Satellite-borne multi-beam interferometric imaging altimeter and oceanic full flow field measurement method

Abstract

This application discloses a spaceborne multibeam interferometric imaging altimeter and a method for measuring the entire ocean current field, belonging to the field of spaceborne microwave remote sensing technology. Based on an interferometric imaging altimeter, this spaceborne multibeam interferometric imaging altimeter maintains a dual-antenna cross-track baseline configuration. In addition to the original front-viewing side-viewing beam pair, it adds two pairs of forward and backward-viewing beams. Using oblique-viewing interferometry, an equivalent in-track baseline is formed, and the ocean current velocity components in two different directions are measured to synthesize the sea surface current field vector. Simultaneously, the sea surface height measured by the front-viewing side-viewing interferometric beams is used to calculate small- and medium-scale eddies and geostrophic currents. This enables the measurement of the entire ocean current field, including sea surface currents, small- and medium-scale eddies, and geostrophic currents. The advantages of this application are: it enables high-resolution, high-precision simultaneous observation of sea surface current vectors and sea surface height, thereby achieving the measurement of the entire ocean current field, including sea surface currents, small- and medium-scale eddies, and geostrophic currents, providing novel observational data for ocean dynamic environment research.

Description

Technical Field

[0001] This invention belongs to the field of spaceborne microwave remote sensing technology, specifically relating to a spaceborne multibeam interferometric imaging altimeter and a method for measuring the entire ocean flow field. Background Technology

[0002] Ocean currents, as one of the main forms of seawater movement, provide crucial information on ocean dynamic parameters, including their velocity and direction, and are key data for marine scientific research. Conducting ocean current observations is of paramount importance for numerical oceanography forecasting, marine ecological protection, maritime navigation, marine fisheries production, and even global climate change research.

[0003] Currently, the primary spaceborne microwave remote sensing method for measuring global ocean current fields utilizes sea surface height data measured by satellite altimeters to calculate geostrophic currents. The new generation of altimeters developed in recent years—interferometric imaging altimeters—can also be used to calculate small- and medium-scale eddies due to their ability to achieve high-resolution, wide-swath sea surface height measurements. However, no satellites currently in orbit are conducting direct observations of global ocean current fields. The shift from indirect inversion to direct detection represents the future trend of satellite-based current field observations, which depends on the development of new payloads. There are two technical approaches to direct ocean current detection: in-orbit interferometry and Doppler centroid migration. In-orbit interferometry is less affected by wind speed and offers higher spatial resolution and accuracy. Several satellites have already conducted experiments on in-orbit interferometry for ocean current measurements, such as the US's SRTM and Germany's TerraSAR. X / TanDEM X, my country's Gaofen-3, etc. The spacecraft launched in March 2023... Figure 1 The satellite also tested the simultaneous inversion of sea surface velocity and sea surface height. However, these satellites can only obtain the velocity component in the radar line-of-sight direction, meaning they can only measure one-dimensional flow fields.

[0004] Internationally, the main satellite programs conceived for ocean current vector measurement include the European Space Agency's Wavemill and SKIM projects, and NASA's WaCM. Wavemill aims to measure the two-dimensional ocean surface current field using in-orbit interferometry and two pairs of forward and backward slanted beams. The latter two employ Doppler technology. SKIM's payload, called a Ka-band Doppler radar, uses a bottom-looking beam and multiple near-nadir rotating beams to measure ocean surface current and wave fields. WaCM's payload is a single-beam Doppler scatterometer, using a pencil-beam rotating antenna to observe the sea surface from multiple azimuth angles, aiming to simultaneously measure sea surface current and wind vectors. These satellites are still in the planning stage and have not yet been formally implemented. Their common limitation is that they can only measure the sea surface current field and cannot obtain full-field information. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned technical defects and design a new type of spaceborne microwave remote sensor to achieve high-resolution and high-precision synchronous observation of sea surface current vectors and sea surface height, thereby realizing the measurement of the entire ocean flow field, including sea surface currents, small and medium-scale eddies, and geostrophic currents.

[0006] To achieve the above objectives, this invention proposes a spaceborne multibeam interferometric imaging altimeter. Based on the interferometric imaging altimeter, it adopts a dual-antenna cross-trajectory baseline configuration, with each antenna having three beams: front side-looking, forward oblique-looking, and rear oblique-looking. Among them, the front side-looking beam pair, the forward oblique-looking beam pair, and the rear oblique-looking beam pair respectively form interferometric measurements. The forward and backward squint beam pairs are used to form an equivalent in-orbit baseline using squint interferometry, and to measure the ocean current velocity components in two different directions, forward and backward, to synthesize the sea surface current field vector. The front-side looking beam pair is used to obtain sea level height through direct cross-track interferometry, and then to calculate small- and medium-scale eddies and geostrophic currents.

[0007] As an improvement to the aforementioned altimeter, the cross-trajectory baseline configuration is implemented using a single-satellite dual-antenna configuration or a dual-satellite formation configuration.

[0008] As an improvement to the aforementioned altimeter, the ground slant angles of the forward and backward slant beam pairs are ±45°, forming measurements in two orthogonal directions.

[0009] As an improvement to the aforementioned altimeter, the sea surface height obtained by interferometry of the front-view beam pair is used to calculate small- and medium-scale eddies and geostrophic currents, and also to calculate the cross-track interference phase in the oblique-view interferometry, thereby separating the oblique-view along-track interference phase from the mixed interference phase of the oblique-view, and then inverting the sea surface current velocity.

[0010] This application also provides a method for measuring the entire ocean current field, based on the aforementioned spaceborne multibeam interferometric imaging altimeter, including: Step 1: Perform interferometric imaging and height inversion processing on the ocean echo signal obtained from the front-side looking beam pair to obtain the sea surface height of the target area; Step 2: Perform interferometric imaging processing on the ocean echo signals obtained from the forward and backward squint beam pairs to obtain the mixed forward and backward squint interferometric phase; Step 3: Calculate the forward and backward oblique view interference phase using the sea surface height; Step 4: Remove the forward and backward squint interference phases from the forward and backward squint mixed interference phases to separate the forward and backward squint in-line interference phases; Step 5: Calculate the forward and backward sea surface velocity components from the forward and backward oblique-view along-orbit interference phases; Step 6: Combine the forward and backward sea surface velocity components into a sea surface flow field vector, which together with the small- and medium-scale eddies and geostrophic flows calculated from the sea surface height to form the full ocean flow field.

[0011] As an improvement to the above method, step 3 includes: Forward and backward oblique gaze intersection phase The calculation formula is: ; in, The operating wavelength of the altimeter. h Sea level r The slant range between the main antenna and the target; H This refers to the satellite's orbital altitude. B The physical baseline length between the two antennas; α This is a ground-level oblique perspective.

[0012] As an improvement to the above method, step 5 includes: The forward and backward sea surface velocity components The calculation formula is: ; in, V For satellite speed; The forward and backward oblique-looking along-orbit interference phases; θ The downward angle of view of the oblique beam in a plane perpendicular to the flight path; γ The angle of view of the beam; β The downward angle of the oblique beam within the oblique plane.

[0013] The advantages of this application are: It can achieve high-resolution and high-precision synchronous observation of sea surface current vectors and sea surface height, thereby enabling the measurement of the entire ocean flow field, including sea surface currents, small and medium-scale eddies, and geostrophic flows, providing new observational data for marine dynamic environment research. Attached Figure Description

[0014] Figure 1 The diagram shows the beam pattern of a spaceborne multibeam interferometric imaging altimeter. Figure 2 The figure shown is a schematic diagram of the oblique observation geometry of a spaceborne multibeam interferometric imaging altimeter. Figure 3 The diagram shown is a flowchart of the ocean full flow field inversion. Figure 4 The image shows a schematic diagram of the simulated sea surface current field vector in the simulation example; Figure 5 The figure shows the forward velocity component obtained by forward-looking beam inversion in a simulation example; Figure 6 The figure shows the backward velocity component obtained by backsight beam inversion in a simulation example; Figure 7 The figure shows the sea surface current field vector obtained by inversion in the simulation example. Detailed Implementation

[0015] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0016] The basic working principle of existing interferometric imaging altimeters is to acquire highly coherent sea surface echoes by incidenting the sea surface at a small angle using dual antennas. High-precision interferometric phases are obtained through echo processing. Based on the interferometric phases, the geometric relationship between the antenna phase center and the measurement point on the sea surface is accurately recovered, thereby determining the sea surface height. The antenna beams are emitted in the frontal side-looking direction, and the interferometric baseline formed by the two antennas is in the cross-track direction; this method can be simply referred to as frontal side-looking cross-track interferometry.

[0017] Example 1 The spaceborne multibeam interferometric imaging altimeter proposed in this invention can be summarized as follows: frontal side-looking cross-track interferometry + forward oblique-looking cross-track interferometry + rear oblique-looking cross-track interferometry. Its two antennas are oriented sideways towards the sea surface for observation, with the interferometric baseline along the cross-track direction. The antennas can transmit / receive beams in multiple directions: frontal side-looking beam, forward oblique-looking beam, and rear oblique-looking beam. A schematic diagram of the multibeam system is shown below. Figure 1 This invention utilizes oblique-looking interferometry to form an equivalent in-orbit baseline based on the cross-orbit baseline, enabling in-orbit interferometry measurement of ocean current velocity. The observation geometry of oblique-looking cross-orbit interferometry is as follows: Figure 2 As shown, taking a forward slant view as an example. In the diagram, the X-axis of the coordinate system is perpendicular to the satellite's orbit, the Y-axis is parallel to the satellite's flight direction, the Z-axis is the altitude direction, O is the origin of the coordinate system, and the XOY plane represents the ground. The satellite's orbital altitude is... H The physical baseline length between antenna 1 and antenna 2 is B, and its direction is parallel to the X-axis; the slant distance between the main antenna and the target is... r The angle of the beam is γ The ground oblique angle is α The downward angle of the slanted beam in the plane perpendicular to the flight path is: θ The downward angle within the oblique plane is β Based on the triangular geometric relationships in the diagram, we can deduce... θ , α , β , γ The relationship between these angles is as follows: , .

[0018] The oblique interference of the crossing baseline can simultaneously generate an equivalent crossing baseline component and an equivalent in-line baseline component. The length of the equivalent crossing baseline can be derived based on the geometric projection relationship. and equivalent in-line baseline length They are respectively: (1) (2) As shown in equation (2), after two projections from the lower and oblique angles, the equivalent in-orbit baseline length will be much shorter than the physical baseline length. Therefore, in practice, a longer physical baseline is required to ensure the accuracy of ocean current velocity measurement. Alternatively, two antennas can be placed on two satellites respectively, forming a long interferometric baseline in a two-satellite formation.

[0019] The phase obtained from oblique-looking interferometry is a mixed phase, which includes both oblique-looking cross-track interferometry phase and oblique-looking along-track interferometry phase. The oblique-looking cross-track interferometry phase is related to sea surface height, while the oblique-looking along-track interferometry phase is related to sea surface current velocity. Derivation of the oblique-looking cross-track interferometry phase. and strabismus along the orbital interference phase The expression is as follows: (3) (4) In the formula The operating wavelength of the altimeter. h Sea level V For satellite speed, This represents the radial velocity of the ocean current (i.e., the velocity component of the actual ocean current velocity in the oblique line of sight).

[0020] In the case of only oblique-looking interferometry, it is impossible to extract the oblique-looking along-track interferometry phase from the oblique-looking mixed interferometry phase. However, multi-beam interferometric imaging altimeter also has a frontal side-looking beam, which can invert the sea level height from the frontal side-looking interferometry phase. h Therefore, the oblique-view cross-track interference phase is calculated according to equation (3). Then, the oblique-view cross-track interference phase is subtracted from the oblique-view mixed interference phase to obtain the oblique-view along-track interference phase. Finally, the radial velocity of the ocean current can be calculated according to equation (4). (5) Furthermore, the radial flow velocity in the XOY plane can be calculated as follows: (6) The formulas for backward strabismus and forward strabismus are the same; you only need to... α and γSimply substitute the negative value. We call the radial velocity obtained from the forward-looking beam the forward velocity component, and the radial velocity obtained from the backward-looking beam the backward velocity component. Based on the one-dimensional flow fields in these two different directions, the two-dimensional sea surface flow field can be calculated. When the ground angles of the forward and backward beams are ±45°, we can obtain the velocity components in two orthogonal directions, which allows for better synthesis of the sea surface current vector.

[0021] Example 2 This application also provides a method for measuring the entire ocean current field, implemented based on the spaceborne multibeam interferometric imaging altimeter of Embodiment 1. The flowchart is shown below. Figure 3 It includes the following steps: 1. Perform interferometric imaging and height inversion processing on the ocean echo signals obtained from the front-side looking beam pair to obtain the sea surface height of the target area; 2. Perform interferometric imaging processing on the ocean echo signals obtained from the forward and backward squint beam pairs to obtain the forward squint hybrid interferometric phase and the backward squint hybrid interferometric phase; 3. Calculate the forward and backward oblique line interference phases according to equation (3); 4. Subtract the cross-track interference phase from the cross-track interference phase to obtain the along-track interference phase: 5. Calculate the forward and backward sea surface velocity components according to equation (6): 6. The forward and backward sea surface velocity components are combined to form the sea surface flow field vector, which together with the small and medium-scale eddies and geostrophic flows calculated from the sea surface height constitute the full ocean flow field.

[0022] The following is a simulation experiment of sea surface current field vector measurement using a multibeam interferometric imaging altimeter. The main simulation parameters are shown in Table 1.

[0023] Table 1 Main Simulation Parameters

[0024] Simulated sea surface current field vectors as follows Figure 4 As shown, the arrow length represents the flow velocity, and the arrow direction represents the flow direction. The flow velocity and direction are consistent throughout the region, at 0.5 m / s and 10° respectively. Based on the parameters, the equivalent cross-track baseline length of the oblique-looking interferometry is calculated to be 35.4 m, and the equivalent in-track baseline length is 1.5 m. The forward flow velocity obtained from the forward oblique-looking echo inversion is as follows... Figure 5 As shown, its direction is 45°, and its average value is 0.42 m / s. The backflow velocity obtained from the backsight echo inversion is as follows: Figure 6 As shown, its direction is -45° and its average value is 0.23 m / s. Figure 5 The forward velocity component shown is Figure 6The sea surface current field vector obtained by synthesizing the backward velocity components is shown in the figure. Figure 7 As shown, the average flow velocity is 0.48 m / s and the average flow direction is 16.6°.

[0025] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit it. Although this application has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of this application do not depart from the spirit and scope of the technical solutions of this application, and should all be covered within the scope of the claims of this application.

Claims

1. A spaceborne multibeam interferometric imaging altimeter for measuring the entire ocean current field, including sea surface currents, small- and medium-scale eddies, and geostrophic currents, characterized in that... Based on the interferometric imaging altimeter, a dual-antenna cross-trajectory baseline configuration is adopted, with each antenna having three beams: frontal side-looking, forward oblique-looking, and backward oblique-looking; among them, the frontal side-looking beam pair, the forward oblique-looking beam pair, and the backward oblique-looking beam pair respectively form interferometric measurements; The forward and backward squint beam pairs are used to form an equivalent in-orbit baseline using squint interferometry, and to measure the ocean current velocity components in two different directions, forward and backward, to synthesize the sea surface current field vector. The front-side looking beam pair is used to obtain sea surface height through direct cross-track interferometry. The obtained sea surface height is used to calculate small- and medium-scale eddies and geostrophic currents, as well as to calculate the cross-track interferometry phase in the oblique-looking interferometry, thereby separating the oblique-looking along-track interferometry phase from the mixed interferometry phase of the oblique-looking interferometry, and then inverting the sea surface current velocity.

2. The spaceborne multibeam interferometric imaging altimeter according to claim 1, characterized in that, The cross-trajectory baseline configuration is achieved by a single satellite with two antennas, or by a two-satellite formation.

3. The spaceborne multibeam interferometric imaging altimeter according to claim 1, characterized in that, The ground slant angles of the forward and backward slant beam pairs are ±45°, forming measurements in two orthogonal directions.

4. A method for measuring the entire ocean current field, implemented based on the spaceborne multibeam interferometric imaging altimeter described in any one of claims 1-3, comprising: Step 1: Perform interferometric imaging and height inversion processing on the ocean echo signal obtained from the front-side looking beam pair to obtain the sea surface height of the target area; Step 2: Perform interferometric imaging processing on the ocean echo signals obtained from the forward and backward squint beam pairs to obtain the mixed forward and backward squint interferometric phase; Step 3: Calculate the forward and backward oblique view interference phase using the sea surface height; Step 4: Remove the forward and backward squint interference phases from the forward and backward squint mixed interference phases to separate the forward and backward squint in-line interference phases; Step 5: Calculate the forward and backward sea surface velocity components from the forward and backward oblique-view along-orbit interference phases; Step 6: Combine the forward and backward sea surface velocity components into a sea surface current field vector, and combine it with the small- and medium-scale eddies and geostrophic flows calculated from the sea surface height to form the full ocean current field; Step 3 includes: Forward and backward oblique gaze intersection phase The calculation formula is: ； in, The operating wavelength of the altimeter. h Sea level r The slant range between the main antenna and the target; H This refers to the satellite's orbital altitude. B The physical baseline length between the two antennas; α This is a ground-level oblique perspective; Forward and backward oblique along-orbit interference phase The expression is: ； in, The radial velocity of the ocean current; θ The downward angle of view of the oblique beam in a plane perpendicular to the flight path; γ The angle of view of the beam; V For satellite speed; Step 5 includes: The formulas for calculating the forward and backward sea surface velocity components are as follows: ； in, β The downward angle of the oblique beam within the oblique plane.

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