A method for constructing a multi-ship composite electromagnetic model

Abstract

The present invention discloses a method for constructing a multi-ship composite electromagnetic model, comprising: step S1, calculating the shaft-frequency electromagnetic field under the condition of multiple ships, including: step S11, constructing the shaft-frequency electromagnetic field based on the amplitude attenuation coefficient and the phase shift constant; step S12, calculating the time series of a single ship at any phase; step S13, calculating the shaft-frequency electromagnetic field time series of multiple ships; step S2, calculating the DC dipole static electric field and static magnetic field of the multiple ships; step S3, calculating the multi-ship composite electromagnetic field model, superimposing the electric field and magnetic field time series of the multiple ships, and obtaining the composite ship electromagnetic field observed by the multiple ships under real conditions. The present invention can improve the simulation accuracy of the ship electromagnetic field, simulate the ship electromagnetic field more consistent with actual observations, and has broad application prospects for monitoring key areas such as ports and waterways.

Description

Technical Field

[0001] The present invention relates to the technical field of electromagnetic field numerical simulation of ships, and in particular to a method for constructing a multi-ship composite electromagnetic model. Background Art

[0002] In today's era of rising strategic importance of the ocean, ships, as core equipment for marine activities, generate electromagnetic fields that have crucial applications in numerous fields and have become a research hotspot in marine science and engineering. The ship's electromagnetic field refers to the sum of the electric and magnetic fields generated by various electrical equipment, electronic systems, and the ship's own metal structure during operation. In-depth research on the characteristics of these electromagnetic fields lays a solid foundation for numerous practical applications. Current research on ship electromagnetic field modeling primarily focuses on single-type physical fields, such as shaft-frequency electric and magnetic fields, static electric and magnetic fields. However, in real-world environments, multiple types of ship electromagnetic fields are intertwined and cannot be observed separately and independently. Therefore, research on composite modeling of multiple electromagnetic fields is necessary.

[0003] Current research on the characteristic electromagnetic field of ships primarily focuses on single characteristic fields. For example, Xiong Lu et al. (2014) constructed a three-dimensional dielectric electric field distribution model based on horizontal DC dipole theory and achieved numerical simulation of the shaft-frequency electric field. Liang Chenghao et al. (2008) used scaled-down model experiments to reveal the modulation mechanism of propeller rotation on corrosion current and cathodic protection current. To inhibit hull corrosion, modern ships generally employ a hybrid protection system combining active impressed current cathodic protection with passive sacrificial anode protection. These active protection systems generate artificially controllable polarization currents during operation, which, together with the natural corrosion current, constitute a unique electrostatic field source for ships. The steady-state currents used for ship corrosion protection also generate corrosion-related static magnetic fields. This research has been conducted, for example, by Jiang Zhiguo et al. (2020) on equivalent simulation methods for corrosion-related electric field sources during natural ship corrosion. However, it is important to note that research on the static magnetic fields generated by the steady-state currents used for corrosion protection is virtually nonexistent. Furthermore, this type of research currently focuses on simulation and characteristic analysis of a single ship's electromagnetic field. However, in actual observations, the electromagnetic receiving system records a composite electromagnetic field consisting of shaft-frequency electromagnetic fields, static electric fields, and static magnetic fields. Furthermore, in key sea areas such as ports and waterways, multiple ships may be within the detection range, necessitating consideration of multiple ship scenarios. Furthermore, characteristic fields such as shaft-frequency electromagnetic fields are time-varying, possessing both amplitude and phase information. Existing ship electromagnetic field models do not consider phase modeling, and phase aliasing in the case of multiple ships results in more complex and variable signals. Therefore, constructing a composite electromagnetic field model for multiple ships is fundamental for practical applications. Summary of the Invention

[0004] The present invention provides a method for constructing a multi-ship composite electromagnetic model, which can improve the simulation accuracy of the ship's electromagnetic field and simulate the ship's electromagnetic field that is more consistent with actual observations. It has broad application prospects in applications such as monitoring of key areas such as ports and waterways.

[0005] The present invention solves the technical problem by adopting the following technical solutions:

[0006] A method for constructing a multi-ship composite electromagnetic model comprises the following steps:

[0007] Step S1, calculating the shaft frequency electromagnetic field in the case of multiple ships, includes:

[0008] Step S11: constructing time-harmonic electric dipole and DC electric dipole models corresponding to multiple ships, as well as a geoelectric model, wherein the time-harmonic electric dipole model includes the time-harmonic dipole frequency, length, current intensity, waveform characteristics, position, phase, and movement speed; the DC electric dipole model includes the DC dipole length, current intensity, position, and movement speed; and the geoelectric model includes seawater depth, resistivity, and the stratification and resistivity distribution of the underground medium;

[0009] Step S12, constructing an axial frequency electromagnetic field based on the amplitude attenuation coefficient and the phase shift constant;

[0010] Step S13, calculating the time series of a single ship at any phase;

[0011] Step S14, calculating the time series of the electromagnetic fields of multiple ships' shaft frequencies;

[0012] Step S2, calculating the static electric field and static magnetic field of the DC electric dipoles of the multiple ships;

[0013] Step S3, calculating the composite electromagnetic field model of multiple ships, superimposing the electric field and magnetic field time series of multiple ships, and obtaining the composite electromagnetic field of multiple ships observed under real conditions.

[0014] Furthermore, in step S12, the method for constructing the shaft frequency electromagnetic field based on the amplitude attenuation coefficient and the phase shift constant is as follows:

[0015] Let the time factor be , in a conductive medium, a multi-frequency time-harmonic electric dipole is constructed to satisfy the electromagnetic field of Maxwell's equations:

[0016] ,

[0017] in, is the electromagnetic field component in the time domain, is the angular frequency, is a plural unit, The initial phase of the goal, is the distance between the target and the receiving point; is the propagation constant, and ; is the alternating electromagnetic field generated by time-harmonic electric dipoles of different electric moment strengths, which has the following form:

[0018] ,

[0019] in, is the DC component of the current generated by the ship, and are the Fourier coefficients, is the harmonic order of the time-harmonic electric dipole, is the index;

[0020] is the magnetic permeability, is the complex dielectric constant, There are the following forms:

[0021] ,

[0022] in, is the dielectric constant of the medium;

[0023] The propagation constant Written in plural form, we have:

[0024] ,

[0025] In the above formula, The amplitude attenuation coefficient indicates the attenuation of the amplitude of the electromagnetic wave per unit length along the propagation direction, with the unit being Np / m or dB; is the phase shift constant, in rad / m, which indicates the radian value of the electromagnetic wave when it propagates one meter;

[0026] The amplitude of the electromagnetic field is determined by the amplitude attenuation coefficient Attenuation, phase according to the phase shift constant changes, and considering the initial phase of the emission source, the electromagnetic field of the multi-frequency time-harmonic electric dipole that satisfies Maxwell's equations is expressed as:

[0027] ,

[0028] in, is the time of the receiving station, Electromagnetic fields emitted by controllable sources The time series of a component in is the angular frequency, The initial phase of the goal; is the amplitude attenuation coefficient, is the phase shift constant.

[0029] Furthermore, in step S13, the method for calculating the time series of a single ship in any phase is as follows:

[0030] By solving Maxwell's equations, the electromagnetic field response of the time-harmonic dipole source in the frequency domain multilayer medium is obtained. Replace it with the amplitude of the shaft frequency electromagnetic field under different transmitting and receiving distances, and Replace with the phase change value under different transmission and reception distances; thus, we get:

[0031] ,

[0032] According to the above formula, the above formula can be re-expressed as:

[0033] ,

[0034] in, is the distance between the target and the receiving point, is the time function; For At this moment, the distance between the target and the receiving point is The electromagnetic field amplitude at ; For At this moment, the distance between the target and the receiving point is The electromagnetic field phase at The initial phase of the target.

[0035] Furthermore, in step S14, the method for calculating the shaft frequency electromagnetic field time series of multiple targets is as follows:

[0036] After the frequency responses of multiple targets are obtained by superimposing the amplitudes and phases of the electromagnetic fields of multiple targets, the multi-target time series can be synthesized:

[0037] ,

[0038] in,

[0039] ,

[0040] in, is the number of targets, For the The frequency domain electromagnetic field of a target, For the The initial phase of a target; The method is obtained by constructing a marine geoelectric model and solving the electromagnetic field of a time-harmonic dipole source at the receiving point at different times in a frequency domain multilayer medium. The electromagnetic field of the relative position of the target and the receiving point at different times is calculated. and They are the amplitude and phase of the multi-target axial frequency electromagnetic field in the frequency domain, that is, the amplitude and phase obtained after the multi-target time series is processed.

[0041] Furthermore, in step S2, the method for calculating the static electric field and static magnetic field of the DC electric dipoles of multiple ships is as follows:

[0042] The electric dipole frequencies corresponding to different ships are set to zero. By solving the Maxwell equations that conform to these electric dipoles, the electric and magnetic field responses of the time-harmonic dipole source in the frequency domain DC electric dipole multilayer medium are obtained. These responses are the static electric and magnetic fields formed by the DC electric dipoles of different ships.

[0043] Beneficial effects of the present invention:

[0044] This paper addresses the mismatch between existing ship electromagnetic field simulation methods and observed ship electromagnetic fields in actual marine environments. It proposes a novel method for rapidly constructing a multi-ship composite electromagnetic model. This model includes four types of electromagnetic field time series: static electric field, static magnetic field, shaft-frequency electric field, and shaft-frequency magnetic field. This method provides a reliable technical solution for simulating the composite electromagnetic field of multiple ships in actual marine environments. It can obtain shaft-frequency electromagnetic field signals with arbitrary initial phases and achieve the superposition of multiple electromagnetic fields of different intensities and types. The simulated electromagnetic field is more consistent with the actual model. Compared with traditional ship electromagnetic field simulation methods, this method is based on solving the Maxwell equations satisfied by different electric dipoles in the frequency domain. It obtains four electromagnetic field models: shaft-frequency electric field, shaft-frequency magnetic field, and static electric field and static magnetic field, with arbitrary initial phases. It can also simulate the electromagnetic field time series of multiple ships. Furthermore, the proposed method can obtain multi-ship time series without using Fourier transforms, greatly reducing the computational effort required to obtain the time series and improving computational efficiency. The proposed multi-ship composite electromagnetic model construction method provides an effective solution for modeling the electromagnetic fields of ships with multiple ships and different phases within the detection range. The simulation method is different from the traditional simulation method based on Fourier transform to obtain time series, which greatly improves the computational efficiency of the simulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 is a flow chart of the method of the present invention;

[0046] Figure 2 This is a schematic diagram of the geoelectric model structure;

[0047] Figure 3 is the simulated three-component axial frequency electric field time series;

[0048] Figure 4 is the simulated three-component axial frequency magnetic field time series;

[0049] Figure 5 is the simulated three-component static electric field time series;

[0050] Figure 6 is the simulated three-component static magnetic field time series;

[0051] Figure 7 is the simulated three-component composite electric field time series;

[0052] Figure 8 is the simulated three-component composite magnetic field time series. DETAILED DESCRIPTION

[0053] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0054] Reference Attachment Figure 1 The present invention provides a method for constructing a multi-ship composite electromagnetic model, comprising the following steps: Step S1, calculating the shaft frequency electromagnetic field under the condition of multiple ships, comprising:

[0055] Step S11: constructing time-harmonic electric dipole and DC electric dipole models corresponding to multiple ships, as well as a geoelectric model. The time-harmonic electric dipole model includes the time-harmonic dipole frequency, length, current intensity, waveform characteristics, position, phase, and moving speed. The DC electric dipole model includes the DC dipole length, current intensity, position, and moving speed. The geoelectric model includes the seawater depth, resistivity, and the stratification and resistivity distribution of the underground medium.

[0056] Step S12: constructing an axial frequency electromagnetic field based on the amplitude attenuation coefficient and the phase shift constant, the method is as follows:

[0057] Let the time factor be , in a conductive medium, a multi-frequency time-harmonic electric dipole is constructed to satisfy the electromagnetic field of Maxwell's equations:

[0058] ,

[0059] in, is the electromagnetic field component in the time domain, is the angular frequency, is a plural unit, The initial phase of the goal, is the distance between the target and the receiving point; is the propagation constant, and ; is the alternating electromagnetic field generated by time-harmonic electric dipoles of different electric moment strengths, which has the following form:

[0060] ,

[0061] in, is the DC component of the current generated by the ship, and are the Fourier coefficients, is the harmonic order of the time-harmonic electric dipole, is the index;

[0062] is the magnetic permeability, is the complex dielectric constant, There are the following forms:

[0063] ,

[0064] in, is the dielectric constant of the medium;

[0065] The propagation constant Written in plural form, we have:

[0066] ,

[0067] In the above formula, The amplitude attenuation coefficient indicates the attenuation of the amplitude of the electromagnetic wave per unit length along the propagation direction, with the unit being Np / m or dB; is the phase shift constant, in rad / m, which indicates the radian value of the electromagnetic wave when it propagates one meter;

[0068] The amplitude of the electromagnetic field is determined by the amplitude attenuation coefficient Attenuation, phase according to the phase shift constant changes, and considering the initial phase of the emission source, the electromagnetic field of the multi-frequency time-harmonic electric dipole that satisfies Maxwell's equations is expressed as

[0069] ,

[0070] in, is the time of the receiving station, Electromagnetic fields emitted by controllable sources The time series of a component in is the angular frequency, The initial phase of the goal; is the amplitude attenuation coefficient, is the phase shift constant.

[0071] Step S13, calculate the time series of a single ship in any phase, the method is as follows:

[0072] By solving Maxwell's equations, the electromagnetic field response of the time-harmonic dipole source in the frequency domain multilayer medium is obtained. Replace it with the amplitude of the shaft frequency electromagnetic field under different transmitting and receiving distances, and Replaced by the phase change value under different transmission and reception distances; thus, we get

[0073] ,

[0074] ,

[0075] According to the above formula, the above formula can be re-expressed as:

[0076] ,

[0077] in, is the distance between the target and the receiving point, is the time function; For At this moment, the distance between the target and the receiving point is The electromagnetic field amplitude at ; For At this moment, the distance between the target and the receiving point is The electromagnetic field phase at ; The initial phase of the target.

[0078] Step S14, calculating the shaft frequency electromagnetic field time series of multiple targets, the method is as follows:

[0079] After the frequency responses of multiple targets are obtained by superimposing the amplitudes and phases of the electromagnetic fields of multiple targets, the multi-target time series can be synthesized:

[0080] ,

[0081] in,

[0082] ,

[0083] ,

[0084] in, is the number of targets, For the The frequency domain electromagnetic field of a target, For the The initial phase of a target; The method is obtained by constructing a marine geoelectric model and solving the electromagnetic field of a time-harmonic dipole source in a multi-layer medium in the frequency domain. The electromagnetic field of the relative position of the target and the receiving point at different times is calculated. and They are the amplitude and phase of the multi-target axial frequency electromagnetic field in the frequency domain, that is, the amplitude and phase obtained after the multi-target time series is processed.

[0085] Step S2, calculating the static electric field and static magnetic field of the DC electric dipoles of multiple ships, the method is as follows:

[0086] The electric dipole frequencies corresponding to different ships are set to zero. By solving the Maxwell equations that conform to these electric dipoles, the electric and magnetic field responses of the time-harmonic dipole source in the frequency domain DC electric dipole multilayer medium are obtained. These responses are the static electric and magnetic fields formed by the DC electric dipoles of different ships.

[0087] Step S3, calculating the composite electromagnetic field model of multiple ships, superimposing the electric field and magnetic field time series of multiple ships, and obtaining the composite electromagnetic field of multiple ships observed under real conditions.

[0088] This paper addresses the mismatch between existing ship electromagnetic field simulation methods and observed ship electromagnetic fields in actual marine environments. It proposes a novel method for rapidly constructing a multi-ship composite electromagnetic model. This model includes four types of electromagnetic field time series: static electric field, static magnetic field, shaft-frequency electric field, and shaft-frequency magnetic field. This method provides a reliable technical solution for simulating the composite electromagnetic field of multiple ships in actual marine environments. It can obtain shaft-frequency electromagnetic field signals with arbitrary initial phases and achieve the superposition of multiple electromagnetic fields of different intensities and types. The simulated electromagnetic field is more consistent with the actual model. Compared with traditional ship electromagnetic field simulation methods, this method is based on solving the Maxwell equations satisfied by different electric dipoles in the frequency domain. It obtains four electromagnetic field models: shaft-frequency electric field, shaft-frequency magnetic field, and static electric field and static magnetic field, with arbitrary initial phases. It can also simulate the electromagnetic field time series of multiple ships. Furthermore, the proposed method can obtain multi-ship time series without using Fourier transforms, greatly reducing the computational effort required to obtain the time series and improving computational efficiency. The proposed multi-ship composite electromagnetic model construction method provides an effective solution for modeling the electromagnetic fields of ships with multiple ships and different phases within the detection range. The simulation method is different from the traditional simulation method based on Fourier transform to obtain time series, which greatly improves the computational efficiency of the simulation.

[0089] Figure 2The geoelectric model constructed consists of an air layer, a seawater layer, sediment layer 1, sediment layer 2, and bedrock. The seawater depth is set at 350 m, with a conductivity varying linearly from 5.8 to 4 S / m. Sediment layer 1 is 50 m thick and has a resistivity of 2 Ωm. Sediment layer 2 is 100 m thick and has a resistivity of 4 Ωm. The bedrock extends infinitely downward and has a resistivity of 100 Ωm. All resistivities are isotropic. The ship is 50 m long, 20 m wide, and has a draft of 5 m. Therefore, the current element length is set to 50 m, the current intensity to 20 A, and the ship moves in the Y direction at a speed of 4 knots (2 m / s). The ship's propeller shaft rotation frequency is 3.5 Hz, the sampling frequency is 20 Hz, and the acquisition time is 600 s. The starting position is (10, -500, 5) and the movement is to (10, 700, 5).

[0090] Figure 3 and Figure 4 are the simulated three-component axial frequency electric and magnetic field time series. Figure 3 and Figure 4 It can be seen that due to the modulation effect of the tail axis of the shaft-frequency electromagnetic field, its time series has obvious vibrations, and the overall shape appears as a "spindle" or "two petals". From the extreme values ​​of the time series, it can be seen that the ship passed directly above the receiving station at about 4.2 minutes. At this time, the electric field component Ey and the magnetic field components Hx and Hz showed maximum values. Due to the orthogonality of the electromagnetic field, the electric field components Ex, Ez and the magnetic field component Hy showed minimum values ​​at this time. Figure 5 and Figure 6 are the simulated three-component static electric field and static magnetic field time series, respectively, Figure 5 and Figure 6 It can be seen that there is no oscillation in the static electric field and static magnetic field components, and their shapes are very close to the envelopes of the corresponding shaft-frequency electric field and shaft-frequency magnetic field, but there are certain differences in the values. Figure 7 and Figure 8 They are respectively the simulated three-component composite electric field time series of the ship. Figure 7 and Figure 8 It can be seen that even after superimposing the shaft-frequency electromagnetic field and the static electromagnetic field, the time series still exhibits oscillations. Comparing the ship's composite electromagnetic field with the single shaft-frequency / static electromagnetic field, the composite electromagnetic field state differs significantly from the shaft-frequency electromagnetic field. This is because the static electromagnetic field, to a certain extent, controls the overall signal shape and intensity. This also demonstrates the importance of constructing a composite electromagnetic field for ships.

[0091] 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 they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method for constructing a multi-ship composite electromagnetic model, characterized in that: The steps include: Step S1, calculating the shaft frequency electromagnetic field in the case of multiple ships, includes: Step S11: constructing time-harmonic electric dipole and DC electric dipole models corresponding to multiple ships, as well as a geoelectric model, wherein the time-harmonic electric dipole model includes the time-harmonic dipole frequency, length, current intensity, waveform characteristics, position, phase, and movement speed; the DC electric dipole model includes the DC dipole length, current intensity, position, and movement speed; and the geoelectric model includes seawater depth, resistivity, and the stratification and resistivity distribution of the underground medium; Step S12: constructing an axial frequency electromagnetic field based on the amplitude attenuation coefficient and the phase shift constant, the method is as follows: Let the time factor be e iωt , in a conductive medium, a multi-frequency time-harmonic electric dipole is constructed to satisfy the electromagnetic field of Maxwell's equations: F=T0expc i(ωt+φ)-kd Where T0 is the time domain electromagnetic field component, ω is the angular frequency, i is the complex unit, φ is the initial phase of the target, d is the distance between the target and the receiving point; k is the propagation constant, and expc is the alternating electromagnetic field generated by time-harmonic electric dipoles of different electric moment strengths, which has the following form: Among them, a dc is the DC component of the current generated by the ship, a n and b n is the Fourier coefficient, n is the harmonic order of the time-harmonic electric dipole, and τ is the exponent; μ is the magnetic permeability, is the complex dielectric constant, There are the following forms: Where ε is the dielectric constant of the medium; Writing the propagation constant k in complex form, we have In the above formula, α is the amplitude attenuation coefficient, which represents the attenuation of the amplitude of the electromagnetic wave per unit length along the propagation direction, and the unit is Np / m or dB; β is the phase shift constant, the unit is rad / m, which represents the radian value of the electromagnetic wave propagating one meter; The electromagnetic field amplitude decays according to the amplitude attenuation coefficient α, and the phase changes according to the phase shift constant β. Considering the initial phase of the emission source, the electromagnetic field of the multi-frequency time-harmonic electric dipole satisfies Maxwell's equations and is expressed as T=T0expc -αd(t) expc -iβd(t) expc (iωt+φ) Where t is the time of the receiving station, T is the electromagnetic field emitted by the controllable source T = (T x ,T y ,T z ), ω is the angular frequency, φ is the initial phase of the target; α is the amplitude attenuation coefficient, and β is the phase shift constant; Step S13, calculating the time series of a single ship at any phase; Step S14, calculating the time series of the electromagnetic fields of multiple ships' shaft frequencies; Step S2, calculating the DC dipole static electric field and static magnetic field of multiple ships; Step S3, calculating the composite electromagnetic field model of multiple ships, superimposing the electric field and magnetic field time series of multiple ships, and obtaining the composite electromagnetic field of ships observed by multiple ships under real conditions.

2. The method for constructing a multi-ship composite electromagnetic model according to claim 1, characterized in that: In step S13, the method for calculating the time series of a single ship in any phase is as follows: By solving Maxwell's equations, we can obtain the electromagnetic field response of the time-harmonic dipole source in the frequency domain multilayer medium. -αd(t) Replace with the amplitude of the axial frequency electromagnetic field under different transmission and reception distances, and replace -βd(t) with the phase change value under different transmission and reception distances; thus, we get T0expc -αd(t) =|F{d(t)}|=|F(d)| βd(t)=-arctan(F(d)) According to the above formula, the above formula can be re-expressed as T=|F(d)|expc iarctan(F(d)) expc (iωt+φ) Where d is the distance between the target and the receiving point, which is a function of time t; F(d) is the amplitude of the electromagnetic field at time t, that is, when the distance between the target and the receiving point is d; arctan(F(d)) is the phase of the electromagnetic field at time t, that is, when the distance between the target and the receiving point is d; φ is the initial phase of the target.

3. The method for constructing a multi-ship composite electromagnetic model according to claim 2, characterized in that: In step S14, the method for calculating the shaft frequency electromagnetic field time series of multiple targets is as follows: After the frequency responses of multiple targets are obtained by superimposing the amplitudes and phases of the electromagnetic fields of multiple targets, the multi-target time series can be synthesized: in, Where N is the number of targets, F j is the frequency domain electromagnetic field of the jth target, φ j is the initial phase of the jth target; F j The method is obtained by constructing a marine geoelectric model and solving the electromagnetic field of a time-harmonic dipole source at the receiving point at different times in a frequency domain multilayer medium. and They are the amplitude and phase of the multi-target axial frequency electromagnetic field in the frequency domain, that is, the amplitude and phase obtained after the multi-target time series is processed.

4. The method for constructing a multi-ship composite electromagnetic model according to claim 3, characterized in that: In step S2, the method for calculating the static electric field and static magnetic field of the DC electric dipole of multiple ships is as follows: The electric dipole frequencies corresponding to different ships are set to zero. By solving the Maxwell equations that conform to these electric dipoles, the electric and magnetic field responses of the time-harmonic dipole source in the frequency domain DC electric dipole multilayer medium are obtained. These responses are the static electric and magnetic fields formed by the DC electric dipoles of different ships.

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