IL201876A - Method for finding the bearing of a sound-emitting target - Google Patents

Abstract

The invention relates to a method for locating a sound projecting target by means of an elongated underwater antenna (10) having a plurality of electroacoustic converters (11), wherein a horizontal target location (ß<SUB>zk</SUB>) is determined in the water as the target in a directionally selective manner from the receiving signals of the converters (11), including a measured variable (C<SUB>mess</SUB>) of the acoustic velocity. In order to compensate the systematic location error inherent in such an underwater antenna, which may lead to erroneous locations particularly in the case of greater locating angles, the acoustic radiation course is calculated in the acoustic distribution direction specified by the target location (ß<SUB>zk</SUB>) by means of an acoustic distribution model, and a vertical acoustic incident angle (?<SUB>k</SUB>) is determined at the location of the antenna from the acoustic radiation course for an estimated distance and depth of the target. A correction factor is deduced from the vertical acoustic incident angle (Yk), and the measured variable (C<SUB>mess</SUB>) of the acoustic velocity is corrected by multiplying the measured variable by said acoustic incident angle. Locating in turn is carried out using the corrected variable (C<SUB>einstell</SUB>) of the acoustic velocity, and an improved target location (ß<SUB>zk</SUB>) is obtained.

Description

A T L A S E L E K T R O N I K G m b H Bremen METHOD FOR FINDING THE BEARING OF A SOUND-EMITTING TARGET The invention relates to a method for finding the bearing of a sound-emitting target by means of an elongated underwater antenna which has a multiplicity of electroacoustic transducers, as claimed in the precharacterizing clause of claim 1.

In a known direction-finding method for passive location of a sound-emitting, that is to say sound-producing, sound-reflecting or sound-scattering target or object (EP 01 308 745 Bl) , a so-called linear antenna is used having a multiplicity of electroacoustic transducers arranged in one or more rows. Linear antennas such as these are, for example, so-called towed arrays or flank arrays arranged on the vessel hull or boat body. The linear antenna covers a reception sector within which incident sound which is emitted from the target or object and propagates in the water is received by the transducers. In order to determine the direction of the incident sound, a fan of directional characteristics or beams which covers the reception sector is generated by means of a so-called beamformer, using the signals received by the electroacoustic transducers. Every directional characteristic, which can be scanned electronically in the horizontal direction with respect to a reference direction, has a relatively small horizontal beam angle and a vertical beam angle of greater or lesser size depending on the number of vertically arranged electroacoustic transducers, as well as a main direction with maximum reception sensitivity. The horizontal direction at right angles to the underwater antenna, also referred to as the lateral direction, is normally chosen as the reference direction. In order to produce the various directional characteristics or beams, the signals received by the transducers are delayed in time, to be precise such that they are in-phase for the respective scan angle of the main direction of the directional characteristic, and the in-phase received signals are added to form so-called array signals, which form the directional characteristic. The delay times for the signals received by the individual transducers are calculated on the basis of the speed of sound measured at the antenna location, the position of the transducers within the underwater antenna, and the scan angle of the respective directional characteristic. When sound is received, the fan of directional characteristics is searched for that directional characteristic in which there is a sound reception maximum. This is determined by detecting the level maximum of the array signals which form the directional characteristics. The scan angle of the main direction of the directional characteristic is emitted as the target bearing. The target bearing is presented in numerical or graphic form on a display.

In linear antennas such as these, both the horizontal and the vertical beam angle of the directional characteristics or beams vary with the scan angle of the main direction of the directional characteristic or the magnitude of the bearing angle. In the case of a bearing angle of 0° laterally with respect to the underwater antenna, the horizontal beam angle is at its narrowest, and the vertical beam angle is at its widest. As the bearing angles increase with respect to the reference direction in the forward or astern directions, the vertical and horizontal beam angles of the directional characteristics approach one another.

By way of example Figure 1 shows the -3dB contour line of a directional characteristic or beam of a linear antenna for finding a target bearing, for three different scan angles of its main direction, that is to say for three different target bearings or bearing angles β. The bearing angle β is plotted on the abscissa, and the vertical sound incidence angle γ is plotted on the ordinate. While the directional characteristic is symmetrical in the vicinity of the lateral direction (β=0°), the -3dB contour line for a target whose bearing is astern or ahead bulges "like a banana". If the sound comes exclusively from the horizontal direction, then the maximum of the directional characteristic always lies on the β coordinate, precisely at the indicated target bearing. However, if the sound comes from a vertical direction, as is indicated by way of example by the dashed straight line running parallel to the β coordinate, then the maximum of the directional characteristic runs along the dashed-dotted line of the maximum reception sensitivity indicated in Figure 1 to greater horizontal bearing angles β, which leads to an offset in the actual bearing, that is to say an excessively large bearing angle β is in principle indicated. The bearing error that results in this case is symbolized in Figure 1 by the double-headed arrow on the β coordinate. As the magnitude of the bearing increases, that is to say as the bearing angle increases, this offset increases. In consequence, in the case of vertical sound incidence, systematic bearing errors occur as a function of the bearing angle β in all linear underwater antennas, or underwater antennas which are similar to a linear form, because of their large vertical beam angle 2Θ_3<3Β of, for example, 75° (± 37.5° upwards and downwards), as is illustrated in the diagram in Figure 2.

The invention is based on the object of specifying a method for finding the bearing of targets of the type mentioned initially, which is subject to less error and therefore produces more accurate bearings.

According to the invention, the object is achieved by the features of claim 1.

The method according to the invention has the advantage that the very large bearing errors which occur in the so-called endfire region of the underwater antenna, particularly for bearing angles which are well away from the lateral direction, are virtually largely eliminated, and equally reliable bearings are therefore obtained in all bearing directions of the linear underwater antenna. At large water depths, in which the sound propagation model calculates a sound ray profile which does not change in an angle range around the measured target bearing, a single correction, derived from the vertical sound incidence angle, to the speed of sound included in the time delays of the received signals is sufficient to obtain a very accurate bearing, even in the "endfire" region, by adaptation of the directional characteristic. At shallow water depths, in which the bottom profile of the water channel can normally change significantly in different bearing directions, and the sound propagation can therefore change significantly, an improved target bearing is obtained by iterative determination of the vertical sound incidence angle and repeated correction of the speed of sound, continually, with this target bearing approaching a convergence value, which indicates the minimized-error bearing to the target, after a small number of iterations.

Expedient embodiments of the method according to the invention together with advantageous developments and refinements of the invention will become evident from the further claims.

According to one advantageous embodiment of the invention, the reciprocal of the cosine of the determined vertical sound incidence angle is used as the correction factor for the multiplication by the sound propagation speed.

The invention will be described in more detail in the following text with reference to one exemplary embodiment, which is illustrated in the drawing, in which : Figure 1 shows, by way of example, a -3dB contour line, plotted against the bearing or the bearing angle β and the vertical sound incidence angle γ, of a directional characteristic of a linear underwater antenna for three different bearing angles , Figure 2 shows a schematic illustration of the bearing error of the linear antenna as a function of the bearing β, Figure 3 shows a block diagram in order to explain the direction-finding method, and Figure 4 shows a schematic illustration of a sound ray profile calculated by means of a sound propagation model within a vertical beam angle 20_3dB of the linear antenna for a bearing angle of β=45°.

The method for finding the bearing of a sound-emitting target by means of an elongated underwater antenna will be described in more detail in the following text using the block diagram illustrated in Figure 3. In this case, a sound-emitting target means an object which is at a location remote from the antenna and produces sound, reflects sound or scatters sound back, which sound propagates in the water and is received by the underwater antenna. Examples of an elongated or linear underwater antenna, or an underwater antenna which is similar to a linear form, are so-called towed arrays, which are towed by a watercraft, or flank arrays which are arranged on the vessel hull or boat body of underwater vehicles, or else PRS (Passive Ranging Sonar) antennas which are fitted to a watercraft.

In principle, in all antenna systems, the target bearing, that is to say the bearing angle, which the bearing direction to the target includes with a reference direction, for example the horizontal perpendicular to the underwater antenna, is determined from the output signals from the electroacoustic transducers, by means of signal processing that is carried out in a bearing apparatus. To do this, it is necessary to enter the current value of the speed of sound at the location of the underwater antenna in the bearing apparatus, with this value generally being measured in advance in the vicinity of the underwater antenna. The bearing apparatus is designed differently depending on the underwater antenna being used and, when using a linear antenna with a multiplicity of transducers arranged in one or more rows alongside one another, is fundamentally different than when using a so-called PRS antenna, which comprises three reception bases, which are arranged at a relatively long distance apart from one another and are composed of electroacoustic transducers which are arranged aligned with one another. The term linear antenna subsumes the abovementioned flank arrays and towed arrays.

Figure 3 schematically illustrates a linear antenna 10 in the form of a flank array and with a multiplicity of electroacoustic transducers 11 arranged in one or more rows alongside one another. In general, the electroacoustic transducers 11 are arranged on an antenna mount 12 at a constant distance d from one another. In the case of a flank array, each of the transducers 11 which are arranged alongside one another on the horizontal, also referred to as a stave, normally comprises a plurality of transducer elements arranged on the vertical, although this is not illustrated any further here. The number of vertical transducer elements, which are preferably located at equal distances from one another, is considerably less than the number of transducers 11 arranged in one or more rows horizontally alongside one another for beam forming. The output signals from the transducer elements which are arranged vertically one above the other are added and normalized and form the signals received by the transducers 11 or staves, referred to in the following text as received signals. In the case of so-called towed arrays, there are no transducer elements arranged vertically one above the other so that, in this case, the output signals from the transducers 11 directly represent the received signals. The bearing apparatus 17 which is connected to the outputs of the transducers 11 or staves comprises a beamformer 18, a reception level measurement device 19 and a level maximum detector 20.

In order to find the bearing of a target, a fan 13 of directional functions or directional characteristics 14, also referred to as a beam fan, is formed in the bearing apparatus 17 by means of appropriate signal processing using the signals received from the electroacoustic transducers 11 or staves, with each directional characteristic 14 (also referred to as a beam) having a narrow horizontal beam angle and a relatively wide vertical beam angle, which is dependent on the number of transducer elements arranged vertically one above the other in each transducer 11. The axes 15 of the directional characteristics 14 or beams represent the main direction of the directional characteristic in which the maximum reception WO 200Θ/138433 - 8 - PCT/EP2008/002640 sensitivity occurs. These are defined and are produced electronically by a horizontal scan angle or beam angle β with respect to the common reference direction 16.

In order to form the directional characteristic 14, the signals received from the transducers 11 or staves are delayed in time or phase in the beamformer 18, to be precise such that they are in-phase for one specific reception direction or bearing direction pj . The delay times which are calculated using: Ti,j = i ■ d · sin β-; · cset (1) are for this purpose stored for all reception directions j (j=l...m) and all transducers 11 or staves i (i=0...n) in the beamformer 18. d is the horizontal transducer separation on the antenna mount 12, and c3et is the value of the speed of sound in water as entered in the beamformer 18. The value of the speed of sound cmeaa as measured at the antenna location before the start of the direction-finding process is assumed as the start value for cset . The in-phase received signals which are obtained in each reception direction in this case are added to form so-called array signals . The levels of the array signals are measured in the reception level measurement device 19 and are stored, associated with the reception directions j or the bearing angles β-j . A level maximum detector 20 determines the level maximum and emits the bearing angle, which is associated with the array signal with the level maximum, as the target bearing zk where k=l,2...K. This array signal with the maximum level represents the directional characteristic 14 in which a sound reception maximum occurs.

In order to reduce the initially described bearing errors of the bearing apparatus 17, which are considerable in particular for relatively large bearing angles β, the sound ray profile for a sound propagation direction which is predetermined by the target bearing βΖ]ί obtained is generated in the block 21 using a sound propagation model. Acoustic sound propagation models such as these are widely known. A listing can be found in: Heinz G. Urban "Handbuch der Wasserschalltechnik" [Manual of Water Sound Technology] STN ATLAS Elektronik GmbH, 2000, pages 305 and 306.

By way of example, Figure 4 schematically illustrates the calculated profile of the sound rays within a vertical beam angle of the linear antenna 10 of about 75° for a sound propagation direction which is predetermined by a target bearing βζι=45°, assumed by way of example. Depth intervals are plotted on the ordinate, and range intervals on the abscissa. The shaded area represents the seabed. A vertical sound incidence angle γ at the antenna location is determined from this sound ray profile for an estimated target range and an estimated target depth. In the illustration in Figure 4, the antenna is located in the vicinity of the origin of the coordinate system. The target range is either known or is estimated by means of other sensors or methods. For example, the target range can be taken from a method, which is also carried out for data support purposes, for passive determination of target data, for example as described in DE 101 29 726 Al . However, it can also be made more precisely from previous target bearings. In the diagram in Figure 4, the target range of the target that is annotated Z is assumed to be 47 000 yards. The target depth is likewise estimated, for example in this case to be 10 m.

From the sound ray diagram, that sound ray originating from the target Z and having the least attenuation is determined. In the example in Figure 4, two sound rays, inter alia, propagate from the target Z which is located at a depth of 10 m and arrive at the antenna location at a vertical sound incidence angle of 3.75° and 22.5°. The high attenuations during the sound propagation are caused by reflections on the water surface and on the seabed. While the sound ray which originates from the target Z and arrives at the antenna at a vertical incidence angle of 3.75° experiences multiple reflections on the water surface, the sound ray which originates from the target Z and arrives at the antenna at a vertical incidence angle of 22.5° for example is attenuated only by propagation losses in the water, and therefore to a considerably lesser extent, because of the lack of bottom reflection and surface reflection. This incidence angle of 22.5° is therefore determined as the vertical sound incidence angle γι associated with the target bearing βΖι· A correction factor is calculated in the correction block 22 from the vertical sound incidence angle γ¾ at the antenna location obtained by means of the sound propagation model in the block 21, as the reciprocal of the cosine of the vertical sound incidence angle ^. This correction factor is used to correct the measured value of the speed of sound cmeas used to form the directional characteristics 14 and the array signals at the start of the direction-finding process. For this purpose, the speed of sound cmeas is multiplied by the correction factor in the speed of sound correction element 23, using: Cset = cmea3 ■ —-— where k = 1,2...K (2) . cosyk This new value of the speed of sound is now set in the beamformer 18. A direction-finding process is now carried 1 out once again in the manner as described above using the delay times Tj,j that have been modified in this way from the stored delay time record, thus resulting in a better target bearing βζ)Ι which, in the abovementioned example with a first target bearing of βζι=45°, now leads to a better target bearing of βζ2=41.3°.

At great water depths, the bottom profile does not change over a relatively large area around the bearing direction, which means that the sound ray profile as calculated by means of the sound propagation model in the block 21, and as illustrated in Figure 4, is still valid for the better target bearing βζ2 and therefore produces an identical vertical sound incidence angle of, for example, γ2=22.5°. The direction-finding process can therefore be ended, and the better target bearing βΖ2 as determined by the bearing apparatus 17 is output and indicated as the bearing of the target βζ.

In shallow water regions, when the water depth is small, the bottom profile changes in different sound propagation directions, as a result of which a different sound ray profile is calculated for the new target bearing in the sound propagation model, and this results in a different vertical sound incidence angle Yk- For the better target bearing that is obtained in the example of βΖ2=41.3°, the sound ray profile calculated once again in block 21 using the sound propagation model results in a different vertical sound incidence angle γ¾ for the same target range and target depth. The better target bearing βζ2 of βζ2=41.3° in the example that is obtained therefore still contains a bearing error, although this bearing error is reduced. In order to eliminate this as well, the better target bearing βζ1: (in the example βΖ2=41.3°) is subjected to the same procedure as the first target bearing βζ ck-i> obtained (in the example βζι=45°) . The sound ray profile for the target Z, which is assumed to be at the same range and depth, is calculated once again for the better target bearing βζ^ (in the example βζ2=41.3°) by means of the sound propagation model in the block 21. A new vertical sound incidence angle of, for example, 73=33.75° is now determined from this sound ray profile, and a new correction factor is calculated in the correction block 22 by calculation of the reciprocal of the cosine of this new vertical sound incidence angle yk. Using the new sound incidence angle, in the example 73=33.75 °, the new value of the speed of sound c3et to be set in the beamformer 18 is once again calculated in the speed of sound correction element 23 using equation (2) , The bearing of the target is once again found using the set of time delays that has been changed as a consequence of the new cset in the beamformer 18, resulting in a new better target bearing, for example βζ3=37.84°. This described process is repeated until the vertical sound incidence angle yk determined using the most recently obtained, and once again improved, target bearing βζΐ£ from the sound propagation model no longer changes within predetermined limits. When this is the case, then the target bearing βζ)ς which has been determined most recently by the bearing apparatus 17 and has once again been improved is output and indicated as the bearing of the target βζ . If the vertical sound incidence angle γ3=33.75° in the last indicated example were no longer to change, then the bearing of the target would be βζ=37.84°.

The equality in the vertical sound incidence angles yk-i and Yk determined in successive runs k using the sound propagation model in the block 21 can be determined in a simple manner by forming the difference between the most recently obtained sound incidence angle yk using the target bearing pzk which has once again been improved, and the vertical sound incidence angle yk-i which was obtained previously using the better target bearing βζθί-υ· I this difference (y - yk-i) is less than a preset value S, then equality has been reached and the most recently obtained, better target bearing βζχ is output as the bearing of the target βζ. For this purpose, by way of example, the previously obtained vertical sound incidence angle yk_i is passed via a memory or a shift register 23 to a subtractor 24, and the subsequently obtained sound incidence angle jk is passed directly to the subtractor 24, and the difference is compared with the preset value S in a comparator 25. If this preset value S has been undershot, then a gate 26 connected downstream from the bearing apparatus 17 is opened, as a result of which the most recently obtained, once again improved, target bearing βζ^ is passed to the display 27, where it is indicated as the bearing of the target β2.

The signal processing in a bearing apparatus which is connected to a so-called PRS antenna comprising three reception bases, which are arranged a long distance apart from one another on an alignment line, with electroacoustic transducers, is described by way of example in US 4 910 719, which is referred to expressly here. In this bearing apparatus as well, the respectively set value of the speed of sound in water is corrected as described above in the blocks 21, 22 and 28 and is input to the bearing apparatus as cset(new). The described iterative process until the bearing of the target βζ is achieved is also the same.

Claims (1)

1. PATENT CLAIMS A method for finding the bearing of a sound-emitting target by means of an elongated underwater antenna which has a plurality of electroacoustic transducers (11) , in which a horizontal target bearing (βΖχ) is determined on a directionally selective basis from the signals received by the transducers and including a predetermined value (cmeas) ' measured in particular at the antenna location, of the speed of sound in water, characterized by the following method steps : the sound ray profile in a sound propagation direction which matches the specific target 1 bearing (βζ*) is calculated using a sound propagation model, a vertical sound incidence angle at the antenna location is determined from the sound ray profile for an estimated target range and an estimated target depth, a correction factor is determined from the vertical sound incidence angle (γ¾) , the predetermined value (cmeaa) of the speed of sound is corrected by multiplication by the correction factor, and a target bearing (βζι<) , which has now been improved, is determined once again using the corrected value of the speed of sound (c3et) · The method as claimed in claim 1, characterized in that the additional method steps are repeated iteratively using the improved target bearing (pzk) until the vertical sound incidence angle (yiJ determined using the respective target bearing (Pzk) which has once again been improved no longer changes or changes only insignificantly, and in that the most recently obtained target bearing (Pzk) is output as the bearing (β2) to the target. The method as claimed in claim 1 or 2, characterized in that the reciprocal of the cosine of the vertical sound incidence angle is calculated as the correction factor. The method as claimed in one of claims 1 to 3, characterized in that the estimated target range and the estimated target depth are determined from previous target bearings or using other sensors and/or target data determination methods. The method as claimed in one of claims 1 to 4, characterized in that the sound propagation model is used to calculate all the sound rays which occur in a vertical reception sector of the underwater antenna (10), and vertical incidence angles and attenuations which result from the sound rays are indicated for each range interval and depth interval, and in that, in order to determine the vertical sound incidence angle (γ^) , the incidence angle is determined of that sound ray which is subject to the least attenuation, starting from the target at the estimated target range and target depth. The method as claimed in one of claims 1 to 5, characterized in that a fan of directional characteristics (14) is formed from the signals received by the transducers (11) in order to determine the target bearing (β21ί) , which directional characteristics (14) have a horizontal and a vertical beam angle and a main direction (15) of maximum reception sensitivity, which is scanned with respect to a common reference line (16) , and the scan angle of the main direction which a sound reception maximum occurs is indicated as the target bearing (p2k) .