Inversion method of electron density in the lower ionosphere based on occultation data

Abstract

The present application relates to a method for inverting the electron density of the lower ionosphere based on occultation data. The method comprises: obtaining secondary data files provided by an occultation project consisting of multiple low-orbit satellites, performing TEC data preprocessing, extracting the ionospheric F layer component from the TEC profile within a certain altitude range based on the preprocessed data using a two-stage linear fit, and obtaining an interpolated ionospheric F layer TEC component; calculating the lower ionospheric TEC component based on the interpolated occultation observations and the ionospheric F layer TEC component; constructing an electron density inversion linear equation system based on the Abel weight function according to the altitude distribution of the lower ionospheric TEC component and solving it using non-negative least squares to obtain an inverted electron density profile; and smoothing the inverted electron density profile using a Gaussian function to obtain an electron density profile product. The present method can improve the accuracy of lower ionospheric electron density inversion.

Description

Technical Field

[0001] The present application relates to the field of satellite navigation technology, and in particular to a method for inverting low ionosphere electron density based on occultation data. Background Art

[0002] The lower ionosphere is a critical plasma region in Earth's near-Earth space. The distribution and variations of its electron density significantly impact systems such as radio communications (especially very low frequency) and satellite navigation and positioning. Due to its low electron density and high density of neutral particles, the lower ionosphere (approximately 60-110 km in altitude) experiences a high frequency of electron collisions, resulting in strong absorption of high-frequency radio waves and even leading to communication disruptions. Therefore, accurately inverting the electron density in the lower ionosphere is crucial for understanding ionospheric physics and ensuring the stability of communication and navigation systems.

[0003] With the advancement of satellite technology, radio occultation (RO) technology based on the Global Positioning System (GNSS) has gradually become one of the primary means of detecting sporadic E-layers. Since the successful demonstration of occultation inversion technology by the GPS / MET (Global Positioning System / Meteorology) constellation, other low-Earth orbit satellites, such as the CHAMP (Challenging Mini-satellite Payload), GRACE (Gravity Recovery and Climate Experiment), and FORMOSAT-3 / COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate), have been launched and have acquired a wealth of observational data. The propagation path of GPS signals through the ionosphere is bent, and the degree of bending is related to the ionosphere's refractive index. The ionosphere's refractive index can be used to decouple the ionospheric electron density. Ionospheric electron density inversion methods can be divided into spherically symmetric inversion methods and non-spherically symmetric inversion methods. Spherically symmetric inversion methods are currently the most commonly used, including the Abel inversion method and the onion layer inversion method. The electron density data inverted by these methods are widely used in studies of ionospheric physics, ionospheric modeling, magnetosphere-ionosphere-middle and upper atmosphere coupling, etc.

[0004] Existing inversion algorithms can determine the electron density of the lower ionosphere. However, because the electron density is inverted from top to bottom using an integral method, inversion errors accumulate, resulting in large errors in the inversion results at low ionosphere altitudes. Currently, there are few methods specifically designed to invert the electron density of the lower ionosphere. A small number of studies have proposed using empirical assumptions to remove the influence of the upper ionosphere on occultation observation data, combining electron density profiles provided by other ionospheric models (such as the International Reference Ionosphere) as a background field to obtain low ionospheric electron density information. Existing occultation observation products suffer from large errors in the electron density of the lower ionosphere, limiting their widespread application. Summary of the Invention

[0005] Based on this, it is necessary to provide a low ionospheric electron density inversion method based on occultation data that can improve the inversion accuracy of the low ionospheric electron density and does not rely on the input of other ionospheric models to address the above technical problems.

[0006] A method for inverting electron density in the lower ionosphere based on occultation data, the method comprising:

[0007] Obtain secondary data files provided by an occultation project consisting of multiple low-orbit satellites; perform TEC data preprocessing on the secondary data files to obtain preprocessed data;

[0008] Based on the preprocessed data, the F-layer component of the ionospheric layer is extracted from the TEC profile within a certain altitude range using a two-segment linear fit to obtain the interpolated F-layer TEC component of the ionospheric layer; the TEC component of the lower ionosphere is calculated based on the interpolated occultation observation value and the interpolated F-layer TEC component of the ionospheric layer;

[0009] A linear equation system is established between the discretized lower ionospheric TEC components and the electron density to be determined and solved using non-negative least squares to obtain the inverted electron density profile.

[0010] The inverted electron density profile is smoothly fitted using a Gaussian function to obtain the electron density profile product.

[0011] The above-mentioned low ionospheric electron density inversion method based on occultation data first interpolates the TEC profile of the occultation observation to grid points with a fixed height range and interval; extracts the ionospheric F layer component in the TEC profile based on two-stage linear fitting, and then obtains the low ionospheric TEC component; constructs an electron density inversion linear equation group based on the Abel weight function according to the height distribution of the low ionospheric TEC component, and solves it using non-negative least squares; finally, uses a Gaussian function to smooth the inverted electron density profile to obtain the electron density profile product. The two-stage linear fitting used in this application can effectively eliminate the ionospheric F layer component in the TEC profile, effectively reduce the cumulative effect 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 accumulation of electron density inversion errors at different heights in the electron density inversion; the use of non-negative least squares solution and Gaussian function fitting can effectively solve the negative value phenomenon in the electron density inversion, which can improve the low ionospheric electron density inversion accuracy and is independent of other ionospheric model inputs. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 1 is a flow chart of a method for inverting the electron density of the lower ionosphere based on occultation data in one embodiment;

[0013] Figure 2 1. A diagram showing the variation of TEC with altitude during a COSMIC occultation event and the two-stage fitting results of the lower ionosphere in one embodiment;

[0014] Figure 3 Schematic diagram showing the comparison of the inverted electron density results, COSMIC electron density profile products, and ground-based altimeter observation results using the method of the present application at different time periods in one embodiment; Figure 3 (a) is a schematic diagram comparing the electron density inversion results using the method of this application, the COSMIC electron density profile product, and the ground-based altimeter observation results on July 15, 2007. Figure 3 (b) is a schematic diagram comparing the electron density inversion results using the method of this application, the COSMIC electron density profile product, and the ground-based altimeter observation results on October 21, 2007. Figure 3 (c) November 14, 2007 is a schematic diagram comparing the inverted electron density results using the method of this application, the COSMIC electron density profile product, and the ground-based altimeter observation results. Figure 3 (d) is a schematic diagram comparing the electron density inversion results using the method of this application, the COSMIC electron density profile product, and the ground-based altimeter observation results on December 12, 2007;

[0015] Figure 4FIG1 is a schematic diagram of the spatial distribution characteristics of electron density of the present application and COSMIC electron density product at an altitude of 100 km in the winter of 2007 in another embodiment; Figure 4 (a) shows the spatial distribution characteristics of electron density at an altitude of 100 km in the winter of 2007 using the LIDIM method proposed in this application; Figure 4 (b) shows the spatial distribution characteristics of electron density at an altitude of 100 km in the winter of 2007 of the COSMIC electron density product. DETAILED DESCRIPTION

[0016] In order to make the purpose, technical solutions and advantages of this application more clear, the following further describes this application in detail with reference to 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 intended to limit this application.

[0017] In one embodiment, Figure 1 As shown in FIG, a method for inverting the electron density in the lower ionosphere based on occultation data is provided, including:

[0018] Step 102: Acquire a secondary data file provided by an occultation project consisting of multiple low-orbit satellites; perform TEC data preprocessing on the secondary data file to obtain preprocessed data.

[0019] In this embodiment, the occultation project selected is the COSMIC constellation, which consists of six low-orbit satellites and can provide approximately 2,000 occultation observation profiles per day. Each profile contains 50 Hz high-resolution observations and 1 Hz low-resolution observation data. This application uses a 1 Hz secondary data file (ionprf) for low-ionospheric electron density inversion. The COSMIC secondary data file ionprf provides the longitude, latitude, altitude, TEC, and electron density products of the occultation event. To ensure the accuracy of the inversion algorithm, it is first confirmed that there are at least three data points in the occultation data with TEC profiles below 50 km. Occultation data that do not meet the conditions will not be further inverted. The TEC profiles that meet the conditions are interpolated to a range of 50-200 km, with an interval of 1 km. It should be noted that the TEC mentioned in this application refers to the total electron density content on the radio wave path between the GPS satellite and the LEO satellite.

[0020] Step 104 : Based on the preprocessed data, the ionospheric F layer component in the TEC profile is extracted according to a two-segment linear fit within a certain altitude range to obtain an interpolated ionospheric F layer TEC component; and the lower ionospheric TEC component is calculated based on the interpolated occultation observation value and the interpolated ionospheric F layer TEC component.

[0021] Existing ionospheric electron density inversion methods based on the assumption of spherical symmetry require integrating the data from the occultation tangent point to the top of the ionosphere. Consequently, the inversion errors introduced by this assumption accumulate gradually, meaning that the error in the inverted electron density profile increases with decreasing altitude. This phenomenon is particularly pronounced 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 eliminated before inverting the low-ionospheric electron density, much of this accumulated error can be eliminated, thereby improving inversion accuracy.

[0022] The TEC profile of an occultation can be divided into contributions from the F layer and the E / D layer (lower ionosphere). If the contribution from the lower ionosphere is ignored and the TEC profile changes only based on the changes in the F layer electron density and the occultation signal path, the TEC variation below lower altitudes is approximately linear. Therefore, we can select regions with low contributions from the lower ionosphere electron density to fit this linear variation and remove the F layer contribution from the ionospheric TEC profile. Because the lower ionosphere is inherently thin and has a much lower electron density than the F region, we assume that the TEC profile in the 50-60 km and 120-130 km ranges is approximately solely contributed by the F layer. A two-segment linear fit is performed on the TEC profile within these altitude ranges:

[0023] (1)

[0024] in, It is the TEC value within the range of 50-60km and 120-130km, is the height of the occultation tangent point, and The linear fitting function is extrapolated to the altitude range of 60-120 km, which is the estimated F region TEC contribution. Subtracting it from the original TEC data can obtain the change of the lower ionospheric TEC profile, as shown in the following formula:

[0025] (2)

[0026] in is the TEC that only includes the contribution from the lower ionospheric electron density, is the interpolated occultation observation value, The TEC values ​​for the lower ionosphere altitude range are obtained by extrapolating formula (1). This process takes advantage of the low electron density of the lower ionosphere itself, which has a weak impact on the lower layers (below 60 km). Furthermore, the fact that only the F region of the ionosphere contributes at higher altitudes (120-130 km) can effectively reduce the influence of the F layer on the inversion of the lower ionosphere electron density. At the same time, the algorithm uses a two-stage linear fit to effectively reduce the impact of nonlinear changes in the F layer TEC contribution caused by occultation tangent point drift and ionospheric inhomogeneities in actual occultation events, greatly improving the stability of the algorithm.

[0027] Step 106: Establish a linear equation system between the discretized lower ionospheric TEC component and the electron density to be determined and solve it using non-negative least squares to obtain the inverted electron density profile.

[0028] Under the assumption of spherical symmetry and ignoring the effect of electron density above the LEO satellite orbital altitude, the relationship between the total electron density content and the electron density of the occultation observation is as follows:

[0029] (3)

[0030] Discretize formula (3) to obtain:

[0031] (4)

[0032] in This is the Abel weight function. In this paper, TEC has been interpolated to grid points with a spacing of 1 km. , formula (4) can be written in matrix form:

[0033] ;

[0034] Where K is the Abel weight function matrix form, which is a lower triangular matrix, is the electron density to be determined. Solving this linear equation system gives the variation of electron density with height.

[0035] To improve the inversion accuracy of electron density, we perform a differential operation on the left and right sides of the linear system before solving it. This has the advantage that after the differential operation, the TEC at a certain height is less affected by the electron density at other heights. The majority of the TEC contribution is determined by the electron density at that height, thus reducing the accumulation of errors during the inversion process and improving the inversion accuracy. This application uses non-negative least squares to solve the linear system of equations after performing the differential operation to obtain the electron density profile.

[0036] Step 108: Use a Gaussian function to perform smooth fitting on the inverted electron density profile to obtain an electron density profile product.

[0037] Considering that the occultation TEC profile will have small irregular changes in the low ionosphere altitude range, which includes the influence of observation errors and occasional irregular bodies such as the E layer, in order to obtain a smooth and stable electron density profile product, the electron density inverted in the previous section needs to be smoothed.

[0038] This application uses a first-order Gaussian function to fit the electron density profile. The form of this function is basically consistent with the electron density height distribution in the lower ionosphere, as shown in the following formula:

[0039] ;

[0040] in, is the maximum electron density in the lower ionosphere, is the height corresponding to the maximum density, is the lower ionospheric electron density elevation. The fitted variation of electron density with altitude is the final inverted electron density product. This fitting effectively reduces the electron density inversion error below 90 km caused by the difference between the two-stage linear fit and the actual nonlinear variation. It is important to note that the RMSE of the fitting result can be used as an indicator to judge the quality of the electron density profile, and different thresholds can be set for screening in different scenarios.

[0041] The above-mentioned low ionospheric electron density inversion method based on occultation data first interpolates the TEC profile of the occultation observation to grid points with a fixed height range and interval; extracts the ionospheric F layer component in the TEC profile based on two-stage linear fitting, and then obtains the low ionospheric TEC component; constructs an electron density inversion linear equation group based on the Abel weight function according to the height distribution of the low ionospheric TEC component, and solves it using non-negative least squares; finally, uses a Gaussian function to smooth the inverted electron density profile to obtain the electron density profile product. The two-stage linear fitting used in this application can effectively eliminate the ionospheric F layer component in the TEC profile, effectively reduce the cumulative effect 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 accumulation of electron density inversion errors at different heights in the electron density inversion; the use of non-negative least squares solution and Gaussian function fitting can effectively solve the negative value phenomenon in the electron density inversion, which can improve the low ionospheric electron density inversion accuracy and is independent of other ionospheric model inputs.

[0042] In one embodiment, the secondary data file includes the longitude, latitude, altitude, TEC profile, and electron density product of the occultation event; and TEC data preprocessing is performed on the secondary data file to obtain preprocessed data, including:

[0043] Confirm that there are at least three data points with TEC profiles below 50 km in the secondary data file, and interpolate the qualified TEC profiles to the range of 50-200 km with an interval of 1 km for inversion.

[0044] In one embodiment, the certain altitude range includes the range of 50-60 km and the range of 120-130 km; based on the preprocessed data, the ionospheric F layer component in the TEC profile is extracted according to the two-segment linear fitting within the certain altitude range, and the interpolated ionospheric F layer TEC component includes:

[0045] Based on the preprocessed data, the ionospheric F layer component in the TEC profile is extracted according to the two-segment linear fitting within a certain altitude range. The interpolated ionospheric F layer TEC component is obtained as follows:

[0046] ;

[0047] in, It is within the range of 50-60km and 120-130km value, is the height of the occultation tangent point, and are the coefficients to be fitted.

[0048] In one embodiment, calculating the lower ionospheric TEC component based on the interpolated occultation observation value and the interpolated ionospheric F layer TEC component includes:

[0049] The TEC value of the lower ionosphere altitude range obtained by extrapolating the linear fitting function to the altitude range of 60-120 km is subtracted from the interpolated occultation observation value to obtain the lower ionosphere TEC component:

[0050] ;

[0051] in, is the lower ionospheric TEC component, is the interpolated occultation observation value, is the interpolated TEC component of the ionospheric F layer, that is, the TEC value within the low ionospheric altitude range.

[0052] In one embodiment, an electron density inversion linear equation system based on an Abel weighting function is constructed according to the height distribution of the lower ionospheric TEC component and solved using non-negative least squares to obtain an inverted electron density profile, including:

[0053] Under the assumption of spherical symmetry and ignoring the effect of electron density above the LEO satellite orbital altitude, the relationship between the total electron density content and electron density in occultation observations is designed.

[0054] The relationship between the total electron density content and the electron density of the occultation observation is discretized to obtain the discretized lower ionospheric TEC component.

[0055] The discretized lower ionospheric TEC component is interpolated to a grid point with a spacing of 1 km. , a set of linear equations containing the electron density to be determined is obtained; non-negative least squares is used to solve it and the inverted electron density profile is obtained.

[0056] In one embodiment, the relationship between the total electron density content and the electron density of the designed occultation observation is:

[0057] ;

[0058] in, is the height of the occultation tangent point, is the electron density to be determined, represents the orbital height of the low-orbit satellite, Indicates height h and electron density function The product of represents the height variable, Indicates GPS satellite altitude.

[0059] In one embodiment, the relationship between the total electron density content and the electron density of the occultation observation is discretized to obtain the discretized lower ionospheric TEC component, including:

[0060] The relationship between the total electron density content and the electron density of the occultation observation is discretized, and the discretized lower ionospheric TEC component is obtained as follows:

[0061] ;

[0062] in, This is the Abel weight function.

[0063] In one embodiment, a linear equation system is established for the discretized lower ionospheric TEC component and the electron density to be determined, including:

[0064] The discretized lower ionospheric TEC component has been interpolated to a grid point with a spacing of 1 km in data preprocessing, that is, , the linear equations containing the electron density to be determined are:

[0065] ;

[0066] Among them, K is the Abel weight function matrix form, which is a lower triangular matrix. is the electron density to be determined.

[0067] The left and right sides of the equation are differentiated, and the electron density is solved using the non-negative least squares method to obtain the low ionospheric electron density profile with a height resolution of 1 km.

[0068] In one embodiment, smooth fitting the inverted electron density profile using a Gaussian function to obtain an electron density profile product includes:

[0069] The inverted electron density profile is smoothly fitted using a Gaussian function to obtain the electron density profile product:

[0070] ;

[0071] in, is the maximum electron density in the lower ionosphere, is the height corresponding to the maximum density, It is the electron density elevation of the lower ionosphere.

[0072] In a specific embodiment, Figure 2 As shown in the figure, the variation of TEC with altitude during a COSMIC occultation event in 2007 is given (black solid line), and the result of the two-stage linear fitting described in this application is also given (orange solid line). The COSMIC constellation consists of 6 low-orbit satellites, which can provide about 2000 occultation observation profiles per day, each profile contains 50Hz high-resolution observation data and 1 Hz low-resolution observation data. Figure 2 As shown in the figure, below approximately 160 km, the F-layer component of TEC decreases approximately linearly with decreasing altitude. This fundamental characteristic is the basic assumption for inverting the electron density in the lower ionosphere. A two-stage linear fit reveals the distribution of this linear relationship between 60 and 150 km, as shown by the orange solid line in the figure. The difference between the two is the TEC component in the lower ionosphere (indicated by the blue box in the figure). It should be noted that in practice, this linear relationship can be affected by factors such as occultation tangent point drift, changes in the ionosphere itself, and observational errors. In some cases, this linear relationship may not hold at all, or the fitted linear variation may differ significantly from the actual variation.

[0073] In order 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 the ground-based altimeter were compared, such as Figure 3 The four cases shown, Figure 3 (a) is a schematic diagram comparing the electron density inversion results using the method of this application, the COSMIC electron density profile product, and the ground-based altimeter observation results on July 15, 2007. Figure 3(b) is a schematic diagram comparing the electron density inversion results using the method of this application, the COSMIC electron density profile product, and the ground-based altimeter observation results on October 21, 2007. Figure 3 (c) November 14, 2007 is a schematic diagram comparing the inverted electron density results using the method of this application, the COSMIC electron density profile product, and the ground-based altimeter observation results. Figure 3 (d) is a schematic diagram comparing the electron density inversion results using the method of this application, the COSMIC electron density profile product, and the ground-based altimeter observation results on December 12, 2007. Since occultation observations are space-based observations and the critical frequency data of the altimeter are ground-based observations, time and space 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 altimeter is within 1 o The difference between the occultation time and the ground-based observation time 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 height corresponding to the critical frequency of the E layer observed by the ground-based altimeter. As can be seen from the figure, the LIDIM method proposed in this application has the following advantages:

[0074] (1) The use of non-negative least squares solution and Gaussian function fitting effectively avoids the negative electron density in COSMIC products;

[0075] (2) In terms of inversion accuracy, the LIDIM method in this paper is closer to the ground-based altimeter observations, and the electron density profile provided has smaller errors.

[0076] This application further counts the spatiotemporal characteristics of the electron density in the lower ionosphere on a global scale and compares 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) in the figure shows the spatial distribution characteristics of electron density at an altitude of 100 km in the winter of 2007 using the LIDIM method proposed in this application and the COSMIC electron density product. As can be seen from the figure, both methods show some basic characteristics of low ionospheric electron density: (1) There are two high-density belt-like regions in high latitudes, which are due to the enhanced electron density of the E layer caused by the precipitation of high-energy particles in the polar regions; (2) The high-value areas of low ionospheric electron density in 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 production of low ionospheric electrons is mainly caused by photoionization. However, the algorithm proposed in this paper has the following advantages over the COSMIC product:

[0077] (1) COSMIC has many negative regions on the night side in mid- and low-latitude regions, but the electron density in the lower ionosphere cannot be negative. The algorithm proposed in this paper solves this problem well.

[0078] (2) COSMIC shows a double-peak structure in low-latitude areas, which is similar to the equatorial anomaly of the F layer in the ionosphere. This phenomenon does not conform to the distribution law of the electron density in the low ionosphere, and this abnormal structure has not been found in the existing international reference ionosphere. According to existing research, this is because the equatorial anomaly structure of the F layer causes a large horizontal gradient of electron density in the ionosphere, which does not conform to the spherical symmetry assumption. This makes the error of the electron density in the low ionosphere of COSMIC products in this area particularly obvious. The inversion method proposed in this application has better eliminated the influence of the equatorial anomaly of the F layer on the electron density in the low ionosphere, and the spatial distribution is basically consistent with the existing cognition, indicating that the method proposed in this paper has high accuracy.

[0079] It should be understood that although Figure 1 The steps in the flowchart are shown in sequence as indicated by the arrows, but these steps are not necessarily executed in the order indicated by the arrows. Unless otherwise specified in this document, there is no strict order restriction for the execution of these steps, and these steps can be executed in other orders. In addition, Figure 1 At least part of the steps may include multiple sub-steps or multiple stages. These sub-steps or stages are not necessarily executed at the same time, but can be executed at different times. The execution order of these sub-steps or stages is not necessarily sequential, but can be executed in turn or alternately with other steps or at least part of the sub-steps or stages of other steps.

[0080] The technical features of the above embodiments can be combined arbitrarily. To make the description concise, 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, they should be considered to be within the scope of this specification.

[0081] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present application. It should be noted that a person of ordinary skill in the art may make various modifications and improvements without departing from the spirit of the present application, and these modifications and improvements fall within the scope of protection of the present application. Therefore, the scope of protection of the present application shall be determined by the appended claims.

Claims

1. A method for inverting the electron density in the lower ionosphere based on occultation data, characterized in that: The method comprises: Acquire 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; Extracting the ionospheric F layer component from the TEC profile according to a two-segment linear fitting within a certain altitude range based on the preprocessed data to obtain an interpolated ionospheric F layer TEC component; and calculating the lower ionospheric TEC component based on the interpolated occultation observation value and the interpolated ionospheric F layer TEC component; A linear equation system is established between the discretized lower ionospheric TEC components and the electron density to be determined and solved using non-negative least squares to obtain the inverted electron density profile. The inverted electron density profile is smoothly fitted using a Gaussian function to obtain an electron density profile product.

2. The method according to claim 1, characterized in that The secondary data files include the longitude, latitude, altitude, TEC profile and electron density products of the occultation event; Perform TEC data preprocessing on the secondary data file to obtain preprocessed data, including: Confirm that there are at least three data points with TEC profiles below 50 km in the secondary data file, and interpolate the qualified TEC profiles to the range of 50 km-200 km with an interval of 1 km for inversion.

3. The method according to claim 1, characterized in that The certain altitude interval includes an interval of 50 km to 60 km and an interval of 120 km to 130 km. Based on the preprocessed data, the ionospheric F layer component in the TEC profile is extracted according to a two-segment linear fitting within the certain altitude interval to obtain the interpolated ionospheric F layer TEC component, including: Based on the pre-processed data, the ionospheric F layer component in the TEC profile is extracted according to a two-segment linear fitting within a certain altitude range, and the interpolated ionospheric F layer TEC component is obtained as follows: ； in, It is within the range of 50km-60km and 120km-130km value, is the height of the occultation tangent point, a0 and a1 are the coefficients to be fitted.

4. The method according to claim 1, wherein Calculating the lower ionospheric TEC component based on the interpolated occultation observation value and the interpolated ionospheric F layer TEC component, including: The TEC value in the lower ionospheric altitude range obtained by extrapolating the linear fitting function to the altitude range of 60km-120km is subtracted from the interpolated occultation observation value to obtain the lower ionospheric TEC component: ； in, is the lower ionospheric TEC component, is the interpolated occultation observation value, is the interpolated ionospheric F layer TEC component, that is, the TEC value within the low ionospheric altitude range, is the height of the occultation tangent point.

5. The method according to claim 1, characterized in that According to the height distribution of the TEC component in the lower ionosphere, a linear equation system for electron density inversion based on the Abel weight function is constructed and solved using non-negative least squares to obtain the inverted electron density profile, including: Under the assumption of spherical symmetry and ignoring the effect of electron density above the LEO satellite orbital altitude, the relationship between the total electron density content and electron density in occultation observations is designed. Discretizing the relationship between the total electron density content and the electron density of the occultation observation to obtain a discretized lower ionospheric TEC component; The discretized lower ionospheric TEC component is interpolated to a grid point with a spacing of 1 km. , a set of linear equations containing the electron density to be determined is obtained; non-negative least squares is used to solve it and the inverted electron density profile is obtained.

6. The method according to claim 5, characterized in that The relationship between the total electron density content and the electron density of the designed occultation observation is: ； in, is the height of the occultation tangent point, is the electron density to be determined, represents the orbital height of the low-orbit satellite, Indicates height h and electron density function The product of represents the height variable, Indicates GPS satellite altitude.

7. The method according to claim 6, characterized in that The relationship between the total electron density content and the electron density of the occultation observation is discretized to obtain the discretized lower ionospheric TEC component, including: The relationship between the total electron density content and the electron density of the occultation observation is discretized, and the discretized lower ionospheric TEC component is obtained as follows: ； in, This is the Abel weight function.

8. The method according to claim 5, characterized in that The linear equations of the discretized lower ionospheric TEC components and the electron density to be determined are established, including: The discretized lower ionospheric TEC component has been interpolated to a grid point with a spacing of 1 km in data preprocessing, that is, , the linear equations containing the electron density to be determined are: ； Among them, K is the Abel weight function matrix form, which is a lower triangular matrix. is the electron density to be determined, is the interpolated occultation observation value; the left and right sides of the equation are differentiated, and the electron density is solved using the non-negative least squares method to obtain the low ionospheric electron density profile with a height resolution of 1 km.

9. The method according to claim 5, characterized in that The inverted electron density profile is smoothly fitted using a Gaussian function to obtain an electron density profile product, including: The inverted electron density profile is smoothly fitted using a Gaussian function to obtain the electron density profile product: ； in, is the maximum electron density in the lower ionosphere, is the height corresponding to the maximum density, is the electron density elevation of the lower ionosphere, Represents the height variable.

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