Cross-rail strabismus equivalent along-rail interference flow measurement method

Abstract

The invention relates to a cross-rail squint equivalent along-rail interference flow measurement method. The method comprises the following steps: step 1, acquiring original echo data of a cross-rail double-antenna squint SAR (Synthetic Aperture Radar); step 2, carrying out imaging processing on the original echo data by adopting a nonlinear CS algorithm to obtain two SAR (Synthetic Aperture Radar) images; 3, performing high-precision image registration on the two SAR images, and performing interference processing to obtain an interference phase; step 4, carrying out flat-ground phase suppression; and 5, extracting the velocity of the flow field, and completing the cross-rail squint equivalent along-rail interference flow measurement. According to the invention, the height and speed information of the flow field can be measured at the same time, or the height and speed of the single flow field can be measured.

Description

Technical Field

[0001] This invention relates to a method for measuring flow in tandem with oblique line of sight, which belongs to the field of signal processing. Background Technology

[0002] The strategic importance of the ocean in modern society is increasingly prominent. As a vital environment covering approximately 75% of the Earth's surface, the ocean is not only a major regulator of climate change but also a crucial resource for human survival and development. Ocean observation and research, especially the velocity vector parameters and height parameters of sea surface currents, are crucial not only in civilian fields such as marine resource development, environmental monitoring, and maritime transportation. However, traditional ocean observation methods, due to their high cost and limited range, are insufficient to meet the demands of modern observation. In recent years, spaceborne interferometry synthetic aperture radar (InSAR) technology, with its all-weather, all-day, wide-range, and high-precision characteristics, has provided a completely new technical means for measuring ocean current velocity and height.

[0003] Along-track interferometric synthetic aperture radar (ATI SAR) uses two or more antennas positioned along the flight direction to acquire interferometric phase information from two observations of the same area within a very short time, thereby inverting parameters of moving targets. For example, in measuring sea surface currents, ATI-SAR can accurately extract the velocity vector of moving targets, providing direct observational data for ocean dynamics research. In 1987, Goldstein and Zebker first proposed ATI-SAR technology and successfully estimated the sea surface current field of San Francisco Bay using the L-band SAR system on NASA's CV990 aircraft. This technology revolutionized ocean dynamics observation. Across-track interferometric synthetic aperture radar (XTI SAR) uses two antennas positioned on a baseline perpendicular to the flight direction to acquire terrain height information using interferometric phase difference. The signal paths received by the two antennas differ, and this path difference is directly related to the terrain of the observation area. Therefore, using known interferometric system parameters, terrain height can be accurately inverted through phase difference. Compared to traditional ocean surface height measurement techniques (such as buoys and current meters), XTI-SAR can perform large-scale observations with higher spatial resolution and lower maintenance costs.

[0004] Geometric relationships of elevation information from spaceborne SAR cross-orbit interferometry, such as Figure 1As shown, and Represents two antennas. Represents the distance between the two antennas. Antenna to target point The distance. From this, we can see that the change in the line-of-sight angle caused by changes in elevation can be expressed as...

[0005] Therefore, with the slope distance remaining constant, the relationship of the interference phase change caused by the elevation change can be expressed as follows:

[0006] This leads to a commonly used index for describing the high sensitivity of the interference phase—the ambiguity height:

[0007] Blur height This refers to the phenomenon that causes interference phase to occur when the slant range remains constant. The phase change is relative to the height change. This index can be used to better characterize the sensitivity of the interferometric phase to changes in elevation.

[0008] Geometric relationships of flow field velocity information obtained by spaceborne SAR in-orbit interferometry, such as Figure 2 As shown, two pairs of antennas ( and The antennas are placed at certain intervals along the platform's flight path. When the two antennas image the same scene, there is a time difference between the two images, and the phase of the backscattered signal from the target point changes due to this time difference.

[0009] Phase difference It can be deduced as

[0010] In the formula, and Representing the orientation of the two images in the direction of the first The pixel and distance direction The complex value of a pixel. Represents complex conjugation. and These represent the total number of pixels in the azimuth and range directions of the image, respectively. Due to the low flow velocity, the corresponding interferometric phase is also small. For ATI SAR, the phase difference mainly stems from the path difference. This is because when two antennas observe a moving target in the same area successively, the target moves a certain distance within the time interval between the two radar detections, causing the distance between the target and the radar to change. Therefore, the arrival times of the two radar echoes are different, and the phases recorded are also different. This is the basic principle of interferometry.

[0011] Estimated line-of-sight (LOS) velocity of scattering points It can be represented as:

[0012] In the formula, The phase difference between the transmitting and receiving antennas. For microwave wavelengths, For radar baseline, Platform velocity. Radial surface velocity. It is the radial LOS velocity. Projection on the sea surface:

[0013] Since 2007, a series of SAR satellites have been launched internationally, such as TerraSAR-X, COSMO-SkyMed, Radarsat-2, SAR-Lupe, TanDEM-X, and RISAT-1. These satellites are characterized by their ability to acquire SAR images with resolutions up to 1 meter. Furthermore, some satellites (such as Radarsat-2) possess full polarization capabilities, while others form interferometric systems (TerraSAR-X and TanDEM-X). This marks a new era for SAR technology, characterized by high resolution. The combination of high resolution with polarization and interferometry in these satellite SAR data has brought new opportunities for marine applications. In 1987, Goldstein and Zebker first proposed ATI-SAR measurement technology. Through interferometry processing of images obtained using an L-band SAR system mounted on a NASA CV990 aircraft, they effectively estimated the ocean surface current field in San Francisco Bay, USA. Subsequently, scholars both domestically and internationally have conducted extensive research on airborne ATI SAR for estimating ocean surface current fields. However, the high cost of airborne SAR and its prevalence in military systems have hindered the further development of this method. Since 2000, the emergence of a series of interferometric SAR satellites has brought unprecedented opportunities for ATI-SAR to measure ocean currents, and many scholars abroad have also carried out a lot of research based on the data from these spaceborne interferometric SAR satellites.

[0014] In 2004, Robert Siegmund first proposed an airborne hybrid baseline mode for measuring high-resolution digital elevation maps of the Wadden Sea using a dual-antenna interferometric synthetic aperture radar system. He also demonstrated for the first time that a hybrid baseline mode combining cross-track and in-track elements could be used to measure both the height and velocity of the current field. Using the AeS-1 airborne InSAR system with dual SAR antennas, the baseline included both in-track and cross-track components. This was the first time a hybrid dual-antenna InSAR system (ATI+XTI) was used to simultaneously measure topographic height and ocean current velocity. However, it's important to note that this was under the assumption that the sea surface current state within the tidal channel remained almost unchanged during the time interval between two flights. This repeated flight method is not suitable for situations with drastic changes in sea surface current velocity. Furthermore, existing spaceborne ATI SAR and XTI SAR systems can only operate independently; XTI SAR can only perform elevation inversion, while ATI SAR can only perform current field inversion, and cannot simultaneously acquire both height and velocity information. In other words, it cannot invert current velocity information simultaneously while performing height field inversion. Summary of the Invention

[0015] The technical problem solved by this invention is to overcome the shortcomings of the prior art and provide a cross-track oblique-view equivalent in-track interferometric flow measurement method, which can simultaneously measure flow field height and velocity information, or measure flow field height and velocity individually. The technical solution adopted in this invention is: A method for measuring current along the track using equivalent oblique line-of-sight interferometry includes the following steps: Step 1: Acquire raw echo data of cross-track dual-antenna slant-look SAR; Step 2: Use the nonlinear CS algorithm to perform imaging processing on the raw echo data to obtain two SAR images; Step 3: Perform high-precision image registration on the two SAR images and perform interferometric processing to obtain the interferometric phase; Step 4: Perform flat-ground phase suppression; Step 5: Extract the flow field velocity and complete the equivalent in-track interferometric flow measurement by crossing the track and looking at it obliquely.

[0016] Furthermore, in step 3, high-precision image registration is performed on the two SAR images, and interferometric processing is performed to obtain the interferometric phase, specifically as follows: One SAR image is used as the main image and the other as the auxiliary image. An image matching metric is selected to calculate the offset between the main and auxiliary images. Then, the images are translated according to the offset so that pixels at the same position in the two images correspond to the same point in the observation area.

[0017] Furthermore, matching measures include: correlation coefficient, maximum interference spectrum, average ripple function, and phase least squares.

[0018] Furthermore, step 4 involves flat-ground phase suppression, specifically: using a geometric method or a frequency shift method to suppress flat-ground phase, calculating the slant range difference and constructing a flat-ground interference phase, and then subtracting the flat-ground interference phase from the registered interference phase to achieve the effect of removing flat-ground interference.

[0019] Furthermore, step 5, which extracts the flow field velocity, specifically involves inverting the interferometric phase to obtain the ocean current velocity distribution on the sea surface, based on the relationship model between the interferometric phase and the ocean current velocity, incident angle, and oblique angle.

[0020] The expression for the velocity of the ocean current from the line of sight to the sea surface is:

[0021] in, For the satellite's orbital velocity, For the in-orbit interference phase, radial surface velocity It is the radial LOS velocity. Projection on the sea surface:

[0022] in, It is a microwave wavelength. It's a bottom-down perspective.

[0023] Secondly, this invention proposes an equivalent in-track interferometric flow measurement system with oblique gaze at intersecting tracks, comprising: Data acquisition module: Acquires raw echo data from cross-track dual-antenna slant-look SAR; Imaging module: The nonlinear CS algorithm is used to process the raw echo data to obtain two SAR images; Registration module: Performs high-precision image registration on two SAR images and performs interferometric processing to obtain the interferometric phase; Phase suppression module: performs flat-ground phase suppression; Flow field velocity extraction module: Extracts flow field velocity and completes equivalent in-track interferometric flow measurement with oblique line of sight.

[0024] The advantages of this invention compared to the prior art are: This method is an interferometric flow measurement approach based on oblique viewing angles along the cross-track, unlike traditional methods that use in-line platforms. This method generates an equivalent in-line component by adding an oblique viewing angle to the cross-track interferometric platform to measure the flow field. This overcomes the limitation of flow field measurement being impossible on cross-track platforms, significantly improving the platform's usability. Furthermore, the proposed method allows for control of the length of the generated in-line component by adjusting the size of the oblique viewing angle, enabling flow field measurements under different sea states. Simulated flow field data processing results validate the effectiveness of the proposed method. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the interference phase change caused by the elevation change; Figure 2 This is a schematic diagram of the ATI SAR geometry. Figure 3 Diagram of the geometric relationship between the two antennas and their oblique-looking interference. Figure 4 This is a flowchart of the method of the present invention; Figure 5 This is a schematic diagram of the background flow field data at the sea surface. Figure 6 Here is the flowchart for the NCS algorithm; Figure 7 This is a schematic diagram of the imaging results from the NCS algorithm. Figure 8 This is a schematic diagram of interferometric registration; Figure 9 This is a schematic diagram of the registration results; Figure 10 A comparison chart of coherence coefficients before and after registration; Figure 11 This is a diagram showing the phase variation of the interference caused by the change in slant range; Figure 12 This is a schematic diagram of the calculated flat terrain phase. Figure 13 A schematic diagram of the interference phase after removing the flat phase; Figure 14 This is a schematic diagram of the flow field inversion results; Detailed Implementation The specific embodiments of the present invention will now be described in further detail with reference to the accompanying drawings.

[0026] This invention provides an equivalent in-orbit interferometric flow measurement method using a cross-track oblique-looking antenna beam. A dual-antenna XTI SAR utilizes the oblique-looking antenna beam to form an equivalent in-orbit baseline component, thus enabling interferometric measurement of sea surface current velocity. When the satellite operates in this mode, the SAR antenna beam needs to be obliquely looked. During normal operation, the satellite measures the altitude field using a normal side-looking antenna, while when flow field measurement is required, the antenna can be adjusted to an oblique-looking mode. This allows for flow field measurement. Compared to traditional single-mode methods, the proposed method can simultaneously measure flow field altitude and velocity information, or measure only flow field altitude and velocity. Simulated radar data processing results verify its effectiveness.

[0027] The geometric relationship of the cross-track oblique-look interferometry of the dual antennas of the spaceborne SAR is as follows: Figure 3 As shown, two pairs of antennas and The distance between them is The two antennas are at the same height. From a bottom perspective, For oblique viewing angles, the system adopts a single-transmitter, dual-receiver mode. and These are the frontal and side-looking distances from the two antennas to the target, respectively. Therefore, Target The distance to the zero Doppler point is (7) Target The distance to the zero Doppler point is (8) Therefore, the equivalent orbital component generated by strabismus can be expressed as: (9) In dual-antenna cross-track interferometry, a along-track component is generated by introducing an oblique angle, thus enabling flow field measurement. However, since it is essentially cross-track interferometry, the obtained interferometric phase includes a ground phase and an elevation phase introduced by the cross-track. To successfully extract the flow field, these two phases need to be suppressed. Furthermore, due to the simultaneous presence of along-track and cross-track components, both range and azimuth axes are shifted for two complex SAR images of the same scene, requiring high-precision registration.

[0028] Secondly, compared to side-looking SAR echoes, the range and azimuth coupling of oblique-looking SAR echoes is more severe. In the standard chirp scaling (CS) algorithm, the variation of the secondary range compression factor with the slant range is ignored; generally, only the secondary range compression factor of the center slant range is used for uniform processing. However, the secondary range compression factor varies more drastically with the slant range in oblique-looking SAR. Using only the secondary range compression factor of the center slant range will cause defocusing of other slant range gates in the range direction. The nonlinear CS algorithm is an improvement on the standard CS algorithm. By incorporating range-direction cubic phase scaling, it compensates for the spatially varying secondary range compression factor. Therefore, this method uses the nonlinear chirp scaling (NCS) algorithm to perform imaging processing on oblique-looking SAR. The complete flowchart of the proposed method is as follows: Figure 4 As shown.

[0029] Figure 4 The flowchart of the proposed oblique-looking equivalent in-orbit interferometric flow measurement algorithm is shown. First, the raw echo data from the cross-orbit dual-antenna oblique-looking SAR is required. Since it is oblique-looking SAR, a nonlinear CS algorithm is used for imaging processing to obtain two SAR image data. Next, high-precision image registration is performed on the two images, and interferometric processing is used to obtain the interferometric phase. Due to the presence of cross-orbit components, flat-ground phase suppression and elevation phase suppression are also required. Finally, the flow field velocity is extracted.

[0030] This method is an interferometric flow measurement method based on oblique viewing angles along the cross-track, unlike traditional flow measurement methods that use a platform along the track. This method generates an equivalent along-track component by adding an oblique viewing angle to the cross-track interferometric platform to measure the flow field. This approach effectively overcomes the limitations of traditional single-platform methods. Velocity measurements can be performed on the cross-track interferometric platform, which greatly improves the platform's usability.

[0031] Meanwhile, traditional spaceborne SAR baselines are too long (tens of meters to hundreds of meters), leading to temporal decoherence at the sea surface and making interferometry impossible. Distributed spaceborne SAR baselines are typically on the order of kilometers, making the conditions for coherent interferometry extremely demanding. The proposed method, however, can meet the coherence requirements by adjusting the oblique viewing angle, significantly expanding the application scope of distributed interferometry of spaceborne SAR at sea.

[0032] The present invention proposes an equivalent in-track interferometric flow measurement method based on oblique line-of-sight, the specific steps of which include: Step 1: Acquire raw echo data of cross-track dual-antenna slant-look SAR; Step 2: Use the nonlinear CS algorithm to perform imaging processing on the raw echo data to obtain two SAR images; Step 3: Perform high-precision image registration on the two SAR images and perform interferometric processing to obtain the interferometric phase; Step 4: Perform flat-ground phase suppression; Step 5: Extract the flow field velocity and complete the equivalent in-track interferometric flow measurement by crossing the track and looking at it obliquely.

[0033] The following detailed description is provided in conjunction with specific examples.

[0034] 1. Simulation of flow field data using dual-antenna interferometric SAR First, the flow field data was simulated using the parameters in Table 1. The simulation adopted a single-transmitter, dual-receiver mode, where one antenna was responsible for both transmitting and receiving echo data, while the other antenna was only responsible for receiving echo data. The baseline length of the dual-antenna intersection was set to 120 m.

[0035] Table 1 Simulation parameters of flow field data

[0036] The flow field velocity set in the simulation is as follows: Figure 5 As shown. The simulation only has the following settings. The velocity in the direction. In the sea surface scattering simulation, based on the wind and wave scattering field generated by the wind field, the background flow field described in this document is further modulated on the scattering field based on the wave-current modulation model, and at the same time superimposed on the velocity field of the sea surface waves, which is used as the velocity to update the position of each surface element in the simulation.

[0037] 2. Nonlinear CS algorithm imaging Compared to side-looking SAR echoes, the coupling in the range and azimuth directions is more severe in oblique-looking SAR echoes. In the standard CS algorithm, the variation of the secondary range compression factor with slant range is ignored; generally, the secondary range compression factor of the center slant range is used for uniform processing. However, the secondary range compression factor varies more drastically with slant range in oblique-looking SAR. Using only the secondary range compression factor of the center slant range will cause defocusing of other slant range gates in the range direction. The nonlinear CS algorithm is an improvement on the standard CS algorithm. By incorporating range-direction third-phase scaling, it compensates for the spatially varying secondary range compression factor. Therefore, the method of this invention requires the use of a nonlinear CS algorithm to achieve imaging processing for oblique-looking SAR.

[0038] The fundamental frequency echo signal after carrier frequency removal processing of the original SAR echo signal can be written as... (10) The two-dimensional spectrum of the echo signal after performing a two-dimensional Fourier transform on the signal is: (11) In equation (11), the first exponential term is a quadratic term that depends only on the range frequency and will be compensated during range compression; the second exponential term represents the target's azimuth position information; the third phase term contains the azimuth and range coupling terms that vary with the range, and only by accurately matching and compensating for them can a fully focused image be obtained. The last exponential term in equation (11) is defined as... ,Right now (12) Expand equation (12) into a third-order Taylor series. (13) Expand equation (13) into a third-order Taylor series. (14) In equation (14) (15) (16) (17) (18) The NCS algorithm considers third-order coupling terms, but assumes that these terms change slowly with distance. A simple filtering function with undetermined coefficients can ensure that these coefficients satisfy subsequent processing requirements, and the filtered signal can be considered an approximately linearly modulated (LFM) signal. The construction of the nonlinear scaling function also uses the undetermined coefficient method. These undetermined coefficients ensure that only distance-independent terms and terms that change linearly with distance are retained in the signal's two-dimensional spectrum, while all other terms are zero.

[0039] like Figure 6 The function expressions used in the text are as follows: (19) (20) (twenty one) Of the four groups of functions above, These are three coefficients to be determined, and their expressions are as follows: (twenty two) In the expression above, For correction factors, The azimuth frequency is used as a reference point. For the equivalent frequency modulation at the reference distance The rate of change of a point with distance is equivalent to the slope of the tangent line of the equivalent frequency modulation at the reference point. For example... Figure 7 The image shows the simulated echo imaging results using the NCS algorithm.

[0040] 3. Interferometric image registration Two antennas acquire echo imaging, resulting in two SAR complex images of the same scene. However, due to subtle variations in the two imaging processes, the same target in the observation area appears in different positions in the two images; that is, the target positions in the two images are not one-to-one. Therefore, in interferometric processing, directly calculating the phase difference between the two images often fails to yield ideal interference fringes. Instead, image registration is necessary to align the target positions in the two images. Image registration is the most fundamental step in interferometric processing. The basic principle of registration is to use one image as the primary image and the other as the secondary image, select an image matching metric, calculate the offset between the primary and secondary images, and then translate the images according to the offset so that pixels at the same position in the two images correspond to the same point in the observation area.

[0041] like Figure 8 As shown, before registration, the phase obtained by conjugate multiplication of the primary and secondary antennas is irregular and cannot be used for interferometry. After registration, the interferogram shows obvious fringe changes, which contain target height and flow field information. The height and velocity of the flow field can be obtained through subsequent processing. Furthermore, it can be seen that the interferogram obtained by conjugate multiplication of the images from the primary and secondary antennas exhibits regular fringe changes, which is due to the flat-ground effect.

[0042] Registration can be divided into two processes: pixel-level coarse registration and sub-pixel-level fine registration.

[0043] Pixel-level coarse registration: Select a series of control points in the main image as a matching window. Similarly, select a relatively large search window in the auxiliary image. Move the matching window in the search window and calculate the matching index for each. The window with the largest matching index value is the one that can be used to calculate the offset. Calculate the difference in the position of the center pixel at this time. This difference in position is the offset to be obtained. Translate the image according to this offset to complete the pixel-level coarse registration.

[0044] Sub-pixel level fine registration: The specific method is the same as coarse registration. However, due to the high precision requirements of fine registration, the complex image needs to be interpolated first to the sub-pixel level. After interpolation, the window matching index is then calculated. Generally, sub-pixel level fine registration requires an accuracy within 1 / 8 of a pixel. After interpolation, the calculation is performed, and the subsequent process is the same as pixel-level registration.

[0045] Commonly used matching measures include: correlation coefficient, maximum interference spectrum, average fluctuation function, and phase least squares. The correlation coefficient is the most basic matching measure; it is simple to operate, highly stable, and widely used in various image registration applications. The correlation coefficient is divided into real correlation coefficient and complex correlation coefficient (coherence coefficient). The complex correlation function is defined as: (twenty three) Complex correlation function registration is simple in concept and quick in calculation, serving as the foundation for many registration algorithms. The registration effect is evaluated using the coherence coefficient of the two SAR image data. The formula for calculating the coherence coefficient of the complex image is as follows: (twenty four) in The coherence coefficient, E For mathematical expectation, , These are two SAR image data sets.

[0046] right Figure 7 Registration was performed, and the registration result is as follows: Figure 9 As shown. From Figure 9 As can be seen from the data, the phase obtained by multiplying the two images before registration is irregular and cannot be used for interferometry. The interferogram obtained after registration shows obvious fringe changes.

[0047] The trend of coherence coefficient changes before and after registration is as follows: Figure 10 As shown in the figure, the azimuth and range coherence coefficients of the registration results are generally above 0.9, meeting the requirements for fine registration of interferometric SAR image data. However, there are jumps at the beginning and end of the azimuth coherence coefficient. This is due to the missing azimuth data at the beginning and end positions caused by oblique viewing, resulting in these jumps.

[0048] 4. Flat-ground phase suppression For cross-track, side-looking InSAR, the diagram illustrating the interferometric phase change caused by slant range variation is as follows: Figure 11 As shown, according to Figure 11 In the radar geometry shown, for a spaceborne InSAR system, due to ,have Therefore, for the target The absolute phase can be expressed as: (25) when Observing targets at a fixed altitude When the slant distance changes, a new target Consistent with the original target, based on the geometric relationships in the image, and determined by the difference in viewing angle. The resulting change in interference phase difference is (26) Based on the radar geometry, we have: (27) Substituting equation (27) into equation (26), we can obtain the interference phase change relationship caused by the change in slant range when the target height remains unchanged: (28) The phenomenon where even perfectly flat terrain with no elevation change causes a linear change in the interferometric phase is called the "flat-ground effect." This phenomenon exacerbates the density of fringes in the interferometric phase diagram, significantly increasing the complexity of InSAR phase filtering and unwrapping. Therefore, it is necessary to eliminate the influence of the flat-ground phase before filtering and unwrapping. Flat-ground effect removal generally utilizes geometric methods and frequency shift methods. The geometric method uses the geometric relationship between the radar's two antennas and the target to remove the flat-ground effect. Because it uses real range information for flat-ground removal, it can be used to remove the flat-ground effect from measured data.

[0049] For a point on the reference plane Target and Antenna The distance is: (29) According to the law of cosines, the target With antenna The distance is: (30) The phase of the interference on flat ground is then: (31) Subtracting the flat-ground interference phase from the initial phase achieves the effect of removing the flat-ground interference. The slant range difference is calculated, and the flat-ground phase is constructed, as follows: Figure 12As shown. Subtracting the flat-ground interferometric phase from the registered interferometric phase then achieves the effect of removing the flat ground effect. For example... Figure 13 As shown, the interference fringes disappear after removing the flat phase.

[0050] Since this method demonstrates flow field measurement by generating an equivalent along-track component through oblique viewing, the simulation does not incorporate sea surface flow height information. Therefore, there is no elevation phase, and thus no elevation phase suppression is required.

[0051] 5. Extracting flow field velocity After removing a series of interference terms, such as the flat-surface phase, from the interferometric phase, what remains is the interferometric phase generated by the flow field. Based on the relationship model between the interferometric phase and ocean current velocity, incident angle, and oblique angle, the ocean current velocity distribution at the sea surface is obtained by inverting the interferometric phase. According to the working principle of in-orbit interferometric SAR ocean current measurement, the expression for the line-of-sight ocean current velocity is: (32) in, For the satellite's orbital velocity, The phase of the interference along the orbit. Radial surface velocity. It is the radial LOS velocity. Projection on the sea surface: (33) in, It is a microwave wavelength. It's a bottom-down perspective.

[0052] like Figure 14 The image shows the extracted Doppler velocities of the flow field.

[0053] The parts of this invention not described in detail are common knowledge to those skilled in the art.

Claims

1. A method for measuring flow in parallel with the track using equivalent oblique line-of-sight interferometry, characterized in that, include: Step 1: Acquire raw echo data of cross-track dual-antenna slant-look SAR; Step 2: Use the nonlinear CS algorithm to perform imaging processing on the raw echo data to obtain two SAR images; Step 3: Perform high-precision image registration on the two SAR images and then perform interferometric processing to obtain the interferometric phase; Step 4: Perform flat-ground phase suppression; Step 5: Extract the flow field velocity and complete the equivalent in-track interferometric flow measurement by crossing the track and looking at it obliquely.

2. The method for measuring flow in parallel with the track using equivalent oblique line-of-sight interferometry according to claim 1, characterized in that: Step 3 involves high-precision image registration of the two SAR images and interferometric processing to obtain the interferometric phase, specifically as follows: One SAR image is used as the main image and the other as the auxiliary image. An image matching metric is selected to calculate the offset between the main and auxiliary images. Then, the images are translated according to the offset so that pixels at the same position in the two images correspond to the same point in the observation area.

3. The method for measuring flow in parallel with the track using equivalent oblique line-of-sight interferometry according to claim 2, characterized in that: Matching measures include: correlation coefficient, maximum interference spectrum, average oscillation function, and phase least squares.

4. The method for measuring flow in parallel with the track using equivalent oblique line-of-sight interferometry according to claim 1, characterized in that: Step 4 involves flat-ground phase suppression, specifically: using a geometric method or a frequency shift method to suppress flat-ground phase, calculating the slant range difference and constructing a flat-ground interference phase, and then subtracting the flat-ground interference phase from the registered interference phase to achieve the effect of removing flat-ground interference.

5. The method for measuring flow in parallel with the track using equivalent oblique line-of-sight interferometry according to claim 1, characterized in that: Step 5, which extracts the flow field velocity, specifically involves inverting the interferometric phase to obtain the ocean current velocity distribution on the sea surface, based on the relationship model between the interferometric phase and the ocean current velocity, incident angle, and oblique angle.

6. The method for measuring flow in parallel with the track using equivalent oblique line-of-sight interferometry according to claim 5, characterized in that: The expression for the velocity of the ocean current from the line of sight to the sea surface is: in, For the satellite's orbital velocity, For the in-orbit interference phase, radial surface velocity It is the radial LOS velocity. Projection on the sea surface: in, It is a microwave wavelength. It's a bottom-down perspective.

7. A cross-track oblique-look equivalent in-track interferometric flow measurement system, characterized in that, include: Data acquisition module: Acquires raw echo data from cross-track dual-antenna slant-look SAR; Imaging module: The nonlinear CS algorithm is used to process the raw echo data to obtain two SAR images; Registration module: Performs high-precision image registration on two SAR images and performs interferometric processing to obtain the interferometric phase; Phase suppression module: performs flat-ground phase suppression; Flow field velocity extraction module: Extracts flow field velocity and completes equivalent in-track interferometric flow measurement with oblique line of sight.

8. The equivalent in-track interferometric flow measurement system based on oblique line-of-sight as described in claim 7, characterized in that: High-precision image registration was performed on the two SAR images, and interferometric processing was then performed to obtain the interferometric phase, specifically as follows: One SAR image is used as the main image and the other as the auxiliary image. An image matching metric is selected to calculate the offset between the main and auxiliary images. Then, the images are translated according to the offset so that the pixels at the same position in the two images correspond to the same point in the observation area. Matching measures include: correlation coefficient, maximum interference spectrum, average oscillation function, and phase least squares.

9. A cross-track oblique-look equivalent in-track interferometric flow measurement system according to claim 7, characterized in that: Flat-ground phase suppression is performed by using a geometric method or a frequency shift method to calculate the slant range difference and construct the flat-ground interferometric phase. Then, the flat-ground interferometric phase is subtracted from the registered interferometric phase to achieve the effect of removing the flat-ground phase.

10. The equivalent in-track interferometric flow measurement system based on oblique line-of-sight according to claim 7, characterized in that: Extracting flow field velocity involves: based on the relationship model between interference phase and ocean current velocity, incident angle, and oblique angle, inverting the interference phase to obtain the ocean current velocity distribution on the sea surface; The expression for the velocity of the ocean current from the line of sight to the sea surface is: in, For the satellite's orbital velocity, For the in-orbit interference phase, radial surface velocity It is the radial LOS velocity. Projection on the sea surface: in, It is a microwave wavelength. It's a bottom-down perspective.

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