GB2503183A

GB2503183A - Unfolding cylindrical sonobuoy

Info

Publication number: GB2503183A
Application number: GB8822845.7A
Authority: GB
Inventors: Guy Riccardi; Alain Lemer
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1987-10-02
Filing date: 1988-09-29
Publication date: 2013-12-25
Anticipated expiration: 2008-09-29
Also published as: GB8822845D0; DE3833272B3; GB2503183B; IT8867861A0

Abstract

In a sonobuoy equipped with unfolding arms (2, figure 1) for carrying the hydrophones 7 of a sonar antenna, said hydrophones 7 are grouped in doublets 10. Processing means (11, 12; figure 3) allow to define for each doublet 10 a directivity pattern in bearing in the form of a cardioid oriented downwards, and to add up these diagrams to obtain an overall diagram (figure 4) slightly inclined downwards and a wide null upwards, which permits to reject the noise from an helicopter from which said sonobuoy is hanging, without increasing the length of said arms (2) with respect to the known methods.

Description

Unfolding eylindrical sonar The present invention relates to unfolding cylindrical sonars that can be used, for example, from an helicopter.

It is known From the rrench patent \O 2 354 920 to cons-truct a sonobuoy including on a body a set of unfolding arms equipped with hydrophones. When the sonobuoy is jettisoned.

the arms are folded back against the body, which facilitate.s its penetration into water and avoids breaking these arms off.

When it is stabili2ed in w;ter at the desired depth, the arms unfold to give a cylindrical listening base with a diameter substantially larger than the body of the sonobuoy.

The electronic processing of the signals received by the hydrophones of each arm allows to localize the noise soures, for example the submarines. It is known, for localizing these noise sources, to form by electronic means channels oriented in bearing. In the case of a sonobuoy or of a sonar hanging on a cable from an helicopter hovering close to the sea sur-face, the noise from the hlicopter, as well as the acticn of the wind From the rotor onto the sea surface, cause a Si- gnificant inerference on reception. It is then known to redu- ce the amplitude of the interfering signals received from abo-ve by forming channels oriented in bearing frbm the signals delivered by the hydrophones of each arm. To obtain a low le- vel of the sidelobes, it is known to weight the spatial dis-tribut ion of the elemental hydrophones within the arms of the I')' NJ -J.S U' C U' C) U' * C I CL:i fl.1) 0) 0 7 V (I) C) It 0. .0 CL 0) 1) rt Ci CD 1 Ii ((El C I t C) III < -7 Cl.i C) ID ID -C) 10' J,1.-(U J P1 -. U) 01 (U (I) U C/) 3 (1. Ct 0) 3 (0 - I CD -3 0) It -:3 LU ii -It CD 3 -If) fl I) to -.-0 0 7 N (I) CO P1 CD CD -r? * p-UI CD 0 I-C 3 -. -çT Ct C 0 ID 0 CO 10 Ct 0 -t, SD i- (0 P1 to 3 ID It 7 0) 3 0) 0 9 7 0 CD Cl_a) ,Q CD CO afl CD P1 P1 CD rt C) CD So 1 -C rr 3 3'< 0 P1 0) -, 0 0) 3 --C 0 ID a 0) Ct CD 710 - 3 -Dl a 9 3 C 3 -0) 0) 10 0 3 CD P1 0 CD t-ID o -It ID CD -, (0 -C -Ct -S CD P1 CD 0.3 0) 10 -t CO 3 -CO CD CD 3 U 10 3 -3 W o a w -0) (0 3 3 3 C 0 0) 01 0 C 3 3 C CD C -C U 3 P1 P1 -, V o to P1 CJ rt Ct 00.. )-.L) CU 0 -.0 0) 3 0 (0 3 P1 0) 10 * * 7 -U C.J It C 0 -.0 U C. -, -* Cj Cl-3 U) -. ID-0 0) P1 C 3 0) 0 -CD W 3 It 3 A (fl CU 0) 3 -.

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-Figure 2 Is a schematic diagram of the set of associated hydrophones carried by an arm of an embodiment of a sonobuoy according to this invention; -Figure 3 is a block diagram of the circuit 11 in Figure 2; and -Figure 4 Is the directivity pattern in elevation of the sonobuoy.

Figure 1 shows the mechanical structure of a sonobuoy with unfolding arms. The sonobuoy comprises a body 1 main-tamed in a vertical position at the end of a cable 8 that permits, for example, to lower it from an helicopter into the sea. Around this body, there are disposed a set of folding arms 2 that are accomodated, in folded position, in a set of recesses 9 located longitudinally alongthe generatrices of the body 1. Each arm 2 comprises a set of two rods 3 attached at one end by two joints 5 to the body 1, and a vertical bar attached to the other end of the rods 3 by two joints 6 so as to form with the rods 3 a parallel motion located in the vertical plane going through the axis of symmetry of the sonobuoy and such that it can fold back against the body and unfold to a position in which the rods 3 are horizontal and the bar 4 is vertical. When the arms 2 are unfolded, the bars 4 of these arms are disposed along a vertical cylindrical ring coaicial with the body 1. These vertical bars are equipped with transducers, namely hydrophones, whose signals are processed by electro-nic circuits contained in the body I to form in the horizontal plane preferential reception directions called "channels't Figure 2 shows a bar 4 of an embodiment of a sonobuoy according to the invention including hydrophones 7 distributed along the height of the column.

In this embodiment, the hydrophones are grouped in doujiets 10 whose signals are applied to electronic processing circuits 11 that allow to obtain a directivity pat-tern in elevation having the shape of acardicid whose maximum is oriented downwards and whose null is oriented upwards.

The electronic processing used to obtain this directivity pattern in the form of a cardioid is performed, for example, by means of the circuit shown in Figure 3.

The two hydrophones of one and the sane doublet 10 are spaced by a distance d, and the signal received in the direc-tion 0 with respect to the vertical is. s(t) Far the higher -hydrophone and s1(t) for the lower hydrophcne. . These two signals are added in a first Edder 30, and sub-tracted from each other in a second adder 60. The signal from the adder 60 is phase-shifted by 51/2 in a ph;se shifter 50 and multiplied by a coefficient/ -cctgCiYd/2)(where 2 is the wavelength cf the received signal) in a muleiolier 70, then the signal thus obtained is added in sn adder 30 to the signal from the adder 30.

If A coscJt is the received signal, the resulting sign x(t) provided by the adder 80 is equal to x(t) 2A cosJt{cos(cosQ) + sin(czs0)].

If d is small compared to 2, this forrnul8 reduces to x(t) r tA coswt(1÷cos), which correscands do a cardioid to which one comes closer as d is smaller. However, there is a lititation on shortening the spacing between the microphones due to the susceptibility to the scatter of the characteristics of these hydrophones, which increases with the value of the coefficient The signals from tre various processing elements 11 (three of them in Figure 2) are then added up in a summing circuit 12 to obtain the output signal S corresponding to the column 4 and that will then be processed in a known manner to obtain the channels in bearing.

As an example, for an active sonar operating at 4kHz, the spacing d is equal to 7 cm, that is about 0.18 \. Each bar includes three doublets spaced by../2, which gives a total height of about 45 cm for the bar.

Figure 4 shows the:verall directivity pattern in bearing for the signal obtained at the output of the summing circuit 12. This diagram is a function of the number of doublets, of the spacing of the doublets and of the spacing of the transdu-cers of each doublet. In this Figure 4, the ordinates are the levels in decibels related to the maximum amplitude of the signal 5, and the abscissas are the angles in degrees bet-ween the vertical upwards, for which QrQ, and the vertical downwards, for which -90°. It can be seen on this diagram that, from 0 to -10°, there is a very significant attenuation of -50 decibels that allows to fully reject the noise:crning, * for example, from an helicopter carrying the sonobuoy. It can also be seen that the maximum of the signal is at _1DC under the horizontal, which Is quite suitable for the detection of submarines targets.

Thus, in this embcdiñient, the overall height of the colunm 4 is reduced by about 25%, In addition to the rejection of the noise coming From abo-ve and the reduction of tie length of the arms and, therefore, possibly of the length of the sonar body, a wider frequency range can be used because of the correction introduced by the coefficient(3 and to use a smiler flumoer of hydrophones For each column.

Of course, the invention is applicable to the case where several bars A are attEched to the end of the rods 3 in a ra-dial plane to form a three-dimensional antenna.

Claims

. . -6-CLAIMS1. An.unfolding cylindrical sonar, comprising a cylindri-cal body, a set of arms articulated on said body and each including a column of transducers and rods at the end of which said columns are attached, said arms being capable of folding back against said body and of unfolding so that the columns form a cylindrical antenna, and means for pro-cessing the signals from said transducers, wherein said transducers are grouped in doublets and said processing means comprise means for obtaining from said signal from each of said doublets, a directivity pattern in bearing substantially in the form of acardioid oriented downwards and having a null towards, anda summing circuit for adding up the output signals of said means for processing the signals from said doublets.
2. A sonar according to claim 1, wherein the number of doublets, the spacing of said doublets and the spacing of *the two transducers of each doublet are set so that the output signal of said summing circuit has a maximum of directivity in bearing slightly inclined downwards.
3. An unfolding cylindrical sonar substantially as herein-before described with reference to the accompanying drawings.CCAmendments to the claims have been filed as follows 1. An unfolding cylindrical sonar, comprising a cylindri-cal body, a set of arms articulated on said body and each including a column of transducers and rods at the end of which. said columns are attached, said arms being capable of folding back against said body and of unfolding so that the columns form a cylindrical antenna, and means for pro-cessing the signals from said transducers, wherein said transducers are grouped in doublets and said* processing means providing an*otut* signal corresponding to each doublet, said output signals having a carthoid directiv-ity pattern oriented in use with a maximum downwards and a null upwards, and a summing circuit for adding up said output signals.2. A sonar according to claim 1, wherein the number of doublets, the spacing of said doublets and the spacing of the two transducers of each doublet are set so that the output signal of said summing circuit has a maximum of directivity in bearing slightly inclined downwards.3. An unfolding cylindrical sonar, substantially as herein-before described with reference to the accompanying drawings.