Correction method and system for sea surface velocity inversion based on synthetic aperture radar data

Abstract

The present invention belongs to the field of marine microwave remote sensing technology and relates to a correction method and system for inverting sea surface current velocity based on synthetic aperture radar data. The method comprises: obtaining the Doppler centroid frequency shift; removing the Doppler frequency shift caused by relative motion between the satellite and the ground, electromagnetic pointing error, and the Doppler frequency shift caused by the sea surface wind and wave field from the Doppler centroid frequency shift to obtain the Doppler frequency shift electromagnetic pointing error caused solely by ocean currents. The method for calculating the electromagnetic pointing error is as follows: when the land-free imaging data and the land-imaged area data are in adjacent areas within the same orbital period, a first-order linear equation is fitted based on the electromagnetic pointing error of the land-imaged area data to calculate the electromagnetic pointing error of the land-free imaging data; and the sea surface current velocity is calculated based on the ratio of the Doppler frequency shift caused solely by ocean currents to the radar wave number and radar incident angle. The correction method proposed by the present invention does not require additional data such as satellite pitch, roll, and yaw attitude parameters and SAR antenna pattern data for electromagnetic pointing error estimation.

Description

Technical Field

[0001] The present invention belongs to the technical field of ocean microwave remote sensing, and in particular relates to a correction method and system for inverting sea surface current velocity based on synthetic aperture radar data. 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] Ocean currents are the most direct physical quantity describing the movement of seawater, governing heat and nutrient transport, as well as geochemical cycles. Operational observations of ocean surface currents facilitate navigation, search and rescue operations, and the prediction of the diffusion and transport pathways of biogeochemical tracers and pollutants such as oil, microplastics, and floating marine debris. Long-term observations indicate that global warming and the input of wind energy are causing ocean current velocities to increase annually. Accurate, large-scale, quantitative ocean current observations are of great research significance and practical application value.

[0004] Synthetic Aperture Radar (SAR) operates around the clock and in all weather conditions, and its high resolution allows it to provide observational data on a wide range of ocean surfaces. The Doppler centroid shift in the SAR system's echo signal, which includes a component caused by the ocean surface current field, can be used to invert ocean current velocities.

[0005] However, SAR antennas are affected by factors such as deformation caused by solar radiation and platform disturbances caused by atmospheric friction, causing the antenna pointing direction to deviate from the designed direction, resulting in electromagnetic pointing errors in the Doppler centroid frequency shift of the echo data. It is usually necessary to use echo information from land (including islands) within the imaging area to determine the electromagnetic pointing error, and then correct the Doppler frequency shift of the echo from the ocean area within the same imaging area. In other words, the SAR imaging data needs to refer to the land area within the imaging area to remove the electromagnetic pointing error and achieve the correction of the inverted sea surface velocity. However, when there is no land (or island) in the imaging area, the current inversion method of ocean currents will not be able to remove the electromagnetic pointing error, which will inevitably lead to large errors in the inverted sea surface velocity. Summary of the Invention

[0006] In response to the shortcomings of existing SAR ocean current inversion methods, the present invention provides a correction method and system for inverting sea surface current velocity based on synthetic aperture radar data, namely, fitting the data of adjacent land imaging areas within the same orbital period in the land-free imaging area to obtain the electromagnetic pointing error equation and apply it to the land-free imaging area.

[0007] To achieve the above objectives, one or more embodiments of the present invention provide the following technical solutions:

[0008] A first aspect of the present invention provides a correction method for inverting sea surface current velocity based on synthetic aperture radar data, comprising:

[0009] Get Doppler centroid frequency shift;

[0010] The Doppler frequency shift caused by the relative motion between the satellite and the ground, the electromagnetic pointing error and the wind and wave field on the sea surface are removed from the Doppler centroid frequency shift to obtain the Doppler frequency shift caused only by the ocean current.

[0011] The method for calculating the electromagnetic pointing error includes: determining whether the imaging area contains land;

[0012] Calculate the electromagnetic pointing errors for the area without land imaging and with land imaging data respectively;

[0013] When the data without land imaging and the data with land imaging are in adjacent areas within the same orbital period, the electromagnetic pointing error of the data without land imaging is calculated by fitting a first-order linear equation with the range pixel position as the independent variable based on the electromagnetic pointing error of the data with land imaging.

[0014] The sea surface velocity is calculated based on the ratio of the Doppler frequency shift caused only by the ocean current to the radar wave number and the radar incident angle.

[0015] A second aspect of the present invention provides a correction system for inverting sea surface current velocity based on synthetic aperture radar data, comprising:

[0016] The Doppler centroid frequency shift acquisition module is configured to: acquire the Doppler centroid frequency shift;

[0017] The Doppler frequency shift calculation module caused by ocean currents is configured to: remove the Doppler frequency shift caused by the relative motion of the satellite and the ground, the electromagnetic pointing error, and the Doppler frequency shift caused by the wind and wave field on the sea surface from the Doppler centroid frequency shift to obtain the Doppler frequency shift caused only by ocean currents;

[0018] The method for calculating the electromagnetic pointing error includes: determining whether the imaging area contains land;

[0019] Calculate the electromagnetic pointing errors for the area without land imaging and with land imaging data respectively;

[0020] When the data without land imaging and the data with land imaging are in adjacent areas within the same orbital period, the electromagnetic pointing error of the data without land imaging is calculated by fitting a first-order linear equation with the range pixel position as the independent variable based on the electromagnetic pointing error of the data with land imaging.

[0021] The sea surface velocity calculation module is configured to calculate the sea surface velocity according to the ratio of the Doppler frequency shift caused only by the sea current to the radar wave number and the radar incident angle.

[0022] One or more of the above technical solutions have the following beneficial effects:

[0023] 1. The correction method for inverting sea surface velocity without land imaging data proposed in the present invention does not require additional auxiliary data such as satellite pitch, rotation and yaw attitude parameters and SAR antenna radiation pattern for electromagnetic pointing error estimation.

[0024] 2. The correction method for inverting sea surface current velocity from landless imaging data proposed in the present invention can accurately remove electromagnetic pointing errors, and can realize the inversion of sea surface current velocity in the open sea without islands or land, thus expanding the scope of application of SAR inversion of ocean currents.

[0025] 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

[0026] 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.

[0027] Figure 1 This is a flow chart of the correction method for inverting sea surface current velocity based on synthetic aperture radar data in the first embodiment.

[0028] Figure 2 This is the SAR echo Doppler centroid frequency shift diagram of the first embodiment.

[0029] Figure 3 (a) and Figure 3 (b) The azimuth and distance f of the land area of ​​track 121IW3 of the first embodiment are respectively em Change graph.

[0030] Figure 4 (a) Figure 4 (b) Figure 4 (c) Figure 4 (d) are scatter plots of the inversion results of the first embodiment without land imaging data and the data with land imaging area in the same period when they are in the same orbits 121, 48, 150, and 77, respectively.

[0031] Figure 5 (a) Figure 5 (b) The f of IW3 in the first embodiment at track 121 and track 48 respectively em Plot of fitted coefficients.

[0032] Figure 6 (a) Figure 6 (b) Figure 6(c) are scatter plots of the inversion results of the first embodiment without land imaging data and the data with land imaging areas in the same period when they are in adjacent orbits 121, 48 and 150, respectively. DETAILED DESCRIPTION

[0033] Example 1

[0034] like Figure 1 As shown, this embodiment discloses a correction method for sea surface velocity inversion based on synthetic aperture radar data, comprising:

[0035] Step 1, obtain the Doppler centroid frequency shift;

[0036] Assume that the actual motion trajectory of the satellite is a local uniform linear motion and the Earth is locally flat and does not rotate. Theoretically, the Doppler centroid frequency shift f dc for:

[0037]

[0038] Where V is the radar velocity, β is the radar instantaneous slant angle, λ is the radar wavelength, and γ is the radar downward angle:

[0039]

[0040] Where H is the distance from the center of the Earth to the SAR sensor, R is the slant range, and R e is the radius of the Earth. Assuming R0 is the tilt distance at a certain instantaneous point, the γ Taylor expansion shown in (2) is given by the following formula:

[0041]

[0042] Where γ0 is the downward viewing angle corresponding to the tilt distance R0.

[0043] From this we can get:

[0044]

[0045]

[0046] 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.

[0047] f dcIt can be obtained not only through the above theoretical formula, but also directly read from the header file or auxiliary file of satellite imaging data. When the synthetic aperture radar data product is released, the header file and auxiliary file will be provided, which include the required parameters such as incident angle, imaging position, satellite attitude, product level, etc. The specific situation depends on the release form of the data. By obtaining synthetic aperture radar data, and then using the header file and auxiliary file to calculate the f at the corresponding azimuth dc .

[0048] The Doppler centroid frequency shift of SAR imaging data on the sea surface can be decomposed into:

[0049] f dc =f geo +f em +f wv +f c (6)

[0050] Where, f dc is the echo Doppler centroid frequency shift, 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, f wv is the Doppler frequency shift caused by the wind and wave field on the sea surface, f c The Doppler shift caused by ocean currents.

[0051] Step 2: Remove the Doppler frequency shift f caused by the relative motion between the satellite and the ground geo ;

[0052] f geo In theory, this can be calculated for a given satellite's attitude and velocity:

[0053]

[0054] 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), β is the argument of latitude, and ψ is the inclination of the satellite orbit plane.

[0055] Divide the theoretical formula to obtain f geo Outside, f geo It can also be calculated from the Doppler coefficient:

[0056] f geo =d0+d1(t SR -t0)+d2(t SR -t0) 2+d3(t SR -t0) 3 +d4(t SR -t0) 4 (8)

[0057] Where, d i is the Doppler coefficient (i=0,1,2,3,4), t SR is the two-way slant range time, t0 is the standard slant range time. Or directly read f from the auxiliary data geo In practical applications, f geo Obtained directly from the header file or auxiliary file of the imaging data.

[0058] Step 3: Remove the electromagnetic pointing error f em ;

[0059] f em The land and ocean areas of the imaging data coexist. Since the land area of ​​the imaging data does not exist ww and f c , in getting f dc and f geo Under the premise of , according to formula (6) we can get f em .

[0060] The imaging data acquisition process is to download directly from the web page. When downloading the data, you can clearly know the location, time and flight orbit period of the imaging data.

[0061] Analysis of imaging data of land areas shows that f em Has the following distribution characteristics: On the one hand, f em The SAR azimuth or flight direction is basically unchanged, and changes linearly in the distance direction. On the other hand, f em The adjacent regions within the same orbital period are basically consistent.

[0062] Among them, the same orbital period refers to the orbital period of the same satellite, that is, the time interval between the last time the satellite was in the orbit and the next time it returned to the orbit. The number of orbits flown within the orbital period is fixed.

[0063] The same orbit refers to the same flight track.

[0064] For example, a satellite needs to fly from Orbit 1 to Orbit 175 in 12 days, and then from Orbit 1 to Orbit 175 again in the next 12 days. 12 days is the satellite's orbital period. Orbit 1, Orbit 2, Orbit 3, etc. are the satellite's different flight paths.

[0065] The electromagnetic pointing error in acquiring data of adjacent land imaging areas within the same orbit period proposed by the present invention has two situations, namely, the data without land imaging and the data of adjacent land imaging areas may be in the same flight orbit or adjacent flight orbits.

[0066] When the data without land imaging and the data with land imaging area are in the same orbit with the same orbit period, according to f em Distribution characteristics, for f with land imaging area data em The fitting is a first-order linear equation with range-direction pixels as the independent variable; the first-order linear equation is Y = Kx + B, where Y is the Doppler shift of the electromagnetic pointing error obtained by fitting, K and B are fitting coefficients, and x is the pixel position in the range direction. In this case, the imaging time interval between the land-free and land-imaged areas is short, resulting in a more accurate electromagnetic pointing error equation and higher accuracy in the corrected inverted ocean current velocity.

[0067] When the data without land imaging and the data with land imaging area are in adjacent orbits with the same orbital period, according to f em Distribution characteristics, also for the land imaging area data f em The fitting is a first-order linear equation with the distance to the pixel as the independent variable;

[0068] The data without land imaging is obtained by obtaining the f of the adjacent area with land imaging data in the same period. em Fit the equation to get f em :

[0069] f em =JY

[0070] Where, J is the same as f dc A matrix with the same number of rows and columns containing all 1s.

[0071] The method provided by the present invention uses "adjacent area data within the same orbital period" to correct the electromagnetic pointing error in the area without land imaging.

[0072] For example, firstly, the land-free imaging data on the flight track 121 is acquired, and the electromagnetic pointing error of the data is calculated using the “adjacent area data within the same orbit period”.

[0073] The "adjacent region data within the same orbital period" may be on flight track 121 or flight track 77. Therefore, there are two situations for calculating electromagnetic pointing errors when there is no land imaging data on track 121: (1) using the same flight track 121 with the same period and containing land imaging data. (2) using the same flight track 77 with the same period and containing land imaging data.

[0074] Step 4: Remove the Doppler frequency shift f caused by the wind and wave field on the sea surface wv ;

[0075] 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. The f generated by wind direction and radar flight direction at different angles wv It is also different. It is largest when it is against the wind (0°), smallest when it is with the wind (180°), and close to 0 when it is crosswind.

[0076] Currently, theoretical calculations cannot accurately obtain f wv . Usually, empirical geophysical models fitted with observational data are used to obtain f wv :

[0077] f wv =α pp F[X(θ,φ,u 10 ,pp)]+β pp (9)

[0078] Where θ is the incident angle, φ is the azimuth angle between the wind direction and the satellite’s viewing direction, and u 10 is the wind speed at 10m above the sea surface, pp is the polarization mode, α pp and β pp is the fitting coefficient. F[x] is the sigmoid function.

[0079] Step 5: Obtain the sea surface velocity V c ;

[0080] 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 sea surface velocity. c , the sea surface velocity V can be obtained through (11) c .

[0081]

[0082] Where ke is the radar wave number and θ is the radar incident angle.

[0083] (1) This experimental example illustrates the technical implementation process when the data without land imaging and the data with land imaging area of ​​the same period are in the same orbit:

[0084] 1. Taking the four ascending orbit imaging data of Sentinel-1A (S1A) in the Gulf Stream as an example, there is no land imaging data in this imaging area, which can be used to verify the accuracy of the correction method for inverting sea surface current velocity in the present invention;

[0085] like Figure 2 As shown, the imaging data is in interferometric wide-swath mode (IW) and contains three sub-swaths (IW1, IW2, and IW3). The imaging data was selected from July 2020 to November 2022. The relative orbit numbers within the imaging data orbit period are 121, 48, 150, and 77 from left to right. Among them, orbit 121 is 6 scenes of continuous imaging data, orbit 48 is 3 scenes of continuous imaging data, orbit 150 is 8 scenes of continuous imaging data, and orbit 77 is 10 scenes of continuous imaging data. The above data are obtained using the secondary ocean (OCN) data of S1A. dc .

[0086] 2. f geo Provided by S1AOCN data.

[0087] 3. f dc Remove f geo After that, the land area only has em The Doppler shift caused by the orbit 121IW3 data is as follows: em Changes such as Figure 3 (a) and Figure 3 (b) is shown. Prove that f em It has a distribution characteristic that remains basically unchanged in the flight direction and decreases linearly with distance.

[0088] According to f em Distribution characteristics, for f em In the azimuth direction, the random error is averaged and the first-order linear equation Y=Kx+B is used for fitting in the range direction. In the formula, Y is the Doppler shift of the electromagnetic pointing error obtained by fitting, K and B are fitting coefficients, and x is the pixel position in the range direction. The electromagnetic pointing error fitting result is shown in the figure below. Figure 3 (b) The black dashed line shows that the f of the track 121 without land imaging scene can be obtained by fitting the first-order linear equation. em .

[0089] 4. Input the S1A radar wavelength, incident angle, and the azimuth between the obtained wind speed, wind direction, and radar line of sight into the empirical geophysical model CDOP to obtain the f caused by the wind and wave field on the sea surface. wv .

[0090] 5. According to formula (6), f dc Remove f geo , f em and f wv Then, the Doppler frequency shift f caused by the sea surface velocity in the radar line of sight is obtained. c , V is obtained from formula (10) cThe sea surface velocity corrected by S1A was verified using the sea surface flow field data of Hybrid Coordinate Ocean Model (HYCOM). The number of matching points (N), bias, root mean square error (rmse), standard deviation (std) and correlation coefficient (r) between the sea surface velocity corrected by S1A and HYCOM data were calculated. Figure 4 (a) Figure 4 (b) Figure 4 (c) and Figure 4 (d) shown.

[0091] (2) This experimental example illustrates the technical implementation process when the data without land imaging and the data with land imaging area in the same period are in adjacent orbits:

[0092] 1. Using the f provided by the S1AOCN data in Example 1 dc .

[0093] 2. Using the f provided by the S1AOCN data in Example 1 geo data.

[0094] 3. IW3 f of Track 121, Track 48, Track 150 and Track 77 em The fitting coefficients under different cycles from July 2020 to November 2022 are as follows: Figure 5 (a) and Figure 5 (b) shows the land imaging area data f of track 121, track 48, track 150 and track 77. em Basically the same, f em The coefficients of the fitted equation are approximate.

[0095] Take orbit 77 as an example, and calculate its periods f em The coefficients of the fitting equation are applied to the f of orbits 121, 48, and 150 with the same period and without land imaging data. em Calculation.

[0096] 4. f calculated using the S1A data in Example 1 wv data.

[0097] 5. f dc Remove f geo 、f em and f wv Then we get f c , V is obtained from formula (10) c The error between the sea surface velocity after S1A correction and HYCOM data was calculated, as shown in the following figure: Figure 6 (a) Figure 6 (b) and Figure 6 (c) shown.

[0098] The results of the above two experimental examples (1) and (2) show that the correction method for inverting sea surface current velocity from land-free imaging data proposed in the present invention has high accuracy, strong reliability and robustness, and can accurately eliminate the influence of electromagnetic pointing errors, thereby realizing error correction of sea current velocity inverted from land-free imaging.

[0099] Example 2

[0100] This embodiment discloses a correction system for inverting sea surface current velocity based on synthetic aperture radar data, comprising:

[0101] The Doppler centroid frequency shift acquisition module is configured to: acquire the Doppler centroid frequency shift;

[0102] The Doppler frequency shift calculation module caused by ocean currents is configured to: remove the Doppler frequency shift caused by the relative motion of the satellite and the ground, the electromagnetic pointing error, and the Doppler frequency shift caused by the wind and wave field on the sea surface from the Doppler centroid frequency shift to obtain the Doppler frequency shift caused only by ocean currents;

[0103] The calculation method of the electromagnetic pointing error includes: determining whether the imaging area contains land;

[0104] Calculate the electromagnetic pointing errors for the area without land imaging and with land imaging data respectively;

[0105] When the land-free imaging data and the land-imaging area data are in the same orbit with the same orbital period, the electromagnetic pointing error of the land-free imaging data is calculated by fitting a first-order linear equation with the range pixel position as the independent variable based on the electromagnetic pointing error of the land-imaging area data;

[0106] When the data without land imaging and the data with land imaging are in adjacent orbits with the same orbital period, the electromagnetic pointing error of the data with land imaging is first calculated; the electromagnetic pointing error of the data without land imaging is calculated by fitting a first-order linear equation based on the electromagnetic pointing error of the data with land imaging.

[0107] The first-order linear equation is: Y = Kx + B, where Y is the electromagnetic pointing error obtained by fitting, K and B are fitting coefficients, and x is the pixel position in the range upward.

[0108] The sea surface velocity calculation module is configured to calculate the sea surface velocity based on the Doppler frequency shift, radar wave number and radar incident angle caused only by the sea current.

[0109] 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.

[0110] 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 correction method for inverting sea surface current velocity based on synthetic aperture radar data, characterized in that: include: Get Doppler centroid frequency shift; The Doppler frequency shift caused by the relative motion between the satellite and the ground, the electromagnetic pointing error and the wind and wave field on the sea surface are removed from the Doppler centroid frequency shift to obtain the Doppler frequency shift caused only by the ocean current. The method for calculating the electromagnetic pointing error includes: determining whether the imaging area contains land; Calculate the electromagnetic pointing errors for the area without land imaging and with land imaging data respectively; When the land-free imaging data and the land-imaging area data are in the same orbit with the same orbital period, the electromagnetic pointing error of the land-free imaging data is calculated by fitting a first-order linear equation with the range pixel position as the independent variable based on the electromagnetic pointing error of the land-imaging area data; When the land-free imaging data and the land-imaging area data are in adjacent orbits with the same orbital period, the electromagnetic pointing error of the land-imaging area data is first calculated; the electromagnetic pointing error of the land-free imaging data is obtained based on the electromagnetic pointing error fitting equation of the land-imaging area data in the adjacent orbits with the same orbital period. The sea surface velocity is calculated based on the Doppler frequency shift, radar wave number and radar incident angle caused only by the ocean current. The first-order linear equation is: Y=Kx+B, where Y is the electromagnetic pointing error obtained by fitting, K and B are fitting coefficients respectively, and x is the pixel position in the distance upward.

2. The method for correcting sea surface velocity based on synthetic aperture radar data inversion according to claim 1, characterized in that: The Doppler frequency shift caused by the relative motion between the satellite and the ground is calculated based on the given satellite attitude and velocity or the Doppler coefficient.

3. The method for correcting sea surface velocity based on synthetic aperture radar data inversion according to claim 2, characterized in that: Calculate the Doppler shift caused by the relative motion of the satellite and the ground based on the given satellite attitude and velocity for: in, is the radar wave number, is the satellite's velocity along the orbital plane, It's an under-the-radar perspective. is the angle between the radial altitude plane and the satellite orbit plane, is the angular velocity of the Earth, is the satellite speed, Indicates radar left view, Indicates radar right view, is the argument of latitude, is the inclination of the satellite's orbital plane.

4. The method for correcting sea surface velocity based on synthetic aperture radar data inversion according to claim 2, wherein: Calculate the Doppler frequency shift caused by the relative motion between the satellite and the ground based on the Doppler coefficient for: in, is the Doppler coefficient ( =0, 1, 2, 3, 4), is the two-way slant range time, is the standard slant range time.

5. The method for correcting sea surface velocity based on synthetic aperture radar data inversion according to claim 1, characterized in that: The Doppler frequency shift caused by the sea surface wind and wave field is obtained by using an empirical geophysical model fitted with observation data.

6. A correction system for inverting sea surface velocity based on synthetic aperture radar data, characterized in that: include: The Doppler centroid frequency shift acquisition module is configured to: acquire the Doppler centroid frequency shift; The Doppler frequency shift calculation module caused by ocean currents is configured to: remove the Doppler frequency shift caused by the relative motion of the satellite and the ground, the electromagnetic pointing error, and the Doppler frequency shift caused by the wind and wave field on the sea surface from the Doppler centroid frequency shift to obtain the Doppler frequency shift caused only by ocean currents; The method for calculating the electromagnetic pointing error includes: determining whether the imaging area contains land; Calculate the electromagnetic pointing errors for the area without land imaging and with land imaging data respectively; When the land-free imaging data and the land-imaging area data are in the same orbit with the same orbital period, the electromagnetic pointing error of the land-free imaging data is calculated by fitting a first-order linear equation with the range pixel position as the independent variable based on the electromagnetic pointing error of the land-imaging area data; When the land-free imaging data and the land-imaging area data are in adjacent orbits with the same orbital period, the electromagnetic pointing error of the land-imaging area data is first calculated; the electromagnetic pointing error of the land-free imaging data is obtained based on the electromagnetic pointing error fitting equation of the land-imaging area data in the adjacent orbits with the same orbital period. The sea surface velocity calculation module is configured to calculate the sea surface velocity based on a ratio of a Doppler frequency shift caused only by the sea current to a radar wave number and a radar incident angle; The first-order linear equation is: Y=Kx+B, where Y is the electromagnetic pointing error obtained by fitting, K and B are fitting coefficients respectively, and x is the pixel position in the distance upward.

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