Space-based Interferometric Imaging Radar Altimeter Calibration Method and System Considering Phase Spatial Variation

Abstract

The present invention discloses a calibration method and system for a spaceborne interferometric imaging radar altimeter considering phase spatial variation, including: obtaining an interferometric phase image containing each reference target by using the spaceborne interferometric imaging radar altimeter, and calculating the radar viewing angle at the pixel center position of the interferometric phase image where each reference target is located; processing the initial radar interferometric parameters respectively with the radar viewing angles of each reference target to obtain the phase error of each reference target, and determining the polynomial fitting coefficients based on the polynomial fitting model, the radar viewing angles of each reference target and the phase error; obtaining an interferometric calibration result including the estimated values of the baseline length error, the equivalent baseline tilt error and the phase spatial variation error of the spaceborne interferometric imaging radar altimeter based on the polynomial fitting coefficients and the phase error model. The present invention can be applied to spaceborne interferometric imaging radar altimeters with horizontal baselines and inclined baselines, reduces the complexity of measurement errors, and improves the accuracy of interferometric calibration.

Description

Technical Field

[0001] The present invention relates to the technical field of interferometric radar calibration, and particularly to a spaceborne interferometric imaging radar altimeter calibration method and system considering phase spatial variation. Background Art

[0002] In recent years, a new type of radar altimeter has been developed, namely a spaceborne interferometric imaging radar altimeter carried on platforms such as satellites or space vehicles. By using small incidence angle observations and interferometric synthetic aperture radar technology, it can obtain two-dimensional height data of the sea surface and land surface water surfaces. The spatial resolution is on the order of 100 meters, and the observed swath width is on the order of 50 kilometers to 100 kilometers. Compared with traditional spaceborne radar altimeters that can only obtain one-dimensional height data of the sub-satellite point along the platform orbit flight direction, it has been greatly improved in both spatial resolution and time resolution, and can quickly obtain medium and small scale ocean height data on a global scale and water surface height data of smaller scale inland lakes and rivers, etc.

[0003] To ensure the interferometric inversion accuracy of the spaceborne interferometric imaging radar altimeter, it is necessary to obtain accurate values of various interferometric parameters. Therefore, accurate interferometric calibration is an essential key step for the spaceborne interferometric imaging radar altimeter. Among the various interferometric parameters affecting the interferometric height measurement accuracy, the phase spatial variation error is a phase error that varies with the radar viewing angle of the observed target in the vertical orbit direction. This error term is not a single value but a series of values. Any unknown phase spatial variation error will seriously affect the estimation of other interferometric error terms, resulting in incorrect estimation and unexpected changes of other interferometric error terms, such as errors in baseline length, baseline tilt, and interferometric phase offset, etc.

[0004] At present, airborne and space-based interferometric imaging radar altimeter flight tests have been carried out in China. The Three-Dimensional Imaging Microwave Altimeter (Interferometric Imaging Radar Altimeter, InIRA) launched on Tiangong-2 in 2016 has become the world's first space-based payload in orbit, adopting a working mode with an inclined baseline. Abroad, airborne flight tests have also been carried out, and satellite operational operations have been carried out, that is, the Surface Water and Ocean Topography (SWOT) satellite jointly developed by NASA of the United States and CNES of France, which was launched in December 2022, adopts a horizontal baseline working mode and first conducts a six-month payload calibration and verification work before formal operation. Relevant interferometric calibration methods at home and abroad are still in the stage of continuous development and technical verification. The domestic interferometric calibration method mainly considers the estimation and correction of error factors such as baseline length, baseline inclination angle, and interferometric phase offset, and does not consider the influence of phase spatially variant errors. Although foreign countries have proposed an estimation method for phase spatially variant errors, this method is only applicable to horizontal baseline interferometric imaging radar altimeters and cannot be used for inclined baseline interferometric imaging radar altimeters.

[0005] Therefore, there is an urgent need to provide a technical solution to solve the above technical problems. Summary of the Invention

[0006] To solve the above technical problems, the present invention provides a calibration method and system for a space-based interferometric imaging radar altimeter considering phase spatial variation.

[0007] The technical solution of the calibration method for a space-based interferometric imaging radar altimeter considering phase spatial variation of the present invention is as follows:

[0008] Within a preset time period, use a space-based interferometric imaging radar altimeter to obtain an interferometric phase image of each reference target in the target observation area, and calculate the radar viewing angle at the pixel center position of each reference target in the interferometric phase image.

[0009] Use the initial radar interferometry parameters to perform combined processing with the radar viewing angle of each reference target to obtain the phase error of each reference target, and determine the polynomial fitting coefficients of the polynomial fitting model based on the polynomial fitting model, the radar viewing angle of each reference target, and the phase error; wherein, the polynomial fitting model is used to describe the spatial variation relationship of the phase error with the radar viewing angle.

[0010] Based on the polynomial fitting coefficients and the preset phase error model, an interferometric calibration result including the estimated value of the baseline length error, the estimated value of the equivalent baseline inclination error, and the estimated value of the phase spatial variation error of the spaceborne interferometric imaging radar altimeter is obtained; wherein, the preset phase error model includes the baseline length error, the equivalent baseline inclination error, and the phase spatial variation error.

[0011] The beneficial effects of the spaceborne interferometric imaging radar altimeter calibration method considering phase spatial variation of the present invention are as follows:

[0012] The method of the present invention reduces the complexity of measurement errors through the phase error model of the spaceborne interferometric imaging radar altimeter defined in the phase domain, and can be applied to the spaceborne interferometric imaging radar altimeter with horizontal baselines and inclined baselines; by simultaneously considering the baseline length error, the equivalent baseline inclination error, and the phase spatial variation error, the accuracy of interferometric calibration is improved.

[0013] On the basis of the above solution, the spaceborne interferometric imaging radar altimeter calibration method considering phase spatial variation of the present invention can also be improved as follows.

[0014] Further, the step of calculating the radar viewing angle of the pixel center position of any reference target in the interferometric phase image includes:

[0015] According to the position information of the any reference target, determine the central position information of the pixel of the interferometric phase image where the reference target is located;

[0016] According to the pixel center position information of the reference target, the height information of the reference target, the orbital position information and speed information of the spaceborne interferometric imaging radar altimeter, and the slant range from the spaceborne interferometric imaging radar altimeter to the reference target, and use a preset cyclic iterative algorithm to iteratively calculate the radar viewing angle of the reference target until the radar viewing angle corresponding to when the preset condition is satisfied is determined as the radar viewing angle of the pixel center position of the interferometric phase image where the reference target is located; wherein, the preset cyclic iterative algorithm is: i is the serial number of the reference target, k is the number of cyclic iterations, θ(i, k + 1) is the radar viewing angle of the i-th reference target at the (k + 1)-th cyclic iteration, θ(i, k) is the radar viewing angle of the i-th reference target at the k-th cyclic iteration, θ inc (i, k) is the radar incident angle of the i-th reference target at the k-th cyclic iteration, r1(i) is the slant range from the spaceborne interferometric imaging radar altimeter to the i-th reference target, Δh(i, k) is the height error of the i-th reference target inverted at the k-th cyclic iteration, Δh(i, k) = h(i, k) - h(i), (Long(i, k), Lat(i, k), h(i, k)) = G -1 {(X s (i), Y s(i), Z s (i)), (V xs (i), V ys (i), V zs (i)), r1(i), θ(i, k)}, G -1 {(X s (i), Y s (i), Z s (i)), (V xs (i), V ys (i), V zs (i)), r1(i), θ(i, k)} represents the theoretical interference inversion of (X s (i), Y s (i), Z s (i)), (V xs (i), V ys (i), V zs (i)), r1(i), θ(i, k). Long(i, k), Lat(i, k), and h(i, k) are respectively the longitude estimate, latitude estimate, and altitude estimate of the pixel center position of the i-th reference target obtained by the theoretical interference inversion in the k-th loop iteration. X s (i), Y s (i), and Z s (i) successively represent the X-axis position component, Y-axis position component, and Z-axis position component when the spaceborne interferometric imaging radar altimeter performs a forward-looking observation of the i-th reference target. V xs (i), V ys (i), and V zs (i) successively represent the X-axis velocity component, Y-axis velocity component, and Z-axis velocity component when the spaceborne interferometric imaging radar altimeter performs a forward-looking observation of the i-th reference target. h(i) is the altitude information of the i-th reference target; R s (i) is the distance from the radar to the center of the earth calculated based on X s (i), Y s (i), and Z s (i). R p (i, k) is the distance from the reference target to the center of the earth calculated based on Long(i, k), Lat(i, k), and h(i, k) and through existing coordinate transformation techniques.

[0017] Furthermore, the preset condition is: when Δh(i, k) in the k-th loop iteration is less than the preset error value, the radar viewing angle corresponding to the end of the k-th loop is determined as the radar viewing angle of the pixel center position of the i-th reference target in the interferometric phase image; θ(i, 0) is the radar viewing angle of the i-th reference target during the initial loop iteration calculation. R p (i, 0) is the initial distance from the reference target to the center of the earth calculated based on the position information and height information of the i-th reference target and through existing coordinate transformation techniques.

[0018] Furthermore, the initial radar interferometry parameters include: the radar carrier frequency of the space-based interferometric imaging radar altimeter, the initial baseline length, and the initial baseline inclination; the steps of obtaining the phase error of the reference target by combining and processing the initial radar interferometry parameters and the radar viewing angle of any reference target include:

[0019] Substitute the initial radar interferometry parameters and the radar viewing angle of any reference target into the theoretical interferometric phase formula to obtain the theoretical interferometric phase of the reference target; where, the theoretical interferometric phase formula is: θ(i) is the radar viewing angle of the i-th reference target, λ is the radar carrier frequency, B is the initial baseline length, α is the initial baseline inclination, and φ(θ(i)) is the theoretical interferometric phase of the i-th reference target;

[0020] Use the interferometric phase bias ambiguity number calculation formula to estimate the interferometric phase bias ambiguity number of any reference target, obtain the corrected interferometric phase bias ambiguity number of the reference target, and use the phase error formula to compare the theoretical interferometric phase of the reference target with the measured interferometric phase corrected by the interferometric phase bias ambiguity number of the reference target to obtain the phase error of the reference target; where, the interferometric phase bias ambiguity number calculation formula is: n(i) is the interferometric phase bias ambiguity number of the i-th reference target, represents taking the integer of φ unw (θ(i)) is the measured interferometric phase of the i-th reference target; the phase error formula is: Δφ unw (θ(i)) = φ unw (θ(i)) - 2πn(i) - φ(θ(i)), Δφ unw (θ(i)) is the phase error of the i-th reference target.

[0021] Furthermore, the steps of determining the polynomial fitting coefficients of the polynomial fitting model based on the polynomial fitting model, the radar viewing angle of each reference target, and the phase error include:

[0022] According to the polynomial fitting model, the radar viewing angle of each reference target, and the phase error, and use the least squares method to determine the polynomial fitting coefficients of the polynomial fitting model; where, the polynomial fitting model is: p j, where \(j = 0, 1, 2, \ldots, N\) are the \(N + 1\) polynomial fitting coefficients of the polynomial fitting model, and \(N\) is the polynomial fitting order; the calculation formula for the polynomial fitting coefficients is: \(\Delta\varphi\) unw \((\theta(i))\), where \(i = 1, 2, \ldots, M\) are the phase errors of the \(M\) reference targets in the target observation area during the preset time period, and \(M\geq(N + 1)\).

[0023] Furthermore, the step of obtaining the interferometric calibration result including the baseline length error estimate, the equivalent baseline tilt angle error estimate, and the phase spatial variation error estimate of the spaceborne interferometric imaging radar altimeter based on the polynomial fitting coefficients and the preset phase error model includes:

[0024] Using the preset phase error model and the polynomial fitting model for derivation to respectively obtain the first calculation formula corresponding to the baseline length error, the second calculation formula corresponding to the equivalent baseline tilt angle error, and the third calculation formula corresponding to the phase spatial variation error;

[0025] Substituting the polynomial fitting coefficients into the first calculation formula, the second calculation formula, and the third calculation formula respectively to obtain the interferometric calibration result including the baseline length error estimate, the equivalent baseline tilt angle error estimate, and the phase spatial variation error estimate of the spaceborne interferometric imaging radar altimeter;

[0026] Among them, the preset phase error model is:

[0027] is the phase error when the radar viewing angle is \(\theta\), \(\varphi\) ps \((\theta)\) is the estimated value of the phase spatial variation error when the radar viewing angle is \(\theta\), \(\varphi\) b \((\theta)\) is the interferometric phase error introduced by the baseline length error and the baseline tilt angle error when the radar viewing angle is \(\theta\), \(\varphi_0\) is the interferometric phase offset, \(\varphi\) n \((\theta)\) is the random phase error introduced by the radar system noise when the radar viewing angle is \(\theta\), \(\Delta B\) is the estimated value of the baseline length error, \(\Delta\alpha\) eff is the estimated value of the equivalent baseline tilt angle error, and the first calculation formula is: The second calculation formula is: The third calculation formula is: \(\varphi\) ps \((\theta)=p_3\theta\) 3 \(+p_2\theta\) 2 .

[0028] Furthermore, it also includes:

[0029] Obtain the estimated values of the baseline length error, the equivalent baseline inclination error, and the phase spatial variation error for the same observation area in multiple time periods, and obtain the estimated values of the baseline length error, the equivalent baseline inclination error, and the phase spatial variation error for multiple observation areas in the corresponding time periods;

[0030] Calculate the average value of all the estimated baseline length errors, the average value of all the estimated equivalent baseline inclination errors, and the average value of all the estimated phase spatial variation errors to optimize the interference calibration result of the spaceborne interferometric imaging radar altimeter.

[0031] The technical solution of the spaceborne interferometric imaging radar altimeter calibration system considering phase spatial variation of the present invention is as follows:

[0032] It includes: an acquisition module, a processing module, and an operation module;

[0033] The acquisition module is used to: within a preset time period, use the spaceborne interferometric imaging radar altimeter to obtain an interference phase image of the target observation area containing each reference target, and calculate the radar viewing angle at the pixel center position of each reference target in the interference phase image;

[0034] The processing module is used to: respectively combine and process the initial radar interference parameters with the radar viewing angle of each reference target to obtain the phase error of each reference target, and based on the polynomial fitting model, the radar viewing angle of each reference target, and the phase error, determine the polynomial fitting coefficients of the polynomial fitting model; wherein, the polynomial fitting model is used to describe the spatial variation relationship of the phase error with the radar viewing angle;

[0035] The operation module is used to: based on the polynomial fitting coefficients and a preset phase error model, obtain an interference calibration result including the estimated values of the baseline length error, the equivalent baseline inclination error, and the phase spatial variation error of the spaceborne interferometric imaging radar altimeter; wherein, the preset phase error model includes a baseline length error, an equivalent baseline inclination error, and a phase spatial variation error.

[0036] The beneficial effects of the spaceborne interferometric imaging radar altimeter calibration system considering phase spatial variation of the present invention are as follows:

[0037] The system of the present invention reduces the complexity of measurement errors through a spaceborne interferometric imaging radar altimeter phase error model defined in the phase domain, and can be applied to spaceborne interferometric imaging radar altimeters with horizontal baselines and inclined baselines; by simultaneously considering the baseline length error, the equivalent baseline inclination error, and the phase spatial variation error, the accuracy of interference calibration is improved.

[0038] Based on the above solutions, the spaceborne interferometric imaging radar altimeter calibration system considering phase spatial variation of the present invention can also be improved as follows.

[0039] Further, the obtaining module is specifically configured to:

[0040] Determine the central position information of the pixel of the interference phase image where the reference target is located according to the position information of any one of the reference targets;

[0041] According to the pixel center position information of the reference target, the height information of the reference target, the orbital position information and velocity information of the spaceborne interferometric imaging radar altimeter, and the slant range from the spaceborne interferometric imaging radar altimeter to the reference target, and using a preset loop iteration algorithm to iteratively calculate the radar viewing angle of the reference target until the radar viewing angle corresponding to when the preset condition is met is determined as the radar viewing angle of the pixel center position of the interference phase image where the reference target is located; wherein, the preset loop iteration algorithm is: i is the serial number of the reference target, k is the number of loop iterations, θ(i, k + 1) is the radar viewing angle of the i-th reference target at the (k + 1)-th loop iteration, θ(i, k) is the radar viewing angle of the i-th reference target at the k-th loop iteration, θ inc (i, k) is the radar incident angle of the i-th reference target at the k-th loop iteration, r1(i) is the slant range from the spaceborne interferometric imaging radar altimeter to the i-th reference target, Δh(i, k) is the height error of the i-th reference target inverted at the k-th loop iteration, Δh(i, k) = h(i, k) - h(i), (Long(i, k), Lat(i, k), h(i, k)) = G -1 {(X s (i) Y s (i), Z s (i)), (V xs (i), V ys (i), V zs (i)), r1(i), θ(i, k)}, G -1 {(X s (i), Y s (i), Z s (i)), (V xs (i), V ys (i), V zs (i)), r1(i), θ(i, k)} represents applying (X s (i), Y s (i), Z s (i)), (V xs (i), V ys (i), V zs(i)), r1(i), θ(i, k) for theoretical interference inversion, Long(i, k), Lat(i, k) and h(i, k) are respectively the longitude estimation value, latitude estimation value and height estimation value of the pixel center position of the i-th reference target obtained by the theoretical interference inversion in the k-th cycle iteration, X s (i), Y s (i) and Z s (i) successively represent the X-axis position component, Y-axis position component and Z-axis position component when the spaceborne interferometric imaging radar altimeter performs forward-looking observation on the i-th reference target, V xs (i), V ys (i) and V zs (i) successively represent the X-axis velocity component, Y-axis velocity component and Z-axis velocity component when the spaceborne interferometric imaging radar altimeter performs forward-looking observation on the i-th reference target, h(i) is the height information of the i-th reference target; R s (i) is the distance from the radar to the center of the earth calculated according to X s (i), Y s (i) and Z s (i), R p (i, k) is the distance from the reference target to the center of the earth calculated according to Long(i, k), Lat(i, k) and h(i, k) and through the existing coordinate transformation technology.

[0042] Further, the preset condition is: when Δh(i, k) in the k-th cycle iteration is less than the preset error value, the radar viewing angle corresponding to the end of the k-th cycle is determined as the radar viewing angle of the pixel center position where the i-th reference target is located; θ(i, 0) is the radar viewing angle of the i-th reference target during the initial cycle iteration calculation, R p (i, 0) is the initial distance from the reference target to the center of the earth calculated according to the position information and height information of the i-th reference target and through the existing coordinate transformation technology. Brief Description of the Drawings

[0043] Figure 1 Shows a schematic flow chart of an embodiment of the spaceborne interferometric imaging radar altimeter calibration method considering phase spatial variation provided by the present invention;

[0044] Figure 2 Shows a schematic diagram of the relationship between the pixel center position and the actual pixel position of the reference target in the embodiment of the spaceborne interferometric imaging radar altimeter calibration method considering phase spatial variation provided by the present invention;

[0045] Figure 3The structural schematic diagram of an embodiment of the spaceborne interferometric imaging radar altimeter calibration system considering phase spatial variation provided by the present invention is shown. Detailed implementation manners

[0046] Figure 1 The flow schematic diagram of an embodiment of a spaceborne interferometric imaging radar altimeter calibration method considering phase spatial variation provided by the present invention is shown. As Figure 1 shown, the method includes the following steps:

[0047] Step 110: Within a preset time period, use the spaceborne interferometric imaging radar altimeter to obtain the interferometric phase image of each reference target in the target observation area, and calculate the radar viewing angle at the pixel center position of each reference target in the obtained interferometric phase image.

[0048] Among them, ① the spaceborne interferometric imaging radar altimeter is the target object to be interferometrically calibrated in this embodiment. ② The reference targets include, but are not limited to, targets in areas such as the ocean or large inland lakes. ③ The target observation area is: a pre-selected observation area. ④ The spaceborne interferometric imaging radar altimeter simultaneously observes the reference target through two radar antennas, obtains radar images of two channels, and obtains the interferometric phase image of the reference target after interferometric processing such as channel matching, phase extraction, phase filtering, and phase unwrapping. ⑤ The relationship between the interferometric phase image and the reference target is one-to-many, that is, a single interferometric phase image may contain multiple reference targets.

[0049] It should be noted that considering the problem that the center position of the pixel of the reference target on the interferometric phase image of the spaceborne interferometric imaging radar altimeter is inconsistent with the actual pixel position of the reference target, in this embodiment, a cyclic iterative algorithm (described in detail later) is defaultly used to accurately estimate the radar viewing angle at the pixel center position of the reference target. Specifically, the relationship between the center position of the pixel of the interferometric phase image containing the reference target obtained by the spaceborne interferometric imaging radar altimeter and the actual pixel position of the reference target is as Figure 2 shown. Figure 2 shows the interferometric phase image pixels represented by 9 squares and a reference target represented by a triangle. The pixel center position of the shown reference target is located at the geometric center position of the pixel in the second row and second column shown; the actual pixel position of the shown reference target is within the pixel in the second row and second column and is in the lower right corner direction, that is, the position where the shown triangle is located; the pixel center position of the reference target is inconsistent with the actual pixel position of the reference target. Taking the InIRA interferometric phase image in China as an example, the spatial resolution in the along-track direction is about 20m, and the spatial resolution in the cross-track direction is not fixed, ranging from about 90m at the near range end to about 30m at the far range end. The interferometric phase obtained by the interferometric imaging radar altimeter is 20×30m 2 to 20×90m2 The comprehensive result of the radar echoes of all observed targets within the range. Therefore, to accurately estimate the variation relationship between the interferometric phase and the radar viewing angle, it is necessary to accurately calculate the radar viewing angle at the pixel center position of the reference target, rather than using the radar viewing angle at the actual pixel position of the reference target.

[0050] Step 120: Combine and process the initial radar interferometric parameters with the radar viewing angle of each reference target respectively to obtain the phase error of each reference target, and based on the polynomial fitting model, the radar viewing angle of each reference target, and the phase error, determine the polynomial fitting coefficients of the polynomial fitting model.

[0051] Among them, ① the polynomial fitting model is used to describe the spatial variation relationship of the phase error with the radar viewing angle. ② The initial radar interferometric parameters include but are not limited to: the radar carrier frequency of the spaceborne interferometric imaging radar altimeter, the initial baseline length, and the initial baseline inclination angle.

[0052] It should be noted that the more the number of reference targets, the more the noise influence can be reduced and the estimation accuracy of the polynomial fitting coefficients can be improved.

[0053] Step 130: Based on the polynomial fitting coefficients and the preset phase error model, obtain the interferometric calibration result including the estimated value of the baseline length error, the estimated value of the equivalent baseline inclination angle error, and the estimated value of the phase spatially-varying error of the spaceborne interferometric imaging radar altimeter.

[0054] Among them, the preset phase error model includes the baseline length error, the equivalent baseline inclination angle error, and the phase spatially-varying error.

[0055] Preferably, the step of calculating the radar viewing angle at the pixel center position of the interferometric phase image where any reference target is located includes:

[0056] According to the position information of the any reference target, determine the center position information of the pixel of the interferometric phase image where the reference target is located.

[0057] Among them, the position information includes: longitude information and latitude information.

[0058] It should be noted that ① the position information and height information of the reference target can be synchronously measured by means or facilities such as traditional spaceborne radar altimeters, airborne lidar, buoys, etc., or calculated by simulating with a sea surface height model, or calculated by averaging historical water level data, etc., without limitation here. ② The process of determining the center position information of the pixel of the interferometric phase image where the reference target is located according to the longitude information and latitude information of the reference target is a prior art and will not be elaborated here.

[0059] According to the pixel center position information of the reference target, the height information of the reference target, the orbital position information and velocity information of the spaceborne interferometric imaging radar altimeter, and the slant range from the spaceborne interferometric imaging radar altimeter to the reference target, and by using a preset cyclic iterative algorithm to iteratively calculate the radar viewing angle of the reference target until the radar viewing angle corresponding to when the preset condition is satisfied is determined as the radar viewing angle at the pixel center position of the interferometric phase image where the reference target is located.

[0060] Among them, the preset cyclic iterative algorithm is as follows: i is the serial number of the reference target, k is the number of cyclic iterations, θ(i, k + 1) is the radar viewing angle of the i-th reference target in the (k + 1)-th cyclic iteration, θ(i, k) is the radar viewing angle of the i-th reference target in the k-th cyclic iteration, θ inc (i, k) is the radar incident angle of the i-th reference target in the k-th cyclic iteration, r1(i) is the slant range from the spaceborne interferometric imaging radar altimeter to the i-th reference target, Δh(i, k) is the height error of the i-th reference target inverted in the k-th cyclic iteration, Δh(i, k) = h(i, k) - h(i), (Long(i, k), Lat(i, k), h(i, k)) = G -1 {(X s (i), Y s (i), Z s (i)), (V xs (i), V ys (i), V zs (i)), r1(i), θ(i, k)}, G -1 {(X s (i), Y s (i), Z s (i)), (V xs (i), V ys (i), V zs (i)), r1(i), θ(i, k)} represents performing theoretical interferometric inversion on (X s (i)Y s (u), Z s (i)), (V xs (i), V ys (i), V zs (i)), r1(i), θ(i, k), Long(i, k), Lat(i, k) and h(i, k) are respectively the longitude estimated value, latitude estimated value and height estimated value of the pixel center position of the i-th reference target obtained by the theoretical interferometric inversion in the k-th cyclic iteration, X s (i), Y s (i) and Z s(i) represent the X-axis position component, Y-axis position component, and Z-axis position component when the space-based interferometric imaging radar altimeter performs a forward-looking observation of the i-th reference target, respectively, V xs (i), V ys (i) and V zs (i) represent the X-axis velocity component, Y-axis velocity component, and Z-axis velocity component when the space-based interferometric imaging radar altimeter performs a forward-looking observation of the i-th reference target, respectively, and h(i) is the altitude information of the i-th reference target; R s (i) is the distance from the radar to the center of the earth calculated based on X s (i), Y s (i) and Z s (i), and R p (i, k) is the distance from this reference target to the center of the earth calculated based on Long(i, k), Lat(i, k), and h(i, k) and through existing coordinate transformation techniques.

[0061] It should be noted that the above-mentioned preset conditions are specifically as follows: when Δh(i, k) in the k-th loop iteration is less than the preset error value, the radar viewing angle corresponding to the end of the k-th loop is determined as the radar viewing angle at the pixel center position of the interferometric phase image where the i-th reference target is located; θ(i, 0) is the radar viewing angle of the i-th reference target during the initial loop iteration calculation, R p (i, 0) is the initial distance from this reference target to the center of the earth calculated based on the position information and altitude information of the i-th reference target and through existing coordinate transformation techniques. Among them, the preset error value is set according to requirements, such as 1 mm, and there is no limit here.

[0062] Preferably, the step of obtaining the phase error of this reference target by combining and processing the initial radar interferometric parameters and the radar viewing angle of any reference target includes:

[0063] Substitute the initial radar interferometric parameters and the radar viewing angle of any reference target into the theoretical interferometric phase formula to obtain the theoretical interferometric phase of this reference target.

[0064] Among them, the theoretical interferometric phase formula is: θ(i) is the radar viewing angle of the i-th reference target, λ is the radar carrier frequency, B is the initial baseline length, α is the initial baseline inclination angle, and φ(θ(i)) is the theoretical interferometric phase of the i-th reference target.

[0065] It should be noted that although the second component on the right side of the theoretical interference phase formula is a small quantity under the condition of B = r1(i) (at the order of one-thousandth under the conditions of a baseline length of about 10 m and a slant range of about several hundred kilometers), this component is retained here for the consideration of the high-precision measurement requirements of the spaceborne interferometric imaging radar altimeter. For the very small incident angles of the spaceborne interferometric imaging radar altimeter, the change of this component in the direction perpendicular to the orbit is very small and can be considered as a constant term.

[0066] Using the interference phase bias ambiguity number calculation formula, estimate the interference phase bias ambiguity number of any of the reference targets to obtain the interference phase bias ambiguity number of this reference target, and use the phase error formula to compare the theoretical interference phase of this reference target with the measured interference phase corrected by the interference phase bias ambiguity number of this reference target to obtain the phase error of this reference target.

[0067] Among them, the interference phase bias ambiguity number calculation formula is: n(i) is the interference phase bias ambiguity number of the i-th reference target, denotes rounding of , φ unw (θ(i)) is the measured interference phase of the i-th reference target; the phase error formula is: Δφ unw (θ(i)) = φ unw (θ(i)) - 2πn(i) - φ(θ(i)), Δφ unw (θ(i)) is the phase error of the i-th reference target.

[0068] Among them, the measured interference phase is obtained after interference processing such as channel registration, phase extraction, phase filtering, and phase unwrapping on the dual-channel complex image data of the spaceborne interferometric imaging radar altimeter.

[0069] It should be noted that the measured interference phase of the reference target at the farthest distance end, that is, with the largest r1(i), is selected to estimate the interference phase bias ambiguity number, and the interference phase bias ambiguity number of this reference target can be directly used for phase error calculation of other reference targets within the same interference phase image. This is because the ambiguity height at the farthest end in the range direction is the largest and the estimation accuracy is least affected by the measured height error of the reference target. For example, the ambiguity height at the farthest end in the range direction of InIRA is nearly 500 m, and in this case, the average water surface height can meet the estimation accuracy requirements.

[0070] Preferably, the steps of determining the polynomial fitting coefficients of the polynomial fitting model based on the polynomial fitting model, the radar viewing angle of each reference target, and the phase error include:

[0071] According to the polynomial fitting model, the radar viewing angles of each reference target, and the phase errors, and using the least squares method to determine the polynomial fitting coefficients of the polynomial fitting model.

[0072] Among them, ① the polynomial fitting model is: p j , j = 0, 1, 2,..., N are the N + 1 polynomial fitting coefficients of the polynomial fitting model, and N is the polynomial fitting order. ② The calculation formula for the polynomial fitting coefficients is: Δφ unw (θ(i)), i = 1, 2,..., M are the phase errors of the M reference targets in the target observation area during the preset time period, and M ≥ (N + 1).

[0073] Preferably, the step of obtaining the interferometric calibration result including the baseline length error estimate, the equivalent baseline inclination error estimate, and the phase spatial variation error estimate of the spaceborne interferometric imaging radar altimeter based on the polynomial fitting coefficients and the preset phase error model includes:

[0074] Using the preset phase error model and the polynomial fitting model for derivation to respectively obtain a first calculation formula for the baseline length error, a second calculation formula for the equivalent baseline inclination error, and a third calculation formula for the phase spatial variation error.

[0075] Substitute the polynomial fitting coefficients into the first calculation formula, the second calculation formula, and the third calculation formula respectively to obtain the interferometric calibration result including the baseline length error estimate, the equivalent baseline inclination error estimate, and the phase spatial variation error estimate of the spaceborne interferometric imaging radar altimeter.

[0076] Among them, the preset phase error model is:

[0077] is the phase error when the radar viewing angle is θ, φ ps (θ) is the estimated value of the phase spatial variation error when the radar viewing angle is θ, φ b (θ) is the interferometric phase error introduced by the baseline length error and the baseline inclination error when the radar viewing angle is θ, φ0 is the interferometric phase offset, φ n (θ) is the random phase error introduced by the radar system noise when the radar viewing angle is θ, ΔB is the estimated value of the baseline length error, Δα eff is the estimated value of the equivalent baseline inclination error, and the first calculation formula is: The second calculation formula is: The third calculation formula is: φ ps (θ) = p3θ 3 + p2θ 2 .

[0078] Specifically, ① according to the theoretical interference phase formula, the interference phase errors introduced by the baseline length error and the baseline tilt error can be decomposed into:

[0079] (where ΔB is the baseline length error and Δα is the baseline tilt error).

[0080] The interferometric imaging radar altimeter operates at a near-nadir angle, and (θ - α) is very small, generally between -2.5° and 2.5°, that is, within the range of -0.0436 rad to 0.0436 rad. Therefore, the phase error introduced by the above interferometric baseline error can be approximated as:

[0081]

[0082] It can be seen that the phase error introduced by the baseline length error has a linear relationship with the radar viewing angle, while the phase error introduced by the baseline tilt error is a constant term.

[0083] ② Considering that the phase error introduced by the interferometric phase bias is also a constant term, the combined effect of the two errors, including the baseline tilt error and the interferometric phase bias, is defined as the equivalent baseline tilt error:

[0084]

[0085] It can be seen that the phase error introduced by the equivalent baseline tilt error is a constant term.

[0086] ③ The remaining phase error, including the quadratic and higher-order terms of the radar viewing angle, is defined as the phase spatially variant error.

[0087] ④ Therefore, the preset phase error model for the change of the interferometric imaging radar altimeter along the vertical orbit direction is defined as:

[0088]

[0089] ⑤ Among the above errors, the phase spatially variant error is a quadratic and higher-order function of the radar viewing angle; the phase error introduced by the baseline length error is linearly related to the radar viewing angle; the equivalent baseline tilt error, including the combined effect of the baseline tilt error and the interferometric phase bias, has a constant phase error introduced; the random phase error introduced by the radar system noise cannot be eliminated and can only be reduced by time and space averaging. To avoid the influence of the radar system noise on the estimation result of the phase spatially variant error caused by overfitting, usually only the quadratic and cubic terms need to be considered, and the higher-order terms of the fourth order and above can be ignored.

[0090] a) Using the first calculation formula, calculate the estimated value of the baseline length error according to the above preset phase error model and the polynomial fitting coefficients:

[0091] b) Using the second calculation formula, calculate the estimated value of the equivalent baseline tilt error according to the above preset phase error model, baseline length error, and polynomial fitting coefficients:

[0092] c) Using the third calculation formula, calculate the estimated value of the phase spatially-varying error φ according to the above preset phase error model and polynomial fitting coefficients: ps (θ) = p3θ 3 + p2θ 2 ;

[0093] It should be noted that: ① According to the relationship that each error term of the polynomial fitting model corresponds equally to the preset phase error model. For example, the quadratic and cubic error terms p3θ 3 + p2θ 2 correspond equally to the phase spatially-varying error φ ps (θ), the first-order error term p1θ corresponds equally to the constant error term p0 corresponds equally to It can be determined that: The first calculation formula is: The second calculation formula is: The third calculation formula is: φ ps (θ) = p3θ 3 + p2θ 2 . ② The estimated value of the phase spatially-varying error can be described by two coefficients p2 and p3, or can be expanded into a series of specific values varying along the vertical orbit direction.

[0094] Preferably, it further includes:

[0095] Obtain the estimated values of the baseline length error, equivalent baseline tilt error, and phase spatially-varying error in the same observation area at multiple time periods respectively, and obtain the estimated values of the baseline length error, equivalent baseline tilt error, and phase spatially-varying error in multiple observation areas at the corresponding time periods respectively;

[0096] Calculate the average value of all the estimated values of the baseline length error, the average value of all the estimated values of the equivalent baseline tilt error, and the average value of all the estimated values of the phase spatially-varying error, so as to optimize the interference calibration result of the spaceborne interferometric imaging radar altimeter.

[0097] Specifically, perform an averaging process on the baseline length error, equivalent baseline tilt error, and phase spatially-varying error obtained at different time periods and different observation areas, reduce the noise influence of the single estimation result, and improve the estimation accuracy of the final result:

[0098]

[0099]

[0100]

[0101]

[0102] where y l , l = 1, 2, ..., L are a total of L times of interferometric calibration time periods, ΔB(t l ) is the estimated value of the baseline length error obtained in the t l time period, Δα eff (t l ) is the estimated value of the baseline tilt error obtained in the t l time period, p2(t l ) and p3(t l ) are two phase spatially variant error coefficients obtained in the t l time period, is the average value of all the estimated values of the baseline length error, is the average value of all the estimated values of the equivalent baseline tilt error, and are respectively the average values of the two phase spatially variant error coefficients after average optimization. According to and the third calculation formula, the average value of the estimated value of the phase spatially variant error is calculated.

[0103] It should be noted that for the long - term errors of the radar system, such as the error drift and variation generated in 1 month or even longer, the variation relationship of each error term with time can be obtained by performing polynomial fitting on the variation relationship of the above - mentioned estimated results with time again.

[0104] The technical solution of this embodiment reduces the complexity of measurement errors through the space - based interferometric imaging radar altimeter phase error model defined in the phase domain, and is applicable to space - based interferometric imaging radar altimeters with horizontal baselines and tilted baselines; by simultaneously considering the baseline length error, equivalent baseline tilt error, and phase spatially variant error, the accuracy of interferometric calibration is improved.

[0105] This embodiment proposes a spaceborne interferometric imaging radar altimeter phase error model defined in the phase domain. Compared with defining measurement errors in the altitude data domain, the complexity of defining measurement errors in the phase data domain is greatly reduced, and the influence of each interferometric error term in two different baseline working modes, namely horizontal and inclined, can be accurately described in a relatively simple form. This embodiment takes into account the influence of phase-variant errors and presents an interferometric calibration method for simultaneously estimating the baseline length error, equivalent baseline inclination error, and phase-variant error of the interferometric imaging radar altimeter. Among them, the baseline length error is a parameter with a single numerical value; the equivalent baseline inclination error includes the influence of both the baseline inclination and the interferometric phase offset, and is also a parameter with a single numerical value; the phase-variant error is a complex system parameter that varies along the vertical orbit direction and consists of a series of numerical values. This embodiment accurately calculates the radar viewing angle of the reference target based on the central position information of the pixels of the reference target projected onto the interferometric phase image of the interferometric imaging radar altimeter, rather than the pixel position information of the reference target itself, so that the extracted phase error and the radar viewing angle are strictly matched, improving the estimation accuracy of interferometric parameter errors.

[0106] Figure 3 FIG. shows a schematic structural diagram of an embodiment of a spaceborne interferometric imaging radar altimeter calibration system considering phase variation provided by the present invention. As Figure 3 shown, the system 200 includes: an acquisition module 210, a processing module 220, and an operation module 230.

[0107] The acquisition module 210 is configured to: within a preset time period, use a spaceborne interferometric imaging radar altimeter to acquire an interferometric phase image of a target observation area containing each reference target, and calculate the radar viewing angle of the center position of the pixels of each reference target in the interferometric phase image;

[0108] The processing module 220 is configured to: perform combined processing on the initial radar interferometric parameters and the radar viewing angle of each reference target respectively to obtain the phase error of each reference target, and determine the polynomial fitting coefficients of the polynomial fitting model based on the polynomial fitting model, the radar viewing angle of each reference target, and the phase error; wherein, the polynomial fitting model is used to describe the spatial variation relationship of the phase error with the radar viewing angle;

[0109] The operation module 230 is configured to: based on the polynomial fitting coefficients and a preset phase error model, obtain an estimated value of the baseline length error, an estimated value of the equivalent baseline inclination error, and an estimated value of the phase-variant error of the spaceborne interferometric imaging radar altimeter; wherein, the preset phase error model includes a baseline length error, an equivalent baseline inclination error, and a phase-variant error.

[0110] Preferably, the acquisition module 210 is specifically configured to:

[0111] Determine the central position information of the pixels of the interference phase image where the reference target is located according to the position information of any of the reference targets;

[0112] According to the pixel center position information of the reference target, the height information of the reference target, the orbital position information and velocity information of the space-based interferometric imaging radar altimeter, and the slant range from the space-based interferometric imaging radar altimeter to the reference target, and using a preset cyclic iteration algorithm to iteratively calculate the radar viewing angle of the reference target until the radar viewing angle corresponding to when the preset condition is satisfied is determined as the radar viewing angle of the pixel center position of the interference phase image where the reference target is located; wherein, the preset cyclic iteration algorithm is: i is the serial number of the reference target, k is the number of cyclic iterations, θ(i, k + 1) is the radar viewing angle of the i-th reference target at the (k + 1)-th cyclic iteration, θ(i, k) is the radar viewing angle of the i-th reference target at the k-th cyclic iteration, θ inc (i, k) is the radar incident angle of the i-th reference target at the k-th cyclic iteration, r1(i) is the slant range from the space-based interferometric imaging radar altimeter to the i-th reference target, Δh(i, k) is the height error of the i-th reference target inverted at the k-th cyclic iteration, Δh(i, k) = h(i, k) - h(i), (Long(i, k), Lat(i, k), h(i, k)) = G -1 {(X s (i), Y s (i), Z s (i)), (V xs (i), V ys (i), V zs (i)), r1(i), θ(i, k)}, G -1 {(X s (u), Y s (i), Z s (i)), (V xs (i), V ys (i), V zs (i)), r1(i), θ(i, k)} represents theoretical interference inversion of (X s (i), Y s (i), Z s (i)), (V xs (i), V ys (i), V zs (i)), r1(i), θ(i, k), Long(i, k), Lat(i, k) and h(i, k) are respectively the longitude estimated value, latitude estimated value and height estimated value of the pixel center position of the i-th reference target obtained by the theoretical interference inversion in the k-th cyclic iteration, X s (i), Ys (i) and Z s (i) represents the X-axis position component, Y-axis position component, and Z-axis position component when the space-based interferometric imaging radar altimeter performs a forward-looking observation of the i-th reference target, respectively. V xs (i), V ys (i) and V zs (i) represents the X-axis velocity component, Y-axis velocity component, and Z-axis velocity component when the space-based interferometric imaging radar altimeter performs a forward-looking observation of the i-th reference target, respectively. h(i) is the height information of the i-th reference target; R s (i) is the distance from the radar to the center of the earth calculated based on X s (i), Y s (i) and Z s (i). R p (i, k) is the distance from the reference target to the center of the earth calculated based on Long(i, k), Lat(i, k), and h(i, k) through existing coordinate transformation techniques.

[0113] Preferably, the preset condition is: when Δh(i, k) at the k-th loop iteration is less than the preset error value, the radar viewing angle corresponding to the end of the k-th loop is determined as the radar viewing angle at the pixel center position of the interferometric phase image where the i-th reference target is located; θ(i, 0) is the radar viewing angle of the i-th reference target during the initial loop iteration calculation, R p (i, 0) is the initial distance from the reference target to the center of the earth calculated based on the position information and height information of the i-th reference target through existing coordinate transformation techniques.

[0114] The technical solution of this embodiment reduces the complexity of measurement errors through the space-based interferometric imaging radar altimeter phase error model defined in the phase domain, and is applicable to space-based interferometric imaging radar altimeters with horizontal baselines and inclined baselines; by simultaneously considering baseline length errors, equivalent baseline inclination errors, and phase-varying errors, the accuracy of interferometric calibration is improved.

[0115] For the parameters and steps of each module in the above-mentioned space-based interferometric imaging radar altimeter calibration system 200 considering phase variation to implement corresponding functions, reference can be made to the parameters and steps in the implementation of the space-based interferometric imaging radar altimeter calibration method considering phase variation in the above text, and details are not repeated here.

[0116] In the description provided herein, numerous specific details are set forth. It will be understood, however, that embodiments of the invention may be practiced without these specific details. Similarly, in order to streamline the present invention and assist in understanding one or more of the various inventive aspects, in the foregoing description of exemplary embodiments of the present invention, the various features of the embodiments of the present invention are sometimes grouped together in a single embodiment, figure, or description thereof. Among them, the claims following the specific implementation are hereby expressly incorporated into the specific implementation, where each claim itself serves as a separate embodiment of the present invention.

[0117] It should be noted that the above embodiments illustrate the present invention rather than limit the present invention, and those skilled in the art can design alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps not listed in the claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The present invention can be implemented by means of hardware including several different elements and by means of a suitably programmed computer. In a unit claim listing several devices, several of these devices may be embodied by the same item of hardware. The use of the words first, second, and third, etc. does not denote any order. These words may be interpreted as names. The steps in the above embodiments, unless otherwise specified, should not be construed as limiting the order of execution.

Claims

1. A calibration method for a space-based interferometric imaging radar altimeter considering phase spatial variation, characterized in that, Including: Within a preset time period, using a space-based interferometric imaging radar altimeter to obtain an interferometric phase image of a target observation area containing each reference target, and calculating the radar viewing angle at the pixel center position of each reference target in the interferometric phase image; Using the initial radar interferometric parameters to perform combined processing with the radar viewing angle of each reference target respectively to obtain the phase error of each reference target, and based on a polynomial fitting model, the radar viewing angle of each reference target, and the phase error, determining the polynomial fitting coefficients of the polynomial fitting model; wherein, the polynomial fitting model is used to describe the spatial variation relationship of the phase error with the radar viewing angle; Based on the polynomial fitting coefficients and a preset phase error model, obtaining an interferometric calibration result including the baseline length error estimation value, equivalent baseline tilt error estimation value, and phase-varying error estimation value of the space-based interferometric imaging radar altimeter; wherein, the preset phase error model includes a baseline length error, an equivalent baseline tilt error, and a phase-varying error.

2. The space-based interferometric imaging radar altimeter calibration method considering phase spatial variation according to claim 1, wherein, The step of calculating the radar viewing angle at the pixel center position of an interferometric phase image where any reference target is located includes: According to the position information of the any reference target, determining the center position information of the pixel of the interferometric phase image where the reference target is located; According to the pixel center position information of the reference target, the height information of the reference target, the orbital position information and velocity information of the space-based interferometric imaging radar altimeter, and the slant range from the space-based interferometric imaging radar altimeter to the reference target, and by using a preset cyclic iteration algorithm to iteratively calculate the radar viewing angle of the reference target until the radar viewing angle corresponding to when the preset condition is satisfied is determined as the radar viewing angle of the pixel center position of the reference target in the interference phase image; wherein, the preset cyclic iteration algorithm is as follows: i is the serial number of the reference target, k is the number of cyclic iterations, θ(i,k + 1) is the radar viewing angle of the i-th reference target in the (k + 1)-th cyclic iteration, θ(i,k) is the radar viewing angle of the i-th reference target in the k-th cyclic iteration, and θ inc (i,k) is the radar incident angle of the i-th reference target in the k-th cyclic iteration, r1(i) is the slant range from the space-based interferometric imaging radar altimeter to the i-th reference target, Δh(i,k) is the height error of the i-th reference target inversely obtained in the k-th cyclic iteration, Δh(i,k)=h(i,k)-h(i), (Long(i,k), Lat(i,k), h(i,k)) = G -1 {(X s (i), Y s (i), Z s (i)), (V xs (i), V ys (i), V zs (i)), r1(i), θ(i,k)}, k)} -1 {(X s (i), Y s (i), Z s (i)), (V xs (i), V ys (i), V zs (i)), r1(i), θ(i,k)} represents theoretical interference inversion for (X s (i), Y s (i), Z s (i)), (V xs (i), V ys (i), V zs (i)), r1(i), θ(i,k). Long(i,k), Lat(i,k) and h(i,k) are respectively the longitude estimated value, latitude estimated value and height estimated value of the pixel center position of the i-th reference target obtained by the theoretical interference inversion in the k-th cyclic iteration. X s (i), Y s (i) and Z s (i) successively represents the X-axis position component, Y-axis position component, and Z-axis position component when the space-based interferometric imaging radar altimeter performs a forward-looking observation of the i-th reference target, V xs (i), V ys (i), and V zs (i) successively represent the X-axis velocity component, Y-axis velocity component, and Z-axis velocity component when the space-based interferometric imaging radar altimeter performs a forward-looking observation of the i-th reference target, and h(i) is the height information of the i-th reference target; R s (i) is the distance from the radar to the center of the earth calculated based on X s (i), Y s (i), and Z s (i), and R p (i,k) is the distance from the reference target to the center of the earth calculated based on Long(i,k), Lat(i,k), and h(i,k) through existing coordinate transformation techniques.

3. The space-based interferometric imaging radar altimeter calibration method considering phase spatial variation according to claim 2, characterized in that, The preset condition is: when Δh(i,k) at the k-th loop iteration is less than a preset error value, the radar viewing angle corresponding to the end of the k-th loop is determined as the radar viewing angle at the pixel center position of the interferometric phase image where the i-th reference target is located; θ(i,0) is the radar viewing angle of the i-th reference target during the initial loop iteration calculation. R p (i,0) is the initial distance from the reference target to the earth's center calculated based on the position information and altitude information of the i-th reference target and through existing coordinate transformation techniques.

4. The space-based interferometric imaging radar altimeter calibration method considering phase spatial variation according to claim 3, characterized in that The initial radar interferometric parameters include: the radar carrier frequency, initial baseline length, and initial baseline tilt of the space-based interferometric imaging radar altimeter; the step of using the initial radar interferometric parameters and the radar viewing angle of the any reference target to perform combined processing to obtain the phase error of the reference target includes: Substitute the initial radar interferometric parameters and the radar viewing angle of any one of the reference targets into the theoretical interferometric phase formula to obtain the theoretical interferometric phase of the reference target; wherein, the theoretical interferometric phase formula is: θ(i) is the radar viewing angle of the i-th reference target, λ is the radar carrier frequency, B is the initial baseline length, α is the initial baseline inclination angle, and φ(θ(i)) is the theoretical interferometric phase of the i-th reference target; Using the interference phase bias ambiguity number calculation formula, estimate the interference phase bias ambiguity number of any one of the reference targets to obtain the interference phase bias ambiguity number of the reference target, and use the phase error formula to compare the theoretical interference phase of the reference target with the measured interference phase corrected by the interference phase bias ambiguity number of the reference target to obtain the phase error of the reference target; wherein, the interference phase bias ambiguity number calculation formula is: n(i) is the interference phase bias ambiguity number of the i-th reference target, denotes taking the integer of , φ unw (θ(i)) is the measured interference phase of the i-th reference target; the phase error formula is: Δφ unw (θ(i)) = φ unw (θ(i)) - 2πn(i) - φ(θ(i)), Δφ unw (θ(i)) is the phase error of the i-th reference target.

5. The spaceborne interferometric imaging radar altimeter calibration method considering phase spatial variation according to claim 4, characterized in that The step of determining the polynomial fitting coefficients of the polynomial fitting model based on the polynomial fitting model, the radar viewing angle of each reference target, and the phase error includes: According to the polynomial fitting model, the radar viewing angle and phase error of each reference target, and using the least squares method to determine the polynomial fitting coefficients of the polynomial fitting model; wherein, the polynomial fitting model is: p j , j = 0, 1, 2, ..., N are the N + 1 polynomial fitting coefficients of the polynomial fitting model, and N is the polynomial fitting order; the calculation formula for the polynomial fitting coefficients is: Δφ unw (θ(i)), i = 1, 2, ..., M are the phase errors of the M reference targets in the target observation area during the preset time period, and M ≥ (N + 1).

6. The space-based interferometric imaging radar altimeter calibration method considering phase spatial variation according to claim 5, wherein The step of obtaining an interferometric calibration result including the baseline length error estimation value, equivalent baseline tilt error estimation value, and phase-varying error estimation value of the space-based interferometric imaging radar altimeter based on the polynomial fitting coefficients and the preset phase error model includes: Using the preset phase error model and the polynomial fitting model for derivation to respectively obtain a first calculation formula corresponding to the baseline length error, a second calculation formula corresponding to the equivalent baseline tilt error, and a third calculation formula corresponding to the phase-varying error; Substituting the polynomial fitting coefficients into the first calculation formula, the second calculation formula, and the third calculation formula respectively to obtain an interferometric calibration result including the baseline length error estimation value, equivalent baseline tilt error estimation value, and phase-varying error estimation value of the space-based interferometric imaging radar altimeter; Wherein, the preset phase error model is: Δφ unw (θ) is the phase error when the radar viewing angle is θ, φ ps (θ) is the estimated value of the phase spatially variant error when the radar viewing angle is θ, φ b (θ) is the interference phase error introduced by the baseline length error and the baseline tilt error when the radar viewing angle is θ, φ0 is the interference phase offset, φ n (θ) is the random phase error introduced by the radar system noise when the radar viewing angle is θ, ΔB is the estimated value of the baseline length error, Δα eff is the estimated value of the equivalent baseline tilt error, and the first calculation formula is: The second calculation formula is: The third calculation formula is: φ ps (θ) = p3θ 3 + p2θ 2 .

7. The method for calibrating a space-based interferometric imaging radar altimeter considering phase spatial variation according to any one of claims 1-6, characterized in that, Also including: Obtain the baseline length error estimation values, equivalent baseline inclination error estimation values, and phase spatial variation error estimation values of the same observation area in multiple time periods respectively, and obtain the baseline length error estimation values, equivalent baseline inclination error estimation values, and phase spatial variation error estimation values of multiple observation areas in the corresponding time periods respectively; Calculate the average value of all baseline length error estimation values, the average value of all equivalent baseline inclination error estimation values, and the average value of all phase spatial variation error estimation values to optimize the interference calibration result of the spaceborne interferometric imaging radar altimeter.

8. A space-based interferometric imaging radar altimeter calibration system considering phase spatial variation, characterized in that, It includes: An acquisition module, a processing module, and an operation module; The acquisition module is used to: within a preset time period, use the spaceborne interferometric imaging radar altimeter to obtain the interferometric phase image of the target observation area containing each reference target, and calculate the radar viewing angle of the pixel center position of each reference target in the obtained interferometric phase image; The processing module is used to: respectively combine and process the initial radar interferometry parameters with the radar viewing angles of each reference target to obtain the phase error of each reference target, and determine the polynomial fitting coefficients of the polynomial fitting model based on the polynomial fitting model, the radar viewing angles of each reference target, and the phase error; wherein, the polynomial fitting model is used to describe the spatial variation relationship of the phase error with the radar viewing angle; The operation module is used to: based on the polynomial fitting coefficients and a preset phase error model, obtain an interference calibration result including the baseline length error estimation value, equivalent baseline inclination error estimation value, and phase spatial variation error estimation value of the spaceborne interferometric imaging radar altimeter; wherein, the preset phase error model includes a baseline length error, an equivalent baseline inclination error, and a phase spatial variation error.

9. The space-based interferometric imaging radar altimeter calibration system considering phase spatial variation according to claim 8, wherein The acquisition module is specifically used to: Determine the center position information of the pixels of the interferometric phase image where a reference target is located according to the position information of any reference target; According to the pixel center position information of the reference target, the height information of the reference target, the orbital position information and velocity information of the spaceborne interferometric imaging radar altimeter, and the slant range from the spaceborne interferometric imaging radar altimeter to the reference target, and by using a preset loop iteration algorithm to iteratively calculate the radar viewing angle of the reference target until the radar viewing angle corresponding to when the preset condition is satisfied is determined as the radar viewing angle of the pixel center position of the reference target in the interferometric phase image; wherein, the preset loop iteration algorithm is: i is the serial number of the reference target, k is the number of loop iterations, θ(i,k + 1) is the radar viewing angle of the i-th reference target in the (k + 1)-th loop iteration, θ(i,k) is the radar viewing angle of the i-th reference target in the k-th loop iteration, θ inc (i,k) is the radar incident angle of the i-th reference target in the k-th loop iteration, r1(i) is the slant range from the spaceborne interferometric imaging radar altimeter to the i-th reference target, Δh(i,k) is the height error of the i-th reference target inversely retrieved in the k-th loop iteration, Δh(i,k)=h(i,k)-h(i), (Long(i,k),Lat(i,k),h(i,k)) = G -1 {(X s (i),Y s (i),Z s (i)),(V xs (i),V ys (i),V zs (i)),r1(i),θ(i,k)}, G -1 {(X s (i),Y s (i),Z s (i)),(V xs (i),V ys (i),V zs (i)),r1(i),θ(i,k)} represents theoretical interferometric inversion of (X s (i),Y s (i),Z s (i)),(V xs (i),V ys (i),V zs (i)),r1(i),θ(i,k), Logn(i,k), Lat(i,k) and h(i,k) are respectively the longitude estimate value, latitude estimate value and height estimate value of the pixel center position of the i-th reference target obtained by the theoretical interferometric inversion in the k-th loop iteration, X s (i), Y s (i) and Z s (i) represent the X-axis position component, Y-axis position component, and Z-axis position component when the space-based interferometric imaging radar altimeter performs a forward-looking observation of the i-th reference target, respectively, V xs (i), V ys (i) and V zs (i) represent the X-axis velocity component, Y-axis velocity component, and Z-axis velocity component when the space-based interferometric imaging radar altimeter performs a forward-looking observation of the i-th reference target, respectively, and h(i) is the altitude information of the i-th reference target; R s (i) is the distance from the radar to the center of the earth calculated based on X s (i), Y s (i) and Z s (i), and R p (i,k) is the distance from this reference target to the center of the earth calculated based on Long(i,k), Lat(i,k), and h(i,k) through existing coordinate transformation techniques.

10. The space-based interferometric imaging radar altimeter calibration system considering phase spatial variation according to claim 9, characterized in that, The preset condition is: when Δh(i,k) at the k-th loop iteration is less than a preset error value, the radar viewing angle corresponding to the end of the k-th loop is determined as the radar viewing angle of the pixel center position of the interferometric phase image where the i-th reference target is located; θ(i,0) is the radar viewing angle of the i-th reference target during the initial loop iteration calculation. R p (i,0) is the initial distance from the reference target to the earth's center calculated based on the position information and altitude information of the i-th reference target and through existing coordinate transformation techniques.

Images