Vector flow inversion method combining single-station ground wave radar and spaceborne SAR (Synthetic Aperture Radar)

Abstract

The invention relates to the field of ocean current remote sensing, and relates to a vector flow inversion method combining a single-station ground wave radar and a satellite-borne SAR, and the method mainly comprises the steps: calculating the frequency deviation of a first-order Bragg peak of the ground wave radar, carrying out the inversion of the sea surface radial flow of the ground wave radar, and carrying out the inversion of the sea surface radial flow of the SAR through employing a Doppler centroid anomaly method; interpolating a ground wave radar radial flow result to an SAR data grid through bilinear interpolation; inverting a single-station vector flow based on a ground wave radar radial flow result, projecting single-station vector flow data with high confidence coefficient to an SAR range direction, calculating a difference mean value of a projection flow velocity and an SAR radial flow velocity, and further calibrating the SAR radial flow; and combining a ground wave radar radial flow result with the calibrated SAR radial flow to invert a vector flow. According to the method, the defect of ocean current observation through a single means is overcome, high-precision inversion of the combined vector flow is achieved, and the method has important significance on ocean environment monitoring, disaster forecasting, shipping safety and the like.

Description

Technical Field

[0001] This invention relates to the field of ocean current remote sensing, and specifically to a method for inverting sea surface vector currents by combining high-frequency ground wave radar and spaceborne synthetic aperture radar (SAR). Background Technology

[0002] Accurate monitoring of ocean currents is of great significance for maritime navigation safety, disaster early warning, and marine engineering operation and maintenance. High-frequency ground wave radar and spaceborne synthetic aperture radar (SAR) are two major remote sensing methods capable of simultaneously achieving large-scale, long-distance marine environmental monitoring.

[0003] Ground-wave radar utilizes the diffraction and propagation characteristics of high-frequency electromagnetic waves on the sea surface to achieve continuous, over-the-horizon observation of the sea surface and real-time inversion of ocean current information; however, its spatial resolution is relatively low, at the kilometer level, making it difficult to obtain fine-grained parameters. Spaceborne SAR, with its high spatial resolution at the meter or sub-meter level, can acquire detailed two-dimensional images of the sea surface, thus enabling the acquisition of high spatial resolution ocean current information; however, SAR has low temporal resolution, making it difficult to achieve continuous observation of areas of interest even through satellite networking, failing to meet emergency observation needs. Therefore, both ground-wave radar and SAR possess global observation capabilities in space, enabling large-scale spatiotemporal matching in nearshore or offshore areas, and demonstrating the feasibility of collaborative detection.

[0004] Ground-wave radar (GWR) uses Doppler shift information in the first-order echo spectrum to calculate radial current velocity. Vector current acquisition is mostly done using bistatic or multistatic radar; however, single-station radar is sometimes used. In such cases, methods such as least squares can be used to invert the vector current, but this sacrifices some spatial or angular resolution, resulting in low accuracy. SAR primarily uses two methods to invert ocean currents: in-orbit interferometry (ATI) and Doppler centroid anomaly (DCA). The ATI method inverts radial current velocity through the phase difference of dual-antenna images; its applicability in practical observations is limited by antenna configuration. The DCA method uses the relationship between sea surface motion and Doppler shift in SAR echoes to invert radial current velocity, suitable for large and medium-scale flow fields; however, Doppler shifts caused by wind and waves can also affect the inverted radial current. Therefore, GWR and SAR can acquire radial flow field results from different lines of sight. Consider using GWR vector current results to optimize SAR radial current, thereby obtaining higher-precision vector current results when both are used in conjunction to observe ocean currents.

[0005] In summary, the collaborative observation of ground wave radar and SAR can obtain higher precision vector flow field results. Based on this, this application will integrate the advantages of ground wave radar and SAR in observing sea conditions and develop a vector flow inversion method that combines single-station ground wave radar and spaceborne SAR to achieve high-precision and high-resolution inversion of joint vector flow. Summary of the Invention

[0006] (a) Technical problems to be solved

[0007] This invention aims to provide a vector flow inversion method that combines single-station ground wave radar and spaceborne SAR to achieve large-scale, high-precision ocean current vector observation, thereby improving the accuracy of ocean current field monitoring and meeting the urgent need for reliable current field data for safe navigation, disaster early warning, and engineering operation and maintenance at sea.

[0008] (II) Technical Solution

[0009] This invention includes the following steps:

[0010] Step 1, Radial flow inversion from ground wave radar and radial flow inversion from SAR:

[0011] Step 1.1: Perform radial flow inversion from ground wave radar. After acquiring the range-Doppler echo spectrum data from the ground wave radar, directly calculate the frequency shift of the first-order Bragg peak. The formula is: In the formula, Indicates the radial velocity of the ocean current. This represents the difference between the actual first-order Bragg peak frequency and the theoretical frequency. This indicates the wavelength of the electromagnetic waves emitted by the radar.

[0012] Step 1.2: Achieve SAR radial flow inversion. Utilize the Doppler centroid anomaly method, i.e., invert sea surface current velocity by analyzing the relationship between sea surface motion and the Doppler frequency shift in the SAR echo. The Doppler frequency shift in the SAR echo... It can be represented as: ,in, The frequency shift caused by the relative motion between the SAR platform and the Earth's surface, i.e., the predicted Doppler, can be calculated using satellite orbit and attitude data. The frequency shift caused by sea surface motion is the result of the combined effects of ocean currents, wind fields, and ocean waves. This indicates the frequency shift of electromagnetic pointing error caused by the satellite platform or antenna attitude being affected by seasonal atmospheric effects and solar radiation, resulting in the antenna pointing deviating from the theoretical direction. This represents the frequency shift caused by inaccurate estimations and unknown biases of non-geophysical terms. It is removed by utilizing the Earth Empirical Physical Model (CDOP). Doppler shift in wind and waves The remaining Doppler frequency shift is generated by ocean current motion.

[0013] The CDOP model function is: ,in, These represent the wind speed and direction at a depth of 10 meters above the sea surface, respectively. Indicates the incident angle of the SAR. This represents the polarization mode of the SAR. Therefore, the inverted radial velocity is: , ,in, Radial flow velocity, For SAR radar electromagnetic wave number, This is the Doppler shift caused by ocean currents.

[0014] Step 2: Spatial matching of ground wave radar and SAR: Spatial registration is performed between the synchronized ground wave radar radial current results and the SAR area. Since the latitude and longitude grids of ground wave radar and SAR data observed in the same sea area are not the same, the radial current results from the ground wave radar need to be matched into the SAR coordinate grid. Here, bilinear interpolation is used between the ground wave radar latitude and longitude matrix and the radial current results to interpolate the radial current results into the uniform latitude and longitude grid of the SAR, ensuring that both methods maintain the same detection range and latitude and longitude grid. The formula for bilinear interpolation is: In the formula, Represents the SAR latitude and longitude points to be matched. , , , Representing four ground-wave radar latitude and longitude points surrounding the SAR latitude and longitude point, the radial flow values ​​at the four points are as follows: , , , .

[0015] Step 3: Constrain and calibrate the SAR radial flow using the vector flow results from a single-station ground wave radar.

[0016] First, the vector flow is inverted using the least squares method.

[0017] Secondly, to ensure the reliability of the vector flow inversion results from a single-station ground wave radar, a quality assessment is necessary. This mainly involves selecting data points with high confidence from the entire flow field obtained in the initial inversion and discarding unreliable points. ,in, For radial velocity residual, For the eastward velocity component of the vector flow, For the northward velocity component of the vector flow, This is the azimuth angle of the radial flow of the ground wave radar. The value is 1 if the condition is met, and 0 otherwise. This represents the standard deviation of the flow direction within a 3x3 grid window. Finally, grid points with a reliability greater than 0.9 are selected, and high-confidence vector flow fields and their spatial locations are output, effectively extracting the reliable portion of the inversion results.

[0018] Finally, the radial velocity of the SAR is calibrated using the mean difference. The high-confidence vector flow is projected onto the SAR range direction to obtain the projected radial component of the ground-wave radar. Then, the mean difference between the SAR and ground-wave radar projected radial components is calculated, and this mean is subtracted from the original SAR velocity as a correction factor to obtain the calibrated velocity. ,in, This indicates the calibrated flow rate. This represents the initial radial velocity obtained using the DCA method. Represents a high-confidence vector flow. Indicates the number of valid data points. This represents the angle between the ground wave radar vector flow and the SAR range direction.

[0019] Step 4: Perform vector flow inversion by combining the radial flow from the single-station ground wave radar and the calibrated SAR radial flow. By unifying the radial flow from the ground wave radar and the calibrated SAR radial flow under the stream function framework, the velocity components are solved using Taylor series expansion, and finally the vector flow field is constructed.

[0020] Stream function It is a function describing the movement of ocean currents, and can be defined by the following formula: , For the ground wave radar radial flow matched in the overlapping region and the calibrated SAR radial flow, , The relationship between the radial velocity and any observation point can be expressed, and the stream function can be expanded using Taylor series: , ,in, for powers of, for powers of, It corresponds The coefficient of the term, Let be the highest order of the Taylor series, set to 3. This includes ground-wave radar and calibrated SAR radial flow at the same observation point. A linear equation can be established for each observation point, and the overdetermined linear equations are finally solved using the least squares method to obtain the stream function coefficients, allowing for the calculation of... and Components, find the vector flow: , ,in, The vector flow velocity obtained from the final joint inversion, This indicates the direction of the vector flow.

[0021] (III) Beneficial Effects

[0022] The advantages of this invention are as follows:

[0023] This invention innovatively combines the advantages of both ground-wave radar and SAR remote sensing methods for observing the sea surface. By spatiotemporally registering the radial flow results from ground-wave radar with SAR data, radial flow results from ground-wave radar that match the SAR data for the same region are obtained. Then, the radial flow retrieved from SAR is constrained and calibrated using vector flow results from a high-confidence single-station ground-wave radar to improve the accuracy of the SAR radial flow. Finally, the vector flow is retrieved by combining the radial flow from ground-wave radar with the calibrated SAR radial flow using a stream function. Attached Figure Description

[0024] Figure 1 The flowchart of the vector flow inversion method for the combined single-station ground wave radar and spaceborne SAR provided by the present invention is shown.

[0025] Figure 2 The high-confidence vector flow results of single-station ground wave radar provided by this invention.

[0026] Figure 3 The absolute error results between the calibrated SAR radial flow and actual ocean current data provided by this invention.

[0027] Figure 4 The absolute error results between the jointly inverted vector current velocity and the actual ocean current data provided by this invention.

[0028] Figure 5 The absolute error results between the vector current direction obtained by the joint inversion provided by this invention and the actual ocean current data. Detailed Implementation

[0029] To make the objectives, contents, and advantages of the present invention clearer, the following description is provided in conjunction with the appendix. Figure 1 The specific embodiments of the present invention will be described in further detail below:

[0030] Taking a set of synchronized ground-wave radar and SAR observation data as an example, the selected area is the west coast of the United States, and the time is 23:30 (UTC) on May 5, 2019. The ground-wave radar data comes from the radial current product of the WERA radar deployed in this sea area, operating at a frequency of 8.328 MHz; the SAR data comes from the Sentinel-1A satellite, is of OCN data type, and uses the interferometric wide swath mode with a spatial resolution of 1 km × 1 km. The time difference between the SAR imaging time and the ground-wave radar data is less than 30 minutes.

[0031] Step 1, inversion of radial flow from ground wave radar and SAR radial flow:

[0032] Step 1.1: Perform radial flow inversion from ground wave radar.

[0033] Step 1.2, perform SAR radial flow inversion. Doppler frequency shift in SAR echo. It can be represented as: ,in, The frequency shift caused by the relative motion between the SAR platform and the Earth's surface, i.e., the predicted Doppler, can be calculated using satellite orbit and attitude data. The frequency shift caused by sea surface motion is the result of the combined effects of ocean currents, wind fields, and ocean waves. This indicates the frequency shift of electromagnetic pointing error caused by the satellite platform or antenna attitude being affected by seasonal atmospheric effects and solar radiation, resulting in the antenna pointing deviating from the theoretical direction. This indicates the frequency shift caused by inaccurate estimations and unknown biases of non-geophysical terms. Since the OCN product data from Sentinel-1 SAR directly provides the Doppler frequency shift... And predicting Doppler Therefore, no additional calculations are required; while the electromagnetic pointing error frequency shift and unknown deviation Land correction is needed to remove the frequency shift, which involves calculating the average Doppler frequency value of the land area in the SAR image and uniformly removing it from the entire scene data. After removing the frequency shift, the Doppler frequency of the sea surface is calculated. At this point, it is necessary to use the Earth Empirical Physical Model (CDOP) to remove the Doppler shift of wind and waves. The remaining Doppler frequency shift is generated by ocean current motion.

[0034] The CDOP model function is: .in, These represent the wind speed and direction at a depth of 10 meters above the sea surface, respectively. Indicates the incident angle of the SAR. This indicates the polarization mode of the SAR. Therefore, the inverted radial velocity is: , .in, Radial flow velocity, For SAR radar electromagnetic wave number, This is the Doppler shift caused by ocean currents.

[0035] Step 2 involves spatially registering the synchronized ground wave radar radial current results with the SAR data. Since the latitude and longitude grids of the ground wave radar and SAR data observed in the same sea area are not identical, the ground wave radar radial current results need to be matched into the SAR coordinate grid after acquisition. Subsequently, bilinear interpolation is performed with the ground wave radar latitude and longitude matrix and the radial current results to interpolate the radial current results into the uniform latitude and longitude grid of the SAR, ensuring that both methods maintain the same detection range and latitude and longitude grid. The formula for bilinear interpolation is: In the formula, Represents the SAR latitude and longitude points to be matched. , , , Representing four ground-wave radar latitude and longitude points surrounding the SAR latitude and longitude point, the radial flow values ​​at the four points are as follows: , , , .

[0036] Step 3: Constrain and calibrate the SAR radial flow using the vector flow results from a single-station ground wave radar.

[0037] First, the least squares method is used to invert the vector flow of a single-station ground wave radar. For each matched radial flow observation point of the ground wave radar, neighborhood data of a certain window size are selected around it. Based on the fact that the radial flow velocity is the projection of the ocean current vector onto the radar beam direction, the corresponding equation is established: .in, The radial velocity of the ground wave radar. The azimuth angle of the radial flow of the ground wave radar. For vector flow, The direction of the vector flow is then determined. Next, the linear equations within this window are solved using least squares. By traversing all valid radial flow observation data through a sliding window, the initial vector flow field of the single-station ground wave radar area can be obtained.

[0038] Secondly, to ensure the reliability of the vector flow inversion results from a single-station ground wave radar, a quality assessment is required: ,in, For radial velocity residual, For the eastward velocity component of the vector flow, This represents the northward velocity component of the vector flow. The value is 1 if the condition is met, and 0 otherwise. This represents the standard deviation of the flow direction within a 3x3 grid window. Finally, grid points with a reliability greater than 0.9 are selected, and high-confidence vector flow fields and their spatial locations are output, effectively extracting the reliable portion of the inversion results. The high-confidence vector flow results from a single-station ground wave radar are shown below. Figure 2 As shown.

[0039] Finally, the radial velocity of the SAR is calibrated using the mean difference. The radial component of the ground-wave radar projection is obtained by projecting the high-confidence vector flow from the ground-wave radar onto the SAR range direction. Then, the mean difference between the SAR and ground-wave radar radial components is calculated, and this mean is subtracted from the original SAR velocity as a correction factor to obtain the calibrated velocity. ,in, This indicates the calibrated flow rate. This represents the initial radial velocity obtained using the DCA method. Represents a high-confidence vector flow. Indicates the number of valid data points. This represents the angle between the ground-wave radar vector current and the SAR range direction. The absolute error results between the calibrated SAR radial current and actual ocean current data are as follows: Figure 3 As shown.

[0040] Step 4: Perform vector flow inversion by combining the radial flow from the single-station ground wave radar and the radial flow from the calibrated SAR. By unifying the radial flow from the ground wave radar and the calibrated SAR radial flow under the stream function framework, the velocity components are solved using Taylor series expansion, and finally, the vector flow is synthesized.

[0041] Stream function It is a function describing the movement of ocean currents, and can be defined by the following formula: , For the ground wave radar radial flow matched in the overlapping region and the calibrated SAR radial flow, , The relationship between the radial velocity and any observation point can be expressed, and the stream function can be expanded using Taylor series: , ,in, for powers of, for powers of, It corresponds The coefficient of the term, Let be the highest order of the Taylor series, set to 3. This includes ground-wave radar and calibrated SAR radial flow at the same observation point. A linear equation can be established for each observation point, and the overdetermined linear equations are finally solved using the least squares method to obtain the stream function coefficients, allowing for the calculation of... and Components, calculate the vector flow: , ,in, The vector flow velocity obtained from the final joint inversion, This indicates the direction of the vector flow.

[0042] The absolute error results of the joint inversion of vector current velocity and actual ocean current data are as follows: Figure 4 As shown, the root mean square error of the current velocity is 0.13 m / s. The absolute error between the jointly inverted vector current direction and the actual ocean current data is shown in the figure. Figure 5 As shown, the root mean square error of the flow direction is 19.37°.

[0043] In summary, the vector currents retrieved by the proposed method have high accuracy in sea state observation.

Claims

1. A vector flow inversion method combining single-station ground-wave radar and spaceborne SAR, characterized in that, Includes the following steps: (1) Radial flow inversion of ground wave radar and radial flow inversion of SAR: For ground wave radar, the frequency shift of the first-order Bragg peak is directly calculated to invert the radial flow of the sea surface; for SAR, the Doppler centroid anomaly method is used, that is, the sea surface velocity is inverted by analyzing the relationship between sea surface motion and Doppler frequency shift in SAR echo. (2) Spatial registration: Spatial registration is performed between the synchronized ground wave radar radial flow results and SAR data. Bilinear interpolation is used to interpolate the ground wave radar radial flow results into the SAR latitude and longitude grid to obtain the ground wave radar radial flow results under the SAR latitude and longitude grid. (3) SAR radial flow calibration: Based on the fact that the radial flow velocity of the ground wave radar is the projection of the ocean current vector on the radar beam direction, the equation between the radial flow and the vector flow of the ground wave radar is established. The initial single-station ground wave radar vector flow is obtained by solving the equation system using the least squares method. Quality assessment is required to select data points with high confidence in the single-station ground wave radar vector flow and remove unreliable points. By projecting the data with high confidence in the vector flow inversion results of a single-station ground wave radar onto the SAR range direction, the projected radial component of the ground wave radar is obtained. The mean difference between the projected radial component of the ground wave radar and the SAR radial velocity obtained in step (1) is calculated, and this mean value is used as a correction amount to be subtracted from the SAR radial velocity to obtain the calibrated SAR velocity. (4) Joint vector flow inversion: By unifying the ground wave radar radial flow matched in step (2) and the calibrated SAR radial flow obtained in step (3) under the stream function framework, the velocity components are solved by Taylor series expansion, and finally the vector flow field is synthesized.

2. The vector flow inversion method combining single-station ground wave radar and spaceborne SAR according to claim 1, characterized in that, The radial flow inversion of ground wave radar and SAR radial flow inversion described in step (1): Ground wave radar sea surface radial flow The calculation formula is as follows: ,in, This represents the difference between the actual first-order Bragg peak frequency and the theoretical frequency. This refers to the wavelength of the electromagnetic wave emitted by the radar; in the SAR method of inverting radial flow using Doppler centroid anomalies, the Doppler frequency shift in the SAR echo is... It can be represented as: ,in, The frequency shift, or predicted Doppler, caused by the relative motion between the SAR platform and the Earth's surface can be calculated using satellite orbit and attitude data. The frequency shift, indicating that it is caused by sea surface motion, is the result of the combined effects of ocean currents, wind fields, and ocean waves. This indicates the frequency shift in electromagnetic pointing error caused by the satellite platform or antenna attitude being affected by seasonal atmospheric effects and solar radiation, resulting in the antenna pointing deviating from the theoretical direction. This indicates the frequency shift caused by inaccurate estimations and unknown biases of non-geophysical terms; since the OCN product data of Sentinel-1 SAR directly provides the Doppler frequency shift. And predicting Doppler Therefore, no additional calculations are required; while the electromagnetic pointing error frequency shift and unknown deviation Land correction is required to remove the frequency shift; the Doppler frequency shift at the sea surface is calculated after removing the above frequency shift. At this point, it is necessary to use the Earth Empirical Physical Model (CDOP) to remove the Doppler shift of wind and waves. The residual Doppler frequency shift is generated by ocean current motion, and the CDOP model function is: ,in, These represent the wind speed and direction at a depth of 10 meters above the sea surface, respectively. Indicates the incident angle of the SAR. Indicates the polarization mode of SAR; the inverted radial velocity is: , ,in, Radial flow velocity, For SAR radar electromagnetic wave number, This is the Doppler shift caused by ocean currents.

3. The vector flow inversion method combining single-station ground wave radar and spaceborne SAR according to claim 1, characterized in that, The spatial registration described in step (2): for the SAR latitude and longitude points to be matched The four ground wave radar range-beam coordinate system latitude and longitude points surrounding it are respectively , , , The corresponding radial flow velocity values ​​are respectively , , , Bilinear interpolation is used to calculate the radial flow value. The formula is: Finally, the radial flow results of the ground wave radar under the SAR latitude and longitude grid were obtained.

4. The vector flow inversion method combining single-station ground wave radar and spaceborne SAR according to claim 1, characterized in that, The SAR radial flow calibration described in step (3): The formula for the radial flow velocity of ground wave radar as the projection of the ocean current vector onto the radar beam direction is: ,in, The radial velocity of the ground wave radar. The azimuth angle of the radial flow of the ground wave radar. For vector flow, The direction of the vector flow is given; the formula for filtering the confidence level of the single-station vector flow results is: ,in, For radial velocity residual, For the eastward velocity component of the vector flow, For the northward velocity component of the vector flow, The value is 1 if the condition is met, and 0 otherwise. This represents the standard deviation of the flow direction within a 3x3 grid window. The confidence level results are used; finally, grid points with a confidence level greater than 0.9 are selected to effectively extract the reliable part of the inversion results; the calibrated SAR radial velocity formula is: ,in, This indicates the calibrated flow rate. This represents the initial radial velocity obtained using the DCA method. This represents the portion of the vector flow from a high-confidence single-station ground wave radar. Indicates the number of valid data points. This represents the angle between the ground wave radar vector flow and the SAR range direction.

5. The vector flow inversion method combining single-station ground wave radar and spaceborne SAR according to claim 1, characterized in that, The joint vector flow inversion described in step (4): Stream function Defined as , For the ground wave radar radial flow matched in the overlapping region and the calibrated SAR radial flow, The relationship between the radial velocity and any observation point can be expressed, and the stream function can be expanded using Taylor series: , ,in, for powers of, for powers of, It corresponds The coefficient of the term, Let be the highest order of the Taylor series, set to 3; Including ground-wave radar and SAR radial flow at the same observation point, a linear equation can be established for each observation point. Finally, the overdetermined linear equation system is solved using the least squares method to obtain the stream function coefficients, from which the flow can be calculated. and Components, calculate the vector flow: , ,in, The vector flow velocity obtained from the joint inversion, The direction of the vector flow obtained from the joint inversion.

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