A Method for Reconstructing Sea Surface Vector Currents by Spaceborne Synthetic Aperture Radar

Abstract

The present application discloses a method for reconstructing the sea surface vector current from a spaceborne synthetic aperture radar, which relates to the field of ocean current observation. The method includes: acquiring a synthetic aperture radar image of a research area; determining the sea surface radial velocity based on the synthetic aperture radar image; determining a preliminary azimuthal velocity based on the geostrophic approximation theory and the sea surface radial velocity; performing non-geostrophic correction on the preliminary azimuthal velocity to obtain the final azimuthal velocity; and reconstructing the sea surface vector current field according to the sea surface radial velocity and the final azimuthal velocity to obtain the magnitude and direction of the velocity of the reconstructed sea surface vector current field. The present application can accurately obtain the sea surface vector current field from a single-antenna spaceborne synthetic aperture radar and improve the accuracy of the sea surface vector current field.

Description

Technical Field

[0001] The present application relates to the field of ocean current observation, and particularly to a method for reconstructing sea surface vector current based on geostrophic approximation and non-geostrophic correction for spaceborne synthetic aperture radar. Background Art

[0002] Ocean currents are complex systems composed of multiple dynamic elements, including geostrophic currents, wind-driven Ekman currents, Stokes drifts caused by waves, inertial motions, and tidal currents, etc. These ocean currents play a crucial role in the global redistribution and exchange of ocean heat, salt, nutrients, carbon sinks, and fresh water. They have a significant impact on ocean dynamic phenomena. In addition, in-depth research on ocean currents has great application value and practical significance for optimizing ship route planning, accurately developing fishery resources, efficiently implementing maritime search and rescue operations, and effectively tracking the diffusion path of pollutants.

[0003] With the rapid development of satellite remote sensing technology, satellites equipped with altimeters have become important tools for mapping the global sea surface current field. Spaceborne synthetic aperture radar (SAR) has the ability to detect the global ocean all day and all weather, and can provide sea surface Doppler shift data with high resolution (1 km) and wide coverage (500 km). However, most currently orbiting SAR satellites are equipped with a single antenna, so they can only obtain one component of the ocean surface current, that is, the radial velocity along the antenna direction, and cannot directly measure the azimuthal velocity along the satellite flight direction. However, in oceanographic research and applications, the sea surface velocity vector is a more ideal data form. Therefore, how to reconstruct the sea surface velocity vector from a single-antenna SAR is a key technical issue for improving the accuracy of numerical model forecasts. Summary of the Invention

[0004] The purpose of the present application is to provide a method for reconstructing sea surface vector current of spaceborne synthetic aperture radar, which can accurately obtain the sea surface vector current field from a single-antenna SAR and improve the accuracy of the sea surface vector current field.

[0005] To achieve the above purpose, the present application provides the following solutions:

[0006] The present application provides a method for reconstructing sea surface vector current of spaceborne synthetic aperture radar, including:

[0007] Obtain a synthetic aperture radar image of the research area;

[0008] Determine the sea surface radial velocity according to the synthetic aperture radar image;

[0009] Based on the geostrophic approximation theory and the sea surface radial velocity, determine the preliminary azimuthal velocity;

[0010] Perform non-geostrophic correction on the preliminary along-track velocity to obtain the final along-track velocity;

[0011] According to the sea surface radial velocity and the final along-track velocity, reconstruct the sea surface vector flow field to obtain the velocity magnitude and direction of the reconstructed sea surface vector flow field.

[0012] According to the specific embodiments provided in this application, this application has the following technical effects:

[0013] This application provides a method for reconstructing the sea surface vector flow using spaceborne synthetic aperture radar. It makes full use of the physical oceanography theory. Based on the geostrophic approximation theory and the sea surface radial velocity, the preliminary along-track velocity is first determined, and then non-geostrophic correction is performed on the preliminary along-track velocity to improve the accuracy of the along-track velocity. Further, according to the sea surface radial velocity and the final along-track velocity, the sea surface vector flow field is reconstructed, so that the sea surface vector flow field can be accurately obtained from a single-antenna SAR, and the accuracy of the sea surface vector flow field is improved. Description of the Drawings

[0014] In order to more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the following will briefly introduce the drawings required in the embodiments. Obviously, the drawings in the following description are only some embodiments of this application. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0015] Figure 1 It is an application environment diagram of a method for reconstructing the sea surface vector flow using spaceborne synthetic aperture radar in an embodiment of this application.

[0016] Figure 2 It is a flowchart of a method for reconstructing the sea surface vector flow using spaceborne synthetic aperture radar provided in an embodiment of this application.

[0017] Figure 3 It is a functional module diagram of a device for reconstructing the sea surface vector flow using spaceborne synthetic aperture radar provided in an embodiment of this application.

[0018] Figure 4 It is a structural diagram of a computer device provided in an embodiment of this application. Detailed Embodiments

[0019] The following will clearly and completely describe the technical solutions in the embodiments of this application with reference to the drawings in the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, rather than all embodiments. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of this application.

[0020] To make the above objects, features, and advantages of the present application more apparent and understandable, the present application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0021] The method for reconstructing the sea surface vector current of a spaceborne synthetic aperture radar provided by the embodiments of the present application can be applied to an application environment as Figure 1 shown. Among them, the terminal 102 communicates with the server 104 through a network. The data storage system can store the data that the server 104 needs to process. The data storage system can be set up separately, integrated on the server 104, or placed on the cloud or other servers. The terminal 102 can send the synthetic aperture radar image of the research area to the server 104. After receiving the synthetic aperture radar image of the research area, the server 104 reconstructs the sea surface vector current field. The server 104 can feedback the reconstructed sea surface vector current field to the terminal 102. In addition, in some embodiments, the method for reconstructing the sea surface vector current of a spaceborne synthetic aperture radar can also be implemented independently by the server 104 or the terminal 102.

[0022] Among them, the terminal 102 can be, but is not limited to, various desktop computers, laptop computers, smartphones, tablets, Internet of Things devices, and portable wearable devices. The Internet of Things devices can be smart speakers, smart TVs, smart air conditioners, smart in-vehicle devices, etc. The portable wearable devices can be smart watches, smart bracelets, head-mounted devices, etc. The server 104 can be implemented by an independent server or a server cluster composed of multiple servers, and can also be a cloud server.

[0023] In an exemplary embodiment, as Figure 2 shown, a method for reconstructing the sea surface vector current of a spaceborne synthetic aperture radar is provided. This method is executed by a computer device, and can specifically be executed independently by a computer device such as a terminal or a server, or jointly executed by the terminal and the server. In the embodiments of the present application, taking this method applied to Figure 1 the server 104 as an example for illustration, it includes the following steps 201 to step 205.

[0024] Step 201, obtain the synthetic aperture radar image of the research area. The synthetic aperture radar image includes the Doppler shift of the ocean surface.

[0025] In a specific application example, the Doppler shift of the ocean surface is measured by a spaceborne synthetic aperture radar to generate a synthetic aperture radar image.

[0026] Step 202, determine the sea surface radial velocity according to the synthetic aperture radar image.

[0027] In a specific application example, the Doppler shift observed by the radar includes a non - geophysical Doppler shift and a geophysical Doppler shift. Among them, the geophysical Doppler shift is caused by ocean currents and wave motions. In order to extract the Doppler shift caused by ocean currents , the non - geophysical Doppler shift and the Doppler shift caused by wave motions in the Doppler shift of the ocean surface are corrected to obtain the Doppler shift caused by ocean currents. For the non - geophysical Doppler shift, the method of combining non - zero Doppler shifts on land with linear fitting can be used for correction; for the Doppler shift caused by wave motions, a semi - empirical Doppler velocity model can be used for correction.

[0028] According to the Doppler shift caused by ocean currents, the formula is used to determine the sea - surface radial velocity. Among them, is the sea - surface radial velocity, the spatial resolution is 1 km, is the Doppler shift caused by ocean currents, is the wave number of the incident electromagnetic wave, is the radar incident angle, and are both known parameters.

[0029] Step 203: Based on the geostrophic approximation theory and the sea - surface radial velocity, determine the preliminary azimuthal velocity.

[0030] In a specific application example, assume that the sea - surface current is quasi - geostrophic and the surface current is the geostrophic current. Since the geostrophic current is non - divergent ( ), and the sea - surface radial velocity has been obtained in step 202, the following formula (1) can be used to determine the preliminary azimuthal velocity.

[0031] (1)

[0032] Among them, is the preliminary azimuthal velocity, is the azimuthal velocity at the ocean lower boundary in the synthetic aperture radar image, is the y axis coordinate at the ocean lower boundary in the synthetic aperture radar image, is the sea - surface radial velocity, and are the coordinates in the rectangular coordinate system, The axis represents the radial direction, that is, the antenna pointing direction, and the

[0033] Specifically, the following formula (2) can be used to obtain the preliminary azimuthal velocity at the position.

[0034] (2)

[0035] Wherein, is the preliminary along-track velocity at the position, and is the along-track velocity at the ocean lower boundary in the synthetic aperture radar image. This unknown quantity can be obtained through the Copernicus Marine Environment Monitoring Service (CMEMS) global high-resolution real-time forecasting system. is the sea surface radial velocity at the

[0036] Step 204: Perform non-geostrophic correction on the preliminary along-track velocity to obtain the final along-track velocity.

[0037] In a specific application example, the ocean surface current is not a strictly geostrophic current and also contains a non-geostrophic component. Therefore, it is necessary to calculate the non-geostrophic flow part to compensate for the preliminary along-track velocity. Step 204 includes the following Steps 41 and 42.

[0038] Step 41: Determine the non-geostrophic flow according to the preliminary along-track velocity by using the surface horizontal momentum balance equation. Its expressions are as shown in Formula (3) and Formula (4).

[0039] (3)

[0040] (4)

[0041] Wherein, is the final along-track velocity, is time, is the Coriolis force, is the gravitational acceleration, is the sea surface height, is the turbulent eddy viscosity coefficient, is the ocean depth.

[0042] By calculating the curl of Formula (3) and Formula (4), the vertical component of the sea surface current vorticity can be obtained, as shown in Formula (5).

[0043] (5)

[0044] Wherein, is the vorticity of the final sea surface current, , is the divergence of the sea surface current, .

[0045] It should be noted that the right side of Equation (3), the right side of Equation (4), and the right side of Equation (5) represent wind-driven currents. The speed of wind-driven currents is approximately 2% of the wind speed. Therefore, a wind speed of 5 m / s generates a flow velocity of approximately 0.1 m / s. The magnitude of mesoscale ocean currents is about 1 - 2 m / s. Therefore, the magnitude of wind-driven currents is very small, so the influence of wind-driven currents is not considered in this application.

[0046] Assume that the ageostrophic current is much smaller than the geostrophic current velocity. Therefore, Equation (5) is linearized to obtain the expression form of Equation (6).

[0047] (6)

[0048] where is the ageostrophic current, is the vorticity of the sea surface current, . Substituting Equation (2) into yields Equation (7).

[0049] (7)

[0050] When estimating the ageostrophic current using Equation (6) and Equation (7), information on the change of vorticity with time is required. However, since SAR observations are instantaneous, this information cannot be obtained. This application uses a mediation parameter to replace the right side term of Equation (6). Among them, is the vorticity of the sea surface current at the position, and

[0051] (8)

[0052] To eliminate the time mediation parameter , combining Equation (2) and Equation (8), the expression related to is as shown in Equation (9).

[0053] (9)

[0054] where is the azimuthal flow velocity at the upper boundary of the ocean in the synthetic aperture radar image. This quantity can be calculated by combining the flow direction of the coastal current with the sea surface radial velocity , is the azimuthal flow velocity at the lower boundary of the ocean in the synthetic aperture radar image, provided by CMEMS, is at the upper boundary of the ocean in the synthetic aperture radar image yAxis coordinates.

[0055] Substitute Equation (9) into Equation (8) to obtain the final expression of the ageostrophic current as shown in Equation (10).

[0056] (10)

[0057] Step 42: Determine the final along-track velocity based on the preliminary along-track velocity and the ageostrophic current.

[0058] In a specific application example, the following Equation (11) is used to determine the final along-track velocity.

[0059] (11)

[0060] Where is the final along-track velocity, is the preliminary along-track velocity, is the ageostrophic current.

[0061] Step 205: Reconstruct the sea surface vector current field based on the sea surface radial velocity and the final along-track velocity to obtain the velocity magnitude and direction of the reconstructed sea surface vector current field.

[0062] In a specific application example, project the sea surface radial velocity and the final along-track velocity onto the east-west vertical coordinate system and the north-south vertical coordinate system respectively to obtain the east-west component and the north-south component of the sea surface current.

[0063] Based on the east-west component and the north-south component, use Equation and to reconstruct the sea surface vector current field to obtain the velocity magnitude and direction of the reconstructed sea surface vector current field; where is the velocity magnitude of the reconstructed sea surface vector current field, is the direction of the reconstructed sea surface vector current field, is the east-west component, is the north-south component.

[0064] The sea surface vector current field reconstructed from the sea surface radial velocity observed by SAR can be used by ocean forecasting operational units and relevant departments such as satellite remote sensing.

[0065] This application first estimates the preliminary along-track velocity based on the geostrophic assumption of mesoscale ocean currents (i.e., the divergence of the ocean current is zero) by solving the divergence equation using the known sea surface radial velocity. Then, the preliminary along-track velocity is corrected for ageostrophy using the surface horizontal momentum equation to obtain a more accurate along-track velocity. Finally, the sea surface radial velocity and the final along-track velocity are vectorially combined to obtain a complete sea surface vector current field.

[0066] This application makes full use of physical oceanography theory. By neglecting terms with smaller magnitudes and using mediation parameters to simplify the equations, a calculation formula for the azimuthal velocity is obtained. This formula combines geostrophic approximation and non-geostrophic correction. It should be noted that relying solely on the preliminary azimuthal velocity results in many inaccurate ocean current estimations, such as underestimation in high-value areas and overestimation in low-value areas. However, after non-geostrophic correction, the results are significantly improved, and the root mean square error is reduced to approximately 0.2 m / s. Through the synergistic effect of step 203 and step 204, it is possible to accurately obtain the sea surface vector current field from a single-antenna SAR, and the spatial resolution of this current field can reach 1 km. The method provided by this application provides more accurate and comprehensive ocean current data support for ocean circulation research, environmental monitoring, disaster warning, and improving the prediction accuracy of ocean numerical models.

[0067] Based on the same inventive concept, an embodiment of this application also provides a spaceborne synthetic aperture radar sea surface vector current reconstruction device for implementing the above-mentioned spaceborne synthetic aperture radar sea surface vector current reconstruction method. The solution provided by this device for solving problems is similar to the solution described in the above method. Therefore, the specific limitations in one or more embodiments of the spaceborne synthetic aperture radar sea surface vector current reconstruction device provided below can refer to the limitations on the spaceborne synthetic aperture radar sea surface vector current reconstruction method in the above text, and will not be elaborated here.

[0068] In an exemplary embodiment, as Figure 3 shown, a spaceborne synthetic aperture radar sea surface vector current reconstruction device is provided, including: an image acquisition module 301, a radial current determination module 302, an azimuthal current initial guess module 303, an azimuthal current correction module 304, and a vector current reconstruction module 305.

[0069] The image acquisition module 301 is used to acquire a synthetic aperture radar image of the research area.

[0070] The radial current determination module 302 is used to determine the sea surface radial velocity according to the synthetic aperture radar image.

[0071] The azimuthal current initial guess module 303 is used to determine a preliminary azimuthal velocity based on the geostrophic approximation theory and the sea surface radial velocity.

[0072] The azimuthal current correction module 304 is used to perform non-geostrophic correction on the preliminary azimuthal velocity to obtain the final azimuthal velocity.

[0073] The vector current reconstruction module 305 is used to reconstruct the sea surface vector current field according to the sea surface radial velocity and the final azimuthal velocity, and obtain the magnitude and direction of the velocity of the reconstructed sea surface vector current field.

[0074] In an exemplary embodiment, a computer device is provided. The computer device can be a server or a terminal, and its internal structure diagram can be as shown in Figure 4 shown. The computer device includes a processor, a memory, an input / output interface (Input / Output, abbreviated as I / O), and a communication interface. Among them, the processor, the memory, and the input / output interface are connected through a system bus, and the communication interface is connected to the system bus through the input / output interface. Among them, the processor of the computer device is used to provide computing and control capabilities. The memory of the computer device includes a non-volatile storage medium and an internal memory. The non-volatile storage medium stores an operating system, a computer program, and a database. The internal memory provides an environment for the operation of the operating system and the computer program in the non-volatile storage medium. The database of the computer device is used to store synthetic aperture radar images of the research area. The input / output interface of the computer device is used to exchange information between the processor and external devices. The communication interface of the computer device is used to communicate with an external terminal through a network connection. When the computer program is executed by the processor, it implements a method for reconstructing sea surface vector flow of spaceborne synthetic aperture radar.

[0075] Those skilled in the art can understand that Figure 4 the structure shown in is only a block diagram of some structures related to the solution of the present application, and does not constitute a limitation on the computer device to which the solution of the present application is applied. The specific computer device may include more or fewer components than those shown in the figure, or combine certain components, or have different component arrangements.

[0076] In an exemplary embodiment, a computer-readable storage medium is provided, storing a computer program, and when the computer program is executed by a processor, the steps in the above method embodiments are implemented.

[0077] In an exemplary embodiment, a computer program product is provided, including a computer program, and when the computer program is executed by a processor, the steps in the above method embodiments are implemented.

[0078] It should be noted that the user information (including but not limited to user device information, user personal information, etc.) and data (including but not limited to data for analysis, stored data, displayed data, etc.) involved in the present application are all information and data authorized by the user or fully authorized by all parties, and the collection, use, and processing of relevant data need to comply with relevant regulations.

[0079] In the present application, all actions of obtaining signals, information, or data are carried out on the premise of complying with the corresponding data protection regulations and policies of the country where it is located and obtaining authorization from the owner of the corresponding device.

[0080] Those of ordinary skill in the art can understand that all or part of the processes in the methods of the above embodiments can be completed by instructing relevant hardware through a computer program. The computer program can be stored in a non-volatile computer-readable storage medium. When the computer program is executed, it can include the processes of the embodiments of the above methods. Among them, any reference to a memory, database, or other medium used in the embodiments provided in the present application can include at least one of non-volatile and volatile memories. Non-volatile memories can include read-only memory (ROM), magnetic tapes, floppy disks, flash memories, optical memories, high-density embedded non-volatile memories, resistive random access memories (ReRAM), magnetoresistive random access memories (MRAM), ferroelectric random access memories (FRAM), phase change memories (PCM), graphene memories, etc. Volatile memories can include random access memory (RAM) or external cache memories, etc. By way of illustration and not limitation, RAM can be in various forms, such as static random access memory (SRAM) or dynamic random access memory (DRAM), etc.

[0081] The databases involved in the embodiments provided in the present application can include at least one of relational databases and non-relational databases. Non-relational databases can include distributed databases based on blockchain, etc., without limitation. The processors involved in the embodiments provided in the present application can be general-purpose processors, central processors, graphics processors, digital signal processors, programmable logics, data processing logics based on quantum computing, etc., without limitation.

[0082] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered to be within the scope described in this specification.

[0083] Specific examples are used in this article to elaborate on the principles and implementation manners of the present application. The description of the above embodiments is only used to help understand the method and its core idea of the present application; at the same time, for those of ordinary skill in the art, according to the idea of the present application, there will be changes in the specific implementation manners and application scopes. In summary, the content of this specification should not be construed as a limitation to the present application.

Claims

1. A method for reconstructing sea surface vector flow from a spaceborne synthetic aperture radar, characterized in that: The spaceborne synthetic aperture radar sea surface vector flow reconstruction method comprises: Acquire synthetic aperture radar images of the study area; determining the sea surface radial velocity according to the synthetic aperture radar image; Based on the geostrophic approximation theory and the sea surface radial velocity, determine the preliminary azimuthal velocity; The preliminary azimuthal flow velocity is subjected to non-geostrophic correction to obtain a final azimuthal flow velocity; the preliminary azimuthal flow velocity is subjected to non-geostrophic correction to obtain a final azimuthal flow velocity, specifically comprising: determining a non-geostrophic flow according to the preliminary azimuthal flow velocity by using a surface horizontal momentum equation; determining a final azimuthal flow velocity according to the preliminary azimuthal flow velocity and the non-geostrophic flow; and determining the non-geostrophic flow by using the following formula: ； in, It is a non-geostrophic flow. is the azimuthal velocity at the upper boundary of the ocean in the synthetic aperture radar image, is the azimuthal velocity at the lower boundary of the ocean in the synthetic aperture radar image, is the upper boundary of the ocean in the synthetic aperture radar image y Axis coordinates, is the lower edge of the ocean in the synthetic aperture radar image y Axis coordinates, is the radial velocity at the sea surface, is the vorticity of the sea surface current, , is the lower boundary of the ocean in the synthetic aperture radar image The azimuthal flow velocity at the location, for The radial velocity of the sea surface at the location, and are coordinates in the rectangular coordinate system, The axis represents the radial direction, The axis represents the direction; The sea surface vector flow field is reconstructed according to the sea surface radial flow velocity and the final azimuthal flow velocity to obtain the flow velocity magnitude and flow direction of the reconstructed sea surface vector flow field.

2. The method for reconstructing sea surface vector flow of spaceborne synthetic aperture radar according to claim 1, characterized in that: The synthetic aperture radar image includes a Doppler shift of the ocean surface; Determining the sea surface radial velocity according to the synthetic aperture radar image specifically includes: The non-geophysical Doppler shift and the Doppler shift caused by wave motion in the Doppler shift of the ocean surface are corrected to obtain the Doppler shift caused by the ocean current; According to the Doppler frequency shift caused by the ocean current, the formula Determine the sea surface radial velocity; where, is the radial velocity at the sea surface, is the Doppler shift caused by the ocean current, is the wave number of the incident electromagnetic wave, is the radar incident angle.

3. The method for reconstructing sea surface vector flow of spaceborne synthetic aperture radar according to claim 1, characterized in that: The preliminary azimuthal flow velocity is determined using the following formula: ； in, is the initial azimuthal velocity, is the azimuthal velocity at the lower boundary of the ocean in the synthetic aperture radar image, is the lower edge of the ocean in the synthetic aperture radar image y Axis coordinates, is the radial velocity at the sea surface, and are coordinates in the rectangular coordinate system, The axis represents the radial direction, The axis represents the direction.

4. The method for reconstructing sea surface vector flow from a spaceborne synthetic aperture radar according to claim 1, characterized in that: The final azimuthal flow velocity is determined using the following formula: ； in, is the final azimuthal flow velocity, is the initial azimuthal velocity, It is a non-geostrophic flow.

5. The method for reconstructing sea surface vector flow of spaceborne synthetic aperture radar according to claim 1, characterized in that: Reconstructing the sea surface vector flow field according to the sea surface radial flow velocity and the final azimuthal flow velocity specifically includes: Projecting the sea surface radial velocity and the final azimuthal velocity into an east-west vertical coordinate system and a north-south vertical coordinate system, respectively, to obtain an east-west component and a north-south component of the sea surface current; Based on the east-west component and the north-south component, the formula and Reconstruct the sea surface vector flow field to obtain the velocity and direction of the reconstructed sea surface vector flow field; is the velocity of the reconstructed sea surface vector flow field, is the direction of the reconstructed sea surface vector flow field, is the east-west component, It is the north-south component.

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