wikipedia.org/wiki/Tide (https://pl.wikipedia.org/wiki/Plywy_morskie)
– (wind, insolation)
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6.1
2016-01-27
Principles of Underwater Acoustics
Propagation velocity
Propagation velocity [32]
Empiric formula [Medwin]
c = 1449.2 + 4.6T − 0.55T 2 + 0.00029T 3 + (1.34 − 0.01T ) · (S − 35) + 1.58 · 10−6 Ph
c – sound velocity in water [m/s]
T – temperature [◦ C]
S – salinity [ppt = 10−3 ]
Ph – hydrostatic pressure [N/m2]
Aproximate formula
c = 1449 + 4.6T + (1.34 − 0.01T ) (S − 35) + 0.016z
where: z – depth [m]
6.2
Sound rays
Sound rays [33]
• Geometric approach – rays
– Assumptions:
∗ channel dimensions are significant in relation to the wavelength and furthermore, in the wavelength scale:
∗ the speed of sound propagation can be considered constant (not changing
significantly)
∗ the wave intensity changes are also negligible
Snell’s law [34]
sin ϑi
sin ϑ
=
= a,
c (z)
c (zi )
ds =
dz
,
cos ϑ
dt =
12
ds
dz
=
,
c (z)
c (z) cos ϑ
dr = dz tan ϑ.
R. Salamon & H. Lasota
Radius of ray path:
2016-01-27
Principles of Underwater Acoustics
[35]
d [c (z)]
= b = gradz c,
dz
r = 1/ab,
Positive and negative curvature radius [36]
13
r=
c
gradz c · sin ϑ
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2016-01-27
Shadow zones [37]
6.3
Sound channels
Refraction in ocean [38]
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2016-01-27
Multipath propagation [39]
6.4
Oceanic sound channel
Layered structure of oceanic waters [40]
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Principles of Underwater Acoustics
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Principles of Underwater Acoustics
Oceanic sound channel [41]
Oceanic sound channel II [42]
Sound velocity distribution in deep (?) oceanic waters has a minimum favoring cylindrical
energy spread .
Deepwater sound channel – SOSUS [43]
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6.5
2016-01-27
Sound attenuation
Sound attenuation in water – fresh water [44]
Sea water [45]
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2016-01-27
Relaxation-induced attenuation [46]
Reminder [47]
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Principles of Underwater Acoustics
R. Salamon & H. Lasota
6.6
2016-01-27
Sea noise
Sea noise [48]
Knudsen noise curves [49]
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Principles of Underwater Acoustics
R. Salamon & H. Lasota
7
2016-01-27
Principles of Underwater Acoustics
Refraction in sonar systems
Acoustic wave refraction [50]
Forecast of sound rays in natural waters
for sonar range estimation
[R. Salamon, Sonar systems]
Typical profile of acoustic wave velocity in ocean [51]
Equiphase surfaces and sound rays [52]
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Principles of Underwater Acoustics
Sound rays due to positive velocity gradient [53]
Sound rays proper to negative velocity gradient [54]
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Surface channel [55]
Acoustic channel [56]
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Principles of Underwater Acoustics
Sound intensity distribution [57]
Depth distributions of sound velocity [58]
Left chart – Wdzydze lake, spring season, right chart – Baltic Sea, summer
season
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Principles of Underwater Acoustics
Sound intensity distribution in Wdzydze lake [59]
Acoustic channel in Southern Baltic [60]
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Principles of Underwater Acoustics
Intensity distribution [61]
Wave emitted by an antenna of defined directivity pattern under a negative gradient of
sound speed.
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Sonar Equations
Part IV
Signal levels in physical space
[Scales, levels]Levels of wave signals in physical space; logarithmic scales [H.
Lasota]Henryk Lasota
Sound levels, izophonic curves, spreading „loss” [63]
Values vs. levels – SPL [64]
SPL Sound Pressure Level
• logarithmic measure of the effective value of the sound pressure in airprms
• "surplus" above the reference value pref = 20 × 10−6 Pa
SP L = 20 log
prms
pref
SIL Sound Intensity Level
measure of sound intensity I referred to Iref = 10−12 W/m2
SIL = 10 log
I
Iref
SWL Sound Power Level
measure of the sound power P referred toPref = 10−12 W
SW L = 10 log
26
P
Pref
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Sonar Equations
Medium characteristic impedance vs. SPL and SIL [65]
Acoustic impedance of the medium for a plane acoustic wave:
Zak = p/v
Acoustic wave intensity:
I = p · v = p2 /Zak
Characteristicq
impedance medium of the medium:
Zak = ρc =
ρ/κ
Air: ρ = 1.2 kg/m3 ;
c = 340m/s;
Zak = 400rayl
It can be calculated that in air the pressure p = 2 × 10−5 Pa corresponds to the intensity I = 10−12
W/m2 , or reference values are equivalent. This means that
SP L = SIL (only in air !)
SWL & SIL [66]
A dependence of SWL and SIL on the distance is illustrated by the following formulae:
SW L = 10 log
P
Pref
I = P/4πr2
Pref = Iref · 1 m2 = Iref · (r1 )2
Whence:
or otherwise:
SW L = 10 log 4π + 20 log r/r1
SIL = SW L − 10 log 4π − 20 log r/r1
Range of communication systems [67]
• Energy balance of a system with isotropic antenna
– lossless medium
– lossy medium
• Antenna directivity
– transmission directivity
– receiving directivity
• Energy balance with directional antennae
• Range equation
– power of acoustic noise
– receiver electrical noise
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Sonar Equations
Energy balance [68]
• comparing the acoustic power of the signal emitted by the transmitting antenna (or
rather of the radiated wave) with the acoustic power of the signal received by the
receiver antenna (segment of the wave reaching the area of the antenna), or else
• weakening of the received signal with increasing the distance between the transmitter
T and the receiver R
• simplifying assumptions:
– propagation medium is :
∗ unlimited, continuous, homogeneous, isotropic, lossless
– the source of the wave is a hypothetical transmitting antenna radiating acoustic
wave omnidirectionally (isotropic) - this means small size of the antenna a
relative to the wavelength λ
– the receiving antenna has a finite equivalent surface (aperture) ΣR
Logarithmic scales [69]
Transmission Loss (TL) [70]
Spreading loss
Plain wave – no loss (0 dB)
Cylindrical wave – TL= 10 log r/r1
Spherical wave – TL = 10 log r/r1
(In wireless communication and radiolocation it is being called "free space attenuation"
- horror!)
Absorption loss
Actual attenuation (conversion of wave energy into heat) – α [dB/m]
Depending on the channel, the transmission loss is being calculated according to one of
the following formulas:
TL= α · r
TL= 10 log r/r1 + α · r
TL= 20 log r/r1 + α · r
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Sonar Equations
Directivity Index (DI) [71]
For sufficiently large aperture:
P
DI = 10 log 4π 2
λ
Therefore:
For a rectangular aperture of relative sides A × B:
DI = 10 log 4π + 10 log A + 10 log B = 10 log AB + 11 dB.
For a circular aperture with a relative radius R:
DI = 20 log 2π + 20 log R = 20 log R + 16 dB.
For a linear aperture having a relative length A:
DI = 10 log 2A = 10 log A + 3 dB.
Transmitting and receiving gain of an antenna [72]
It can be shown that for a transmitting - receiving antenna with a given aperture, the coefficients
K and κ are equal. For this reason, there is a practice to use a concept of the gain G as a numerical
parameter characterizing the antenna directivity:
G = K = κ = 4π/Ω0
In the logarithmic measure
DIR = 10 log K
DIR 10 log κ
Glog = 10 log G [dB]
Energetic gain - applies to the electric power.
Directivity gain - applies to the power stream on the side of the field, does not take into account the
conversion efficiency of the antenna
Power level (SWL) [73]
SWL – power level (water – re 1W; air – re 1 pW)
SW Lel = 10 log
SW Lak = 10 log
PTel
Pref
PTak
PT · ηea
= 10 log el
Pref
Pref
SW Lak = SW Lel + EF
EF = 10 log ηea – logarithmic measure of electric/acoustic conversion efficiency (negative value)
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Sonar Equations
SIL in water [74]
Sound intensity level:
SIL = 10 log
I
Iref
For sound in water Iref has been adopted corresponding to pressure:
pref = 10−6 Pa = 1 µPa
or pref = 1 Pa
and since
2
Iref = (pref ) /Zak
Zak = %c = 1.5 · 106 rayl,
so:
2
Iref = (1 µPa) /1.5 · 106 W/m2 = 0.67 · 10−18 W/m2
10 log
or:
Iref
1 W/m2
= −182 dBre1
W/m2
2
Iref = (1 Pa) /1.5 · 106 W/m2 = 0.67 · 10−6 W/m2 ≈ 1 µW/m2
10 log
Iref
1 W/m2
= −62 dBre1
W/m2
Madness of the civilization, [75]
The difference of 120 dB - in a simple and obvious matter of a technical
standard
2
−6
2
A reasonable reference value of 1 µW/m 0.67 · 10 W/m
is being pushed by a value of10−6 pW/m2 (60 dB below the threshold of hearing 1 pW/m2 !
Euro – the common currency of ambiguously sounding name
juro, ojro, orro, jewro, euro
and Asians can easily come up with the names of cars (almost) equally sounding in all European
languages!
Source Level (SL) 1 [76]
SIL1 – Sound pressure level of an isotropic quasi-point source measured in a unit distance r1
IT 1
PT
= 10 log
Iref
4π · r12 Iref
SIL1 = 10 log
SL – Source Level = SIL1 directive source
SL = 10 log
IT 1
PT · K
= 10 log
Iref
4πr12 Iref
DI – Directivity Index
EF – logarithmic measure of conversion efficiency
SL = SIL1 + DIT (+EF )
Source Level (SL) 2 [77]
In view of the large number of possible reference values, numerous forms of SL. formula can be found
Most often it is
SL [dBre1µPa ] = SW Lel [dBre1W ]+EF +182−10 log 4π+DIT = SW Lel [dBre1W ]+10 log ηea +171+DIT
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Sonar Equations
Distance measurement [78]
Echo signals arrive at the receiver with a delay TR from the time of emission of the sounding signal,
resulting from the finite wave propagation speed c in the medium:
TR = 2R/c,
hence the distance R to the heterogeneity of/in the resort which is the source of the echo signal is:
R = cT R/2
In the case of simple detection of a signal of time length (duration) Ti , the distance resolution (longitudinal, range resolution) δR is:
δR = cTi /2
in the case of correlation reception of special wideband signals with Ti B 1, the range resolution is:
δR = c/2B
Direction measurement [79]
• 1) Rotating antenna or a directional beam sweeping the observation sector
– measurement of the direction from which the echo comes means a search, in subsequent
sounding signal transmissions, for its maximum level
–
–
–
–
the same principle applies to passive listening systems
the direction is determined with a precision (accuracy) ∆θ equal to the beamwidth Θ
also the angular resolution δθ is equal to the width of the beam Θ
a sweep with the beam narrower to the size of the object leads to determine its contour
(shape)
• 2) Two receiving points
– the direction θ from which the echo comes is determined from the time difference ∆TR
between its arrival to two points spaced at a distance d one from another (or, in the case of
monochromatic signals, from the phase difference ∆ϕ between two points):
∆TR = d sin θ/c
θ = arc sin(∆TR c/d)
θ = arc sin(∆ϕλ/2πd)
(very good measurement precision bur poor angular resolution equal Θ )
Speed measurement [80]
Waves generated by a moving object have in the medium the frequency displaced relative to the original
one. The frequency difference fD between that of a source moving at a radial speed vR , and the received
frequency fR is called the Doppler shift:
fR ≈ fS (1 + vR /c)
fD = fS –fR ≈ fS vR /c
Waves reflected from a moving object have Doppler shift doubled:
fD ≈ 2fS vR /c
Radial velocity vR of the object, means the projection of its velocity vector v onto the axis pointing
towards the observer:
vR = v cos α
where α- angle between the motion vector and the direction to the observer.
Sonars vs radars [81]
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Sonar Equations
cem = 0.3 · 109 m/s
cha = 1.5 · 103 m/s
The waiting time for echo (and the rate of space scan) differs 200 000 times!
TR
ha
TR
= 1.3 s/km of range
em
= 6.7 µs/km of range
(6.7 ms / 1000 km of range)
Very different sounding pulse repetition frequency
Tp > 3TR ⇒ Fp ha < 1 Hz
Fp ha = 1 − 5 Hz
for a given size of the antenna, sonar achieves the same directivity as radar, with much lower frequencies: fha = fem · 5 · 10−6
at the same frequency, acoustic systems have a much better resolution (acoustic microscope)λha =
λem · 5 · 10−6
usually fha fem and λha λem
Specificity of sonar technology - easiness [82]
much lower frequency
fha = 500Hz − 300kHz
⇔
fem = 100M Hz − 30GHz
λha = 3m − 5mm
⇔
λe m = 3m − 10mm
longer durations of pulses Ti (LF technology vs. nanosecond technology)
Ti
ha
= 20 µs − 20 ms
⇔
Ti
em
= 0.1 ns − 10 ns
good resolution achieved with relatively small aperture diameters:
• angular resolution δθ ≈ 1/A = λ/a
• the potential transversal resolution δx ≈ λ
Specificity of sonar technology - difficulties [83]
• long time to wait for an echo, hence special methods of time- and space processing to reduce time
of space search:
– acquiring maximum information from the observed sector within a single transmission of
sounding pulse:
∗ monopulse techniques
∗ multi-beam technology
– use of accumulated information from numerous soundings:
∗
∗
∗
∗
complex techniques of imaging / exposure
recording of echo signals from multiple soundings
multidimensional analysis (time, frequency, statistics)
multimonitor, multiple observation posts
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Sonar Equations
Operating area search time – head sonar [84]
• Exemplary requirements
–
–
–
–
–
sonar range: RM = 150 m ⇒ waiting time for echo: TR = 0.2 s
safety factor (time reserve): n = 3
dimensions of the search area: ΘSH × ΘSV = 60◦ × 20◦
beam width: ΘH × ΘV = 5◦ × 5◦
number of transmissions at one direction: 1
• Search time
–
–
–
–
number of transmissions into the area: N × M = ΘSH /ΘH × ΘSV /ΘV = 12 × 4 = 48
time of area search (scan, inspection): TS = TR · N · M ≈ 10 s
search time with a margin of safety: TΣ = n · TS = 0.5 min
extension of the range to do RM = 1 500 m increases search time TΣ to 5 min, the use of a
16-beam sonar reduces it to about 20 s
Monostatic, bistatic, and multistatic echolocation [85]
• Transmitter and receiver of echolocation system (its transmitting and receiving antennae) can be,
in general, in two different locations. We are dealing with bistatic echolocation. If the receiver
receives signals coming from several transmitting antennae of different locations, the system works
as multistatic..
• Classical radar / sonar operate in a monostatic configuration using the same transmitting and
receiving antenna, or two different antennae in a common location (eg. a common mast).
• Transmitting antenna emits a wave that "illuminates" whole the chosen sector of observation within
a single transmission.
• Receiving antenna working with a multi-channel receiver can "generate" simultaneously a number
of narrow beams covering the sector. The aperture of such an antenna is much larger than the
aperture of the sector antenna.
Sonar energy balance 1 [86]
• The energy balance of an echolocation system is performed in two stages, according to the scheme
of a cascade of two communication configurations:
– from the transmitting antenna to the area of the “illuminated” heterogeneity,
– from the heterogeneity as an isotropic secondary source to the receiving antenna.
• IntensityISi of the incident wave in the area of heterogeneity equals:
ISi =
PT e ηea K
exp (−nr)
4πr2
• Equivalent power PS of the secondary point source is: PS = ISi σS
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Sonar Equations
Sonar energy balance 2 [87]
• Intensity IR of the wave reaching the receiving antenna:
IR =
PS
exp (−nr)
4πr2
• Power PR of the secondary point source is: PR = IR ΣR hence:
PR =
PT e ηea K
2
(4π) r4
σS ΣR exp (−2nr)
• Energy balance of a sonar as a surplus of the radiated acoustic power over the power perceived by
the receiving antenna
2
(4π)
PT
=
r4 exp (2nr)
PR
KσS ΣR
Effective scattering area of a target [88]
Effective scattering area σS of an object, known also as its “radar cross-section”, is defined as the ratio of
the power PS radiated isotropically from the scattering element (target), to the intensity ISi of a plane
wave incident on the element:
PS
σS =
ISi
In the general case of bistatic configuration and a target (object, heterogeneity) of complex shape,
σS it depends both on the direction (ϑST , ϕST ) of the object exposure relative to its proper reference
system, ie. selected axis and reference plane, and the direction of observation (ϑSR , ϕSR ).
For objects with spherical symmetry (ball), the effective scattering surface is only a function of the
difference between the angle of observation and angle of exposure:
σS (ϑST ϕST ; ϑSR ϕSR ) = σs (ϑSR − ϑST )
Scattering, reflection σs (ϑSR = ϑST ) [89]
Radar cross-section of a ball σS (a/λ), monostatic obsewrvation:
4
2
σS → 9π (ka) for k → 0
σS → π (a) for ka → ∞
Scattering cross-sections – addendum [90]
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Air bubble in water
at resonance
σS ≈ 400πa2 !!
Scattering of acoustic wave in aerated water causes
an increase in attenuation (multiple reflections)
Rigid ball
4
– σS → 9π (ka)
for ka → 0
– σS → πa2
for ka → ∞
Scattering, reflection [91]
σS (ϑSR − ϑST )
Bistatic observation of dielectric spheres in linearly
polarized light
Radar cross section of a plane [92]
σS (ϑS )
35
Sonar Equations
R. Salamon & H. Lasota
2016-01-27
Target strength of a submarine [93]
T S (ϑS )
36
Sonar Equations
R. Salamon & H. Lasota
2016-01-27
Modern Technologies
Part V
Technology progress
in modern hydrolocation systems
[2016_01_25_EchoMeth_Add_script_HL]Modern technology and techniques in
hydrolocation systems [R. Salamon, H. Lasota]R. Salamon, H. Lasota 2016-01-27
Overview [95]
1.
2.
3.
4.
5.
8
Traditional solution
New approach
Envelope detection (see part III, sec. 17)
Single beam echosounder (see part VI, sec. 26)
Low frequency echosounder (see part II, sec. 6, part IV sec 20-21)
Side scan sonar (see part VII, sec. 36)
High power sonar
Matched filtering (see part III, sec. 18)
Multibeam echosounder (see part VI, sec. 27-28)
Parametric echosounder
Synthetic aperture sonar (see part VII, sec. 31,35)
Quiet sonar (see sec. 41)
Matched filtering
Matched filtering [96]
Disadvantages of envelope detection
• range depends on the power of sounding signal (limited transmitter power, cavitation)
• distance resolution depends on the duration of the sounding pulse (for short pulses bandwidth of
the receiver must be broad – the noise level increases)
• need to use excessively wide bandwidth of the receiver due to the Doppler effect
Example:
• operating frequency f = 15 kHz
• speed of observation object (target) v = 10 m/s
• duration of the sounding pulse τ = 100 ms
• optimum bandwidth of the receiver B = 1/τ = 10 Hz
• Doppler shift ∆f = (2v/c)f = 200 Hz
• necessary bandwidth B = 400 Hz
• increase of the noise level in the receiver ∆SN R = 10 log 40 = 16 dB
The principle of matched filtering (correlation reception) [97]
1. The transmitter emits a sounding signal with a linear or hyperbolic frequency modulation s (t)
Bt
s (t) = S0 sin 2π f −
t
T
2. The receiver calculates the Fourier transform of the echo signal
X (jω) = F {x (t)}
and S (jω) = F {s (t)}
3. The spectrum X (jω) is multiplied by the spectrum S ∗ (jω) and the inverse Fourier transform
y (t) = F −1 {X (jω) S ∗ (jω)} is calculated
n
o
4. If X (jω) = S (jω), than y (t) = F −1 |X (jω)|2 = rxx (t) – autocorrelation function of the
sounding signal.
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Modern Technologies
[98]
Noise in the 30 kHz frequency band
Matched filtering advantages [99]
• the range depends on the energy of sounding signal (long pulse with a limited power provides high
energy)
•
SN R0 =
E
= Bτ SN Ri
M
• distance resolution depends on the spectrum width of the sounding pulse (∆R = c/2B – does not
depend on the pulse duration, therefore the sounding signal can be very long)
• Doppler effect reduces the output signal with linear frequency modulation and does not reduce the
signal with hyperbolic frequency modulation
Example:
• operating frequency f = 15 kHz
• speed of observation object (target) v = 10 m/s
• sounding pulse width τ = 1 s
• receiver bandwidth B = 3 Hz
• Doppler shift ∆f = (2v/c)f = 200 Hz – a slight influence of the Doppler effect
• Bτ = 3000 (35 dB)
• range increases approximately 6 times with the same transmitter power!
9
9.1
Multibeam echosounder
Beamforming
Multibeam echosounder [100]
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Modern Technologies
Disadvantages of single beam echo
sounder
• small scanned volume (contained in a single,
narrow beam)
• distance measurement only in the direction
of the beam axis
• two-dimensional cross-section image of the
scanned volume
Advantages:
• Simple construction, low cost.
Working principle of a narrowband, delay-and-sum beamformer [101]
Beamformer producing beams in one plane
Signal at the output of the n-th antenna element
sn (t, θ) = S0 sin {2πf0 [t − τgn (θ)] + ϕ}
“Geometric” delay
nd
τgn (θ) = −
sin θ
c
In order to produce one deflected beam, the signals in each channel signal are delayed in such a way
that the total delay is equal in each channel. All thus delayed signals are summed to give the signal of a
tilted/deflected beam.
[102]
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Modern Technologies
Operating scheme of a digital phase beamformer [103]
The Hilbert transformers can be replaced by 90◦ phase-shift circuits, but this deteriorates beamformer
parameters, especially when the signal spectrum is relatively wide.
[104]
Advantages
echosounder
of
a
multibeam Disadvantages
• wide angle of simultaneous observation (the
number of deflected beams exceeds one hundred)
• high (good) angular resolution (the width of
a single beam – a few degrees)
• possibility of three-dimensional exposition
9.2
• Extensive design (hence high costs):
–
–
–
–
a multi-element antenna,
a multi-channel receiver,
high performance computation system,
electronic stabilization of beam position
Acoustic camera
Adaptive Resolution Imaging Sonar (Sound Metrics) [105]
http: ARIS camera
http: Seeing with sound
http: High-resolution sonar-images
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Side-looking camera images [106]
41
Modern Technologies
R. Salamon & H. Lasota
2016-01-27
Modern Technologies
Acoustic camera parameters [107]
10
Parametric echosounder
Parametric echosounder [108]
Purpose
Examination of the internal structure of seabed (sea bottom)
Basic requirement
Sounding pulse frequency as low as possible to ensure deep penetration of acoustic waves into sediments.
Disadvantages of low-frequency echosounder
• very large dimensions of the antenna (Example:f = 7.5 kHz, wavelength λ = 0.2m, beamwidthi
Θ = 10 deg, antenna length L ∼
= 1 m)
• multi-element antenna built of the sandwich-type transducers
• large wieght of antenna
• large cost
The operating principle of parametric echosounder [109]
The transmitting transducer emits, within a narrow beam, sounding pulses filled with a superposition
of sine waves of high frequencies, f1 and f2 , with a very high amplitude of acoustic pressure. In the
near-field of the trancducer, the wave is almost plane, thus its pressure does not decrease. It is so high
that it falls within the range of medium nonlinearity. Due to the nonlinear phenomena, a low frequency
sine wave is effectively created within the beam, of frequency F = f1 − f2 , being the difference of the
original ones.
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Modern Technologies
Features of parametric echosounder [110]
• Positive
–
–
–
–
low difference frequency (a few kilohertz)
small antenna size
beam width equal to the width of the beam at high frequencies (typically a few degrees)
lack of side lobes (in conventional systems the side lobe level is reduced by amplitude weighing
in multi-element antennas)
– possibility of obtaining a relatively broad spectrum of difference signal
– possibility of using high-power primary signals (higher cavitation threshold at high frequencies)
• Negative
– low efficiency of power conversion of high frequency signals into the signal of low, difference
frequency
[111]
Echosounder f = 100 kHz
Parametric echosounder f = 5 kHz
Courtesy of Professor. G. Grelowska and Professor E. Kozaczka
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2016-01-27
Modern Technologies
Synthetic aperture sonar
Synthetic aperture sonar [112]
Drawbacks of side scan sonar (SSS)
• lateral resolution worsening with distance
(resulting from a constant angular resolution)
• nonlinear scale of bottom-projected object
position vs. sounding distance:
p
x = r 2 − d2
where r – measured distance , d – depth (can
be linearized in the receiver)
Synthetic aperture radars and sonars [113]
In common radars and sonars, the lateral resolution depends on the beam width and deteriorates with
the distance from the system antenna. Synthetic aperture radars (SAR) and sonars (SAS) are used to
increase the lateral (transversal) resolution.
The general working principle of SAR and SAS involves collecting, recording and processing of the
echo signals by a small antenna with a wide beam,
in subsequent points of the trajectory traveled by
the antenna installed on a platform (airplane, underwater vehicle, satellite) moving in a straight line.
Thus the antenna is apparently extended in space,
that reduces the beam width and thereby improves
the lateral resolution of the system (azimuthal in
the figure).
Synthetic aperture working principle [114]
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Modern Technologies
Time-correlated pulse compression & space-correlated focusing
Complex amplitude is “naturally” recorded in subsequent positions. The amplitude-and-phase knowledge of the received signals makes the SA technique to be a holography on the radio waves.
Signal processing in SAR and SAS [115]
Sounding signal with linear frequency modulation s (t, x)
Distance compression – by time (matched filtering)
S (f, x) = It {s (t, x)}
2
Y (f, x) = S (f, x) Sp∗ (f, x) ∼
= |S (f, x)|
B = 15 kHz
Distance resolution
δr = 5 cm
y (t, x) = I−1 {Y (f, x)} = rss (t, x)
Pulse duration y (t, x) T = 1/B
Azimuthal (lateral) compression – by path x (space focussing)
Z (t, u) = Ix {y (t, x)}
∼ |Z (t, u)|2
Q (t, u) = Z (t, u) Zp∗ (t, u) =
q (t, x) = I−1 {Z (t, u)} = ryy (t, x)
The frequency of the y(t, x) signal is changing linearly as a result of Doppler effect. The longer the
pathway, the wider is the bandwidth and the better
is the resolution.
Azimuthal resolution = half actual length of the antenna.
[116]
Advantages:
• constant, very good (“high”) lateral resolution
• constant, very high longitudinal (distance, depth) resolution
Disadvantage:
• High cost resulting from complex design and high performance computing
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12
2016-01-27
Modern Technologies
Silent sonar CW FM
Silent sonar CW FM [117]
Purpose: Hindering an opponent the detection of
sounding signal by an intercepting sonar.
Factors hindering the detection by an intercept
sonar:
• possibly low power
• continuous signal (constant wave)
• widest possible spectrum
Linear frequency modulation:
B
B
fl (t) = sin 2π f0 − +
t t
2
2T
Hiperbolic frequency modulation:
B
f1 fh
T ln 1 −
t
fh (t) = sin 2π
B
fh T
.
[118]
Signal processing in the receiver – matched filtering
y (n) = I−1 {F (k) X ∗ (k)}
y (n) = X0 rf f (n − n0 )
The influence of the Doppler effect when observing moving targets
Distance measurement error: ∆Rm ∼
= −vT
f0
1
+
B
2
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MES Dept. Sonars
Part VI
MES Department sonars
[Polish Navy sonars]MES Department sonars [MES Dept.]Marine Electronic Systems Department 21 I 2015
13
Operational and technical parameters in underwater sounding
13.1
Polish Navy sonars (developed by the MES Department,
ETI Faculty, GUT)
Sonars developed by the MES Department, ETI Faculty, GUT [119]
All the MES sonar systems are in a fully operational service on board of Polish Navy anti-submarine
warfare (ASW) and mine counter-measure (MCM) ships of both US and Soviet origin, meeting critical
requirements of the highest technology readiness level (TRL 9).
I
II
III
IV
V
13.2
Long-range active ASW sonar
Medium-range MCM sonar
Helicopter dipping ASW sonar for submarine detection and tracking
Passive ASW towed-array sonar
Side-scan MCM sonar
Long-range active ASW sonar
I Long-range active ASW sonar [120]
The hull-mounted anti-submarine warfare (ASW) long-range,
multi-beam sonar systems with full angular range of target tracking are designed for the detection, localization and tracking of ships
and other objects.
The long-range active ASW system is equipped with a cylindrical acoustic antenna cooperating with
a beam-former, which allows simultaneous observation of the targets at all bearings around the ship.
Automatic position stabilization of the antenna enables conducting a continuous survey of targets and
determining their position with a good accuracy even at stormy weather. Very high energy, frequencymodulated sounding pulses used in sonar, combined with the correlation detection, assure a long range (up
to 32 km), even in difficult propagation conditions with a high level of acoustic noise and reverberation.
The system is equipped with a modern, ergonomic imaging assembly with 4 color display screens and
control panels, operated by two operators.
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MES Dept. Sonars
Long-range active ASW sonar – operating consoles [121]
Long-range active ASW sonar – parameters 1 [122]
Transmitter
Operation frequency (nominal)
Sweep frequency range
Transmitting beam horizontal width
Transmitting beam vertical width
Number of transmitting channels
In-pulse electric power
Source level in dB re 1 Pa @ 1m
Antenna
Ultrasonic transducer sensitivity
Receiving beam horizontal width
Receiving beam vertical width
Observation sector width
48
8,5 kHz
7,75 ÷ 9,25 kHz
360°
12° ± 2,5°
30
≥ 10 kW
≥ 95 dB
500 µV/Pa
12° ± 2,5°
12° ± 2,5°
360°
R. Salamon & H. Lasota
2016-01-27
MES Dept. Sonars
Long-range active ASW sonar – parameters 2 [123]
Receiver
Receiving band (3dB)
Number of receiving channels
Processing
Beamforming technology
7,5 ÷ 9,5 kHz
30
Number of beams
Post-beamforming filtering for “chirp” signals
Post-beamforming filtering for “ping” signals
Operational range
Time-width of sounding pulses
Precision of distance measurement
Precision of bearing measurement
Number of simultaneously tracked targets
13.3
digital, in the frequency-domain
with second-order sampling
90
digital correlation filtering
in the time domain
envelope detection with digital
low-pass filtering
1, 2, 4, 8, 16, 32 km
50, 100, 200, 400, 800, 1600 ms
1% of nominal range
1°
max. 12
Medium-range MCM sonar
II Medium-range MCM sonar [124]
The mine counter-measure (MCM) sonar systems are designed for
searching, detecting, and localizing bottom and contact mines,
specifically in shallow waters with strong bottom reverberations
and substantial deflection of acoustic wave propagation routes.
The MCM sonar is a multi-transmitting and multi-receiving beam systems designed using real-time
microprocessor technology. On the transmitting side, the RDT technique of electronically rotated beam
is used. The generation of sounding signals and rotation of electronic beam rotation is implemented
using the direct digital synthesis (DDS) controlled by single-chip microprocessors. On the receiving side,
a multi-processor DSP system is used for the algorithmic implementation of a beam-former. The latter is
implemented in the frequency domain with the second order sampling and 14 bit resolution. A significant
reduction of beam pattern side lobes both on the transmitting and receiving side is achieved. Two LCD
monitors are used for supporting the console operators in detection, identification and tracking of objects.
Information exchange between the sonar and other on-board systems is also provided.
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MES Dept. Sonars
Medium-range MCM sonar – operating consoles [125]
Medium-range MCM sonar – parameters 1 [126]
Transmitter
Operation frequency
Transmitting beam horizontal width
Transmitting beam vertical width
Number of transmitting channels
In-pulse electric power
Source level in dB re 1 Pa @ 1m
Antenna
Angular range of antenna declination (vertical)
Angular range of antenna deflection (horizontal)
Receiving beam horizontal width
Receiving beam vertical width
Angular width of simultaneous observation sector
Overall observation sector width
50
41 ÷ 46 kHz
60°
9° ± 1°
36
≥ 10 kW
≥ 110 dB
+5 lub -55°
± 60°
3°
9° ± 1°
60°
180°
R. Salamon & H. Lasota
2016-01-27
MES Dept. Sonars
Medium-range MCM sonar – parameters 2 [127]
Receiver
Receiving band (3dB)
Number of receiving channels
Processing
Beamforming technology
41 ÷ 46 kHz
36
digital, in the frequency-domain
with second-order sampling
Number of beams
61
Nominal operation ranges
100, 200, 400, 800, 1600 m
Precision of distance measurement 0,5% of nominal range
Precision of bearing measurement 1°
A-type display target resolution
15, 37.5, 75 cm
relative to the pulse-width
after compression
13.4
Helicopter dipping ASW sonar
III Helicopter dipping ASW sonar [128]
The dipping sonar for helicopters are new systems designed for detection and tracking submarines in
both active and passive modes, equipped with an additional gradient passive array and a meter of speed
velocity distribution in water.
All electronic systems in the transducer, receiver and imaging system are designed using real-time
microprocessor technology. The on-deck transmitter using a direct digital synthesis (DDS) modulator
generates a broadband, frequency-modulated sounding signal. The sonar receives echo signals from a
revolving ultrasonic transducer operating in the active mode and 4 signals from hydrophones of the
passive array. After the first stage of preliminary magnification and analog filtering the received signals is
subject to analog-to-digital conversion. Consequently, computer-aided processing and imaging is applied.
The pulse compression technique used the in active mode significantly improves detection performance
and increases the maximum range of detecting and tracking submarines, which can be displayed at console
monitor.
Helicopter dipping ASW sonar – operating consoles [129]
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MES Dept. Sonars
Helicopter dipping ASW sonar – parameters 1 [130]
Transmitter
Center frequency of the sounding signal
Sounding signal modulation
Sounding signal sweep range
Transmitting beam horizontal width
Transmitting beam vertical width
Number of transmitting channels
In-pulse electric power
Sounding pulse maximum energy
Antenna
Receiving beam horizontal width
Receiving beam vertical width
Observing sector angular range
Scanning technique
Receiver
3dB receiver band
Number of receiving channels
15,5 kHz
linear frequency modulation
(LMF)
14 to 17kHz
15°
15°
1
≥ 1 kW
0.5 kJ
15°
15°
360°
mechanical rotation
14 to 17 kHz
1
Helicopter dipping ASW sonar – parameters 2 [131]
Processing
Number of beams
Filtering for a “chirp” signal
1
digital correlation filtering
in the frequency domain
Filtering for a “ping” signal
envelope detection with digital
low-pass filtering
Operational range
1.5, 3, 6, 12 km
Time-width of sounding pulses
50, 125, 250, 500 ms
Precision of distance measurement
1% of nominal range
Precision of bearing measurement
< 5°
Number of simultaneously tracked targets max. 4
Passive operation (intercept)
Listening band
5 ÷ 300 Hz
Listening time
1, 2, 4 s
Bearing accuracy:
SNR > 20dB, f = 100 Hz
1°
SNR > 10dB, f = 100 Hz
3°
Spectrum resolution
1, 0.5, 0.25 Hz
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2016-01-27
MES Dept. Sonars
Towed-array passive sonar
IV Towed-array passive sonar [132]
The SQR-19PG is a passive anti-submarine warfare (ASW) towedarray side-scan sonar system designed for detecting and tracking
submarines.
The SQR-19PG sonar is a low-frequency, broadband, passive real-time system designed using digital
microprocessor technology. Increased angular resolution has been achieved by generating more receiving
beams using effective methods of digital signal processing. New effective beam-forming algorithms for
broadband signals, as well as methods for high-resolution spectrum estimation were developed and applied. This results in a precise measurement of the incoming acoustic wave bearing. Novel algorithms for
automatic tracking of selected targets were developed and applied. The adequate data transmission rate
from the transducer array, thru the towing cable-line is assured by using the VDSL (Very High Speed
Digital Subscriber Line) transmission technique.
Towed-array passive sonar [133]
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Towed-array passive sonar – parameters 1 [134]
Maximum operation depth
Antenna modules filling liquid
Number of intercept bands
Frequency range of intercept bands:
Effective antenna diameter in the specific bands:
610 m
ISOPAR M
4
I: 10 – 175 Hz
II: 175 – 350 Hz
III: 350 – 700 Hz
IV: 700 – 1400 Hz
I: 195 m
II: 97,5 m
III: 48,8 m
IV: 24,4 m
Number of processed acoustic channels
120
Non-acoustic data transferred from the antenna immersion depth
magnetic heading
water temperature
Number of beams formed
91
Observation sector
360°
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Towed-array passive sonar – parameters 2 [135]
Bearing ambiguity port side / starboard
ability to determine the heading side
after a change of the course
from ± 2° in orthogonal axis
up to ± 4° in sectors
current bearing
bearing rate of change
1s, 2s, 4s
Target bearing accuracy
Motion parameters being measured
Listening time
Frequency resolution in specific bands
for a given listening time:
Resolution of analog-to-digital conversion
Sampling frequency of acoustic signals
Data transmission down-link band (to antenna)
Data transmission up-link band (from antenna)
Modulation scheme of digital signal transmission
Number of simultaneously tracked targets
13.6
I: 1, 0,5, 0,25 Hz
II: 1, 0,5 , 0,25 Hz
III: 2, 1, 0,5 Hz
IV: 4, 2, 1 Hz
16 bis
4096 Hz
1 ÷ 3,5 MHz
3,8 ÷ 6 MHz
VDSL – QAM
max.. 16
Side-scan MCM sonar
V Side-scan MCM sonar [136]
The side-scan mine counter-measure sonar systems use a very effective underwater acoustic method for detecting and localizing motionless underwater objects.
The side scan sonar is an active system designed using real-time microprocessor technology. Multielement ultrasonic transducers are towed behind the ship above the bottom in a so-called tow fish. The
beam patterns are diagonally directed to the bottom, on the right and left. The dynamic beam width
control ensures constant linear resolution throughout the entire search operation. The echo signals from
the towed transducers are converted into the digital form and sent to the on-board device using the
VDSL (Very High Speed Digital Subscriber Line) data transmission technology. The survey results and
other information are displayed on two-monitor operator console with such functions as zoom, short-term
memory, dimensioning, multiple windows, etc. The side-scan method is used effectively in deep water and
on the bottom (e.g. contact and bottom mines, shipwrecks, underwater structures). It enables identifying
the bottom topography (for hydrographic purposes – making seabed maps) on the area of several hundred
meters wide on both sides of the sounding vessel.
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MES Dept. Sonars
Side-scan MCM sonar – towfish and sounding exposition [137]
Side-scan MCM sonar – parameters [138]
Side-scan sonar:
Source level (dB re 1 Pa @ 1m)
≥ 90 dB
Time-width of sounding pulses
0.2, 0.5, 1, 2 ms
Operation frequency
on the left side of the towfish
154 ± 5 kHz
on the right side of the towfish
175 ± 5 kHz
Angular resolution in the horizontal plane
dynamically varying with the range:
1° for 200 m
constant
4°
Linear resolution in the horizontal plane
with dynamically varying beam width:
ca. 3.5 m
Range resolution:
switched
0,5 or 1,5 m
Vertical width of observation beam
50°
Display exposition range
150 or 300 m
Reservoir depth
5 to 100 m
Towing speed
max. 6 knots
Echosounder:
Operation frequency
200 kHz
Reservoir depth
5 to 100 m
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HPEC Systems
Part VII
High Performance Embedded
Computing
[HPEC]High Permormance Embedded Computing [I. Kochanska]Iwona Kochanska
High Performance Embedded Computing (HPEC) [140]
One of the major goals of HPEC:
to deliver ever greater levels of functionality to embedded signal and image processing (SIP) applications.
High performance SIP algorithms demand throughputs ranging from hundrads of millions of operations
per second (MOPS) to trillions of OPS (TOPS).
HPEC challenges [141]
HPEC is particularly challenging:
• high throughout requirements,
• real-time deadlines
• form-factor constraints.
HPEC is a juggling act that must deal with all three challenges at once.
HPEC latencies [142]
HPEC latencies:
• milliseconds for high pulse repetition frequency (PRF) tracking radars
• a few hundred milliseconds in surveillance radars
• minutes for sonar systems
The best designs will satisfy both latency and throughput requirements while
minimizing:
• hardware resources
• software complexity
• form factor (HPEC systems must fit into spaces ranging from less than a cubic foot to a few tens
of cubic feet, and must operate on power budgets of a few watts to a few kilowatts)
With these size and power constraints, achievable computational power efficiency, measured in operations per second per unit power (OPS/watt), and computational density, measured in operations per
second per unit volume (OPS/cubic foot), determine the overall technology choice.
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HPC latencies (2) [143]
The front-end and back-end latency allowances are determined by the overall latency requirement and
must be traded off against each other.
For example, tracking radars may have to close the tracking loop in a few
milliseconds.
• he amount of time the radar has to detect the target, update its track, generate a new position
prediction, and then direct its antenna to point in a new direction (to keep the target in the radar
beam) must not exceed a few milliseconds
• This update rate is driven by the dynamics of the target being tracked. For a slow-moving target,
a few hundred milliseconds or even a few seconds may be appropriate. For a highly maneuverable
target such as a fighter aircraft, firecontrol radars may need to operate with millisecond latencies
HPEC canonical architecture [144]
HPEC processing stages [145]
Front-end signal and image processing stage
extract information from a large volume of input data (performs stream-based signal and image
processing).
• removal of noise and interference from signals and images
• detection of targets
• extraction of feature information from signals and images
back-end data processing stage
Further refine the information so that an operator, the system itself, or another system can then act
on the information to accomplish a system-level goal (knowledge-based processing).
• parameter estimation
• target tracking
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HPEC Systems
• fusion of multiple features into objects
• object classification and identification
• other knowledge-based processing tasks
• display processing
• interfacing with other systems
HPEC technology [146]
The technology choices for front-end processors:
• full-custom very-large-scale integration (VLSI)
• application specific integrated circuits (ASICs)
• field-programmable gate array (FPGAs)
• programmable digital signal processors (DSPs)
• microprocessor units (MPUs)
• hybrid designs that incorporate a combination of these technologies
The technology chosen for the back-end data processing:
• programmable multicomputer composed of DSPs or MPUs
• shared memory multiprocessor
Typically, front-end processing requires significantly greater computational throughput, whereas
back-end processing has greater program complexity.
Throughput is usually measured in terms of OPS.
• front-end algorithms can require anywhere from a few billion OPS to as many as a few trillion
OPS
• back-end algorithms tend to require an order of magnitude or two fewer OPS
HPEC data rates [147]
• Front-end computations perform operations that transform raw data into information.
• The analog-to-digital converters (ADCs) are themselves highly sophisticated components that set
limits on the precision and bandwidth of the signals that can be digitally processed.
• SIP algorithms remove noise and interference, and extract higher level information, such as target
detections or communication symbols, from a complex environment being sensed by a multidimensional signal, image, or communication sensor.
• In phased-array radars, for example, signals with data rates in the 100s of millions of samples per
second (MSPS) arrive at the front-end of the digital signal processor from tens of receiver channels.
This results in an aggregate sample rate of billions of samples per seconds (GSPS)
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Data reduction in front-end stages [148]
ECCM (electronic counter-countermeasures): beamforming operation in which the channels are combined
to form a small number of beams
Detection stage – algorithm rejects noise and identifies the range gates that contain targets. The
number of targets is significantly less than the total number of range gates. Front-end output/input data
rate is typically less than 0.5%.
(Source:
Martinez, D. R., Bond, R.A., Vai, M.M.,High Performance Embedded Computing Handbook: A Systems Perspective, CRC
Press 2008.)
Surface moving-target indication (SMTI) surveillance radar example [149]
• SMTI radars are used to detect and track targets moving on the earth’s surface
• Radar signal consisting of a series of pulses from a coherent processing interval (CPI) is transmitted
• The pulse repetition interval (PRI) determines the time interval between transmitted pulses. Multiple pulses are transmitted to permit moving-target detection
• The pulsed signals reflect off targets, the earth’s surface (water and land), and man-made structures
such as buildings, bridges, etc.; a fraction of reflected energy is received by the radar antenna
• The goal of the SMTI radar is to process the received signals to detect targets (and estimate their
positions, range rates, and other parameters) while rejecting clutter returns and noise. The radar
must also mitigate interference from unintentional sources such as RF systems transmitting in the
same band and from jammers that may be intentionally trying to mask targets
(SMTI) surveillance radar [150]
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HPEC Systems
(Source: Martinez, D. R., Bond, R.A., Vai, M.M.,High Performance Embedded Computing Handbook: A Systems
Perspective, CRC Press 2008.)
SMTI analog processing [151]
• The radar antenna typically consists of a two-dimensional array of elements (1000s of elements)
• The signals from these elements are combined in a set of analog beamformers to produce subarray
receive channels, thereby reducing the number of signals that need to be converted to the digital
domain for subsequent processing. For example: 20 vertical subarrays are created that span the
horizontal axis of the antenna system. Employed in an airborne platform, the elevation dimension
is covered by the subarray analog beamformers, and the azimuthal dimension is covered by digital
beamformers
• The channel signals subsequently proceed through a set of analog receivers that perform downconversion and band-pass filtering
• The signals are then digitized by analog-to-digital converters (ADCs) and input to the high performance digital front-end
SMTI digital processing [152]
• Channelizer process divides the wideband signal into narrower frequency subbands
• Filtering and beamformer front-end mitigates jamming and clutter interference, and localizes return signals into range, Doppler, and azimuth bins
• Constant-false-alarm-rate (CFAR) detector (after the subbands have been recombined)
• Post-processing stage that performs such tasks as target tracking and classification
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Contents
I
Doppler effect
1
1 Waves and motion
1.1 Doppler effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1
3
2 Continuous wave radars and sonars
4
II
6
Echolocation in nature
3 Echolocation in nature
3.1 Bats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Dolphins & Whales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 Other spieces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
6
7
8
III
9
Principles of underwater acoustics
4 Underwater environment
4.1 Propagation conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 (Hydro)acoustic waveguides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
9
10
5 Sound modes
10
6 Refraction
6.1 Propagation velocity .
6.2 Sound rays . . . . . .
6.3 Sound channels . . . .
6.4 Oceanic sound channel
6.5 Sound attenuation . .
6.6 Sea noise . . . . . . .
11
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7 Refraction in sonar systems
IV
20
Signal levels in physical space
V Technology progress
in modern hydrolocation systems
26
37
8 Matched filtering
37
9 Multibeam echosounder
9.1 Beamforming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.2 Acoustic camera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
38
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40
10 Parametric echosounder
42
11 Synthetic aperture sonar
44
12 Silent sonar CW FM
46
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MES Department sonars
13 Operational & technical parameters
13.1 Polish Navy sonars . . . . . . . . . .
13.2 Long-range active ASW sonar . . . .
13.3 Medium-range MCM sonar . . . . .
13.4 Helicopter dipping ASW sonar . . .
13.5 Towed-array passive sonar . . . . . .
13.6 Side-scan MCM sonar . . . . . . . .
VII
HPEC Systems
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High Performance Embedded Computing
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