Long synthetic aperture time SAR imaging airborne test verification method

Abstract

The invention relates to a long synthetic aperture time SAR imaging airborne test verification method, and belongs to the field of satellite overall design. Building a ground test system; designing a frequency band of the SAR antenna; designing a polarization mode of the ground test system; designing the flight height of a flight platform in the ground test system; designing an incident angle range of an SAR antenna beam in the ground test system; designing the flight speed of the flight platform; designing a radar slope distance in the ground test system; designing the breadth of the ground test system; designing a beam incident angle of the ground test system; the synthetic aperture time of the ground test system is designed; designing the azimuth resolution of the ground test system; designing the bandwidth of the ground test system; the flight platform flies according to the design parameters, test original echo data are obtained, a final imaging image is obtained, and verification of the imaging performance of the SAR satellite with the long synthetic aperture time is completed; according to the method, the long synthetic aperture time SAR satellite imaging performance and function are verified in the satellite ground development stage.

Description

Technical Field

[0001] This invention belongs to the field of satellite overall design and relates to an airborne experimental verification method for long synthetic aperture time SAR imaging. Background Technology

[0002] High-resolution SAR satellites and medium-to-high orbit SAR satellites represent the development trend of microwave remote sensing. These satellites all have the characteristic of long synthetic aperture time, and the resulting time-varying ground object scattering characteristics, orbit curvature, two-dimensional time-varying Doppler characteristics, and non-"stop-and-go" signal models are all significant influences.

[0003] SAR satellites typically require ground-based verification tests, such as airborne flight calibration, during the development phase to verify the functional performance, technical system, and image indicators of the SAR payload. For SAR satellites with long synthetic aperture time characteristics, the primary consideration in conducting airborne flight calibration tests is the equivalence of the test. A comprehensive equivalence analysis is performed on aspects such as imaging mechanisms, models, and parameters. Then, the parameter design of the airborne experimental SAR system is completed, and finally, the experimental verification is completed.

[0004] Currently, there are no suitable airborne flight calibration or other ground verification tests, so the equivalence of the tests cannot be fully achieved, and a comprehensive equivalence analysis cannot be conducted from the aspects of imaging mechanism, model, and parameters. Summary of the Invention

[0005] The technical problem solved by this invention is to overcome the shortcomings of the prior art and propose an airborne test verification method for long synthetic aperture time SAR imaging, which enables the verification of the imaging performance and function of long synthetic aperture time SAR satellites during the satellite ground development stage.

[0006] The solution of the present invention is:

[0007] Airborne experimental verification methods for long synthetic aperture time-of-flight SAR imaging include:

[0008] Build a ground test system, including a SAR antenna and a flight platform; mount the SAR antenna onto the flight platform;

[0009] Design the frequency band for the SAR antenna;

[0010] Design the polarization mode of the ground test system;

[0011] Design the flight altitude of the flight platform in the ground test system;

[0012] Design the incident angle range of the SAR antenna beam in the ground test system;

[0013] Design the flight speed of the flight platform;

[0014] Design the radar slant range in the ground test system;

[0015] Design the width of the ground test system;

[0016] Design the beam incident angle of the ground test system;

[0017] Design the synthetic aperture time for the ground test system;

[0018] Design the azimuth resolution of the ground test system;

[0019] Design the bandwidth of the ground test system;

[0020] The flight platform flew according to the above design parameters, obtained the raw echo data of the test, obtained the final imaging image, and completed the verification of the long synthetic aperture time SAR satellite imaging performance.

[0021] In the above-mentioned airborne experimental verification method for long synthetic aperture time SAR imaging, the frequency band used is the L band; the polarization mode is HH polarization.

[0022] In the above-mentioned airborne experimental verification method for long synthetic aperture time SAR imaging, the flight altitude design method of the flight platform is as follows:

[0023] Based on the satellite's orbital altitude of 35,792 km, the flight altitude of the flight platform is designed to be 3 km above the ground.

[0024] In the above-mentioned airborne experimental verification method for long synthetic aperture time SAR imaging, the design method for the incident angle range of the SAR antenna beam is as follows:

[0025] The satellite incident angle range is 10°-60°, with a corresponding downward angle of 2.21°-7.30°. To ensure that the ground object scattering characteristics obtained from the airborne test are as consistent as possible with those on the satellite, the incident angle range is designed to be 10°-70°. The SAR antenna beam's 3dB main lobe is within this incident angle range.

[0026] In the above-mentioned airborne experimental verification method for long synthetic aperture time SAR imaging, the design method for the flight speed of the flight platform is as follows:

[0027] Based on the satellite's flight speed above the ground being 100m / s-800m / s, the flight platform's flight speed is designed to vary between 2 and 10m / s; the error range for flight speed control is within ±25%.

[0028] In the above-mentioned airborne test verification method for long synthetic aperture time SAR imaging, based on the typical slant range of the satellite being 39,000 km, the radar slant range in the ground test system was designed to be 6 km.

[0029] In the above-mentioned airborne test verification method for long synthetic aperture time SAR imaging, based on the satellite swath width of 500km, the swath width of the ground test system is designed to be 3-5km.

[0030] In the above-mentioned airborne experimental verification method for long synthetic aperture time SAR imaging, the incident angle of the satellite is in the range of 10°-60°, and the beam center incident angle of the ground test system is designed to be 55°.

[0031] In the above-mentioned long synthetic aperture time SAR imaging airborne test verification method, based on the typical value of satellite synthetic aperture time being 200s, the synthetic aperture time of the ground test system is designed to be 100s to 200s.

[0032] In the above-mentioned airborne experimental verification method for long synthetic aperture time SAR imaging, the azimuth resolution of the ground test system is designed to be 1m based on a satellite azimuth resolution of 20m; based on a satellite bandwidth of 70MHz, the range resolution under slant-out conditions is guaranteed to be 20m, the maximum range resolution of the ground test system is 1m, and the bandwidth of the ground test system is designed not to exceed 250MHz.

[0033] The advantages of this invention compared to the prior art are:

[0034] (1) This invention proposes an equivalent ground verification method based on "mounting SAR payload equipment on a slow flight platform", which realizes the verification of long synthetic aperture time SAR satellite imaging performance and function during the satellite ground development stage;

[0035] (2) Based on the six aspects of equivalence proposed in this paper, this invention conducts targeted analysis; and based on the requirements after the equivalence analysis, it completes the design of 11 parameters of the ground test system to ensure the accuracy of the ground test;

[0036] (3) Based on the equivalence analysis and the requirements of 11 parameters of the ground test system, this invention selects a spacecraft that meets the requirements, carries a test SAR payload that meets the requirements, and selects a test site scenario that meets the requirements, thus building complete verification conditions and finally realizing the verification of the long synthetic aperture time SAR satellite imaging performance. Attached Figure Description

[0037] Figure 1 This is a flowchart of the airborne test verification process for long synthetic aperture time SAR imaging of the present invention. Detailed Implementation

[0038] The present invention will be further described below with reference to the embodiments.

[0039] This invention provides an airborne experimental verification method for long synthetic aperture time SAR imaging and proposes an effective ground-based calibration and verification method for long synthetic aperture time SAR imaging.

[0040] Conducting an equivalence analysis based on the technical characteristics of geostationary orbit SAR satellites is a necessary prerequisite. This will be explained through a comparative analysis of six aspects:

[0041] First aspect: Long synthetic aperture time imaging characteristics. Taking the L-band 20° orbital inclination geosynchronous orbit SAR satellite system as an example: the average time to acquire a 10m resolution synthetic aperture is 460s, the average time to acquire a 20m resolution synthetic aperture is 220s, and the average time to acquire a 50m resolution synthetic aperture is 105s.

[0042] To simulate this synthetic aperture time, a ground-based flight system needs a flight speed of no more than 10 m / s. Fixed-wing aircraft (conventional aircraft) have speeds far exceeding this value, thus failing to meet the requirements.

[0043] Secondly, the characteristics of curved trajectory imaging. Traditional low-Earth orbit satellites have synthetic aperture times (SAPs) in the order of seconds, so the curvature of the orbit is negligible and can be considered a straight line. However, since SAPs reach the order of hundreds of seconds, the satellite flight trajectory is much longer, and the curvature of the orbit can no longer be ignored. Therefore, the ground-based flight system needs to simulate an arc-shaped flight trajectory, the curvature of which can be calculated based on the altitude ratio between the satellite and the spacecraft. This can be achieved through trajectory pre-setting, allowing the flight trajectory on the ground control platform to approximately simulate the characteristics of the satellite's curved trajectory.

[0044] Thirdly, the characteristics of slow, variable-speed motion. Low-Earth orbit SAR satellites fly at approximately 7000 m / s, with a speed variation of about 30-40 m / s over their entire orbital period. Geostationary orbit SAR satellites fly at speeds relative to Earth ranging from approximately 100-800 m / s, exhibiting a large dynamic range. Therefore, the ground-based flight system needs to simulate these slow, variable-speed motion characteristics, which can be achieved through preset flight speeds, allowing the flight speed on the ground control platform to vary between 2 and 10 m / s.

[0045] Fourthly, the characteristics of large oblique angle imaging. Geostationary orbit SAR satellites operate in oblique angle mode most of the time, with a maximum geosliding angle of ±60°, corresponding to a maximum oblique angle of approximately 8°. This large oblique angle leads to severe range / azimuth signal coupling. Therefore, the ground-based flight system needs to simulate the characteristics of large oblique angle imaging by adjusting the antenna beam pointing to achieve the same oblique angle, approximating the coupling state of higher-order oblique angle quantities in high-orbit operation.

[0046] Fifth aspect: Time-varying scattering characteristics of multiple ground features. Geostationary orbit SAR satellites have a wide imaging swath, therefore targets within the beam coverage area exhibit composite characteristics of multiple ground features. Furthermore, over a long synthetic aperture time (SAP) period, the scattering characteristics of these ground features change, leading to decreased coherence. Therefore, this ground-based airborne calibration experiment needs to conduct composite scene imaging of stationary targets in complex scenarios (e.g., composite scenes including bare land, crops, and water, and composite scenes including bare land, buildings, and water). Simultaneously, the experiment will conduct long SAP time imaging to observe the changes in the scattering characteristics of ground features.

[0047] Sixthly: The ground-based airborne flight calibration test system needs to be consistent with the on-board system in terms of system bandwidth, polarization mode, and incident angle, so as to complete the equivalence verification.

[0048] Based on the equivalence analysis of the above six aspects, an airborne experimental verification method for long synthetic aperture time-of-flight SAR imaging was carried out, such as... Figure 1 As shown, the specific steps include the following:

[0049] Build a ground test system, including a SAR antenna and a flight platform; mount the SAR antenna onto the flight platform.

[0050] The frequency band for designing the SAR antenna is the L-band.

[0051] Design the polarization mode of the ground test system; the polarization mode is HH polarization.

[0052] Design the flight altitude of the flight platform in the ground test system. The method for designing the flight altitude of the flight platform is as follows:

[0053] Based on the satellite's orbital altitude of 35,792 km, the flight altitude of the flight platform is designed to be 3 km above the ground.

[0054] Design the incident angle range of the SAR antenna beam in the ground test system.

[0055] The design method for the incident angle range of SAR antenna beams is as follows:

[0056] The satellite incident angle range is 10°-60°, with a corresponding downward angle of 2.21°-7.30°. To ensure that the ground object scattering characteristics obtained from the airborne test are as consistent as possible with those on the satellite, the incident angle range is designed to be 10°-70°. The SAR antenna beam's 3dB main lobe is within this incident angle range.

[0057] Design the flight speed of the flight platform.

[0058] The design method for the flight speed of the flight platform is as follows:

[0059] Based on the satellite's flight speed above the ground being 100m / s-800m / s, the flight platform's flight speed is designed to vary between 2 and 10m / s; the error range for flight speed control is within ±25%.

[0060] Design the radar slant range in the ground test system.

[0061] Based on the typical operational slant range of a satellite being 39,000 km, the radar slant range in the ground test system was designed to be 6 km.

[0062] Design the swath width of the ground test system. Based on the satellite swath width of 500 km, design the swath width of the ground test system to be 3-5 km.

[0063] Design the beam incidence angle of the ground test system. Based on the satellite's incidence angle range of 10°-60°, design the beam center incidence angle of the ground test system to be 55°.

[0064] Design the synthetic aperture time of the ground test system. Based on the typical value of 200s for satellite synthetic aperture time, design the synthetic aperture time of the ground test system to be 100s to 200s.

[0065] Design the azimuth resolution of the ground test system. Based on the satellite azimuth resolution of 20m, the azimuth resolution of the ground test system is designed to be 1m.

[0066] Design the bandwidth of the ground test system; based on the satellite bandwidth of 70MHz, ensure a distance resolution of 20m under oblique line-of-sight conditions, and the highest distance resolution of the ground test system is 1m, and design the bandwidth of the ground test system to not exceed 250MHz.

[0067] The flight platform flew according to the above design parameters, obtained the raw echo data of the test, obtained the final imaging image, and completed the verification of the long synthetic aperture time SAR satellite imaging performance.

[0068] Taking the geosynchronous orbit SAR satellite calibration test system as an example, this invention proposes an example test system and puts forward an equivalent ground verification method based on "loading SAR payload equipment on a slow flight platform", which realizes the verification of long synthetic aperture time SAR satellite imaging performance and function during the satellite ground development stage.

[0069] Based on the six aspects of equivalence proposed in this paper, this invention conducts targeted analysis; and based on the requirements after the equivalence analysis, it completes the design of 11 parameters of the ground test system, ensuring the accuracy of the ground test.

[0070] Based on equivalence analysis and the requirements for 11 parameters of the ground test system, this invention selects a suitable aircraft, a suitable test SAR payload, and a suitable test site scenario to build complete verification conditions, ultimately realizing the verification of the long synthetic aperture time SAR satellite imaging performance.

[0071] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications to the technical solutions of the present invention by utilizing the methods and techniques disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall fall within the protection scope of the technical solutions of the present invention.

Claims

1. A method for airborne test and validation of long synthetic aperture time (SAR) imaging, characterized in that: The application relates to a long synthetic aperture time SAR satellite imaging performance verification method. The application comprises the following steps: Building a ground test system, including a SAR antenna and a flight platform; loading the SAR antenna on the flight platform; Designing a frequency band of the SAR antenna; Designing a polarization mode of the ground test system; Designing a flight height of the flight platform in the ground test system; Designing an incident angle range of a SAR antenna beam in the ground test system; Designing a flight speed of the flight platform; Designing a radar slant range in the ground test system; Designing a width of the ground test system; Designing a beam incident angle of the ground test system; Designing a synthetic aperture time of the ground test system; Designing an azimuth resolution of the ground test system; Designing a bandwidth of the ground test system; 2. The long synthetic aperture time (SAR) imaging airborne test and validation method of claim 1, wherein: The flight platform flies according to the above design parameters, original echo data of the test are obtained, final imaging images are obtained, and the long synthetic aperture time SAR satellite imaging performance verification is completed.

3. The long synthetic aperture time (SAR) imaging airborne test and validation method of claim 1, wherein: The frequency band adopts an L frequency band; and the polarization mode is HH polarization. The flight height design method of the flight platform is as follows:

4. The long synthetic aperture time (SAR) imaging airborne proof-of- concept method of claim 1, wherein: According to the satellite orbit height of 35792km, the height of 3km away from the ground of the flight platform is designed as the flight height of the flight platform. The incident angle range design method of the SAR antenna beam is as follows:

5. The long synthetic aperture time (SAR) imaging airborne test and validation method of claim 1, wherein: The satellite incident angle range is 10-60 degrees, and the corresponding downward viewing angle is 2.21-7.30 degrees; in order to ensure that the ground object scattering characteristics obtained by the airborne test are consistent with those on the satellite as much as possible, the incident angle range is designed as 10-70 degrees; and the 3dB main lobe of the SAR antenna beam is within the incident angle range. The flight speed design method of the flight platform is as follows:

6. The long synthetic aperture time (SAR) imaging airborne proof of concept method of claim 1, wherein: According to the satellite flight speed of 100-800m / s, the flight speed of the flight platform is designed to change between 2-10m / s; and the error range of the flight speed control is within + / -25%.

7. The long synthetic aperture time (SAR) imaging airborne proof of concept method of claim 1, wherein: According to the satellite typical action slant range of 39000km, the radar slant range in the ground test system is designed as 6km.

8. The long synthetic aperture time (SAR) imaging airborne proof of concept method of claim 1, wherein: According to the satellite width of 500km, the width of the ground test system is designed as 3-5km.

9. The long synthetic aperture time (SAR) imaging airborne proof of concept method of claim 1, wherein: Based on the satellite incident angle range of 10-60 degrees, the beam center incident angle of the ground test system is designed as 55 degrees.

10. The long synthetic aperture time (SAR) imaging airborne proof-of- concept method of claim 1, wherein: Based on the satellite synthetic aperture time typical value of 200s, the synthetic aperture time of the ground test system is designed as 100-200s. Based on the satellite azimuth resolution of 20m, the azimuth resolution of the ground test system is designed as 1m; based on the satellite bandwidth of 70MHz, the distance resolution is ensured to be 20m under the slant condition, the highest distance resolution of the ground test system is 1m, and the bandwidth of the ground test system is designed to be not more than 250MHz.

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