RU2161319C1 - Method for detection of underwater objects on sea bound in shallow sea - Google Patents

Abstract

FIELD: detection of underwater objects. SUBSTANCE: hydroacoustic pulses directed to the upper half-space are radiated from a number of points with known coordinated located on the bottom in the bound line, echo signals reflected from the objects are received in the same points, and location of the object is judged by the coordinates of the receiver that has detected the echo signal, pulses are radiated in the adjacent points at different frequencies and in all points simultaneously, echo signals of natural frequency and at least of two other frequencies are received in each point from detectors on each side adjacent in the line, the distance between the adjacent points of reception-radiation are selected to be equal to a double sea depth in the bound line; the time of arrival of echo signals is limited by the time of arrival of pulses reflected from the sea surface at each frequency received in the given point, and the location of the detected object is defined more exactly by the coordinates of the radiator operating at the frequency of the received echo signals. The level of direct hydroacoustic pulses received from the adjacent radiator is watched, and in case of variation of this level the appearance of an object near the bottom is judged. EFFECT: enhanced truth and continuity of monitoring, prevented zone of unsure reception. 2 cl, 3 dwg

Description

 The invention relates to sonar methods for detecting small underwater objects in the shallow sea - underwater swimmers, small submarines, submarine scooters, etc.

 To detect underwater objects at a specific sea line, a set of active sonar stations are used, which are usually installed at the bottom, forming a barrier line of the line. In order to reduce the number of stations forming a barrier line, they strive to increase the range of each station by increasing its energy potential (radiated power, increasing the directivity of emitting and receiving antennas, etc.). To increase the range of the stations also use the separation of radiation and reception along the barrier line, i.e. bistatic detection systems. The coordinates of the detected targets relative to the location points of the stations are determined in such systems by the time and direction of arrival of the echo signal from the target. The range of these systems, as a rule, significantly exceeds the depth of the sea at the place of their installation, and the reliability of detection significantly depends on the hydrological characteristics of the sea along the propagation path of signals that are subject to variability during the year, time of day, and other factors. At the same time, echo signals are detected against the background of surface and bottom reverberation, which substantially complicate the detection of a useful signal. To combat reverberation in these systems, methods are used that are based on reducing simultaneously voiced volume, sea surface and bottom. This is achieved by reducing the duration of the pulses, using directional scanning in space radiation and reception. A similar method is used in the "Bistatic-monostatic sonar barrage system" according to US Pat. US N 5305286 with a priority of 05/21/1992, as well as in the "Bistatic sonar" located on the platform according to US Pat. U.S. N 5237541 dated 05/21/1992

 In the shallow sea, the influence of hydrology on the path of sound propagation, surface and bottom reverb is especially strong. To ensure the inspection of the entire controlled water volume, it is necessary to scan a narrow beam in the vertical plane and in azimuth, which requires a complex scanning device and does not provide constant observation of the entire field along the boundary.

 A known method of detecting small objects in the shallow sea, used in the "Detection System swimmers" (reference Yanes 1995-96, p.105). In this system, non-directional or non-directional radiation is carried out from a number of points located at the bottom on the line of the guarded barrier, echo signals are reflected from objects at the same points, and the location of the object is judged by the coordinates of the receiver that detected the echo signal. The method can be applied in shallow water with a fairly frequent arrangement of the reception-emission points with a distance between them comparable to depth (see figure 1.). The method allows to exclude the effect of changes in hydrology on the sound propagation in the detection zone due to the small refraction of sound during vertical propagation from the bottom to the surface.

 The disadvantages of the method include the formation of zones of uncertain detection, in cases where the distance to the object exceeds the depth of the installation site and the detection is carried out against the background of surface reverb, especially at large angles of incidence of sound on the surface, characteristic of lighting from below (see figure 1.) With simultaneous emission from all active points of the barrier line, crosstalk is generated, which requires sequential operation of underwater acoustic modules at the points of the barrier line, which also eliminates continuous ny control space.

 Thus, it is desirable to obtain a method for detecting underwater objects at the sea frontier in the shallow sea, which would ensure reliable and continuous control of the space and exclude the presence of zones of uncertain reception.

 In the proposed method for detecting underwater objects at the sea frontier in the shallow sea, as well as in the known one, hydroacoustic pulses directed to the upper half-space emit from a number of points with known coordinates located at the bottom on the boundary line, receive echo signals reflected from objects in these the same points and the position of the object are judged by the coordinates of the receiver that detected the echo signal.

 In order to ensure reliable and continuous control of the space without areas of poor reception in the proposed method, radiation at neighboring points is produced at different frequencies and at all points simultaneously receive at each point a signal of natural frequency and at least two other frequencies from the adjacent radiators on each side of the line, the time of reception of echo signals is limited by the time of arrival of pulses reflected from the sea surface at each of the frequencies received at a given point, the distance between adjacent radiation points is chosen equal to the double depth of the sea along the boundary line, and the location of the object is specified by the coordinates of the emitter operating at the frequency of the received echo signals.

 Improving the reliability of detection is achieved due to the fact that the reception of echo signals is performed only before the occurrence of the interfering effect of surface reverberation and due to the simultaneous emission of sounding signals of different frequencies at neighboring points that differ by a value that allows filtering by the frequency of the received echo signals. The reception of echo signals at each point is carried out simultaneously at least at three frequencies: the "natural" frequency of the adjacent radiator on the left and the frequency of the neighboring radiator on the right along the barrier line. Thus, a set of alternating monostatic and bistatic systems is implemented along the barrier. Figure 2 shows the operating conditions of a fragment of the barrier line with several points of reception-radiation. Confident reception zones (before surface reverberation) for monostatic systems are spheres of radii equal to the depth of the sea. Confidence areas for bistatic systems are ellipsoids with foci at the points of emission and reception at the same frequency. Figure 2 shows that the zones of uncertain (against the background of surface reverberation) reception between the spheres of monostatic systems are overlapped by ellipsoids of the zones of reliable reception of bistatic emitter-receiver pairs, and, thus, the entire water volume along the barrier line is simultaneously controlled.

где C - скорость звука в воде.The time during which echoes without surface reverb are received in the monostatic system T _m is determined by the sea depth at the installation site - H and can vary from 0 to

where C is the speed of sound in water.

до

где d - расстояние, равное половине большой оси эллипса.For a bistatic emitter-receiver pair, the reception time from a reverberation ellipsoid can vary from

before

where d is the distance equal to half the major axis of the ellipse.

до

, мы обеспечим зондирование поля до возникновения мешающего влияния поверхностной реверберации.Thus, limiting the reception time for a monostatic receiver to T _m , and for a bistatic receiver from

before

, we will provide sounding of the field before the occurrence of the interfering effect of surface reverb.

 The use of interconnected monostatic and bistatic systems in the method allows you to specify the coordinates of the detected object according to the coordinates of the emitter, the signal of which was received by the receiver at the frequency of this emitter.

 If the target is detected by only one monostatic system, i.e. the frequency of the received signal coincides with the frequency of the emitter located at the same point as the receiver, the target is in the sphere (see figure 2) of this system.

 If the target is detected by a receiver located at an adjacent point with the point of the emitter emitting signals of this frequency, i.e. only a bistatic system, which means it is in the shaded area between the points where the receiver and emitter of this bistatic pair are located (Fig. 2).

 And finally, if the target is detected by both monostatic and bistatic systems, then the coordinates of the target are specified in the zone of joint action of the monostatic and bistatic systems (Fig. 2).

 With the diversity of the radiation-reception system, it becomes possible to control the near-bottom space between the emitter and the receiver by directly shading the object with a direct sonar pulse from the emitter to the adjacent receiver. If we monitor the level of received hydroacoustic pulses, then when this level changes, we can judge the appearance of an object near the bottom.

 In FIG. Figure 1 shows the operating conditions of unidirectional emitters located at points on the bottom and radiating to the upper half-space. Figure 1 shows: 1 - bottom, 2 - water surface, 3 - receiving-emission points, 4 - zone (shaded), from which the echo signal will come to the receivers at points 3 against the background of surface reverb (uncertain reception zone).

 In FIG. Figure 2 shows the operating conditions of a fragment of the barrier sonar line, consisting of several acoustic modules with monostatic and bistatic reception. Designations: 6 - spheres of monostatic systems, 5 - ellipsoids of bistatic systems.

At point K-1, the frequency f _K -1 is emitted, and reception is carried out at frequencies f _K-1 , f _K-2 >, f _K.

At point K, a frequency f _{K is} emitted, and reception is carried out at frequencies f _K , f _K-1 , f _{K + 1} .

At the point K + 1, the frequency f _K + 1 is emitted, and reception is carried out at frequencies f _{K + 1} , f _K , f _{K + 2} .

 The proposed method can be implemented in the form of a barrier line for the detection system of underwater objects in the shallow sea, a fragment of which is shown in Fig.3.

 The barrier system consists of N receiving-emitting hydroacoustic modules 7 located at the bottom and connected by a cable line 8 with the equipment of the shore station 9. Each of the hydroacoustic modules 7 consists of a receiving-emitting hydroacoustic antenna 16 that emits and receives hydroacoustic signals from the upper half-space. Through the reception and transmission switch 17, the antenna 16 is connected to the input signal amplifier 18 and the signal generator 19, the power of which is supplied from the cable communication line from the shore post power supply device. Echo signals received and amplified by amplifier 18 are transmitted via cable line 8 to shore station equipment 9 from each receiver to their own processing channel, consisting of a set of frequency filters 10, where the received signals are separated by frequency. Then, the signals of each of the frequencies arrive at the gating devices 11, which transmit signals to the display device only during the time from the moment of emission to the moment of arrival of the signal reflected from the sea surface at a given frequency.

 The display device 12 represents to the operator the received echo signals from all N receiving-emitting modules, at all operating frequencies in each channel. When an echo appears in any channel at any frequency, the operator decides to detect it and determines the number of the channel that received the echo and the operating frequency at which the signal was received. The channel numbers and the combination of frequencies determine the location of the detected object. The coordinates of the locations of the hydroacoustic modules at the bottom of the sea are determined when the modules are installed.

 To count the time from the start of radiation, synchronizing pulses from the synchronizing device 14 are supplied to the display device 9.

 The power of the equipment of the coastal post and receiving-emitting hydroacoustic modules is carried out from the power device 15. The control device 13 enables and disables the power and controls the system.

 The generator 19 upon arrival of the synchronizing pulse via the cable communication line 8 from the synchronization device 14 generates a probing signal, which, through the receive-transmit switch 17, is transmitted to the antenna 16 and radiated into the sea.

 The display device 12 also monitors the level of the direct signal received at the frequencies of adjacent hydroacoustic modules, and, in the case of a level change, the operator concludes that the object intersects the space between these modules near the bottom.

Claims (2)

 1. A method for detecting underwater objects at a sea line in a shallow sea, in which hydroacoustic pulses directed to the upper half-space emit from a number of points with known coordinates located at the bottom on the line, receive echo signals reflected from objects at these points and about the place the position of the object is judged by the coordinates of the receiver that detected the echo signal, characterized in that they emit pulses at neighboring points at different frequencies and at all points at the same time, receive the echo signals proper at each point the frequency and at least two other frequencies from the adjacent emitters on each side of the line, the distance between adjacent points of reception and emission is chosen equal to the double depth of the sea along the boundary line, the arrival time of the echo signals is limited by the arrival time of the pulses reflected from the sea surface on each from the frequencies received at this point, and the location of the detected object is specified by the coordinates of the emitters, operating at the frequency of the received echo signals.  2. A method for detecting underwater objects at a sea line in a shallow sea according to claim 1, characterized in that they monitor the level of direct sonar pulses received from a neighboring emitter, and when this level changes, the appearance of an object near the bottom is judged.

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