SYNTHETIC ANTENNA SONAR AND METHOD FOR FORMING
SYNTHETIC ANTENNA BEAMS
The present invention relates to the field of sonar imaging and to
5 the detection and classification of objects by means of a synthetic aperture
sonar. It more particularly relates to mine warfare and to the detection and
classification of mines by means of a synthetic aperture sonar.
The question of classification of objects is a problem that is difficult
to solve especially for bottom mines placed on a textured seabed, or for
10 stealth mines. The use of a sonar in a very-high-resolution side-scan
synthetic-aperture-sonar mode is one response to this problem, but one that
remains unsatisfactory. By "side-scan sonar" what is meant is a sonar that
emits acoustic pulses along a sighting axis having a bearing angle
substantially equal to 90°, i.e. that is substantially perpendicular to the path of
15 a carrier on board of which the sonar is installed. The sonar is positioned on
one of the sides of a fish or carrier that is submerged. The carrier may be
autonomous or towed by a surface vessel. By "very-high-resolution", what is
generally meant is a resolution lower than 10 em for a sonar having a
frequency higher than 100 kHz.
20 The aim of synthetic aperture sonar is to improve resolution, at a
given range, without increasing the linear dimension of the receiving antenna.
The principle of synthetic aperture sonar consists in using a physical antenna
formed by a linear array of N transducers. In this type of sonar, during the
advance of the carrier, an emitting device, or emitting antenna, emits P
25 successive pulses in an elementary sector that remains stationary with
respect to the carrier. The signals received by the N transducers of the
physical receiving antenna at P instants, and therefore in P successive
locations, are used to form the beams of the synthetic antenna. The
resolution of the images obtained, i.e. the resolution of the beams of
30 synthetic antennae, is substantially equivalent to that of a virtual antenna the
length of which corresponds to about twice the length travelled by the
physical antenna during these P successive instants.
The beams of the synthetic antenna are constructed by a method
for processing the backscattered signals measured by the antenna, which
35 method is called a "synthetic aperture processing method". This type of
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method is known in the art. To form a beam of the synthetic antenna of the
sonar, the signals measured by the receiving antenna are added using
delays corresponding to the direction of the formed beam and to the various
locations of the transducers of the antenna, which locations depend on the
5 positions of the transducers in the physical antenna and on the movement of
the latter.
The main difficulty experienced when applying the syntheticantenna
principle resides in the determination of the delays to be used in the
beam formation. Whereas with a conventional antenna these delays depend
10 only on the distance and direction of the sighting point, those of a synthetic
antenna depend on the movement of the carrier during the formation time.
The longer this formation time, i.e. the higher the number of pings, which
goes hand-in-hand with a better resolution, the more difficult it is to determine
these delays.
15 The basis of the patent application having the publication number
FR 2769372 is the observation that the precision required in the
measurement of the position of the receiving antenna is not obtainable with
an inertial navigation system (INS) because the error in the measurement of
the spatial position of a carrier equipped with an INS is too large. Moreover,
20 in this application it is observed that, in what are called autocalibrating or
autofocusing processes, which allow the position of the antenna to be
obtained by processing of the various signals measured by the antenna, the
precision of the angle of rotation of the antenna between two pings is the
factor limiting the precision of the process. To remedy these drawbacks, it
25 therefore proposes a method for correcting for the effects of parasitic
antenna movements in a synthetic aperture sonar, i.e. for correcting for
effects due to angular variations of the antenna, in which method a synthetic
antenna is formed over M pings of the sonar and the variations in the
movement of the physical antenna are corrected for by carrying out an
30 autocalibration by intercorrelation of the successive pings using a
measurement of the rotation of the antenna, which measurement is obtained
by means of a gyrometer, and by measuring the elevation angle of the
backscattered signal with an auxiliary antenna that is perpendicular to the
physical antenna. This method allows the resolution of the sonar image
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obtained by way of the synthetic aperture beams to be substantially
improved.
The method described in the patent application having the
publication number FR 2769372 needs the sonar to be equipped with an
5 auxiliary antenna that is perpendicular to the physical antenna.
One aim of the present invention is to provide a sonar that makes
it possible to omit an auxiliary antenna.
To this end, one subject of the invention is a synthetic aperture
sonar intended to move along a first axis, the sonar comprising an emitting
10 device configured to emit, in each ping, at least one acoustic pulse toward an
observed zone in a set of sectors comprising at least one sector, the sonar
comprising a first physical receiving antenna extending along the first axis
allowing measurements of backscattered signals generated by said pulse to
be acquired and a processing device configured to form, over R pings, for
15 each sector of the set of sectors, synthetic aperture beams from
measurements of signals backscattered by the observed zone and generated
by acoustic pulses emitted in said sector, the sonar comprising at least one
gyrometer, characterized in that said processing device is configured so as to
correct for variations in the movement of the first receiving antenna during
20 the formation of the synthetic aperture beams of said set of sectors by
carrying out an autocalibration by intercorrelation of the successive pings
using measurements of rotation of the first receiving antenna, which
measurements are obtained with said at least one gyrometer, and using
estimations of the elevation angles of the backscattered signals to determine
25 image planes of the backscattered signals and to project said rotation
measurements onto said image planes, the projections obtained being used
to carry out the autocalibration, and in which, during the formation of the
synthetic aperture beams of at least one sector of the set of sectors, which
sector is called the bathymetric sector, estimations of elevation angles of
30 backscattered signals are used, said estimations being obtained from a
bathymetric chart comprising the three-dimensional positions, defined in the
terrestrial reference frame, of a plurality of points of the observed zone.
Advantageously, the emitting device is configured to emit, in each
35 ping, in different respective sectors comprising a first sector and at least one
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second sector, distinguishable acoustic pulses toward an observed zone,
along a first sighting axis and a second sighting axis having different bearing
angles, respectively, wherein said at least one bathymetric sector comprises
at least one second sector, and wherein the bathymetric chart is obtained
s from measurements of first elevation angles of first backscattered signals
generated by acoustic pulses emitted in said first sector.
The sonar may comprise an array of transducers comprising a
plurality of transducers distributed along a second axis perpendicular to the
10 first axis, said transducers forming the array of transducers being
dimensioned and configured so that their receiving lobes cover the first
sector but so that said at least one second sector is located at least partially
beyond their receiving lobes, the first backscattered signals being acquired
by means of the array of transducers.
IS
More precisely, the physical receiving antenna may comprise a
first elementary physical antenna formed from first transducers dimensioned
and configured so that their receiving lobes cover the first sector but so that
said at least one second sector is at least partially located beyond their
20 receiving lobes. The sonar comprises a second elementary physical antenna
formed from second transducers dimensioned and configured so that their
receiving lobes cover the first and second sectors. The processing device is
configured so as to form, during the formation of the synthetic aperture
beams, beams of a first synthetic antenna from measurements of first
25 backscattered signals generated in the first sector and acquired by means of
the first elementary antenna, and beams of a second synthetic antenna from
measurements of second backscattered signals generated by pulses emitted
in said second sector and acquired by means of the second elementary
antenna.
30
35
Advantageously, the array of transducers is formed by the first
elementary antenna and another antenna that is identical to the first
elementary antenna and superposed on the first elementary physical antenna
along the second axis.
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Advantageously, the bathymetric chart is stored in a memory of
the sonar before the observed zone is imaged.
Another subject of the invention is a sonar system comprising the
5 sonar of the invention, and a carrier, the sonar being installed in the carrier.
Another subject of the invention is a method for forming synthetic
aperture beams of a sonar over R pings of the sonar, the sonar being
intended to move along a first axis, the sonar comprising an emitting device
10 configured to emit, in each ping, at least one acoustic pulse toward an
observed zone in a set of sectors comprising at least one sector, the sonar
comprising a first physical receiving antenna extending along the first axis
allowing measurements of backscattered signals generated by said at least
one pulse to be acquired and a processing device configured to form, over R
15 pings, for each sector of the set of sectors, synthetic aperture beams from
measurements of signals backscattered by the observed zone and generated
by acoustic pulses emitted in said sector, the sonar comprising at least one
gyrometer, the method comprising a forming step in which, for each sector
over R pings, synthetic aperture beams are formed from measurements of
20 signals backscattered by the observed zone and generated by acoustic
pulses emitted in said sector, in which variations in the movement of the first
receiving antenna during the formation of the synthetic aperture beams of
said set of sectors are corrected for by carrying out an autocalibration by
intercorrelation of the successive pings using measurements of rotation of
25 the first receiving antenna, which measurements are obtained with said at
least one gyrometer, and using estimations of the elevation angles of the
backscattered signals to determine image planes of the backscattered
signals and to project said rotation measurements onto said image planes,
the projections obtained being used to carry out the autocalibration, and in
30 which, during the formation of the synthetic aperture beams of at least one
sector of the set of sectors, which sector is called the bathymetric sector,
estimations of elevation angles of backscattered signals are used, said
estimations being obtained from a bathymetric chart comprising the threedimensional
position of a plurality of points of the observed zone.
35
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Advantageously, the emitting device is configured to emit, in each
ping, in different respective sectors comprising a first sector and at least one
second sector, distinguishable acoustic pulses toward an observed zone,
along a first sighting axis and a second sighting axis having different bearing
5 angles, respectively. Said at least one bathymetric sector comprises at least
one second sector. The bathymetric chart is obtained from measurements of
first elevation angles of first backscattered signals generated by acoustic
pulses emitted in said first sector.
10 Advantageously, the sonar comprises an array of transducers
comprising a plurality of elementary transducers distributed along a second
axis perpendicular to the first axis, said transducers forming the array of
transducers being dimensioned and configured so that their receiving lobes
cover the first sector but so that said at least one second sector is located at
15 least partially beyond their receiving lobes, the first backscattered signals
being acquired by means of the array of transducers, the method
advantageously comprising, for each ping, a step of measuring first elevation
angles offirst backscattered signals by means of the array of transducers, a
step of calculating estimations of first elevation angles, consisting in
20 transposing the measurements of first elevation angles to a terrestrial
reference frame. The method furthermore comprises a step of producing the
bathymetric chart from the estimations of the first elevation angles, the
bathymetric chart comprising three-dimensional coordinates, in the terrestrial
reference frame, of probe points having backscattered the first backscattered
25 signals.
Advantageously, the method comprises a step of estimating, from
the bathymetric chart, the elevation angles of the backscattered signals
generated by pulses emitted in said bathymetric sector. The method
30 comprises, for each of the backscattered signals, a step of calculating the
position of that point Mp of the bathymetric chart which is closest to a section
of a circle Cp obtained by rotating, about the first axis, a point B located on
the other sighting axis at a distance from the antenna corresponding to the
distance separating the antenna from a probe point having generated the
35 backscattered signal, a step of calculating a first point of intersection lp
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between the bathymetric chart and the section of the circle Cp on the basis of
the closest point Mp, and a first step of calculating, in the terrestrial reference
frame, the elevation angle of the point of intersection .
. 5 The point of intersection lp may be the point of intersection
between a horizontal plane, in the terrestrial reference frame, passing
through the closest point Mp, and the section of the circle Cp.
Advantageously, the method comprises a second step of
10 calculating a second point of intersection lp between the bathymetric chart
and the section of the circle Cp on the basis of the closest point Mp and other
points of the bathymetric chart, and, if a second point of intersection is
obtained, a second step of calculating the elevation angle of the second point
of intersection.
15
The physical receiving antenna may comprise a first elementary
physical antenna formed from first transducers dimensioned and configured
so that their receiving lobes cover the first sector but so that said at least one
second sector is at least partially located beyond their receiving lobes. The
20 step of forming beams then comprises a step of forming beams of a first
synthetic antenna from measurements of backscattered signals generated by
pulses emitted in said first sector and acquired by means of the first
elementary antenna, in which step the estimations of backscattered signal
elevation angles used to determine the image planes of the backscattered
25 signals and to project said rotation measurements onto said image planes
are estimations of first elevation angles of the first backscattered signals, the
first backscattered signals being generated by pulses emitted in said first
sector, the estimations of the first elevation angles being transpositions of the
measurements of the first elevation angles into the terrestrial reference
30 frame.
35
The last subject of the invention is a computer program product
comprising programming code instructions for executing the steps of the
method according to the invention when the program is run on a computer.
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The proposed invention makes it possible to omit the auxiliary
antenna. It also makes it possible not to decrease the resolution of the
synthetic aperture sonar, i.e. without decreasing its detection and
classification capabilities. In other words, the obtained resolution is
5 comparable to that obtained by means of the method described in document
FR 2769372, i.e. to the resolution obtained by means of a method using an
autofocusing process in which the parasitic movements of the antenna are
corrected for.
10 Other features and advantages of the invention will become apparent
15
20
25
on reading the following detailed description, which is given by way of
nonlimiting example and with reference to the appended drawings, in which:
- figure 1 schematically shows the components of an exemplary
sonar according to the invention;
- figure 2 schematically shows the sonar in figure 1 installed on a
carrier as seen from above during the emission of acoustic
pulses in three sectors;
- figure 3 schematically shows, from the side, the first physical
receiving antenna and a second receiving antenna of the sonar
in figure 1;
- figure 4 schematically shows an elevation angle of a signal
backscattered by a target A, such as calculated and used in
the method according to invention;
- figure 5 shows a block diagram of an exemplary method
according to invention;
- figure 6 schematically shows the construction of the bathymetric
chart; and
- figure 7 schematically shows the calculation of the position of
probe points for the second elevation angle.
30 From one figure to the next, the same elements have been referenced
by the same references.
The invention relates to a mono-aspect or multi-aspect synthetic
aperture sonar. By "mono-aspect synthetic aperture sonar", what is meant is
35 a synthetic aperture sonar intended to move along a first axis, the sonar
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comprising an emitting device configured to emit, in each ping, an acoustic
pulse toward an observed zone in a single sector, the sonar comprising a first
physical receiving antenna allowing measurements of backscattered signals
generated by said pulse to be acquired and a processing device configured
5 to form, over R pings, synthetic aperture beams from measurements of
signals backscattered by the observed zone, which signals are generated by
acoustic pulses emitted in said sector. The acoustic pulses are emitted along
a single sighting axis in a single sector surrounding the sighting axis. This
sighting axis may be attached to the antenna or be directed in a fixed
10 direction in the terrestrial reference frame, for example by means of a
stabilizing device. By "sector in which an acoustic pulse is emitted", what is
meant is the sector of -3 dB aperture in which the main lobe of the emitted
acoustic pulse is emitted.
The performance of a mono-aspect synthetic aperture sonar
15 proves to be unsatisfactory in the step of classifying objects detected in sonar
images. By "classifying", what is meant is the characterization of the nature of
the object detected in the image (such as for example its size and/or its
shape or even the characterization of the object as a mine or not a mine). In
order to improve classification performance, the viewpoints of the detected
20 objects are multiplied. The greater the number of observations of a given
object at different angles, the easier it is to classify this object. To multiply
viewpoints, one solution consists in using a multi-aspect synthetic aperture
sonar. This solution does not require the sonar to be passed over the
observation area a plurality of times along different paths. It moreover has a
25 low power consumption. Therefore, this solution is suitable for installation on
autonomous submarine vessels. It does not require the absolute position of
the carrier to be known with high precision or registration techniques to be
implemented in order to associate various views of a given object together. It
furthermore makes it possible to improve the rate of detection of objects in
30 sonar images.
Figure 1 shows the constituent components of an exemplary sonar
1 according to the invention. In this example, the sonar is a multi-aspect
sonar. It comprises an emitting device 2, comprising one or more emitting
35 antennae. The emitting device 2 is configured so as to emit, in each ping,
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acoustic pulses toward an observed zone, for example a seabed. The pulses
emitted in one ping are emitted in a set of sectors comprising a plurality of
sectors. In each ping, the emissions emitted in the respective sectors are
distinguishable. For example, the pulses emitted in the respective sectors are
5 emitted with carriers that are distinct from one another, i.e. located in
separate frequency bands. As a variant, the pulses are emitted with carriers
having one and the same carrier frequency but are distinguished from one
another by orthogonal codes, i.e. by orthogonal modulations. The signals
backscattered by the seabed and originating from the various sectors are
10 then distinguishable in the same way as the pulses emitted in these various
sectors, for example by filtering or by demultiplexing. In each ping, the pulses
emitted in the various sectors are, for example, emitted simultaneously or
substantially simultaneously.
Figure 2 shows the sectors S 1, S2 and S3 in which the emitting
15 device 2 of the sonar according to the invention emits the acoustic pulses in
each ping. The emitting device 2 emits 3 pulses in three respective sectors
S1, S2, S3 in each ping along the respective sighting axes v1, v2, v3. The
sonar 1 is intended to move along a first axis X1 during the emission of the
acoustic pulses in the successive pings. The sonar 1 is mounted on a carrier
20 PO. In the embodiment in the figure, the first axis X1 is parallel to the
direction X of movement of the carrier PO. The sighting axes v1, v2, v3 make
different respective bearing angles 81, 82, 83 to the first axis X1.
Advantageously, the sighting axes v1, v2, v3 have the same elevation angle,
the elevation angle being defined in the terrestrial reference frame. As a
25 variant, the sighting axes have the same what is called local elevation angle
in the reference frame associated with the sonar, i.e. they make the same
angle to a plane parallel to the axis X1 and perpendicular to the plane formed
by the active areas of the transducers.
The sighting axes v1, v2, v3 comprise a lateral sighting axis v1
30 that is substantially perpendicular to the first axis X1, and two additional
sighting axes v2 and v3 that are symmetric with one another about a plane of
symmetry that is perpendicular to the direction of advance X and that passes
through the axis v1. In other words, the bearing angle 81 of the axis v1 is
equal to 90°. It will be called the lateral sighting axis below. The axes v2 and
35 v3 for example make bearing angles 8s to the axis v1 of -35° and 35°,
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respectively. As a variant, the additional sighting axes are not symmetric with
one another about the plane of symmetry. The axis v2 is called the forward
sighting axis and the sighting axis v3 is called the rearward sighting axis. The
swing-off of the sighting axes v2 and v3 with respect to the first axis v1 is
5 achieved electronically or mechanically. In the latter case, the emitting device
comprises three emitting antennae steered along three different sighting
axes.
In the embodiment in figure 2, the sectors S1, S2, S3 do not
adjoin. The apertures of two adjacent sectors are smaller than the angle
10 made between them in the reference plane. Advantageously, the angular
aperture of each sector is small, i.e. smaller than 1 oo. These characteristics
make it possible to limit backscattered-signal processing cost by limiting the
total size of the insonified sector while maximizing the total effective
clockwise coverage of the sonar. Generally, the aperture of the sectors must
15 be sufficiently large to obtain the desired resolution at the emission frequency
of the sonar. Limiting the width of the insonified sector makes it possible to
limit the elevation between the hydrophones of the first receiving antenna
and therefore their number and cost. As a variant, the sectors adjoin or
partially overlap pairwise. The sectors S1 to S3 for example have the same
20 bearingwise aperture and the same elevationwise aperture. As a variant, the
sectors have different bearingwise and/or elevationwise apertures.
The number of sectors is as a variant different from 3 and for
example equal to 5 or to 2. What is important is that, in each ping, the
emitting device 2 emits distinguishable acoustic pulses in respective sectors
25 comprising at least one first sector and another sector distinct from the first
sector.
The sonar 1 comprises a first physical receiving antenna 3
allowing the signals backscattered by the seabed and generated by the
acoustic pulses emitted in the various sectors in each ping to be measured.
30 The sonar 1 also comprises a processing device 4, for example comprising at
least one computer, configured so as to form the beams of a synthetic
antenna for each of the sectors. In other words, the processing device 4 is
configured so as to form synthetic aperture beams, this consisting in forming,
for each sector, the beams of a synthetic antenna from measurements of
35 backscattered signals generated by acoustic pulses emitted in the sector in
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question, i.e. from measurements of signals backscattered by the observed
zone in the sector in question. The processing device 4 is configured to form,
over R pings, the beams of a first synthetic antenna from measurements of
backscattered signals generated by acoustic pulses emitted in the first sector
5 81, and the beams of at least one other synthetic antenna, the beams of
each other synthetic antenna being formed from measurements of
backscattered signals generated by acoustic pulses emitted in one of the
other sectors. The measurements of backscattered signals used are
measurements carried out by the first receiving antenna 3. In the case shown
10 in figure 2, the processing device 4 therefore forms the beams of three
synthetic antennae, one for each of the sectors 81, 82, 83.
In figure 2, the first receiving antenna 3 is placed on the starboard
side, the emitting device 2 emitting acoustic pulses on the starboard side. As
a variant, the sonar comprises two emitting devices, one emitting port-side
15 and one emitting starboard-side, and two receiving antennae, one port-side
and one starboard-side.
The first receiving antenna 3 is a longitudinal antenna extending
linearly along a first axis X1. The first axis X1 is substantially parallel to the
direction X of advance of the carrier PO. The receiving antenna comprises
20 N+M sensors. It comprises, generally, one or more elementary physical
receiving antennae.
Figure 3 shows a side view of the receiving antennae of the sonar
according to the invention. The first receiving antenna 3 is a composite
antenna formed from a linear array of N+M transducers. It comprises a first
25 elementary antenna 5 that comprises a linear array of M (here 4) identical
first transducers T5 spaced apart along the first axis X 1, and a second
elementary antenna 6 comprising a linear array of N {here 4) identical second
transducers T6 spaced apart along the first axis X 1. The first transducers T5
are separated pairwise by a second transducer T6 along the first axis X1 and
30 the second transducers T6 are separated pairwise by a first transducer T5
along the axis X1. In other words, the linear array of N+M transducers along
the axis X1 comprises, in alternation, along the first axis X1, a first transducer
and then a second transducer. Consecutive transducers are separated by a
space having a set length e along the first axis so that the first elevation P5
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between the first transducers is equal to the second elevation P6 between
the second transducers.
In the embodiment in figure 3, the first transducers T5 have a
larger width L5 than the width L6 of the second transducers T6 along the first
5 axis X 1. Therefore, the bearingwise aperture of the receiving lobes of the first
transducers T5 of the first elementary antenna 5 is smaller than the
bearingwise aperture of the receiving lobes of the second transducers T6 of
the second elementary antenna 6. Advantageously, the first transducers T5
are dimensioned and configured so that only the first sector 81 is included in
10 their receiving lobes and so that the other sectors 82, 83 are located outside
of the receiving lobes of the first transducers T5 forming the first elementary
antenna 5. In other words, the first transducers have a directivity that allows
the first antenna 5 to image the first sector 81 but that does not allow the
other sectors to be imaged simultaneously. In contrast, the signal-to-noise
15 ratio of the first elementary antenna is higher than the signal-to-noise ratio of
the second antenna. The bearingwise aperture of the transducers of the first
elementary antenna 5 is advantageously substantially equal to the
bearingwise aperture of the first sector 81. As a variant, the first transducers
T5 are dimensioned and configured so that the first sector is included in the
20 receiving lobes of the first transducers T5 of the first elementary antenna 5
and so that the other sectors 82, 83 are at least partially included in the
receiving lobes of the first transducers T5 of the first elementary antenna.
This variant generates a synthetic first antenna having a worse signal-tonoise
ratio but a lower cost.
25 The elementary antennae 5, 6 each allow the backscattered
signals generated by all the pulses emitted in one ping by the emitting device
2 to be measured. The processing device 4 is configured so as to distinguish
the measurements of signals backscattered by the seabed and originating
from the respective pulses and to generate the beams of three synthetic
30 antennae. The processing device 4 comprises a first module 40 allowing the
measurements of backscattered signals to be distinguished depending on the
acoustic-pulse sector in which they were generated, i.e. the sector in which
the target that backscattered the signal is found, and a second module 41
allowing the beams of the synthetic antennae to be generated from
35 measurements of the signals backscattered in the respective sectors. The
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second module 41 is configured so as to generate the beams of the first
synthetic antenna from first measurements of first signals backscattered by
the observed zone and generated by pulses emitted in the first sector S 1,
said first measurements being acquired by the first elementary antenna 5.
5 The second module 41 is configured so as to generate the beams of the
second and third synthetic antennae from second measurements of signals
backscattered by the seabed and generated by pulses emitted in the second
and third sectors S2 and S3, the second measurements being acquired by
the second elementary antenna 6. This arrangement and the associated
10 processing mode make it possible to obtain a first synthetic antenna of high
resolution and having a very good signal-to-noise ratio and other synthetic
antennae having a very high resolution, without having to excessively sample
the receiving antenna, i.e. without having to provide an inter-transducer
elevation with a value of about half the wavelength of the acoustic pulse
15 used. This makes it possible to limit the required number of transducers, this
having advantages in terms of cost and power consumption.
The processing device 4 also allows an image representing the
synthetic aperture beams of each synthetic antenna to be generated. These
20 synthetic images are, for example, but not necessarily, waterfall type images.
They represent the beams of the synthetic antennae over R pings, and in the
R+1th ping the ping of index 1 disappears from the screen in order to allow
the representation of the R+1th ping to appear. These images are not
focused on a particular point in the geocentric reference frame. They thus
25 have the advantage of allowing objects to be detected and not solely a
previously detected object to be classified. They represent a number of
viewpoints of an observed zone equal to the number of sectors, the
viewpoints being acquired substantially simultaneously. The sonar 1
according to the invention comprises a displaying device 10 allowing said
30 synthetic images to be displayed simultaneously. It allows an operator to
simultaneously observe various synchronized viewpoints of an observed
zone, thereby making object detection and classification operations easier for
him.
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The modules are for example computational functions of a given
computer or different computers. The first module may comprise filters and/or
a demultiplexer.
As for mono-aspect sonars, the resolution of the synthetic
5 antennae of a multi-aspect sonar is limited by deviations of the receiving
antenna from a rectilinear and uniform navigational path. Therefore, the
formation of the beams of each of the synthetic antennae is carried out while
correcting for the effects of parasitic movements of the receiving antenna 3
using the correcting principal described in patent application FR 2769372.
10 The processing device 4 is configured so that variations in the movement of
the first physical receiving antenna 3 are corrected for, for each synthetic
antenna, by performing an autocalibration by intercorrelation of the
successive pings using measurements of rotations of the receiving antenna
between successive pings, said measurements being obtained by means of
15 at least one gyrometer 9, and using estimations, in the terrestrial reference
frame, of the elevation angles of signals backscattered between these two
pings. Each gyrometer for example forms part of an inertial navigation
system 9. The gyrometers together advantageously form an inertial
navigation system.
20 Figure 4 shows a terrestrial reference frame x, y, z, representing
the vertical direction in the terrestrial reference frame and the plane (x,y) a
horizontal plane in the same reference frame. The elevation angle q> i.e. the
elevation angle defined in a terrestrial reference frame, of a signal
backscattered by a target A is, in the present patent application, the angle
25 made between the image plane PI, which is the plane containing the target A
and the first axis X1, and the horizontal plane (x, y). The elevation angles or
elevationwise inclinations of the backscattered signals correspond to the
elevation angles of the image planes or sighting planes of the antenna, which
planes are defined for the sighting points having generated these
30 backscattered signals.
In the sonar according to the invention, in each ping, estimations
of the elevation angles of backscattered signals are used to define the image
planes of the backscattered signals and to project the rotation measurements
obtained by means of the gyrometers onto the obtained image planes, as is
35 described in patent application FR 2769372. Next, on the basis of the
wo 2015/136089 16 PCT/EP2015/055334
projections of the obtained rotation measurements, parameters I and -r are
estimated for each synthetic antenna by a conventional autocalibrating
process, -r being the difference in the round-trip propagation time of the sonar
pulse for a given point of reflection in the observed zone (here the seabed)
5 between two successive pings, and I being the longitudinal movement of the
receiving antenna, along the first axis X1, between two successive pings.
These parameters make it possible to correct for variations in the movement
of the physical antenna during the formation of the beams of the synthetic
antennae. The same process is used in the case of mono-aspect sonars.
10 By "autocalibrating process", what is meant is a process that
determines these coefficients from measurements of backscattered signals
acquired by the receiving antenna. Among such processes, processes
exploiting the intercorrelation of the acoustic field over the antenna over two
successive pings are in particular known. When the longitudinal movement
15 between two pings is smaller than half the length of the receiving antenna,
the field at the front end of the first ping is highly correlated with the field at
the back end. The length Lc of the two correlated ends of the field of the
antenna is then given by the formula: Lc = L - 2. V. Tr. Such a process
exploits this correlation to estimate the longitudinal movement I, the
20 difference -r in the round-trip propagation time of the sonar pulse for a given
point of reflection from the seabed, and the rotation J3 of the sighting .
direction, between the two pings. One example of such a method is
described in United States patent US-A-4 244 036 (Raven).
The use of the elevation angles of the backscattered signals to
25 form the beams of the various synthetic antennae thus allows synthetic
aperture beams and synthetic images having a very high resolution to be
obtained.
In summary, the estimations of the elevation angles of the
backscattered signals are used in order to project the rotation measurements
30 obtained by the gyrometer(s) onto the image planes of the backscattered
signals, the projections obtained being used to carry out the autocalibration.
The projections of the rotation measurements onto the image planes are the
only data required to generate the autocalibrated synthetic antennae. The
use of projections of the rotation measurements onto the image planes
35 allows synthetic aperture beams having a better resolution to be obtained
wo 2015/136089 17 PCT/EP2015/055334
than when the rotation measurements obtained by means of gyrometers are
used. This method makes it possible to improve the resolution of the sonar
image obtained from the synthetic aperture beams. The elevation angles are
defined in the terrestrial reference frame.
5 According to the invention, during the formation of the synthetic
aperture beams of at least one sector of the set of sectors, which sector is
called the bathymetric sector, estimations of elevation angles of
backscattered signals are used, which estimations are taken from among
said elevation-angle estimations used during the formation of beams of at
10 least one of the synthetic antennae. These estimations are obtained from
a bathymetric chart comprising three-dimensional positions defined in the
terrestrial reference frame of respective points of the observed zone.
According to the invention, it is for example possible to make
provision for all the insonified sectors to be bathymetric sectors or indeed
15 for the only insonified sector (mono-aspect case) to be a bathymetric
sector.
The invention has the advantage of allowing an auxiliary
antenna, such as described in patent application FR 2769372, comprising
a plurality of sensors distributed along an axis perpendicular to the axis of
20 the first receiving antenna, to be omitted. It is for example possible to use
a pre-existing bathymetric chart of the observed zone, which chart is
stored in a memory of the sonar before the observed zone is imaged. This
bathymetric chart may be obtained from an atlas of bathymetric charts or
by a survey by a hydrographic ship. As a variant, the bathymetric chart
25 may be obtained by means of a multibeam probe or of another side-scan
sonar, for example, without a bathymetric capability, or by means of a
device for measuring the altitude of the sonar, assuming that the observed
zone has a constant altitude. In the latter three cases, the bathymetric
chart may be constructed during R pings of the sonar or before the R
30 pings. In summary, the bathymetric chart may be obtained by means of a
system external to the sonar according to the invention. Advantageously,
the sonar is devoid of an auxiliary antenna comprising a plurality of
sensors distributed along an axis perpendicular to the axis of the first
receiving antenna. Such a sonar has both a low hardware cost and a low
35 processing cost, a low bulk, a low weight, and consumes little power
5
10
wo 2015/136089 18 PCT/EP2015/055334
because of the decrease in the processing cost and of the decrease in
bulk and weight.
As a variant, the sonar according to the invention comprises an
auxiliary antenna such as described in patent application FR 2769372.
According to one particular embodiment relating to the multiaspect
sonar such as described above, elevation-angle estimations obtained
from the bathymetric chart are used to correct for variations in the movement
of the first receiving antenna during the formation of the beams of sectors S2
and S3 or of at least one of these two sectors.
The bathymetric chart may be constructed by various means, for
example by means of a device external to the sonar.
Advantageously, the measurements of first elevation angles used
to estimate the elevation angles are measurements of the first elevation
angles of the backscattered signals originating from the first sector, i.e.
15 generated by acoustic pulses emitted in the first sector. Estimating the
elevation angles of the backscattered signals generated by pulses emitted in
the sectors S2 and S3 on the basis of first elevation angles measured for
backscattered signals generated by acoustic pulses emitted in the first sector
allows synthetic images having a much better resolution to be obtained for
20 these sectors S2, S3 without needing to correct for the parasitic movements
of the receiving antenna during the formation of the beams of the synthetic
antennae corresponding to these sectors, these images being similar to
those that would have been obtained if elevation-angle measurements
obtained directly by an auxiliary antenna having a receiving lobe covering
25 these sectors were used.
According to one particular embodiment, the bathymetric chart is
obtained from measurements of first elevation angles of first backscattered
signals acquired in the first sector S1. The measurements of the first
elevation angles are obtained by means of measurements of backscattered
30 signals obtained by means of an array 11 of transducers T5, T7 comprising a
plurality of transducers distributed along a second axis Z2 perpendicular to
the first physical receiving antenna 3, i.e. perpendicular to the first axis X1. In
other words, the array 11 comprises a stack of transducers in the direction
Z2. Such a distribution of transducers makes it possible to take
35 measurements of first elevation angles of backscattered signals since the
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array 11 of transducers has a directivity along the axis Z2. The signal-tonoise
ratio of the elevation-angle measurements performed by this antenna is
clearly higher than that obtained with the receiving antenna 3. The axis Z2 is
perpendicular to the first axis X1 and is parallel, locally, to the plane of the
5 first physical antenna, i.e. to the plane formed by the membranes of the
transducers 5, 6. The axis Z2 is parallel to the vertical axis z defined in a
terrestrial reference frame, when the angle of elevation of the receiving
antenna 3 is zero. Preferably, the array 11 has, along the axis Z2, a height
larger than the height of the first receiving antenna 3.
10
Advantageously, the transducers T5, T7 forming the array 11 of
transducers are dimensioned and configured so that only the first sector S1 is
completely comprised in their receiving lobes. We will see that this makes it
possible to limit the required number of sensors, and therefore the cost of the
15 sonar, while allowing synthetic images and synthetic aperture beams of very
high resolution to be obtained.
By "configuration of the transducers", what is meant is their
positions with respect to the receiving antenna and their boresight directions.
In other words, the transducers of the array 11 of transducers are
20 dimensioned and configured so that the first sector S1 is comprised,
bearingwise, in their main receiving lobes and so that the other sectors S2,
S3 are at least partially located, bearingwise, outside of their main receiving
lobes. Advantageously, the bearingwise aperture of the receiving lobes of the
transducers of the array 11 is substantially equal to the bearingwise aperture
25 of the first sector S1.
In the embodiment in figure 3, the other sectors S2, S3 are located
completely outside the main lobes of the transducers forming the array 11 of
transducers. In this figure, the sonar 1 according to the invention comprises a
second receiving antenna 12. This second receiving antenna 12 is a physical
30 antenna identical to the first elementary antenna 5 and superposed on the
first elementary antenna 5 along the second axis Z2. It comprises third
transducers T7 distributed along a third axis X3 parallel to the first axis X1.
The third transducers T7 are identical to the transducers T5 and spaced
apart by the same elevation along the axis X 1. The array 11 of transducers
35 by means of which the first elevation angles are measured comprises the
5
10
wo 2015/136089 20 PCT /EP2015/055334
transducers of the first elementary antenna 5 and of the second elementary
antenna 12. In other words, the array 11 of transducers is formed by the first
elementary receiving antenna 5 and by the second receiving antenna 12.
These two antennae form an interferometric antenna.
As a variant, the second receiving antenna 12 is shorter, along the
axis X1, then the first receiving antenna. In other words, it comprises fewer
sensors along the axis X1. In another variant, the second receiving antenna
12 comprises transducers having a different size, in the direction X1 and/or in
the direction Z2, than the first transducers T5.
In another variant, the array 11 of transducers comprises only one
transducer in the direction X 1 and a linear array of spaced-apart transducers
along the axis Z2. The array of transducers optionally comprises one of the
transducers of the first receiving antenna 3. However, these antennae are not
selective bearingwise, and only allow synthetic aperture beams of lower
15 bearingwise resolution to be obtained.
The transducers forming the array 11 may extend linearly along
the second axis Z2 or indeed form a curved surface following the curvature of
the cylindrical hull but having an extension along the second axis Z2.
In summary, the transducers forming the array 11 of transducers
20 are configured and dimensioned so that the array 11 allows the elevation
angles of signals backscattered by targets located in only one and not all of
the sectors 81, 82, 83 to be estimated directly. In the non limiting example of
this patent application, this sector is the sector 81, i.e. the side-scan sector.
This solution is economical from a software point of view and from a
25 hardware point of view since it does not require provision to be made for an
array of transducers allowing all the sectors to be covered. It is for example
more economical than a solution consisting in forming an interferometric
antenna from the first receiving antenna and a second identical receiving
antenna superposed on the first receiving antenna in the direction Z2. The
30 number of transducers of the second receiving antenna would then be twice
the number of transducers of the second receiving antenna of the sonar
according to the invention, and this would be more expensive from a
hardware point of view and from the point of view of data processing, and
would increase bulk. In contrast, the proposed solution based on the
35 interferometric-antenna array 11 according to the invention allows elevation
wo 2015/136089 21 PCT /EP2015/055334
angles to be obtained, in the first sector S1, with a resolution identical to that
of an interferometric antenna obtained by superposing a first receiving
antenna 3 and another identical antenna. The proposed solution does not
require an interferometric antenna that is costly and oversampled along the
5 axis X1.
The invention also relates to a method for forming a synthetic
antenna of a sonar according to the invention over R pings of the sonar. The
sonar described above is able to implement the method according to the
IO invention. Figure 5 shows a block diagram of this method.
15
20
25
30
35
The beams are formed from the measurement signals obtained
from R pings.
The method comprises, in each ping r where r = 1 to R,
a step 100 of the emitting distinguishable acoustic pulses in
each sector S1, S2, S3, by means of the emitting device 2, as
the sonar 1 advances along the axis X 1 ;
- a step 101 of acquiring measurements of signals backscattered
by the observed zone, by means of the first receiving antenna
3;
- a step 102 of distinguishing between the measurements of the
signals acquired by the first receiving antenna, for example by
means of the first module 40, possibly carried out after step
103;
- a step 103 of storing the measurements of the signals acquired
by the first receiving antenna 3, for example in a first memory
70;
- a step 104 of measuring (roll, pitch and yaw) rotations of the
first receiving antenna or of the carrier PO by means of at least
one gyrometer;
- a step 105 of storing the rotation measurements, for example in
a second memory 71, which may optionally be the first
memory;
- a step 106 of measuring the position of the carrier, or of the
receiving antenna 3, in a terrestrial reference frame by means
of a device 72 for measuring position. This step allows the
5
wo 2015/136089 22 PCT/EP2015/055334
position of the carrier PO in latitude, longitude and depth to be
measured in a terrestrial reference frame;
- a step 107 of storing the measurements of the position of the
sonar, for example in a third memory 73, which may optionally
be the first memory and/or the second memory.
The method also comprises a step 120, 121, 122 of forming the
beams of the synthetic antennae from measurements of backscattered
signals acquired by the first receiving antenna 3 over R successive pings of
10 the sonar 1. The method also comprises a step 130, 131, 132 of forming
synthetic images 11, 12, 13 from the beams of the respective synthetic
antennae in respective steps 130, 131, 132.
In the case of a mono-aspect sonar, the step 102 is not
implemented, the step 120, 121, 122 is a step of forming the beam of one
15 synthetic antenna and step 130, 131, 132 is a step offorming the associated
synthetic image.
The invention relates to a method comprising a step 120, 121, 122
of forming, over R pings and for each corresponding synthetic antenna of the
sonar in question, beams from measurements of signals backscattered by
20 the observed zone and generated by the acoustic pulses emitted in each
section in question. In this step, variations in the movement of the first
receiving antenna during the formation of the beams of the synthetic
antennae are corrected for as explained above. According to the invention,
during the formation of the beams of at least one synthetic antenna,
25 estimations of elevation angles of backscattered signals are used, these
estimations being obtained from a bathymetric chart comprising the threedimensional
positions of a plurality of points of the observed zone.
We will now describe the other steps of the method according to
the invention in the case of a multi-aspect sonar in which the beams of the
30 synthetic antennae corresponding to the second and third sectors are
constructed from a bathymetric chart obtained from measurements of first
elevation angles, which measurements are obtained by means of the array
11 of transducers.
The invention relates to a method for forming beams of corresponding
35 synthetic antennae in step 120,121, 122. The step 120, 121, 122 of forming
tl wo 2015/136089 23 PCT/EP2015/055334
the beams of the synthetic antennae over R successive pings comprises, a
step 120 of forming the beams of the first synthetic antenna from
measurements of first backscattered signals generated in the first sector S 1
and steps 121, 122 of forming beams of two other synthetic antennae from
5 second measurements of second and respectively third backscattered
signals generated by acoustic pulses emitted in the second and respectively
third sector, in which steps variations in the movement of the first receiving
antenna are corrected for by carrying out an autocalibration by
intercorrelation of the successive pings using measurements of rotation of
10 the receiving antenna, which measurements are obtained by means of said
at least one gyrometer, and using, to determine second and third image
planes, estimations of second and respectively third elevation angles of
backscattered signals calculated from a bathymetric chart. The rotation
measurements are then projected onto the second and third image planes
15 and used to carry out the autocalibration associated with the second and
respectively third synthetic antenna. The bathymetric chart is for example
obtained from the measurements of the first elevation angles. Steps 120 to
122 are carried out by the second module 41. The steps 120, 121, 122 are
preceded by a distinguishing step, carried out by means of the first module
20 40, for distinguishing between the measurements of the signals depending
on the acoustic-pulse sector in which they were generated. Each step 120,
121, 122 comprises a step (not shown) of selecting the signals required to
form the beams of the synthetic antenna in question from the signals
measured by the first physical antenna 3.
25 The method also comprises, prior to steps 120, 121, 122, a step 108,
carried out for each ping, of measuring first elevation angles of first signals
backscattered at a number P of probe times tp, where p = 1 toP, which times
are spaced apart pairwise by a predefined elementary period T starting at a
first probe time t1 subsequent to the time of emission of the associated
30 acoustic pulse and spaced apart from the latter by a predefined duration D. In
other words, in each ping, first elevation angles of first signals backscattered
by P probe points Pp, which signals are measured by the first receiving
antenna 3, are measured at P probe times tp. These measurements are
carried out in the reference frame of the array 11 by the array 11. In the case
35 in figure 3, the first elevation angles are estimated on the basis of first
wo 2015/136089 24 PCT /EP2015/055334
measurements of first backscattered signals, which measurements are
performed by the first elementary antenna 5, and on the basis of additional
measurements of the first backscattered signals, which measurements are
carried out by the second receiving antenna 12, in one ping.
5 The step 120 of forming beams of the first synthetic antenna is carried
out on the basis of first measurements of first backscattered signals
generated by pulses emitted in the first sector during the R pings. The first
measurements are carried out, in the embodiment shown in the figures, by
means of the first elementary receiving antenna 5. In this step, variations in
10 the movement of the receiving antenna are corrected for by carrying out an
autocalibration by intercorrelation of the successive pings. To correct for
these variations, measurements of the rotations of the receiving antenna,
which are obtained with said at least one gyrometer, are used and the
estimations of the first elevation angles of the first backscattered signals
15 generated by the acoustic pulses emitted in the first sector S 1 are used to
determine the first image planes onto which the rotation measurements
obtained by means of the gyrometer must be projected to obtain the
projections that are used to carry out the autocalibration of the first synthetic
antenna. The estimations of the first elevation angles correspond to the
20 measurements of the first elevation angles carried out by the array of
transducers 11 and transposed, in step 110, to the terrestrial reference frame
on the basis of the measurements of position and rotations of the sonar
carried out in steps 104 and 106. This method makes it possible to obtain, for
the first synthetic antenna, beams having a very high resolution identical to
25 the beams of a conventional synthetic antenna the variations in the
movement of the physical antenna of which are corrected for by means of the
method described in patent application FR 2769372.
The method comprises a step 111 a of producing, from the estimations
of the first elevation angles, which estimations are obtained during the R
30 pings, the bathymetric chart of the observed zone, and a step 111 b of storing
the bathymetric chart, for example in a fourth memory 74. The bathymetric
chart comprises a set of three-dimensional positions of probe points Pp in the
terrestrial reference frame.
Step 111 consists, for each ping, in positioning, in a terrestrial
35 reference frame, the probe points that caused the first backscattered signals
wo 2015/136089 25 PCT /EP20 15/055334
measured for the ping in question, this positioning being carried out on the
basis of the measurements of the first elevation angles carried out in step
108 and on the basis of the measurements of measured positions and
rotation carried out in steps 104, 106, or indeed on the basis of the
5 estimations obtained in step 110 of the first elevation angles in the terrestrial
reference frame. Figure 6 shows with circles the positions on each ping of the
probe points Pp, where p = 1 to 6, in a terrestrial reference frame x, y, z, the
path TS of the carrier PO and the positions of the carrier PO on each ping r (r
= 1 to 5). For greater clarity, the circles associated with even pings are
10 colored white and the circles associated with uneven pings are colored gray.
For each ping, the limits of the first sector S1 in the vertical plane y, z
containing the first sighting axis v1 have been represented by solid lines and
the line lr (where I = 1 to 5) contained in this plane and passing through the
probe points Pp obtained for said ping has been represented by a dotted line.
15 The dotted lines lr corresponding to the various pings are not parallel to one
another because the path of the carrier is not exactly rectilinear. The threedimensional
mesh formed by the probe points is not necessarily regular in
the x, y plane because of rotations and/or changes of speed of the carrier.
For greater clarity, the positions of the probe points Pp have only been
20 referenced for the first ping.
The method comprises a step 112 of estimating, for each ping, second
elevation angles of second backscattered signals measured by the second
elementary antenna 6 at P probe times tp, where p = 1 toP, which times are
spaced apart pairwise by a predefined elementary period T starting at a first
25 probe time t1 subsequent to the time of emission of the corresponding
second acoustic pulse and separated from the latter by the duration D. It also
comprises a step 113 of estimating third elevation angles of third
backscattered signals measured by the second elementary antenna 6 at P
probe times tp, where p = 1 to P, which times are spaced apart pairwise by a
30 predefined elementary period T starting at a first probe time t1 subsequent to
the time of emission of the corresponding third acoustic pulse and separated
from the latter by the duration D. These steps are carried out on the basis of
the bathymetric chart and of the measurements of position and attitude of the
carrier during the R pings, which measurements are carried out in steps 104
35 and 106 of the corresponding ping. Steps 121 and 122 respectively use the
wo 2015/136089 26 PCT/EP2015/055334
estimations of the second and third elevation angles to improve the precision
of the measurement of the rotations, which measurement is obtained by the
gyrometer.
We will now describe the step 112 of estimating second elevation
5 angles. The step 113 of estimating third elevation angles is carried out in the
same way, but on the basis of the backscattered signals generated in the
third sector S3. It will not be precisely described. The step 112 comprises, for
each ping and for each probe time tp, a step 112p of estimating the second
elevation angle of a second backscattered signal of an acoustic pulse emitted
10 in the second sector S2 and measured by the first receiving antenna 3 at the
15
20
25
30
probe time tp. This step 112p comprises:
- a step 112a of calculating the position of that point Mp of the
bathymetric chart which is closest to the position of the probe point
Pp that generated the second backscattered signal by determining
the position of that point of the bathymetric chart which is closest
to the section of a circle Cp that is obtained by rotating a point B
located at a distance pp from the center 0 of the receiving antenna
3 along the second sighting axis v2 about the first axis X 1, the
distance pp being the distance by which the probe point Pp needs
to be separated from the center 0 of the first receiving antenna 3
for the first receiving antenna to measure the second signal
backscattered by the probe point Pp at the probe time tp;
- a step 112b of calculating a point of intersection lp between the
bathymetric chart and the section of the circle Cp on the basis of
the point Mp, the point lp corresponding to the estimated position
of the probe point that backscattered the second signal;
- a step 112c of calculating the elevation angle of the point lp, in
the terrestrial reference frame, on the basis of the position of lp, on
the basis of the measurement of the position of the carrier Po or of
the receiving antenna 3 and especially its altitude with respect to
the seabed and on the basis of the distance PP·
The circle Cp is located in a plane PC perpendicular to the axis
X1. Figure 7 shows the circle Cp containing the points located at a distance
pp from the center 0 of the antenna and having a bearing angle 8p of 55°, the
35 closest point Mp and the intersection lp between Cp and the bathymetric
wo 2015/136089 27 PCT /EP2015/055334
chart CB. The known points of the bathymetric chart CB are the points of
intersection of the grid Q. The used section of the circle is that section of the
circle which is located starboard-side but the entirety of the circle Cp could
also be used. The circle Cp is an estimation of the location of possible
5 positions of the probe point Pp that caused the second backscattered signal.
It is a question of all the points on a cone the axis of which is the axis X 1 and
the generatrice of which has a bearing angle equal to that of the second
sighting axis v2, and which are located in the plane PC. In other words, this
amounts to estimating, in the step 112, second elevation angles of second
10 backscattered signals generated by acoustic pulses emitted along the
second sighting axis v2, 92 and located at the distance Pp from the center of
the receiving antenna 3.
The bathymetric chart must be stored in memory over the minimum
number Nm of pings that allows the Pp probe points of the current ping to be
15 positioned in the step of estimating third elevation angles (i.e. in the step of
estimating elevation angles for the rearward mode). This number is the
number of pings required when the average rotation of the sonar over its path
is zero and it advances at the minimum speed Vmin (least favorable case):
20 N = I1+ _Pm_ax_ si_n .~Bs.l_rII~
111 V·Tr mm·
Pmax is the maximum range of the sonar (it is a maximum
distance with respect to the center of the antenna referred to as the oblique
distance) and e8 is the relative bearing angle between the first sighting axis
25 v1 and the rearward sighting axis v3. Tr is the time interval between two
successive pings. Once the bathymetric chart has been produced for Nm
pings, the estimation of elevation angles for the current ping for the rearward
mode and the formation of the beam of the third synthetic antenna for the
current ping, which is carried out using these elevation angles, may start. For
30 the forward mode (formation of the beams of the second synthetic antenna),
the calculation of the elevation angles cannot start immediately because the
zone observed by the sonar along the second sighting axis v2 is located in
front of the zone explored in side-scan mode (along v1). All the
measurements of position, of rotation and of the backscattered signals must
wo 2015/136089 28 PCT /EP2015/055334
be kept in memory over Nm pings until the bathymetric chart corresponding
to the zone sighted by the second axis v2 in the current ping has been
constructed.
Advantageously, step 112 comprises, for each ping and for each time
5 tp of order higher than 1, before the step 112a, a step (not shown) of
extracting a section of the bathymetric chart that is smaller than the
bathymetric chart, steps 112a and 112b then being carried out on the basis
of the section of the bathymetric chart. This step makes it possible to
accelerate the processing time. In one nonlimiting example, the entire
10 bathymetric chart is used for tp when p = 1 and then, for times tp of higher
orders, a section of the bathymetric chart which is located at a horizontal
distance below a preset threshold from the point of intersection obtained at
the time of lower order is used.
15 Step 112a is carried out by calculating the distance between each
point of the bathymetric chart (or of the bathymetric subchart) and the circle
(or circular arc). Thus, the point Mp that is that point of the bathymetric chart
which is closest to the section in question of the circle Cp is obtained. Step
112b is for example carried out by calculating the point of intersection lp
20 between a horizontal plane (parallel to the plane (x, y)) passing through the
point Mp and the section of the circle Cp. This amounts to approximating the
bathymetric chart by a horizontal plane in the vicinity of Mp. This step could
be carried out more precisely using a plurality of points of the bathymetric
chart to estimate the surface formed by the bathymetric chart in the vicinity of
25 Mp.
In a first embodiment, the estimation of the second elevation angle is
the angle calculated in step 112c.
In one variant (not shown), step 112 comprises the step 112a of
calculating a first point Mp, a first step 112b of calculating a first point of
30 intersection lp, and a first step 112c of calculating a first elevation angle, in
which steps the point lp is calculated from the horizontal plane passing
through the point M. Step 112 also comprises a second step 112b of
calculating a second point of intersection, in which step a second point of
intersection between a second section of the circle Cp and a surface formed
35 from the point M and other points of the bathymetric chart is sought in order
wo 2015/136089 29 PCT/EP2015/055334
to improve the precision of the positioning of the point lp, and, if this step
converges, a second step of calculating the elevation angle, in the terrestrial
reference frame, of the second point of intersection. The second elevation
angle is then the elevation angle calculated for the second point of
5 intersection. This method allows more precise estimations of elevation angles
to be obtained.
Advantageously, the steps 112p are carried out for each time tp
starting at a start time and while scanning the times in increasing order until a
last time (p=P) and while scanning the times, from the time preceding the
10 start time, in decreasing order to the first time (p=1), the start time being
different from the first time and the last time. This method makes it possible
to increase robustness.
Another subject of the invention is a computer program product
15 comprising programming code instructions for executing the steps of the
method according to the invention when the program is run on a computer.
The steps of the described process may be implemented by
means of one or more programmable processors that run a computer
program in order to perform the functions of the invention by operating on
20 input data (especially the backscattered signals, the gyrometer data and the
bathymetric chart) and generating output data (synthetic aperture beams). A
computer program may be written in any form of programming language,
including compiled or interpreted program languages and the computer
program may be deployed in any form including as an autonomous program
25 or a subroutine, a component or another unit usable in a programmable
environment. A computer program may be deployed to be run on one
computer, or on a plurality of computers on one site or distributed over a
plurality of sites and interconnected by a communication network.

CLAIMS
1. A synthetic aperture sonar (1) intended to move along a first axis (X1),
the sonar (1) comprising an emitting device (2) configured to emit, in
5 each ping, at least one acoustic pulse toward an observed zone in a set
of sectors comprising at least one sector, the sonar (1) comprising a first
physical receiving antenna (3) extending along the first axis (X1) allowing
measurements of backscattered signals generated by said pulse to be
acquired and a processing device (4) configured to form, over R pings,
10 for each sector of the set of sectors, synthetic aperture beams from
measurements of signals backscattered by the observed zone and
generated by acoustic pulses emitted in said sector, the sonar (1)
comprising at least one gyrometer, characterized in that said processing
device (4) is configured so as to correct for variations in the movement of
15 the first receiving antenna during the formation of the synthetic aperture
beams of said set of sectors by carrying out an autocalibration by
intercorrelation of the successive pings using measurements of rotation
of the first receiving antenna (3), which measurements are obtained with
said at least one gyrometer, and using estimations of the elevation
20 angles of the backscattered signals to determine image planes of the
backscattered signals and to project said rotation measurements onto
said image planes, the projections obtained being used to carry out the
autocalibration,
and in which, during the formation of the synthetic aperture beams of at
25 least one sector of the set of sectors, which sector is called the
bathymetric sector, estimations of elevation angles of backscattered
signals are used, said estimations being obtained from a bathymetric
chart comprising the three-dimensional positions, defined in the terrestrial
reference frame, of a plurality of points of the observed zone.
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2. The sonar as claimed in claim 1, wherein the emitting device (2) is
configured to emit, in each ping, in different respective sectors (S1, S2,
S3) comprising a first sector (S1) and at least one second sector (S2,
S3), distinguishable acoustic pulses toward an observed zone, along a
35 first sighting axis (v1) and a second sighting axis (v2, v3) having different
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bearing angles, respectively, wherein said at least one bathymetric sector
comprises at least one second sector, and wherein the bathymetric chart
is obtained from measurements of first elevation angles of first
backscattered signals generated by acoustic pulses emitted in said first
5 sector.
3. The sonar as claimed in claim 2, comprising an array (11) of transducers
comprising a plurality of transducers distributed along a second axis (Z2)
perpendicular to the first axis (X1), said transducers forming the array of
10 transducers being dimensioned and configured so that their receiving
lobes cover the first sector (81) but so that said at least one second
sector (82) is located at least partially beyond their receiving lobes, the
first backscattered signals being acquired by means of the array ( 11) of
transducers.
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4. The synthetic aperture sonar as claimed in claim 3, wherein the physical
receiving antenna (3) comprises a first elementary physical antenna (5)
formed from first transducers (T5) dimensioned and configured so that
their receiving lobes cover the first sector (8 1) but so that said at least
20 one second sector (82) is at least partially located beyond their receiving
lobes, wherein the sonar (1) comprises a second elementary physical
antenna (6) formed from second transducers (T6) dimensioned and
configured so that their receiving lobes cover the first and second sectors
(81, 82), and wherein the processing device (4) is configured so as to
25 form, during the formation of the synthetic aperture beams, beams of a
first synthetic antenna from measurements of first backscattered signals
generated in the first sector (81) and acquired by means of the first
elementary antenna (5), and beams of a second synthetic antenna from
measurements of second backscattered signals generated by pulses
30 emitted in said second sector (82, 83) and acquired by means of the
second elementary antenna (6).
5. The synthetic aperture sonar as claimed in the preceding claim, wherein
the array (11) of transducers is formed by the first elementary antenna
35 (5) and another antenna (12) that is identical to the first elementary
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antenna {5) and superposed on the first elementary physical antenna (5)
along the second axis (Z2).
6. The synthetic aperture sonar as claimed in either one of claims 1 to 2,
5 wherein the bathymetric chart is stored in a memory of the sonar before
the observed zone is imaged.
7. A sonar system comprising the sonar (1) as claimed in any one of the
preceding claims, and a carrier (PO), the sonar (1) being installed in the
10 carrier (PO).
8. A method for forming synthetic aperture beams of a sonar over R pings
of the sonar, the sonar (1) being intended to move along a first axis (X1),
the sonar (1) comprising an emitting device (2) configured to emit, in
15 each ping, at least one acoustic pulse toward an observed zone in a set
of sectors comprising at least one sector, the sonar (1) comprising a first
physical receiving antenna (3) extending along the first axis (X1) allowing
measurements of backscattered signals generated by said at least one
pulse to be acquired and a processing device (4) configured to form, over
20 R pings, for each sector of the set of sectors, synthetic aperture beams
from measurements of signals backscattered by the observed zone and
generated by acoustic pulses emitted in said sector, the sonar (1)
comprising at least one gyrometer, the method comprising a forming step
(120, 121, 122) in which, for each sector over R pings, synthetic aperture
25 beams are formed from measurements of signals backscattered by the
observed zone and generated by acoustic pulses emitted in said sector,
in which variations in the movement of the first receiving antenna during
the formation of the synthetic aperture beams of said set of sectors are
corrected for by carrying out an autocalibration by intercorrelation of the
30 successive pings using measurements of rotation of the first receiving
antenna {3), which measurements are obtained with said at least one
gyrometer, and using estimations of the elevation angles of the
backscattered signals to determine image planes of the backscattered
signals and to project said rotation measurements onto said image
35 planes, the projections obtained being used to carry out the
autocalibration, and in which, during the formation of the synthetic
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aperture beams of at least one sector of the set of sectors, which sector
is called the bathymetric sector, estimations of elevation angles of
backscattered signals are used, said estimations being obtained from a
bathymetric chart comprising the three-dimensional position of a plurality
5 of points of the observed zone.
9. The method for forming synthetic aperture beams as claimed in claim 8,
wherein the emitting device (2) is configured to emit, in each ping, in
different respective sectors (81, 82, 83) comprising a first sector (81)
10 and at least one second sector (82, 83), distinguishable acoustic pulses
toward an observed zone, along a first sighting axis (v1) and a second
sighting axis (v2, v3) having different bearing angles, respectively,
wherein said at least one bathymetric sector comprises at least one
second sector, and wherein the bathymetric chart is obtained from
15 measurements of first elevation angles of first backscattered signals
generated by acoustic pulses emitted in said first sector (81).
10. The method for forming synthetic aperture beams as claimed in claim
9, wherein the sonar comprising an array (11) of transducers comprising
20 a plurality of elementary transducers distributed along a second axis (Z2)
perpendicular to the first axis (X1), said transducers forming the array of
transducers being dimensioned and configured so that their receiving
lobes cover the first sector (81) but so that said at least one second
sector (82) is located at least partially beyond their receiving lobes, the
25 first backscattered signals being acquired by means of the array ( 11) of
transducers, the method comprising, for each ping, a step (108) of
measuring first elevation angles of first backscattered signals by means
of the array (11) of transducers, a step (11 0) of calculating estimations of
first elevation angles, consisting in transposing the measurements of first
30 elevation angles to a terrestrial reference frame, the method comprising a
step (111) of producing the bathymetric chart from the estimations of the
first elevation angles, the bathymetric chart comprising three-dimensional
coordinates, in the terrestrial reference frame, of probe points having
backscattered the first backscattered signals.
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11. The method as claimed in the preceding claim, compns1ng a step
(112) of estimating, from the bathymetric chart, the elevation angles of
the backscattered signals generated by pulses emitted in said
bathymetric sector, comprising, for each of the backscattered signals, a
5 step (112a) of calculating the position of that point Mp of the bathymetric
chart which is closest to a section of a circle Cp obtained by rotating,
about the first axis (X1), a point B located on the other sighting axis (v2)
at a distance from the antenna corresponding to the distance separating
the antenna from a probe point having generated the backscattered
10 signal, a step (112b) of calculating a first point of intersection lp between
the bathymetric chart and the section of the circle Cp on the basis of the
closest point Mp, and a first step (112c) of calculating, in the terrestrial
reference frame, the elevation angle of the point of intersection (112c).
15 12. The method as claimed in the preceding claim, wherein the point of
intersection lp is the point of intersection between a horizontal plane, in
the terrestrial reference frame, passing through the closest point Mp, and
the section of the circle Cp.
20 13. The method as claimed in claim 11, comprising a second step of
calculating a second point of intersection lp between the bathymetric
chart and the section of the circle Cp on the basis of the closest point Mp
and other points of the bathymetric chart, and, if a second point of
intersection is obtained, a second step (112c) of calculating the elevation
25 angle of the second point of intersection (112c).
14. The method as claimed in any one of claims 9 to 13, wherein the
physical receiving antenna (3) comprises a first elementary physical
antenna (5) formed from first transducers (T5) dimensioned and
30 configured so that their receiving lobes cover the first sector (S1) but so
that said at least one second sector (S2) is at least partially located
beyond their receiving lobes, the step (120, 121, 122) of forming beams
comprising a step (120) of forming beams of a first synthetic antenna
from measurements of backscattered signals generated by pulses
35 emitted in said first sector (S 1) and acquired by means of the first
elementary antenna (5), in which step the estimations of backscattered
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signal elevation angles used to determine the image planes of the
backscattered signals and to project said rotation measurements onto
said image planes are esti111ations of first elevation angles of the first
backscattered signals, the first backscattered signals being generated by
s pulses emitted in said first sector, the estimations of the first elevation
angles being transpositions of the measurements of the first elevation
angles into the terrestrial reference frame.
15. A computer·. program product comprising programming code
to instructions for executing the steps of the method according to any one of
claims 8 to 14 when the program is run on a computer.