Geo-sar satellite system ambiguity optimization method based on dual-band reflector antenna

Abstract

A method for ambiguity optimization in a Geo-SAR satellite system based on a dual-band reflector antenna is disclosed. The method includes: calculating the ratio of the signal energy intensity of all ambiguous points in the range direction to the target signal energy intensity to obtain the range ambiguity of each distributed target within the imaging range; calculating the ratio of the sum of the signal energy of all ambiguous points in the azimuth direction to the target energy to obtain the azimuth ambiguity of each distributed target within the imaging range; and calculating the ratio of the sum of the signal energy of each ambiguous point to the target energy, excluding the range and azimuth directions, to obtain the cross-ambiguity of each distributed target in the azimuth direction within the imaging range. This disclosure presents a system design architecture based on dual-band Geo-SAR and provides a method for ambiguity optimization calculation for Geo-SAR satellites. This method has good versatility and can improve the readability of system images.

Description

Technical Field

[0001] This invention relates to the field of SAR satellite overall design, and in particular to a method for ambiguity optimization of Geo-SAR satellite systems based on dual-band reflector antennas. Background Technology

[0002] Geo-SAR satellites represent the future trend in microwave remote sensing. These satellites can quickly point and image within a ±8.2° conical field of view of the Earth.

[0003] Blur is an unavoidable problem in spaceborne SAR, and the degree of blur is one of the important indicators for evaluating the quality of spaceborne SAR images.

[0004] According to classical theory, ambiguity alters the energy of pixels in the useful echo signal image or superimposes "ghosting" of strong point targets onto the useful echo signal image, affecting SAR image quality. Ambiguity Score (ASR) is a fundamental parameter characterizing the ambiguity performance of spaceborne SAR and an important indicator for evaluating radar image quality. It is defined as the ratio of the ambiguity signal intensity to the main signal intensity within a SAR image resolution cell, with the classical expression being:

[0005]

[0006]

[0007] In the formula, B p For azimuth processing bandwidth, G 2 (f d ,τ) is the radiation pattern of a two-way far-field antenna, σ0(f d ,τ) is the radar backscattering coefficient, ρ a It is the azimuth resolution, ρ gr It is the ground distance resolution, R st (f d ,τ) is the slant range from the imaging point or blurred point to the radar, f d τ is the Doppler frequency, τ is the echo delay time, PRF is the pulse repetition frequency, m is the Doppler frequency domain ambiguity number, and n is the echo time domain ambiguity number.

[0008] Fuzzy signals can be divided into three categories:

[0009] (1) Distance ambiguity signal: When m=0 and n≠0, the echo signal of the ambiguity point that has the same Doppler frequency as the imaging point and whose echo delay differs by an integer multiple of the pulse repetition time is the distance ambiguity signal.

[0010] (2) Azimuth blur signal: When m≠0 and n=0, the echo signal of the blur point that has the same slant distance as the imaging point but whose Doppler frequency differs from the pulse repetition frequency by an integer multiple, i.e., is located on the same equidistant line, is the azimuth blur signal.

[0011] (3) Cross-ambiguity signal: When m≠0 and n≠0, the echo signal of the blurred point that differs from the Doppler frequency of the imaging point by an integer multiple of the pulse repetition frequency and whose echo delay differs from the pulse repetition time by an integer multiple of the pulse repetition time is a cross-ambiguity signal.

[0012] Low-Earth orbit (LEO) spaceborne SAR typically only needs to consider range and azimuth ambiguities, which only require them to meet threshold constraints. Cross-ambiguity is generally much smaller than the former two and can be ignored. For Geo-SAR, firstly, due to the high orbital altitude, the spherical indices of the Earth's surface must be considered to accurately calculate the signal energy intensity distribution in each ambiguity region. Secondly, due to the large yaw angle, simplifying the antenna pattern into two one-dimensional sinc functions in the range and azimuth directions will produce significant errors. It is necessary to combine attitude control algorithms and orbital position to accurately calculate the intensity distribution and time-varying characteristics of the two-dimensional pattern on the Earth's surface. Thirdly, cross-ambiguity is significantly stronger for Geo-SAR, and the system design needs to comprehensively consider the effects of range, azimuth, and cross-ambiguity ambiguities. Summary of the Invention

[0013] This disclosure provides a method for ambiguity optimization of Geo-SAR satellite systems based on dual-band reflector antennas, which can optimize the calculation of range, azimuth, and cross ambiguities.

[0014] The ambiguity optimization method for Geo-SAR satellite systems based on dual-band reflector antennas disclosed herein includes the following steps:

[0015] The distance ambiguity of each distributed target within the imaging range is obtained by calculating the ratio of the signal energy intensity of all blurred points to the signal energy intensity of the target.

[0016] The azimuth ambiguity of each distributed target within the imaging range is obtained by calculating the ratio of the sum of the signal energy of each ambiguous point in the azimuth direction to the energy of the target point.

[0017] In addition to the range and azimuth directions, the ratio of the sum of the signal energy of each blurred point to the energy of the target point is calculated to obtain the cross-ambiguity of each distributed target in the azimuth direction within the imaging range.

[0018] Furthermore, the distance ambiguity is calculated according to the following formula:

[0019]

[0020] In the formula, the subscript containing the suffix Abm indicates a distance-ambiguous signal; otherwise, it indicates the imaging target signal.ea R represents the antenna gain distribution along the equidistant circle in the two-dimensional radiation pattern of a two-way antenna; st θ represents the slant distance of the imaging target point or the blurred point; in Indicates the angle of incidence.

[0021] Furthermore, the two-dimensional radiation pattern G of the two-way antenna ea The calculation methods include:

[0022]

[0023] In the formula, D r and D a These represent the antenna dimensions in the range and azimuth directions, respectively; assuming the straight line from the satellite to the Earth's center is the reference line, the elevation angle ξ of the antenna pattern is... e and azimuth ζ a Based on this, ξ e0 and ζ a0 The expressions for the elevation and azimuth angles representing the beam centerline are:

[0024]

[0025] In the formula, γ t θ represents the downward angle of view of the beam centerline. dt0 This represents the corresponding tilt angle; for example, in a frontal or side-view image, this value is 0; θ BFP This indicates the beam's ground projection rotation angle.

[0026] Furthermore, the azimuth ambiguity of each distributed target is calculated according to the following formula:

[0027]

[0028] In the formula, the subscript ta represents a distributed target within the azimuth range, kp and M represent the maximum number of ambiguities in the azimuth and elevation dimensions, respectively, and k and m represent integers not exceeding kp and M, respectively. e and ζ a This indicates the elevation and azimuth angles of the beam.

[0029] Furthermore, the cross-ambiguity of each distributed target in the azimuth direction within the imaging range is calculated according to the following formula:

[0030]

[0031] In the formula, N represents the maximum number of intersecting fuzzy points, and n represents an integer not exceeding N.

[0032] Compared with the prior art, the beneficial effects of this disclosure are: (1) The cross ambiguity, azimuth ambiguity and range ambiguity of Geo-SAR are almost of the same order of magnitude and need to be considered in the system design process. Based on the system design architecture of dual-band Geo-SAR, this disclosure gives a method for ambiguity optimization calculation of Geo-SAR satellites; (2) It has good versatility; (3) It can improve the readability of system images. Attached Figure Description

[0033] The above and other objects, features and advantages of this disclosure will become more apparent from the more detailed description of exemplary embodiments of this disclosure taken in conjunction with the accompanying drawings, in which the same reference numerals generally represent the same components.

[0034] Figure 1 A flowchart of the Geo-SAR satellite system ambiguity optimization calculation based on this disclosure;

[0035] Figure 2 Ground coverage map of L and P band antenna beams;

[0036] Figure 3 (a) represents the L-band payload distance ambiguity;

[0037] Figure 3 (b) represents the P-band payload distance ambiguity;

[0038] Figure 4 (a) represents the L-band payload azimuth ambiguity;

[0039] Figure 4 (b) represents the azimuth ambiguity of the P-band payload;

[0040] Figure 5 (a) represents the L-band payload cross-ambiguity;

[0041] Figure 5 (b) represents the cross-ambiguity of the P-band payload. Detailed Implementation

[0042] Preferred embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While preferred embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the present disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art.

[0043] This disclosure provides a method for optimizing the calculation of ambiguity in a Geo-SAR satellite system based on a dual-band reflector antenna.

[0044] The dual-band SAR system uses a radar architecture that shares a large reflector, meaning two SAR systems share a common reflector antenna. This maximizes the utilization of overall satellite resources and optimizes the overall satellite structure. The Geo-SAR satellite employs a dual-band design architecture to acquire dual-band scattering information from the ground and form microwave images. Both SAR payloads use a radar architecture with a large deployable reflector and a phased array feed antenna. The large deployable reflector is shared for both frequencies, saving satellite resources and improving efficiency.

[0045] High-orbit SAR has an imaging range on the order of 39,000 km. To meet the requirements of imaging signal-to-noise ratio and system NEσ° (and system sensitivity), large-aperture, high-gain antennas and high transmit power must be used. SAR payloads typically use frequency bands such as P, L, S, C, X, and Ku. This disclosure uses the L and P bands as examples, but practical applications are not limited to these combinations.

[0046] For L-band payloads, a combination of channel combining and spatial combining can be used to achieve peak radiated power in the tens of kilowatts range. The combined power output is fed into the antenna feed source through a solid-state amplifier. The feed source forms a beam that radiates into space, and the radiated power after spatial combining reaches the tens of kilowatts level, thus meeting the requirements of Geo-SAR imaging.

[0047] For P-band payloads, the radar system also employs a large truss-type deployable reflector plus a feed antenna. Its power combining and beamforming methods are consistent with those of L-band payloads.

[0048] Based on considerations of overall satellite weight and space optimization, the P-band and L-band payloads share a common link with the radar central control processor, receiver, large reflector antenna, and other related equipment. This enables the system to possess both P-band and L-band SAR capabilities, and allows for simultaneous measurement of the absolute and relative changes in ionospheric TEC along the radar signal propagation path during P-band or L-band SAR imaging. The system employs HH polarization for L-band SAR and circular polarization for P-band to reduce the impact of ionospheric attenuation on signal power.

[0049] Since the P-band and L-band payloads share a single reflector antenna, but their feed arrays cannot be shared, one payload is a prime feed and the other is an offset feed. This disclosure uses an L-band prime feed and a P-band offset feed as an example. A schematic diagram of the beam footprint coverage area on the ground for dual-band payloads sharing an antenna is shown below. Figure 2 As shown in the figure below, the solid circle represents the 3dB coverage footprint of the L-band payload antenna beam on the ground, and the dashed circle represents the 3dB coverage footprint of the P-band payload antenna beam on the ground. The solid circle in the figure represents the projection of the reflector antenna normal onto the ground. It can be seen that the center of the L-band beam footprint coincides with the normal, while the center of the P-band beam footprint deviates from the normal.

[0050] This disclosure optimizes the image ambiguity index based on the new system architecture design. Image ambiguity is a crucial aspect of system design. Due to the large yaw angles that Geo-SAR satellites experience drastic changes, the system design must consider not only azimuth and range ambiguities but also the impact of cross-ambiguities.

[0051] According to this disclosure, the ambiguity calculation method based on the L and P dual-band Geo-SAR system design architecture includes the following steps:

[0052] 1. First, calculate the distance ambiguity, which can be expressed as:

[0053]

[0054] In the formula, θ in Indicates the angle of incidence. The subscript containing the suffix "Abm" indicates a distance-ambiguous signal; otherwise, it indicates the imaging target signal. R st It represents the slant distance of the imaging target point or the blurred point.

[0055] 2. Calculate the azimuth ambiguity. The expression for the azimuth ambiguity of each distributed target within the imaging range is:

[0056]

[0057] In the formula, the subscript ta represents a target distributed within the azimuth range. It should be noted that G in formula (4) ea It is the antenna gain distribution value along the equidistant circle after comprehensively considering the antenna's two-dimensional radiation pattern and the antenna beam's ground projection azimuth angle.

[0058] 3. Calculate cross-ambiguity. The cross-ambiguity of Geo-SAR satellites cannot be ignored, and it is also necessary to use the two-dimensional radiation pattern G. ea To improve computational accuracy, the cross-ambiguity expression for each distributed target in the azimuth direction within the imaging range can be obtained as follows:

[0059]

[0060] In the formula, the subscript ta represents a target distributed within a certain azimuth range; R st θ represents the slant distance of the imaging target point or blurred point. in The subscript indicates the angle of incidence. If the subscript includes the suffix Abm, it indicates a cross-ambiguity signal; otherwise, it indicates a target point signal.

[0061] In the optimization calculation of dual-band Geo-SAR satellite ambiguity, both satellite trajectory and surface curvature are indispensable. The accuracy of calculating the location of the ambiguity point and the antenna pattern gain at that location determines the accuracy of the Geo-SAR ambiguity calculation. This disclosure focuses on solving the calculation method of Geo-SAR ambiguity, and the antenna pattern is characterized by an ideal sinc function. The two-way antenna pattern is shown in the following equation:

[0062]

[0063] Neither the main signal nor the range ambiguity signal of Geo-SAR can be calculated relying on the one-dimensional range pattern profile, as this would lead to significant errors. Therefore, it is necessary to calculate the two-dimensional range pattern G. ea In the formula, D r and D a These represent the antenna dimensions in the range and azimuth directions, respectively. Assuming the straight line from the satellite to the Earth's center is the reference line, the elevation and azimuth angles of the antenna pattern are based on this line. ξ e0 and ζ a0 The expressions for the elevation and azimuth angles representing the beam centerline are:

[0064]

[0065] In the formula, γ t θ represents the downward angle of view of the beam centerline. dt0 This indicates the corresponding tilt angle; for example, in a frontal or side-view image, this value is 0.

[0066] It is evident that the Geo-SAR satellite fuzzy energy calculation process has the following distinct characteristics:

[0067] (1) Distance and orientation ambiguities must be determined using a two-dimensional orientation map G. ea Calculations using the one-dimensional main profile of the antenna elevation and azimuth patterns will result in significant errors.

[0068] (2) Due to the influence of the large antenna beam ground projection direction angle, the cross ambiguity signal of Geo-SAR satellites cannot be ignored and must be considered in the system design process;

[0069] (3) Geo-SAR satellite orbital altitude is much greater than the Earth's radius, and the slant range does not change significantly within the incident angle range (18°, 60°). Therefore, the range ambiguity signal strength is mainly determined by the antenna pattern gain distribution itself and has a weak correlation with the incident angle.

[0070] The simulation results obtained using the simulation parameters shown in Table 1 are attached. Figure 3-5 As shown.

[0071] Table 1 Geo-SAR Satellite Simulation Parameters

[0072] Parameter name value Semi-major axis of the track (km) 42163.5 Track inclination angle (°) 28 Orbital eccentricity 0.001 Angle of incidence (°) 45.5 Argument of perigee (°) 90 Earth Model WSG84

[0073] The simulation results show that the ambiguity is less than -20dB, which can guarantee the quality of SAR satellite images.

[0074] The above technical solutions are merely exemplary embodiments of the present invention. For those skilled in the art, based on the application methods and principles disclosed in the present invention, it is easy to make various types of improvements or modifications, and not limited to the methods described in the specific embodiments of the present invention. Therefore, the methods described above are only preferred and not restrictive.

Claims

1. A method for ambiguity optimization in a Geo-SAR satellite system based on a dual-band reflector antenna, comprising the following steps: The distance ambiguity of each distributed target within the imaging range is obtained by calculating the ratio of the signal energy intensity of all blurred points to the signal energy intensity of the target. The azimuth ambiguity of each distributed target within the imaging range is obtained by calculating the ratio of the sum of the signal energy of each ambiguous point in the azimuth direction to the energy of the target point. The cross-ambiguity of each target in the imaging range is obtained by calculating the ratio of the sum of the signal energy of each cross-ambiguity point to the energy of the target point. The cross-ambiguity of each distributed target in the azimuth direction within the imaging range is calculated according to the following formula: In the formula, N represents the maximum number of intersecting fuzzy points, and n represents an integer not exceeding N; kp and M represent the maximum number of ambiguities in the azimuth and elevation dimensions, respectively, and k and m represent integers not exceeding kp and M, respectively. ξ e and ζ a Indicates the elevation and azimuth angles of the beam; G ea The antenna gain distribution value along the equidistant circle in the two-dimensional radiation pattern of the two-way antenna; ξ e and ζ a Indicates the elevation and azimuth angles of the beam; R st The slant distance represents the target point or blurred point in the image; θ in Indicates the angle of incidence; Subscript includes suffix Abm If it indicates a distance-ambiguous signal, then it indicates an imaging target signal; otherwise, it indicates an imaging target signal.

2. The method according to claim 1, characterized in that, in, Distance ambiguity is calculated using the following formula: In the formula, the subscript includes the suffix. Abm If it indicates a distance-ambiguous signal, otherwise it indicates the imaging target signal; G ea The antenna gain distribution value along the equidistant circle in the two-dimensional radiation pattern of the two-way antenna; R st The slant distance represents the target point or blurred point in the image; θ in Indicates the angle of incidence.

3. The method according to claim 2, characterized in that, in, Two-way antenna two-dimensional radiation pattern G ea The calculation methods include: In the formula, D r and D a These represent the range and azimuth antenna dimensions, respectively; assuming the straight line from the satellite to the Earth's center is the reference line, the elevation angle of the antenna pattern is... ξ e and azimuth ζ a Based on this, ξ e0 and ζ a0 The expressions for the elevation and azimuth angles representing the beam centerline are: In the formula, γ t The downward angle representing the beam centerline; θ dt0 This indicates the corresponding tilt angle; for frontal or side-view imaging, this value is 0. θ BFP This indicates the beam's ground projection rotation angle.

4. The method according to claim 2 or 3, characterized in that, The azimuth ambiguity of each distributed target is calculated according to the following formula: In the formula, the subscript ta represents a distributed target within the azimuth range, kp and M represent the maximum number of ambiguities in the azimuth and elevation dimensions, respectively, and k and m represent integers not exceeding kp and M, respectively. ξ e and ζ a This indicates the elevation and azimuth angles of the beam.

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