Low ionosphere electron density inversion method based on occultation data

Abstract

The invention relates to a low ionized layer electron density inversion method based on occultation data. The method comprises the following steps: obtaining a secondary data file provided by an occultation item consisting of a plurality of low-orbit satellites, then carrying out TEC data preprocessing, extracting an F-layer component of an ionized layer in a TEC profile according to two-section linear fitting within a certain height interval range based on the preprocessed data, and obtaining an F-layer TEC component of the ionized layer after interpolation; calculating the TEC component of a low ionized layer according to the occultation observation value after interpolation and the TEC component of the F layer of the ionized layer; constructing an electron density inversion linear equation set based on an Abel weighting function according to the height distribution of the TEC component of the low ionized layer, and solving by adopting non-negative least squares to obtain an inverted electron density profile; and performing smooth fitting on the inverted electron density profile by using a Gaussian function to obtain an electron density profile product. By adopting the method, the inversion precision of the electron density of the low ionosphere can be improved.

Description

Technical Field

[0001] This application relates to the technical field of satellite navigation, and particularly to a method for inverting the electron density of the lower ionosphere based on occultation data. Background Art

[0002] The lower ionosphere is an important plasma region in the near-Earth space of the Earth. The distribution and variation of its electron density have a significant impact on systems such as radio communication (especially very low frequency) and satellite navigation and positioning. Due to the low electron density, high neutral particle density, and large electron collision frequency in the lower ionosphere (at an altitude of about 60 - 110 km), it strongly absorbs high-frequency radio waves, even causing communication interruptions. Therefore, accurately inverting the electron density of the lower ionosphere is crucial for understanding the physical processes of the ionosphere and ensuring the stability of communication and navigation systems.

[0003] With the development of satellite technology, the radio occultation (RO) technology based on the Global Navigation Satellite System (GNSS) has gradually become one of the main detection means for the sporadic E layer. Since the successful verification of the occultation inversion technology by GPS / MET (Global Positioning System / Meteorology), other near-Earth orbit satellites such as CHAMP (Challenging Mini-satellite Payload), GRACE (Gravity Recovery and Climate Experiment), FORMOSAT-3 / COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) and other constellations have been successively launched and a large amount of observational data has been obtained. When the GPS signal passes through the ionosphere, its propagation path will bend, and the degree of bending is related to the refractive index of the ionosphere. The electron density of the ionosphere can be obtained by the refractive index of the ionosphere. Ionospheric electron density inversion methods can be divided into spherically symmetric assumption inversion methods and non-spherically symmetric inversion methods. Among them, the spherically symmetric inversion method is the most commonly used method at present, such as the Abel inversion method, the onion layer inversion method, etc. The electron density data obtained by these methods are widely used in research such as ionospheric physics, ionospheric modeling, and magnetosphere-ionosphere-thermosphere coupling.

[0004] Existing inversion algorithms can solve the electron density in the lower ionosphere. However, since the integral method is used to invert the electron density from top to bottom, the inversion error will accumulate continuously, resulting in a large error in the inversion result at the lower ionosphere altitude. Currently, there are few methods specifically for inverting the electron density in the lower ionosphere. A small amount of research has proposed using empirical assumptions to remove the influence of the upper ionosphere on occultation observation data, and combining the electron density profiles provided by other ionosphere models (such as the International Reference Ionosphere) as the background field to comprehensively obtain the electron density information of the lower ionosphere. The error of the electron density in the lower ionosphere in existing occultation observation products is large, which limits their wide application. Summary of the Invention

[0005] Based on this, in view of the above technical problems, it is necessary to provide a method for inverting the electron density of the lower ionosphere based on occultation data, which can improve the inversion accuracy of the electron density of the lower ionosphere and does not depend on the input of other ionosphere models.

[0006] A method for inverting the electron density of the lower ionosphere based on occultation data, the method comprising: Obtain a secondary data file provided by an occultation project composed of multiple low-orbit satellites; perform TEC data preprocessing on the secondary data file to obtain preprocessed data; Based on the preprocessed data, extract the ionospheric F-layer component in the TEC profile according to two-segment linear fitting within a certain altitude range to obtain the interpolated ionospheric F-layer TEC component; calculate the lower ionosphere TEC component according to the interpolated occultation observation value and the interpolated ionospheric F-layer TEC component; Establish a linear equation set of the discretized lower ionosphere TEC component and the electron density to be solved, and use non-negative least squares to solve it to obtain the inverted electron density profile; Use a Gaussian function to perform smooth fitting on the inverted electron density profile to obtain an electron density profile product.

[0007] The above method for inverting the electron density of the lower ionosphere based on occultation data first interpolates the TEC profile observed by occultation to grid points with a fixed height range and interval; extracts the ionospheric F-layer component in the TEC profile according to two-stage linear fitting, and then obtains the TEC component of the lower ionosphere; constructs a linear equation system for electron density inversion based on the Abel weight function according to the height distribution of the TEC component of the lower ionosphere, and uses non-negative least squares for solution; finally, uses a Gaussian function to smooth and fit the inverted electron density profile to obtain the electron density profile product. The two-stage linear fitting adopted in this application can effectively eliminate the ionospheric F-layer component in the TEC profile, effectively reduce the cumulative influence of the upper electron density inversion error, thereby improving the inversion accuracy; the electron density solving method based on the Abel weight function using differential processing can effectively reduce the cumulative error of electron density inversion at different heights in electron density inversion; using non-negative least squares solution and Gaussian function fitting can effectively solve the negative value phenomenon existing in electron density inversion, can improve the inversion accuracy of the lower ionosphere electron density and does not depend on the input of other ionospheric models. Description of the Drawings

[0008] Figure 1 It is a schematic flowchart of a method for inverting the electron density of the lower ionosphere based on occultation data in an embodiment; Figure 2 It is a diagram showing the variation of TEC with height and the two-stage fitting result of the lower ionosphere in a COSMIC occultation event in an embodiment; Figure 3 It is a schematic diagram comparing the inverted electron density results, COSMIC electron density profile products and ground-based ionosonde observation results using the method of this application at different time periods in an embodiment; Figure 3 (a) It is a schematic diagram comparing the inverted electron density results, COSMIC electron density profile products and ground-based ionosonde observation results using the method of this application on July 15, 2007, Figure 3 (b) It is a schematic diagram comparing the inverted electron density results, COSMIC electron density profile products and ground-based ionosonde observation results using the method of this application on October 21, 2007, Figure 3 (c) It is a schematic diagram comparing the inverted electron density results, COSMIC electron density profile products and ground-based ionosonde observation results using the method of this application on November 14, 2007, Figure 3 (d) It is a schematic diagram comparing the inverted electron density results, COSMIC electron density profile products and ground-based ionosonde observation results using the method of this application on December 12, 2007; Figure 4 It is a schematic diagram showing the spatial distribution characteristics of the electron density at an altitude of 100 km in winter 2007 of this application and the COSMIC electron density product in another embodiment;Figure 4 Figure (a) shows the spatial distribution characteristics of electron density at an altitude of 100 km in winter 2007 by the LIDIM method proposed in this application; Figure 4 Figure (b) shows the spatial distribution characteristics of electron density at an altitude of 100 km in winter 2007 by the COSMIC electron density product. Detailed implementation manners

[0009] In order to make the objectives, technical solutions and advantages of this application clearer and more understandable, the following further details this application in combination with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not used to limit this application.

[0010] In one embodiment, as Figure 1 shown, a method for retrieving low ionospheric electron density based on occultation data is provided, including: Step 102: Obtain a secondary data file provided by an occultation project composed of multiple low-orbit satellites; perform TEC data preprocessing on the secondary data file to obtain preprocessed data.

[0011] In this embodiment, the selected occultation project is the COSMIC constellation. The COSMIC constellation consists of 6 low-orbit satellites, which can provide approximately 2,000 occultation observation profiles per day. Each profile contains high-resolution observations at 50 Hz and low-resolution observation data at 1 Hz. This application selects the 1 Hz secondary data file (ionprf) for retrieving low ionospheric electron density. The ionprf file of the COSMIC secondary data file provides the longitude, latitude, altitude, TEC, and electron density products of occultation events. To ensure the accuracy of the inversion algorithm, first confirm that there are at least 3 data points in the TEC profile below 50 km in the occultation data. The occultation data that does not meet the conditions will not be further inverted. Interpolate the qualified TEC profiles to the range of 50 - 200 km at an interval of 1 km. It should be noted that the TEC mentioned in this application refers to the total electron content of the radio wave path between the GPS satellite and the LEO satellite.

[0012] Step 104: Based on the preprocessed data, extract the ionospheric F-layer component in the TEC profile according to two-segment linear fitting within a certain altitude range to obtain the interpolated ionospheric F-layer TEC component; calculate the low ionospheric TEC component according to the interpolated occultation observation values and the interpolated ionospheric F-layer TEC component.

[0013] Existing ionospheric electron density inversion methods based on the spherical symmetry assumption need to solve the integral from the occultation tangent point height to the ionosphere top. Therefore, the inversion errors brought by the spherical symmetry assumption accumulate gradually, that is, the error of the inverted electron density profile increases gradually with the decrease of height. This phenomenon is particularly significant in the low-latitude equatorial anomaly region (where the horizontal distribution of electron density is uneven). If the influence of the F-layer electron density can be removed before the inversion of the low ionospheric electron density, most of the error accumulation can be eliminated, thus improving the inversion accuracy.

[0014] The TEC profile of occultation can be divided into the contribution of the F-layer and the contribution of the E / D layer (low ionosphere). Without considering the contribution of the low ionosphere, when the change of the TEC profile only considers the change of the F-layer electron density and the occultation signal path, the change of TEC is approximately linear below a relatively low height. Therefore, a region with a relatively small contribution of the low ionospheric electron density can be selected to fit this linear change and remove the contribution of the F-layer from the TEC profile of the ionosphere. Since the low ionosphere itself is relatively thin and the electron density is much smaller than that of the F-region, it is assumed that the TEC profile is approximately only the contribution of the F-layer in the range of 50 - 60 km and 120 - 130 km, and a two-segment linear fitting is performed on the TEC profile in this height range: (1) where are the TEC values in the range of 50 - 60 km and 120 - 130 km, is the occultation tangent point height, and are the coefficients to be fitted. The linear fitting function is extrapolated to the height range of 60 - 120 km. This is the estimated TEC contribution of the F-region, which can be subtracted from the original TEC data to obtain the change of the low ionospheric TEC profile, as shown in the following formula: (2) where is the TEC only containing the contribution of the low ionospheric electron density, is the interpolated occultation observation value, is the TEC value in the low ionospheric height range extrapolated using formula (1). In this process, the characteristic that the electron density of the low ionosphere itself is small is utilized, so the influence on the lower layer (below 60 km) is weak, and the characteristic that only the F-region of the ionosphere contributes at a higher height (120 - 130 km) is also utilized, which can better reduce the influence of the F-layer on the inversion of the low ionospheric electron density. At the same time, the algorithm adopts two-segment linear fitting, which can better reduce the influence brought by the non-linear change of the F-layer TEC contribution caused by the drift of the occultation tangent point and the ionospheric inhomogeneity in actual occultation events, and greatly improves the stability of the algorithm.

[0015] Step 106: Establish a linear equation system for the discretized low ionospheric TEC component and the electron density to be solved, and use non - negative least squares to solve it to obtain the inverted electron density profile.

[0016] Under the condition of spherical symmetry assumption and ignoring the influence of electron density above the LEO satellite orbit altitude, the relationship between the total electron content and electron density of occultation observations is as follows: (3) Discretize formula (3) to obtain: (4) Where is the Abel weight function. In this paper, TEC has been interpolated to grid points at 1 - km intervals. Formula (4) can be written in matrix form: ; Where K is the matrix form of the Abel weight function, which is a lower triangular matrix, is the electron density to be solved. Solving this linear equation system can obtain the variation of electron density with altitude.

[0017] To improve the inversion accuracy of electron density, before solving the linear equation system, differential processing is performed on both the left and right sides of the equation. The advantage is that after differentiation, the TEC at a certain altitude is less affected by the electron density at other altitudes, and most of the TEC contribution is determined by the electron density at that altitude. Therefore, it can reduce the accumulation problem of errors in the inversion process and improve the inversion accuracy. In this application, non - negative least squares is used to solve this linear equation system after differential processing to obtain the electron density profile.

[0018] Step 108: Use a Gaussian function to smooth and fit the inverted electron density profile to obtain the electron density profile product.

[0019] Considering that the occultation TEC profile will have small - scale irregular variations in the low ionospheric altitude range, including observation errors and irregularities such as sporadic E - layer, etc., in order to obtain a smooth and stable electron density profile product, it is necessary to smooth the electron density inverted in the previous section.

[0020] This application uses a first - order Gaussian function to fit the electron density profile. This function form is basically consistent with the altitude distribution of low ionospheric electron density, as shown in the following formula: ; Where, is the maximum electron density in the low ionosphere, is the altitude corresponding to the maximum density, is the scale height of the low ionosphere electron density. The final electron density product of the inversion is the variation of the electron density with height after fitting. This fitting can effectively reduce the error of the electron density inversion below 90 km caused by the difference between the two-segment linear fitting and the actual non-linear variation. It should be noted that the RMSE of the fitting result can be used as an index to judge the quality of the electron density profile, and different thresholds can be set for screening in different scenarios.

[0021] The above method for inverting the low ionosphere electron density based on occultation data first interpolates the TEC profile observed by occultation to grid points with a fixed height range and interval; extracts the ionospheric F-layer component in the TEC profile according to the two-segment linear fitting, and then obtains the low ionosphere TEC component; constructs a linear equation set for electron density inversion based on the Abel weight function according to the height distribution of the low ionosphere TEC component, and uses non-negative least squares for solution; finally, smooths and fits the inverted electron density profile with a Gaussian function to obtain the electron density profile product. The two-segment linear fitting adopted in this application can effectively remove the ionospheric F-layer component in the TEC profile, effectively reduce the cumulative influence of the upper electron density inversion error, and thus improve the inversion accuracy; the electron density solution method based on the Abel weight function using differential processing can effectively reduce the cumulative error of the electron density inversion at different heights; the use of non-negative least squares solution and Gaussian function fitting can effectively solve the negative value phenomenon existing in the electron density inversion, can improve the inversion accuracy of the low ionosphere electron density and does not depend on the input of other ionosphere models.

[0022] In one embodiment, the secondary data file includes the longitude, latitude, altitude, TEC profile and electron density product of the occultation event; performing TEC data preprocessing on the secondary data file to obtain the preprocessed data, including: Confirm that there are at least 3 data points in the TEC profile of the secondary data file below 50 km, and interpolate the qualified TEC profile to the range of 50 - 200 km with an interval of 1 km for inversion.

[0023] In one embodiment, a certain height interval range includes the range of 50 - 60 km and 120 - 130 km; based on the preprocessed data, extract the ionospheric F-layer component in the TEC profile according to the two-segment linear fitting in a certain height interval range, and the interpolated ionospheric F-layer TEC component includes: Based on the preprocessed data, extract the ionospheric F-layer component in the TEC profile according to the two-segment linear fitting in a certain height interval range, and the obtained interpolated ionospheric F-layer TEC component is: ; wherein, is in the range of 50 - 60 km and 120 - 130 km value is the occultation tangent point height and are the coefficients to be fitted

[0024] In one embodiment, calculating the low ionosphere TEC component according to the interpolated occultation observation value and the interpolated ionosphere F-layer TEC component includes: Subtracting the TEC value in the low ionosphere height range obtained by extrapolating the linear fitting function to the height range of 60 - 120 km from the interpolated occultation observation value, the low ionosphere TEC component is obtained as: ; wherein is the low ionosphere TEC component is the interpolated occultation observation value is the interpolated ionosphere F-layer TEC component, that is, the TEC value in the low ionosphere height range

[0025] In one embodiment, constructing a linear equation set for electron density inversion based on the Abel weight function according to the height distribution of the low ionosphere TEC component and solving it by non - negative least squares to obtain the inverted electron density profile, includes: Under the condition of spherical symmetry assumption and ignoring the influence of electron density above the LEO satellite orbit height, designing the relationship between the total electron density content and electron density of occultation observation; Discretizing the relationship between the total electron density content and electron density of occultation observation to obtain the discretized low ionosphere TEC component; Interpolating the discretized low ionosphere TEC component to grid points at 1 km intervals , obtaining a linear equation set containing the electron density to be solved; solving it by non - negative least squares to obtain the inverted electron density profile

[0026] In one embodiment, the relationship between the total electron density content and electron density of occultation observation is designed as: ; wherein is the occultation tangent point height is the electron density to be solved represents the LEO satellite orbit height represents height h is the product of the height and the electron density function represents the height variable represents the GPS satellite height

[0027] ​In one embodiment, the relationship between the total electron content and the electron density of occultation observations is discretized to obtain the discretized low ionosphere TEC component, including: The relationship between the total electron content and the electron density of occultation observations is discretized to obtain the discretized low ionosphere TEC component as: ; Wherein, is the Abel weight function.

[0028] In one embodiment, a linear equation system of the discretized low ionosphere TEC component and the electron density to be solved is established, including: The discretized low ionosphere TEC component has been interpolated to grid points at 1 km intervals during data preprocessing, that is , and the linear equation system containing the electron density to be solved is obtained as: ; Wherein, K is the matrix form of the Abel weight function and is a lower triangular matrix, is the electron density to be solved.

[0029] The left and right sides of the equation are differenced, and the non - negative least - squares method is used to solve for the electron density, obtaining the low ionosphere electron density profile with a height resolution of 1 km.

[0030] In one embodiment, the inverted electron density profile is smoothed and fitted using a Gaussian function to obtain the electron density profile product, including: The inverted electron density profile is smoothed and fitted using a Gaussian function to obtain the electron density profile product as: ; Wherein, is the maximum electron density of the low ionosphere, is the height corresponding to the maximum density, is the electron density scale height of the low ionosphere.

[0031] In a specific embodiment, as Figure 2 shows, the variation of TEC with height in a COSMIC occultation event in 2007 (black solid line) is given. At the same time, the fitting result of the two - stage linear fitting described in this application to it (orange solid line) is given. The COSMIC constellation consists of 6 low - earth orbit satellites, which can provide about 2000 occultation observation profiles per day. Each profile contains high - resolution observations at 50 Hz and low - resolution observation data at 1 Hz. As Figure 2As shown, below about 160 km, the F-layer component of TEC basically decreases linearly with the decrease of altitude. This basic feature is the basic assumption for inverting the electron density in the lower ionosphere. Through two-stage linear fitting, the distribution of this linear relationship between 60 - 150 km can be given, as shown by the orange solid line in the figure. The difference between the two is the TEC component in the lower ionosphere (the part in the blue box in the figure). It should be noted that in fact, this linear relationship may also be affected by factors such as occultation tangent point drift, changes in the ionosphere itself, and observation errors. In some cases, it may even lead to the complete invalidation of the linear relationship or a large difference between the fitted linear change and the actual situation.

[0032] To verify the inversion accuracy of this application, the electron density inversion results of this application, the electron density products provided by COSMIC, and the observation results of ground-based ionosondes were compared, as Figure 3 shown in the following four cases. Figure 3 (a) is a schematic diagram comparing the inverted electron density results using the method of this application, the COSMIC electron density profile products, and the observation results of ground-based ionosondes on July 15, 2007. Figure 3 (b) is a schematic diagram comparing the inverted electron density results using the method of this application, the COSMIC electron density profile products, and the observation results of ground-based ionosondes on October 21, 2007. Figure 3 (c) is a schematic diagram comparing the inverted electron density results using the method of this application, the COSMIC electron density profile products, and the observation results of ground-based ionosondes on November 14, 2007. Figure 3 (d) is a schematic diagram comparing the inverted electron density results using the method of this application, the COSMIC electron density profile products, and the observation results of ground-based ionosondes on December 12, 2007. Since occultation observation is a space-based observation method and the critical frequency data of the ionosonde are ground-based observation data, space-time matching is required before comparison. The matching principle in this article is: the difference between the average longitude and latitude of the occultation tangent point and the longitude and latitude of the ground-based ionosonde is within 1 o degree, and the difference between the occultation and ground-based observation times is within 1 hour. The red solid line in the figure is the electron density inversion result of the LIDIM method of this application, the blue solid line is the electron density profile product provided by COSMIC, and the red dots are the electron density and altitude corresponding to the E-layer critical frequency observed by the ground-based ionosonde. It can be seen from the figure that the LIDIM method proposed in this application has the following advantages: (1) Using non-negative least squares solution and Gaussian function fitting effectively avoids the situation of negative electron density in the COSMIC products; (2) In terms of inversion accuracy, the LIDIM method in this article is closer to the observation of the ground-based ionosonde, and the error of the provided electron density profile is smaller.

[0033] This application further statistically analyzed the spatio-temporal characteristics of the low ionospheric electron density on a global scale and compared them with the existing distribution characteristics to further confirm the performance of the algorithm proposed in this application. Figure 4 (a) and Figure 4 (b) respectively show the spatial distribution characteristics of the electron density at an altitude of 100 km in winter 2007 by the LIDIM method proposed in this application and the COSMIC electron density product. It can be seen from the figure that both show some basic characteristics of the low ionospheric electron density: (1) There are two high-density-like banded regions in the high-latitude regions, which is due to the enhanced E-layer electron density caused by the precipitation of high-energy particles in the polar regions; (2) The high-value regions of the low ionospheric electron density in the mid- and low-latitude regions mainly appear on the dayside, and the electron density on the nightside approaches 0, which is consistent with the theory that the generation of low ionospheric electrons is mainly caused by photoionization. However, the algorithm proposed in this paper has the following advantages compared with the COSMIC product: (1) There are many negative value regions in the nightside of the mid- and low-latitude regions in COSMIC, while the low ionospheric electron density cannot be negative. The algorithm proposed in this paper solves this problem well; (2) COSMIC shows a double-peak structure similar to the equatorial anomaly of the ionospheric F layer in the low-latitude regions. This phenomenon does not conform to the distribution law of the low ionospheric electron density, and this abnormal structure has not been found in the existing International Reference Ionosphere; according to the existing research, this is because the equatorial anomaly structure of the F layer leads to a large horizontal gradient of the electron density in the ionosphere, that is, it does not conform to the spherical symmetry hypothesis condition, which makes the error of the low ionospheric electron density of the COSMIC product particularly obvious in this region; the inversion method proposed in this application preferably eliminates the influence of the equatorial anomaly of the F layer on the low ionospheric electron density, and the spatial distribution is basically consistent with the existing cognition, indicating that the method proposed in this paper has high accuracy.

[0034] It should be understood that although Figure 1 the steps in the flowchart of Figure 1 are shown in sequence according to the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless there is a clear indication in this article, the execution of these steps has no strict order limit, and these steps can be executed in other orders. Moreover,

[0035] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope recorded in this specification.

[0036] The above-described embodiments merely represent several implementation manners of the present application. The description thereof is relatively specific and detailed, but it should not be construed as a limitation on the scope of the present application. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several modifications and improvements can still be made, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the appended claims.

Claims

1. A method for inverting the electron density of the lower ionosphere based on occultation data, characterized in that The method includes: Obtaining a secondary data file provided by an occultation project composed of multiple low-earth orbit satellites; performing TEC data preprocessing on the secondary data file to obtain preprocessed data; Based on the preprocessed data, extracting the ionospheric F-layer component in the TEC profile according to two-segment linear fitting within a certain altitude range to obtain the interpolated ionospheric F-layer TEC component; calculating the low ionosphere TEC component according to the interpolated occultation observation value and the interpolated ionospheric F-layer TEC component; Establishing a linear equation system of the discretized low ionosphere TEC component and the electron density to be solved, and using non-negative least squares to solve it to obtain the inverted electron density profile; Performing smooth fitting on the inverted electron density profile using a Gaussian function to obtain an electron density profile product.

2. The method according to claim 1, wherein The secondary data file includes the longitude, latitude, altitude, TEC profile, and electron density product of the occultation event; Performing TEC data preprocessing on the secondary data file to obtain preprocessed data, including: Confirming that there are at least 3 data points in the TEC profile of the secondary data file below 50 km, and interpolating the qualified TEC profile into the range of 50 - 200 km at an interval of 1 km for inversion.

3. The method according to claim 1, wherein The certain altitude range includes the ranges of 50 - 60 km and 120 - 130 km; based on the preprocessed data, extracting the ionospheric F-layer component in the TEC profile according to two-segment linear fitting within a certain altitude range to obtain the interpolated ionospheric F-layer TEC component, including: Based on the preprocessed data, extracting the ionospheric F-layer component in the TEC profile according to two-segment linear fitting within a certain altitude range to obtain the interpolated ionospheric F-layer TEC component as: ； Among them, is the value in the range of 50 - 60 km and 120 - 130 km value, is the occultation tangent point height, and are the coefficients to be fitted.

4. The method according to claim 1, characterized in that, Calculating the low ionosphere TEC component according to the interpolated occultation observation value and the interpolated ionospheric F-layer TEC component, including: Subtracting the TEC value in the low ionosphere altitude range obtained by extrapolating the linear fitting function to the altitude range of 60 - 120 km from the interpolated occultation observation value to obtain the low ionosphere TEC component as: ； Among them, is the low ionosphere TEC component, is the occultation observation value after interpolation, is the TEC component of the ionospheric F layer after interpolation, that is, the TEC value within the low ionosphere height range.

5. The method according to claim 1, wherein Constructing a linear equation system for electron density inversion based on the Abel weight function according to the altitude distribution of the low ionosphere TEC component and using non-negative least squares to solve it to obtain the inverted electron density profile, including: Under the condition of spherical symmetry assumption and ignoring the influence of electron density above the LEO satellite orbit altitude, designing the relationship between the total electron content of occultation observation and electron density; Performing discretization processing on the relationship between the total electron content of occultation observation and electron density to obtain the discretized low ionosphere TEC component; Interpolate the discretized low ionosphere TEC components to grid points at 1 km intervals , obtaining a system of linear equations containing the electron density to be determined; solve it using non - negative least squares to obtain the inverted electron density profile.

6. The method according to claim 5, wherein Designing the relationship between the total electron content of occultation observation and electron density as: ； Among them, is the occultation tangent point height, is the electron density to be determined, represents the low Earth orbit satellite altitude, represents altitude h multiplied by the electron density function and represents the altitude variable, represents the GPS satellite altitude.

7. The method according to claim 6, wherein Performing discretization processing on the relationship between the total electron content of occultation observation and electron density to obtain the discretized low ionosphere TEC component, including: Performing discretization processing on the relationship between the total electron content of occultation observation and electron density to obtain the discretized low ionosphere TEC component as: ； Among them, is the Abel weight function.

8. The method according to claim 5, wherein Establish a linear equation system for the discretized low ionosphere TEC component and the electron density to be solved, including: The discretized low ionosphere TEC component has been interpolated onto grid points at 1 km intervals during data preprocessing, that is , and the resulting system of linear equations containing the electron density to be solved for is: ； wherein, K is in the form of an Abel weight function matrix and is a lower triangular matrix, which is the electron density to be solved; Perform difference processing on both sides of the equation, and use non-negative least squares method to solve the electron density, obtaining a low ionosphere electron density profile with a height resolution of 1 km.

9. The method according to claim 5, characterized in that, Use a Gaussian function to perform smooth fitting on the inverted electron density profile to obtain an electron density profile product, including: The electron density profile product obtained by using a Gaussian function to perform smooth fitting on the inverted electron density profile is: ； wherein, is the maximum electron density of the lower ionosphere, is the height corresponding to the maximum density, is the scale height of the electron density of the lower ionosphere.

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