Method for calculating electromagnetic reflection coefficient of three-dimensional rough sea surface based on wind direction factor

Abstract

The present application relates to a three-dimensional rough sea surface electromagnetic reflection coefficient calculation method based on wind direction factors, comprising the following steps: collecting radar frequency, polarization mode, incident angle, wind speed and wind direction data; constructing a PM-ITTC directional sea wave spectrum model; inputting the wind speed and wind direction vector data into the PM-ITTC directional sea wave spectrum model to obtain a three-dimensional sea surface roughness correction factor; inputting the radar frequency, polarization mode and incident angle into the Fresnel formula to obtain the Fresnel reflection coefficient of the electromagnetic wave on the sea surface; and calculating the three-dimensional rough sea surface electromagnetic reflection coefficient based on the wind direction factors according to the three-dimensional sea surface roughness correction factor and the Fresnel reflection coefficient. The present application considers the influence of wind direction on the roughness correction factor, vectorizes the wind speed and wind direction information, and combines the sea wave parameter information to correct the roughness correction factor, thereby improving the accuracy of the sea surface electromagnetic reflection coefficient calculation.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of electromagnetic wave scattering, and particularly relates to a three-dimensional rough sea surface electromagnetic reflectivity calculation method based on a wind direction factor. BACKGROUND

[0002] Sea surface electromagnetic reflectivity is a key parameter in the fields of marine remote sensing, marine environment monitoring, marine resource exploration, and climate change research. The sea surface reflection characteristics are affected by the frequency, polarization mode, incident angle, dielectric constant of seawater, and sea surface roughness of the incident wave.

[0003] Traditional rough sea surface reflectivity calculation methods mainly include measurement methods, analytical approximation methods, and numerical calculation methods. In existing research, the asymmetric influence of wind direction on sea wave energy distribution is ignored, while wind direction is a key factor affecting the distribution of sea surface roughness, determining the propagation direction and energy concentration area of sea waves, and further affecting the scattering characteristics of electromagnetic waves. SUMMARY

[0004] The technical problem of the present application is to consider the influence of wind direction on sea surface reflectivity and improve the accuracy and efficiency of calculating rough sea surface reflectivity by processing the full grazing angle range.

[0005] The technical solution of the present application is a three-dimensional rough sea surface electromagnetic reflectivity calculation method based on a wind direction factor, comprising the following steps:

[0006] S1: Collect radar frequency, polarization mode, incident angle, wind speed, and wind direction data;

[0007] S2: Construct a PM-ITTC directional sea wave spectrum model;

[0008] S3: Input the wind speed and wind direction vector data into the PM-ITTC directional sea wave spectrum model to obtain a three-dimensional sea surface roughness correction factor;

[0009] S4: Input the radar frequency, polarization mode, and incident angle into the Fresnel formula to obtain the Fresnel reflectivity of electromagnetic waves on the sea surface;

[0010] S5: Calculate the three-dimensional rough sea surface electromagnetic reflectivity based on the wind direction factor according to the three-dimensional sea surface roughness correction factor and the Fresnel reflectivity.

[0011] The PM-ITTC directional sea wave spectrum model in step S2 is optimized, which contains PM spectrum and ITTC directional extension function, and is used to describe sea surface roughness by introducing wind speed and wind direction, and further simulate the frequency spectrum and directional distribution of sea waves.

[0012] Further, the PM spectrum, in the form of natural exponential function, characterizes the spectral properties of wind waves and quantifies the energy distribution at different wave numbers, which is further used to analyze the scattering and reflection properties of electromagnetic waves by sea waves. The PM spectrum is expressed as:

[0013] ;

[0014] where exp( ) represents the natural exponential function, represents the wave number vector, represents the modulus of the wave number vector, and represents the empirical constant, represents the wind speed vector, U 10 represents the wind speed.

[0015] The optimized ITTC is a directional spreading function, which describes the energy distribution of sea waves in different directions by the angle between the direction of sea waves and the wind direction, and reflects the roughness characteristics in three-dimensional space. The expression is:

[0016] ;

[0017] where is the angle between the direction of sea waves and the wind direction.

[0018] Further, the step S3 includes inputting the wind speed and wind direction vector data into the PM-ITTC directional sea wave spectrum model to obtain the spectral density function of the sea surface, and using the spectral density function to calculate the root mean square height and the correlation length, and then calculate the sea surface structure parameters;

[0019] 1) The calculation formula of the spectral density function of the sea surface is:

[0020] ;

[0021] where A and B represent constants, f p represents the peak frequency, f represents the radar frequency, and exp( ) represents the exponential calculation;

[0022] 2) The calculation formula of the root mean square height is:

[0023] ;

[0024] where f represents the radar frequency; θ represents the direction angle of the sea waves, and S( ) is the spectral density function.

[0025] 3) The correlation length is calculated by adjusting the wave energy at all frequencies and directions. The calculation formula of the correlation length is:

[0026] ;

[0027] where f represents the radar frequency, unit Hz; θ represents the direction angle of the sea wave, unit radian. S(f, θ) represents the spectral density function, corresponding to the wave energy distribution under different frequencies and directions at the corresponding direction angle θ, the integral range is from 0 to infinite frequency domain frequency and from 0 to 2π direction range, represents the adjustment of the spectral density function.

[0028] 4) The calculation formula for characterizing the sea surface structure parameter is:

[0029] ;

[0030] where W( ) represents a three-dimensional sea surface roughness correction factor containing wind direction information, represents the wind direction angle of the wind speed, represents the incident angle, represents the root mean square height of the sea wave, represents the correlation length of the sea wave, represents the wavelength of the radar electromagnetic wave, represents the exponential operation.

[0031] Further, in step S4, the following sub-steps are included:

[0032] 1) Calculate the wavelength of the electromagnetic wave, the calculation formula is:

[0033] ;

[0034] where c represents the speed of light, and f represents the radar frequency.

[0035] 2) The dielectric constant of seawater, the relative dielectric constant of seawater is calculated using an empirical formula, and the dielectric constant is divided into real part dielectric constant and imaginary part dielectric constant, and the calculation formulas are respectively:

[0036] ;

[0037] ;

[0038] where, represents the real part dielectric constant, represents the imaginary part dielectric constant, where and represent the constants related to temperature and salinity, and i represents the count unit.

[0039] Optimized, in step S4, the Fresnel formula is used to calculate the reflection characteristics of the electromagnetic wave under different polarization states respectively, and the calculation formula is:

[0040] ;

[0041] ;

[0042] wherein, represents the vertical polarization result, represents the horizontal polarization result.

[0043] In the optimization step S5, the calculation is based on the wind direction factor of the three-dimensional rough sea surface electromagnetic reflection coefficient, and the calculation formula is:

[0044]

[0045] wherein, represents the Fresnel reflection coefficient of the electromagnetic wave on the sea surface.

[0046] Compared with the prior art, the beneficial effects of the present application include:

[0047] 1) The present application considers the influence of wind direction on the roughness correction factor, vectorizes the wind speed and wind direction information, and combines the sea wave parameter information to correct the roughness correction factor, thereby improving the accuracy of the sea surface electromagnetic reflection coefficient calculation.

[0048] 2) The present application uses an analytical approximation method, which avoids complex numerical calculations and improves the efficiency of sea surface electromagnetic reflection coefficient calculation.

[0049] 3) The present application is suitable for electromagnetic waves of different frequencies, different polarization modes and different incident angles, and improves the practicability of calculating the sea surface electromagnetic reflection coefficient. BRIEF DESCRIPTION OF DRAWINGS

[0050] The present application will be further described below in conjunction with the drawings and examples.

[0051] Figure 1 The flow chart of the method for calculating the sea surface electromagnetic reflection coefficient of the embodiment of the present application.

[0052] Figure 2 The rough sea surface model and the reflection coefficient calculation diagram of the embodiment of the present application.

[0053] Figure 3 The three-dimensional PM-ITTC directional wave number spectrum of the embodiment of the present application combined with the ITTC directional spreading function.

[0054] Figure 4 The sea surface model comparison diagram of the embodiment of the present application under different wind speed and wind direction. DETAILED DESCRIPTION

[0055] As Figure 1 shown, the three-dimensional rough sea surface electromagnetic reflection coefficient calculation method based on the wind direction factor includes the following steps:

[0056] ​S1: Collecting radar frequency, polarization mode, incident angle, wind speed and wind direction data;

[0057] The preset wind speed, wind direction, latitude and longitude sea surface environment parameters are as shown in the following table. Figure 2

[0058] S2: Constructing a PM-ITTC directional sea wave spectrum model;

[0059] The PM-ITTC directional sea wave spectrum model in step S2 contains a PM spectrum and an ITTC directional spreading function, and is used to depict sea surface roughness by introducing wind speed and wind direction, so as to simulate the frequency spectrum and direction distribution of sea waves.

[0060] The PM spectrum represents the frequency spectrum characteristics of wind waves in the form of a natural exponential function, and quantifies the energy distribution at different wave numbers, and is further used to analyze the scattering and reflection characteristics of sea waves on electromagnetic waves. The expression of the PM spectrum is as follows:

[0061] ;

[0062] In the formula, exp( ) represents a natural exponential function, represents a wave number vector, represents the modulus of the wave number vector, and represents an empirical constant, represents a wind speed vector, U 10 represents the wind speed.

[0063] The ITTC is a directional spreading function, which describes the energy distribution of sea waves in different directions through the angle between the direction of sea waves and the wind direction, and reflects the roughness characteristics in three-dimensional space. The expression is as follows:

[0064] ;

[0065] In the formula, is the angle between the direction of sea waves and the wind direction.

[0066] S3: Inputting the wind speed and wind direction vector data into the PM-ITTC directional sea wave spectrum model to obtain a three-dimensional sea surface roughness correction factor;

[0067] As shown in the following table, Figure 3 Step S3 includes inputting the wind speed and wind direction vector data into the PM-ITTC directional sea wave spectrum model to obtain the spectral density function of the sea surface, and using the spectral density function to calculate the root mean square height and the correlation length, and then calculating the sea surface structure parameters;

[0068] 1) The calculation formula of the spectral density function of the sea surface is as follows:

[0069] ; ​

[0070] In the formula, A and B represent constants, and f p represents the peak frequency, f represents the radar frequency, and exp() represents exponential calculation;

[0071] 2) The formula for calculating the root mean square height is:

[0072] ;

[0073] In the formula, f represents the radar frequency, θ represents the direction angle of the sea waves, and S() is the spectral density function.

[0074] 3) The formula for calculating the relevant length is:

[0075] ;

[0076] In the formula, f Let θ represent the radar frequency, θ represent the direction angle of the sea waves, and S() represent the spectral density function.

[0077] 4) The formulas for calculating the parameters characterizing sea surface structure are:

[0078] ;

[0079] In the formula, W() represents the three-dimensional sea surface roughness correction factor containing wind direction information. Wind direction angle, representing wind speed Indicates the angle of incidence. This represents the root mean square height of the ocean waves. Indicates the relevant length of the ocean wave. The wavelength of radar electromagnetic waves. This indicates the exponentiation operation.

[0080] S4: Input the radar frequency, polarization mode and incident angle into the Fresnel formula to obtain the Fresnel reflection coefficient of electromagnetic waves on the sea surface;

[0081] Step S4 includes the following sub-steps:

[0082] 1) Calculate the wavelength of the electromagnetic wave using the following formula:

[0083] ;

[0084] In the formula, c represents the speed of light, and f represents the radar frequency.

[0085] 2) The dielectric constant of seawater is calculated using empirical formulas. The dielectric constant is divided into the real part and the imaginary part, and the calculation formulas are as follows:

[0086] ;

[0087] ;

[0088] In the formula, Represents the real part of the dielectric constant. Denotes the imaginary part of the dielectric constant, where and This represents a constant related to temperature and salinity, where i represents the counting unit.

[0089] In step S4, Fresnel's formula is used to calculate the reflection characteristics of electromagnetic waves under different polarization states. The calculation formula is as follows:

[0090] ;

[0091] ;

[0092] In the formula, This indicates the vertical polarization result. This indicates the result of horizontal polarization.

[0093] S5: Calculate the electromagnetic reflection coefficient of a three-dimensional rough sea surface based on wind direction factors, using the three-dimensional sea surface roughness correction factor and Fresnel reflection coefficient.

[0094] In step S5, the calculation of the three-dimensional rough sea surface electromagnetic reflection coefficient based on wind direction is performed using the following formula:

[0095] ;

[0096] In the formula, This represents the Fresnel reflection coefficient of electromagnetic waves on the sea surface.

[0097] To verify the effectiveness of the proposed method, data with different wind speeds, wind directions, and radar frequencies were selected as input parameters. As shown in Table 1, wind speeds of 3 m / s, 6 m / s, 10 m / s, and 15 m / s, wind directions ranging from 0° to 360° with a step size of 40°, radar frequencies of 1 GHz, 5 GHz, 10 GHz, and 20 GHz, and polarizations of HH and VV were selected as input parameters.

[0098] Table 1

[0099]

[0100] The influence of sea surface roughness is considered by introducing a correction factor, and the Fresnel reflection coefficient is combined to calculate the final corrected sea surface electromagnetic reflection coefficient, as shown in Table 2.

[0101] Table 2

[0102]

[0103] Traditional models cannot reflect the directional changes of the reflection coefficient on the sea surface under different wind speeds and wind directions. This invention can accurately reflect the changes in different wind directions and speeds, closely resembling the actual undulations of the sea surface.

[0104] like Figure 4 As shown, the method for calculating the electromagnetic reflection coefficient of a three-dimensional rough sea surface based on wind direction proposed in this invention effectively solves the error problem existing in traditional methods by introducing a more accurate sea surface model and a dynamic wind direction correction factor, providing a more reliable theoretical basis and calculation tool for fields such as marine remote sensing and environmental monitoring.

[0105] The above embodiments are merely preferred technical solutions of the present invention and should not be considered as limitations on the present invention. The scope of protection of the present invention should be the technical solution described in the claims, including equivalent substitutions of the technical features described in the claims. That is, equivalent substitutions and improvements within this scope are also within the scope of protection of the present invention.

Claims

1. A method for calculating electromagnetic reflection coefficient of three-dimensional rough sea surface based on wind direction factor, characterized in that, The method comprises the following steps: S1: collecting radar frequency, polarization mode, incident angle, wind speed and wind direction data; S2: constructing a PM-ITTC directional sea wave spectrum model; S3: inputting the wind speed and wind direction vector data into the PM-ITTC directional sea wave spectrum model to obtain a spectrum density function of the sea surface, and using the spectrum density function to calculate a root mean square height and a correlation length, and then calculating a sea surface structure parameter; a calculation formula of a three-dimensional sea surface roughness correction factor is: ； where W() represents a three-dimensional sea surface roughness correction factor containing wind direction information, represents the wind direction angle of the wind speed, represents the incident angle, represents the root mean square height of the sea wave, represents the correlation length of the sea wave, represents the wavelength of the radar electromagnetic wave, represents the exponential operation; S4: inputting the radar frequency, polarization mode and incident angle into a Fresnel formula to obtain a Fresnel reflection coefficient of an electromagnetic wave on the sea surface; S5: calculating a three-dimensional rough sea surface electromagnetic reflection coefficient based on a wind direction factor according to the three-dimensional sea surface roughness correction factor and the Fresnel reflection coefficient.

2. The method of claim 1, wherein the method is based on a wind direction factor. In step S2, the PM-ITTC directional sea wave spectrum model comprises a PM spectrum and an ITTC directional extension function, and is used for describing sea surface roughness by introducing wind speed and wind direction, and then simulating the spectrum and direction distribution of sea waves.

3. The method of claim 2, wherein the method is based on a wind direction factor. The PM spectrum is in the form of a natural exponential function, and is used for characterizing the spectrum characteristics of wind waves and quantifying the energy distribution under different wave numbers, and then is used for analyzing the scattering and reflection characteristics of sea waves on electromagnetic waves, and a PM spectrum expression is: ； where exp( ) denotes the natural exponential function, denotes the wave number vector, denotes the modulus of the wave number vector, and denotes an empirical constant, denotes the wind velocity vector, U 10 denotes the wind velocity magnitude.

4. The method of claim 2, wherein the method is characterized by: The ITTC is a directional extension function, and is used for describing the energy distribution of sea waves in different directions through the angle between the direction of the sea waves and the wind direction, and reflecting the roughness characteristics in three-dimensional space, and an expression is: ； wherein is the angle between the direction of the sea waves and the wind direction.

5. The method of claim 1, wherein the method is based on a wind direction factor. In step S4, the Fresnel formula is used for calculating the reflection characteristics of electromagnetic waves under different polarization states, and a calculation formula is: ； ； wherein represents the vertical polarization result, represents the horizontal polarization result.

6. The method of claim 1, wherein the method is based on a wind direction factor. In step S5, the calculation of the three-dimensional rough sea surface electromagnetic reflection coefficient based on the wind direction factor is a calculation formula: ； wherein denotes the Fresnel reflection coefficient of the electromagnetic wave on the sea surface.

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