A method and system for controlling the gain of a spaceborne synthetic aperture radar.

Abstract

This invention discloses a method and system for gain control of a spaceborne synthetic aperture radar (SAR), relating to the field of satellite remote sensing. The method includes obtaining a target-to-ground motion model based on the positional relationship between the ground-illuminated target and the satellite, and a satellite-to-ground motion model; calculating the incident angle of the SAR based on the target-to-ground motion model; calculating the scattering coefficient of the ground-illuminated target's location based on the environment of the target's location, or based on the incident angle and the environment of the target's location; calculating the cross-sectional area of ​​the target; calculating gain control parameters based on the incident angle, scattering coefficient, and target cross-sectional area; and adjusting the gain of the SAR based on the gain control parameters. This invention pre-calculates the gain value of the region based on existing knowledge, eliminating the need to wait for the echo signal to arrive before controlling echo sampling, thereby compensating for the high latency defect of the GEOSAR system.

Description

Technical Field

[0001] This invention relates to the field of satellite remote sensing, and in particular to a method and system for controlling the gain of a spaceborne synthetic aperture radar. Background Technology

[0002] Spaceborne SAR (Synthetic Aperture Radar) systems receive echo signals with large amplitude variations, requiring appropriate radar gain settings based on ground object echo power for subsequent digital processing. Improper receiver gain adjustment can lead to quantization distortion of the raw data, affecting the SAR system's image signal-to-noise ratio and radiometric accuracy. High-orbit SAR systems suffer from significant time delays; AGC (AppGallery Connect) only functions upon receiving echoes. Failure to promptly control echo sampling results in inaccurate predictions of abrupt changes in terrain. Summary of the Invention

[0003] The purpose of this invention is to provide a method and system for controlling the gain of a spaceborne synthetic aperture radar (SAR), which can calculate the gain value (MGC) of the region in advance based on existing knowledge, thereby compensating for the time delay of the SAR system and improving the accuracy of data processing.

[0004] To achieve the above objectives, the present invention provides the following solution:

[0005] A method for gain control of a spaceborne synthetic aperture radar, the method comprising:

[0006] Based on the positional relationship between the ground-illuminated target and the satellite, and the satellite-ground motion model, the function of the target-ground distance over time is determined, resulting in the target-ground motion model. The target-ground distance represents the distance between the ground-illuminated target and the Earth's center. The positional relationship between the ground-illuminated target and the satellite is determined based on the relative positional relationship between the ground-illuminated target and the spaceborne synthetic aperture radar. The ground-illuminated target is the ground area illuminated by the satellite antenna. The satellite-ground motion model, a function of the satellite-ground distance over time, is obtained based on the satellite's orbital parameters. The satellite-ground distance represents the distance between the satellite and the Earth's center.

[0007] Based on the target-ground motion model, calculate the incident angle of the spaceborne synthetic aperture radar;

[0008] The scattering coefficient of the ground-irradiated target is calculated based on the environment of the location of the ground-irradiated target, or based on the incident angle and the environment of the location of the ground-irradiated target; the environment of the location of the ground-irradiated target includes at least one of dry snow, grassland, shrubs, soil and rock surface or forest;

[0009] Calculate the cross-sectional area of ​​the ground-irradiated target;

[0010] Calculate the gain control parameters based on the incident angle, the scattering coefficient, and the cross-sectional area of ​​the ground-irradiated target;

[0011] The gain of the spaceborne synthetic aperture radar is adjusted according to the gain control parameters.

[0012] Optionally, before determining the function of destination-to-location distance over time, the method further includes:

[0013] Based on the satellite's orbital parameters, the function of the satellite-to-ground distance changing with time is determined, and a satellite-to-ground motion model is obtained.

[0014] The positional relationship between the ground-illuminated target and the satellite is determined based on the relative positional relationship between the ground-illuminated target and the spaceborne synthetic aperture radar.

[0015] Optionally, the expression for the star-ground motion model is:

[0016]

[0017] Among them, R s (t a ) for t a The distance between the satellite and the Earth's center at any given time; θ f (t a ) represents the true anomaly angle of the satellite orbit; a s denoted as the semi-major axis of the satellite's orbit; e is the satellite's orbital eccentricity.

[0018] Optionally, the incident angle can be calculated using the following formula:

[0019]

[0020] Where θ is the incident angle of the satellite antenna illuminating the ground target; R e Here is the target-to-ground motion equation; h is the orbital altitude; From a perspective.

[0021] Optionally, the scattering coefficient at the location of the ground-illuminated target is calculated based on the environment at the location of the ground-illuminated target, or based on the incident angle and the environment at the location of the ground-illuminated target, specifically including:

[0022] When the environment of the ground-irradiated target is any of the following: dry snow, grassland, shrubs, or soil and rock surface, the scattering coefficient of the ground-irradiated target location is calculated using the Ulaby model.

[0023] When the environment of the ground-irradiated target is a forest, the scattering coefficient of the target location is calculated using a forest-scattering characteristic model; the forest-scattering characteristic model is a model representing the relationship between forest biomass and scattering characteristics.

[0024] When the environment of the target location includes two or more environments such as dry snow, grassland, shrubs, soil and rock surfaces, and forest, the scattering coefficient of the target location is calculated using the surface element model method.

[0025] Optionally, the forest-scattering characteristic model formula is:

[0026]

[0027] Where, σ 0 denoted as the normalized backscattering coefficient, B as the forest biomass, and a, b, c as the fitted parameters.

[0028] Optionally, the calculation of the scattering coefficient at the target's location using the element-size model method specifically includes:

[0029] The ground-illuminated target is modeled using a digital elevation model to obtain a three-dimensional model of the ground-illuminated target;

[0030] The three-dimensional model is divided into multiple triangular facets using the facet modeling method;

[0031] For each triangular facet element, the scattering characteristics at the location of the triangular facet element are calculated by selecting either the Ulaby model or the forest-scattering characteristic model, depending on its location environment.

[0032] Based on the scattering characteristics at the location of the triangular facet, the scattering coefficient at the target's location is calculated using the summation method.

[0033] Optionally, calculating the cross-sectional area of ​​the ground-illuminated target specifically includes:

[0034] The ground illumination target is divided into multiple triangular elements using the triangular element method.

[0035] For each triangular facet element, the cross-sectional area of ​​the triangular facet element is calculated using the scattering cross-sectional area calculation formula.

[0036] Based on the cross-sectional area of ​​the triangular facet, the cross-sectional area of ​​the ground-illuminated target is calculated using the summation method.

[0037] Optionally, the calculation formula for the gain control parameter includes:

[0038]

[0039] Where MGC represents the gain control parameter; A represents the illumination portion of the ground-illuminated target; G cg σ represents the channel gain, λ represents the wavelength, and σ represents the channel gain. 0 P represents the backscattering coefficient. n K represents system noise. L P represents system losses. t G represents the radar transmit power. 2 (θ a ,φ a ) represents the antenna's two-way gain, and θ represents the incident angle. G cg , λ, P t G 2 (θ a ,φ a ), P n K L These are all basic parameters of the radar system, obtained from radar design manuals.

[0040] The present invention also provides a system for gain control of a spaceborne synthetic aperture radar, the system comprising:

[0041] The target-to-ground motion model acquisition module is used to determine the target-to-ground distance as a function of time based on the positional relationship between the ground-illuminated target and the satellite, and the satellite-to-ground motion model, thereby obtaining the target-to-ground motion model. The target-to-ground distance represents the distance between the ground-illuminated target and the Earth's center. The positional relationship between the ground-illuminated target and the satellite is determined based on the relative positional relationship between the ground-illuminated target and the spaceborne synthetic aperture radar. The ground-illuminated target is the ground area illuminated by the satellite antenna. The satellite-to-ground motion model, a function of the satellite-to-ground distance over time, is obtained based on the satellite's orbital parameters. The satellite-to-ground distance represents the distance between the satellite and the Earth's center.

[0042] An incident angle calculation module is used to calculate the incident angle of the spaceborne synthetic aperture radar based on the target-ground motion model.

[0043] The scattering coefficient calculation module is used to calculate the scattering coefficient of the ground-irradiated target location based on the environment of the location of the ground-irradiated target, or based on the incident angle and the environment of the location of the ground-irradiated target location; the environment of the location of the ground-irradiated target location includes at least one of dry snow, grassland, shrubs, soil and rock surface or forest;

[0044] The target cross-sectional area calculation module is used to calculate the cross-sectional area of ​​the ground-irradiated target;

[0045] The gain control parameter calculation module is used to calculate the gain control parameters based on the incident angle, the scattering coefficient, and the cross-sectional area of ​​the ground-irradiated target.

[0046] A gain control module is used to adjust the gain of the spaceborne synthetic aperture radar according to the gain control parameters.

[0047] According to specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0048] This invention provides a method and system for controlling the gain of a spaceborne synthetic aperture radar (SAR). The method first calculates the incident angle of the satellite antenna illuminating the ground target, the scattering coefficient of the ground target's location, and the cross-sectional area of ​​the ground target. Then, it calculates the gain control parameters based on these three parameters. This approach can pre-calculate the gain value (MGC) of the region using existing knowledge, eliminating the need to wait for the echo signal to arrive before controlling echo sampling. This compensates for the high latency of the GEOSAR system and improves the accuracy of subsequent data processing. Attached Figure Description

[0049] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. 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.

[0050] Figure 1 Here is a flowchart of the spaceborne synthetic aperture radar gain control method provided in Embodiment 1 of the present invention;

[0051] Figure 2 This is a structural diagram of the spaceborne synthetic aperture radar gain control system provided in Example 2. Detailed Implementation

[0052] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0053] The purpose of this invention is to provide a method and system for controlling the gain of a spaceborne synthetic aperture radar (SAR). By calculating the gain value (MGC) of the region in advance based on existing knowledge, the time delay defect of the SAR system can be compensated for, thereby improving the accuracy of data processing.

[0054] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0055] Example 1

[0056] like Figure 1 As shown, this embodiment provides a method for gain control of a spaceborne synthetic aperture radar, including:

[0057] S1. Based on the positional relationship between the ground-illuminated target and the satellite, and the satellite-ground motion model, determine the function of the target-ground distance changing with time to obtain the target-ground motion model; the target-ground distance represents the distance between the ground-illuminated target and the Earth's center; the positional relationship between the ground-illuminated target and the satellite is determined based on the relative positional relationship between the ground-illuminated target and the spaceborne synthetic aperture radar; the ground-illuminated target is the ground area illuminated by the satellite antenna; the satellite-ground motion model is a function of the satellite-ground distance changing with time, obtained based on the satellite's orbital parameters; the satellite-ground distance represents the distance between the satellite and the Earth's center.

[0058] S2. Calculate the incident angle of the spaceborne synthetic aperture radar based on the target-ground motion model.

[0059] S3. Calculate the scattering coefficient of the location of the ground-illuminated target based on the environment of the location of the ground-illuminated target, or based on the incident angle and the environment of the location of the ground-illuminated target; the environment of the location of the ground-illuminated target includes at least one of dry snow, grassland, shrubs, soil and rock surface or forest.

[0060] S4. Calculate the cross-sectional area of ​​the ground-irradiated target.

[0061] S5. Calculate the gain control parameters based on the incident angle, the scattering coefficient, and the cross-sectional area of ​​the ground-irradiated target.

[0062] S6. Adjust the gain of the spaceborne synthetic aperture radar according to the gain control parameters.

[0063] As an optional implementation, before step S1, the method further includes:

[0064] S01. Based on the satellite's orbital parameters, determine the function of the satellite-to-ground distance changing with time to obtain the satellite-to-ground motion model.

[0065] The motion of a satellite relative to the Earth is generally described in geocentric rectangular coordinates. The geocentric rectangular coordinates take the Earth's center as the origin, the X-axis is located in the equatorial plane and points in the positive direction towards the Greenwich Meridian, the Z-axis is the Earth's rotation axis and points in the positive direction towards the North Pole, and the Y-axis follows the right-hand rule.

[0066] Under the influence of gravity, if we simplify the Earth and the artificial satellite into two point masses and consider them as a two-body problem, assuming the Earth is a perfect sphere with a radius of 6371 km and a rotational angular velocity of 7.292 × 10⁻⁶ km², then... -5Given a satellite's orbital eccentricity of e (rad / s), the distance between the satellite and the Earth's center can be expressed by the following formula:

[0067]

[0068] Where, θ f (t a ) represents the true anomaly angle of the satellite orbit, a s It is the semi-major axis of the track.

[0069] As can be seen from the above formula, the star-ground motion model in this embodiment is essentially a function of the star-ground distance changing with time.

[0070] S02. Determine the positional relationship between the ground-illuminated target and the satellite based on the relative positional relationship between the ground-illuminated target and the spaceborne synthetic aperture radar.

[0071] Ground-illuminated targets are geometrically positioned based on their relative relationship to the spaceborne synthetic aperture radar (SAR). In the azimuth direction, the azimuth of a ground-illuminated target is always represented by its angle of deviation from the SAR center, while in the range direction it can be represented by the beam's angle of view. To facilitate subsequent analysis of the impact of attitude errors on imaging, ground-illuminated targets are described in a satellite-fixed coordinate system. The ground-illuminated targets then fall within the azimuth beam plane of the SAR at different angles. Transforming this to geocentric rectangular coordinates yields the equations of motion of the ground-illuminated targets relative to the Earth, which are related to the azimuth angle and angle of view of the SAR beam.

[0072] Ground targets are located geometrically by their relative position to spaceborne synthetic aperture radar (SAR). In geostationary satellite systems, due to the curvature of the Earth's surface, unlike traditional airborne SAR, the antenna viewing angle of GEO SAR has a geometric relationship with the angle of incidence. Based on this, in step S2, given the orbital altitude h and viewing angle… The angle of incidence θ can then be calculated. Specifically, the formula for calculating the angle of incidence is:

[0073]

[0074] Where θ is the incident angle of the satellite antenna illuminating the ground target; R e Here is the target-to-ground motion equation; h is the orbital altitude; From a perspective.

[0075] Factors affecting manual gain control include the scattering characteristics of the ground surface, the radar wave position (i.e., the incident angle θ), the radar transmit power, and the antenna's nominal gain. Since the radar transmit power and antenna nominal gain are nominal values, they generally do not change. Since the incident angle has already been calculated in step S2, the gain control parameters can be calculated by solving for the scattering characteristics in step S3.

[0076] When solving for the scattering characteristics of ground objects, an existing classical model is selected and combined with the physical optics method to calculate the scattering characteristics of ground objects in the target area.

[0077] Specifically, in step S3, when the irradiated target is a single type of terrain, different land cover types will have different backscattering coefficient values. Depending on the terrain, an appropriate model is selected to calculate the scattering coefficient. When the environment of the irradiated target is any of dry snow, grassland, shrubbery, or soil / rock surface, the Ulaby model is used to calculate the scattering coefficient at the target's location. When the environment of the irradiated target is a forest, the forest-scattering characteristic model is used to calculate the scattering coefficient at the target's location. The forest-scattering characteristic model is a model representing the relationship between forest biomass and scattering characteristics.

[0078] When using the classic Ulaby model to calculate the scattering characteristics of typical ground features in the L-band, the expression is as follows:

[0079] σ 0 (θ)=P1+P2 exp(-P3θ)+P4cos(P5θ+P6)

[0080] Where, σ 0 (θ) represents the scattering coefficient; P1 to P6 are parameters obtained by fitting statistical data from actual experiments; θ is the incident angle.

[0081] When the forest-scattering characteristic model is selected to calculate the scattering characteristics, the model formula is:

[0082]

[0083] Where, σ 0 denoted as the normalized backscattering coefficient, B as the forest biomass, and a, b, c as the fitted parameters.

[0084] It should be noted that when radar illuminates a forest, different forests have different scattering characteristics. The forest-scattering characteristics model suggests that the different scattering characteristics are mainly related to forest biomass.

[0085] In step S3, when calculating scattering characteristics, in addition to a single terrain, when the irradiated area is large, there are also cases where two or more terrains are included at the same time, that is, the calculation of scattering characteristics of complex scenes.

[0086] When the environment of the target location includes two or more environments such as dry snow, grassland, shrubs, soil and rock surfaces, and forest, the scattering coefficient of the target location is calculated using the surface element model method.

[0087] Specifically, as an optional implementation method, the calculation method for scattering characteristics of complex scenes includes: modeling the ground-illuminated target using a digital elevation model to obtain a three-dimensional model of the ground-illuminated target; dividing the three-dimensional model into multiple triangular facets using a facet model method; for each triangular facet, selecting either the Ulaby model or the forest-scattering characteristic model based on its location environment to calculate the scattering characteristics at the location of the triangular facet; and calculating the scattering coefficient at the target location using a summation method based on the scattering characteristics at the location of the triangular facet.

[0088] By approximating the actual natural scene with these triangular facets, and then combining them with physical optics methods to calculate its scattering characteristics.

[0089] In step S4, when calculating the total target cross-sectional area of ​​the ground-illuminated target area, based on the physical optics method, to simplify the solution of the Stratton-Chu integral equation, the following assumptions are now made:

[0090] 1) The total field in the dark area on the surface of the scatterer is zero;

[0091] 2) In terms of wavelength and the relative size of the scatterer, the observation point is far from the scatterer;

[0092] 3) The total field on the surface of the scatterer is represented by the incident field.

[0093] Step S4 may specifically include: dividing the ground-illuminated target into multiple triangular elements using the triangular element method; calculating the cross-sectional area of ​​each triangular element using the scattering cross-sectional area calculation formula; and calculating the cross-sectional area of ​​the ground-illuminated target using the summation method based on the cross-sectional area of ​​the triangular elements.

[0094] The radar cross section of a target can be calculated using the following formula:

[0095]

[0096] in, For the total electric field of scattering, The incident electric field is expressed as:

[0097]

[0098]

[0099] In the formula, A is the illumination part of the target, Z0 is the intrinsic impedance, and k is the propagation constant in free space. is the unit vector pointing in the direction of the incident wave; r is the distance from the far field; It is the position vector at the point of incidence of the incident wave; E is the unit vector of the incident wave electric field; E0 is its amplitude at the origin of the coordinate system. This represents the surface current, and its expression is as follows:

[0100]

[0101]

[0102] in, R is the total magnetic field. n For the reflection coefficients of different media, It is the normal vector of the surface element.

[0103] The target cross-sectional area of ​​the illuminated region can be obtained by solving for the radar cross-section of each surface element and then summing them. Substituting this, along with other parameters, into the manual gain control parameter expression yields the control parameters.

[0104] In step S5, the incident angle θ and the scattering coefficient σ are... 0 By summing up the target cross-sectional area and combining it with other known satellite parameters, radar parameters, geographical parameters, etc., and substituting them into the corresponding synthetic aperture radar equations, the manual gain control parameters of the target area can be obtained.

[0105] In some embodiments, the expression for the manual gain control parameter is obtained based on the definition of the manual gain control parameter and the radar equation as follows:

[0106]

[0107] Where MGC represents the gain control parameter; A represents the illumination portion of the ground-illuminated target; G cg σ represents the channel gain, λ represents the wavelength, and σ represents the channel gain. 0 P represents the backscattering coefficient. n K represents system noise. L P represents system losses. t G represents the radar transmit power. 2 (θ a ,φ a ) represents the antenna's two-way gain, and θ represents the incident angle. G cg , λ, P t G 2 (θ a ,φ a ), P n K L These are all basic parameters of the radar system, obtained from radar design manuals.

[0108] The expressions for the manual gain control parameters mentioned above are derived from the received power expression of the synthetic aperture radar equation, which is shown below:

[0109]

[0110] The manual gain control parameters can be calculated based on step S5. In step S6, the gain of the spaceborne synthetic aperture radar is adjusted based on the manual gain control parameters in S5.

[0111] This embodiment has the following technical effects:

[0112] (1) The AGC control circuit is, in principle, a negative feedback electronic circuit. Its internal components (resistors and capacitors) cause a time delay between the input signal and the output signal, resulting in a high delay defect. This patent calculates the gain value (MGC) of the region in advance based on existing knowledge, eliminating the need to wait for the echo signal to arrive before controlling echo sampling. Compared to the AGC control circuit, this compensates for the high delay defect of systems like GEO SAR.

[0113] (2) Negative feedback control circuits are quite sensitive. Any change in the input will cause a change in the output through the negative feedback circuit. If the input signal is unstable (oscillating), the output signal will also oscillate, which is detrimental to subsequent data processing. This patented solution does not use an AGC negative feedback control circuit, thus making the amplitude adjustment of the echo more stable. Due to the stability of the received echo, the quality of subsequent data processing is improved.

[0114] (3) Traditional automatic gain control frequently adjusts the gain, introducing unnecessary amplitude modulation, which reduces image quality. This patent calculates the gain value in advance and then performs manual gain control based on that value to achieve gain regulation of the spaceborne synthetic aperture radar, avoiding the defect of introducing unnecessary amplitude modulation and reducing image quality in the prior art.

[0115] Example 2

[0116] This embodiment provides a system for gain control of a spaceborne synthetic aperture radar. Please refer to [link / reference]. Figure 2 The system includes:

[0117] The target-to-ground motion model acquisition module M1 is used to determine the target-to-ground distance as a function of time based on the positional relationship between the ground-illuminated target and the satellite, and the satellite-to-ground motion model, thereby obtaining the target-to-ground motion model. The target-to-ground distance represents the distance between the ground-illuminated target and the Earth's center. The positional relationship between the ground-illuminated target and the satellite is determined based on the relative positional relationship between the ground-illuminated target and the spaceborne synthetic aperture radar. The ground-illuminated target is the ground area illuminated by the satellite antenna. The satellite-to-ground motion model, a function of the satellite-to-ground distance over time, is obtained based on the satellite's orbital parameters. The satellite-to-ground distance represents the distance between the satellite and the Earth's center.

[0118] The incident angle calculation module M2 is used to calculate the incident angle of the spaceborne synthetic aperture radar based on the target-ground motion model.

[0119] The scattering coefficient calculation module M3 is used to calculate the scattering coefficient of the ground-irradiated target location based on the environment of the location of the ground-irradiated target, or based on the incident angle and the environment of the location of the ground-irradiated target location; the environment of the location of the ground-irradiated target location includes at least one of dry snow, grassland, shrubs, soil and rock surface or forest;

[0120] The target cross-sectional area calculation module M4 is used to calculate the cross-sectional area of ​​the ground-irradiated target;

[0121] The gain control parameter calculation module M5 is used to calculate the gain control parameters based on the incident angle, the scattering coefficient, and the cross-sectional area of ​​the ground-irradiated target.

[0122] The gain control module M6 is used to adjust the gain of the spaceborne synthetic aperture radar according to the gain control parameters.

[0123] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the systems disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the descriptions are relatively simple; relevant parts can be referred to the method section.

[0124] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for gain control of a spaceborne synthetic aperture radar, characterized in that, The method includes: Based on the positional relationship between the ground-illuminated target and the satellite, and the satellite-ground motion model, the function of the target-ground distance over time is determined, resulting in the target-ground motion model. The target-ground distance represents the distance between the ground-illuminated target and the Earth's center. The positional relationship between the ground-illuminated target and the satellite is determined based on the relative positional relationship between the ground-illuminated target and the spaceborne synthetic aperture radar. The ground-illuminated target is the ground area illuminated by the satellite antenna. The satellite-ground motion model, a function of the satellite-ground distance over time, is obtained based on the satellite's orbital parameters. The satellite-ground distance represents the distance between the satellite and the Earth's center. Based on the target-ground motion model, calculate the incident angle of the spaceborne synthetic aperture radar; The scattering coefficient of the ground-irradiated target is calculated based on the environment of the location of the ground-irradiated target, or based on the incident angle and the environment of the location of the ground-irradiated target; the environment of the location of the ground-irradiated target includes at least one of dry snow, grassland, shrubs, soil and rock surface or forest; The calculation of the cross-sectional area of ​​the ground-illuminated target specifically includes: dividing the ground-illuminated target into multiple triangular elements using the triangular element method; calculating the cross-sectional area of ​​each triangular element using the scattering cross-sectional area calculation formula; and calculating the cross-sectional area of ​​the ground-illuminated target using the summation method based on the cross-sectional area of ​​the triangular elements. Based on the incident angle, the scattering coefficient, and the cross-sectional area of ​​the ground-irradiated target, the gain control parameters are calculated; the calculation formula for the gain control parameters includes: Where MGC represents the gain control parameter; A represents the illumination portion of the ground-illuminated target; G cg σ represents the channel gain, λ represents the wavelength, and σ represents the channel gain. 0 P represents the backscattering coefficient. n K represents system noise. L P represents system losses. t G represents the radar transmit power. 2 (θ a ,φ a ) represents the antenna's two-way gain, and θ represents the incident angle; G cg , λ, P t G 2 (θ a ,φ a ), P n K L These are all basic parameters of the radar system, obtained from radar design manuals; The gain of the spaceborne synthetic aperture radar is adjusted according to the gain control parameters.

2. The method according to claim 1, characterized in that, Before determining the function of destination-to-location distance over time, the method further includes: Based on the satellite's orbital parameters, the function of the satellite-to-ground distance changing with time is determined, and a satellite-to-ground motion model is obtained. The positional relationship between the ground-illuminated target and the satellite is determined based on the relative positional relationship between the ground-illuminated target and the spaceborne synthetic aperture radar.

3. The method according to claim 2, characterized in that, The expression for the star-ground motion model is: Among them, R s (t a ) for t a The distance between the satellite and the Earth's center at any given time; θ f (t a ) represents the true anomaly angle of the satellite orbit; a s denoted as the semi-major axis of the satellite's orbit; e is the satellite's orbital eccentricity.

4. The method according to claim 1, characterized in that, The angle of incidence is calculated using the following formula: Where θ is the incident angle of the satellite antenna illuminating the ground target; R e Here is the target-to-ground motion equation; h is the orbital altitude; From a perspective.

5. The method according to claim 1, characterized in that, Based on the environment at the location of the ground-illuminated target, or based on the incident angle and the environment at the location of the ground-illuminated target, the scattering coefficient at the location of the ground-illuminated target is calculated, specifically including: When the environment of the ground-irradiated target is any of the following: dry snow, grassland, shrubs, or soil and rock surface, the scattering coefficient of the ground-irradiated target location is calculated using the Ulaby model. When the environment of the ground-irradiated target is a forest, the scattering coefficient of the target location is calculated using a forest-scattering characteristic model; the forest-scattering characteristic model is a model representing the relationship between forest biomass and scattering characteristics. When the environment of the target location includes two or more environments such as dry snow, grassland, shrubs, soil and rock surfaces, and forest, the scattering coefficient of the target location is calculated using the surface element model method.

6. The method according to claim 5, characterized in that, The forest-scattering characteristic model formula is as follows: Where, σ 0 denoted as the normalized backscattering coefficient, B as the forest biomass, and a, b, c as the fitted parameters.

7. The method according to claim 5, characterized in that, The calculation of the scattering coefficient at the target's location using the surface element model method specifically includes: The ground-illuminated target is modeled using a digital elevation model to obtain a three-dimensional model of the ground-illuminated target; The three-dimensional model is divided into multiple triangular facets using the facet modeling method; For each triangular facet element, the scattering characteristics at the location of the triangular facet element are calculated by selecting either the Ulaby model or the forest-scattering characteristic model, depending on its location environment. Based on the scattering characteristics at the location of the triangular facet, the scattering coefficient at the target's location is calculated using the summation method.

8. A system for gain control of a spaceborne synthetic aperture radar, characterized in that, The system includes: The target-to-ground motion model acquisition module is used to determine the target-to-ground distance as a function of time based on the positional relationship between the ground-illuminated target and the satellite, and the satellite-to-ground motion model, thereby obtaining the target-to-ground motion model. The target-to-ground distance represents the distance between the ground-illuminated target and the Earth's center. The positional relationship between the ground-illuminated target and the satellite is determined based on the relative positional relationship between the ground-illuminated target and the spaceborne synthetic aperture radar. The ground-illuminated target is the ground area illuminated by the satellite antenna. The satellite-to-ground motion model, a function of the satellite-to-ground distance over time, is obtained based on the satellite's orbital parameters. The satellite-to-ground distance represents the distance between the satellite and the Earth's center. An incident angle calculation module is used to calculate the incident angle of the spaceborne synthetic aperture radar based on the target-ground motion model. The scattering coefficient calculation module is used to calculate the scattering coefficient of the ground-irradiated target location based on the environment of the location of the ground-irradiated target, or based on the incident angle and the environment of the location of the ground-irradiated target location; the environment of the location of the ground-irradiated target location includes at least one of dry snow, grassland, shrubs, soil and rock surface or forest; The target cross-sectional area calculation module is used to calculate the cross-sectional area of ​​the ground-irradiated target, specifically including: dividing the ground-irradiated target into multiple triangular elements using the triangular element method; calculating the cross-sectional area of ​​each triangular element using the scattering cross-sectional area calculation formula; and calculating the cross-sectional area of ​​the ground-irradiated target using the summation method based on the cross-sectional area of ​​the triangular elements. The gain control parameter calculation module is used to calculate the gain control parameters based on the incident angle, the scattering coefficient, and the cross-sectional area of ​​the ground-irradiated target; the calculation formula for the gain control parameters includes: Where MGC represents the gain control parameter; A represents the illumination portion of the ground-illuminated target; G cg σ represents the channel gain, λ represents the wavelength, and σ represents the channel gain. 0 P represents the backscattering coefficient. n K represents system noise. L P represents system losses. t G represents the radar transmit power. 2 (θ a ,φ a ) represents the antenna's two-way gain, and θ represents the incident angle; G cg , λ, P t G 2 (θ a ,φ a ), P n K L These are all basic parameters of the radar system, obtained from radar design manuals; A gain control module is used to adjust the gain of the spaceborne synthetic aperture radar according to the gain control parameters.

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