Compact omnidirectional antenna for dipping sonar
The present invention relates to the general field of sonar
detection, in particular implemented in anti-submarine warfare. It more
particularly relates to the field of airborne sonars, referred to as "dipping
sonars", implemented from a helicopter.
5 In the context of anti-submarine warfare activities, in order to be
able to detect submerged submarines in a given area, sonars are generally
employed, in particular active sonars. In this context, the deployment of
sonars from airborne platforms, airplanes or helicopters, has been proven to
be especially effective, as such platforms have high mobility with respect to
l o submarines.
Thus, maritime patrol airplanes deploy sonobuoys that are
composed of acoustic sensors, and sometimes transmitters, and a VHF
system acting as a relay for communication to the aircraft.
In an analogous manner, helicopters may also be used to
15 implement sonar transmitters and receivers that are linked, by a cable, to
their platform, i.e. to the helicopter. These are then referred to as "dipping
sonars". The submerged cable-linked sub-assembly is henceforlh referred to
as an antenna. It comprises the sonar transmitters and receivers per se, and
potentially electronic equipment associated with the transmitters and
20 receivers. It may also comprise environmental sensors.
The dipping of these antennas into the water from the platform, the
control thereof once immersed and their recovery are carried out by means of
a winch located inside the helicopter. In addition to antenna deployment and
recovery functions, the winch cable generally conveys the sonar signals as
25 well as the electrical power that is required for acoustic transmission and
operation of the receivers. Moreover, equipment required for generating
acoustic signals and processing received acoustic data is located on board
the helicopter.
The increased acoustic stealth of .modern submarines has
30 necessitated an evolution of the detection techniques employed toward highpower
sonars operating at low frequency. This evolution translates into an
increase in the dimensions and mass of the various sub-assemblies forming
the sonar. For example, for the antenna, the fact of lowering its operating
frequency has tended to increase its dimensions. Antennas have, for
example, been developed in which the sound wave receivers, or
hydrophones, are arranged on arms that are deployed during operation.
Between sonar operating phases, the arms are folded back and the antenna,
5 wound back in by the winch to a position referred as the storage position, is
stowed inside the helicopter. It is sometimes difficult to house all of the subassemblies
of a sonar inside a helicopter. The antenna, which is generally
cylindrical in order to be omnidirectional in terms of bearing and directive in
terms of elevation, is suspended by the cable that bears it. The greatest
10 vertical dimension of the sonar is restricted by the height of the antenna at
which the cable attachment at the top of the antenna must be added and, at
least in part, by a winch pulley whose diameter depends on the minimum
radius of curvature to which the cable may be subjected. This dimension
must be able to fit, in terms of height, in the helicopter.
15 Regarding the increase in the mass of sonar sub-assemblies, this
affects the duration of missions that a carrier is able to carry out with its
sonar.
The invention aims to reduce the bulk and mass of certain sub-
20 assemblies of a dipping sonar, in particular the mass and height of the
antenna, while retaining analogous acoustic performance. The invention also
aims to reduce the complexity of certain sub-assemblies, in particular of the
antenna. More specifically, the invention allows the articulated arms bearing
the hydrophones to be omitted. By avoiding such moving parts, the reliability
25 of the sonar is improved.
To this end, a subject of the invention is an omnidirectional
antenna intended to be equipped by a dipping sonar, the antenna comprising
multiple elementary transmission rings that are formed around a longitudinal
axis of the antenna and multiple hydrophones that are distributed around the
30 longitudinal axis, the antenna being intended to be dipped in water, the
hydrophones being separate from the elementary transmission rings, the
hydrophones and the elementary transmission rings being fixed in the
antenna, characterized in that the elementary transmission rings and the
hydrophone rings are interlinked along one and the same height measured
35 along the longitudinal axis, in that the elementary transmission rings and the
hydrophones operate at a working frequency of less than 8 kHz, in that a first
minimum cylindrical volume around the longitudinal axis occupied by the
elementary transmission rings and a second minimum cylindrical volume
around the longitudinal axis occupied by the hydrophones have a diameter
5 ratio of less than 30%, and in that the diameter ratio of the two volumes is
constant regardless of whether the antenna is in operation or in a storage
position.
The range of a sonar antenna is linked to the working frequency of
the hydrophones. An antenna according to the invention may operate at a
10 working frequency of less than 8 kHz. Stated otherwise, the elementary
transmission rings and the hydrophones operate at a working frequency of
less than 8 kHz. Advantageously, it is possible to go down to frequencies of
less than 6 kHz, or even less than 4 kHz.
There exist acoustic components, commonly referred to by the
15 term Tonpilz, that allow both the transmission and reception of sound waves
to be carried out. For frequencies of less than 8 kHz, this type of component
would be much too bulky and the antenna would be unsuited to a dipping
sonar, in particular one on board a helicopter. For working at low frequency, it
is advantageous to separate the transmission rings from the hydrophones.
20 In the prior art of low-frequency dipping sonar antennas, in
particular in cylindrical antennas, the transmission rings occupy a volume that
is separate from that of the hydrophones. The term "volume" is understood to
mean a space with a convex contour. In contrast, according to the invention
the elementary transmission rings and the hydrophones are interlinked, i.e.
25 the volumes occupied by the transmission rings and the volume occupied by
the hydrophones have shared portions. The distribution of the hydrophones
and the transmission rings may or may not be regular. The fact of interlinking
the elementary transmission rings and the hydrophone rings allows the
acoustic transmitters and receivers each to be distributed over a greater
30 volume.
More specifically, a first minimum cylindrical volume occupied by
the elementary transmission rings and a second minimum cylindrical volume
occupied by the hydrophones are defined. These two volumes are the
smallest possible cylinders that include the transmission rings or the
35 hydrophones.
Advantageously, the projections from the phase centers of the
hydrophones in a horizontal plane that is perpendicular to the longitudinal
axis, or in a vertical plane containing the longitudinal axis, are spaced apart
by less than AI3, A being the wavelength at the working frequency. Reducing
5 the distance between hydrophones in a horizontal plane allows the directivity
of the antenna in terms of bearing to be improved and between hydrophones
in a vertical plane the directivity of the antenna in terms of elevation to be
improved. Depending on the mission for which the antenna is intended, it
may be chosen to favor one directivity or the other. It is of course possible to
10 reduce the distance between hydrophones in both planes.
Another subject of the invention is a sonar comprising an antenna
according to the invention. Advantageously, the sonar comprises a computer
configured to carry out sonar beamforming, beamforming parameters being
calculated on the basis of the covariance of a spatially correlated noise
I5 according to an adaptive processing formalism that is robust with respect to
antenna calibration errors.
Of course, the invention is not limited to a sonar equipped by a
helicopter. The carrier equipped with a dipping sonar according to the
invention may be of any nature. By reducing the mass of a dipping sonar, it
20 is, for example, possible for a drone to be equipped with a sonar according to
the invention.
The invention will be better understood and further advantages will
become apparent upon reading the detailed description of one embodiment
25 given by way of example, which description is illustrated by the attached
drawing in which:
figure 1 shows a helicopter equipped with a dipping sonar;
figures 2a, 2b, 2c and 2d show a first embodiment of an antenna
belonging to the sonar of figure 1;
30 figure 3 shows a second embodiment of an antenna belonging to
the sonar of figure 1;
figure 4 shows a third embodiment of an antenna belonging to the
sonar of figure 1.
For the sake of clarity, the same elements will bear the same
35 references in the various figures.
Figure 1 shows a helicopter 10 hovering above the water. The
helicopter 10 is equipped with an active dipping sonar 11. This type of sonar
allows submarine objects to be detected and classified, in particular.
5 The sonar 11 essentially comprises a winch 12 installed on board
the helicopter 10, a cable 13 and an omnidirectional antenna 14, shown
submerged in figure 1. The antenna 14 is suspended from the cable 13 and
the winch 12 allows the cable 13 to be wound in and wound out depending
on the depth to which it is desired to submerge the antenna 14. The winch
l o also allows the antenna 14 to be wound back inside the helicopter 10. The
sonar 11 also comprises electronic equipment (not shown) on board the
helicopter 10. The equipment allows the sonar to be operated, in particular
for generating sound waves and for making use of the waves received as
echoes of the transmitted waves. The equipment also allows the antenna 14
15 to be supplied with electrical power.
The cable 13 fulfills two functions, first mechanically supporting the
antenna 14 and subsequently electrically connecting the electronic
equipment.positioned on board the helicopter 10 to the antenna 14. The
electrical connection encompasses the supply of electrical power and the
20 transmission of data to the antenna 14 or originating therefrom. Alternatively,
it is possible to produce an autonomous antenna 14 without electrical
connection with the carrier, the cable then being solely supporting. The
antenna 14 then has its own source of electrical power, e.g. in the form of a
battery. Means for transmitting data, e.g. by radio wave, may be
25 implemented.
The sonar 11 comprises a computer 15, e.g. positioned at the
base of the winch 12. The computer 15 is configured to generate the data
transmitted to the antenna 14 and to process the data from the antenna 14.
The computer 15 is in particular configured to carry out sonar beamforming.
30 The computer 15 is connected to the cable 13 via a slip ring placed on the
winch 12, for example. The computer 15 is advantageously connected to a
screen allowing a sonar image to be viewed. The computer 15 comprises, for
example, a memory containing instructions and a processor that is capable of
implementing the instructions allowing the sonar beamforming to be
35 calculated.
Figures 2a, 2b, 2c and 2d show a first embodiment of the antenna
14. Figure 2a shows a diagrammatic view of the outside of the active portion
of the antenna 14. Figures 2b and 2c diagrammatically show a cross section
of the antenna 14 and figure 2d shows a perspective view of the antenna 14.
5 The antenna 14 is essentially cylindrical. It extends along a
longitudinal axis 20. When the antenna 14 is suspended by its own weight by
the cable 13, the latter also extends along the longitudinal axis 20.
The active portion of the antenna 14 is formed from sound
transmitters and receivers. The transmitters are formed from elementary
l o sound wave transmission rings 21 that are formed around the axis 20 and the
receivers are formed from hydrophones 22 that are distributed in rings
formed around the axis 20. An exemplary implementation of the elementary
transmission rings 21 is, for example, described in the patent EP 0 799 097
61. The hydrophones 22 are uniformly distributed around the axis 20. The
15 hydrophones 22 are, for example, insulated in a polyurethane-based resin or
immersed in an oil bath contained in a flexible envelope.
According to the invention, the elementary transmission rings 21
and the hydrophone rings 22 are distributed along one and the same height
H measured along the longitudinal axis 20. This distribution over the entire
20 height H allows the directivity of acoustic reception in terms of elevation to be
improved for the hydrophones.
Figure 26 shows the antenna 14 in cross section in a plane V
containing the longitudinal axis 20, referred to as the vertical plane, and
figure 2c shows the antenna 14 in cross section in a plane P that is
25 perpendicular to the longitudinal axis 20, referred to as the horizontal plane.
A spacing p l separating two neighboring hydrophones 22 in one
and the same vertical plane containing the longitudinal axis 20 may be
defined. In the cross-sectional figure 2c, a ring of 12 hydrophones 22 that are
regularly distributed on the perimeter of the ring appears. The various rings of
30 hydrophones are advantageously identical. A spacing p2 separating two
neighboring hydrophones in one and the same ring is defined. The spacing
p2 separates the phase centers of each of the hydrophones 22. It is defined
by the length of the chord across the perimeter of the ring separating two
neighboring hydrophones. The spacings p l and p2 are advantageously
regular. It is nonetheless possible to opt for an irregular distribution of the
spacings p l and p2.
In the example shown in figures 2a, 2b, 2c and 2d, the rings 21
5 and 22 are alternately arranged. More specifically, the dimensions of the
elementary transmission rings 21 and of the hydrophone rings 22 are
substantially the same in terms of diameter around the axis 20. The
elementary transmission rings 21 and the hydrophone rings 22 are
alternatively stacked on top of one another.
10 The smallest cylindrical volume on axis 20, referred to as the
minimum volume, occupied by the elementary transmission rings 21 bears
the reference 23. Its diameter around the axis 20 is denoted by Dl. The
smallest cylindrical volume on axis 20, referred to as the minimum volume,
occupied by the hydrophones 22 bears the reference 24. Its diameter around
15 the axis 20 is denoted by D2. The diameters Dl and D2 have a ratio of less
than 30%. Stated otherwise, the difference in absolute value between the two
' diameters Dl and D2 remains less than 30% of the smallest of the two
diameters Dl and 02. In the example shown, the diameter Dl is smaller than
the diameter D2. We therefore have:
20 (D2 - Dl) I Dl < 30%
Advantageously, in order to improve the hydrodynamics of the
antenna, this ratio may be less than 20%, or even 10% and ideally less than
5%.
The diameter ratio of the two volumes 23 and 24 is constant
25 regardless of whether the antenna is in operation or in a storage position.
Stated otherwise, the antenna comprises no folding arms as in the prior art
known for this type of antenna operating at low frequency.
Advantageously, the elementary transmission rings 21 and the
30 hydrophone rings 22 form a tube 25 that extends along the longitudinal axis
20 between two ends 26 and 27. The tube has the largest diameter, Dl or D2
(D2 in the example shown) and is limited by the height H. The antenna 14
comprises two structures 28 and 29 that close the tube 25, each at one of the
ends 26 and 27 of the tube 25. The interior of the tube 25 is thus isolated
35 from the environment into which the antenna 14 is dipped. In order to ensure
good watertightness of the outer surface of the tube 25 and also to ensure
the mechanical protection of the elementary transmission rings 21 and
hydrophone rings 22, the cylindrical outer surface of the tube 25 may be
covered with an elastomer material such as, for example, a polyurethane-
5 based material. The two structures 28 and 29 may be one-piece metal parts
made, for example, of molded aluminum alloy. The upper structure 29 may
comprise vertical fins allowing the hydrodynamics of the antenna 14 to be
improved while moving through water, in particular during descent and
ascent when the winch 12 is in action.
10 When the inside of the elementary transmission rings 21 is
isolated from the environment into which the antenna 14 is dipped, the
transmission rings 21 operate according to a technique in which air is located
on the inside of the rings 21. This technique is known in the literature by the
term "air-backed ring" or ABR.
15 When the tube 25 is closed at both of its ends, it forms a watertight
enclosure inside which electronic equipment may be positioned. By way of
example, the antenna 14 comprises an electronic transmitter 31 connected to
the elementary transmission rings 21 and an electronic receiver 32
connected to the hydrophones 22. The transmitter 31 and the receiver 32 are
20 positioned inside the tube 25. Other components may be positioned inside
the tube 25, such as, for example, a battery 33. Environmental sensors 34
may also be placed inside the tube 25.
Figure 3 shows a second embodiment of an antenna 40 according
25 to the invention. In this embodiment, the hydrophones 22 are concentrically
arranged on each of the elementary transmission rings 21. An elementary
transmission ring 21 is located inside a ring of hydrophones 22. The
elementary transmission rings 21 and the hydrophone rings 22 are, as in the
first embodiment, distributed along the height H.
30 It is possible to position the elementary transmission rings 21 so
that they are in contact with one another. The elementary transmission rings
21 then occupy the entire height H. The same applies for the hydrophone
rings 22. An antenna 40 that is very compact in terms of height is thus
obtained. In this arrangement, the tube 25 may, as above, be watertight and
35 the interior of the tube 25 may be used for positioning electronic equipment
therein. The elementary transmission rings 21 then operate according to the
ABR technique.
Alternatively, in this second embodiment, inner walls of the
elementary transmission rings 21 are in contact with a fluid in the liquid state.
5 This liquid may be enclosed inside the tube 25. The presence of liquid allows
the acoustic performance of the antenna to be improved. In order to benefit
from the advantages of the presence of a liquid without increasing the mass
of the antenna, it is possible to allow the water into which the antenna 14 is
dipped to come into contact with the inner walls of the elementary
10 transmission rings 21. To this end, the antenna 40 comprises openings 41
that are arranged between the elementary transmission rings 21. These
openings allow the water into which the antenna 40 is dipped to flow along
inner walls 42 of the elementary transmission rings 21. Thus, when the
antenna 14 is not submerged, the water bathing the interior of the antenna
15 disappears and does not increase the mass of the antenna. When the inside
of the elementary transmission rings 21 is bathed in the environment into
which the antenna 40 is dipped, the transmission rings 21 operate according
to a technique in which water flows freely around the transmission ring 21.
This technique is known in the literature by the term "free-flooded ring" or
20 FFR.
In order to allow the presence of the openings 41, the antenna 40
comprises multiple supports 44 linking the two structures 28 and 29. The
transmission rings 21 are fixed to the supports 44.
In figure 3, the openings 41 are radial. It is also possible to make
25 these openings in the structures 28 and 29.
By implementing the FFR technique, the internal space of the
transmission rings 21 is no longer available for positioning electronic
equipment therein, which may be placed in watertight compartments made in
the structures 28 and 29.
30 The FFR technique may be implemented in the first embodiment
shown with the aid of figures 2a, 2b and 2c by providing one or more
openings allowing the interior of the tube 25 to communicate with the
exterior.
Figure 4 shows a third embodiment of an antenna 50 according to
the invention. The transmission rings 21 and the two structures 28 and 29 are
shown again. In this embodiment, the hydrophones 22 are not arranged in
rings, but on bars 51 that are fixed to the transmission rings 21. The bars 51
5 may be parallel to the longitudinal axis 20. In this embodiment, it is possible
to keep the supports 44 separate from the bars 51. Alternatively, the bars 51
may be used to connect the two structures 28 and 29 and replace the
supports 44. The.-bars 51 may be positioned inside or outside the
transmission rings 21.
In the various embodiments and in particular those implementing
the FFR technique, it is possible to streamline the antenna in order to
improve its hydrodynamic behavjor.
The abandonment of the deployable antenna principle leads to a
reduction in the diameter of the antenna 14, and hence an enlargement of
the main lobe in the directivity diagram of the beam formed by adding up the
signals from the hydrophones 22 after compensating for propagation delays
for a signal incoming in the setpoint direction. At 4 kHz, the width of this
20 diagram at -3 dB in a horizontal plane that is perpendicular to the axis 20
thus goes from 22" to 52" when switching from a cylindrical antenna with a
diameter of 700 mm composed of 12 deployable arms to a compact antenna
with a fixed cylindrical geometry and a diameter of 300 mm and composed of
12 columns, such as described in the present invention. Such a decrease in
25 resolution in terms of bearing poses a problem, as it not only translates into a
lower signal-to-noise ratio at the beamforming output, about -3 dB in terms of
isotropic noise if the deployable arms of the antenna and the columns of the
compact antenna are each composed of four hydrophones spaced apart by
160 mm along the vertical axis 20, but also into a loss of precision in the
30 angular location of a nearby target by a factor of three.
In the invention, use is made of the fact that the horizontal spacing
in a sub-array is much smaller than the half-wavelength, and that as a
consequence the ambient noise at two neighboring hydrophones is strongly
correlated, to carry out beamforming that maximizes the signal-to-noise ratio,
35 such beamforming being different from prior conventional beamforming that
maximizes the signal-to-noise ratio when the noise is spatially decorrelated.
The complex coefficients for baseband beamforming may be calculated from
the noise covariance matrix according to the well-known processing
formalism for adaptive antennas. According to the prior art, the covariance
5 matrix may either be calculated from the assumed angular distribution of
ambient noise, or estimated on the basis of signals from the antenna. With
respect to the conventional processing, which is the specific case of this
processing for a noise covariance matrix that is equal to the product of the
identity matrix and of the power of the noise at the output of a hydrophone,
10 the gain in signal-to-noise ratio for isotropic noise, i.e. directive gain, is
increased by 5 dB and the opening at -3 dB in the directivity diagram in the
horizontal plane is reduced to 23", thereby bringing the directive gain of the
compact antenna to 2 dB above that of the deployable antenna. The same
adaptive processing on the deployable antenna only leads to a gain of barely
15 more than 1 dB due to the low spatial correlation of the noise on this
antenna, for which the spacing between hydrophones is close to the halfwavelength.
, .,
In the prior art for sonar, groups of a small number of hydrophones
20 have a spacing that is smaller than the half-wavelength and their signals are
combined by means of cardioid-type beamforming whose directivity diagram
has zeros in predetermined directions. An antenna may be composed of
multiple groups of this type, the value of the spacing between groups being
close to the half-wavelength at the highest frequency in the band of the
25 sonar. In the invention, the horizontal spacing p2 between two neighboring
hydrophones is advantageously less than one third of the wavelength M3, it
is about Af5 in the example above, as is the spacing p l between neighboring
hydrophones of one and the same vertical column. To measure the spacing
between hydrophones, the projections from the phase centers of the
30 respective hydrophones is considered. The wavelength may vary depending
on numerous parameters, such as water temperature and pressure. For the
geometric definition of the spacing between hydrophones, the speed c of a
sound wave in water is taken to be 1500 mls. The wavelength A is given by
the formula A = clf. For an antenna of circular cross section, the horizontal
35 spacing between two hydrophones is the length of the chord between two
neighboring hydrophones over the diameter in question. For a working
frequency of 4 kHz, the wavelength is 0.375 m. For this working frequency,
the horizontal spacing between two neighboring hydrophones is therefore
advantageously less than M3, i.e.: 0.125 m. For a compact antenna with a
5 diameter of 300 mm working at 4 kHz and having 12 hydrophones 22 in each
horizontal plane, a spacing of the order of 0.08 m is obtained, i.e. of the order
of M5.
If the columns of the compact antenna in our example are
composed of seven hydrophones with a spacing of 80 mm instead of four
10 hydrophones with a spacing of 160 mm as first assumed, with the height
between the two end hydrophones of a column remaining equal to 480 mm,
the gain, with respect to the conventional processing, of the processing that
is optimized with respect to the spatial correlation of an isotropic ambient
noise becomes higher than 6 dB.
15 The directivity gain due to narrowing the spacing between
hydrophones and taking the spatial correlation of the isotropic noise into
account in the calculation of the beamforming coefficients is particularly
sensitive to antenna calibration errors. Beamforming with coefficients that are
calculated according to the same formalism as that which is optimum for a
20 perfectly calibrated antenna then gives performance that may be lower than
that of the conventional processing, for which reason this type of antenna has
not been employed until now. In the invention, this sensitivity may be
remedied by means of a coefficient calculation that aims to attenuate the
negative effect thereof according to a processing formalism referred to as an
25 "adaptive robust" formalism, which remains based on the noise covariance
matrix but takes the uncertainty in the antenna's response in the direction of
the beam into account. There exist multiple coefficient calculation variants.
The processing described in the article "On Robust Capon Beamforming and
Diagonal Loading" by Jian LEE, published on July 7 2003 (IEEE
30 Transactions on signal processing, Vol. 51 No. 7) may, for example, be used.
With respect to the ideal case in which antenna calibration is perfect and for
realistic calibration errors, for example with a standard deviation of 1 dB in
terms of gain and 10" in terms of phase, in the preceding example we lose
the advantage in directive gain of the compact antenna with a diameter of
35 300 mm composed of 12 columns of seven hydrophones over the deployable
antenna with a diameter of 700 mm composed of 12 arms each bearing four
hydrophones, both antennas then having almost equivalent directive gains.
CLAIMS
1. An omnidirectional antenna intended to be equipped by a dipping sonar
( I ? ) , the antenna (14; 40, 50) comprising multiple elementary
transmission rings (21) that are formed around a longitudinal axis (20)
of the antenna (14; 40; 50) and multiple hydrophones (22) that are
distributed around the longitudinal axis (20), the antenna (14; 40; 50)
being intended to be dipped in water, the hydrophones (22) being
separate from the elementary transmission rings (21), the hydrophones
(22) and the elementary transmission rings (21) being fixed to one
another in the antenna (14; 40; 50), characterized in that the elementary
transmission rings (21) and the hydrophones (22) are interlinked along
one and the same height (H) measured along the longitudinal axis (20),
in that the elementary transmission rings (21) and the hydrophones (22)
operate at a working frequency of less than 8 kHz, in that a first
minimum cylindrical volume around the longitudinal axis (20) occupied
by the elementary transmission rings (21) and a second minimum
cylindrical volume around the longitudinal axis (20) occupied by the
hydrophones (22) have a diameter ratio of less than 30%, in that the
diameter ratio of the two volumes is constant regardless of whether the
antenna (14; 40, 50) is in operation or in a storage position and in that
the projections from the phase centers of the hydrophones (22) in a
horizontal plane (P) that is perpendicular to the longitudinal axis (20), or
in a vertical plane (V) containing the longitudinal axis (20), are spaced
apart by less than N3, A being the wavelength at the working frequency.
25 2. The antenna as claimed in claim 1, characterized in that the
hydrophones (22) are arranged on bars (51) that are fixed to the
transmission rings (21).
3. The antenna as claimed in claim 1, characterized in that the
30 hydrophones (22) are distributed in rings that are formed around the
longitudinal axis (20).
4. The antenna as claimed in claim 3, characterized in that the
elementary transmission rings (21) and the hydrophone rings (22) are
arranged alternately along the height (H).
5 5. The antenna as claimed in any one of claims 1 to 3, characterized in
that the hydrophones (22) are superposed over, the elementary
transmission rings (21).
6. The antenna as claimed in either of claims 3 and 4, characterized in
that the elementary transmission rings (21) and the hydrophone rings
(22) form a tube (25) that extends along the longitudinal axis (20)
between two ends (26, 27), in that the antenna (14, 40, 50) comprises
two structures (28, 29) that close the tube (25), each at one of the ends
(26, 27) of the tube (25).
7. The antenna as claimed in any one of claims 1 to 5, characterized in
that the inner walls of the elementary transmission rings (21) are in
contact with a fluid in the liquid state.
20 8. The antenna as claimed in claim 7, characterized in that it comprises
openings (41) that allow the water into which the antenna (40) is dipped
to flow along inner walls of the elementary transmission rings (21).
9. The antenna as claimed in claim 8, characterized in that the openings
25 (41) are arranged between the transmission rings (21).
10. A dipping sonar (11) comprising a cable (13) and an antenna (14) as
claimed in one of the preceding claims, the antenna (14; 40, 50) being
suspended from the cable (13).
30
11. The dipping sonar (1 1) as claimed in claim 10, characterized in that it
additionally comprises a winch (12) that allows the cable (13) to be
wound in and wound out.
35 12.The dipping sonar ( I I) comprising a computer (15) and an antenna (14)
as claimed in one of claims 1 to 9, the computer (14) being configured
to carry out sonar beaniforming, beamforniing parameters being
calculated on the basis o'f the covariance of a spatially correlated noise
according to an adaptive processing formalism that is robust with
respect to antenna calibration errors (14).