An Inversion Method for Evaporation Duct Profiles Based on Cooperative Marine Radar Signals

Abstract

The present invention relates to a method for retrieving the evaporation duct profile based on cooperative marine radar signals. By using the methods of time accumulation, direction locking, and beam timing, on the basis of known parameters of the target ship's marine radar, path loss data on the monitoring link is obtained. By establishing an evaporation duct profile model with horizontal non-uniformity on the monitoring link, and combining with advanced electromagnetic wave propagation models, objective functions, global optimization algorithms, interpolation assimilation algorithms, etc., taking the inversion result in a horizontally uniform environment as the initial value, setting the search threshold of the objective function, the non-uniform evaporation duct distribution on the monitoring link is retrieved. Reasonably utilizing the known information of the ship's marine radar, passively receiving radio frequency signals in real time, retrieving the evaporation duct profile, reducing the inversion difficulty, improving the inversion efficiency, realizing the passive signal inversion of the evaporation duct profile, obtaining the environmental characteristics of electromagnetic wave propagation at sea, and ensuring the over-the-horizon working requirements of the marine electromagnetic system.

Description

Technical Field

[0001] The present invention belongs to the technical fields of near-sea evaporation duct, marine atmosphere, ocean physics, etc. It belongs to the evaporation duct profile inversion method and relates to an evaporation duct profile inversion method based on cooperative marine radar signals. Background Art

[0002] Evaporation duct is a relatively common marine atmospheric duct phenomenon that often appears in the troposphere. The generation of this phenomenon stems from the interaction between meteorological factors such as temperature, humidity, and pressure in the atmosphere. Due to the evaporation of seawater, the interaction between sea and vapor occurs on the sea surface, and the vapor continuously diffuses. As the diffusion height increases, the atmospheric refractive index in the atmospheric environment continuously decreases. When reaching a certain height, the phenomenon that the refractive index is less than the curvature of the earth's sea surface appears, and electromagnetic waves are trapped in this layer, thus realizing over-the-horizon propagation.

[0003] Currently, the active inversion method of evaporation duct requires the transmitting end to emit electromagnetic wave signals and calculate through the received path loss data, which increases the complexity and difficulty of monitoring. Marine radar is a commonly used detection device configured on marine ships, with high signal intensity, and the radio frequency signal frequency of X-band marine radar is within the optimal frequency band of evaporation duct. Therefore, the method of using marine radar to passively invert the evaporation duct profile has natural technical advantages.

[0004] Since the current methods for obtaining the evaporation duct profile all have certain defects, it is impossible to obtain the true profile of the high-precision evaporation duct on the sea in real time, covertly and efficiently, and over a large range for a long time, making it difficult to meet the actual needs of aspects such as calibration of the evaporation duct prediction model, comparison of monitoring link data, optimization of the inversion model, and passive monitoring of the evaporation duct channel. Summary of the Invention

[0005] Technical Problems to be Solved

[0006] In order to avoid the deficiencies of the prior art, the present invention proposes an evaporation duct profile inversion method based on cooperative marine radar signals, which is applicable to aspects such as evaporation duct profile inversion, comparison and testing of experimental data for evaporation duct monitoring, and prediction of the feasibility of evaporation duct communication, and has the advantages of long-term real-time, covert and efficient, wide range, etc.

[0007] Technical Solution

[0008] An evaporation duct profile inversion method based on cooperative marine radar signals, characterized by the following steps:

[0009] Step 1: Establish a copy field database: Calculate the atmospheric modified refractive index distribution of the evaporation duct height h t = z EDH at set intervals:

[0010]

[0011] where: M0 and M(z) are the atmospheric modified refractive indices at the sea surface and at height z respectively, and the value of M0 is 330; h t = z EDH is the evaporation duct height, the roughness length z0 = 1.5×10 -4 , ΔM = M(h1) - M0 is the evaporation duct strength, and M(h1) is the modified refractive index at the evaporation duct height;

[0012] The above obtained data is used as the environmental data of the horizontally uniform evaporation duct for establishment;

[0013] Step 2, path loss simulation: Set the simulation parameters of the emission source according to the target ship information, substitute them into the advanced propagation model, and calculate the path loss corresponding to the evaporation duct profile:

[0014]

[0015] PL fs is the free space loss, and the unit is dB: PL fs = 32.45 + 20log f + 20log r;

[0016] where: f is the frequency, and the unit is MHz, r is the distance between the transmitting antenna and the receiving antenna, and the unit is km;

[0017] The F is the propagation factor:

[0018] where: |u(x,z)| represents the electromagnetic field strength distribution in the evaporation duct environment calculated by using the parabolic equation model; x represents the propagation distance, and the unit is m;

[0019] Step 3, calculate the measured path loss: According to the rotation characteristics of the marine radar, extract the maximum monitored signal level and its occurrence time within a radar rotation period, and combine the position information, transmission power, antenna height, and antenna gain provided by the target ship to calculate the path loss of the monitoring link:

[0020]

[0021] where: t represents the time parameter corresponding to time accumulation, t = 1, 2,..., N t ; is the actually measured path loss, and the unit is dB; P t is the transmission signal power, and the unit is dBm; G t is the transmitting antenna gain, and the unit is dBi; G r is the receiving antenna gain, and the unit is dBi;a is the gain of the received signal amplifier, with the unit of dB; P r is the level value of the received signal, with the unit of dBm; IL is the equivalent system insertion loss of the whole system, with the unit of dB;

[0022] Step 4: Establish the inversion objective function

[0023]

[0024] where p represents the evaporation duct profile model parameter, N t is the number of searches in the copy field database;

[0025] Using the global optimization algorithm, search and solve for a set of optimal solutions that minimize to inversely obtain the uniform evaporation duct modified refractive index profile on a single link;

[0026] Step 5: Calculate the copy field vector: Use the inversion result, i.e., the uniform evaporation duct modified refractive index profile on a single link, as the initial value to establish a horizontally non-uniform evaporation duct modified refractive index profile on the monitoring link: M = [M1, M2, …, M N , and substitute the calculated N evaporation duct profiles and the transmitter parameters into the advanced propagation model to calculate the copy field vector for path loss inversion;

[0027] Step 6: Inversely obtain the evaporation duct distribution in the area: Repeat Steps 3 to 5, calculate the uniform evaporation duct modified refractive index profile for each inverted link, and then interpolate and assimilate the inversion results of each link to obtain the regional distribution of the evaporation duct.

[0028] The propagation factor F d and F r represent the antenna direction factors of the direct and refracted rays respectively, and Ω represents the total phase angle of the direct and refracted rays.

[0029] The maximum monitored signal level within the radar rotation period refers to the level at the center frequency of the radar RF signal, which can reach the maximum value within one radar rotation period, i.e., the signal level when the main beam of the radar antenna is completely aligned with the receiving antenna.

[0030] The evaporation duct height h t = z EDH ranges from 0 to 50 m.

[0031] The set interval is 0.1 m.

[0032] The transmitter power mentioned above is the average power of the radar RF to cope with the pulse desensitization effect exhibited by the radar RF signal in the frequency spectrum.

[0033] The described marine radar includes two types: X-band and C-band. Since the RF signal of the X-band radar is within the optimal frequency band of the evaporation duct, it is more suitable for use in the inversion of the evaporation duct profile. Therefore, the X-band marine radar signal is used as the inversion target signal.

[0034] The node spacing of the corrected refractive index profile of the evaporation duct described above can be 25 - 50 km. It is impossible to have both the resolution of the inversion distance step and the accuracy resolution of the evaporation duct profile.

[0035] Beneficial effects

[0036] An evaporation duct profile inversion method based on cooperative marine radar signals proposed by the present invention uses methods such as time accumulation, direction locking, and beam timing. Based on the known parameters of the target ship's marine radar, path loss data on the monitoring link is obtained. By establishing a horizontally non-uniform evaporation duct profile model on the monitoring link and combining electromagnetic wave advanced propagation models, objective functions, global optimization algorithms, interpolation assimilation algorithms, etc., with the inversion result of a horizontally uniform environment as the initial value and setting the search threshold of the objective function, a non-uniform evaporation duct distribution on the monitoring link is inverted. It reasonably utilizes the known information of the ship's marine radar, passively receives RF signals in real time, inverses the evaporation duct profile, reduces the inversion difficulty, improves the inversion efficiency, realizes the passive signal inversion of the evaporation duct profile, obtains the environmental characteristics of electromagnetic wave propagation at sea, and ensures the over-the-horizon working requirements of the marine electromagnetic system. It can be used in aspects such as model calibration, data comparison, inversion optimization, scientific research exploration, and auxiliary decision-making of the marine electromagnetic system related to the evaporation duct.

[0037] The present invention proposes an evaporation duct profile inversion method based on cooperative marine radar signals. By the path loss data of the target ship's marine radar signal, a non-uniform evaporation duct distribution on the monitoring link is inversed. There is no need to actively transmit signals, avoiding equipment frequency interference, solving the problem that it is difficult for existing evaporation duct channel monitoring means to be applied in a long-term real-time, concealed, efficient, and large-area manner, and can be used as an auxiliary decision-making means for the marine electromagnetic system. Description of the drawings

[0038] Figure 1 is the flowchart of the present invention

[0039] Figure 2 is the conceptual diagram of collecting navigation radar signals

[0040] Figure 3 is the simulated path loss when the evaporation duct height is 20 m

[0041] Figure 4 is the spectrum of the marine radar RF signal

[0042] Figure 5is the modified refractive index profile of the uniform evaporation duct obtained by inversion

[0043] Figure 6 is the modified refractive index profile of the non-uniform evaporation duct obtained by inversion

[0044] Figure 7 is the inversion assimilation result of the regional evaporation duct Detailed implementation mode

[0045] The present invention will be further described below in conjunction with embodiments and drawings:

[0046] An evaporation duct profile inversion method based on cooperative marine radar signals, characterized in that by using the measured shipborne marine radar signal level data, combining methods such as non-uniform evaporation duct profile models, advanced propagation models, inversion objective functions, and global optimization algorithms, the evaporation duct distribution of a single link is inverted, and through the interpolation and assimilation algorithms of the inversion results of multiple links, the evaporation duct distribution in the monitored sea area is obtained. The specific steps are as follows:

[0047] Step 1: Establish a copy field database. Use formula (1) to calculate the atmospheric modified refractive index distribution with the evaporation duct height ranging from 0 to 50 m and an interval of 0.1 m, and establish a horizontally uniform evaporation duct environment.

[0048]

[0049] In formula (1), M0 and M(z) are the atmospheric modified refractive indices at the sea surface and at height z respectively, M0 takes the value of 330; h t = z EDH is the evaporation duct height, the roughness length z0 = 1.5×10 -4 , ΔM = M(h1) - M0 is the evaporation duct strength, and M(h1) is the modified refractive index at the evaporation duct height.

[0050] Step 2: Path loss simulation. Set the emission source simulation parameters according to the target ship information, substitute them into the advanced propagation model, and use formulas (2) to (7) to calculate the path loss corresponding to the evaporation duct profile.

[0051] PL fs = 32.45 + 20log f + 20log r (2)

[0052]

[0053] In formulas (2) to (3), f is the frequency, the unit is MHz, r is the distance between the transmitting antenna and the receiving antenna, the unit is km, and PL fs is the free space loss, the unit is dB, is the calculated path loss in dB. F is the propagation factor, which has two forms in the advanced propagation model, namely Equation (4) and Equation (5). F in the parabolic equation space is calculated by Equation (4), in the ray space by Equations (5) and (6), in the flat earth space by Equations (5) and (7), and in the super-optical space far beyond the evaporation duct layer, it is approximately calculated by the ray space. Among them, the flat earth space is used when the antenna elevation angle is greater than 5° or the distance is less than 2.5 km, the ray space is used when the antenna elevation angle is less than 5° and greater than 2° and the distance is greater than 2.5 km. The low-altitude area outside the ray space is the parabolic equation space, and the high-altitude area is the super-optical space.

[0054]

[0055]

[0056]

[0057]

[0058] In Equations (4) to (7), |u(x,z)| represents the electromagnetic field strength distribution in the evaporation duct environment calculated using the parabolic equation model; x represents the propagation distance in m; F d and F r represent the antenna directivity factors of the direct and refracted rays respectively, Ω represents the total phase angle of the direct and refracted rays, β d and β r represent the propagation angles of the direct and refracted rays respectively, R and are the level and the phase lag of the Fresnel reflection coefficient respectively, α is the receiving elevation angle, α1 and α0 represent the angles at the start and end of each ray tracing step respectively, X represents the calculated distance, S represents the spherical propagation factor, D represents the optical path length difference, r1 and r2 are the path lengths of the direct and refracted rays respectively, a represents the average radius of the earth, k e represents the effective earth radius coefficient.

[0059] Step 3: Calculate the measured path loss. According to the rotation characteristics of the marine radar, extract the maximum monitored signal level and its occurrence time within a radar rotation period, and combine the position information, transmit power, antenna height, and antenna gain provided by the target ship to calculate the path loss of the monitoring link by Equation (8).

[0060]

[0061] In the formula, t represents the time parameter corresponding to the time accumulation, t = 1, 2, …, N t ; is the actually measured path loss in dB; P tis the transmitted signal power, with the unit of dBm; G t is the transmitting antenna gain, with the unit of dBi; G r is the receiving antenna gain, with the unit of dBi; G a is the gain of the receiving signal amplifier, with the unit of dB; P r is the level value of the received signal, with the unit of dBm; IL is the equivalent system insertion loss of the whole system, with the unit of dB.

[0062] Step 4: Establish the inversion objective function

[0063]

[0064] where p represents the evaporation duct profile model parameter, N t represents the number of searches, and its size is related to the copy field database established in Step 1. Using the global optimization algorithm, search and solve for a set of optimal solutions that minimize to inversely obtain the evaporation duct modified refractive index profile on a single link.

[0065] Step 5: Calculate the copy field vector. Take the inversion result of the uniformly distributed evaporation duct as the initial value, and establish a horizontally non-uniform evaporation duct modified refractive index profile on the monitoring link: M = [M1, M2, …, M N , substitute the calculated N evaporation duct profiles and the transmitter end parameters into the advanced propagation model to calculate the copy field vector for path loss inversion.

[0066] Step 6: Inversely obtain the evaporation duct distribution in the area. Use the shore-based monitoring points, mobile monitoring points, island reef monitoring points, etc. to form a monitoring network, inversely obtain the non-uniform evaporation duct distribution of the monitoring links between each monitoring point and the cooperative ship respectively, and combine the interpolation and assimilation algorithms to inversely obtain the evaporation duct distribution in the monitoring area. Specific embodiment:

[0068] This method is characterized in that it uses the measured sea-borne navigation radar signal level data, combines methods such as the non-uniform evaporation duct profile model, the advanced propagation model, the inversion objective function, and the global optimization algorithm, inversely obtains the evaporation duct distribution on a single link, and through the interpolation and assimilation algorithms of the inversion results of multiple links, obtains the evaporation duct distribution in the monitored sea area. Figure 1 is the flow chart of the method of the present invention.

[0069] The steps of the embodiment of this method are as follows:

[0070] Step 1: Establish a copy field database. Use Equation (1) to calculate the atmospheric modified refractive index distribution with the evaporation duct height ranging from 0 to 50 m and an interval of 0.1 m, and establish a horizontally uniform evaporation duct environment.

[0071] Step 2: In this example, the height of the navigation radar antenna of the target ship is 20 m, and the height of the receiving end antenna is 15 m. Substitute these values into the advanced propagation model and use formulas (2) to (7) to calculate the path loss of the corresponding evaporation duct profile. Figure 2 It is a schematic diagram of the path loss when the evaporation duct height is 20 m.

[0072] Step 3: In this example, the target ship is 50 km away from the monitoring point. The average transmit power of the navigation radar is 15 dBm, the antenna height is 20 m, and the antenna gain is 30 dBi; the receiving end antenna gain is 25 dBi, and the receiving signal amplifier gain is 25 dB. The conceptual diagram of signal acquisition is as shown in Figure 3 shown, and the received signal spectrum is as shown in Figure 4 shown. The equivalent insertion loss of the system is 5 dB. The path loss of the monitoring link is calculated by formula (8) to be 156.94 dB.

[0073] Step 4: Establish an inversion objective function

[0074] Use the global optimization algorithm to search for and solve a set of optimal solutions that minimize Figure 5 It is the corrected refractive index profile of the evaporation duct obtained by inversion on a single link, and the evaporation duct height is 22.9 m.

[0075] Step 5: Take the inversion result of the uniformly distributed evaporation duct as the initial value, and establish a horizontally non-uniform corrected refractive index profile of the evaporation duct on the monitoring link: M = [M1, M2, …, M N , and the node spacing is 25 km. Figure 6 It is the non-uniform corrected refractive index profile of the evaporation duct obtained by inversion on a single link.

[0076] Step 6: Use shore-based monitoring points, mobile monitoring points, island reef monitoring points, etc. to form a monitoring network, and respectively invert the non-uniform evaporation duct distribution of the monitoring links between each monitoring point and the cooperative ship. Combine interpolation and assimilation algorithms to obtain the evaporation duct distribution of the monitoring area, as shown in Figure 7 shown.​

Claims

1. A method for retrieving the evaporation duct profile based on cooperative marine radar signals, characterized in that The steps are as follows: Step 1. Establish a copy field database: Calculate the evaporation duct height h at a set interval t = z EDH for the atmospheric modified refractive index distribution: Where: M0 and M(z) are the atmospheric modified refractive indices at the sea surface and at height z respectively, and the value of M0 is 330; h t = z EDH is the evaporation duct height, the roughness length z0 = 1.5×10 -4 , ΔM = M(h1) - M0 is the evaporation duct strength, and M(h1) is the modified refractive index at the evaporation duct height; The data obtained above is used as the data for establishing a horizontally uniform evaporation duct environment. Step 2, path loss simulation: Set the simulation parameters of the emission source according to the target ship information, substitute them into the advanced propagation model, and calculate the path loss of the corresponding evaporation duct profile: PL fs is the free space loss, with the unit of dB: PL fs = 32.45 + 20 log f + 20 log r; Where: f is the frequency, with the unit of MHz, and r is the distance between the transmitting antenna and the receiving antenna, with the unit of km. The F is the propagation factor: Where: |u(x,z)| represents the electromagnetic field strength distribution in the evaporation duct environment calculated using the parabolic equation model; x represents the propagation distance, with the unit of m. Step 3, calculate the measured path loss: According to the rotation characteristics of the marine radar, extract the maximum monitoring signal level and its occurrence time within a radar rotation period, and combine the position information, transmit power, antenna height, and antenna gain provided by the target ship to calculate the path loss of the monitoring link: where: t represents the time parameter corresponding to time accumulation, t = 1, 2, …, N t ; is the actually measured path loss, in dB; P t is the transmit signal power, in dBm; G t is the transmit antenna gain, in dBi; G r is the receive antenna gain, in dBi; G a is the receive signal amplifier gain, in dB; P r is the level value of the received signal, in dBm; IL is the equivalent system insertion loss of the entire system, in dB; Step 4: Establish the inversion objective function where p represents the evaporation duct profile model parameter, N t the number of searches in the copy field database; Using a global optimization algorithm, search and solve for a set of optimal solutions that minimize and inversely obtain the modified refractive index profile of the uniform evaporation duct on a single link; Step 5: Calculate the copy field vector: Use the inversion result, i.e., the uniform evaporation duct modified refractive index profile on a single link, as the initial value to establish a horizontally non-uniform evaporation duct modified refractive index profile on the monitoring link: M = [M1, M2, …, M N , substitute the calculated N evaporation duct profiles and the transmitter parameters into the advanced propagation model to calculate the copy field vector for path loss inversion; Step 6, invert the evaporation duct distribution in the area: Repeat steps 3 to 5, calculate and invert the uniform evaporation duct modified refractive index profile of each link, and then interpolate and assimilate the inversion results of each link to obtain the regional distribution of the evaporation duct.

2. The evaporation duct profile inversion method based on cooperative marine radar signals according to claim 1, characterized in that: The propagation factor F d and F r respectively represent the antenna directivity factors of the direct and refracted rays, and Ω represents the total phase angle of the direct and refracted rays.

3. The evaporation duct profile inversion method based on cooperative marine radar signals according to claim 1, characterized in that: The maximum monitoring signal level within the radar rotation period refers to the level at the center frequency of the radar RF signal, which can reach the maximum value within a radar rotation period, that is, the signal level when the main beam of the radar antenna is completely aligned with the receiving antenna.

4. The evaporation duct profile inversion method based on cooperative marine radar signals according to claim 1, wherein: The height h of the evaporation duct t = z EDH ranges from 0 to 50 m.

5. The evaporation duct profile inversion method based on cooperative marine radar signals according to claim 1, characterized in that: The set interval is 0.1 meters.

6. The evaporation duct profile inversion method based on cooperative marine radar signals according to claim 1, wherein: The transmit power at the transmitting end is the average power of the radar RF to cope with the pulse desensitization effect shown by the radar RF signal in the frequency spectrum.

7. The evaporation duct profile inversion method based on cooperative marine radar signals according to claim 1, characterized in that: The marine radar includes two types: X-band and C-band. Since the RF signal of the X-band radar is within the optimal frequency band of the evaporation duct and is more suitable for use in inverting the evaporation duct profile, the X-band marine radar signal is used as the inversion target signal.

8. The evaporation duct profile inversion method based on cooperative marine radar signals according to claim 1, characterized in that: The node spacing of the evaporation duct modified refractive index profile can be 25 - 50 km, and it is impossible to have both the resolution of the inversion distance step and the accuracy resolution of the evaporation duct profile.

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