An ionospheric tomography method based on GEO SAR image sequences

Abstract

The present invention provides an ionospheric tomography method based on GEO SAR image sequences. Based on the tomographic geometry of image sequence tomography, multiple SAR images of a GEO SAR system are used to estimate and obtain a large amount of TEC data on observation paths, thereby increasing the data volume. At the same time, the present invention also proposes a judgment standard for similar ray paths, eliminates redundant TEC data of similar ray paths, and retains only one, thereby improving tomography efficiency. Finally, tomography is performed using the screened TEC observations to achieve accurate and efficient ionospheric tomography.

Description

Technical Field

[0001] The invention belongs to the technical field of synthetic aperture radar, and in particular relates to an ionospheric tomography method based on GEO SAR image sequences. Background Art

[0002] Ionospheric monitoring plays an important role in monitoring the interstellar environment and assisting earthquake prediction. Traditional ionospheric observation methods based on TEC and methods that use ionospheric layered structure models to describe ionospheric characteristics cannot reflect the vertical structure of the ionosphere. Ionospheric tomography technology can effectively solve this problem.

[0003] Three-dimensional ionospheric tomography uses satellite signals to achieve three-dimensional (altitude, longitude, and latitude) spatial imaging of ionospheric electron density. Existing three-dimensional ionospheric tomography methods are based on GNSS data (such as GPS and BeiDou). They measure TEC using ground-based receivers and then perform tomographic inversion of electron density. However, this method is significantly limited due to the impossibility of deploying receivers at high density over a large area in practical engineering. To overcome the unreliability of tomography data in areas without observation stations, ionospheric tomography can be performed using the PS points in the scene based on the GEO SAR system. This method is independent of ground-based receivers and effectively addresses the shortcomings of GNSS ionospheric tomography methods.

[0004] However, existing GEO SAR ionospheric tomography methods use only a single SAR image to estimate TEC data for tomography. This limited amount of observational data and the relatively single observation path angle limit the accuracy of the tomography. Therefore, it is necessary to explore new models for utilizing GEO SAR images in ionospheric tomography to increase the amount of data and achieve more accurate and efficient ionospheric tomography. Summary of the Invention

[0005] To solve the above problems, the present invention provides an ionospheric tomography method based on GEO SAR image sequences, which can achieve more accurate and efficient ionospheric tomography.

[0006] An ionospheric tomography method based on GEO SAR image sequences comprises the following steps:

[0007] From N image A total of N GEO SAR images are obtained image ·N PS ray paths and obtain the TEC observation value on each ray path, where N PS is the number of permanent scatterers in each GEO SAR image;

[0008] Divide the ionospheric tomography area of ​​interest into grids, and record the number of ionospheric tomography grids obtained as N voxel ;

[0009] Obtain the intercept of each ray path on each ionospheric tomography grid;

[0010] After combining each ray path in pairs, perform similar path judgment operation, and perform redundancy removal operation on the combination with similar paths to complete the ray path screening; the similar path judgment operation is to determine whether the number of grids penetrated by the two ray paths in the combination is not less than the set value. The two ray paths are at N voxel Is the sum of the intercept differences on the ionospheric tomography grids not greater than the set value A? thr The redundancy removal operation is as follows: similar path combinations containing the same ray path are merged, and then only one ray path in the union is randomly retained and the rest of the ray paths in the union are deleted;

[0011] The TEC observations on the remaining ray paths are used as the input of the MART algorithm, and the electron density corresponding to each ionospheric tomography grid is calculated according to the MART algorithm.

[0012] Furthermore, N image The setting method is as follows:

[0013]

[0014] in, To round down, t aim is the time resolution of electron density desired by the user, t s is the single accumulation time of GEOSAR image.

[0015] Furthermore, N image The method for acquiring a GEO SAR image is as follows:

[0016] Calculate the latitude and longitude center of the ionospheric tomography area of ​​interest to the user as

[0017] λ center =(λ min +λ max ) / 2

[0018]

[0019] Among them, λ center is the longitude center, is the latitude center, λ min is the lower limit of the longitude of the ionospheric tomography region, λ max is the upper limit of the longitude of the ionospheric tomography region, is the lower latitude limit of the ionospheric tomography region, is the upper latitude limit of the ionospheric tomography region;

[0020] Let the GEO SAR imaging center be the latitude and longitude center And make GEO SAR satellite continuously observe the ionospheric tomography area, every accumulated t s Perform imaging until N image The echo data of GEO SAR images, where t s is the single accumulation time of GEO SAR image.

[0021] Furthermore, the intercept of each ray path on each ionospheric tomography grid is obtained as follows:

[0022] Get the number of ionospheric tomography grids N voxel as follows:

[0023]

[0024] in, is the upper latitude limit of the ionospheric tomography region, is the lower latitude limit of the ionospheric tomography region, λ max is the upper limit of the longitude of the ionospheric tomography region, λ min is the lower limit of the longitude of the ionospheric tomography region, h max is the upper limit of the ionospheric tomography region, h min is the lower limit of the ionospheric tomography region, is the latitude interval in the electron density spatial resolution, Δλ is the longitude interval in the electron density spatial resolution, and Δh is the altitude interval in the electron density spatial resolution;

[0025] Get the i=1,2,...,Nth image ·N PS The ray path and the j=1,2,...,N voxel The entry points of the ionospheric tomography grid and exit intersection

[0026] According to the two intersection points corresponding to each ray path, the intersection points of each ray path at j=1, 2, N are obtained respectively. voxel The intercept on the ionospheric tomographic grid

[0027] Furthermore, the method for obtaining the number Ns of the same grids penetrated by the two ray paths is as follows:

[0028]

[0029]

[0030] Among them, δ() is an auxiliary function, is the first auxiliary variable, is the second auxiliary variable, and there is

[0031]

[0032]

[0033] Where akj is the intercept of the first ray path k in the combination on the j-th ionospheric tomographic grid, and aqj is the intercept of the second ray path q in the combination on the j-th ionospheric tomographic grid.

[0034] Furthermore, the sum of the intercept differences Ap of the two ray paths on the Nvoxel ionospheric tomography grids is obtained as follows:

[0035]

[0036] Among them, Ng is the number of meshes that penetrate the same grid and is not less than the set value The number of combinations, is the intercept of the first ray path k in the pth combination on the jth ionospheric tomographic grid, is the intercept of the second ray path q in the pth combination on the jth ionospheric tomographic grid.

[0037] Beneficial effects:

[0038] The present invention provides an ionospheric tomography method based on GEO SAR image sequences. Based on the tomographic geometry of image sequence tomography, multiple SAR images of a GEO SAR system are used to estimate and obtain a large amount of TEC data on observation paths, thereby increasing the data volume. At the same time, the present invention also proposes a judgment standard for similar ray paths, eliminates redundant TEC data of similar ray paths, and retains only one, thereby improving tomography efficiency. Finally, tomography is performed using the screened TEC observations to achieve accurate and efficient ionospheric tomography. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is a flow chart of the ionospheric tomography method for GEO SAR long-term multi-image combination;

[0040] Figure 2 Schematic diagram of GEO SAR single image tomography and three-image joint tomography observation geometry;

[0041] Figure 3 is the electron density variation of the vertical profile above (7.4°N, 124.2°N);

[0042] Figure 4is the average relative error of GEO SAR single image tomography and three-image joint tomography at different heights. DETAILED DESCRIPTION

[0043] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application.

[0044] When performing ionospheric tomography, the user first determines the ionospheric tomography area of ​​interest, the temporal and spatial resolution of electron density desired by the user, and the accumulation time of each GEO SAR imaging. The specific parameters are shown in Table 1 (all are user input).

[0045] Table 1

[0046]

[0047] Based on this, Figure 1 As shown, the present invention provides an ionospheric tomography method based on a GEO SAR image sequence, comprising the following steps:

[0048] S1: From N image A total of N GEO SAR images are obtained image ·N PS ray paths and obtain the TEC observation value on each ray path, where N PS is the number of permanent scatterers in each GEO SAR image;

[0049] Furthermore, the number of images N in the GEO SAR image sequence used for ionospheric tomography is image The time resolution t of the electron density desired by the user aim Calculation yields:

[0050]

[0051] in, To round down, t aim is the time resolution of electron density desired by the user, t s is the single accumulation time of GEOSAR image.

[0052] N image The method for acquiring a GEO SAR image is as follows:

[0053] According to the latitude and longitude range of the ionospheric tomography area in Table 1, the latitude and longitude center of the target area is calculated as follows: The calculation method is:

[0054] λ center =(λ min +λmax ) / 2

[0055]

[0056] Among them, λ center is the longitude center, is the latitude center, λ min is the lower limit of the longitude of the ionospheric tomography region, λ max is the upper limit of the longitude of the ionospheric tomography region, is the lower latitude limit of the ionospheric tomography region, is the upper latitude limit of the ionospheric tomography region;

[0057] Let the GEO SAR imaging center be the latitude and longitude center And make GEO SAR satellite continuously observe the ionospheric tomography area, every accumulated t s Perform an imaging and observe the geometry as Figure 2 As shown, until N is obtained image The echo data of GEO SAR images, where t s is the single accumulation time of GEO SAR image. At the same time, there are N PS permanent scatterers, and finally, from N image A total of N image ·N PS TEC observations along the ray paths.

[0058] It should be noted that the TEC on the ray path of the permanent scatterer PS point is obtained from GEO SAR echo data. The specific method is shown in the reference: C. Hu, Y. Tian, ​​X. Dong, R. Wang, and T. Long, "Computerized ionospheric tomography based on geosynchronous SAR," Journal of Geophysical Research: Space Physics, vol. 122, no. 2, pp. 2686-2705, 2017.

[0059] S2: Divide the ionospheric tomography area of ​​interest into grids, and record the number of ionospheric tomography grids obtained as N voxel ;

[0060] Among them, the number of ionospheric tomography grids N voxel The specific calculation formula is as follows:

[0061]

[0062] in, is the upper latitude limit of the ionospheric tomography region, is the lower latitude limit of the ionospheric tomography region, λ max is the upper limit of the longitude of the ionospheric tomography region, λ min is the lower limit of the longitude of the ionospheric tomography region, h max is the upper limit of the ionospheric tomography region, h min is the lower limit of the ionospheric tomography region, is the latitude interval in the electron density spatial resolution, Δλ is the longitude interval in the electron density spatial resolution, and Δh is the altitude interval in the electron density spatial resolution.

[0063] S3: Obtain the intercept of each ray path on each ionospheric tomography grid, specifically:

[0064] Get the i=1,2,...,N image ·N PS The ray path and the j=1,2,...,N voxel The entry points of the ionospheric tomography grid and exit intersection It should be noted that calculating the intersection of the ray path and each ionospheric tomography grid is a common mathematical problem of solving the intersection of a straight line (ray path) and a sphere (altitude) or a plane (longitude and latitude). For specific methods, see the reference: Wen Debao. Research on ionospheric tomography algorithm based on GPS and its application [D]. Hubei: Institute of Geodesy and Geophysics, Chinese Academy of Sciences, 2007.

[0065] According to the two intersection points corresponding to each ray path, each ray path is obtained at the j=1,2,.N. v.o,x The intercept on the ionospheric tomographic grid

[0066] S4: Combine each ray path in pairs and perform similar path judgment operation, and perform redundancy removal operation on the combination with similar paths to complete the ray path screening; the similar path judgment operation is to determine whether the number of grids penetrated by the two ray paths in the combination is not less than the set value N. s thr , the two ray paths are at N voxel Is the sum of the intercept differences on the ionospheric tomography grids not greater than the set value A? thr The redundancy removal operation is as follows: similar path combinations containing the same ray path are merged, and then only one ray path in the union is randomly retained and the rest of the ray paths in the union are deleted;

[0067] Specifically, each time two ray paths are taken out, the loop is repeated until all combinations are taken out, and the number of the same grids penetrated by each group (two) of rays is calculated. Let the labels of the two ray paths taken out be k and q, and the number of the same grids penetrated by the two ray paths N is s The calculation formula is as follows:

[0068]

[0069]

[0070] Among them, δ( ) is an auxiliary function, is the first auxiliary variable, is the second auxiliary variable, and there is

[0071]

[0072]

[0073] Among them, a kj is the intercept of the first ray path k in the combination on the jth ionospheric tomographic grid, a qj is the intercept of the second ray path q in the combination on the jth ionospheric tomographic grid.

[0074] Furthermore, the two ray paths are voxel The sum of the intercept differences A on the ionospheric tomography grids p The method to obtain is:

[0075]

[0076] Among them, N g The number of meshes that penetrate the same grid is not less than the set value The number of combinations, is the intercept of the first ray path k in the pth combination on the jth ionospheric tomographic grid, is the intercept of the second ray path q in the pth combination on the jth ionospheric tomographic grid.

[0077] That is to say, when performing similar path judgment operations, set the threshold of the same number of grids and intercept threshold A thr ,when And A p ≤A thr When , record the labels of the two ray paths in the current combination until all combinations are cycled through, and record a total of N g_final Similar ray path combinations. For N g_finalFor each group of similar ray paths, only one ray path data is retained and the data of other ray paths are deleted.

[0078] S5: The TEC observations on the remaining ray paths are used as input to the MART algorithm, and the electron density corresponding to each ionospheric tomography grid is calculated according to the MART algorithm.

[0079] It should be noted that the MART algorithm, an iterative algorithm, uses the TEC observations along the filtered ray paths, setting the initial value and the upper limit of the iteration number, and iteratively calculates the electron density corresponding to each ionospheric tomography grid. The MART algorithm is a conventional ionospheric tomography algorithm. The specific method can be found in the reference: Xiao Hongbo. Ionospheric Tomography and Occultation Inversion Method [D]. Shaanxi: Xidian University, 2007. DOI: 10.7666 / d.Y1035699.

[0080] At this point, all steps are completed.

[0081] Next, an implementation example is given with specific parameters.

[0082] In this example, the present invention considers a GEO SAR satellite operating in a small 8-orbit with a circular antenna diameter of 22 meters, an incidence angle of 30 degrees, and an accumulation time of 200 seconds for one imaging. The ionospheric tomography region of interest is 3.04° to 13.04° latitude, 120.9° to 130.9° longitude, and 200km to 600km altitude. The desired electron density time resolution is 10 minutes, and the spatial resolution is 0.2° × 0.25° × 10km (latitude × longitude × altitude). The electron density distribution at 12:00 UTC on June 18, 2019 is used as the initial value, and the electron density distribution at the same location at 12:00 UTC on June 21, 2019 is inverted.

[0083] First, according to step 1, determine the ionospheric tomography area of ​​interest to the user, the temporal and spatial resolution of the electron density desired by the user, and the accumulation time for each GEO SAR imaging. The specific parameters are shown in Table 2 (all are user input).

[0084] Table 2

[0085]

[0086] Execute step 2 to calculate the number of images in the GEO SAR image sequence used for ionospheric tomography based on the user's desired electron density time resolution of 10 minutes and the GEO SAR single imaging accumulation time of 200 seconds.

[0087] Execute step 3. First, according to the latitude and longitude range of the ionospheric tomography area in Table 2, calculate the latitude and longitude center of the target area as Secondly, let the GEOSAR imaging center be (8.04°N, 125.90°E), and let the GEO SAR satellite continuously observe the target area. After every 200s of accumulation, an image is taken. The observation geometry is as follows: Figure 2 As shown in Figure 3, echo data from three GEO SAR images were obtained. Each GEO SAR image contains 900 PS points. Finally, TEC observations along a total of 2700 ray paths were obtained from the three GEO SAR images.

[0088] Execute step 4. First, calculate the number of grid cells based on the range of the ionospheric tomography region and the desired spatial resolution of electron density:

[0089]

[0090] Secondly, the intersection points of the 2700 ray paths with the 80,000 grid cells are calculated. Finally, the intercepts of the 2700 ray paths at each ionospheric tomography grid cell are calculated.

[0091] Execute step 5 and set the threshold of the same number of grids according to the intercept of the ray path on each ionospheric tomography grid. Intercept threshold A thr = 5 km, similar ray paths are screened and deleted, leaving only one, and finally 2435 rays remain.

[0092] Perform step 6. Using the TEC observations on the 2435 screened ray paths, employ the MART algorithm. With the initial iteration value set to the electron density distribution at 12:00 UTC on June 18, 2019, and an upper limit of 20 iterations, iteratively calculate the electron density distribution at 12:00 UTC on June 21, 2019, for each ionospheric tomography grid, with a time resolution of 10 minutes.

[0093] At this point, all steps are completed.

[0094] Figure 3 The electron density changes in the vertical profile over (7.4°N, 124.2°N) are shown when using GEO SAR single image tomography and three-image joint tomography. The results based on the three-image joint tomography are significantly better than those based on the GEO SAR single image tomography. Figure 4 The average relative error of GEO SAR single image tomography and three-image joint tomography at different heights is shown. The average relative error based on three-image joint tomography is lower, which proves the effectiveness of this algorithm.

[0095] Thus, the present invention provides an ionospheric tomography method based on GEO SAR image sequences. By combining multiple SAR images of the GEOSAR system, a large amount of TEC data on observation paths is obtained. Redundant paths are eliminated, and one similar path is screened and retained. The TEC observation data on the screened ray path is used to perform ionospheric tomography, thereby improving the tomography accuracy while ensuring the tomography efficiency.

[0096] Of course, the present invention may have many other embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art may of course make various corresponding changes and modifications based on the present invention, but these corresponding changes and modifications should all fall within the scope of protection of the claims attached to the present invention.

Claims

1. An ionospheric tomography method based on GEO SAR image sequences, characterized in that: The following steps are involved: From N image A total of N GEO SAR images are obtained image ·N PS ray paths and obtain the TEC observation value on each ray path, where N PS is the number of permanent scatterers in each GEO SAR image; Divide the ionospheric tomography area of ​​interest into grids, and record the number of ionospheric tomography grids obtained as N voxel ; Obtain the intercept of each ray path on each ionospheric tomography grid; After combining each ray path in pairs, perform similar path judgment operation, and perform redundancy removal operation on the combination with similar paths to complete the ray path screening; the similar path judgment operation is to determine whether the number of grids penetrated by the two ray paths in the combination is not less than the set value. The two ray paths are at N voxel Is the sum of the intercept differences on the ionospheric tomography grids not greater than the set value A? thr The redundancy removal operation is as follows: similar path combinations containing the same ray path are merged, and then only one ray path in the union is randomly retained and the rest of the ray paths in the union are deleted; The TEC observations on the remaining ray paths are used as the input of the MART algorithm, and the electron density corresponding to each ionospheric tomography grid is calculated according to the MART algorithm.

2. The ionospheric tomography method based on GEO SAR image sequence according to claim 1, characterized in that: N image The setting method is as follows: in, To round down, t aim is the time resolution of electron density desired by the user, t s is the single accumulation time of GEO SAR image.

3. The ionospheric tomography method based on GEO SAR image sequence according to claim 1, characterized in that: N image The method for acquiring a GEO SAR image is as follows: Calculate the latitude and longitude center of the ionospheric tomography area of ​​interest to the user as Among them, λ center is the longitude center, is the latitude center, λ min is the lower limit of the longitude of the ionospheric tomography region, λ max is the upper limit of the longitude of the ionospheric tomography region, is the lower latitude limit of the ionospheric tomography region, is the upper latitude limit of the ionospheric tomography region; Let the GEO SAR imaging center be the latitude and longitude center And make GEO SAR satellite continuously observe the ionospheric tomography area, every accumulated t s Perform imaging until N image The echo data of GEO SAR images, where t s is the single accumulation time of GEOSAR image.

4. The ionospheric tomography method based on GEO SAR image sequence according to claim 1, characterized in that: The intercept of each ray path on each ionospheric tomography grid is obtained as follows: Get the number of ionospheric tomography grids N voxel as follows: in, is the upper latitude limit of the ionospheric tomography region, is the lower latitude limit of the ionospheric tomography region, λ max is the upper limit of the longitude of the ionospheric tomography region, λ min is the lower limit of the longitude of the ionospheric tomography region, h max is the upper limit of the ionospheric tomography region, h min is the lower limit of the ionospheric tomography region, is the latitude interval in the electron density spatial resolution, Δλ is the longitude interval in the electron density spatial resolution, and Δh is the altitude interval in the electron density spatial resolution; Get the i=1,2,...,N image ·N PS The ray path and the j=1,2,…,N voxel The entry points of the ionospheric tomography grid and exit intersection According to the two intersection points corresponding to each ray path, the intersection points of each ray path at j=1, 2, ..., N are obtained respectively. voxel The intercept on the ionospheric tomographic grid 5. The ionospheric tomography method based on GEO SAR image sequence according to claim 1, characterized in that: The number of identical grids N that the two ray paths penetrate s The method to obtain is: Among them, δ() is an auxiliary function, is the first auxiliary variable, is the second auxiliary variable, and there is Among them, a kj is the intercept of the first ray path k in the combination on the jth ionospheric tomographic grid, a qj is the intercept of the second ray path q in the combination on the jth ionospheric tomographic grid.

6. The ionospheric tomography method based on GEO SAR image sequence according to claim 1, characterized in that: The two ray paths are at N voxel The sum of the intercept differences A on the ionospheric tomography grids p The method to obtain is: Among them, N g The number of meshes that penetrate the same grid is not less than the set value The number of combinations, is the intercept of the first ray path k in the pth combination on the jth ionospheric tomographic grid, is the intercept of the second ray path q in the pth combination on the jth ionospheric tomographic grid.

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