Note: Descriptions are shown in the official language in which they were submitted.
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FORWARD SCANNING SONAR SYSTEM AND METHOD WITH ANGLED FAN BEAMS
FIELD OF THE INVENTION
The present invention relates to underwater sonar systems, and more
particularly, to a forward
scanning sonar systems and method with angled fan beams.
BACKGROUND OF THE INVENTION
Detailed, gap-free forward sonar imaging along the path of a vessel is highly
desirable in
numerous applications such as, for example, navigation, obstacle avoidance,
surveying, search
and rescue operation, and treasure hunting.
Unfortunately, while there are various sonar systems available for sector
scanning in a forward
direction such as, for example, multi-beam, short aperture bathymetric,
electronically or
mechanically steered sonar systems, or combinations thereof, none of these
sonar systems
produce a frontal scanning view of the mapped area, nor can they accommodate
large aperture,
high resolution transducers. All these sonar systems produce a sectoral field
of view where
lateral selectivity is rapidly diminishing with the range. While some sonar
systems such as, for
example, multi-beam altimeters, are capable of producing 3-D mapped area in
the forward
direction, they lack detailed resolution and come at significant cost due to a
large number of
channels needed in the system.
On the other hand, existing side scan sonar systems, while being of high
resolution and widely
available, cannot be utilized for forward mapping due to the loss of
selectivity in the forward
direction, and are not capable of simultaneous depth profiling. Furthermore,
its port and
starboard imaging data suffer wide data voids at nadir direction, leaving the
resulting image
dissected in the middle. Attempts to mitigate this problem with supplementary
nadir gap filler
sonars lack resolution and come at added cost.
It is desirable to provide a sonar system and method that enable use of high
resolution sonar
transducers for forward mapping.
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It is also desirable to provide a forward scanning sonar system and method
that provide gap-free
mapping.
It is also desirable to provide a forward scanning sonar system and method
that are simple and
cost effective to implement.
It is also desirable to provide a forward scanning sonar system and method
that enable depth
profiling along the path ahead.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide a forward
scanning sonar system
and method that enable use of high resolution sonar transducers for forward
mapping.
Another object of the present invention is to provide a forward scanning sonar
system and
method that provide gap-free forward mapping.
Another object of the present invention is to provide a forward scanning sonar
system and
method that are simple and cost effective to implement.
Another object of the present invention is to provide a forward scanning sonar
system and
method that enable depth profiling along the path ahead.
According to one aspect of the present invention, there is provided a forward
scanning sonar
system. The forward scanning sonar system comprises at least an elongated
sonar transducer, wet
side electronics, top side computer processor, data and telemetry
uplink/downlink, advanced
visualization software, and a support structure having the at least a sonar
transducer mounted
thereto. The at least a sonar transducer is configured such that, while during
scanning operation
the sonar transducer is moved along a forward moving direction, a fan-shaped
beam of the sonar
transducer is forming a plane oriented forwardly downwardly such that the fan-
shaped beam
forms a scan line oriented at a scan angle to the forward moving direction
with the scan angle
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being greater than 0 and smaller than it/2 such that scan line intersects the
forward direction at a
point ahead of the transducer.
According to the aspect of the present invention, there is provided a forward
scanning sonar
system. The forward scanning sonar system comprises a port sonar transducer
and a starboard
sonar transducer mounted to a support structure in an angled forward,
descending triangle
formation. The sonar transducers are configured such that, while during
scanning operation the
sonar transducers are moved along a forward moving direction, fan-shaped beams
of the sonar
transducers are forming planes oriented forwardly downwardly such that each of
the fan-shaped
beams forms a scan line oriented at a scan angle to the forward moving
direction with the scan
angle being greater than 0 and smaller than rc/2, and such that an
intersecting point of the scan
lines with each other is ahead of the sonar transducers towards the forward
moving direction.
The port sonar transducer and the starboard sonar transducer each comprise a
transmit/receive
sonar transducer element for transmitting and receiving sonar pulses of the
fan-shaped beam in a
plane oriented substantially perpendicular to a longitudinal extension thereof
The port sonar
transducer is mounted to the support structure such that the longitudinal
extension is oriented
rearwardly downwardly and is oriented towards port at a port angle to the
vertical plane
containing the forward moving direction with the port angle being greater than
0 and smaller
than rr/2. The starboard sonar transducer is mounted to the support structure
such that the
longitudinal extension is oriented rearwardly downwardly and is oriented
towards starboard at a
starboard angle to the vertical plane containing the forward moving direction
with the starboard
angle being greater than 0 and smaller than it/2.
According to the aspect of the present invention, there is provided a forward
scanning sonar
system. The forward scanning sonar system comprises a port sonar transducer
and a starboard
sonar transducer mounted to a support structure in an angled forward,
ascending triangle
formation. The sonar transducers are configured such that, while during
scanning operation the
sonar transducers are moved along a forward moving direction, fan-shaped beams
of the sonar
transducers are forming planes oriented forwardly downwardly such that each of
the fan-shaped
beams forms a scan line oriented at a scan angle to the forward moving
direction with the scan
angle being greater than 0 and smaller than 71/2, and such that an
intersecting point of the scan
lines with each other is ahead of the sonar transducers towards the forward
moving direction.
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The port sonar transducer and the starboard sonar transducer each comprise a
transmit/receive
sonar transducer element for transmitting and receiving sonar pulses of the
fan-shaped beam in a
plane oriented substantially perpendicular to a longitudinal extension
thereof. The port sonar
transducer is mounted to the support structure such that the longitudinal
extension is oriented
forwardly upwardly and is oriented towards port at a port angle to the
vertical plane containing
the forward moving direction with the port angle being greater than 0 and
smaller than 7c/2. The
starboard sonar transducer is mounted to the support structure such that the
longitudinal
extension is oriented forwardly upwardly and is oriented towards starboard at
a starboard angle
to the vertical plane containing the forward moving direction with the
starboard angle being
greater than 0 and smaller than n/2.
According to the aspect of the present invention, there is provided a forward
scanning sonar
method. At least a sonar transducer mounted to a support structure is moved
along a forward
moving direction. While moving along the forward moving direction, the at
least a sonar
transducer transmits sonar pulses in the form of angled fan-shaped beam. The
angled fan-shaped
beam of the sonar transducer forms a plane oriented forwardly downwardly and
at a scan angle
to the forward moving direction with the scan angle being greater than 0 and
smaller than 762
and such that the scan line intersects the forward direction at a point ahead
of the transducer.
While moving along the forward moving direction, sonar return echo sequences
from the sonar
pulses are received, converted into raw sonar return data and provided to a
computer processor.
Using the computer processor, imaging data are determined in dependence upon
the sonar return
data and passed on to a computer monitor for real time visualization or
playback.
According to the aspect of the present invention, there is provided a forward
scanning sonar
method. A port sonar transducer and a starboard sonar transducer mounted to a
support structure
in an angled forward, triangle formation are moved along a forward moving
direction. While
moving along the forward moving direction, the sonar transducers transmit
sonar pulses in the
form of fan-shaped beams. The fan-shaped beams of the sonar transducer form
planes oriented
forwardly downwardly such that each of the fan-shaped beams forms a scan line
oriented at a
scan angle to the forward moving direction with the scan angle being greater
than 0 and smaller
than n/2, and such that an intersecting point of the scan lines with each
other is ahead of the
sonar transducers towards the forward moving direction. While moving along the
forward
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moving direction, the port sonar transducer receives port sonar return echo
sequences and the
starboard sonar transducer receives starboard sonar return echo sequences, and
both transducers
receive port and starboard sonar return echo sequences along the intersect
line of the two sonar
beams. The port sonar return echo sequences and the starboard sonar return
echo sequences are
converted into raw digital port sonar return data and starboard sonar return
data, respectively,
and provided to a processor. Using the processor, first imaging data are
determined in
dependence upon the port sonar return data and second imaging data are
determined in
dependence upon the starboard sonar return data, and the profile data is
determined upon sonar
return data along the intersect line of the two sonar beams. The first imaging
data and the second
imaging data are then combined and displayed on a computer monitor along with
the profile data
in the forward direction.
According to another aspect of the present invention, there is provided a
rearward scanning sonar
method. At least a sonar transducer mounted to a support structure is moved
along a forward
moving direction. While moving along the forward moving direction, the at
least a sonar
transducer transmits sonar pulses in the form of angled fan-shaped beam. The
angled fan-shaped
beam of the sonar transducer forms a plane oriented rearwardly downwardly and
at a scan angle
to the forward moving direction with the scan angle being greater than 0 and
smaller than rr/2
and such that the scan line intersects the forward direction at a point behind
the transducer.
While moving along the forward moving direction, sonar return echo sequences
from the sonar
pulses are received, converted into raw sonar return data and provided to a
computer processor.
Using the computer processor, imaging data are determined in dependence upon
the sonar return
data and passed on to a computer monitor for real time visualization or
playback.
The advantage of the present invention is that it provides a forward scanning
sonar system and
method that enable use of high resolution sonar transducers for forward
mapping.
A further advantage of the present invention is that it provides a forward
scanning sonar system
and method that provide gap-free forward mapping.
A further advantage of the present invention is that it provides a forward
scanning sonar system
and method that simple and cost effective to implement.
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A further advantage of the present invention is that it provides a forward
scanning sonar system
and method that enable depth profiling along the path ahead.
BRIEF DESCRIPTION OF THE DRAWINGS
A preferred embodiment of the present invention is described below with
reference to the
accompanying drawings, in which:
Figures la and lb are simplified block diagrams illustrating in top
perspective views the
forward scanning process using the forward scanning sonar system according to
a
preferred embodiment of the invention;
Figure 1 c is a simplified block diagram illustrating in a top view of the
scanning process
using the forward scanning sonar system according to a preferred embodiment of
the
invention;
Figure ld is a simplified block diagram illustrating imaging results of the
scanning
process using the forward scanning sonar system and method according to a
preferred
embodiment of the invention;
Figure le is a simplified block diagram illustrating in a top view the
scanning process for
employing advanced graphics processes using the forward scanning sonar system
according to a preferred embodiment of the invention;
Figure 2a is a simplified block diagram illustrating a sonar transducer having
fan-shaped
directional beam employed in the forward scanning sonar system according to a
preferred
embodiment of the invention, all near-field effects in the directivity
ignored;
Figures 2b and 2c are simplified block diagrams illustrating in a perspective
view a first
and a second arrangement, respectively, of the sonar transducers employed in
the forward
scanning sonar system according to a preferred embodiment of the invention;
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Figures 3a to 3d are simplified block diagrams illustrating implementations of
the
forward scanning sonar system according to a preferred embodiment of the
invention
having the sonar transducers mounted to a submersible glider, a towfish, a
submarine,
and a surface vessel, respectively; and,
Figure 4a is a simplified block diagram illustrating a preferred
implementation of data
acquisition and processing for the forward scanning sonar system according to
a preferred
embodiment of the invention;
Figures 4b and 4c are simplified block diagrams illustrating plotting of
waterfall traces
for forward scan setup and side scan setup, respectively, using the preferred
implementation of data acquisition and processing illustrated in Figure 4a;
Figures 4d to 4f are simplified block diagrams illustrating example frequency
modulated
and match-filtered sonar return data, data after cross-correlation, and the
result of the hit-
crossing module after cross correlation, respectively, using the preferred
implementation
of data acquisition and processing illustrated in Figure 4a;
Figure 5a is a simplified block diagram illustrating in a top view sensor
elements of a
sonar transducer capable of producing one or two independent fan beams
employed for
3D imaging in the forward scanning sonar system according to a preferred
embodiment
of the invention; and,
Figure 5b is a simplified block diagram illustrating the scanning process for
employing
3D bathymetry sonar processes using the forward scanning sonar system
according to a
preferred embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Unless defined otherwise, all technical and scientific terms used herein have
the same meaning
as commonly understood by one of ordinary skill in the art to which the
invention belongs.
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Although any methods and materials similar or equivalent to those described
herein can be used
in the practice or testing of the present invention, the preferred methods and
materials are now
described.
While the description of the preferred embodiments hereinbelow is with
reference to a forward
scanning sonar system and a forward scanning sonar method for simplicity, it
will become
evident to those skilled in the art that the embodiments of the invention are
not limited thereto,
but are also adaptable for implementing a rearward scanning sonar system and a
rearward
scanning sonar method by reversing the movement of the sonar transducers in
the direction
opposite to the forward moving direction indicated by the block arrow in
Figures la to id, 2b,
and 2c. Furthermore, the preferred embodiments may also be employed for side
scan
applications.
Referring to Figures 1 a to ld, a forward scanning sonar system 100 and a
forward scanning sonar
method according to a preferred embodiment of the invention are provided. The
forward
scanning sonar system 100 comprises a port sonar transducer 102p and a
starboard sonar
transducer 102s mounted to a support structure such as, for example, a
submersible glider, using
standard underwater technologies known to one skilled in the art. The sonar
transducers 102p,
102s and the support structure are configured such that longitudinal
extensions of the sonar
transducers 102p, 102s form an angled forward triangle. While during scanning
operation the
sonar transducers 102p,102s are moved along a forward moving direction 10.1 ¨
indicated by the
block arrow in Figures la to lc, fan-shaped beams 104p, 104s transmitted from
the sonar
transducers 102p, 102s are forming two planes oriented forwardly downwardly
such that the fan-
shaped beams 104p, 104s form scan lines 106p, 106s oriented at a scan angle ri
' to a vertical
projection 10.2 of the forward moving direction 10.1 onto the sea floor 12
with the scan angle ii'
being greater than 0 and smaller than 7c/2. The fan-shaped beams 104p, 104s
intersect each other
along intersecting line 108 ¨ which is angled at angle a to the forward moving
direction 10.1 or
its vertical projection 10.2, crosses the sea bottom floor at intersecting
point F of the scan lines
106p, 106s - such that the intersecting point F of the scan lines 106p, 106s
is ahead of the sonar
transducers 102p,102s in the forward moving direction 10.1, 10.2.
As illustrated in Figure lb, the sonar transducers 102p, 102s are located at
point A which is at
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distance h - between points A and G - above the sea floor 12. The distance r ¨
between points F
and G ¨ is the forward horizontal range to the intersecting point F. The
distance s ¨ between
points I and K ¨ is combined lateral swath provided by the transducers 102p,
102s. Angle a ¨
between lines FA and FG ¨ is the altitude of the transducers 102p, 102s as
seen from the focal
point F; it is noted that angle a is a function of angles co and 0, a = fico,
0). It is also noted that
range r is proportionate to h and inversely proportionate to a as r =
h/tan(a), with smaller a
yielding longer range r at given depth. It is also noted that parameters r and
s are in an inverse
relationship: a longer r leads to a shorter s, and vice versa, whereas the
mapped area r*s is
defined by the signal-to-noise ratio (SNR). The area defined by the points I,
K, B, D, L, M, Q
and N is the gap-free imaged/mapped sea floor area ahead of the sonar
transducers 102p, 102s
which is displayed on a computer monitor, for example, as illustrated in
Figure id. The
imaged/mapped gap-free sea floor area has an aspect ratio of FG/IK or r/s. It
is preferred to
optimize a towards higher s for side scan application, or higher r for forward
scan application,
and anywhere in between for a mixed, side and forward, application.
Following transmission of sonar pulses from the port 102p and the starboard
transducerl 02s, the
processed sonar echo return signals are colour coded based on signal strength,
and provided to a
computer monitor for imaging as angled 'water fall traces' drawn at angles n'
to the forward
direction 10.2 to form a scaled down 2-D image of the mapped area of depth r
and width s
resulting in an undistorted, overlapped, gap-free frontal view of the mapped
area - I, K, B, D, and
F - in front of the sonar transducers 102p,102s as they are moved forward 10.2
at a constant
speed, revealing structures/objects 14.
Figure ld schematically illustrates a snapshot of seabed image with the
monitor area KK'I'I
schematically displaying a forward scan imaging field KDMNQLBI. The sonar
transducer 102p,
102s position is marked by the point G, with the sonar transducers 102p,102s
as being moved
towards the point F along the forward direction 10.2. GF and KI is the swath
range r and width s,
respectively. Darkened areas along the lines KD and TB represent propagation
delays due to
depth h. Areas along the lines MN and LQ may be affected by low SNR. N'NQQ' is
the
overlapped area between port and starboard.
By transmitting a sonar pulse from the one of the port and the starboard sonar
transducer 102p,
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102s and timing the receipt of the echo return on the other sonar transducer,
the forward depth hF
is determined for the front bottom segment along the intersecting line 108 of
the two angled fan-
shaped beams 104p and 104s, hF = c* t *Sin(a)/2, where c is the speed of
sound, t is
propagation delay. This results in anticipated depth profile along the forward
path 10.2. The
depth hF is then displayed, for example, on a subplot 122 or as image overlay
124, as illustrated
in Figure id.
Preferably, the geometry of the fan-shaped beams 104p, 104s transmitted from
the sonar
transducers 102p, 102s is exploited using advanced graphics processes to
improve visualization
and readability of the displayed sonar images.
Referring to Figure 1 e, as the sonar moves along path 10.1 and transmits two
consecutive pings
at points Li and L2 in the proximity to vertical pole 16 of height Ah
positioned at point Ti. Both
pings hit the target 16 at two different bearings 131 and 132, 132> 131. If
conditions (1), (2) hold true,
13 5_ 7E/2 -17' (1)
AL * tan(a) (2)
then both pings hit the target 16 at two different slant ranges R1 and R2, so
that R2 < R1 :
R2 = VR? ¨ 2R1ALcos131+ (AL)2 ;
131= 0: R1- R2 = AL;
131= 762: R1- R2 = 0.
Because of the range difference AR, echo returns are displayed as two separate
targets at points
Ti and T2 which results in a line T1-T2 with the length depending on the
bearing angle 13 to the
target. All conditions equal, the line Ti-T2 of targets of the same height
will be displayed the
longest at bearing angle 13 = 0 (direct forward scanning) and be zero at
bearing angler. =7E/2
(direct side scanning), with targets at other bearing angles 13 being of
intermediate length. This
phenomenon is exploited using the fan-shaped beams 104p, 104s at bearing
angles 13> 0 and
advanced graphics processes for better visualizing vertically oriented targets
by displaying
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vertically oriented lines as lines and vertically oriented planes as planes.
Furthermore, line Ti-Ti' is an extension of Range R1 and corresponds to the
cast shadow from
the pole 16, which may also be displayed using the advanced graphics processes
to further
improve visualization in combination with the use of the fan-shaped beams
104p, 104s at bearing
angles 0 <13 < n/2. It is noted that cast shadows are always cast away from
the sonar source.
Condition (1) sets general limitation on bearing angle during forward
scanning: it shows that
wider scan angle n' leads to a narrower field of view to avoid detection
ambiguity, and vice
versa.
Condition (2) provides theoretical threshold criteria on target height during
forward scanning
with angled beams, it shows that the smaller forward looking angle a is, the
smaller target
height Ah can be detected. Practical value of Aft will be further limited by
sonar directivity and
signal-to-noise ratio as described by standard sonar equations.
Figure 2a illustrates a state of the art sonar transducer 102 comprising an
elongated and
streamlined housing 102.1 having disposed therein a transmit/receive sonar
transducer element
102.2. RF power and data transfer is enabled via cable 102.3. The transducer
element 102.2
transmits sonar pulses forming a fan-shaped beam 104 - having beam spread y ¨
in a plane
oriented substantially perpendicular to the longitudinal extension 1 of the
sonar transducer 102. In
an example implementation of the forward scanning sonar system 100 high
resolution side scan
sonar transducers - Jetasonic 1240 PX ¨ having length 1 of 30" and beam
spread y of 600 have
been employed.
Figure 2b illustrates a first arrangement of the sonar transducers 102p, 102s
for realizing the
forward scan sonar system 100 described hereinabove using the sonar transducer
102 illustrated
in Figure 2a. The sonar transducers 102p, 102s are arranged forming angled
forward descending
triangle AGD oriented rearwardly downwardly at angle 0 to the forward
direction 10.1 -
indicated by the block arrow. Line BC represents base distance b between
transducers 102p and
102s . The port sonar transducer 102p is oriented towards port at angle cop to
the vertical plane
containing the forward direction 10.1, with cop being greater than 0 and
smaller than 7c/2, and the
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starboard sonar transducer102s is oriented towards starboard at angle gos to
the vertical plane
containing the forward direction 10.1, with cos being greater than 0 and
smaller than 7r12.
Preferably, the port sonar transducer 102p and the starboard sonar transducer
102s are of the
same length land oriented rearwardly downwardly at a same angle 0 and the port
angle and the
starboard angle are a same angle yE, = cos = go/2 resulting in an angled
forward, descending
isosceles triangle.
If distance b and transducer length land are small compared to the forward
range r. b <<r and
1<< r, the position and orientation of the fan beams 104p, 104s is then
defined by a set of three
angles (cp, 0, y), based on the geometries illustrated in Figures lb and 2b.
The arrangement, as
illustrated in Figure 2b, creates two converging fan beams 104p, 104s which
are angled
forwardly downwardly and intersect each other along the line 108.
Figure 2c illustrates a second arrangement of the sonar transducers 102p, 102s
for realizing the
forward scan sonar system 100 described hereinabove using the sonar transducer
102 illustrated
in Figure 2a. The sonar transducers 102p, 102s are arranged forming angled
forward ascending
triangle ACE oriented forwardly upwardly at angle 0 to the forward direction
10.1 -- indicated by
the block arrow. The port sonar transducer 102p is oriented towards port at
angle cop to the
vertical plane containing the forward direction 10.1, with cop being greater
than 0 and smaller
than n/2, and the starboard sonar transducer102s is oriented towards starboard
at angle gos to the
vertical plane containing the forward direction 10.1 with gos being greater
than 0 and smaller than
n/2. Preferably, the port sonar transducer 102p and the starboard sonar
transducer 102s are of the
same length land oriented forwardly upwardly at a same angle 0 and the port
angle and the
starboard angle are a same angle cop = gos = g9/2 resulting in an angled
forward, ascending
isosceles triangle. Line AD represents base distance b between transducers
102p and 102s .
If the same conditions apply, b <<r and 1 << r, the position and orientation
of the fan beams
104p, 104s is then defined by a set of three angles (go, 0, y), based on the
geometries illustrated in
Figures lb and 2c. The arrangement illustrated in Figure 2c creates two
converging fan beams
104p, 104s which are angled forwardly downwardly and intersect along the line
108.
By varying the angles y, go and 0, a wide range of aspect ratios r/s is
achieved. For example, for
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y = = 600 and 0 = 150 a mapped seabed area es = 38*26 m2 per every 10m
of water column is
achieved while a= 15.8 and the scan angle ii' = 24.10. It is noted that, a
higher r/s ratio is
preferred for a long range forward scan application, and a lower r/s ratio is
preferred for a wide
swath side scan sonar application.
Optionally, phased arrays may be used instead of fixed beams to vary the
bearing of the beam
intersect enabling multiple depth readings across the mapped field.
Further optionally, the orientation of the port and starboard sonar
transducers may be different,
resulting in an asymmetrical field of view.
Further optionally, more than two sonar transducers may be employed, added in
pairs, for
example, with each pair of sonar transducers having its own orientation go, 0,
and spread y.
Further optionally, only one sonar transducer may be employed for imaging,
creating an
asymmetric field of view and at the loss of up to 50% of data. It is noted
that depth profiling
requires at least two intersecting beams.
The sonar transducers 102p, 102s may be incorporated into respective leading
edges 22p, 22s of
wings 20p, 20s of various underwater vehicles such as, for example, a
submersible glider, a
towfish, or a submarine, as illustrated in Figures 3a to 3c, respectively. It
is noted that in Figures
3a to 3c the leading edges 22p, 22s are oriented rearwardly downwardly
allowing implementation
of the arrangement illustrated in Figure 2b.
Alternatively, the wings 20p, 20s are oriented upwardly enabling orientation
of the leading edges
22p, 22s forwardly upwardly for implementing the arrangement illustrated in
Figure 2c.
Preferably, the sonar transducers 102p, 102s have a streamlined front enabling
seamless
incorporation into the leading edges 22p, 22s.
The sonar transducers 102p, 102s may also be mounted to a keel or respective
port and starboard
hull sections 30p, 30s of a surface vessel, as illustrated in Figure 3d, for
example, implementing
the arrangement illustrated in Figure 2c.
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It is noted, that Figures 3a to 3d illustrate only examples for deploying the
forward scanning
sonar system 100 but is not limited thereto, and that it will become evident
to those skilled in the
art that the embodiments of the invention are not limited thereto, but may be
deployed in various
other ways such as, for example using a boom mounted to various types of
marine vessels.
Besides transducers, the forward scanning sonar system 100 uses standard
system blocks that can
be found, by way of example, in side scan sonar systems such as, among others,
tuning networks,
power amplifier, analog front end (AFE), A/D and D/A converters, digital
signal processor
(DSP), field-programmable gate array (FPGA), communication ports, top side PC
computer,
sensors (compass, GPS, pressure, pitch/roll), and may include various
firmware, middleware and
software. For use with the forward scanning sonar system, a graphic user
interface (GUI) and
advanced visualization software for high-resolution, gap-free imaging and
forward profiling has
been designed using standard computer and programming technologies known to
one skilled in
the art.
Signal generation and data acquisition in the forward scanning sonar system
100 is performed in
a way that can be found in a side scan sonar. For example, using pulse
compression port imaging
data are determined in dependence upon port sonar return signals and starboard
imaging data are
determined in dependence upon the starboard sonar return signals and passed on
to a topside PC
via communication port. The port imaging data and the starboard imaging data
are then
combined and displayed on a computer monitor by the visualization software as
a gap-free, range
calibrated, imaged and profiled dataset ahead of the sonar as illustrated in
Figure ld.
The forward imaging process is performed as follows. A port sonar transducer
102r and a
starboard sonar transducer 102s mounted to a support structure are moved along
a forward
moving direction 10.1. While moving along the forward moving direction 10.1,
the sonar
transducers 102p, 102s transmit sonar pulses in the form of fan-shaped beams
104. The sonar
pulses may be transmitted AM or FM modulated, or a combination thereof. The
fan-shaped
beams 104 of the sonar transducers form converging beam planes oriented
forwardly
downwardly and at a scan angle to the forward moving direction 10.1 with the
scan angle being
greater than 0 and smaller thann/2 and intersect each other. While moving
along the forward
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moving direction, the port sonar transducer 102p receives port sonar return
signals from the port
or starboard sonar pulses and the starboard sonar transducer 102s receives
starboard sonar return
signals from the starboard or port sonar pulses depending on user controls and
transducer
configuration. The port sonar return signals and the starboard sonar return
signals are received,
converted into port sonar return data and starboard sonar return data,
respectively, or vice versa,
and provided to a DSP processor or FPGA. Using the processor, port imaging
data are
determined in dependence upon the port sonar return data and starboard imaging
data are
determined in dependence upon the starboard sonar return data. The port
imaging data and the
starboard imaging data are then combined and passed on to a topside computer
for real time
visualization, playback, and storage.
When using sequential data acquisition and processing, the port sonar
transducer 102p and the
starboard sonar transducer 102s are configured to transmit the sonar pulses in
an alternating
fashion while moving along the forward moving direction 10.1 to enable forward
profiling by
metering propagation delay of side-to-side pulses, establishing the direction
of movement and
generation of geo-referenced, gap-free imaging when combining the port and the
starboard
imaging data along the forward path 10.2. However, the alternating
transmission of the port and
starboard sonar pulses reduces the resolution of the generated image compared
to simultaneous
transmission of the port and starboard sonar pulses, thus requiring reducing
the speed of the
movement of the sonar transducers to obtain the same resolution.
Referring to Figure 4a, a preferred data acquisition/processing system and
method for use in the
forward scanning sonar system 100 is provided, enabling generation of gap-free
images while the
port and starboard sonar pulses are transmitted simultaneously. The system
comprises two
channels ¨port (1) and starboard (2) ¨ interposed between the respective sonar
transducers 102p,
102s and processor 152 such as, for example, a FPGA, with both channels having
similar
components. The two channels are synchronized and operable in simultaneous
fashion. Each
channel is of standard design and comprises, for example, the following
components: D/A
convertor (DAC); High-voltage transmitter (Tx); T/R switch; Amplifiers (Gain);
Low-pass filter
(LPF); A/D converter (ADC), digital Demodulator (Dem); and serializer (not
shown). The
components of the two channels and the processor 152 are, for example,
disposed on a Printed
Circuit Board (PCB) 150.
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The AID converters sample incoming data at a sampling rate Fs that is
sufficient for non-aliased
envelop reconstruction. The port and starboard transducers 102p, 102s may
employ a common (as
shown) or split transmitter/receiver but otherwise have close characteristics
including electrical
impedance, Transmit Voltage Response (TVR), Open-Circuit Voltage (OCV)
sensitivity, beam
directivity and operating frequency bands. Waveform 1 and Waveform 2 pattern
generators
generate waveforms that differ in the time domain, but have matched
characteristics in the
frequency domain. Preferably, the waveforms have mirrored impulse responses,
such as linear
up- and down- chirp signals of the same duration and rate, and occupy the same
frequency band.
Alternatively, transducers may have unmatched frequency responses to operate
with AM or FM
modulated waveforms that occupy parted frequency bands.
Following transmission of the frequency modulated sonar pulses, the channels
wait for their
respective sonar return signals. Upon receipt, each channel converts the
respective sonar return
signals into sonar return data and provides the same to the processor 152. The
processor 152
performs correlation/matched filtering (C) of the input frames using overlap-
add or overlap-save
techniques before multiplexing (Mux) and transmitting the filtered and down-
converted data to
the top computer via common interface link.
At the same time, the port and starboard sonar return data are cross-
correlated (XC) between
port-starboard data and starboard-port data followed by hit-crossing modules
at (XCorr). Cross-
correlating between the mirrored pulses increases the SNR without compromising
port/starboard
performances, thus leading to detection of sonar returns along the common
intersecting line 108
of the two beams 104p and 104s, while both sonar sides operate concurrently.
Both cross-
correlations (Xcor 1, Xcor2) yield similar results for simultaneous sonar
pulses and can be
averaged, enabling measuring and 2-D plotting of the anticipated depth profile
along the forward
path 10.2, as well as establishing the direction of movement for correct
overlay of the combined
port/starboard imaging data along the forward path 10.2. The anticipated depth
profile is
calculated along the intersecting line 108 of the two fan beams 104p and 104s
extended in the
angled forward direction as the sonar moves on.
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Based on, for example, a hit-crossing control threshold, the crossover points
of the two
overlapping data sets are determined in the hit-crossing modules in order to
reduce data
redundancy and to form limiting bounds for geo-referenced, gap-free overlay of
the port and
starboard imaging data in the forward half-plane.
Figures 4b and 4c illustrate the plotting of the traces as the sonar ¨
indicated by black dots - is
moved along the forward path 10.2 as indicated by the block arrow.
In forward scan setup (forwardly angled traces), the port (P) and starboard
(S) data is plotted
from the hit-crossing bound (forward path 10.2) and down to the first
received, as indicated by
the dashed block arrows in Figure 4b. Data beyond the bound is redundant but
can be used to
extend the forward range.
In side scan setup (inverted traces), the data is plotted from the hit-
crossing bound (forward path
10.2) on, as indicated by the dashed block arrows in Figure 4c. Data before
the bound has limited
usage.
Figures 4d to 4f illustrate in an example frequency modulated and match-
filtered sonar return
data, data after cross-correlation, and the result of the hit-crossing module
after cross correlation,
respectively, for port side. The cross-correlation filter blocks any return
signals except breaking
through via beam intersection, and then finds sample number Np (in this case
Np= 146847)
corresponding to the forward direction by applying a threshold above the noise
level, which is
done by the hit crossing module detecting when the input reaches the threshold
offset parameter
value. The same process is applied to the starboard side to find sample number
Ns, with ideally
Ns=Np=n0 for two concurrent sonar pulses.
At the same time, anticipated forward depth ahead of the sonar is calculated
as
11¨(c*n0)/(2*Fs)*sin(a), where c is speed of sound, Fs is sampling rate and a
is the angle
between beam intersection line 108 and horizontal plane. In the above example
c=1500 m/s,
Fs=4MHz and a =17 , then h=8.05 meters. Horizontal range r to the depth-
measured segment of
sea bottom is given by r=h/tan(a)=26.33 meters. The depth profiling is
performed
simultaneously with the imaging process above.
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Optionally, the imaging process is omitted and the forward scanning sonar
system 100 is
employed for depth profiling of the path ahead for obstacle avoidance, for
example, for use with
surface vessels and submarines.
While the above description of the preferred embodiments of the forward
scanning sonar system
100 is with reference to a sonar transducer 102 having only one
transmit/receive sonar transducer
element, it will become evident to those skilled in the art that the
embodiments of the invention
are not limited thereto, but may employ sonar transducers having more than one
transmit/receive
element operating in the same or different frequency bands or separate
transmitter and receiver
elements (for example, one transmitter element and two or more receiver
elements), as long as
they are placed in close proximity to each other, configured in angled
triangular formation and
produce fan-shaped beams as illustrated in Figures 2a to 2c.
Referring to Figures 5a and 5b, the forward scanning sonar system 100 and the
forward scanning
sonar method associated therewith is adapted for performing 3D bathymetry to
measure
directions to targets T and determine 3D images of the sea bottom as the sonar
moves along path
10.1. Here, the sonar transducers 102 comprise two or more parallel sensor
elements 103.1,
103.2, each producing a fan beam such that all beams are overlapped. To avoid
angular
ambiguity, the sensor elements 103.1, 103.2 are acoustically isolated and the
distance DSE
between the sensor elements 103.1, 103.2 is DSE < = 212, where X is the
acoustic wavelength. It is
noted that Figure 5a is not drawn to scale, i.e. 1>>DsE.
A reflected sonar signal at a distance from its source ¨ target T ¨ can be
considered to have a
plane wave-front W, enabling determination of the angle at which the signal is
radiating with
respect to the sonar transducer 102 based on the time delay between the
arrivals of the reflected
sonar signal at the sensor elements 103.1, 103.2. Since one sensor element
103.1 is closer to the
source ¨ target T ¨ than the other, the reflected sonar signal received by the
more distant sensor
element 103.2 is delayed by the time At. Hence, the angle is = arcsin (d/b),
where d = c * At,
with c being the speed of sound in water.
Alternatively, there could be more than two parallel elements with unequal
spacing and
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coincided acoustic beams to resolve angles of arrival within common acoustic
plane by method
of triangulation resulting in 3D imaging of the target locations as the sonar
is moved along.
Alternatively, 3D images can be determined based on the signals provided by
the two or more
sensor elements 103.1, 103.2 using processing techniques implemented for
bathymetry sonar as
disclosed, for example, by Paul Kraeutner and John Bird in US Patent 6,130,
641.
Unlike conventional bathymetry sonar, the angled beam forward scanning sonar
system 100
attacks targets T at an angle to the forward direction, which has the
advantages of minimizing the
surface backscatter. A further advantage is that the angled sonar beams lead
to hitting targets T
multiple times as the sonar system 100 progresses in the forward direction,
thus compounding
detection at various angles.
The present invention has been described herein with regard to preferred
embodiments.
However, it will be obvious to persons skilled in the art that a number of
variations and
modifications can be made without departing from the scope of the invention as
described
herein.
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