SGP4 model-based space target shadow time calculation method

Abstract

The invention discloses a space target ground shadow time calculation method based on an SGP4 model, relates to the technical field of space target ground shadow time calculation, and thoroughly solves the problem of wrong shutdown caused by strong reflection of an ice surface by fusing a high-resolution ice cover database and a seasonal modulation function and embedding a ground surface reflection physical quantity into a ground shadow criterion. The integrity of polar region monitoring data is ensured; a refraction compensation look-up table is constructed based on an atmospheric thermodynamic model, experience correction is replaced with physical driving, and the sensor damage risk caused by the pseudo dawn effect is remarkably inhibited; a dynamic threshold value and probabilistic output are innovatively adopted, the oscillation problem of binary judgment of a penumbra area is eliminated, and smooth transition and redundancy protection of a satellite energy system are achieved in combination with a hierarchical control strategy.

Description

Technical Field

[0001] This invention relates to the field of spatial target shadow time calculation technology, and in particular to a spatial target shadow time calculation method based on the SGP4 model. Background Technology

[0002] In missions such as polar monitoring and meteorological observation, polar-orbiting satellites need to frequently traverse high-latitude regions. These satellites rely on accurate Earth shadow time calculations to control payload power-on and power-off to avoid energy waste or damage to optical components. The mainstream approach is based on the SGP4 model to analyze TLE data, determine the illumination status through the geometric projection of the Earth's umbra / penumbra, and directly output UTC time. This model is widely used in low-Earth orbit constellations and single-satellite missions due to its high computational efficiency and mature engineering.

[0003] The traditional SGP4 model assumes that the Earth's surface is a uniform light absorber, but seasonal ice sheets in polar regions form strong reflective surfaces with an albedo of over 0.8. When a satellite is in the theoretical shadow area, the sunlight reflected from the ice surface can still provide effective illumination, causing the model to misjudge it as being in the shadow. Existing newer solutions, such as dynamic atmospheric compensation or space-based collaborative sensing, focus on atmospheric or orbital corrections and do not address the interference from surface reflections.

[0004] To address this, some solutions introduce surface albedo databases, such as the NSIDC ice map, but static data cannot match the daily dynamic changes in ice sheet melting; other solutions add spaceborne shortwave radiometers for real-time monitoring, but hardware costs have skyrocketed and miniaturization is difficult. It is evident that existing solutions are limited by model-environment decoupling design and fail to embed surface reflection as a core variable into the ground shadow criterion system; therefore, a spatial target ground shadow time calculation method based on the SGP4 model is urgently needed to solve this problem. Summary of the Invention

[0005] In view of the aforementioned existing problems, the present invention is proposed.

[0006] This invention provides a method for calculating the time of shadow of a space target based on the SGP4 model, which solves the problems of existing SGP4 models neglecting surface reflection and atmospheric refraction, leading to polar orbit misjudgment, low-orbit timing deviation, and high-orbit control oscillation.

[0007] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0008] This invention provides a method for calculating the time of shadow of a spatial target based on the SGP4 model, which includes:

[0009] Step S1: Input the TLE orbital data of the space target;

[0010] Step S2: Call the SGP4 model to calculate the target's orbital position and geocentric solar angle within a preset time window;

[0011] Step S3: Based on the geometric projection model of the Earth's umbra and penumbra, output the UTC time series of the target entering / leaving the Earth's shadow.

[0012] As a preferred embodiment of the spatial target shadow time calculation method based on the SGP4 model described in this invention, in which: after calculating the geocentric solar angle in step S2, atmospheric refraction compensation is performed:

[0013] Based on the target's real-time altitude and latitude, the refraction time delay value is determined using a preset altitude-latitude compensation coefficient table;

[0014] The delay value is superimposed on the original ground shadow time output of SGP4.

[0015] As a preferred embodiment of the spatial target shadow time calculation method based on the SGP4 model described in this invention, wherein: during the atmospheric refraction compensation process, the standard atmospheric pressure-temperature model is incorporated into the refractive index formula:

[0016] ,

[0017] in, Indicates height Electromagnetic refractive index, dimensionless. Represents the refractive constant of dry air, numerical value , This represents the standard atmospheric pressure at sea level, measured in hPa. Indicates the pressure scale height coefficient, unit , Indicates the target orbital altitude, in km. This represents the standard sea-level temperature, in Kelvin (K). Indicates the rate of temperature lapse, in units of ;

[0018] The light path is bent using the third-order tangent approximation:

[0019] ,

[0020] in, Indicates at altitude Zenith distance The angle of refraction at the bottom, in rad. This represents the first-order altitude attenuation coefficient, in rad. Indicates the pressure scale height coefficient, unit , Indicates the target orbital altitude, in km. This represents a first-order constant term, in rad. This represents the apparent zenith distance from the sun, measured in rad. This represents the third-order altitude attenuation coefficient, in rad. Represents a third-order constant term, in rad;

[0021] In the formula:

[0022] , ,

[0023] , ,

[0024] in, Represents the first-order refractive ratio constant, numerically , This represents the standard atmospheric pressure at sea level, measured in hPa. This represents the standard sea-level temperature, in Kelvin (K). Indicates the rate of temperature lapse, in units of , Indicates the target orbital altitude, in km. Represents the third-order refractive index constant, numerically. ;

[0025] Angular offset mapped using the angular velocity from the Earth's perspective (Sun's perspective):

[0026] ,

[0027] in, The time delay of the Earth's shadow caused by refraction is expressed in seconds (s). This represents the angle of refraction obtained in step two, in rad. Represents the angular velocity of the sun from the Earth's center, a constant. , Indicates the target orbital altitude, in km. Indicates the target's geographical latitude, in degrees. This represents the apparent zenith distance from the sun, measured in rad.

[0028] At sunset geometric limit Define the compensation coefficient:

[0029] ,

[0030] in, This represents the refraction time delay compensation coefficient based on latitude and altitude, in seconds (s). The refraction time delay at the ultimate zenith distance is expressed in seconds (s).

[0031] Will Discretized to Using a step length of 25km, The two-dimensional matrix is ​​calculated with a step size of 5°. ;like Record it as 0, and keep other values ​​to three decimal places. Use bilinear interpolation to fill in the lookup outside the table to form the table of altitude latitude refraction time delay compensation coefficients.

[0032] As a preferred embodiment of the spatial target ground shadow time calculation method based on the SGP4 model described in this invention, surface reflectance compensation is performed to address surface reflection interference.

[0033] Preloaded with a surface reflectance database containing ice sheet extents, indexed by Julian day and latitude / longitude;

[0034] When the absolute value of the target latitude is greater than 60°, the reflectance compensation amount is retrieved based on the target location and the current Julian day.

[0035] The reflectivity compensation amount is applied to the geocentric solar angle determination threshold.

[0036] As a preferred embodiment of the spatial target ground shadow time calculation method based on the SGP4 model described in this invention, the reflectivity compensation amount is dynamically adjusted according to the ice cover coverage:

[0037] The compensation amount for the ice sheet region is set to a typical value of 0.05-0.08;

[0038] The compensation amount for non-ice-covered areas is set to 0;

[0039] The amount of compensation follows the changes in Julian Japan.

[0040] As a preferred embodiment of the spatial target ground shadow time calculation method based on the SGP4 model described in this invention, the generation process of the seasonal reflectance compensation amount of the ice sheet includes:

[0041] Searching for related data in the ice sheet raster database corresponding Unit, and for which Calculate the average of the pixels:

[0042] ,

[0043] in, Indicating Julian Japan Time and location Ice cover coverage, dimensionless. For the first A pixel-sized ice cap symbol, 1 for ice surface, 0 for non-ice surface. The total number of unit pixels, Geographical longitude, Geographical latitude, The Julian Day number is 1-366;

[0044] Multiplying the coverage rate by a cosine-type seasonal function to reflect the changes in polar day and polar night, the formula is:

[0045] ,

[0046] in, The seasonal modulation coefficient is dimensionless. The Julian Day corresponds to the winter solstice in the Northern Hemisphere, 355° in the Northern Hemisphere and 172° in the Southern Hemisphere.

[0047] The final compensation amount is written as:

[0048] ,

[0049] in, This is the compensation amount for surface reflectance. This is a latitude threshold function. If the value is 1, then 0. To compensate for the lower limit constant of 0.05°, To compensate for the upper limit constant of 0.08°.

[0050] As a preferred embodiment of the spatial target ground shadow time calculation method based on the SGP4 model described in this invention, the ground shadow boundary determination uses a dynamic threshold instead of a fixed threshold.

[0051] The dynamic threshold is generated by superimposing the base value of 0.264°, the seasonal compensation term for the Earth's perihelion, and the oblateness compensation term;

[0052] The seasonal compensation term is related to the Earth's orbital parameters;

[0053] The flattening compensation term is calculated from the difference between the Earth's equatorial radius and polar radius.

[0054] As a preferred embodiment of the spatial target shadow time calculation method based on the SGP4 model described in this invention, when outputting the shadow state, the binary judgment is extended to a continuous probability value:

[0055] When the central solar angle is within a 0.05° range before and after the dynamic threshold, it is mapped to a probability value of 0-1 through the S-shaped function;

[0056] The sigmoid function is defined as a sigmoid function.

[0057] As a preferred embodiment of the spatial target shadow time calculation method based on the SGP4 model described in this invention, the probability value output is associated with the satellite energy control strategy:

[0058] When the probability value > 0.3, the load protection preparatory command is triggered;

[0059] When the probability value is greater than 0.7, the load is de-energized.

[0060] The load protection preparatory instructions include: reducing the optical load gain and activating the backup power buffer; the load power-off operation is completed within 50ms after the UTC time stamp.

[0061] This invention also discloses a spatial target ground shadow time calculation system based on the SGP4 model, including: a TLE data parsing module;

[0062] Configured as a processing module for implementing the spatial target ground shadow time calculation method;

[0063] And an interface module for outputting UTC time series with probability labels.

[0064] The beneficial effects of this invention are as follows: By integrating a high-resolution ice sheet database with a seasonal modulation function, this invention embeds surface reflection physical quantities into the shadow criterion, completely solving the problem of false shutdown caused by strong ice surface reflection and ensuring the integrity of polar monitoring data; Based on an atmospheric thermodynamic model, a refraction compensation lookup table is constructed, replacing empirical correction with physical drive, significantly suppressing the risk of sensor damage caused by the false dawn effect; Innovatively, dynamic thresholds and probabilistic outputs are adopted to eliminate the oscillation problem of binary judgment in the penumbra region, and combined with a hierarchical control strategy, a smooth transition and redundancy protection of the satellite energy system are achieved; In addition, while retaining the efficient analytical framework of SGP4, this invention achieves plug-and-play upgrades through a modular compensation mechanism (atmosphere / surface / threshold), avoiding the reconstruction of the core orbital algorithm and significantly reducing the cost of onboard system modification. Attached Figure Description

[0065] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0066] Figure 1 This is a flowchart illustrating the spatial target ground shadow time calculation method based on the SGP4 model in Example 1. Detailed Implementation

[0067] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0068] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0069] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0070] Example 1, referring to Figure 1 This embodiment provides a method for calculating the time of the shadow of a spatial target based on the SGP4 model, including:

[0071] Step S1: Input the TLE orbital data of the space target;

[0072] Step S2: Call the SGP4 model to calculate the target's orbital position and geocentric solar angle within a preset time window;

[0073] After calculating the geocentric solar angle in step S2, atmospheric refraction compensation is performed:

[0074] Based on the target's real-time altitude and latitude, the refraction time delay value is determined using a preset altitude-latitude compensation coefficient table;

[0075] Add the delay value to the original SGP4 ground shadow time output;

[0076] In the atmospheric refraction compensation process, the standard atmospheric pressure-temperature model is incorporated into the refractive index formula:

[0077] ,

[0078] in, Indicates height Electromagnetic refractive index, dimensionless. Represents the refractive constant of dry air, numerical value , This represents the standard atmospheric pressure at sea level, measured in hPa. Indicates the pressure scale height coefficient, unit , Indicates the target orbital altitude, in km. This represents the standard sea-level temperature, in Kelvin (K). Indicates the rate of temperature lapse, in units of The thermodynamic dependence of pressure and temperature is directly maintained, causing an exponential decay between refractive index and altitude, thus ensuring physical continuity within the range of 0-1000 km.

[0079] The light path is bent using the third-order tangent approximation:

[0080] ,

[0081] in, Indicates at altitude Zenith distance The angle of refraction at the bottom, in rad. This represents the first-order altitude attenuation coefficient, in rad. Indicates the pressure scale height coefficient, unit , Indicates the target orbital altitude, in km. This represents a first-order constant term, in rad. This represents the apparent zenith distance from the sun, measured in rad. This represents the third-order altitude attenuation coefficient, in rad. Represents a third-order constant term, in rad;

[0082] In the formula:

[0083] , ,

[0084] , ,

[0085] in, Represents the first-order refractive ratio constant, numerically , This represents the standard atmospheric pressure at sea level, measured in hPa. This represents the standard sea-level temperature, in Kelvin (K). Indicates the rate of temperature lapse, in units of , Indicates the target orbital altitude, in km. Represents the third-order refractive index constant, numerically. Here, the coefficients are explicitly separated into three factors: pressure, temperature, and altitude. The formula retains both first-order and third-order terms, which enhances the fitting accuracy in the low-altitude range and allows for automatic convergence in the high-altitude range.

[0086] Angular offset mapped using the angular velocity from the Earth's perspective (Sun's perspective):

[0087] ,

[0088] in, The time delay of the Earth's shadow caused by refraction is expressed in seconds (s). This represents the angle of refraction obtained in step two, in rad. Represents the angular velocity of the sun from the Earth's center, a constant. , Indicates the target orbital altitude, in km. Indicates the target's geographical latitude, in degrees. This represents the apparent zenith distance from the sun, measured in rad.

[0089] At sunset geometric limit Define the compensation coefficient:

[0090] ,

[0091] in, This represents the refraction time delay compensation coefficient based on latitude and altitude, in seconds (s). The refraction delay at the extreme zenith distance is expressed in seconds. The spatial angle is directly linearly projected into the time domain using a constant solar angular velocity, which can be easily superimposed with the SGP4 orbital epoch time. The latitude-related extreme angle makes the compensation value geographically continuous, avoids sudden jumps in polar regions, and improves the smoothness of the dynamic threshold method.

[0092] Will Discretized to Using a step length of 25km, The two-dimensional matrix is ​​calculated with a step size of 5°. ;like Record it as 0, and keep other values ​​to three decimal places. Use bilinear interpolation to fill the lookup outside the table to form the table of altitude latitude refraction time delay compensation coefficients.

[0093] Specifically, the discretization selection takes into account the common resolutions of LEO and MEO, and the matrix size is controlled at 41×37, which is convenient for embedding on-board firmware.

[0094] Step S3: Based on the geometric projection model of the Earth's umbra and penumbra, output the UTC time series of the target entering / leaving the Earth's shadow;

[0095] To address surface reflection interference, surface reflectance compensation is performed:

[0096] Preloaded with a surface reflectance database containing ice sheet extents, indexed by Julian day and latitude / longitude;

[0097] When the absolute value of the target latitude is greater than 60°, the reflectance compensation amount is retrieved based on the target location and the current Julian day.

[0098] When outputting the ground shadow state, the binary judgment is expanded to a continuous probability value:

[0099] When the central solar angle is within a 0.05° range before and after the dynamic threshold, it is mapped to a probability value of 0-1 through the S-shaped function;

[0100] The S-type function is defined as the Sigmoid form;

[0101] Probability value output associated with satellite energy control strategy:

[0102] When the probability value > 0.3, the load protection preparatory command is triggered;

[0103] When the probability value is greater than 0.7, the load is de-energized.

[0104] The load protection preparatory instructions include: reducing the optical load gain and activating the backup power buffer; the load power-off operation is completed within 50ms after the UTC time stamp.

[0105] The reflectivity compensation is applied to the threshold for determining the geocentric solar angle.

[0106] The surface reflectance database refers to the daily ice sheet extent raster dataset released by the Arctic Council, with a latitude and longitude resolution of 0.25°×0.25° and a Julian day index covering days 1-366.

[0107] The reflectivity compensation amount is dynamically adjusted based on the ice cover coverage:

[0108] The compensation amount for the ice sheet region is set to a typical value of 0.05-0.08;

[0109] The compensation amount for non-ice-covered areas is set to 0;

[0110] The amount of compensation follows the changes in Julian Japan;

[0111] The generation process of seasonal reflectance compensation in ice sheets includes:

[0112] Searching for related data in the ice sheet raster database corresponding Unit, and for which Calculate the average of the pixels:

[0113] ,

[0114] in, Indicating Julian Japan Time and location Ice cover coverage, dimensionless. For the first A pixel-sized ice cap symbol, 1 for ice surface, 0 for non-ice surface. The total number of unit pixels, Geographical longitude, Geographical latitude, The Julian Day number is 1-366;

[0115] Multiplying the coverage rate by a cosine-type seasonal function to reflect the changes in polar day and polar night, the formula is:

[0116] ,

[0117] in, The seasonal modulation coefficient is dimensionless. The Julian Day corresponds to the winter solstice in the Northern Hemisphere, 355° in the Northern Hemisphere and 172° in the Southern Hemisphere.

[0118] The final compensation amount is written as:

[0119] ,

[0120] in, This is the compensation amount for surface reflectance. This is a latitude threshold function. If the value is 1, then 0. To compensate for the lower limit constant of 0.05°, To compensate for the upper limit constant of 0.08°;

[0121] Specifically, a high-resolution ice sheet grid is used to assess the actual ice surface ratio at the pixel level, which can distinguish between broken sea ice and permanent ice sheets, avoiding misjudgments caused by binarization and coarsening of the terrain. Subsequently, a cosine seasonal function is used to smoothly modulate the compensation amplitude with a six-month cycle, so that the trigger threshold gradually rises and falls with the transition from winter to spring, eliminating command jumps. The linear interval mapping corresponds the empirical range of 0.05° to 0.08° with the actual ice surface coverage, which maintains expert experience while ensuring that some ice surface areas are not over-corrected. The latitude threshold function automatically filters low-latitude data, reducing the burden of irrelevant calculations. The entire process requires only one database query, one weighted average, and two multiplication and addition operations, and can be output in real time at a refresh rate of one second on the satellite FPGA, complementing the preceding atmospheric refraction compensation and further improving the accuracy of ground shadow determination.

[0122] For determining the ground shadow boundary, a dynamic threshold is used instead of a fixed threshold:

[0123] The dynamic threshold is generated by superimposing the base value of 0.264° (standard ground shadow angle threshold), the Earth's perihelion seasonal compensation term, and the flattening compensation term.

[0124] The seasonal compensation term is related to the parameters of Earth's orbit around the sun;

[0125] The flattening compensation term is calculated from the difference between the Earth's equatorial radius and polar radius;

[0126] The calculation of the Earth's perihelion seasonal compensation term is related to the Earth's orbital eccentricity and the Julian sun parameters at perihelion; the Earth's equatorial radius in the oblateness compensation term is 6378.137 km as defined in WGS-84, and the polar radius is 6356.752 km;

[0127] This embodiment also provides a spatial target ground shadow time calculation system based on the SGP4 model, including: a TLE data parsing module;

[0128] Configured as a processing module for implementing the spatial target ground shadow time calculation method;

[0129] And an interface module for outputting UTC time series with probability labels.

[0130] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for calculating the time of spatial target shadows based on the SGP4 model, characterized in that, include, Step S1: Input the TLE orbital data of the space target; Step S2: Call the SGP4 model to calculate the target's orbital position and geocentric solar angle within a preset time window; Step S3: Based on the geometric projection model of the Earth's umbra and penumbra, output the UTC time series of the target entering / leaving the Earth's shadow.

2. The method for calculating the time of spatial target shadow based on the SGP4 model as described in claim 1, characterized in that, After calculating the geocentric solar angle in step S2, atmospheric refraction compensation is performed: Based on the target's real-time altitude and latitude, the refraction time delay value is determined using a preset altitude-latitude compensation coefficient table; The delay value is superimposed on the original ground shadow time output of SGP4.

3. The method for calculating the time of spatial target shadow based on the SGP4 model as described in claim 2, characterized in that, In the atmospheric refraction compensation process, the standard atmospheric pressure-temperature model is incorporated into the refractive index formula: ， in, Indicates height Electromagnetic refractive index, dimensionless. Represents the refractive constant of dry air, numerical value , This represents the standard atmospheric pressure at sea level, measured in hPa. Indicates the pressure scale height coefficient, unit , Indicates the target orbital altitude, in km. This represents the standard sea-level temperature, in Kelvin (K). Indicates the rate of temperature lapse, in units of ; The light path is bent using the third-order tangent approximation: ， in, Indicates at altitude Zenith distance The angle of refraction at the bottom, in rad. This represents the first-order altitude attenuation coefficient, in rad. Indicates the pressure scale height coefficient, unit , Indicates the target orbital altitude, in km. This represents a first-order constant term, in rad. This represents the apparent zenith distance from the sun, measured in rad. This represents the third-order altitude attenuation coefficient, in rad. Represents a third-order constant term, in rad; In the formula: ， ， ， ， in, Represents the first-order refractive ratio constant, numerically , This represents the standard atmospheric pressure at sea level, measured in hPa. This represents the standard sea-level temperature, in Kelvin (K). Indicates the rate of temperature lapse, in units of , Indicates the target orbital altitude, in km. Represents the third-order refractive index constant, numerically ; Angular offset mapped using the angular velocity from the Earth's perspective (Sun's perspective): ， in, The time delay of the Earth's shadow caused by refraction is expressed in seconds (s). This represents the angle of refraction obtained in step two, in rad. Represents the angular velocity of the sun from the Earth's center, a constant. , Indicates the target orbital altitude, in km. Indicates the target's geographical latitude, in degrees. This represents the apparent zenith distance from the sun, measured in rad. At sunset geometric limit Define the compensation coefficient at this point: ， in, This represents the refraction time delay compensation coefficient based on latitude, in seconds (s). The refraction time delay at the ultimate zenith distance is expressed in seconds (s). Will Discretized to Using a step length of 25km, The two-dimensional matrix is ​​calculated with a step size of 5°. ;like Record it as 0, and keep other values ​​to three decimal places. Use bilinear interpolation to fill in the lookup outside the table to form the table of altitude latitude refraction time delay compensation coefficients.

4. The method for calculating the time of spatial target shadow based on the SGP4 model as described in claim 1, characterized in that, To address surface reflection interference, surface reflectance compensation is performed: Preloaded with a surface reflectance database containing ice sheet extents, indexed by Julian day and latitude / longitude; When the absolute value of the target latitude is greater than 60°, the reflectance compensation amount is retrieved based on the target location and the current Julian day. The reflectivity compensation amount is applied to the geocentric solar angle determination threshold.

5. The method for calculating the time of spatial target shadow based on the SGP4 model as described in claim 4, characterized in that, The reflectivity compensation amount is dynamically adjusted based on the ice cover coverage: The compensation amount for the ice sheet region is set to a typical value of 0.05-0.08; The compensation amount for non-ice-covered areas is set to 0; The amount of compensation follows the changes in Julian Japan.

6. The method for calculating the time of spatial target shadow based on the SGP4 model as described in claim 5, characterized in that, The process of generating the seasonal reflectivity compensation of the ice sheet includes: Searching for related data in the ice sheet raster database corresponding Unit, and for which Calculate the average of the pixels: ， in, Indicating Julian Japan Time and location Ice cover coverage, dimensionless. For the first A pixel-sized ice cap symbol, 1 for ice surface, 0 for non-ice surface. The total number of unit pixels, Geographical longitude, Geographical latitude, The Julian Day number is 1-366; Multiplying the coverage rate by a cosine-type seasonal function to reflect the changes in polar day and polar night, the formula is: ， in, The seasonal modulation coefficient is dimensionless. The Julian Day corresponds to the winter solstice in the Northern Hemisphere, 355° in the Northern Hemisphere and 172° in the Southern Hemisphere. The final compensation amount is written as: ， in, This is the compensation amount for surface reflectance. This is a latitude threshold function. If the value is 1, then 0. To compensate for the lower limit constant of 0.05°, To compensate for the upper limit constant of 0.08°.

7. The method for calculating the time of spatial target shadow based on the SGP4 model as described in claim 1, characterized in that, For determining the ground shadow boundary, a dynamic threshold is used instead of a fixed threshold: The dynamic threshold is generated by superimposing the base value of 0.264°, the seasonal compensation term for the Earth's perihelion, and the oblateness compensation term; The seasonal compensation term is related to the Earth's orbital parameters; The flattening compensation term is calculated from the difference between the Earth's equatorial radius and polar radius.

8. The method for calculating the time of spatial target shadow based on the SGP4 model as described in claim 1, characterized in that, When outputting the ground shadow state, the binary judgment is expanded to a continuous probability value: When the central solar angle is within a 0.05° range before and after the dynamic threshold, it is mapped to a probability value of 0-1 through the S-shaped function; The sigmoid function is defined as a sigmoid function.

9. The method for calculating the time of spatial target shadow based on the SGP4 model as described in claim 8, characterized in that, The probability value output is associated with the satellite energy control strategy: When the probability value > 0.3, the load protection preparatory command is triggered; When the probability value is greater than 0.7, the load is de-energized. The load protection preparatory commands include: reducing the optical load gain and activating the backup power buffer; The load power-off operation is completed within 50ms after the UTC time stamp.

10. A spatial target ground shadow time calculation system based on the SGP4 model, based on the spatial target ground shadow time calculation method based on the SGP4 model according to any one of claims 1 to 9, characterized in that, include: TLE data parsing module; Configured as a processing module for implementing the spatial target ground shadow time calculation method; And an interface module for outputting UTC time series with probability labels.

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