Computational method of wind-wave coupled sea surface scattering and high-resolution SAR imaging under high sea conditions

Abstract

The present invention relates to a method for calculating wind-rain-wave coupled sea surface scattering and high-resolution SAR imaging under high sea conditions. The method comprises the following steps: establishing a time-varying nonlinear rainfall sea surface geometric model under high sea conditions using white canopy coverage and a rainfall-corrected sea spectrum; solving for sea surface scattering using a rainfall-corrected dual-scale model, taking into account the corrective effect of the rainfall annular spectrum on the sea surface capillary wave spectrum; solving for the scattering and attenuation of rainfall particles in the atmosphere, as well as the coupled scattering effects of rain columns, secondary splashed seawater droplets, and foam on the sea surface at the sea-air interface using vector radiative transfer theory; and performing SAR imaging of the rain-induced sea surface under high sea conditions using a velocity spotlight imaging algorithm. Advantageously, the method utilizes vector radiative transfer theory in combination with a three-layer random particle model to provide an accurate and effective method for calculating wind-rain-wave coupled sea surface scattering and high-resolution SAR imaging under high sea conditions, with high computational speed.

Description

Technical Field

[0001] The present invention belongs to the field of ocean remote sensing, and mainly relates to a method for calculating wind-wave-coupled sea surface scattering and high-resolution SAR imaging under high sea conditions. Background Art

[0002] Improving radar's ability to observe and monitor the ocean, particularly the scattering mechanisms of the sea surface under high sea conditions and rainfall, has long been one of the most important research topics in ocean remote sensing. In the past decade, the use of high-resolution synthetic aperture radar backscatter to infer wind speed and direction under high sea conditions and even hurricane-like conditions has attracted widespread attention.

[0003] The impact of rainfall on sea surface radar echoes is primarily manifested in two aspects: (1) volume scattering caused by raindrops and the attenuation effect in the atmosphere. (2) Raindrops at the sea-air interface impact the water surface, forming ring waves, rain columns, secondary seawater droplets, and other structures that affect radar scattering echoes. Therefore, establishing a suitable rainfall sea surface scattering model is of great significance for the study of sea surface weather and climate.

[0004] Current research on rainfall over sea surfaces, both domestically and internationally, primarily involves modifying statistical models of sea surface scattering based on measured rainfall data. However, rainfall under high sea conditions makes radar detection challenging, and research on the scattering mechanisms of high-resolution, deterministic rainfall over sea surfaces based on physical processes is still lacking. Furthermore, no studies have been conducted on the high-resolution rainfall over sea surface scattering mechanisms under high sea conditions that consider the multiple coupling effects of breaking waves and foam. Therefore, research on wind-wave coupled sea surface scattering and high-resolution SAR imaging computational methods under high sea conditions has significant academic value and broad application prospects. Summary of the Invention

[0005] Technical problems to be solved

[0006] In order to overcome the shortcomings of the prior art, the present invention provides a method for calculating wind-wave-coupled sea surface scattering and high-resolution SAR imaging under high sea conditions.

[0007] Technical Solution

[0008] A method for calculating wind-wave-coupled sea surface scattering and high-resolution SAR imaging under high sea conditions, characterized by the following steps:

[0009] Step 1: Use the whitecap coverage and rainfall-corrected ocean spectrum to establish a time-varying nonlinear rainfall sea surface geometry model under high sea conditions;

[0010] Step 2: Considering the correction effect of the rainfall annular spectrum on the sea surface capillary wave spectrum, the rainfall-corrected two-scale model is used to solve the sea surface scattering coefficient;

[0011] Step 3: Use vector radiative transfer theory to solve for the scattering and attenuation of rainfall particles in the atmosphere, as well as the coupled scattering between rain columns, secondary splashing seawater droplets, foam, and the sea surface at the sea-air interface;

[0012] Step 4: Use the velocity spotlight imaging algorithm to perform high-resolution SAR imaging of the sea surface under high sea conditions.

[0013] A further technical solution of the present invention is as follows: in the step 1, the time-varying nonlinear rainfall sea surface geometric modeling under high sea conditions is performed, a rainfall-corrected sea spectrum model is adopted, a linear filtering method is combined with Hilbert transform to obtain a nonlinear sea surface, and the white crown coverage rate is used in combination with the slope criterion to obtain the deterministic distribution of breaking waves and foam on the rainfall sea surface.

[0014] A further technical solution of the present invention: In step 2, the total scattering coefficient of the sea surface The mirror scattering coefficient of each small sea surface cell and perturbation coefficient The superposition form:

[0015]

[0016] in, is the unit vector of the incident and scattering directions, A represents the area of ​​the entire sea surface calculation area, ΔxΔy is the area of ​​a single small facet; P and Q are the polarization modes in the global coordinate system, M and N are the number of sampling points along the x and y directions respectively, and Their respective expressions are as follows:

[0017]

[0018]

[0019] Among them, p, q represent the polarization mode of the local small surface element, k is the wave number of the incident electromagnetic wave, ε is the dielectric constant of seawater, is the polarization factor under Kirchhoff approximation, is the polarization factor of the local small surface element; is the sea surface slope, The probability density distribution function of S ζ (q l ) is with Haipu sea (k) related parameters, whose expression is, S ζ (q l )=Ψ sea (k) / q l q l q = k s -k i Projection on the tangent plane; According to the rainfall annular spectrum Ψring (k) Correction effect on the ocean spectrum, S ζ (q l ) needs to be corrected to S ζ (q l )=[Ψ sea (k)+Ψ ring (k)] / q l .

[0020] A further technical solution of the present invention is as follows: in step 3, vector radiation transfer theory is used to solve the scattering and attenuation of rainfall particles in the atmosphere, as well as the interaction between rain columns, secondary splashing seawater droplets, foam and the sea surface at the sea-air interface;

[0021] The scattering coefficient of rainfall particles in the atmosphere is:

[0022]

[0023] The attenuation coefficient of rainfall particles in the atmosphere is:

[0024]

[0025] Therefore, the sea surface scattering coefficient after scattering and attenuation by rainfall in the atmosphere is:

[0026]

[0027] Among them, ε rain is the relative dielectric constant of raindrops, n0 is the number of raindrops per unit volume, θ is the angle of incidence, H is the rainfall thickness, and is the scattering matrix parameter related to rainfall rate RR and raindrop size; is the sea surface scattering coefficient at the sea-air interface, which is composed of sea surface scattering considering the disturbance of rainfall ring wave, foam particle scattering, and rain column and secondary splashing sea droplet scattering components;

[0028] For a surface element without foam determined by the white canopy coverage, the backscatter coefficient is expressed as the sea surface scattering coefficient attenuated by the random particle layer of rain column and secondary splash seawater. Multiple coupled scattering The sum of

[0029]

[0030] in

[0031]

[0032]

[0033]

[0034] in is the scattering coefficient of each small sea surface element obtained in step 2, d sd is the thickness of the random particle layer composed of rain column and secondary splashing seawater droplets, κ s , κ e is the equivalent scattering coefficient and equivalent extinction coefficient obtained based on the probability density distribution of the rain column and the secondary splashed seawater droplets, |R h0 |with|R v0 | is the modulus of the Fresnel reflection coefficient; for the surface element containing foam determined by the white crown coverage, the backscattering coefficient can be Replaced with the sea surface scattering coefficient including breaking waves and foam

[0035] A further technical solution of the present invention is as follows: In step 4, based on the velocity beamforming model, assuming that V is the projection velocity of the platform on the ground, and the distance between the sea surface moving target and the radar platform in the upward direction at time t=0 is R, then the intensity distribution in the two-dimensional sea surface image plane is as follows:

[0036]

[0037] Among them, ρ aN is the resolution of the still sea surface in azimuth, ρ a ' N (x0) is the azimuth resolution of the moving sea surface after N-view processing, The sea surface velocity along the radar line of sight, σ(x0, y0) is the scattering coefficient distribution after wave modulation, which is calculated by the wind-wave coupled sea surface scattering coefficient under high sea conditions in step 3. σ(x0, y0) is the wind-wave coupled sea surface scattering coefficient under high sea conditions in step 3. pp The distribution at the coordinate position (x0, y0), δ(y-y0) is the range resolution function.

[0038] A computer system, characterized in that it includes: one or more processors, and a computer-readable storage medium for storing one or more programs, wherein when the one or more programs are executed by the one or more processors, the one or more processors implement the above-mentioned method.

[0039] A computer-readable storage medium is characterized by storing computer-executable instructions, which are used to implement the above method when executed.

[0040] Beneficial effects

[0041] The present invention provides a method for calculating wind-wave-coupled sea surface scattering and high-resolution SAR imaging under high sea conditions, which solves the problems of coupled scattering of multiple random particle layers at the sea-air interface and the sea surface under high sea conditions and high-resolution SAR imaging of the sea surface under rainfall conditions.

[0042] The method of the present invention utilizes vector radiation transfer theory combined with a three-layer random particle model to provide an accurate and effective calculation method for wind-wave-coupled sea surface scattering and high-resolution SAR imaging under high sea conditions, with a fast calculation speed. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The accompanying drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like parts throughout the drawings.

[0044] Figure 1 It is a calculation flow chart of the present invention;

[0045] Figure 2 It is a three-layer random particle model of the wind-wave coupled rainfall sea surface (rainfall layer, rain column-secondary splash layer, foam layer). DETAILED DESCRIPTION

[0046] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only intended to illustrate the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0047] Reference Figure 1 and Figure 2 , the specific implementation steps of the present invention are as follows:

[0048] Step 1: The linear filtering method considers the ocean waves as a superposition of a series of harmonics with different wavelengths, different periods and different initial phases. The amplitudes of the harmonics are independent Gaussian random variables {γ n}, its real and imaginary parts respectively satisfy the standard normal distribution. Using the spectrum filter, we can get the frequency domain expression of time t

[0049]

[0050] Among them, sea (k x ,k y ) is the ocean spectrum in the rectangular coordinate system. According to the rainfall annular spectrum form:

[0051]

[0052] The angular frequency ω and wave number k are determined by the dispersion relation: T is the surface tension (T = 7.9 × 10 -2 N·m -1 ), ρ w is the water density, N(D) is the raindrop distribution function, which is related to the rainfall rate RR (mm / h) and the raindrop terminal velocity W(D). J2(kR) is the first kind of second order Bessel function, and has d mix is the layer thickness, v e is the viscosity coefficient. Therefore, when generating the sea surface, the sea spectrum model needs to be corrected to Ψ sea (k)+Ψ ring (k).

[0053] Using the Hilbert transform on the linear rainfall sea surface, we can derive a frequency-domain representation of the nonlinear sea surface. Applying the IFFT function to this frequency-domain representation yields the nonlinear sea surface fluctuation height. Once the two-dimensional sea surface height fluctuation is obtained, the sea surface breaking location and foam coverage distribution can be determined based on the wave crest foam coverage Fc%, the static foam coverage Fs%, and the sea surface slope. Specifically, assuming the number of sea surface bins is N, select N*Fc% bins with the largest sea surface slope and replace them with breaking waves; select N*Fs% bins with the second largest sea surface slope and cover them with a layer of static foam; and treat the remaining bins as normal sea surfaces. This yields a deterministic distribution of breaking waves and foam on the rainfall sea surface.

[0054] Step 2: Total scattering coefficient of sea surface It can be written as:

[0055]

[0056] in, is the unit vector of the incident and scattering directions, A represents the area of ​​the entire sea surface calculation area, ΔxΔy is the area of ​​a single small surface element. P and Q are the polarization modes in the global coordinate system (H or V are possible), M and N are the number of sampling points along the x and y directions respectively. and They represent the mirror scattering coefficient of each small surface element and the perturbation coefficient of the small surface element, respectively, and their respective expressions are as follows:

[0057]

[0058]

[0059] Where p and q represent the polarization mode of the local small element (which can be h or v); k is the wave number of the incident electromagnetic wave, and ε is the dielectric constant of seawater. is the polarization factor under Kirchhoff approximation, is the polarization factor of the local small facet. is the sea surface slope, The probability density distribution function of S can be obtained from step 1; ζ (q l ) is with Haipu sea (k) related parameters, whose expression is, S ζ (q l )=Ψ sea (k) / q l q l q = k s -k i The projection on the tangent plane. Therefore, when calculating the perturbation coefficient of the small surface element, S ζ (q l ) needs to be corrected to S ζ (q l )=[Ψ sea (k)+Ψ ring (k)] / q l .

[0060] Step 3: Use vector radiative transfer theory to solve the scattering and attenuation of rainfall particles in the atmosphere, as well as the interaction between rain columns and secondary splashing seawater droplets at the sea-air interface.

[0061] The scattering coefficient of rainfall particles in the atmosphere is:

[0062]

[0063] The attenuation coefficient of rainfall particles in the atmosphere is:

[0064]

[0065] Therefore, the sea surface scattering coefficient after scattering and attenuation by rainfall in the atmosphere is:

[0066]

[0067] Among them, ε rain is the relative dielectric constant of raindrops, n0 is the number of raindrops per unit volume, θ is the angle of incidence, H is the rainfall thickness, and is the scattering matrix parameter related to the rainfall rate RR (mm / h) and raindrop size. is the sea surface scattering coefficient at the sea-air interface, which is composed of sea surface scattering considering rainfall ring wave disturbance, foam particle scattering, and rain column and secondary splashing seawater droplet scattering components.

[0068] For a surface element without foam determined by the white canopy coverage, the backscattering coefficient can be expressed as the sea surface scattering coefficient attenuated by the random particle layer of rain column and secondary splash seawater. Multiple coupled scattering The sum of

[0069]

[0070] in

[0071]

[0072]

[0073]

[0074] in is the scattering coefficient of each simple sea surface small cell obtained in step 2, d sd is the thickness of the random particle layer composed of rain column and secondary splashing seawater droplets, κ s , κ e is the equivalent scattering coefficient and equivalent extinction coefficient obtained based on the probability density distribution of the rain column and the secondary splashed seawater droplets, |R h0 |with|R v0 | is the modulus of the Fresnel reflection coefficient. It should be noted that for the surface element containing foam determined by the white crown coverage, the backscattering coefficient can be Replaced with the sea surface scattering coefficient including breaking waves and foam

[0075] Step 4: Simulate Creamer nonlinear rainfall sea surface SAR imaging. According to the velocity beam imaging model, assuming that V is the projection velocity of the platform on the ground, the distance between the sea surface moving target and the radar platform in the range direction at time t = 0 is R, T is the integration time, and the incident wavelength is λ, then the resolution of the still sea surface in the azimuth direction is

[0076]

[0077] Then the resolution of the moving sea surface in azimuth after N-view processing is

[0078]

[0079] Among them, a r (x0) is the acceleration of the sea surface along the radar line of sight. τ s is the sea surface related time. Then the intensity distribution in the two-dimensional sea surface image plane is as follows

[0080]

[0081] in, The sea surface velocity along the radar line of sight, σ(x0,y0) is the scattering coefficient distribution after wave modulation, which can be calculated by the wind-wave coupled sea surface scattering coefficient under high sea conditions described in step 3. That is, the wind-wave coupled sea surface scattering coefficient under high sea conditions described in step 3 is pp The distribution at the coordinate position (x0, y0), δ(y-y0) is the range resolution function.

[0082] The above description is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with this technical field can easily think of various equivalent modifications or replacements within the technical scope disclosed in the present invention, and these modifications or replacements should all be included in the scope of protection of the present invention.

Claims

1. A method for calculating wind-wave-coupled sea surface scattering and high-resolution SAR imaging under high sea conditions, characterized by Here are the steps: Step 1: Use the whitecap coverage and rainfall-corrected ocean spectrum to establish a time-varying nonlinear rainfall sea surface geometry model under high sea conditions; Step 2: Considering the correction effect of the rainfall annular spectrum on the sea surface capillary wave spectrum, the rainfall-corrected two-scale model is used to solve the sea surface scattering coefficient; Step 3: Use vector radiative transfer theory to solve for the scattering and attenuation of rainfall particles in the atmosphere, as well as the coupled scattering between rain columns, secondary splashing seawater droplets, foam, and the sea surface at the sea-air interface; Step 4: Use the velocity spotlight imaging algorithm to perform high-resolution SAR imaging of the sea surface under high sea conditions.

2. The method for calculating wind-wave-coupled sea surface scattering and high-resolution SAR imaging under high sea conditions according to claim 1, characterized in that: In the step 1, the time-varying nonlinear rainfall sea surface geometry modeling under high sea conditions is carried out, and a rainfall-corrected sea spectrum model is adopted. The linear filtering method is combined with the Hilbert transform to obtain the nonlinear sea surface. The white crown coverage rate is used in combination with the slope criterion to obtain the deterministic distribution of breaking waves and foam on the rainfall sea surface.

3. The method for calculating wind-wave-coupled sea surface scattering and high-resolution SAR imaging under high sea conditions according to claim 1, characterized in that: In step 2, the total scattering coefficient of the sea surface The mirror scattering coefficient of each small sea surface cell and perturbation coefficient The superposition form: in, is the unit vector of the incident and scattering directions, A Represents the area of ​​the entire sea surface calculation area, is the area of ​​a single small surface element; P 、 Q is the polarization mode in the global coordinate system, M 、 N are the number of sampling points along the x and y directions, and Their respective expressions are as follows: in, p , q represents the polarization mode of the local small surface element, is the wave number of the incident electromagnetic wave, is the dielectric constant of seawater, is the polarization factor under Kirchhoff approximation, is the polarization factor of the local small surface element; is the sea surface slope, The probability density distribution function of For Haipu The relevant parameters are expressed as follows: for Projection on the tangent plane; According to the rainfall annular spectrum The correction effect on the sea spectrum, Need to be corrected to .

4. The method for calculating wind-wave-coupled sea surface scattering and high-resolution SAR imaging under high sea conditions according to claim 1, characterized in that: In step 3, vector radiative transfer theory is used to solve the scattering and attenuation of rainfall particles in the atmosphere, as well as the interaction between rain columns, secondary splashing seawater droplets, foam and the sea surface at the sea-air interface; The scattering coefficient of rainfall particles in the atmosphere is: The attenuation coefficient of rainfall particles in the atmosphere is: Therefore, the sea surface scattering coefficient after scattering and attenuation by rainfall in the atmosphere is: in, is the relative dielectric constant of raindrops, is the number of raindrops per unit volume, is the angle of incidence, H is the rainfall thickness, and is the scattering matrix parameter related to rainfall rate RR and raindrop size; is the sea surface scattering coefficient at the sea-air interface, which is composed of sea surface scattering considering the disturbance of rainfall ring wave, foam particle scattering, and rain column and secondary splashing sea droplet scattering components; For a surface element without foam determined by the white canopy coverage, the backscatter coefficient is expressed as the sea surface scattering coefficient attenuated by the random particle layer of rain column and secondary splash seawater. Multiple coupled scattering The sum of in in is the scattering coefficient of each small sea surface element obtained in step 2, is the thickness of the random particle layer composed of rain column and secondary splashing seawater droplets, are the equivalent scattering coefficient and equivalent extinction coefficient obtained based on the probability density distribution of rain column and secondary splashed seawater droplets, and is the modulus of the Fresnel reflection coefficient; for the surface element containing foam determined by the white crown coverage, the backscattering coefficient needs to be replaced by Replaced with the sea surface scattering coefficient including breaking waves and foam .

5. The method for calculating wind-wave-coupled sea surface scattering and high-resolution SAR imaging under high sea conditions according to claim 1, characterized in that: In step 4, according to the velocity beamforming model, it is assumed that is the projection speed of the platform on the ground, The distance between the moving target on the sea surface and the radar platform at the moment is , then the intensity distribution in the two-dimensional sea surface image plane is as follows: in, is the resolution of the still sea surface in azimuth, is the azimuth resolution of the moving sea surface after N-view processing, The velocity of the sea surface along the radar line of sight, is the scattering coefficient distribution after wave modulation, which is calculated from the wind-wave coupled sea surface scattering coefficient under high sea conditions. That is, the wind-wave coupled sea surface scattering coefficient under high sea conditions At coordinate position The distribution of is the range resolution function.

6. A computer system, characterized in that include: One or more processors, and a computer-readable storage medium for storing one or more programs, wherein when the one or more programs are executed by the one or more processors, the one or more processors are enabled to implement the method of claim 1.

7. A computer-readable storage medium, characterized in that Computer-executable instructions are stored, and when the instructions are executed, they are used to implement the method of claim 1.

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