AU701700B2 - Reception method with ambiguity removal for a towed linear acoustic antenna - Google Patents

Description

;s Reception Method with Ambiguity Removal for a Towed Linear Acoustic Antenna The present invention relates to reception processes which make it possible to remove the ambiguity customarily present in the detection signals derived from the signals received by a towed linear acoustic array. It applies more particularly to location by a very low frequency active sonar.

Towed linear acoustic arrays are known, formed by a set of hydrophones arranged in a very long flexible tube whose buoyancy is set such as to be substantially neutral in the medium, the sea generally, in which it is submerged. These acoustic arrays are towed behind a hauling boat, taking maximum care that they remain as straight as possible. The signals received by the hydrophones are equally well acoustic noises arising from a source of noise, such as the screws of a boat, as the echoes arising from the reflection of an acoustic signal emitted by an acoustic emitter fixed to the hull of the boat or towed together with the array. In the first case the array operates in passive mode and in the second case in active mode. The dimensions, several hundred metres or even several kilometres, which these arrays may take allow operation at very low frequency, and particularly in active mode, thus allowing long-range detectiop.

The signals received by the hydrophones are converted by them into electrical signals which are then processed in suitable electronic circuits. It is known to form reception channels with this processing, corresponding to directions which are more or less inclined to the axis of the array. Since the hydrophones used do not generally have any particular directivity in themselves, especially at low frequency, Sthese channels in fact have a syiimetry of revolution about the axis of the array and therefore form cones of revolution centred thereon. Since the sound source, can arise from any point situated in the cone, there is p RIZ0 therefore a considerable ambiguity within a channel. In -i

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N,6 14 12 practice and on account of a number of asymmetries, in particular of the surface of the sea on which most boats lie, this ambiguity is limited essentially to a right/left ambiguity, which has to be removed.

Various means of solving this problem are known in the art: It is known for example in the case of an active sonar to use a directional emission which makes it possible to project sound in a preferential manner into each of the two half-spaces situated to the right and to the left of the axis of the array. As described in particular in French Patent Application No. 91 03853 filed by the applicant on 29 March 1991 and granted on 14 May 1993 as No. 2 674 717, use is then made of orthogonal codes usually amounting to different frequencies. Under these conditions the signals obtained after processing to form a channel differ depending on whether the object giving rise to the echo is to the right or to the left. However, it is difficult to produce such a dual directional emission into the two half-spaces. To do so it is necessary to use, especially at very low frequency, emission arrays of large dimensions containing numerous transducers, the complication of which contrasts with the simplicity of the omnidirectional emission array generally used.

Furthermore, in order to separate the two emissions they have to correspond to two disjoint frequency bands and since the complete frequency band av2 ble is n,-t very large and is already completely ed in tne customary systems, it is in fact require, to emit on each side in half the possible band, thereby correspondingly reducing the processing gain.

As described in particular in French Patent Application No. 91 03853 filed by the applicant on 29 March 1991 and granted on 14 May 1993 as No. 2 674 717.

Another technique consists in producing an array or an array system which exhibits intrinsic right/left directivity. This requires the use of linked I t) -3hydrophones which may either be distributed among separate linear arrays towed in parallel, according to the so-called rake technique, or combined within one and the same linear array of bigger transverse dimensions, for example according to a technique described in French Patent Application No. 89 11749 filed by the applicant on 8 September 1989 and granted on 17 April 1992 as No. 2 651 950. In the case of multiple linear arrays it is difficult to maintain or ascertain their relative positions.

This then leads to the use either of complicated mechanical systems for keeping them truly parallel, or complicated real-time telemetry systems for determining the relative positions and applying the corrections corresponding to their variations. In the case of pick-ups with intrinsic directivity, these are difficult to produce and furthermore have lower r,)'ormance in terms of sensitivity and are bulkier than a conventional hydrophone whilst also being far more expensive. Such hydrophones are in particular j described in French Patent Application No. 88 16803 filed on 20 December 1988 by the applicant and granted on 26 July 1991 as No. 2 640 842 as well as in Patent i Application No. 91 16162 filed by the applicant on 26 December 1991 and granted on February 1994 as No. 2 185 848.

i Summary of the Invention In accordance with the invention, there is provided a process for improving 20 directional reception of acoustic signals using a towed linear acoustic array, the towed linear acoustic array including a set of linearly spaced apart hydrophones for receiving the acoustic signals and converting them to electrical signals, the process including thei steps of: i causing the towed linear acoustic array to undulate along its length; and delaying and summing the electric signals generated by the respective hydrophones whilst taking into account the undulations of the towed linear acoustic array, thereby to generate one or more reception channels, wherein the undulations are of sufficient amplitude to generate a main lobe and sidelobes in a spectrum of the s reception channel or channels, the relative differences in amplitude between the main l ir 16 r_ 3a lobe and lie side lobes being sufficiently high that a direction from which the acoustic signals are received by the hydrophones is ascertainable.

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17 Rn xamle bY 4 According to another characteristic the curvature due to the undulating path is sufficiently high for the reception channels to comprise a main lobe and sidelobes whose level is sufficiently small relative to the level of the main lobe as to allow the ambiguity in reception between these two lobes to be removed.

According to another characteristic the curvature makes it possible to obtain an attenuation of -9 dB in the level of the sidelobes relative to the level of the main lobe.

According to another characteristic the curvature due to the undulating trajectory is sufficiently low for the whole of the array to follow substantially the trajectory of the tip of this array.

According to another characteristic in order to tow the array a fish is used which is steered so as to make it possible to obtain the said undulating motion.

According to another characteristic hydrodynamic flaps are used to steer the fish.

According to another characteristic the motion of the fish is controlled by an open-loop system and a set of pick-ups is used to reconstruct the trajectory of the fish.

According to another characteristic the determination of the trajectory of the array is refined by using a hydrodynamic model which makes it possible to determine the deviations r2lative to the trajectory of the fish.

According to another characteristic in order to obtain the undulating trajectory means are used to oscillate the point of anchoring to the towing boat of the cable serving to haul the array.

According to another characteristic these means of oscillation consist of an arm which oscillates in the horizontal plane relative to the longitudinal axis of the towing boat and to the end of which is fixed the hauling cable.

18 According to another characteristic the linear array is used in active mode to receive the echoes arising from the reflection of the acoustic signals I emitted by an omnidirectional array and the instants of emission of this omnidirectional array are adjusted so as to maximize the ratio between the level of reception of this echo on the main reception lobe on the one hand and the sidelobes on which this echo is also received on the other hand.

Other features and advantages of the invention will emerge clearly in the following description i presented by way of non-limiting example with regard to the appended figures which represent: Figure 1, the shape of a trajectory imposed i 15 on the array according to the invention; Figure 2, the deviation between the actual shape of the array and the trajectory of its tip; Figures 3 and 4, two diagrammatic S0 representations of a linear array hauled by a boat; 20 Figure 5, a side view of a fish serving to tow an array according to the invention; Figure 6, the schematic diagram of a system for retrieving the geometry of the array; Figure 7, the chart of a reception channel i mounting [sic] the main lobe and the sidelobe; Figure 8, a schematic diagram of the i formation of the channels and of the detection of the echoes in these channels; andi Figure 9, a schematic diagram of a variant embodiment of the invention. i It is known that, to form a reception channel in an array, delays are imparted to the various signals received by the pick-ups of this array before summing these signals. These delays depend on the siting of the pick-ups with respect to one another and on the direction of the channel to be obtained. Therefore, in general they determine a single direction only. j In the case of a linear array, more S particularly an acoustic array, it is the symmetry rCi 19

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6- imparted by the alignment of the pick-ups on the axis of the array which leads to the obtainment of a coneshaped reception channel.

When the symmetry is broken, for example following accidental curvature of the array, the processing gain 2 side the cone decreases rapidly in most directions, thereby leaving only one favoured direction forming a single reception channel, with more or less attenuated sidelobes distributed in an approximate manner in the initial cone. As it is not I known where this channel points, and since in particular it may very well be directed towards the surface or towards the bottom, which is generally of no interest, every means is employed to endeavour to

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retain the straightness of the array in the most perfect way possible.

The invention runs counter to these customs by proposing to afford the linear array a curved and known shape in the horizontal plane for example, in such a way that the processing of the signals from the hydrophones makes it possible to form separate right and left channels no longer exhibiting right/left ambiguity. To do this a first processing operation forms a left channel and a second processing operation a right channel. In fact each of these processing operations entails both the formation of a main lobe on i one side corresponding to the channel formed and that of a sidelobe on the other side corresponding to the initial ambiguity, but the level of the sidelobe is low i enough to be able to remove the ambiguity by comparing the reception level on the two sides.

The invention furthermore proposes to impose a low-amplitude oscillatory motion on the tip of the array in the horizontal plane, in such a way that the shape of the array is known at each instant from the motion of this tip, this making it possible continuously to match the processing of the signals from the hydrophones so as to form the desired Z channels.

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7 Thus, conventional linear arrays whose diameter is small compared with their length and which are correctly equilibrated, that is to say whose density at every point is very close to that of water, exhibit the feature that, when the distortion of the array is small, this corresponding to a low angle of incidence at all points of the array (the speed vector of each point of the array remains substantially parallel to the mean longitudinal axis of the array at this point), the trajectory of the array is such that each successive point along it follows very substantially the trajectory of the tip as it advances. Thus the array behaves somewhat like a train which is following rails which are not straight, or like an earthworm advancing in the hole which it digs as it progresses.

Therefore, Lo obtain a given shape of array it then suffices for the tip to follow the requisite trajectory, the shape of the array at each instant subsequently coinciding very substantially with this trajectory.

It is of course possible to imagine all kinds of trajectories meeting these conditions, but since what matters in essence is that the shape of the array is not straight, those trajectories which are simplest to obtain will be as good as any others and therefore a priori the best since they will make it possible simultaneously to use simple means to obtain them and simple means to perform the channel formation calculations.

Among the various simple shapes, the invention proposes to select more particularly a sinusoidal trajectory such as that of Figure 1 which represents a substantially sinusoidal trajectory 101, with different scales in respect of distance, along the abscissa, and deformed shape, along the ordinate. The array which follows this trajectory is represented at two different instants which correspond to two different shapes 102 and 103.

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i .i 8 By way of example, for a linear array 100 m long towed at a speed of 10 kn, i.e. 5 m per second, with the aid of a hydrodynamic body situated at the tip of this array and the motion of which is controlled by steering gear so as to obtain a trajectory in the horizontal plane representing a sinusoid of period 100 m likewise and with amplitude 2 m, the time period of the motion will be 20 seconds and the maximum angle between the axis of the array and its mean straight trajectory will be of the order of 40. It is thus appreciated that the angle of incidence remains very small everywhere and that therefore the trajectory of the hydrodynamic body situated at the tip of the array and the shape of the array remain very similar.

Furthermore, extremely accurate hydrodynamic models are known of a highly elongate ody such as an array of this type, which make it possible to calculate to an accuracy of a few centimetres the shape of this body as a function of the trajectory followed by the tip under the abov defined conditions of low curvature. The result of the modelling calculations applied to the array corresponding to the above numerical values is represented in Figure 2 by the curve 202 which therefore represents the actual shape 25 of the array relative to the trajectory of the tip of the latter represented by the curve 201. It may be seen that the deviation, which moreover is perfectly predictable and stable, is extremely tiny between these two curves.

In order to implement the invention the simplest system is represented in Figure 3. In this figure a boat 301 sailing on the surface tows a linear acoustic array 302 at a relatively low and constant submersion depth, hauled from the boat with the aid of a towing cable 303. This towing cable is hitched to the boat with the aid of a device 304 which makes it possible to vary the point of anchoring to the boat on either side of the axis of the latter according to a periodic and symmetric motion. This device can consist lr~

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9for example of an arm fixed on a vertical axle and to the end of which is hitched the cable 303. This arm is driven by means such as a cam for example, which allow the end of the arm to which the cable is hitched to be to describe the alternating motion in question.

The profile of the cam will then be set so that the tip of the arm describes a motion which, in view of the advancing of the boat, corresponds to the desired deformed shape of the cable, a sinusoid for example.

The cable then follows this motion and transmits it to the tip of the towed array 302, the body of which itself also follows the motion.

This device is simple but has the drawback that the ielationship between the motion of the end of the arm and that of the tip of the array is not as strict as in the case in which the motion of the tip of this array is excited directly in the manner which will be described later. Furthermore, the motion of the tip of the arm is perturbed by the motions of the boat, due in particular to the waves. This motion can of course be compensated for by appropriate mechanical means of correction, but this is done to the detriment of the simplicity of the system. However, when these motions are compensated for, or else when the sea is sufficiently calm, the relationship between the motion of the tip of the arm and that of the tip of the array is sufficiently constant for it to be possible, if need be by prior calibration, to regard the motion of the array as stable and well known.

It is pointed out in particular that this embodiment is especially suited to the -ase of an acoustic linear array operating passively and so not requiring the use of a submerged acoustic emitter as in the preferred embodiment which will now be described, since it is no longer necessary to use a body' intended to receive the emission array.

This preferred embodiment is represented A diagrammatically in Figure 4: a boat 401 tows an Sacoustic linear array 402 via a towing cable 403 and -if the link between the array and the cable is effected with the aid of a towed body 4u4, represented diagrammatically here in the form of a ball. In the known manner this towed body exhibits both a considerable mass anc considerable hydrodynamic drag, thus making it possible to stabilize the motion of the array despite jerks arisii.g from the towing.

Furthermore, and also in the known manner, the streamlining of this body is designed so that it exhibits a negative lift enabling the array to be maintained at a stable and specified depth beneath the surface of the water, view of the dimensions and mass of this body, also called a "fish", the acousti emission means necessary when it is desired to use the linear array in an active mode are generally placed inside this bod-'> The invention then proposes to use this fish also to apply the necessary motion to the tip of the a array, intended to have the deformed shape enabling ambiguity removal to be achieved. One of the means for achieving this effect consists in furnishing the fish with hydrodynamic steering gear which is actuated in such a way as to make the fish undertake an undulating motion in the horizuntal plane, this being transmitted to the array.

Represented in Figure 5 is a side view of a particular embodiment of such a fish. The latter comprises a streamlined body 501 having the shape o a vertical symmetric aircraft wing which serves as carrier structure to the other elements of tle fish and which includes inside it an omnidirectional very low frequency emission array having the sh-pe of a cylinder 502 of elliptic cross-section so as to best fill the free space inside che body 501.

Hitching means 503 make it possible to fix the i towing cable 403 to the body of the fish in an articulated manner with two axes of Lt-eedom. O- of I these axes allows the anchoring point of the c )le to be inclined towards the surface thereby leaving the '3;4i~z. e ilc ie ffaj i:

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1 11 fish free to navigate horizontally, and the other axis enables the fish, under the action of the steering geEr described later, to orient itself about the vertical in such a way as to initiate the undulating motion necessary to the invention.

Anchoring means 504 situated at the rear of the body 501 ma:,e it possible to fix the array 402 to the latter so as to tow it. These anchoring mneans allow for the passage of the electrical connections of the array which pass through the body and are plugged into the towing cable 403 by way of fixing means 503.

The upper end of the body 501 is topped off witn a stabilizing dihedral 505 in the form of an inverted wing. It makes it possible both to maintain the body in its vertical position and to exert a pattern oC negative lift which tends to sink the entire fish to the bottom of the water. This depressor action allows the array 402 to be maintained with the desired submersion.

According to the invention, the fish furthermore includes vertical steering gear formed by a rudder blade 506 supplemented with a trailing edge flap 507. This steering gear is actuated by a motor sit.iated in the body 501 and obeying control signals coming from the hauling boat via the towing cable 403. Under the action of these signals the motor turns the rudder blade 506, either directly or indirectly by way of the flap 507, in such a way as to send the fish to the left and to the right of the trajectory followed by the boat. The signals applied enable an undulating motion, preferably sinusoidal, of the fish in the horizontal plane to be obtained thus. As seen earlier, this motion is communicated to the tip of the array and the remainder of the body of the array follows the motion so as to obtain the effect desired in the invention.

This embodiment thus described is merely a particular example and many others may be conceived. It is in particular possible to conceive a body whose hydrodynamic shape gives rise to an instability in the ji it n I P--LL~-9 L I jl 2/8

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0 ft 12 horizontal direction manifested as an oscillati.on naturally giving a sinusoidal motion.

Thus, in order to form the channels with sufficient accuracy it is necessary to reconstruct the shape of the array with an accuracy of the order of 1/10th of the acoustic wavelength. This necessitates direct measurement of the displacement of the tip of the array, and hence that of the fish, and it therefore matters little whether the fish obeys the commands of the steering gear with high accuracy or exhibits an intrinsic undulating motion.

The schematic diagram of a system for controlling and measuring the trajectory of the array is represented by way of example in Figure 6.

The parameters of the trajectory, that is to say essentially the amplitude and frequency in the case of a sinusoidal motion, are applied to a steering gear control block 601 which delivers signals enabling the steering gear 506/507 to be oriented at angles corresponding to the motions to be obtained, given the known parameters of the fish. As explained earlier this steering is carried out in open loop since it would be unfounded to wish to install servocontrol in the strict sense.

25 The fish also comprises a set of sensors, collected together in the block 602 in Figure 6, the readings from which enable the trajectory of the fish to be reconstructed. These sensors can be of various kinds, and the simplest embodiment, given the accuracy to be obtained, consists for example of a three-axis accelerometer combination, preferably of very low characteristic frequency, in order to reconstruct this trajectory by double integration, ini a marner similar to that used in inertial platforms. Given the current 35 orders of magnitude, an exaiwple of which was cited earlier, the accuracy required for these accelerometers in order to obtain the necessary accuracy in the trajectory is of the order of a milli-g, this being entirely achievable with current accelerometers. The i s 1

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3/8 1; 'egjRd~ 13 readings from these accelerometers can be supplemented with attitude sensors which enable a second-order correction to be made to the calculations making it possible to retrieve the trajectory on the basis of the signals from the accelerometers. These signals are applied to a calculation block 603 which makes it possible to reconstruct the trajectory of the fish on the basis of the transfer functions established by prior calibration between the motions of the platform and the signals delivered by the pick-ups. This trajectory will be calculated, using preferably digital filtering, over a duration equivalent to the time of transit of the linear array. This time of transit is

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given by where L is the length of the streamer and v v the speed of the hauling boat. The accuracy obtained in the trajectory gives a relative accuracy in the positions over this duration and not a long-term accuracy, this being normal when accelerometers are used as pick-up. However, in any event the long-term accuracy is given essentially by the fact that, since the fish is towed, its mean trAjectory is essentially that of the hauling boat since because it is linked to the latter by a cable there cannot be any drifting which distances it lastingly from the boat.

The shape of the array at each instant is reconstructed in a block 604 from the trajectory of the fish as determined in the block 603. This is done using the methods of calculation which link the trajectory of the tip of the array to the trajectory of each part thereof, more particularly the parts which contain the hydrophones. These methods of calculation, which were mentioned earlier, are well known in the art and they make it possible to obt-in an accuracy of better than a few centimetres over the entire length of the array, the main perturbations being taken into account by the measured trajectory of the fish. The three-dimensional shape is reconstructed, although the invention requires only a curvature in the horizontal plane, since it is -i1 i

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14 inevitable, in particular giver the jerkiness arising from the hauling boat, that the fish will have a slight spurious motion in the vertical direction, communicated to the array. This spurious motion affects the shape of the array and hence the accuracy of the detection channels. Since it is known how to take it into account in order to form these channels in such a way as to obtain the desired directions, and since this accounting of the vertical motion takes up virtually no hardware but merely a little calculation time, it is actually preferable to take it into account.

The signals from the attitude sensors of the array, when the latter is provided therewith, can moreover be used in the block 604. Generally these sensors make it possible to determine the heading and submersion of the array and their use will make it possible to fit the parameters of the model enabling the shape of the array to be reconstructed from the trajectory of the fish, and possibly these parameters to be corrected in real time. A table determining the relative positions in space of the various hydrophones of the array is therefore obtained at the output of this block 604, with the recurrence necessary for the digital formation )f the reception channels of this array.

The values of the angles of the steering gear delivered by the block 601 can also be used, as the case may be, in the block 603 to refine reconstruction of the trajectory of the fish. The results, obtained in the block 604, of the reconstruction of the shape of the array can also be fed back to the steerilig gear control block 601 so as to correct the trajectory parameters in order to correct any slow variations over time in the overall transfer function of the system, which would be manifested for example as a left or right drifting of the fish/array assembly corresponding to a non-zero mean deflection of the steering gear contrlling the fish.

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15 As explained earlier, when the linear array has a sufficiently curved geometry, the directivity function of the channels which can be formed on taking this geometry into account in an exact manner no longer exhibits a symmetry of revolution about the axis of the array, and this directivity function therefore possesses a main lobe with a single maximum in the direction for which the channel is formed and a variable number of sidelobes in varying directions, the levels of which are lower than that of the main lobe.

When an echo is received from an acoustically reflecting source, it is received on the various lobes and experience shows that with a deviation of a few dB between the level of this echo in the main lobe and the levels of the same echo in the sidelobes of the other channels, it is possible to determine with a sufficiently low probability of error that this echo is properly situated in the main lobe of the channel in question. These discrimination tests are known in the art of sonars and start from an implicit assumption according to which the echo is a single echo, at least in the sectors in which there is a risk of there being an ambiguity for a given distance.

RFresented in Figure 7 by way of example is the secti_, for a zero elevation, through the directivity function of a deformed array having a pseudo-sinusoidal shape of wavelength 75 m with an amplitude of 0.75 m. The array itself measures 75 m and the sampling o, the signals from the hydrophones corresponds to an acoustic wavelength of 1.50 m. It may be seen in this chart that the deviation in level between the main lobe 701 an4 the sidelobe of highest level 702 is well over 9 dB and therefore allows discrimination between the two directions corresponding to these lobes in a single recurrence.

Represented in Figure 8 is a schematic diagram of the processing of the signals from the hydrophones o.f a deformed array according to the invention.

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gbmw 6/8 16 The array shape data, coming for example from the block 604 of Figure 6, are applied to a block 701.

These data in fact correspond to the relative positions of the hydrophones in space at the instant t of sampling the signals from the hydrophones. The block therefore makes it possible to calculate from these positions the temporal or phase delays to be a-.plied to the signals from the hydrophones in order to form the channels in the desired directions. This calculation is performed repetitively according to the recurrence of the samples from the hydrophones and while taking into account the position of the hydrophones at each of these instants. The calculation also makes it possible to obtain the amplitude weighting coefficient [sic] for the hydrophone signals which make it possible, according to a known technique, to reduce the level of the sidelobes.

The signals from the hydrophones are then delayed, and possibly weighted, in a channel formation block 702 so as to thus obtain a set of n reception channels.

The signals corresponding to these n channels are subsequently processed according to a code matched to the emission code in a block 703, according to a known technique. The energy level received in each distance cell, possibly after an integration, is available for each channel at the output of this block.

Lastly a final processing operation, itself also conventional, is performed in the block 704 which, after normalization and detection, makes it possible to retain for each distance cell the n strrongest detections and subsequently to perform a test making it possible to determine, as a function of the level of the echo and of the signal-to-noise ratio estimated in each channel, in which channel the source of the echo is most probably situated.

Fine measurement calculations in terms of bearing and distance can subsequently be performed on these elementary detections and their associated 1;

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17 measurements by any known methods, for example by interpolation.

In a variant of the invention, the amplitude of the deformation is constrained by acting on the steering gear commands, possibly with the aid of a servocontrol, such that: it is sufficiently small as to cause a negligible loss of detection if it is not compensated for during formation of the channels; it is nevertheless sufficient to allow removal of the right/left ambiguity in the echoes when it is taken into account in the formatj of the A channels. I Thus the right and left channels .ed on compensating for the deformation exhibit at output I strongly correlated noise so that a small rejection t value suffices to remove the ambiguity with sufficient probability. The amplitude of the corresponding deformed shape is then small (typically and the signal losses from the channels formed when neglecting the deformation are then less than 1 dB.

Processing is then performed in two steps which 7 makes it possible to remove the ambiguity without knowing all the parameters (in particular the amplitude) of the deformation. i The first step consists in forming ambiguous channels, without taking the deformed shape into account, followed by filtering matched to the code i emitted (or by any other temporal processing), and then steps of normalization, detection, measurements. The second step, shown diagrammatically in Figure 9, requires prior storage (901) of the i hydrophone signals of the recurrence in progress, as well as accelerometric and/or attitude pick-ups (902).

For each of the echoes detected, a restricted number of pairs of right and left channels (904) is formed in the Sneighbourhood of the mean bearing of the echo measured i J at the moment (905) of ambiguous reception. These pairs I 1 i 18 of channels take into account the attitude of each pick-up of the array, reconstructed (903) on the basis of the acceleration and/or attitude sensors of the fish and/or of the array. The amplitude of the deformation is a parameter which is optimized as a function of the output of the pairs of channels formed, with regard to the criterion of the maximum level of the echo at output.

Reading the channel formation coefficients of the channel in which the echo is a maximum makes it possible to determine (906) the direction (right or left) of the target.

The process according to the invention basically makes it possible to remove the right/left ambiguity in reception by a towed linear array.

Furthermore, it offers a number of derived advantages: as has been seen, the precise shape of the array is continuously evaluated from the navigation data of the towed body, and even from certain attitude data for the array, and this evaluation does not in fact depend on the motion of the hauling ship. Any motions of this ship are therefore automatically compensated for;, whether caused by heaving due to the sea or by manoeuvres of this boat, for example a change of heading. This advantage is particularly important since in the system used hitherto, in which the array had to be maintained as straight as possible, reception became totally incoherent when the hauling boat changed direction.

Moreover, as has been seen, the main lobe of the channels thus formed also exhibits elevational directivity. This enables the echo/reverberation ratio to be considerably improved, this being particularly valuable when navigating in shallow waters where the reverberation on the bottom represents a considerable level of noise.

Finally, since the shape of the array is changing continuously, as may be seen in Figure 1, the reception level on the sidelobes varies periodically as 19- X a function of the changes of shape, themselves linked with the advancing of the array. Hence, when a target has already been detected, where [sic] presumed detected, the detection and measurement performance of the array can be optimized by synchronizing the emission of the sound projector, in the case of an active sonar, in such a way that at the scheduled instant of reception the array has the optimum shape which maximizes the attenuation of the image lobes relative to the main lobe. The existence of the source of the echo can thus be confirmed and specific pursuit undertaken in respect of this source, making it possible to exceed the range and signal/noise ratio limits generally obtained. 4 iA ai ,I

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Claims (7)

1. A process for improving directional reception of acoustic signals using a towed linear acoustic array, the towed linear acoustic array including a set of linearly spaced apart hydrophones for receiving the acoustic signals and converting them to electrical signals, the process including the "teps of: causing the towed linear acoustic array to undulate along its length; and delaying and summing the electric signals generated by the respective hydrophones whilst taking into account the undulatiens of the towed linear acoustic array, thereby to generate one or more reception channels, wherein the undulations are of sufficient amplitude to generate a main lobe and sidelobes in a spectrum of the reception channel or channels, the relative differences in amplitude between the main lobe and the side lobes being sufficiently high that a direction from which ;he acoustic signals are received by the hydrophones is ascertainable. S* 2. A process according to claim 1, wherein the undulation provides a relative attenuation of -9dB between the main lobe and the side lobes.

3. A process according to claim 1 or 2, wherein the relative amplitude of the undulations with respect to the length of the towed linear acoustic array is sufficiently small that paths traversed by various points on the linear acoustic array correspond substantially with a path traversed by a forward end of the towed linear acoustic array. C 25 4. A process ac-ording to any one of claims 1 to 3, wherein a steerable fish is used at or adjacent a forward end of the towed acoustic array, the path of the steerable fish being controlled to impart the undulation to the towed linear acoustic array. II:IB INMlA1HO1 in34MXL -21- .A A process according to claim 4, further including the step of using hydrodynamic flaps to steer the fish along a desired path.

6. A process according to claim 4 or claim 5, wherein the steerable fish is controlled by an open loop system, and the resultant path of the steerable fish through the water is determined using pickups associated with the steerable fish.

7. A process according to claim 6, further including the step of using a hydrodynamic model to determine the paths traversed by various points of the array, based on the determined path of the fish.

8. A process according to any one of claims I to 3, wherein undulation imparting means mounted to a boat towing the towed linear acoustic array laterally oscillate with respect to the boat, thereby to impart the undulations to the towed linear acoustic array. Sthereto, and a hauling line extending between a distal end of the arm or boom and a forward end of the towed linear acoustic array. o

9. A process according to any one of the preceding claim wherein the undulation imprting towed linear acoustic array is used in an active mode to receive echoes arising fromespect 25 the aeousto, signals being dynamically adjusted to maximise the ratio betworen the maind a lobe and side lobes generated by reception of the echoes by the towed linear acoustic array. array. [N:\LIBEIO1134:MXL bi r L -22-

11. A process for improving the directional reception of acoustic signals substantially as herein described with reference to anyone of the embodiments of the invention shown in the accompanying drawings. DATED this Twenty-fourth Day of November 1998 Thomson-CSF Patent Attorneys for the Applicant SPRUSON FFRGUSON R At 0 C so i S S.. i w /:3 4: s 22 ABSTRACT RECEPTION PROCESS WITH REMOVAL OF AMBIGUITY FOR A TOWED LINEAR ACOUSTIC ARRAY The invention relates to processes which make it possible to remove the ambiguity which customarily exists in reception signals of towed acoustic linear arrays (402) It consists making this array traverse a deliberately deformed trajectory (101) and in forming the reception channels whilst taking account of this deformation. Under these conditions these channels are deformed such as to exhibit a main lobe (701) and sidelobes (702) which are attenuated relative to the main lobe, thus making it possible to remove the ambiguity of reception. It makes it possible to improve the detection of the positioning of the sources of the echoes received by such a towed acoustic linear array. FIGURE 1 I U I '4i 8P 2 4 j,