Two-dimensional ocean current inversion method and system based on multi-azimuth synthetic aperture radar

Abstract

The present invention proposes a two-dimensional ocean current inversion method and system based on multi-azimuth synthetic aperture radar (SAR). The method comprises: obtaining one-dimensional sea surface velocities observed at different observation azimuths using the Doppler centroid anomaly method or along-track interferometry; synthesizing two-dimensional ocean currents based on the one-dimensional ocean current velocities at different observation azimuths, including: calculating the eastward and northward components based on the one-dimensional sea surface velocities observed at two non-parallel observation azimuths; and calculating the velocity and direction of the two-dimensional ocean current based on the eastward and northward components. The present invention inverts one-dimensional ocean current velocities from data observed by multiple SAR satellites at different azimuths on the same sea surface, and synthesizes the two-dimensional ocean current based on the radar observation azimuth vector. By using the ATI method to invert ocean currents, the present invention can effectively improve the spatial resolution of ocean current inversion.

Description

Technical Field

[0001] The present invention belongs to the technical field of ocean remote sensing, and in particular relates to a two-dimensional ocean current inversion method and system based on multi-azimuth synthetic aperture radar. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] Global ocean circulation transports heat and salt, influencing both local and global climate change, and is a key physical quantity in ocean dynamics. Accurately measuring sea surface currents is crucial for understanding multiscale ocean dynamics, ocean-atmosphere interactions, and ocean energy cycles.

[0004] With further exploration of the ocean, there is a growing demand for large-scale, high-precision sea surface current data. However, existing in-situ ocean current data provided by ships, moored buoys, and Lagrange drifters suffer from limited observation range, short observation times, and high costs. Spaceborne altimeters measure changes in sea surface altitude at the subsatellite point to obtain geostrophic currents. However, these currents have a low spatial resolution and are difficult to discern, making it difficult to distinguish ocean dynamic phenomena smaller than sub-mesoscales. Typical spatial scales of sea surface currents range from a few kilometers to several thousand kilometers, with periods ranging from one day to several years. Sea surface currents modulate the ocean wave spectrum, which is then detected by spaceborne synthetic aperture radar (SAR). Compared to other observation methods, spaceborne SAR offers all-day, all-weather measurement capabilities, covering a wide width (500 km), and with a high spatial resolution (several meters). It has become a key data source for global sea surface current monitoring.

[0005] There are two common methods for inverting sea surface velocity using synthetic aperture radar: Doppler centroid anomaly (DCA) and along-track interferometric (ATI). The DCA method uses the Doppler centroid frequency shift of the echo from single-antenna synthetic aperture radar imaging data to invert ocean current velocity, but has a low spatial resolution (1-2 km). The ATI method uses dual antennas to acquire sea surface imaging data and inverts sea surface velocity using the interferometric phase difference between the two images, achieving a high spatial resolution (10-100 m). Both methods share a common shortcoming: they can only obtain the velocity component of the two-dimensional ocean current in the radar's line of sight, but cannot determine the direction and magnitude of the true two-dimensional ocean current field.

[0006] Chinese patent application number 201510852862.1 discloses a spaceborne synthetic aperture radar system and method for ocean current inversion based on angle diversity. This system uses a single satellite with dual antennas to perform time-sharing observations of the same ocean area at two azimuths. The DCA method is used to obtain current components at two different azimuths, and then vector-synthesizes two-dimensional ocean currents. However, the DCA method is the only method used to invert ocean currents, which has limited spatial resolution and can only illuminate the same ocean area at different times, requiring the dynamic ocean to undergo real-time changes during this time.

[0007] Chinese patent application number 201910840751.7 discloses a single-channel synthetic aperture radar two-dimensional flow field inversion method and system based on Doppler center shift. It uses sub-aperture segmentation technology to decompose the single-star synthetic aperture radar image into sub-images of different azimuth angles, perform ocean current inversion on each sub-image, and then vector-synthesize the two-dimensional ocean current. However, the shortcomings are: first, the spatial resolution of the segmented sub-aperture image is reduced, which inevitably leads to a decrease in the accuracy of the inverted ocean current. Second, the azimuth angle interval between the segmented sub-aperture images is small. Within a range of less than 10°, the vector synthesis results of different components within a small angle range will have a large error with the actual ocean current. Third, the DCA method is also only applicable to single-star synthetic aperture radar data, and its overall spatial resolution is limited. Summary of the Invention

[0008] To address the problem of insufficient spatial dimensionality in existing SAR inversion of sea surface flow fields, the present invention proposes a two-dimensional ocean current inversion method and system based on multi-azimuth synthetic aperture radar. This method inverts one-dimensional ocean current velocities from data observed by multiple SAR satellites at different azimuths on the same sea surface, and then synthesizes two-dimensional ocean currents based on the radar observation azimuth vectors. This method is applicable to both DCA and multi-satellite ATI methods with different headings.

[0009] To achieve the above objectives, one or more embodiments of the present invention provide the following technical solutions:

[0010] A first aspect of the present invention provides a two-dimensional ocean current inversion method based on multi-azimuth synthetic aperture radar, comprising:

[0011] The one-dimensional sea surface velocity at different observation azimuths is obtained using the Doppler centroid anomaly method or along-track interferometry.

[0012] The one-dimensional ocean current velocity at different observation azimuths is synthesized into a two-dimensional ocean current, including:

[0013] If the observation azimuths are two non-parallel observation azimuths, the eastward and northward current components are calculated based on the observed one-dimensional sea surface current velocities at the two non-parallel observation azimuths;

[0014] Calculate the speed and direction of two-dimensional ocean currents based on eastward and northward current components.

[0015] A second aspect of the present invention provides a two-dimensional ocean current inversion system based on multi-azimuth synthetic aperture radar, comprising:

[0016] The one-dimensional sea surface velocity calculation module is configured to: obtain the one-dimensional sea surface velocity observed at different observation azimuths using the Doppler centroid anomaly method or the along-track interferometry method;

[0017] The two-dimensional ocean current synthesis module is configured to synthesize the two-dimensional ocean current according to the one-dimensional ocean current velocity at different observation azimuths, including:

[0018] If the observation azimuths are two non-parallel observation azimuths, the easting and northing components are calculated based on the observed one-dimensional sea surface current velocity at the two non-parallel observation azimuths;

[0019] Calculate the speed and direction of two-dimensional ocean currents based on their easting and northing components.

[0020] One or more of the above technical solutions have the following beneficial effects:

[0021] (1) The present invention uses the ATI method to invert ocean currents based on single-satellite multi-beam interferometry or dual-satellite companion interferometry data. This method has the flexibility to target different data sources and can realize the inversion of the global two-dimensional ocean surface flow field based on SAR remote sensing data.

[0022] (2) The present invention can effectively improve the spatial resolution of ocean current inversion by using the ATI method to invert ocean currents.

[0023] (3) A multi-satellite and multi-task SAR operation system can be designed to achieve simultaneous or short-interval observation of the same sea area with cross-tracks of different headings, thereby improving the timeliness of ocean current inversion.

[0024] Advantages of additional aspects of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The accompanying drawings, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0026] Figure 1 This is a flow chart of the two-dimensional ocean current inversion method based on multi-azimuth synthetic aperture radar in the first embodiment.

[0027] Figure 2 This is a Doppler centroid frequency shift diagram at different observation azimuth angles using the DCA method of the first embodiment.

[0028] Figure 3 (a) and Figure 3 (b) Doppler frequency shift diagrams caused by the relative motion between the synthetic aperture radar satellite observation platform and the earth in observation directions 1 and 2 of the first embodiment respectively.

[0029] Figure 4 (a) and Figure 4 (b) Electromagnetic pointing error diagrams for observation direction 1 and observation direction 2 of the first embodiment, respectively.

[0030] Figure 5 This is a schematic diagram of synthesizing a two-dimensional ocean current by observing one-dimensional velocity vectors at different azimuth angles in the first embodiment.

[0031] Figure 6 (a) and Figure 6 (b) Scatter plots of the two-dimensional ocean current velocity and direction inverted by the DCA method of the first embodiment.

[0032] Figure 7 (a) and Figure 7 (b) Interference phase diagrams of observation direction 1 and observation direction 2 of the first embodiment respectively.

[0033] Figure 8 (a) and Figure 8 (b) Scatter plots of the inverted two-dimensional ocean current velocity and direction of the first embodiment. DETAILED DESCRIPTION

[0034] Example 1

[0035] like Figure 1 As shown, this embodiment discloses a two-dimensional ocean current inversion method based on multi-azimuth synthetic aperture radar, including:

[0036] Step 1: Use the Doppler centroid anomaly method or the along-track interferometry method to obtain the one-dimensional sea surface velocity at different observation azimuths;

[0037] Step 2: Synthesize the one-dimensional ocean current velocities at different observation azimuths into a two-dimensional ocean current.

[0038] In step 1, the Doppler centroid anomaly method is used to obtain the one-dimensional sea surface velocity observed at different observation azimuths, including:

[0039] Step 101-1, obtaining Doppler centroid frequency shifts observed at different azimuth angles;

[0040] Assuming that the actual motion trajectory of the satellite is a local uniform linear motion and the earth is locally flat and does not rotate, the Doppler centroid frequency shift (f dc )for:

[0041]

[0042] Where V is the radar speed, β is the instantaneous slant angle of the radar, and λ is the radar wavelength. This is the following perspective:

[0043]

[0044] Where H is the distance from the center of the Earth to the synthetic aperture radar sensor, R is the slant range; R e is the radius of the Earth. Assume that R0 is the slope distance at a certain instantaneous point, and replace (2) Taylor expansion, given by:

[0045]

[0046] Where, is the downward viewing angle corresponding to the tilt distance R0. From this we can get:

[0047]

[0048]

[0049] Where f0 represents the center of the Doppler centroid frequency shift at a certain instantaneous point in the visual angle, which can be approximated as a constant;

[0050] f dc Not only can it be obtained through theoretical formulas, but also the f of different azimuth observations can be obtained from the header file or auxiliary file of the imaging data. dc When SAR data products are released, header files and auxiliary files are provided, which include the required parameters such as incident angle, imaging position, satellite attitude, product level, etc. The specific situation depends on the data release format. By obtaining SAR data at different azimuth angles, and then using the header files and auxiliary files to calculate the f at the corresponding azimuth angle dc This calculation method is more convenient than the calculation method based on theoretical formulas.

[0051] Step 101-2: Remove the Doppler frequency shift caused by the relative motion between the satellite and the ground, the electromagnetic pointing error caused by the SAR antenna fluctuation, and the Doppler frequency shift caused by sea surface wind and waves from the Doppler centroid frequency shift to obtain the Doppler frequency shift caused only by the ocean current velocity.

[0052] f of the sea surface area dc It is caused by the relative motion between the synthetic aperture radar observation platform and the rough scattering surface modulated by the ocean current, and can be decomposed into the following parts:

[0053] f dc =f geo +f em +f wv+f c (6)

[0054] Where, f geo is the Doppler frequency shift caused by the relative motion between the satellite and the ground, f em is the electromagnetic pointing error caused by the fluctuation of the synthetic aperture radar antenna, f wv is the Doppler shift caused by wind and waves on the sea surface, f c is the Doppler frequency shift caused by the ocean current velocity. According to formula (6), f dc Remove f geo 、f em 、f wv After that, get f c And invert the sea surface velocity.

[0055] f geo In theory, this can be calculated for a given satellite's attitude and velocity:

[0056]

[0057] Where k r is the radar wave number, V SC is the satellite's velocity along the orbital plane, γ is the radar's viewing angle, α is the angle between the radial altitude plane and the satellite's orbital plane, is the angular velocity of the Earth, is the satellite velocity, ε represents the radar left view (ε = -1) and right view (ε = +1), and β is the argument of latitude. geo It can also be calculated from the Doppler coefficient:

[0058] f geo =d0+d1(t SR -t0)+d2(t SR -t0) 2 +d3(t SR -t0) 3 +d4(t SR -t0) 4 (8)

[0059] Where, d i is the Doppler coefficient (i=1,2,3,4), t SR is the two-way slant distance time, t0 is the standard slant distance time. geo It can also be provided by the header file or auxiliary file of the imaging data.

[0060] The synthetic aperture radar antenna or platform is affected by external factors, and the antenna pointing deviates from the theoretical direction, resulting in f dc There is f em, which causes errors in the inverted ocean current. In order to ensure that the ocean current velocity is relative to the land, this error needs to be removed. em There are both ocean and land areas in the entire imaging scene. There is no land area in the synthetic aperture radar image. ww and f c , so when obtaining f from synthetic aperture radar data dc and obtain f by calculating geo Under the premise of , according to formula (6) we can get f em The distribution of land areas in the imaging scene and fitting f em Equation. The f of the ocean area can be obtained by fitting the equation em .

[0061] The interaction between the wind field and waves on the sea surface modulates the capillary waves and long waves on the sea surface, resulting in the presence of f in the Doppler centroid frequency shift caused by the influence of wind and waves. wv For C-band radar at an incident angle of 20 to 40°, f wv It gradually decreases with the increase of the incident angle. Under the wind speed condition of 1~20m / s, f wv It is positively correlated with wind speed. Wind direction and the f generated by radar at different azimuths wv It is also different. It is the largest (0°) when it is against the wind, the smallest (180°) when it is with the wind, and close to 0 when it is crosswind. The current theoretical calculation cannot accurately obtain f wv , usually the empirical geophysical model fitted with observational data is used to obtain f wv .

[0062] Step 101-3: Invert the one-dimensional sea surface velocity at a certain observation azimuth angle based on the Doppler frequency shift caused by the ocean current velocity.

[0063] After removing the Doppler frequency shift caused by non-ocean current factors, the echo signal in the sea surface area only has the f caused by the ocean current velocity. c , the one-dimensional sea surface velocity at a certain observation azimuth can be obtained through formula (9).

[0064]

[0065] Where ke is the radar wave number and θ is the radar incident angle.

[0066] In step 1, the one-dimensional sea surface velocity at different observation azimuths is obtained using the along-track interferometry method, including:

[0067] Step 102-1, respectively obtaining phases of synthetic aperture radar observation data of the same target sea surface at different azimuth angles;

[0068] The single look complex (SLC) data of the synthetic aperture radar observed by two antennas (primary satellite and auxiliary satellite) at different azimuth angles on the same target sea surface are obtained respectively. The phase of the corresponding data is:

[0069]

[0070] Where, and They represent the phase of the observation data of the primary and auxiliary satellites respectively, R1 and R2 are the slant distances from the primary and auxiliary satellite antennas to the target respectively, and Δt is the time difference between the two antenna imaging.

[0071] Step 102-2: Calculate the interference phase difference between the phases of the two observation data

[0072] Step 102-3, removing the flat ground phase caused by the flat ground effect, the phase difference caused by terrain changes, and the phase difference caused by wind and waves on the sea surface from the interference phase difference, and obtaining the phase difference caused only by the ocean current velocity;

[0073] Specifically, such as Figure 2 As shown in the figure, the phase difference of the echo signal in the sea surface area is caused by the displacement of the rough scattering surface modulated by factors such as ocean current. In order to obtain the phase difference caused by the one-dimensional ocean current velocity, it is necessary to remove the irrelevant terms:

[0074]

[0075] Where, is the flat earth phase caused by the flat earth effect, It is the phase difference caused by the response of the cross-track baseline to the terrain change. is the phase difference caused by the influence of wind and waves on the sea surface, is the phase difference caused by ocean current.

[0076] When calculating the flat-earth interferometry phase, the slant distance R of the pixel corresponding to the SLC data of the primary and auxiliary satellites is calculated according to the synthetic aperture radar satellite orbit parameters and the WGS84 reference ellipsoid. 11 、R 22 The corresponding slant distance difference can be obtained.

[0077]

[0078] The along-track baseline and the cross-track baseline may coexist between the primary and secondary satellites. The terrain changes and the presence of the cross-track baseline will produce terrain phase. The phase difference except for the sea surface elevation needs to be obtained according to the elevation phase measurement principle:

[0079]

[0080] Where B ⊥ is the intersection baseline, H is the sea surface elevation, and θ is the radar incident angle.

[0081] There are contributions from the phase velocity of large-scale waves and Bragg waves on the sea surface. It can also be obtained from empirically fitted geophysical models.

[0082] Step 102-4: Calculate the one-dimensional sea surface velocity at a certain observation azimuth based on the phase difference caused by the ocean current velocity

[0083] Where V is the radar speed, λ is the radar wavelength, and θ is the radar incident angle. is the phase difference caused by ocean current, and B is the length of the along-track baseline.

[0084] In step 2, the one-dimensional ocean current velocities at different observation azimuths are synthesized into a two-dimensional ocean current, including:

[0085] If the observation azimuths are two non-parallel observation azimuths, the eastward and northward current components are calculated based on the observed one-dimensional sea surface current velocities at the two non-parallel observation azimuths;

[0086] Calculate the speed and direction of two-dimensional ocean currents based on easterly and northerly current components;

[0087] If the observation azimuth angle is greater than two, vector synthesis is performed through the permutation and combination of the two observation azimuth angles to obtain the average east and north components;

[0088] Specifically, the magnitude of the two-dimensional ocean current (V curr ) and direction (α curr ). As long as the actual observation direction is not perpendicular to the two-dimensional ocean current direction, the component of the two-dimensional ocean current in that direction can be observed. This component is also the sum of the projections of U and V in that direction. Based on the above principle, at least two non-parallel observation azimuths are required to obtain the ocean current velocities U and V. When there are more than two observation azimuths, U and V can be calculated for each azimuth by permutation and combination, and then the average U and V can be obtained, and finally the two-dimensional ocean current can be obtained by vector synthesis. Increasing the observation azimuth can improve the synthesis accuracy of the two-dimensional ocean current field.

[0089] Combine Figure 5 It can be seen that the one-dimensional ocean current velocities V1 and V2 obtained under the two observation azimuths are different from the two-dimensional ocean current V curr The relationship is shown in formula (15):

[0090]

[0091] After expansion and organization, the relationship between U and V is shown in the following formula. By solving the formulas together, U and V can be obtained.

[0092]

[0093] Where α1 and α2 are two non-parallel observation azimuths.

[0094] The two-dimensional ocean current after vector synthesis is:

[0095]

[0096]

[0097] Where, and They represent the average values ​​of U and V obtained by synthesizing multiple groups of vectors.

[0098] The following two examples verify the reliability of this method:

[0099] Example 1:

[0100] This example uses ascending and descending orbit observation data of a single synthetic aperture radar satellite over the same sea area to illustrate the implementation of the DCA method of the present invention to invert two-dimensional ocean currents.

[0101] 1. Take the observation azimuth data of Sentinel-1A (S1A) ascending orbit (observation direction 1) and descending orbit (observation direction 2) as an example. dc It can be obtained from the level 2 data of S1A with a spatial resolution of about 1.5 km, such as Figure 2 shown.

[0102] f geo Available from the Level 2 data of S1A, f in observation direction 1 and observation direction 2 geo Respectively as Figure 3 (a) and Figure 3 (b)

[0103] S1A synthetic aperture radar data is released in different levels: Level 0 raw echo data, Level 1 imaging focus data, and Level 2 ocean physical parameter data. Level 1 data is generated from Level 0 data, and Level 2 data is generated from Level 1 data. This embodiment uses Level 2 data.

[0104] Select by f dc Remove f geo The Doppler frequency shift of the land imaging area is fitted with a first-order linear equation with the range pixel as the independent variable. This equation is used to obtain the f in the entire imaging scene (including the ocean area). em ,like Figure 4 (a) and Figure 4 (b) shown.

[0105] The wavelength, incident angle, wind speed, wind direction, azimuth angle between radar sighting direction and S1A radar wavelength are input into the geophysical model CDOP to obtain the contribution component f of sea surface wind and wave factors in the echo signal. wv .

[0106] f dc Remove f geo , f em and f wv Then we can get the Doppler frequency shift f caused by the sea surface current in the radar line of sight. c According to formula (9), the velocity components of the two-dimensional ocean current in the two radar lines of sight are obtained respectively.

[0107] 2. According to Figure 5 The calculation method of formula (17), (18) and (19) is used to vector synthesize the two-dimensional ocean current. The high-frequency (HF) radar is used to match and verify the two-dimensional ocean current measured in the imaging area with the two-dimensional ocean current inverted by multi-azimuth observation results. Figure 6 (a) and Figure 6 As shown in (b), the deviation and root mean square error of the inverted two-dimensional ocean current velocity are 0.07 m / s and 0.1 m / s, respectively, and the deviation and root mean square error of the flow direction are -8.54° and 20°, respectively.

[0108] Example 2:

[0109] This example uses simulation data to illustrate the implementation of the ATI ocean current inversion method of the present invention. The simulation parameters are shown in Table 1:

[0110] Table 1

[0111]

[0112] 1. Based on the simulation parameters in Table 1, the simulation data of the primary and secondary satellites under two observation angles are obtained respectively. The resolution after spatial averaging is about 100m. The interference phase difference is obtained after conjugate interferometry. like Figure 7 (a) and Figure 7 (b) shown.

[0113] 2. According to formula (11) and the orbit parameters of the synthetic aperture radar satellite, calculate the slant distance of each pixel corresponding to the main and auxiliary images one by one and obtain the corresponding Since the intersection baseline is set to 0, =0.

[0114] The radar observation azimuth, wind speed and wind direction are input as parameters into the geophysical model CDOP to obtain the Doppler frequency caused by the wind and waves on the sea surface, and the frequency is converted into

[0115] Removal and Then we get the phase difference caused by the sea surface velocity in the radar line of sight According to formula (14), the velocity components V1 and V2 of the ocean current at the two radar observation azimuths are obtained respectively.

[0116] 3. The sea surface velocity obtained at different azimuths is vector-synthesized into two-dimensional ocean currents according to equations (15), (16), (17), and (18). The ATI two-dimensional ocean current inversion method proposed in this invention is compared and verified with the set two-dimensional ocean current parameters. Figure 8 (a) and Figure 8 As shown in (b), the deviation and root mean square error of the inverted two-dimensional ocean current velocity are 0.03 m / s and 0.1 m / s, respectively, and the deviation and root mean square error of the flow direction are 0.17° and 11.28°, respectively.

[0117] In summary, the two-dimensional ocean current inverted by the DCA and ATI methods based on multi-satellite multi-azimuth synthetic aperture radar proposed in the present invention is basically consistent with the HF measured and simulated two-dimensional ocean current in terms of flow velocity and flow direction, verifying the reliability of this method.

[0118] Example 2

[0119] This embodiment discloses a two-dimensional ocean current inversion system based on multi-azimuth synthetic aperture radar, comprising:

[0120] The one-dimensional sea surface velocity calculation module is configured to: obtain the one-dimensional sea surface velocity observed at different observation azimuths using the Doppler centroid anomaly method or the along-track interferometry method;

[0121] The two-dimensional ocean current synthesis module is configured to synthesize the two-dimensional ocean current according to the one-dimensional ocean current velocity at different observation azimuths, including:

[0122] If the observation azimuths are two non-parallel observation azimuths, the eastward and northward current components are calculated based on the observed one-dimensional sea surface current velocities at the two non-parallel observation azimuths;

[0123] Calculate the speed and direction of two-dimensional ocean currents based on eastward and northward current components.

[0124] Those skilled in the art will appreciate that the modules or steps of the present invention described above can be implemented using a general-purpose computer device. Alternatively, they can be implemented using program code executable by a computing device, which can then be stored in a storage device and executed by the computing device. Alternatively, they can be fabricated into separate integrated circuit modules, or multiple modules or steps can be fabricated into a single integrated circuit module for implementation. The present invention is not limited to any specific combination of hardware and software.

[0125] Although the above describes the specific embodiments of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without any creative work are still within the scope of protection of the present invention.

Claims

1. A two-dimensional ocean current inversion method based on multi-azimuth synthetic aperture radar, characterized in that: include: The one-dimensional sea surface velocity at different observation azimuths is obtained using the Doppler centroid anomaly method or along-track interferometry. The one-dimensional ocean current velocity at different observation azimuths is synthesized into a two-dimensional ocean current, including: If the observation azimuths are two non-parallel observation azimuths, the eastward and northward current components are calculated based on the observed one-dimensional sea surface current velocities at the two non-parallel observation azimuths; Calculate the speed and direction of two-dimensional ocean currents based on easterly and northerly current components; If the observation azimuth angle is greater than two, vector synthesis is performed through the permutation and combination of the two observation azimuth angles to obtain the average eastward and northward current components; Calculate the speed and direction of two-dimensional ocean currents based on the average easterly and northerly current components; One-dimensional ocean current velocity obtained from two non-parallel observation azimuths and and two-dimensional ocean current velocity The relationship is: One-dimensional ocean current velocity obtained at two observation azimuths and Eastward and northward current components U and V The relationship is: Where, α 1, α 2 are two non-parallel observation azimuths, is the direction of the two-dimensional ocean current; The simultaneous equations are solved to obtain the two-dimensional ocean current; The speed of the synthesized two-dimensional ocean current and direction They are: in, and Respectively represent the eastward and northward current components obtained by synthesizing multiple sets of vectors U and V average value.

2. The method for inverting two-dimensional ocean currents based on multi-azimuth synthetic aperture radar according to claim 1, wherein: The Doppler centroid anomaly method is used to obtain one-dimensional sea surface velocity at different observation azimuths, including: Obtain the Doppler centroid frequency shift observed at different azimuth angles; The Doppler frequency shift caused by the relative motion between the satellite and the ground, the electromagnetic pointing error caused by the fluctuation of the synthetic aperture radar antenna, and the Doppler frequency shift caused by the wind and waves on the sea surface are removed from the Doppler centroid frequency shift to obtain the Doppler frequency shift caused only by the ocean current velocity. The one-dimensional sea surface velocity at a certain observation azimuth is inverted based on the Doppler frequency shift caused by the ocean current velocity.

3. The method for inverting two-dimensional ocean currents based on multi-azimuth synthetic aperture radar according to claim 1, wherein: The one-dimensional sea surface velocity at different observation azimuths is obtained using the along-track interferometry method, including: Obtain the phase of the observation data of the same target sea surface from two antennas of the synthetic aperture radar at different observation azimuths respectively; Calculate the interferometric phase difference between the phases of two observation data; The phase difference caused by the flat ground effect, the phase difference caused by terrain changes and the phase difference caused by the wind and waves on the sea surface are removed from the interference phase difference to obtain the phase difference caused only by the ocean current velocity; The one-dimensional ocean current velocity at a certain observation azimuth is calculated based on the phase difference caused by the ocean current velocity.

4. The method for inverting two-dimensional ocean currents based on multi-azimuth synthetic aperture radar according to claim 3, wherein: The slant ranges of the pixels corresponding to the main and auxiliary images are calculated according to the synthetic aperture radar satellite orbit parameters and the reference ellipsoid, and the flat earth phase caused by the flat earth effect is obtained through the slant range difference between the main and auxiliary images.

5. The method for inverting two-dimensional ocean currents based on multi-azimuth synthetic aperture radar according to claim 3, characterized in that: The phase difference caused by terrain changes is calculated based on the principle of elevation phase measurement.

6. The method for two-dimensional ocean current inversion based on multi-azimuth synthetic aperture radar according to claim 3, characterized in that: The phase difference caused by the influence of wind and waves on the sea surface is obtained based on the geophysical model fitted empirically.

7. A two-dimensional ocean current inversion system based on multi-azimuth synthetic aperture radar, using the method according to any one of claims 1 to 6, characterized in that: include: The one-dimensional sea surface velocity calculation module is configured to: obtain the one-dimensional sea surface velocity observed at different observation azimuths using the Doppler centroid anomaly method or the along-track interferometry method; The two-dimensional ocean current synthesis module is configured to synthesize the two-dimensional ocean current according to the one-dimensional ocean current velocity at different observation azimuths, including: If the observation azimuths are two non-parallel observation azimuths, the eastward and northward current components are calculated based on the observed one-dimensional sea surface current velocities at the two non-parallel observation azimuths; Calculate the speed and direction of two-dimensional ocean currents based on eastward and northward current components.

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