A method for simulating reverberation signals at the interface of a single-base active sonar with separate transmitters and receivers

Abstract

The present invention relates to the field of underwater acoustic engineering technology, and provides a method for simulating interface reverberation signals of a single-base active sonar with separate transmitters and receivers, including: establishing a method for determining the minimum scattering ellipsoid of the seabed and sea surface scattering of the single-base sonar with separate transmitters and receivers and a method for determining the subsequent scattering ellipsoid; proposing a scattering unit division method based on an interface scattering elliptical ring; establishing a calculation model for the coordinates of the scattering unit boundary points and the scattering unit center points; establishing calculation models for the scattering unit transmission incident angle, receiving scattering angle, distance to the transmitting and receiving point, reverberation delay, etc.; establishing a calculation model for the seabed and sea surface reverberation signals; and updating the interface reverberation calculation end condition. The present invention can calculate the comprehensive reverberation signal generated by the seabed and sea surface scattering, greatly improves the accuracy of the reverberation arrival time, reverberation intensity, and Doppler calculation in the single-base working mode of separate transmitters and receivers, and enhances the reliability of the reverberation model.

Description

Technical Field

[0001] The invention relates to the technical field of underwater acoustic engineering, and in particular to a method for simulating interface reverberation signals of a single-base active sonar with separate transmitters and receivers. Background Art

[0002] Ocean reverberation is the sum of scattered signals generated by the seabed, sea surface, and the inhomogeneity of the ocean water body after the active sonar transmits the signal. It is an inevitable underwater acoustic phenomenon. Whether in shallow or deep sea, the interface reverberation caused by the scattering of the seabed and sea surface is the main interference signal of active sonar, which has an important impact on the detection of active target echo signals. Reverberation signal modeling and simulation is an important topic in the field of underwater acoustics. It has two main uses: one is to verify and improve the anti-reverberation signal processing technology of active sonar; the other is to be used for the deduction and evaluation of active sonar systems, and to evaluate the detection effect of sonar systems in reverberation environments.

[0003] The single-base sonar system with separate transmitters and receivers is a common form of new active sonar. The transmitting array and receiving array of this sonar system are carried by the same platform. The transmitting unit and the receiving array work independently, and the two are a certain distance apart (generally 100-600 meters). Therefore, it is different from the traditional single-base (the transmitting single array and the receiving array are on the same platform, shared, and in the same position), and also different from the multi-base sonar (the transmitting and receiving are located on different platforms, and the distance between the two is generally far, more than a few kilometers). At present, the modeling and simulation of active sonar reverberation is mainly aimed at two situations: single-base sonar with combined transmitters and receivers or dual-base (the transmitter and receiver are separated by a long distance). In order to simplify the calculation, it is usually treated as a single-base with combined transmitter and receiver, which seriously affects the reliability of the simulation. The existing reverberation signal simulation methods mainly have the following problems:

[0004] 1. For a single-base sonar with a transmission and reception distance of 100-600 meters, the incident angle of the transmitted signal reaching the scatterer (seabed, sea surface and water body) is very different from the scattering angle of the scatterer reaching the receiving point. The conventional single-base concept is used for reverberation modeling, and only the one-way double geometric propagation loss is considered, which causes errors in the scattered signal intensity and the scattered signal arrival time. Especially in shallow sea environments, the impact is more serious;

[0005] 2. Shallow water active sonar reverberation is mainly caused by seabed scattering. The simulation of the Doppler characteristics of the seabed reverberation signal cannot be ignored. Due to the large transmission and reception separation angle in the shallow water environment, the Doppler modeling of the reverberation signal using the combined transmission and reception will cause the Doppler simulation error of the reverberation signal, which will affect the reliability of the deduction and evaluation effect, and even make it impossible to use this method to deduce and evaluate the sonar with anti-reverberation signal processing capability.

[0006] 3. Dual-base sonar reverberation signal modeling method: Because the distance between the dual-base transceiver unit is several kilometers, dozens of kilometers, or even farther, this modeling method cannot be used for a single-base with a shorter transceiver distance. Moreover, most of the existing dual-base reverberation signal modeling methods do not consider the control of the vertical beam angle of the transmission, and the vertical direction is omnidirectional, which has a great impact on the reverberation intensity and the arrival time of the reverberation signal, affecting the reliability and accuracy of the reverberation signal simulation;

[0007] 4. Existing reverberation models are all modeled for traditional short-pulse sonar (pulse length τ is generally tens of milliseconds to hundreds of milliseconds). For short-pulse reverberation calculations, the distance interval between scattering rings can be calculated according to cτ / 2, so that the instantaneous value of the reverberation signal obtained conforms to the Gaussian distribution and the envelope conforms to the Rayleigh distribution. However, the pulse length of new active sonars is getting longer and longer, and the pulse length is generally 1-10 seconds or even longer. In this case, if the interval of the scattering rings is calculated according to the existing method, the reverberation signal obtained will show obvious jagged shapes, which cannot meet the basic characteristic requirements of the reverberation signal that the instantaneous value conforms to the Gaussian distribution and the envelope conforms to the Rayleigh distribution.

[0008] In view of the above practical needs and existing problems, a patent application for the present invention is hereby filed. Summary of the invention

[0009] The present invention mainly solves the above four technical problems existing in the sonar reverberation signal simulation method of the prior art, and proposes a method for simulating reverberation signals at the interface of a single-base active sonar with separate transmitters and receivers, so as to improve the reverberation arrival time and reverberation intensity, enhance the reliability of the reverberation model, ensure the accuracy of the reverberation intensity, and better meet the needs of engineering realization such as the calculation accuracy and real-time performance of the reverberation model.

[0010] The present invention provides a method for simulating reverberation signals of a single-base active sonar interface with separate transmitters and receivers, comprising:

[0011] Establish the interface scattering ellipsoid model of single-base sonar with separate transmitter and receiver;

[0012] Determining a minimum scattering ellipsoid of seabed / sea surface scattering based on the scattering ellipsoid model, and obtaining subsequent scattering ellipsoids according to the minimum scattering ellipsoid;

[0013] Dividing each of the scattering ellipsoids according to the azimuth to form a plurality of scattering units whose distances are determined by the scattering elliptical rings and whose directions are determined by the azimuth regions, and calculating the coordinates of the boundary points and the center points of each of the scattering units;

[0014] According to the coordinates of the boundary points and the center point of the scattering unit, the transmitting incident angle, the scattering angle, the receiving scattering angle, the reverberation delay, and the distance from each scattering unit to the receiving-transmitting point and the area of ​​the region where the scattering unit is located are calculated;

[0015] Establishing a Doppler frequency shift model, and calculating the normalized bistatic Doppler frequency shift and the time domain Doppler factor of the scattered echo at the scattering unit;

[0016] Calculate the scattering loss of the scattering unit according to the transmission incident angle, scattering angle, receiving scattering angle, reverberation delay, distance to the transmitting and receiving point, and area of ​​the scattering unit; calculate the reverberation signal of the scattering unit according to the scattering loss and the Doppler frequency shift and time domain Doppler factor;

[0017] When the calculated reverberation signal satisfies that the reverberation level is lower than the difference between the ocean environment noise level and the array directivity DI, the reverberation simulation program of the scattering unit ends.

[0018] Furthermore, the establishment of the interface scattering ellipsoid model of the single-base sonar with separate transmitter and receiver includes:

[0019] Assume that the X-axis is positive along the moving direction of the towing platform, the transmitting array is in front and the receiving array is in the back, the z-axis is positive toward the water surface and negative toward the seabed; T and R are the positions of the transmitting array and the receiving array, respectively, and the depths of the transmitting array and the receiving array are h T and h R The oblique distance between the transmitting and receiving stations is 2L, and the inclination angle between the connecting line and the horizontal plane is δ.

[0020] Further, the determining of the minimum scattering ellipsoid of seabed / sea surface scattering based on the scattering ellipsoid model includes:

[0021] Under the condition that the vertical beam of the transmission is ±Ψ / 2, there is a seabed / surface silent scattering circle corresponding to the single-base sonar with separate transmitters and receivers. The silent scattering circle is a circle with T′ as the center and L y The radius of the seabed silent scattering circle is:

[0022] L y =(Hh T )tan(90-Ψ / 2)=(Hh T )ctan(Ψ / 2) (1-1)

[0023] The radius of the sea surface silent scattering circle is expressed as:

[0024] L y =h T tan(90-Ψ / 2)=h T ctan(Ψ / 2) (1-2)

[0025] L y The intersection of the silent scattering circle with a radius of and the line connecting T'-R' is taken as the scattering point;

[0026] The minimum slant distance R from the transmitted sound to the seabed scattering point Tmin It can be expressed as:

[0027]

[0028] The minimum slant distance R of the transmitted sound to the scattering point on the sea surface Tmin It can be expressed as:

[0029]

[0030] The slant distance R from the seabed scattering point to the receiving point Rmin It can be expressed as:

[0031]

[0032] The slant distance R from the sea surface scattering point to the receiving point Rmin It can be expressed as:

[0033]

[0034] Wherein, 2L' is the horizontal distance between the transmitting point T and the receiving point R, that is, the distance between T' and R';

[0035] The seabed / sea surface scattering ellipsoid that first reaches the receiving point is the minimum scattering ellipsoid, and its semi-major axis is expressed as:

[0036]

[0037] The corresponding two semi-minor axes are:

[0038]

[0039] In equations (4) and (5), a0, b0, and c0 define the minimum scattering ellipsoid.

[0040] Further, obtaining a subsequent scattering ellipsoid according to the minimum scattering ellipsoid includes:

[0041] The semi-major axis and semi-minor axis of the outer ring of the first scattering ellipsoid in the subsequent scattering ellipsoid are respectively:

[0042]

[0043] The semi-major axis and semi-minor axis of the outer ring of the i=1,2,... scattering ellipsoid are:

[0044]

[0045] Furthermore, the step of dividing each of the scattering ellipsoids according to the orientation to form a plurality of scattering units whose distances are determined by the scattering elliptical rings and whose directions are determined by the orientation regions, and calculating the coordinates of the boundary points and the center points of each of the scattering units, comprises:

[0046] The method of dividing the scattering ellipsoid according to the azimuth is: if the change of the Doppler shift in a certain azimuth range is within the allowable range, then the azimuth area is determined as an independent scattering unit cell;

[0047] The method for calculating the boundary points of each scattering unit is as follows:

[0048] The cross section of the scattering ellipsoid on the seabed / sea surface is an ellipse on the xoy plane, and the equation of the ellipse is:

[0049]

[0050] In the formula, when calculating the seabed reverberation, the calculation is performed through the cross section of the i=1, 2, .... scattering ellipsoid at the seabed z=-(H-H0); when calculating the sea surface reverberation, the calculation is performed through the cross section of the i=1, 2, .... scattering ellipsoid at the seabed z=H0;

[0051] Each scattering ellipse is divided into K regions, then the boundary point B of the kth scattering unit on the outer ring of the i-th scattering ellipse is ik The polar angle is ρ bk , the coordinates of the kth scattering unit are bx ik ,by ik , calculated by equations (9)-(10):

[0052] bx ik =by ik cot(ρ bk ) or by ik =bx ik tan(ρ bk ) (9)

[0053]

[0054] The method for calculating the center point coordinates of each scattering unit is as follows:

[0055] Let S ik is the scattering center position of the kth scattering unit on the i-th scattering ellipse, and its coordinates are expressed as (sx ik ,sy ki ,z) indicates that the semi-major axis and semi-minor axis of the central ring of the i=1,2,....scattering elliptical ring are respectively:

[0056]

[0057] Let the polar angle of the center of the kth scattering unit be ρ k , then S ik The coordinates are:

[0058] or

[0059] One of the two formulas in the above formula (12) can be combined with the ellipse equation to solve the center point coordinates (sx ik ,sy ik ), expressed as:

[0060]

[0061] Further, the calculation of the transmission incident angle, scattering angle, receiving scattering angle, reverberation delay, distance from each scattering unit to the transmit-receive point and the area of ​​the region where the scattering unit is located, and the calculation of the normalized bistatic Doppler frequency shift of the scattered echo at the scattering unit according to the coordinates of the boundary points and the center point of the scattering unit includes:

[0062] (1) Establish a calculation model for the distance between scattering units and their propagation delay:

[0063] The coordinates of the emission point T are (-Lcosδ, 0, Lsinδ) to the scattering unit S ik The distance to the center point is:

[0064]

[0065] S ik The distance to the receiving point R with coordinates (Lcosδ,0,-Lsinδ) is:

[0066]

[0067] From the transmitting point T to the seafloor scattering unit S ik , and then the propagation time to the receiving point R is:

[0068]

[0069] Due to the limitation of the vertical transmission beam, there is an acoustic scattering shadow area on the seabed directly below the transmission point. The sound wave cannot reach this area. No scattering occurs in this area. It should not be included in the process of expanding the scattering ellipsoid to calculate the scattering intensity. For a flat seabed, the basic criterion is:

[0070] R Tik >R Tmin (17)

[0071] (2) Calculation model of incident angle and scattering angle of scattering unit:

[0072] The incident light at point T is incident on the scattering element S. ik The vertical angle of incidence is:

[0073] Seafloor scattering:

[0074] Sea Surface Scattering:

[0075] Scattering unit S ik The vertical scattering angle to the receiving point R is:

[0076] Seafloor scattering:

[0077] Sea Surface Scattering:

[0078] (3) Calculation model of the area where each scattering unit is located:

[0079] According to the calculated boundary point coordinates of the i-th elliptical ring and the k-th scattering unit, (bx ik ,by ik ), and further calculate the area of ​​the i-th elliptical ring and the k-th sector:

[0080]

[0081] The scattering unit S ik The area is:

[0082] A bik =A ik -A (i-1)k

[0083]

[0084] When calculating the area of ​​a sector, proceed incrementally according to the subscript number in the same ellipse. Considering the symmetry of the region, only 1 / 4 of the sector needs to be calculated, and the region with the same area can be directly used.

[0085] Furthermore, the establishing of the Doppler frequency shift model and the calculation of the normalized bistatic Doppler frequency shift and the time domain Doppler factor of the scattered echo at the scattering unit include:

[0086] Under the same towing platform motion condition, the transmitting station speed, the receiving station motion speed and the platform motion speed v are the same, then the scattering point S ik The normalized bistatic Doppler frequency of the scattered echo at is expressed as:

[0087]

[0088] The time domain Doppler factor is expressed as:

[0089]

[0090] In the formula, α ik T reaches the scattering unit S ik The grazing angle, β ik Scattering unit S ik The grazing angle to R; θ Tik T' reaches the scattering unit S ik The side angle of Rik R' reaches the scattering unit S ik side angle.

[0091] Further, the scattering loss of the scattering unit is calculated according to the transmission incident angle, scattering angle, receiving scattering angle, reverberation delay, distance to the receiving-transmitting point and the area of ​​the region of the scattering unit, including:

[0092] In the deep sea, the seabed / surface scattering unit S ik The total scattering loss is:

[0093] TLR ik (t) = TS bik (t)-20lgR Tik (t)-20lg R Rik (t)-10 -3 α(f)(R Tik (t)+R Rik (t))(24-1)

[0094] In shallow waters, the expansion loss obeys the 1.5th power law, that is, between plane waves and spherical waves, and the seabed / sea surface scattering unit S ik The total scattering loss is:

[0095] TLR ik (t) = TS bik (t)-15lgR Tik (t)-15lg R Rik (t)-10- 3 α(f)(R Tik (t)+R Rik (t))(24-2)

[0096] Among them, TS bik Scattering unit S ik The seafloor scattering intensity is expressed as:

[0097] TS bik =S bik +10lg A bik (25-1)

[0098] For sea surface scattering, the scattering unit S in equation (24) ik TSbik The scattering intensity TS of the sea surface sik replace:

[0099] TS sik =S sik +10lg A sik (25-2)

[0100] A in formula (27) bik and A sik is the area of ​​the kth seafloor and sea surface scattering unit in the i-th scattering ring, S bik , S sik are the seabed scattering intensity and the sea surface scattering coefficient respectively, which are expressed by formula (26):

[0101] S bik =10lg(μsinα ik sinβ ik ) (26-1)

[0102]

[0103] Where, β = 158 (vf 1 / 3 ) -0.58 , α ik is the grazing angle of the incident wave of the scattering unit, β ik is the grazing angle of the scattered wave of the scattering unit, v represents the sea surface wind speed, and f represents the center frequency;

[0104] α(f) is the sound absorption coefficient of seawater, which can be expressed using the Thorp empirical formula for the center frequency as follows:

[0105]

[0106] Scattering unit S ik The scattering loss of the scattered signal is:

[0107]

[0108] Further, the calculating the reverberation signal of the scattering unit according to the scattering loss, the Doppler frequency shift, and the time domain Doppler factor includes:

[0109] Assume that the transmitted signal is a signal s(t) with a sound source level of SL, and do not consider the horizontal directivity of the transmission. The vertical directivity of the transmission is in accordance with the previous definition, and the Doppler effect of the scattering unit is considered. The transmission at point T passes through the seabed / sea surface scattering unit S ik The scattered signal reaching the receiving point R can be expressed as:

[0110]

[0111] or:

[0112]

[0113] In the formula, f dik is the Doppler shift, Δ ik is the corresponding Doppler factor (in time domain), φ ik is a random phase that obeys uniform distribution, uniformly distributed between 0-2π, τ ik is the round trip delay of the signal from the Sik scattering unit to the receiving point, calculated according to formula (16);

[0114] The calculation model of the scattered signal of the i-th scattered elliptical ring is expressed as:

[0115]

[0116] AB i is the random amplitude of the scattering signal generated by the i-th scattering ring, which obeys the normal distribution;

[0117] The calculation model of the reverberation signal scattered by the seabed / sea surface is expressed as:

[0118] Seabed reverberation signal:

[0119] Sea surface reverberation signal:

[0120] N b (t), N s (t) are the total number of scattering elliptical rings of the seabed scattering signal and the sea surface scattering signal reaching the receiving end at time t;

[0121] Adding the signals of equations (31-1) and (31-2) at the same time, we can get the total reverberation signal generated by the seabed and the sea surface:

[0122] y(t)=y b (t)+y s (t) (32).

[0123] Further, when the calculated reverberation signal satisfies the condition that the reverberation level is lower than the ocean ambient noise level, the reverberation simulation program of the scattering unit ends, including:

[0124] Taking into account the array processing capability of the active sonar receiver, the directional DI gain of the array processing is increased to make the interface reverberation and the ocean reverberation connect more smoothly. When the condition of formula (33) is met, the reverberation simulation program of the scattering unit ends;

[0125]

[0126] The present invention provides a method for simulating interface reverberation signals of a single-base active sonar with separate transmitters and receivers. The method can comprehensively obtain reverberation signals generated by interface scattering of the seabed and the sea surface, thereby changing the current status of using a single-base or multi-base active sonar with separate transmitters and receivers for reverberation modeling. The method greatly improves the working mode of the single-base active sonar with separate transmitters and receivers, improves the accuracy of the reverberation arrival time, reverberation intensity, and Doppler simulation calculation of the interface reverberation signal, and enhances the reliability of the reverberation model.

[0127] Based on the scattering ellipsoid model, the present invention proposes a method for determining the minimum ellipsoid and subsequent scattering elliptical rings that form the active reverberation of a single-base sonar with separate transmitters and receivers, and proposes a method for determining the earliest arrival time (reverberation delay) of the reverberation signal at the seabed (sea surface). The reverberation arrival time reflects the position of the reverberation signal arriving at the receiving point, and the delay accuracy directly affects the time and energy relationship of the reverberation signal, which is very important for modeling the reverberation signal of any separate transmitter-receiver system;

[0128] In view of the limited vertical transmission beam, the present invention proposes a criterion for determining whether a scattering unit is within the vertical beam range during dynamic calculation. If it is within the vertical beam range, it participates in the reverberation intensity calculation, otherwise it does not participate in the calculation, thereby better ensuring the accuracy of the reverberation intensity.

[0129] The present invention proposes a method for dividing scattering ellipse units, which ensures that the instantaneous amplitude of the reverberation signal meets the requirements of Gaussian distribution and the envelope meets the requirements of Rayleigh distribution, and provides a principle for determining the long axis increment (for the range of the scattering ellipse ring) of the expansion of the scattering ellipsoid caused by sound propagation, thereby solving the problem of how to determine the range of the interface scattering ring caused by long pulses and the lack of basis for reference;

[0130] The present invention proposes a method for dividing each scattering ring into scattering cells according to azimuth intervals, which can better meet the characteristics of a horizontal omnidirectional transmitting array (i.e., the transmitting array has no horizontal beam). The azimuth interval can be set according to the requirements of the interface reverberation Doppler characteristics on azimuth resolution accuracy and real-time calculation of reverberation signals, so as to better meet the needs of engineering realization such as reverberation model calculation accuracy and real-time performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0131] Figure 1 It is a flow chart of the method of the present invention.

[0132] Figure 1-1 This is an interface reverberation diagram of the embodiment, with a sea depth of 200 meters and considering the "silent scattering circle" (from left to right, the interface reverberation intensity diagram calculated by the model, the interface reverberation signal diagram, and the reverberation intensity diagram calculated by the reverberation signal);

[0133] Figure 1-2This is an interface reverberation diagram in the embodiment, at a sea depth of 200 meters, without considering the "silent scattering circle" (from left to right, the interface reverberation signal diagram and the reverberation intensity diagram calculated by the reverberation signal);

[0134] Figure 1-3 This is an interface reverberation diagram of the embodiment, with a sea depth of 5000 meters and considering the "silent scattering circle" (from left to right, the interface reverberation intensity diagram calculated by the model, the interface reverberation signal diagram, and the reverberation intensity diagram calculated by the reverberation signal);

[0135] Figure 1-4 This is an interface reverberation diagram in the embodiment, at a sea depth of 5000 meters, without considering the "silent scattering circle" (from left to right, the interface reverberation signal diagram and the reverberation intensity diagram calculated by the reverberation signal);

[0136] Figure 1-5 : is a distribution diagram of boundary points (solid points) and center points (hollow circles) of seabed scattering units at a depth of 200 meters in the embodiment;

[0137] Figure 1-6 : is a distribution diagram of boundary points (solid points) and center points (hollow circles) of sea surface scattering units at a depth of 200 meters in the embodiment;

[0138] Figure 1-7 is a comparison diagram of the instantaneous amplitude histogram and Gaussian distribution of the normalized interface reverberation signal in an embodiment;

[0139] Figure 1-8 is a comparison diagram of a normalized interface reverberation signal envelope histogram and a Rayleigh distribution in an embodiment;

[0140] Figure 2-1 1 is a schematic diagram of a vertical cross-section geometry of a seabed scattering system with separate transmitters and receivers in an embodiment;

[0141] Figure 2-2 is a three-dimensional schematic diagram of a transmit-receive split scattering unit in an embodiment;

[0142] Figure 2-3 is a schematic diagram of the division of the scattering elliptical ring and the scattering unit in an embodiment;

[0143] Figure 2-4 1 is a flow chart for calculating the receiving and transmitting split reverberation signal in an embodiment. DETAILED DESCRIPTION

[0144] In order to make the technical problems solved by the present invention, the technical solutions adopted and the technical effects achieved clearer, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It is understood that the specific embodiments described herein are only used to explain the present invention, rather than to limit the present invention. It should also be noted that, for the convenience of description, only the parts related to the present invention are shown in the accompanying drawings, rather than all the contents.

[0145] like Figure 1 As shown, the method for simulating the interface reverberation signal of a single-base active sonar with separate transmitters and receivers provided in an embodiment of the present invention includes:

[0146] 101. Establish an interface scattering ellipsoid model for a single-base sonar with separate transmitters and receivers;

[0147] Specifically, the geometric configuration vertical profile of the sonar system is as follows: Figure 2-1 Assume that the X-axis is positive along the moving direction of the towing platform, the transmitting array is in front and the receiving array is in the back, the z-axis is positive toward the water surface and negative toward the seabed; T and R are the positions of the transmitting array and the receiving array respectively, and the depths of the transmitting array and the receiving array are h T and h R The oblique distance between the transmitting and receiving stations is 2L, and the inclination angle between the connecting line and the horizontal plane is δ.

[0148] Drawing on the idea of ​​reverberation modeling based on unit scattering, such scattering points satisfy the same sound propagation time condition of "transmit-scattering unit-receive". According to the principle of geometry, in three-dimensional space, the scattering points that arrive synchronously in time in the transmitting and receiving sonar system are ellipsoids with the midpoint O of the receiving-transmitting line as the center and T and R as the focus, and the ellipse where the ellipsoids intersect on the seabed or sea surface is the seabed or sea surface scattering ellipse. If the ocean sound speed does not change with the direction and distance of sound propagation, the sound propagation time from the transmitting point T to any point on the scattering ellipse and then to the receiving point R is the same, which is referred to as the "time-isochronous ellipsoid". This is the basis for establishing a scattering model for the transmitting and receiving system. T' and R' represent the feet of the perpendiculars of the transmitting and receiving stations on the seabed projection plane, and O' is the center of the ellipse formed by the projection of the isochronous scattering ellipsoid on the seabed. Ψ / 2 is the maximum grazing angle determined by the vertical beam (±Ψ / 2). The sound line emitted by T along this solid angle forms a circle with T' as the center and |AB| as the diameter on the seabed. This circle is called the "silent scattering circle", which means that the sound energy does not reach the arc, or no scattering occurs on all areas within the circle. Figure 2-2 shown.

[0149] 102. Determine the minimum scattering ellipsoid of seabed / sea surface scattering based on the scattering ellipsoid model, and obtain subsequent scattering ellipsoids based on the minimum scattering ellipsoid;

[0150] Specifically, (1) determine the minimum scattering ellipsoid of seabed and sea surface scattering

[0151] Determining the minimum scattering ellipsoid is an important basis for reverberation modeling, because it involves the time of the first arriving scattered wave and thus affects the signal energy of the subsequent scattered waves.

[0152] Under the condition that the vertical beam of the transmission is ±Ψ / 2, there is a seabed / surface silent scattering circle corresponding to the single-base sonar with separate transmitters and receivers. The silent scattering circle is a circle with T′ as the center and L y The radius of the seabed silent scattering circle is:

[0153] L y =(Hh T )tan(90-Ψ / 2)=(Hh T )ctan(Ψ / 2) (1-1)

[0154] The radius of the sea surface silent scattering circle is expressed as:

[0155] L y =h T tan(90-Ψ / 2)=h T ctan(Ψ / 2) (1-2)

[0156] L y The intersection of the silent scattering circle with a radius of and the line connecting T'-R' is taken as the scattering point;

[0157] The minimum slant distance R from the transmitted sound to the seabed scattering point Tmin It can be expressed as:

[0158]

[0159] The minimum slant distance R of the transmitted sound to the scattering point on the sea surface Tmin It can be expressed as:

[0160]

[0161] The slant distance R from the seabed scattering point to the receiving point Rmin It can be expressed as:

[0162]

[0163] The slant distance R from the sea surface scattering point to the receiving point Rmin It can be expressed as:

[0164]

[0165] Wherein, 2L' is the horizontal distance between the transmitting point T and the receiving point R, that is, the distance between T' and R';

[0166] No matter how the transceiver unit is configured, there must be an ellipsoid that has just touched the seabed (or sea surface). The first point to touch the seabed (or sea surface) is just a point, that is, there is a scattering point that first reaches the receiving point. The seabed (or sea surface) scattering ellipsoid determined by this point is actually the minimum scattering ellipsoid. Taking the ellipsoid as the starting point, the ellipsoid formed by the major axis of the ellipsoid continues to expand, and the seabed (or sea surface) cross section forms a seabed (or sea surface) scattering ellipse. The scattering ellipses of the two ellipsoids extended with a certain semi-major axis spacing Δa on the seabed (or sea surface) form a scattering ellipse ring.

[0167] Determination of the minimum scattering ellipsoid and scattering point. The seabed / sea surface scattering ellipsoid that first reaches the receiving point is the minimum scattering ellipsoid, expressed as:

[0168]

[0169] The corresponding two semi-minor axes are:

[0170]

[0171] In equations (4) and (5), a0, b0, and c0 define the minimum scattering ellipsoid.

[0172] (2) Determine the scattering ellipsoid interval and subsequent scattering ellipsoids

[0173] Conventional single-base sonars with combined transmission and reception usually emit short pulse signals of less than 100 milliseconds, and the reverberation modeling is calculated based on the scattering circle with a radius interval of cτ / 2. However, for pulse signals of several seconds to more than 10 seconds, the radius interval of the scattering circle calculated in this way is several kilometers to more than ten kilometers, and the delay of the subsequent scattering signals arriving in each scattering ring is several seconds to more than 10 seconds. The resulting reverberation signal will be obviously sawtooth-shaped, and the statistical characteristics of the reverberation signal amplitude and envelope do not meet the characteristics of the reverberation signal.

[0174] The present invention conducts simulation research on shallow sea and deep sea reverberation, and expands the interval of subsequent scattering ellipsoid rings (interval of semi-major axis a of scattering ellipsoid) between transmitter and receiver according to Δa=0.1c~0.2c meters (c is the speed of sound). The reverberation signal calculated thereby meets the basic characteristic requirements of sonar reverberation signal.

[0175] The subsequent scattering ellipsoids are obtained according to the minimum scattering ellipsoid, including: the semi-major axis and semi-minor axis of the outer ring of the first scattering ellipsoid in the subsequent scattering ellipsoid are respectively:

[0176]

[0177] The semi-major axis and semi-minor axis of the outer ring of the i=1,2,... scattering ellipsoid are:

[0178]

[0179] 103. Divide each scattering ellipsoid according to the azimuth to form a number of scattering units whose distances are determined by the scattering elliptical rings and whose directions are determined by the azimuth regions, and calculate the coordinates of the boundary points and the center points of each scattering unit;

[0180] Specifically, (1) the method of dividing the scattering ellipsoid by azimuth is: if the change of the Doppler offset in a certain azimuth range is within the allowable range, then the azimuth area is determined as an independent scattering unit cell. This is because it is determined by the relative position (azimuth, distance) of the Doppler right scatterer of the reverberation. In theory, the smaller the azimuth area, the more accurate the calculation, and the larger the single calculation amount, which affects the real-time performance of the calculation. If the transmitting array has the ability to scan the horizontal beam, the size of the azimuth area is determined by the width of the horizontal scanning beam; otherwise, the size of the azimuth area is determined by the sensitivity of the Doppler characteristics to the change in azimuth, that is, if the change of the Doppler offset in a certain azimuth range is within the allowable range, then the azimuth area is determined as an independent scattering unit cell.

[0181] (2) The method for calculating the boundary points of each scattering unit is as follows:

[0182] The cross section of the scattering ellipsoid on the seabed / sea surface is an ellipse on the xoy plane, and the equation of the ellipse is:

[0183]

[0184] In the formula, when calculating the seabed reverberation, the calculation is performed through the cross section of the i=1, 2, .... scattering ellipsoid at the seabed z=-(H-H0); when calculating the sea surface reverberation, the calculation is performed through the cross section of the i=1, 2, .... scattering ellipsoid at the seabed z=H0;

[0185] Each scattering ellipse is divided into K regions, then the boundary point B of the kth scattering unit on the outer ring of the i-th scattering ellipse is ik The polar angle is ρ bk , the coordinates of the kth scattering unit are bx ik ,by ik , calculated by equations (9)-(10):

[0186] bx ik =by ik cot(ρ bk ) or by ik =bx ik tan(ρ bk ) (9)

[0187]

[0188] (3) The method for calculating the center point coordinates of each scattering unit is as follows:

[0189] Let S ik is the scattering center position of the kth scattering unit on the i-th scattering ellipse, and its coordinates are expressed as (sx ik ,sy ki ,z) indicates that the semi-major axis and semi-minor axis of the central ring of the i=1,2,....scattering elliptical ring are respectively:

[0190]

[0191] Let the polar angle of the center of the kth scattering unit be ρ k , then S ik The coordinates are:

[0192] or

[0193] One of the two formulas in the above formula (12) can be combined with the ellipse equation to solve the center point coordinates (sx ik ,sy ik ), expressed as:

[0194]

[0195] 104. Calculate the transmission incident angle, scattering angle, receiving scattering angle, reverberation delay, and the distance from each scattering unit to the receiving-transmitting point and the area of ​​the region where the scattering unit is located according to the coordinates of the boundary points and the center point of the scattering unit;

[0196] Specifically, (1) a calculation model for the distance of the scattering unit and its propagation delay is established:

[0197] The coordinates of the emission point T are (-Lcosδ, 0, Lsinδ) to the scattering unit S ik The distance to the center point is:

[0198]

[0199] S ik The distance to the receiving point R with coordinates (Lcosδ,0,-Lsinδ) is:

[0200]

[0201] From the transmitting point T to the seafloor scattering unit S ik , and then the propagation time to the receiving point R is:

[0202]

[0203] Due to the limitation of the vertical transmission beam, there is an acoustic scattering shadow area on the seabed directly below the transmission point. The sound wave cannot reach this area. No scattering occurs in this area. It should not be included in the process of expanding the scattering ellipsoid to calculate the scattering intensity. For a flat seabed, the basic criterion is:

[0204] R Tik >R Tmin (17)

[0205] (2) Calculation model of incident angle and scattering angle of scattering unit:

[0206] The incident light at point T is incident on the scattering element S. ik The vertical angle of incidence is:

[0207] Seafloor scattering:

[0208] Sea Surface Scattering:

[0209] Scattering unit S ik The vertical scattering angle to the receiving point R is:

[0210] Seafloor scattering:

[0211] Sea Surface Scattering:

[0212] (3) Calculation model of the area where each scattering unit is located:

[0213] like Figure 2-3 As shown, the calculated boundary point coordinates of the i-th elliptical ring and the k-th scattering unit are (bx ik ,by ik ), and further calculate the area of ​​the i-th elliptical ring and the k-th sector:

[0214]

[0215] The scattering unit S ik The area is:

[0216] A bik =A ik -A (i-1)k

[0217]

[0218] When calculating the area of ​​a sector, proceed incrementally according to the subscript number in the same ellipse. Considering the symmetry of the region, only 1 / 4 of the sector needs to be calculated, and the region with the same area can be directly used.

[0219] 105. Establish a Doppler frequency shift model and calculate the normalized bistatic Doppler frequency shift and time domain Doppler factor of the scattered echo at the scattering unit;

[0220] Specifically, under the same towing platform motion condition, the transmitting station speed, the receiving station motion speed and the platform motion speed v are the same, then the scattering point S ik The normalized bistatic Doppler frequency of the scattered echo at is expressed as:

[0221]

[0222] The time domain Doppler factor is expressed as:

[0223]

[0224] In the formula, α ik T reaches the scattering unit S ik The grazing angle, β ik Scattering unit S ik The grazing angle to R; θ Tik T' reaches the scattering unit S ik The side angle of Rik R' reaches the scattering unit S ik Obviously, different directions have different Doppler shifts.

[0225] 106. Calculate the scattering loss of the scattering unit according to the transmission incident angle, scattering angle, receiving scattering angle, reverberation delay, distance to the transmitting and receiving point, and the area of ​​the region where the scattering unit is located; calculate the reverberation signal of the scattering unit according to the scattering loss, Doppler frequency shift, and time domain Doppler factor;

[0226] Specifically, in the deep sea, the seabed / surface scattering unit S ik The total scattering loss is:

[0227] TLR ik (t) = TS bik (t)-20lgR Tik (t)-20lg R Rik (t)-10 -3 α(f)(R Tik (t)+R Rik (t))(24-1)

[0228] In shallow waters, the expansion loss obeys the 1.5th power law, that is, between plane waves and spherical waves, and the seabed / sea surface scattering unit S ik The total scattering loss is:

[0229] TLR ik (t) = TS bik (t)-15lgRTik (t)-15lg R Rik (t)-10- 3 α(f)(R Tik (t)+R Rik (t))(24-2)

[0230] Among them, TS bik Scattering unit S ik The seafloor scattering intensity is expressed as:

[0231] TS bik =S bik +10lg A bik (25-1)

[0232] For sea surface scattering, the scattering unit S in equation (24) ik TS bik The scattering intensity TS of the sea surface sik replace:

[0233] TS sik =S sik +10lg A sik (25-2)

[0234] A in formula (27) bik and A sik is the area of ​​the kth seafloor and sea surface scattering unit in the i-th scattering ring, S bik , S sik are the seabed scattering intensity and the sea surface scattering coefficient respectively, which are expressed by formula (26):

[0235] S bik =10lg(μsina ik sinβ ik ) (26-1)

[0236]

[0237] Where, β = 158 (vf 1 / 3 ) -0.58 , α ik is the grazing angle of the incident wave of the scattering unit, β ik is the grazing angle of the scattered wave of the scattering unit, v represents the sea surface wind speed (in knots), and f represents the center frequency (in Hz).

[0238] α(f) is the sound absorption coefficient of seawater, which can be expressed using the Thorp empirical formula for the center frequency as follows:

[0239]

[0240] Scattering unit S ikThe scattering loss of the scattered signal is:

[0241]

[0242] Assume that the transmitted signal is a signal s(t) with a sound source level of SL, and do not consider the horizontal directivity of the transmission. The vertical directivity of the transmission is in accordance with the previous definition, and the Doppler effect of the scattering unit is considered. The transmission at point T passes through the seabed / sea surface scattering unit S ik The scattered signal reaching the receiving point R can be expressed as:

[0243]

[0244] or:

[0245]

[0246] In the formula, f dik is the Doppler shift, Δ ik is the corresponding Doppler factor (in time domain), φ ik is a random phase that obeys uniform distribution, uniformly distributed between 0-2π, τ ik is the round trip delay of the signal from the Sik scattering unit to the receiving point, calculated according to formula (16);

[0247] The calculation model of the scattered signal of the i-th scattered elliptical ring is expressed as:

[0248]

[0249] AB i is the random amplitude of the scattering signal generated by the i-th scattering ring, which obeys the normal distribution;

[0250] The calculation model of the reverberation signal scattered by the seabed / sea surface is expressed as:

[0251] Seabed reverberation signal:

[0252] Sea surface reverberation signal:

[0253] N b (t), N s (t) are the total number of scattering elliptical rings of the seabed scattering signal and the sea surface scattering signal reaching the receiving end at time t;

[0254] Adding the signals of equations (31-1) and (31-2) at the same time, we can get the total reverberation signal generated by the seabed and the sea surface:

[0255] y(t)=y b (t)+y s (t) (32).

[0256] 107. When the calculated reverberation signal satisfies that the reverberation level is lower than the difference between the ocean environment noise level and the array directivity DI, the reverberation simulation program of the scattering unit ends.

[0257] Specifically, the determination of N(t): As the grazing angle becomes smaller and the scattering element becomes farther and farther, the scattering intensity becomes weaker and weaker. It is not necessary to calculate all grazing angles within the vertical opening angle in the simulation, so it is necessary to determine when to terminate the calculation.

[0258] Basic criterion: the reverberation level is lower than the ocean ambient noise level (same as the active sonar operating frequency band), that is, when the reverberation masking level changes to the noise masking level, the reverberation simulation program ends. Considering the array processing capability of the active sonar receiver, the directional DI gain of the array processing is increased to make the interface reverberation and the ocean reverberation connect more smoothly. When the condition of formula (33) is met, the reverberation simulation program of the scattering unit ends;

[0259]

[0260] The overall process is as follows Figure 2-4 .

[0261] according to Figure 2-4 Perform reverberation signal simulation calculation. The implementation process is as follows:

[0262] 1. Set the background parameters for reverberation calculation

[0263] (1) The active transmission signal is an HFM signal with a frequency range of 900Hz-1500Hz and a pulse length of 3 seconds.

[0264] (2) The transmitting sound source level SL = 210 dB, and the transmitting signal sound pressure amplitude is calculated based on the sound source level;

[0265] (3) According to the sampling frequency f s =25000H, generating signal waveform s(t).

[0266] (4) Transmission vertical angle (±Ψ / 2): omnidirectional, ±Ψ / 2 = ±90°; restricted: ±Ψ / 2 = ±30°.

[0267] (5) Sea area depth H: (1) 200 meters in shallow sea; (2) 5000 meters in deep sea.

[0268] (6) Level 2 sea condition, sea surface wind speed v = 13.3 kn.

[0269] (7) Transmitter array depth h T =70 meters, receiving array depth h R =50 meters, H0=(70+50) / 2=60 meters.

[0270] (8) The slant distance between the transmitting and receiving points is 2L = 400 meters.

[0271] (9) The increment of the semi-major axis of the extended ellipsoid is Δa = 150 m.

[0272] (10) The speed of sound c = 1500 m / s.

[0273] (11) The number of each scattering unit is K=12.

[0274] (12) Seabed scattering backscatter intensity 10logμ=-27.

[0275] 2. Determine the minimum scattering ellipsoid of the seafloor z = -(H-H0) and its subsequent scattering ellipsoid

[0276] (1) Determine the minimum scattering ellipsoid parameters according to formulas (1)-(5);

[0277] (2) Determine the ellipsoid parameters of the subsequent i=1, 2, ... seafloor scattering according to formulas (6)-(8).

[0278] 3. For all seafloor scattering ellipsoid rings, perform the following cyclic calculations:

[0279] 3.1 Calculate the coordinates of the boundary points of the seafloor scattering ellipse unit

[0280] The seafloor scattering ellipse ring is divided into K = 12 azimuth regions, and the azimuth angle of each region is Δρ = 360 / K = 30°. Then the boundary point B of the kth scattering unit on the outer ring of the i-th scattering ellipse is ik The polar angle is ρ bk =0°, 3°0, 60°, ..., 330°, by combining equations (9) to (10), we can calculate the boundary coordinates of the scattering cell (bx ik ,by ik ).

[0281] 3.2 Calculate the center point coordinates of each scattering unit of the i-th scattering ellipse ring on the seabed

[0282] According to formula (11), calculate the semi-major axis and semi-minor axis of the central ring of the i=1, 2, .... scattering ellipsoid;

[0283] Combining equations (12) and (13), calculate the scattering center position S of the kth scattering unit (azimuth area) on the i-th scattering ellipse ring: ik The coordinates (sx ik ,sy ki ), the ρ in the calculation formula k =ρ bk +15°.

[0284] 3.3 Calculation of scattering unit distance and propagation delay parameters

[0285] (1) Calculation of scattering unit distance and propagation delay

[0286] According to formula (14), the distance R from the transmitting point T with coordinates (-Lcosδ, 0, Lsinδ) to the center point of the scattering unit Sik is calculated: Tik .

[0287] According to formula (15), calculate the distance R from Sik to the receiving point R with coordinates (Lcosδ, 0, -Lsinδ) Rik .

[0288] According to formula (16), the propagation time τ from the transmitting point T to the seafloor scattering unit Sik and then to the receiving point R is calculated: ik .

[0289] 3.4 Calculate the incident angle and scattering angle of the scattering unit

[0290] According to formulas (18) and (19), point T is incident on the scattering unit S ik The vertical angle of incidence α ik and scattering unit S ik Vertical scattering angle β to the receiving point R ik .

[0291] 3.5 Calculation of scattering intensity of scattering unit

[0292] (1) Calculate the scattering unit S ik The area of ​​the region

[0293] The scattering cell boundary coordinates (bx) calculated in step 3.1 ik ,by ik ), and use formulas (20)-(21) to calculate the area of ​​each scattering unit.

[0294] (2) Calculate the seafloor scattering intensity of the kth scattering unit Sik in the i-th scattering ring

[0295] Calculate α according to step 3.4 ik and β ik , calculate the scattering coefficient S of the scattering unit Sik according to formula (26) bik .

[0296] (3) According to the scattering unit area A bik and the interface scattering intensity S bik , calculate the scattering intensity TS of the scattering unit according to formula (25) bik .

[0297] 3.6 Calculation of the Doppler shift of the scattering element Sik

[0298] Calculate the scattering point S according to formula (22): ik The normalized bistatic Doppler frequency f of the scattered echo at dik .

[0299] 3.7 Calculation of scattering loss of scattering unit

[0300] (1) Based on the center frequency f, the sound absorption coefficient α(f) of seawater is calculated according to formula (27).

[0301] (2) The scattering unit distance R obtained in step 3.4 Tik and R Rik , and TS obtained in step 3.5 bik , calculate the scattering loss TLr of the scattering unit Sik in decibels according to formula (24) ik .

[0302] If R Tik If equation (17) is satisfied, the scattering unit is included in the reverberation calculation, otherwise it is not calculated.

[0303] (3) Then, the sound pressure amplitude coefficient SB of the scattering loss is converted by formula (28) ik .

[0304] 3.8 Calculation of the reverberation signal of a single scattering unit

[0305] (1) Calculate the reverberation signal of a single scattering unit according to formula (29):

[0306] (2) Calculate the reverberation signal of the i-th scattering ring according to formula (30):

[0307] (3) Calculate the seafloor reverberation signal at time t according to formula (31):

[0308] 3.9 Determine whether the reverberation calculation meets the end conditions

[0309] Determine whether the termination condition is met according to (32).

[0310] If the end condition is met, the reverberation calculation ends; otherwise, repeat 3.1 to 3.9 to continue calculating the reverberation signal.

[0311] 4 Set z = H0 and calculate the sea surface reverberation signal according to steps 2 to 3 (3.1 to 3.9).

[0312] The corresponding formula for calculating the sea surface reverberation signal is replaced by the sea surface scattering related formula.

[0313] 5 According to the same time sampling points, the sea surface and seabed reverberation signals are superimposed to form the total ocean interface reverberation signal.

[0314] 6 Calculation result diagram:

[0315] Figure 1-1 and Figure 1-2 The following are the interface reverberation results at a depth of 200 meters, with and without considering the "silent scattering circle". Because it is a shallow sea, the seabed reverberation is dominant, and the reverberation arrival time and reverberation intensity in the two cases are not much different and are relatively close.

[0316] Figure 1-3 and Figure 1-4 These are the interface reverberation results at a depth of 5,000 meters, with and without the "silent scattering circle". This is the deep sea, the first to arrive is the sea surface reverberation, and the last to arrive is the seabed reverberation. In these two cases, the arrival time and reverberation intensity of the sea surface reverberation are basically the same; the arrival time and reverberation intensity of the seabed reverberation are significantly different. The case of considering the "silent scattering circle" is more in line with the actual simulation calculation.

[0317] Figure 1-5 and Figure 1-6 They are the coordinate position maps of the seabed, sea surface scattering ellipse, scattering center position point, and scattering area boundary point at a sea depth of 200 meters.

[0318] Figure 1-7 and Figure 1-8 They are the comparison diagram of the normalized interface reverberation signal instantaneous amplitude histogram and Gaussian distribution, and the comparison diagram of the normalized interface reverberation signal envelope histogram and Rayleigh distribution. It can be seen from the two figures that the instantaneous amplitude of the interface reverberation signal calculated by this patent conforms to the Gaussian distribution, and the reverberation signal envelope conforms to the Rayleigh distribution, which meets the statistical characteristics of the reverberation signal.

[0319] Beneficial effects:

[0320] 1. The present invention establishes a method for determining the minimum scattering ellipsoid of the seabed and sea surface scattering of a single-base sonar with separate transmitters and receivers, and a method for determining the subsequent scattering ellipsoid. The method for determining the minimum scattering ellipsoid solves the problem of determining the interface scattering signals that arrive first and subsequently arrive at the single-base sonar with separate transmitters and receivers; the method for determining the subsequent scattering ellipsoid can solve the problem of calculating the reverberation signal of a long-pulse active signal to a limited extent, so that the characteristics of the generated reverberation signal are more consistent with the statistical characteristics of the reverberation. The combined advantages of the two are to improve the accuracy of the arrival time of the reverberation signal, the intensity accuracy of the superposition of multiple reverberation signals, and the authenticity of the reverberation signal. The invention is also applicable to the modeling and simulation of interface reverberation of multi-base sonars.

[0321] 2. The present invention proposes a scattering unit division method based on an interface scattering elliptical ring. The method divides the entire horizontal plane into K azimuth regions for a horizontal omnidirectional active sonar, so that K scattering units are formed on each interface scattering elliptical ring. The number of regions K can be determined based on comprehensive factors such as the beam width of the receiving array, the Doppler frequency shift resolution of the interface reverberation signal, and the time cost of the reverberation calculation.

[0322] 3. The present invention establishes a calculation model for the coordinates of the scattering unit boundary points and the scattering unit center points, laying the foundation for calculating the scattering unit area, the scattering unit Doppler frequency shift, etc.

[0323] 4. The present invention establishes calculation models such as the scattering unit's transmitting incident angle, receiving scattering angle, distance to the transmitting-receiving point, and reverberation delay, laying the foundation for the calculation of scattering loss and reverberation signal of each scattering unit.

[0324] 5. The present invention establishes a calculation model for the reverberation signal of the seabed and the sea surface. (1) A Doppler frequency shift calculation model based on the transmission and reception separation at the center of the scattering unit is proposed, and the frequency domain and time domain expressions are given, which can be used for narrowband signals and wideband signals respectively; (2) A calculation formula for the sea surface scattering intensity of the transmission and reception separation is given, which can provide support for the modeling of sea surface reverberation of various single-base or multi-base sonars with transmission and reception separation.

[0325] 6. The present invention updates the interface reverberation calculation termination condition. The present invention takes into account the array processing capability of the active sonar receiver and increases the directivity DI factor of the array processing, so that the interface reverberation and the ocean reverberation are connected more smoothly.

[0326] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that modifying the technical solutions described in the aforementioned embodiments, or replacing some or all of the technical features therein by equivalents, does not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for simulating reverberation signals at the interface of a single-base active sonar with separate transmitters and receivers, characterized in that: The method comprises: Establish the interface scattering ellipsoid model of single-base sonar with separate transmitter and receiver; Determining a minimum scattering ellipsoid of seabed / sea surface scattering based on the scattering ellipsoid model, and obtaining subsequent scattering ellipsoids according to the minimum scattering ellipsoid; Dividing each of the scattering ellipsoids according to the azimuth to form a plurality of scattering units whose distances are determined by the scattering elliptical rings and whose directions are determined by the azimuth regions, and calculating the coordinates of the boundary points and the center points of each of the scattering units; According to the coordinates of the boundary points and the center point of the scattering unit, the transmitting incident angle, the scattering angle, the receiving scattering angle, the reverberation delay, and the distance from each scattering unit to the transmitting-receiving point and the area of ​​the region where the scattering unit is located are calculated; Establishing a Doppler frequency shift model, and calculating the normalized bistatic Doppler frequency shift and the time domain Doppler factor of the scattered echo at the scattering unit; Calculate the scattering loss of the scattering unit according to the transmission incident angle, scattering angle, receiving scattering angle, reverberation delay, distance to the transmitting and receiving point, and area of ​​the scattering unit; calculate the reverberation signal of the scattering unit according to the scattering loss and the Doppler frequency shift and time domain Doppler factor; When the calculated reverberation signal satisfies that the reverberation level is lower than the difference between the ocean environment noise level and the array directivity DI, the reverberation simulation program of the scattering unit ends.

2. The method for simulating reverberation signals of a single-base active sonar interface with separate transmitters and receivers according to claim 1, characterized in that: The method of establishing a single-base sonar interface scattering ellipsoid model with separate transmitters and receivers comprises: Assume that the X-axis is positive along the moving direction of the towing platform, the transmitting array is in front and the receiving array is in the back, the z-axis is positive toward the water surface and negative toward the seabed; T and R are the positions of the transmitting array and the receiving array, respectively, and the depths of the transmitting array and the receiving array are h T and h R The oblique distance between the transmitting and receiving stations is 2L, and the inclination angle between the connecting line and the horizontal plane is δ.

3. The method for simulating reverberation signals at the interface of a single-base active sonar with separate transmitters and receivers according to claim 2, characterized in that: The step of determining the minimum scattering ellipsoid of seabed / sea surface scattering based on the scattering ellipsoid model comprises: Under the condition that the vertical beam of the transmission is ±Ψ / 2, there is a seabed / surface silent scattering circle corresponding to the single-base sonar with separate transmitters and receivers. The silent scattering circle is a circle with T′ as the center and L yi is a circle with radius i=1,2, The radius of the seabed silent scattering circle is expressed as: THE y1 =(Hh T )tan(90°-Ψ / 2)=(Hh T )ctan(Ψ / 2)(1-1) The radius of the sea surface silent scattering circle is expressed as: L y2 = h T tan(90°-Ψ / 2)= h T ctan(Ψ / 2) (1-2) L yi The intersection of the silent scattering circle with a radius of and the line connecting T′-R′ is taken as the scattering point; The minimum slant distance R from the transmitted sound to the seabed scattering point Tmin1 It can be expressed as: The minimum slant distance R of the transmitted sound to the scattering point on the sea surface Tmin2 It can be expressed as: The slant distance R from the seabed scattering point to the receiving point Rmin1 It can be expressed as: The slant distance R from the sea surface scattering point to the receiving point Rmin2 It can be expressed as: Wherein, 2L' is the horizontal distance between the transmitting point T and the receiving point R, that is, the distance between T' and R'; The seabed / sea surface scattering ellipsoid that first reaches the receiving point is the minimum scattering ellipsoid, and its semi-major axis is expressed as: The corresponding two semi-minor axes are: In equations (4) and (5), a0, b0, and c0 define the minimum scattering ellipsoid.

4. The method for simulating reverberation signals at the interface of a single-base active sonar with separate transmitters and receivers according to claim 3 is characterized in that: The obtaining a subsequent scattering ellipsoid according to the minimum scattering ellipsoid comprises: The semi-major axis and semi-minor axis of the outer ring of the first scattering ellipsoid in the subsequent scattering ellipsoid are respectively: The semi-major axis and semi-minor axis of the outer ring of the i=1,2,... scattering ellipsoid are:

5. The method for simulating reverberation signals at the interface of a single-base active sonar with separate transmitters and receivers according to claim 4 is characterized in that: The step of dividing each scattering ellipsoid according to the azimuth to form a plurality of scattering units whose distance is determined by the scattering elliptical ring and whose direction is determined by the azimuth region, and calculating the coordinates of the boundary points and the center point of each scattering unit comprises: The method of dividing the scattering ellipsoid according to the azimuth is: if the change of the Doppler shift in a certain azimuth range is within the allowable range, then the azimuth area is determined as an independent scattering unit cell; Let the coordinates of the cell boundary point of the i-th scattering ellipse be (bx i ,by i ), the method for calculating the boundary point coordinates of each scattering unit is as follows: The cross section of the scattering ellipsoid on the seabed / sea surface is an ellipse on the xoy plane, and the equation of the ellipse is: In the formula, z j , j = 1, 2; for the calculation of seabed reverberation, the calculation is performed through the cross section of the i = 1, 2, .... scattering ellipsoid at the seabed z1 = -(H-H0); for the calculation of sea surface reverberation, the calculation is performed through the cross section of the i = 1, 2, .... scattering ellipsoid at the seabed z2 = H0; Each scattering ellipse is divided into K regions, then the boundary point B of the kth scattering unit on the outer ring of the i-th scattering ellipse is ik The polar angle is ρ bk , the coordinates of the kth scattering unit are bx ik ,by ik , calculated by equations (9)-(10): bx ik = by ik cot(ρ bk ) or by ik = bx ik tan(ρ bk )(9) The method for calculating the center point coordinates of each scattering unit is as follows: Let S ik is the scattering center position of the kth scattering unit on the i-th scattering ellipse, and its coordinates are expressed as (sx ik ,sy ki ,z j ) indicates that z j They represent the z-axis coordinates of the scattering centers at sea bottom j=1 and sea surface j=2 respectively; the semi-major axis and semi-minor axis of the center ring of the i=1, 2, .... scattering elliptical rings are: Let the polar angle of the center of the kth scattering unit be ρ k , then S ik The coordinates are: or One of the two formulas in the above formula (12) can be combined with the ellipse equation to solve the center point coordinates (sx ik ,sy ik ), expressed as:

6. The method for simulating reverberation signals at the interface of a single-base active sonar with separate transmitters and receivers according to claim 5, characterized in that: The method calculates the transmission incident angle, scattering angle, receiving scattering angle, reverberation delay, distance from each scattering unit to the transmitting-receiving point and the area of ​​the region where the scattering unit is located according to the coordinates of the boundary points and the center point of the scattering unit, and calculates the normalized bistatic Doppler shift of the scattered echo at the scattering unit, including: (1) Establish a calculation model for the distance between scattering units and their propagation delay: The coordinates of the emission point T are (-Lcosδ, 0, Lsinδ) to the scattering unit S ik The distance to the center point is: S ik The distance to the receiving point R with coordinates (Lcosδ,0,-Lsinδ) is: From the transmitting point T to the seafloor scattering unit S ik , and then the propagation time to the receiving point R is: Due to the limitation of the vertical transmission beam, there is an acoustic scattering shadow area on the seabed directly below the transmission point. The sound wave cannot reach this acoustic scattering shadow area. This acoustic scattering shadow area does not scatter and should not be included in the process of calculating the scattering intensity by expanding the scattering ellipsoid. For a flat seabed, the basic criterion is: R Tik >R Tmin (17) (2) Calculation model of incident angle and scattering angle of scattering unit: The incident light at point T is incident on the scattering element S. ik The vertical angle of incidence is: Seafloor scattering: Sea Surface Scattering: Scattering unit S ik The vertical scattering angle to the receiving point R is: Seafloor scattering: Sea Surface Scattering: (3) Calculation model of the area where each scattering unit is located: According to the calculated boundary point coordinates of the i-th elliptical ring and the k-th scattering unit, (bx ik ,by ik ), and further calculate the area of ​​the i-th elliptical ring and the k-th sector: Then the scattering unit S ik The area is: When calculating the area of ​​a sector, proceed incrementally according to the subscript number in the same ellipse. Considering the symmetry of the region, only 1 / 4 of the sector needs to be calculated, and the region with the same area can be directly used.

7. The method for simulating reverberation signals at the interface of a single-base active sonar with separate transmitters and receivers according to claim 6, characterized in that: The step of establishing a Doppler frequency shift model and calculating a normalized bistatic Doppler frequency shift and a time domain Doppler factor of the scattered echo at the scattering unit includes: Under the same towing platform motion condition, the transmitting station speed, the receiving station motion speed and the platform motion speed v are the same, then the scattering point S ik The normalized bistatic Doppler frequency of the scattered echo at is expressed as: The time domain Doppler factor is expressed as: In the formula, The incident light from the seafloor point T to the scattering unit S ik The vertical angle of incidence, The incident light from point T on the sea surface to the scattering unit S ik The vertical angle of incidence; is the seafloor scattering unit S ik The vertical scattering angle to the receiving point R; is the sea surface scattering unit S ik Vertical scattering angle to receiving point R; θ Tik T' reaches the scattering unit S ik The side angle of Rik R' reaches the scattering unit S ik side angle.

8. The method for simulating reverberation signals at the interface of a single-base active sonar with separate transmitters and receivers according to claim 7, characterized in that: The calculating of the scattering loss of the scattering unit according to the transmission incident angle, scattering angle, receiving scattering angle, reverberation delay, distance to the receiving-transmitting point and the area of ​​the region of the scattering unit comprises: In the deep sea, the seabed / surface scattering unit S ik The total scattering loss is: TLr ik (t)=TS bik (t)-20lgR Tik (t)-20lgR Rik (t)-10 -3 α(f)(R Tik (t)+R Rik (t))(24-1) In shallow waters, the expansion loss obeys the 1.5th power law, that is, between plane waves and spherical waves, and the seabed / sea surface scattering unit S ik The total scattering loss is: TLr ik (t)=TS bik (t)-15lgR Tik (t)-15lgR Rik (t)-10 -3 α(f)(R Tik (t)+R Rik (t))(24-2) Among them, TS bik Scattering unit S ik The seafloor scattering intensity is expressed as: TS bik =S bik +10lgA bik (25-1) For sea surface scattering, the scattering unit S in equation (24) ik TS bik The scattering intensity TS of the sea surface sik replace: TS sik =S sik +10lgA sik (25-2) A in formula (25) bik and A sik is the area of ​​the kth seafloor and sea surface scattering unit in the i-th scattering ring, S bik , S sik are the seabed scattering intensity and the sea surface scattering coefficient respectively, which are expressed by formula (26): Where, β = 158 (vf 1 / 3 ) -0.58 , The incident light from the seafloor point T to the scattering unit S ik The vertical angle of incidence, The incident light from point T on the sea surface to the scattering unit S ik The vertical angle of incidence; is the seafloor scattering unit S ik The vertical scattering angle to the receiving point R; is the sea surface scattering unit S ik The vertical scattering angle to the receiving point R; v represents the sea surface wind speed, and f represents the center frequency; α(f) is the sound absorption coefficient of seawater, which can be expressed using the Thorp empirical formula for the center frequency as follows: Scattering unit S ik The scattering loss of the scattered signal is:

9. The method for simulating reverberation signals at the interface of a single-base active sonar with separate transmitters and receivers according to claim 8, characterized in that: The step of calculating the reverberation signal of the scattering unit according to the scattering loss, the Doppler frequency shift, and the time domain Doppler factor comprises: Assume that the transmitted signal is a signal s(t) with a sound source level of SL, and do not consider the horizontal directivity of the transmission. The vertical directivity of the transmission is in accordance with the previous definition, and the Doppler effect of the scattering unit is considered. The transmission at point T passes through the seabed / sea surface scattering unit S ik The scattered signal reaching the receiving point R can be expressed as: or: In the formula, f dik is the Doppler shift, Δ ik is the corresponding Doppler factor in the time domain, φ ik is a random phase that obeys uniform distribution, uniformly distributed between 0-2π, τ ik The signal is transmitted to the S ik The round-trip delay of the total acoustic path from the scattering unit to the receiving point is calculated according to formula (16); The calculation model of the scattered signal of the i-th scattered elliptical ring is expressed as: AB i is the random amplitude of the scattering signal generated by the i-th scattering ring, which obeys the normal distribution; The calculation model of the reverberation signal scattered by the seabed / sea surface is expressed as: Seabed reverberation signal: Sea surface reverberation signal: N b (t), N s (t) are the total number of scattering elliptical rings of the seabed scattering signal and the sea surface scattering signal reaching the receiving end at time t; Adding the signals of equations (31-1) and (31-2) at the same time, we can get the total reverberation signal generated by the seabed and the sea surface: y(t)=y b (t)+y s (t)(32)。 10. The method for simulating reverberation signals at the interface of a single-base active sonar with separate transmitters and receivers according to claim 9, characterized in that: When the calculated reverberation signal satisfies the condition that the reverberation level is lower than the ocean ambient noise level, the reverberation simulation program of the scattering unit ends, including: Taking into account the array processing capability of the active sonar receiver, the directional DI gain of the array processing is increased to make the interface reverberation and the ocean reverberation connect more smoothly. When the condition of formula (33) is met, the reverberation simulation program of the scattering unit ends;

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