CN116451465A - Satellite-borne SAR mesoscale vortex imaging simulation method and system - Google Patents
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Abstract
The invention discloses a simulation method and a simulation system for satellite-borne SAR mesoscale vortex imaging. The method comprises the following steps: for the mesoscale vortex actually existing in the ocean, a high-resolution three-dimensional ocean numerical model is adopted to simulate and obtain mesoscale vortex sea surface flow field data; calculating a balanced sea wave spectrum; calculating the disturbance quantity of the balanced wave spectrum based on the mesoscale vortex sea surface flow field data to obtain an unbalanced wave spectrum; calculating a sea surface backscattering coefficient based on the unbalanced sea wave spectrum; calculating a modulated sea surface backscatter coefficient based on the sea surface backscatter coefficient; and performing mesoscale vortex imaging simulation based on the modulated sea surface backscattering coefficient. The invention starts from the ocean mesoscale vortex truly existing in a typical sea area, utilizes a high-resolution three-dimensional ocean numerical model to simulate the obtained refined vortex sea surface flow field structure, and combines a sea surface microwave scattering model to develop SAR vortex imaging simulation research.
Description
Technical Field
The invention relates to the technical field of ocean mesoscale vortex, in particular to a satellite-borne SAR mesoscale vortex imaging simulation method and system.
Background
As an important marine phenomenon, the mesoscale vortex in the ocean carries a significant portion of the energy of the world ocean, which plays an important role not only in ocean loop structures and marine ecology, but also in the atmospheric phenomena of wind farms, clouds, rainfall, etc. through sea-gas interactions. Therefore, the mesoscale vortex plays an important role in the aspects of marine economy, offshore engineering, marine military and the like, and has great significance on the research of global climate change.
The spaceborne SAR has the characteristics of all weather, all-day time, high spatial resolution and the like, and has obvious advantages in aspects of marine mesoscale vortex dynamic monitoring and characteristic parameter quantitative extraction. The SAR remote sensing imaging of the mesoscale vortex is mainly affected by three mechanisms of wave-flow interaction, sea surface biomembrane or oil film accumulation in a flow field radial aggregation area and atmospheric stability above sea, so that the SAR is greatly affected by the ocean dynamic environment in the process of the mesoscale vortex remote sensing imaging, and the characteristics shown in the SAR image are different, so that a certain difficulty is brought to interpretation of the ocean vortex characteristics on the SAR image. The simulation of the marine mesoscale vortex imaging based on the SAR image can break through the bottleneck of limiting the interpretation capability of the satellite-borne SAR mesoscale vortex target.
Disclosure of Invention
Based on the above, the invention aims to provide a simulation method and a simulation system for satellite-borne SAR mesoscale vortex imaging.
In order to achieve the above object, the present invention provides the following solutions:
a satellite-borne SAR mesoscale vortex imaging simulation method comprises the following steps:
for the mesoscale vortex actually existing in the ocean, a high-resolution three-dimensional ocean numerical model is adopted to simulate and obtain mesoscale vortex sea surface flow field data;
calculating a balanced sea wave spectrum;
calculating the disturbance quantity of the balanced wave spectrum based on the mesoscale vortex sea surface flow field data to obtain an unbalanced wave spectrum;
calculating a spatially averaged sea surface backscatter coefficient based on the unbalanced sea wave spectrum;
calculating a modulated sea surface backscatter coefficient based on the spatially averaged sea surface backscatter coefficient; the modulation includes tilt modulation, hydrodynamic modulation, and velocity beaming modulation;
and performing mesoscale vortex imaging simulation based on the modulated sea surface backscattering coefficient.
Optionally, the calculation formula of the balanced ocean wave spectrum is as follows:
W(k)＝(B L +B H )/k 3
G(k,ψ)＝[1+Δ(k)cos(2ψ)]/2π
wherein,,represents the balanced wave spectrum, k represents the wave number of the wave spectrum, < ->Representing wave direction, W (k) representing a spectral model function, G (k, ψ) representing a propagation function, B L Representing the contribution of low frequency waves to spectral energy, B H The contribution of the high frequency wave to the spectral energy is shown, and delta (k) represents the inverse crosswind ratio.
Optionally, calculating a spatially averaged sea surface backscatter coefficient based on the unbalanced sea wave spectrum specifically includes:
according to sea surface wind field data, radar observation frequency, radar observation incidence angle, the unbalanced sea wave spectrum and space scale parameters, carrying out interpolation calculation on spectrum disturbance space to obtain a space position;
obtaining a local incident angle of each space position through observation geometric calculation;
based on the local angle of incidence and the unbalanced ocean wave spectrum, a spatially averaged sea surface backscatter coefficient is calculated with a modified combined surface model.
Optionally, the calculation formula of the modulated sea surface backscattering coefficient is as follows:
where σ represents the modulated sea surface backscatter coefficient,representing spatially averaged sea surface backscattering coefficients, FFT representing Fourier transform, +.>Representing the modulation transfer function>Representing the velocity beaming modulation function, F (k, phi) represents the unbalanced ocean wave spectrum.
Optionally, the expression of the tilt modulation transfer function is as follows:
the expression of the hydrodynamic modulation transfer function is as follows:
the expression of the velocity beamformed modulation transfer function is as follows:
wherein T is tilt Representing the tilt modulation transfer function, i representing the imaginary part of the complex number, k l Representing the component of the incident wave number vector in the radar view direction, θ representing the local incident angle, HH representing the polarization of the radar transmit signal, VV representing the polarization of the radar receive signal, T h Represents the hydrodynamic modulation transfer function, ω represents the angular frequency, μ represents the relaxation rate factor, k represents the wave number of the ocean wave spectrum,representing the velocity beaming modulation transfer function, R representing the distance between the target and the stage, and V representing the stage velocity.
The invention also provides a satellite-borne SAR mesoscale vortex imaging simulation system, which comprises:
the data acquisition module is used for simulating mesoscale vortex sea surface flow field data obtained by adopting a high-resolution three-dimensional ocean numerical model for mesoscale vortex truly existing in the ocean;
the balance wave spectrum calculation module is used for calculating a balance wave spectrum;
the unbalanced wave spectrum calculation module is used for calculating the disturbance quantity of the balanced wave spectrum based on the mesoscale vortex sea surface flow field data to obtain an unbalanced wave spectrum;
the space average sea surface backscattering coefficient calculation module is used for calculating the space average sea surface backscattering coefficient based on the unbalanced sea wave spectrum;
the sea surface backscattering coefficient calculation module is used for calculating the sea surface backscattering coefficient after modulation based on the sea surface backscattering coefficient after spatial averaging; the modulation includes tilt modulation, hydrodynamic modulation, and velocity beaming modulation;
and the imaging simulation module is used for performing mesoscale vortex imaging simulation based on the modulated sea surface backscattering coefficient.
Optionally, the spatially averaged sea surface backscattering coefficient calculation module specifically includes:
the space position determining unit is used for carrying out interpolation calculation on the spectrum disturbance space according to sea surface wind field data, radar observation frequency, radar observation incident angle, the unbalanced sea wave spectrum and space scale parameters to obtain a space position;
the local incidence angle calculation unit is used for obtaining the local incidence angle of each space position through observation geometric calculation;
a spatially averaged sea surface backscatter coefficient calculation unit for calculating a spatially averaged sea surface backscatter coefficient with an improved combined surface model based on the local angle of incidence and the unbalanced sea wave spectrum.
According to the specific embodiment provided by the invention, the invention discloses the following technical effects:
the invention starts from the ocean mesoscale vortex truly existing in a typical sea area, utilizes a high-resolution three-dimensional ocean numerical model to simulate the obtained refined vortex sea surface flow field structure, and combines a sea surface microwave scattering model to develop SAR vortex imaging simulation research. In this way, the system discusses the influence of sea dynamic environments such as sea surface wind fields and radar parameters such as radar incidence angles, polarization modes, wave bands and the like on the mesoscale eddy SAR imaging in real sea, and the low wind speed, C wave band and VV polarization modes are found to be beneficial to SAR mesoscale eddy remote sensing imaging.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions of the prior art, the drawings that are needed in the embodiments will be briefly described below, it being obvious that the drawings in the following description are only some embodiments of the present invention, and that other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
Fig. 1 is a flowchart of a simulation method of on-board SAR mesoscale vortex imaging according to a first embodiment of the present invention;
FIG. 2 is a schematic diagram of SAR observation geometry;
FIG. 3 is a schematic diagram of Bragg resonance scattering geometry;
FIG. 4 is a schematic diagram of a combined surface model.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
In order that the above-recited objects, features and advantages of the present invention will become more readily apparent, a more particular description of the invention will be rendered by reference to the appended drawings and appended detailed description.
Example 1
In the prior SAR mesoscale vortex imaging simulation research, a theoretical model is mostly adopted to develop mesoscale vortex sea surface flow field simulation, and the flow field structure of the real ocean mesoscale vortex is simplified to a certain extent in the research process. The invention starts from the ocean mesoscale vortex truly existing in a typical sea area, utilizes a high-resolution three-dimensional ocean numerical model to simulate the obtained refined vortex sea surface flow field structure, and combines a sea surface microwave scattering model to develop SAR vortex imaging simulation research.
The embodiment provides a simulation method for satellite-borne SAR mesoscale vortex imaging, as shown in fig. 1, comprising the following steps:
step 101: and simulating the mesoscale vortex which really exists in the ocean by adopting a high-resolution three-dimensional ocean numerical model to obtain mesoscale vortex sea surface flow field data.
Taking MITgcm (MIT general circulationmodel) as an example, the spatial resolution is 0.01 degrees, 50 standard layers are vertical, and the thickness of each layer is increased from 4m on the sea surface to 300m on the sea bottom. The eastern open border gives temperature, salinity and horizontal velocity fields from the ocean current and month-averaged ocean analysis data provided by climate estimation project Estimation of the Circulation and Climate of the Oceanproject (Kohl and Stammer, 2008). The model is driven by sea surface wind, air temperature, specific humidity, precipitation, downward short wave radiation and long wave radiation, and the data is weather research forecast products with 10km spatial resolution every 3 hours, and the products are obtained by reducing the scale of ERA-Interim products provided by ECMWF (European Centre forMedium-Range weather forecsts). The model is widely applied to various researches through various observation verification.
Step 102: and calculating the balanced sea wave spectrum.
Two-dimensional wave spectrum (i.e. balanced wave spectrum) proposed by Elfouhaily et al is adopted, and the influence of small-scale gravitational waves is fully considered in the deduction process of the wave spectrum.
Two-dimensional wave spectrumCan be expressed as a multiplication of a non-directional spectral model function W (k) and a propagation function G (k), and in polar coordinates the Elfouhaily spectral model can be expressed as:
wherein,,
W(k)＝(B L +B H )/k 3 (2)
G(k,ψ)＝[1+Δ(k)cos(2ψ)]/2π (3)
where k is the wave number of the ocean wave spectrum,is the wave direction, B L Representing the contribution of low frequency waves, such as gravitational waves, to spectral energy, and B H Represents the contribution of the high frequency wave, i.e. capillary wave, to the spectral energy, delta (k) represents the inverse crosswind ratio.
Step 103: and calculating the disturbance quantity of the balanced wave spectrum based on the mesoscale vortex sea surface flow field data to obtain an unbalanced wave spectrum.
According to the theory of conservation of wave action spectrum, the change of the offshore flow field can change the spectrum energy of the sea surface equilibrium state, which can be defined as the action spectrum density N (k, x, t) and meets the requirement
Where k is a wave number vector, x= (x, y) is a two-dimensional horizontal space coordinate, and U is a sea surface flow field vector.
It is assumed that the change in the (k, x) space of the effective spectral density over a relaxation time satisfies the following relationship:
here N eq For the equilibrium state of the applied spectral density, τ is the relaxation time and has:
wherein c g Is the group velocity of the wave.
Then there are:
thus, the relationship between the sea surface flow field change caused by the ocean vortex and the action spectrum density and the unbalanced sea wave spectrum is established. By numerically solving the equation, the disturbance quantity of the balanced wave spectrum caused by the change of the ocean vortex, namely the unbalanced wave spectrum F, can be obtained.
Step 104: and calculating a spatially averaged sea surface backscatter coefficient based on the unbalanced sea wave spectrum.
According to the vortex sea surface wave spectrum disturbance data calculated in the front, sea surface wind field data and the like, parameters such as radar observation wave bands, polarization modes, geometric parameters, spatial resolution and the like are combined, sea surface backscattering coefficients are calculated, simulated sea surface backscattering coefficients are obtained, and influences of different radar parameters, different wind speeds and wind directions on a mesoscale vortex imaging simulation effect are discussed.
According to input sea surface wind field data, radar observation frequency, radar observation incidence angle, disturbed sea wave spectrum and space scale parameters, carrying out linear interpolation calculation on spectrum disturbance space to obtain a space position to be calculated; obtaining local incidence angles of each spatial position through observation geometric calculation; calculating disturbed wave spectrums and related functions at different spatial positions; calculating the sea surface backscattering coefficient of each pixel by adopting an approximate electromagnetic scattering model such as a Bragg resonance scattering model, an improved combined surface model and the like; and finally outputting the calculated backscattering coefficient.
1) Observation geometry calculation
The observation geometry of SAR is shown in fig. 2. The included angle between the central direction of the antenna beam and the point below the satellite, namely the pitch angle is alpha, and the local incident angle is theta. The local angle of incidence θ is greater than the pitch angle α due to the effect of ground curvature. R is R a Is the earth radius and h is the satellite orbit altitude. The geometric relationship of θ to α can be expressed as:
the distance d of the antenna footprint from the nadir may be expressed as:
d＝R a ·η＝R a ·(θ-α) (9)
the skew distance r can be expressed as:
2) Spatially averaged backscattering coefficient calculation
The simulation system mainly adopts a Bragg scattering model to calculate the sea surface backscattering coefficient. At low wind speeds (here representing wind speeds at elevations above average sea level), capillary waves of wavelength and short gravitational waves are widely distributed over the sea surface. If the wavelength of the radar wave and the sea surface wavelength are in the same range, a special scattering mechanism, namely Bragg resonance scattering or Bragg scattering for short, is triggered at the moment, and the geometrical relationship is shown in figure 3. R in FIG. 3 1 、R 2 Representative radarIncident wave, θ is incident angle, λs is sea surface wavelength, N 0 Represents the sea surface normal, δr represents the optical path difference.
Its normalized sea surface backscattering coefficientCan be calculated by the following formula:
wherein g PP Is a polarization function, subscripts HH and VV denote the polarization of the radar transmit and receive signals, for horizontal polarization:
for vertical polarization:
wherein ε s Is the complex dielectric constant of seawater.
Step 105: calculating a modulated sea surface backscatter coefficient based on the spatially averaged sea surface backscatter coefficient; the modulation includes tilt modulation, hydrodynamic modulation, and velocity beaming modulation. Under these three modulations, the radar backscatter coefficients can be expressed as:
wherein,,the spatial average of the scattering coefficient can be calculated by Bragg resonance scattering theory; />For modulating transfer function of real aperture radar, including tilt modulation T tilt And hydrodynamic modulation T h Two parts, i.e. a->The transfer function is modulated for velocity beaming.
1) Tilt modulation and hydrodynamic modulation
According to the sea surface electromagnetic scattering double-scale theory, the local incidence angle of scattering surface elements can be changed by large-scale waves, so that oblique modulation is generated on a scattering field; in addition, under the action of the track speed, large-scale waves can generate hydrodynamic modulation on small-scale waves subjected to Bragg resonance scattering, so that scattered fields are converged or scattered. Under these two modulations, the radar backscatter coefficient can be expressed as:
wherein,,the spatial average of the scattering coefficient can be calculated by Bragg resonance scattering theory or a combined surface model; />Representing a velocity beaming modulation function, F (k, phi) representing an unbalanced ocean wave spectrum; />For modulating transfer function of real aperture radar, including tilt modulation T tilt And hydrodynamic modulation T h Two parts. Assuming the sea surface is an ideal conductor, the tilt modulation component can be expressed as:
wherein k is l Is the component of the incident wave number vector in the radar view direction.
Hydrodynamic modulation transfer function T h Can be obtained by hydrodynamic interactions between short and long waves:
wherein k is x Is the distance component of the long wave number; μ is a relaxation factor, which is generally considered to be equivalent to the growth rate of wind.
2) Velocity beaming modulation
The velocity beamformed modulation transfer function may be expressed as:
step 106: and performing mesoscale vortex imaging simulation based on the modulated sea surface backscattering coefficient.
SAR imaging simulation is carried out on SAR images based on the sea surface backscattering coefficients calculated in advance according to satellite platform parameters, SAR sensor parameters (C and L wave bands, VV and HH polarization), a system error model, image speckle noise, stripe noise and other information, so that simulated SAR images are obtained.
Example two
In order to execute the corresponding method of the above embodiment to achieve the corresponding functions and technical effects, a satellite-borne SAR mesoscale vortex imaging simulation system is provided below.
The system comprises:
the data acquisition module is used for simulating mesoscale vortex sea surface flow field data obtained by adopting a high-resolution three-dimensional ocean numerical model for mesoscale vortex truly existing in the ocean;
the balance wave spectrum calculation module is used for calculating a balance wave spectrum;
the unbalanced wave spectrum calculation module is used for calculating the disturbance quantity of the balanced wave spectrum based on the mesoscale vortex sea surface flow field data to obtain an unbalanced wave spectrum;
the space average sea surface backscattering coefficient calculation module is used for calculating the space average sea surface backscattering coefficient based on the unbalanced sea wave spectrum;
the sea surface backscattering coefficient calculation module is used for calculating the sea surface backscattering coefficient after modulation based on the sea surface backscattering coefficient after spatial averaging; the modulation includes tilt modulation, hydrodynamic modulation, and velocity beaming modulation;
and the imaging simulation module is used for performing mesoscale vortex imaging simulation based on the modulated sea surface backscattering coefficient.
The sea surface backscattering coefficient calculation module for spatial averaging specifically comprises:
the space position determining unit is used for carrying out interpolation calculation on the spectrum disturbance space according to sea surface wind field data, radar observation frequency, radar observation incident angle, the unbalanced sea wave spectrum and space scale parameters to obtain a space position;
the local incidence angle calculation unit is used for obtaining the local incidence angle of each space position through observation geometric calculation;
a spatially averaged sea surface backscatter coefficient calculation unit for calculating a spatially averaged sea surface backscatter coefficient with an improved combined surface model based on the local angle of incidence and the unbalanced sea wave spectrum.
In the present specification, each embodiment is described in a progressive manner, and each embodiment is mainly described in a different point from other embodiments, and identical and similar parts between the embodiments are all enough to refer to each other. For the system disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant points refer to the description of the method section.
The principles and embodiments of the present invention have been described herein with reference to specific examples, which are intended to be only illustrative of the methods and concepts underlying the invention, and not all examples are intended to be within the scope of the invention as defined by the appended claims.
Claims (7)
1. The simulation method for the satellite-borne SAR mesoscale vortex imaging is characterized by comprising the following steps of:
for the mesoscale vortex actually existing in the ocean, a high-resolution three-dimensional ocean numerical model is adopted to simulate and obtain mesoscale vortex sea surface flow field data;
calculating a balanced sea wave spectrum;
calculating the disturbance quantity of the balanced wave spectrum based on the mesoscale vortex sea surface flow field data to obtain an unbalanced wave spectrum;
calculating a spatially averaged sea surface backscatter coefficient based on the unbalanced sea wave spectrum;
calculating a modulated sea surface backscatter coefficient based on the spatially averaged sea surface backscatter coefficient; the modulation includes tilt modulation, hydrodynamic modulation, and velocity beaming modulation;
and performing mesoscale vortex imaging simulation based on the modulated sea surface backscattering coefficient.
2. The simulation method of on-board SAR mesoscale vortex imaging according to claim 1, wherein the calculation formula of said balanced ocean wave spectrum is as follows:
W(k)＝(B L +B H )/k 3
G(k,ψ)＝[1+Δ(k)cos(2ψ)]/2π
wherein,,represents the balanced wave spectrum, k represents the wave number of the wave spectrum, < ->Representing wave direction, W (k) representing a spectral model function, G (k, ψ) representing a propagation function, B L Representing the contribution of low frequency waves to spectral energy, B H The contribution of the high frequency wave to the spectral energy is shown, and delta (k) represents the inverse crosswind ratio.
3. The method for simulating satellite-borne SAR mesoscale vortex imaging according to claim 1, wherein calculating a spatially averaged sea surface backscattering coefficient based on said unbalanced sea wave spectrum comprises:
according to sea surface wind field data, radar observation frequency, radar observation incidence angle, the unbalanced sea wave spectrum and space scale parameters, carrying out interpolation calculation on spectrum disturbance space to obtain a space position;
obtaining a local incident angle of each space position through observation geometric calculation;
based on the local angle of incidence and the unbalanced ocean wave spectrum, a spatially averaged sea surface backscatter coefficient is calculated with a modified combined surface model.
4. The simulation method of on-board SAR mesoscale vortex imaging according to claim 1, wherein the formula of calculation of the modulated sea surface backscattering coefficient is as follows:
where σ represents the modulated sea surface backscatter coefficient,representing spatially averaged sea surface backscattering coefficients, FFT representing Fourier transform, +.>Representing the modulation transfer function>Representing the velocity beaming modulation function, F (k, phi) represents the unbalanced ocean wave spectrum.
5. The method of simulating satellite-borne SAR mesoscale vortex imaging according to claim 4, wherein said oblique modulation transfer function is expressed as follows:
the expression of the hydrodynamic modulation transfer function is as follows:
the expression of the velocity beamformed modulation transfer function is as follows:
wherein T is tilt Representing the tilt modulation transfer function, i representing the imaginary part of the complex number, k l Representing the component of the incident wave number vector in the radar view direction, θ representing the local incident angle, HH representing the polarization of the radar transmit signal, VV representing the polarization of the radar receive signal, T h Represents the hydrodynamic modulation transfer function, ω represents the angular frequency, μ represents the relaxation rate factor, k represents the wave number of the ocean wave spectrum,representing the velocity beaming modulation transfer function, R representing the distance between the target and the stage, and V representing the stage velocity.
6. A satellite-borne SAR mesoscale vortex imaging simulation system, comprising:
the data acquisition module is used for simulating mesoscale vortex sea surface flow field data obtained by adopting a high-resolution three-dimensional ocean numerical model for mesoscale vortex truly existing in the ocean;
the balance wave spectrum calculation module is used for calculating a balance wave spectrum;
the unbalanced wave spectrum calculation module is used for calculating the disturbance quantity of the balanced wave spectrum based on the mesoscale vortex sea surface flow field data to obtain an unbalanced wave spectrum;
the space average sea surface backscattering coefficient calculation module is used for calculating the space average sea surface backscattering coefficient based on the unbalanced sea wave spectrum;
the sea surface backscattering coefficient calculation module is used for calculating the sea surface backscattering coefficient after modulation based on the sea surface backscattering coefficient after spatial averaging; the modulation includes tilt modulation, hydrodynamic modulation, and velocity beaming modulation;
and the imaging simulation module is used for performing mesoscale vortex imaging simulation based on the modulated sea surface backscattering coefficient.
7. The on-board SAR mesoscale vortex imaging simulation system of claim 6, wherein the spatially averaged sea surface backscatter coefficient calculation module specifically comprises:
the space position determining unit is used for carrying out interpolation calculation on the spectrum disturbance space according to sea surface wind field data, radar observation frequency, radar observation incident angle, the unbalanced sea wave spectrum and space scale parameters to obtain a space position;
the local incidence angle calculation unit is used for obtaining the local incidence angle of each space position through observation geometric calculation;
a spatially averaged sea surface backscattering coefficient calculation unit for calculating a sea surface backscattering coefficient spatially averaged with an improved combined surface model based on the local angle of incidence and the unbalanced sea wave spectrum.
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