Method for measuring target strength of underwater object

Abstract

FIELD: hydroacoustics.SUBSTANCE: invention which can be used to measure the target strength of autonomous unmanned underwater vehicles (AUVs), scale models of underwater craft, and other underwater objects. A method is proposed in which an underwater object installed at a given depth is irradiated with a pulsed acoustic signal from a distance determined by the boundaries of its Fraunhofer diffraction region (far field). Using a multi-channel measuring probe, made in the form of a generating sphere, installed in the immediate vicinity of an underwater object in its Fresnel diffraction region (near field), the amplitudes of the pulsed acoustic signal irradiating the underwater object and the pulsed acoustic signal reflected from the underwater object on a virtual spherical surface are measured, then, using the Kirchhoff integral formula, the amplitude of the acoustic signal reflected from the underwater object in the Fraunhofer diffraction region in an arbitrary plane of space is calculated. Next, using the ratio of the amplitudes of the reflected and radiating acoustic signals, the values of the radius of the equivalent sphere and the target strength of the underwater object in an arbitrary plane of space are calculated.EFFECT: transferring the measurement zone of the signal reflected from an underwater object from the Fraunhofer diffraction region (far field) to the Fresnel diffraction region (near field) and using a virtual spherical measuring surface ensures the measurement of the target strength values of an underwater object in an arbitrary plane of space with high accuracy by reducing the influence of signal reflections from the boundaries of the water area, as well as fluctuations in the signal reflected from the underwater object.1 cl, 3 dwg

Description

[0001] The invention relates to the field of hydroacoustics and can be used to measure such a parameter as the target strength of autonomous unmanned underwater vehicles (AUVs), scale models of underwater vehicles, and other underwater objects.

[0002] Currently, the use of underwater vehicles such as AUVs in both search and rescue and research missions has become widespread. Their timely detection and tracking is often achieved using hydroacoustic systems operating on the principle of echolocation.

[0003] The reflectivity of an underwater object irradiated by an underwater sound wave is estimated by the target strength parameter, equal to the ratio of the intensity of the sound wave reflected from the object, reduced to a distance of 1 meter, to the intensity of the sound wave incident on it.

[0004]

[0005] where T is the target strength of the object;

[0006] J1 - the intensity of the sound wave reflected from an underwater object, reduced to a distance of 1 meter;

[0007] Jп - intensity of the sound wave incident on an underwater object. [Urick Robert J. Fundamentals of Hydroacoustics. L., Shipbuilding, 1978].

[0008] In practice, the target strength (reflectivity) of underwater objects is compared to the target strength (reflectivity) of an ideal reflecting sphere of a given radius. This allows the target strength to be expressed through the radius of an equivalent sphere with the same reflectivity as the underwater object being studied.

[0009]

[0010] where R ЭКВ _ radius of the equivalent sphere.

[0011] The numerical value of the target strength (the radius of the equivalent sphere) is used in the development of any hydroacoustic system operating on the principle of echolocation, without exception, to determine the potential range and resolution of such a system. Therefore, reliable measurement of the target strength of underwater objects is of such great importance in the process of adapting existing hydroacoustic echolocation systems and designing new ones [Rumynskaya I.A. Fundamentals of Hydroacoustics. - Leningrad, Shipbuilding. - 1979].

[0012] The numerical value of the target strength obtained for the case of mismatch between the directions (angles) of the sound waves incident on an underwater object and reflected from it is used in a similar way in the creation of hydroacoustic echolocation systems operating in bistatic mode (separation of the emitting and receiving hydroacoustic antennas) [Sheinman E.L., Matveeva I.V., Shkolnikov I.S.; Joint-Stock Company "Okeanpribor Concern". Method for determining the current coordinates of a target in bistatic sonar mode. Patent No. 2653956 of the Russian Federation, IPC G01S 15 / 87. No. 2017112183; Claimed 10.04.2017; Published 15.05.2018, Bulletin No. 14].

[0013] A known method for measuring the target strength of submarines [Urick Robert J. Fundamentals of Hydroacoustics. - L., Shipbuilding. - 1978], involves irradiating an object and receiving an echo signal from it at a sufficiently large distance, determined by the size of the Fraunhofer diffraction zone of the object, and then recalculating the signal intensity that would occur at a unit distance.

[0014] The disadvantage of this method is the need to carry out measurements at a sufficient distance from the underwater object, determined by the inequality:

[0015]

[0016] where R is the distance between the underwater object and the emitter; L is the maximum overall size of the underwater object; R is the length of the sound wave.

[0017] This distance significantly reduces measurement accuracy due to the influence of signal reflections from the water area boundaries, as well as signal fluctuations over time due to the characteristics of their propagation. This significantly limits the applicability of the method and requires multiple measurements.

[0018] A similar method is known [Salin B.M., Kemarskaya O.N., Salin M.B. Near-field measurement of scattering characteristics of a moving object based on Doppler signal filtering / / Acoustic journal. - Scientific and technical collection: issue 6 (T56). - Moscow: 2010], which involves measuring the target strength of an underwater moving object when it is irradiated with a continuous tone signal and the reflected signal is received by an antenna located in the near field. Separation of the direct and reflected tone signals is achieved by separating the frequencies due to the occurrence of the "Doppler effect".

[0019] The disadvantage of this method is the need for precise positioning of the transmitting and receiving antennas relative to the underwater object's path. Furthermore, to ensure acceptable measurement accuracy, the object's movement must be linear and uniform, following a strictly specified course and depth, which is difficult to achieve in the open waters of the measurement site.

[0020] Furthermore, the specified analogs do not allow measuring the target strength values ​​obtained for cases of mismatch between the directions (angles) of sound waves incident on an underwater object and reflected from it.

[0021] The closest method in terms of the number of common features is the method for determining the target strength on a scale model [Mayorov V.S., Valyantinas K.I., Lisenkov N.M., Korolishin D.Z., Klimenkov A.S.; Russian Federation, represented by the Ministry of Industry and Trade of the Russian Federation. Method for measuring target strength on a scale model. Patent No. 2796238 of the Russian Federation, IPC G01S 15 / 02. No. 2022128866; Claimed 09.11.2022; Published 18.05.2023, Bulletin No. 14].

[0022] This method is based on measuring the amplitude of the signal reflected from an object and comparing it with the amplitude of the signal reflected from a reference spherical reflector with a known target strength. This method requires preliminary calibration of the measurement path using an additional hydrophone.

[0023] To implement the measurement method described in the prototype, perform the following steps.

[0024] Using a lifting and rotating device, the reference reflector is installed at a given depth, and an acoustic signal emitter and receiver (hereinafter hydrophone No. 1) are installed at the same depth at a distance determined by expression (1).

[0025] Hydrophone #1 measures the amplitude of the pulse signal emitted by the transmitter and reflected from the reference reflector. The underwater object being measured is then placed in place of the reference reflector at the same depth, and an additional acoustic signal receiver (hereinafter referred to as hydrophone #2) is installed above it. Hydrophone #2 then measures the amplitude of the pulse signal emitted by the transmitter.

[0026] The amplitude of the pulse signal emitted by the emitter, measured by hydrophone No. 2, is compared with the amplitude of the pulse signal reflected from the reference reflector, measured by hydrophone No. 1, their amplitudes are equalized by pre-amplifiers, this procedure is the calibration of the measuring path.

[0027] Next, in the process of direct measurements of the target strength of the underwater object, hydrophone No. 1 measures the amplitude of the pulse signal A OT , reflected from the underwater object, at the same time, hydrophone No. 2 measures the amplitude of the signal emitted by the emitter, and comparing it with the amplitude of the signal obtained during the calibration of the measuring path, recalculates to the current conditions A ИЗ .

[0028] Based on the measurements taken, the values ​​of the equivalent sphere radius and the target strength of the underwater object are calculated.

[0029]

[0030] where R ЭКВ - radius of the equivalent sphere;

[0031] A OT - the amplitude of the signal reflected from the underwater object;

[0032] A ИЗ - the amplitude of the signal emitted by the emitter, recalculated to the current conditions.

[0033] The disadvantages of the prototype are:

[0034] - the need to use a reference reflector to calibrate the measuring path, while in order to reduce the methodological component of the measurement error, the boundaries of the Fraunhofer diffraction zones of underwater objects and reference reflectors must be close, which in turn requires the equality of their geometric dimensions.

[0035] - the need to measure signals reflected from an underwater object and a reference reflector in the far field (in the Fraunhofer diffraction zone) of the underwater object, the boundary of which is determined by expression (1). This distance significantly reduces measurement accuracy due to the influence of signal reflections from the boundaries of the water area, as well as signal fluctuations over time associated with the characteristics of their propagation. Furthermore, for underwater objects with low target strength, the levels of reflected signals measured at the required distances in the far field can be comparable to the level of external noise in the water area, which leads to a sharp decrease in measurement accuracy, making them practically impossible in some cases.

[0036] - using a rotating device rod to attach an underwater object, which introduces additional error in measuring the target strength.

[0037] - impossibility of measuring the target strength values ​​in cases where the directions (angles) of the sound waves incident on the underwater object and reflected from it do not match.

[0038] The objective of the invention is to provide the ability to measure the target strength of underwater objects, including those with low target strength, in an arbitrary plane of space with increased accuracy.

[0039] The technical result obtained by implementing the invention is the provision of the possibility of measuring the target force in an arbitrary plane of space and increasing the accuracy of measuring the target force of an underwater object in an arbitrary plane of space.

[0040] In order to ensure the claimed technical result in the method for measuring the target strength, which consists in the fact that an underwater object is installed at a given depth using a lifting and rotating device, a radiation source is installed at the same depth, the amplitude of the pulse signal irradiating the underwater object is measured, the amplitude of the pulse signal reflected from the underwater object is measured, and the target strength of the underwater object is determined based on the measured amplitudes of these signals, new features are introduced, namely: the measurement of the amplitudes of the pulse signals irradiating the underwater object and reflected from it is carried out using a hydroacoustic receiver fixed on the rod of a rotating device, made in the form of a multichannel measuring probe (MSP), having the shape of a generatrix of a sphere, with the convex side facing the radiation source, along the generatrix of the MIP sphere, the diameter of which is greater than the linear size of the underwater object, equidistantly with a step of no more than λ / 2,where λ is the wavelength of the sound wave of radiation, hydrophones are installed at points with specified coordinates in the elevation plane, the underwater object is installed inside the hemisphere formed by the rotation of the MIZ in the azimuthal plane, on the axis connecting the center of the MIZ with the source of the radiating signal, the amplitude A is measured using the central hydrophone of the MIZ, ИЗ acoustic pulse signal irradiating an underwater object, rotate the rod of the rotary device in a sector of angles from -90° to 90° relative to the direction of the emitter and measure the amplitude of the acoustic pulse signal reflected from an underwater object, received by the MIZ hydrophones and stored in the form of a two-dimensional array of the amplitudes of the received signals , where θ0 is the angular coordinate in the azimuthal plane on a spherical surface, determined by the angular sensor of the rotary device, <р0 is the angular coordinate in the elevation plane on a spherical surface, determined by the position of the hydrophone in the MIZ, calculation of the amplitude of the signal reflected from an underwater object in the Fraunhofer diffraction zone produced according to the integral formula (5) of Kirchhoff [Smaryshev M.D., Dobrovolsky Yu.Yu. Hydroacoustic antennas / / Handbook. - L.: Shipbuilding. - 1984], for the selected angular coordinates (θ, ϕ) in the azimuthal plane within the range from -90° to 90° relative to the direction of arrival of the irradiating signal and within the range from 0° to 180° in the elevation plane, and the target strength of the underwater object T is determined by the formula , Where R1 is the distance between the emitter and the central hydrophone of the MIZ; R0 is the radius of the MIZ.

[0041] Eliminating the need to measure the amplitude of the signal reflected from the reference reflector and then calibrate the measuring path significantly simplifies the measurement procedure and eliminates errors associated with calibration.

[0042] Transferring the measurement zone of the signal reflected from an underwater object from the Fraunhofer diffraction zone (far field) to the Fresnel diffraction zone (near field) significantly reduces the influence of signal reflections from the boundaries of the water area, as well as signal fluctuations over time associated with the characteristics of their propagation, and, as a result, increases the noise immunity and accuracy of the measurement method, which in turn makes it possible to measure the target strength of underwater objects with low reflectivity.

[0043] The use of a multi-channel measuring probe ensures, when it rotates in the azimuthal plane, the measurement of the values ​​of the signal amplitudes reflected from an underwater object in the Fresnel diffraction zone on the surface of a virtual sphere, both in the azimuthal plane and in the elevation plane, which in turn makes it possible to calculate the values ​​of the target strength of an underwater object in the Fraunhofer diffraction zone in arbitrary planes with increased accuracy.

[0044] The essence of the invention is explained by Fig. 1, 2, 3, wherein Fig. 1 shows a diagram of the implementation of the proposed method for measuring the target strength of an underwater object, Fig. 2 is a structural diagram of the measurement path implementing the proposed method, Fig. 3 is the result of measuring the values ​​​​of the target strength of an underwater object in the form of a thin-walled steel cylinder, in the frequency range of (20-60) kHz by the proposed method. In Fig. 3, the solid line indicates the theoretical values ​​​​of the target strength of an underwater object in the form of a thin-walled steel cylinder, calculated by a known method [Urick Robert J. Fundamentals of Hydroacoustics. - L.: Shipbuilding. - 1978]. The dotted line indicates the values ​​​​of the target strength of an underwater object, measured by the proposed method.

[0045] The implementation diagram of the proposed method for measuring the target strength (Fig. 1) contains a rotating device 7, equipped with a vertical rod 10, on which the MIZ 1 is fixed, an underwater object 8, installed at a given depth with the help of two steel cables 9. The emitter 6 is intended for irradiating the underwater object 8.

[0046] In the immediate vicinity of the underwater object 8, a micro-isolator 1 is located in the form of a hemisphere with its open side facing the underwater object 6. The diameter of the micro-isolator 1 must be greater than the linear size of the underwater object 8. The hydrophones located in the micro-isolator are arranged uniformly and equidistantly with a step of no more than λ / 2, where λ is the length of the sound wave irradiating the underwater object.

[0047] In accordance with the structural diagram of the measurement path (Fig. 2), the outputs of the hydrophones MIZ 1 are connected to the input of the multichannel analog-to-digital converter (ADC) 2, the output of which is connected to the personal computer 3. The personal computer 3 is also connected to the input of the master oscillator 4, the output of which is connected to the input of the power amplifier 5. The output of the power amplifier 5 is connected directly to the emitter 6.

[0048] To transmit and record the current angular coordinates, the rotary device 7 is connected to the personal computer 3.

[0049] All devices used in the implementation scheme of the proposed method (Fig. 1) and in the structural diagram of the measurement path (Fig. 2) are known and do not require separate development [Handbook of Hydroacoustics L., Shipbuilding, 1988].

[0050] In accordance with the implementation diagram of the proposed method (Fig. 1) and the structural diagram of the measurement path (Fig. 2), the proposed method is implemented as follows.

[0051] The emitter 6 is placed in the aquatic environment at the depth of the underwater object 8 at a distance determined by the boundaries of the Fraunhofer diffraction zone of the underwater object 8. Upon command from the personal computer 3, a radio pulse signal of a given frequency, amplitude and duty cycle is generated at the output of the master oscillator 4, then this signal is fed to the power amplifier 5, where it is amplified and fed to the input of the emitter 6. The MIZ 1 is placed in the aquatic environment in the immediate vicinity of the underwater object 8.

[0052] Using the rotating device 7, the rod 10 is rotated in such a way that the center of the MIZ 1 fixed to it is located on the line connecting the emitter 6 with the underwater object 8.

[0053] The direct pulse signal coming from the emitter 6 is received by the central hydrophone MIZ 1, goes to the input of the analog-to-digital converter 2, where it is digitized and its amplitude A is recorded in the memory of the personal computer 3 ИЗ .

[0054] After this, the rotation of the rod 10 of the rotary device 7 with the MIZ 1 fixed to it is started in a sector of angles from -90° to 90° relative to the direction to the emitter.

[0055] Personal computer 3 receives data on the current angular coordinates of the rod in the azimuthal plane.

[0056] Pulse signals reflected from underwater object 8 are received by all hydrophones of MIZ 1 and fed to the input of analog-to-digital converter 2, where they are digitized and recorded in the memory of personal computer 3 in the form of an array of amplitude values

[0057] Temporal selection of direct and reflected signals from an underwater object, received by MIZ hydrophones, is carried out by selecting the duration and repetition period of pulse signals emitted by the emitter.

[0058] After completing the measurement, using expression (5), the amplitude values ​​of the acoustic signal reflected from the underwater object in the Fraunhofer diffraction zone are calculated.

[0059]

[0060] Where,

[0061] - array of Green's function values;

[0062] After calculating the amplitude value of the signal reflected from the underwater object, using expressions (6) and (7) for the conditions of the Fraunhofer diffraction zone, the values ​​of the radius of the equivalent sphere R are calculated ЭКB and the target force T of an underwater object in an arbitrary plane of space in the direction determined by the coordinates 9 and (p) in the azimuthal and elevation planes, respectively.

[0063]

[0064] where R ЭКB - the radius of the equivalent sphere for an underwater object;

[0065]

[0066] where T is the target strength of the underwater object;

[0067] The results of the completed measurement of the target strength values ​​of an underwater object in the form of a thin-walled steel cylinder, in the frequency range of (20-60) kHz (Fig. 3), namely, minor differences between both the calculated and measured values, largely determined by the non-ideality of the geometry of the thin-walled steel cylinder, confirm the possibility of using expressions (5), (6), (7) and the proposed implementation scheme of the method (Fig. 1) and the structural diagram of the measurement path (Fig. 2), for measuring the target strength of an underwater object by the proposed method.Thus, the combination of the results of measuring the target strength of an underwater object in the form of a thin-walled steel cylinder, as well as the applied technical solutions such as: transferring the measurement zone of the signal reflected from the underwater object from the Fraunhofer diffraction zone (far field) to the Fresnel diffraction zone (near field) and the use of a virtual spherical measuring surface ensures a significant reduction in the influence of signal reflections from the boundaries of the water area, as well as fluctuations in the signal reflected from the underwater object over time, thereby increasing the noise immunity and accuracy of the proposed method for measuring the target strength, while allowing measurements of the target strength values ​​of an underwater object to be performed in an arbitrary plane of space, thereby achieving the specified technical result.

[0068] The proposed method can be used in the development of methods for measuring the target strength of autonomous unmanned underwater vehicles (AUVs), scale models of underwater craft, as well as other underwater objects with low reflectivity in experimental pools and measurement ranges.

Claims

A method for measuring the target strength, which consists in the fact that an underwater object is installed at a given depth using a lifting and turning device, a radiation source is installed at the same depth, the amplitude of the pulse signal irradiating the underwater object is measured using a hydroacoustic receiver, the amplitude of the pulse signal reflected from the underwater object is measured, and the target strength of the underwater object is determined based on the measured amplitudes of these signals, characterized in that the hydroacoustic receiver is made in the form of a multichannel measuring probe (MMP) installed at a distance R1 from the radiation source, having the shape of a generatrix of a sphere, the diameter of which is greater than the linear size of the underwater object, with the convex side facing the radiation source, along the generatrix of which the hemisphere is equidistant with a step of no more than λ / 2, where λ is the length of the sound wave of the radiation, hydrophones are installed at points with given coordinates in the elevation plane,the underwater object is installed inside the area covered by the MIZ, on the axis connecting the center of the MIZ with the source of the irradiating signal, the amplitude A is measured using the central hydrophone of the MIZ, ИЗ an acoustic pulse signal irradiating an underwater object, rotate the rod of the lifting and turning device on which the MIZ is installed in a sector of angles from -90° to 90° relative to the direction of the emitter, and measure the amplitudes of the acoustic pulse signal reflected from the underwater object and received by the MIZ hydrophones, and store them in the form of a two-dimensional array of the amplitudes of the received signals A БП (θ0,ϕ0), where θ0 is the angular coordinate in the azimuthal plane on a spherical surface, determined by the angular sensor of the rotary device, ϕ0 is the angular coordinate in the elevation plane on a spherical surface, determined by the position of the hydrophone in the MIZ, calculation of the amplitude of the signal reflected from an underwater object in the Fraunhofer diffraction zone A ОТР(θ,ϕ), is produced according to the integral Kirchhoff formula for the selected angular coordinates (θ,ϕ) in the azimuthal plane within the range from -90° to 90° relative to the direction of arrival of the irradiating signal and within the range from 0° to 180° in the elevation plane, and the target strength of the underwater object T is determined by the formula , Where , R0 - radius of the MIZ, R ЭКВ - the radius of the equivalent sphere of the underwater object.

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