Satellite-borne sar satellite imaging performance closed-loop test method and system

Abstract

The application provides a kind of spaceborne SAR satellite imaging performance closed loop test method and system, and main steps are as follows: utilize dynamics simulator and GNSS simulator closed loop, record ephemeris, position, speed parameter;Utilize the above parameters to carry out echo simulation;Utilize fixed time delay standard linear frequency modulation signal to measure link time delay;Target echo simulator compensates time delay;Utilize dynamics simulator, GNSS simulator, target echo simulator closed loop to carry out imaging test.The application solves the whole satellite stage imaging quality test and on-board SAR data processing imaging problem.The method proposed in the application solves the problems such as inconsistency between echo auxiliary data and simulation results, inconsistency between actual link time delay and echo simulation, and inability to support on-board SAR data processing imaging in ground SAR imaging quality test, which can greatly improve the imaging quality test efficiency and improve the test coverage.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of signal and information processing, and in particular to a spaceborne SAR satellite imaging performance closed-loop test method and system. BACKGROUND

[0002] Spaceborne SAR satellites have the advantages of all-weather, all-time data acquisition, and highly automated data processing, and have become one of the remote sensing detection means widely researched and developed by countries around the world.

[0003] Spaceborne SAR uses a SAR antenna to transmit signals, which are received by the radar after being diffusely reflected by the ground, forming echo data containing ground object scattering information. After imaging processing of the echo, a SAR image can be generated. In the process of ground testing of the SAR load, due to the absence of a space-ground signal link, in order to test the SAR imaging quality, a target echo simulator is needed to simulate the echo. As one of the ground test equipment for SAR load integration test and on-orbit test, the SAR target echo simulator is mainly used to receive the signals transmitted by the SAR, simulate the echo signals and feed them into the SAR receiving channel, so as to test and evaluate the function and performance indicators of the SAR system.

[0004] Currently, when testing SAR imaging, the target echo data simulated in advance is used to access the spaceborne SAR system through the simulator. Since the simulated data is inconsistent with the actual ephemeris and GNSS data at the time of testing, the auxiliary data domain obtained in the SAR data is simulated inconsistently, and the auxiliary data cannot be directly read for imaging during imaging processing, but needs to be externally inputted with ephemeris and GNSS data, which is inconsistent with the actual on-orbit process, causing inconvenience for imaging quality verification. At the same time, since the simulator accesses through the on-board SAR calibration system, it causes an additional time delay error, resulting in inconsistency between the actual time delay of the test data and the simulation results, which also needs to be compensated in the subsequent imaging processing process. These result in the difference between the ground imaging processing software processing flow and the actual on-orbit data processing flow, and an additional set of imaging software adapted to ground processing is needed, and there is a problem of insufficient verification of on-orbit imaging software.

[0005] Real-time processing on board is one of the hotspots and directions of subsequent SAR satellite development, but due to the inconsistency between the echo auxiliary data and the simulation results, and the inconsistency between the actual link time delay and the echo simulation, the data obtained by the ground spaceborne SAR cannot meet the test requirements of the real-time processing system on board, and the real-time processing system on board can only use its own ground equipment for imaging function and performance self-verification, and cannot perform imaging function and performance test in the whole satellite test, which has the risk of insufficient test.

[0006] CN110501734A Double Star Formation SAR Satellite Joint Test System and Method specifies a double star imaging system and method, but does not solve the problem of inconsistency between double star dynamics simulator, GNSS simulator and echo signal, and does not have time delay calibration and compensation function. CN208921860U only proposes a design method of a wideband radar target echo simulator microwave assembly system, and does not solve the problem of inconsistency between double star dynamics simulator, GNSS simulator and echo signal. CN108008366B proposes a radar target echo simulation method and system, CN104833962B proposes a radar echo simulation test system, and CN102866389A proposes a double-channel radar echo signal simulation method and system, but the above-mentioned methods and systems do not have time delay calibration, compensation and ignore pulse functions, and cannot meet the test requirements of real-time processing and imaging on board.

[0007] At present, there are problems in the quality test of spaceborne SAR imaging, such as inconsistency between echo auxiliary data and simulation results, inconsistency between actual link time delay and echo simulation, and inability to support on-board SAR data processing and imaging. At present, there is no effective imaging quality test method that completely simulates the real processing process on orbit.

[0008] Therefore, a new technical solution is needed to improve the above technical problems. SUMMARY

[0009] In view of the defects in the prior art, the purpose of the present application is to provide a spaceborne SAR satellite imaging performance closed-loop test method and system.

[0010] According to the spaceborne SAR satellite imaging performance closed-loop test method provided by the present application, the method comprises the following steps:

[0011] Step S1: Use the dynamics simulator and GNSS simulator to close loop and record ephemeris, position, speed and attitude parameters;

[0012] Step S2: Use ephemeris, position, speed and attitude parameters to simulate echo;

[0013] Step S3: Measure the link time delay by using a fixed time delay standard linear frequency modulation signal;

[0014] Step S4: Compensate the time delay by using a target echo simulator;

[0015] Step S5: Use the dynamics simulator, GNSS simulator and target echo simulator to close loop for imaging test.

[0016] Preferably, the step S1 sets a dynamic simulator, a GNSS simulator closed loop, after the dynamic simulator sets initial values, time, position and speed parameters are sent to the GNSS simulator, after the GNSS simulator adds navigation information, navigation text signals are sent to a satellite GPS receiver, the GPS receiver generates position and speed information in a WGS-84 system; an on-board attitude control subsystem generates ephemeris and attitude signals under a yaw guide rule, and extracts in-orbit ephemeris, position, speed and attitude parameters through a bus monitor.

[0017] Preferably, the step S2 interpolates the ephemeris, position, speed and attitude parameters obtained in the step S1, and uses the interpolated data to simulate echo, the imaging time t0 and the simulated echo data time delay t10=t1 b +t1 s , wherein t1 b is a time delay corresponding to a gate opening time, i.e. a large time delay, the time delay can be set on a simulator interface, t1 s is a decimal part of the time delay, i.e. a micro time delay, the time delay is realized by echo.

[0018] Preferably, in the step S3, since the echo is coupled to a receiving channel through an internal calibration network, in an actual test process, a received echo signal time delay includes a simulated theoretical time delay t10, an internal calibration network time delay, a test cable time delay and a simulator hardware time delay, the part of the time delay needs to be calibrated and compensated in the test; the actual test data time delay t1 is t1=t10+t1 d +t1 m +t1 l , wherein t1 d is a time delay introduced by the calibration network, t1 m is a time delay introduced by the simulator, and t1 l is a time delay introduced by the test cable.

[0019] Preferably, the step S4 sets an overall time delay and a time delay compensation setting bar in a simulator interface, wherein the simulator overall time delay is set as t1 b , the time delay compensation is set as dt, and t1 b >dt.

[0020] The application also provides a satellite-borne SAR satellite imaging performance closed loop test system, the system comprises the following modules:

[0021] Module M1: using a dynamic simulator and a GNSS simulator closed loop to record ephemeris, position, speed and attitude parameters;

[0022] Module M2: using ephemeris, position, speed and attitude parameters to simulate echo;

[0023] Module M3: Measure link delay using a fixed-delay standard linear frequency modulation signal;

[0024] Module M4: Compensates for time delay using a target echo simulator;

[0025] Module M5: Uses a dynamic simulator, GNSS simulator, and target echo simulator to perform imaging tests.

[0026] Preferably, module M1 sets up a closed loop for a dynamic simulator and a GNSS simulator. After the dynamic simulator is set with initial values, it sends time, position, and velocity parameters to the GNSS simulator. After the GNSS simulator adds navigation information, it sends navigation message signals to the satellite GPS receiver. The GPS receiver generates position and velocity information in the WGS-84 system. The onboard attitude control subsystem generates ephemeris and attitude signals under yaw guidance, and extracts ephemeris, position, velocity, and attitude parameters within one orbit through the bus monitor.

[0027] Preferably, module M2 interpolates the ephemeris, position, velocity, and attitude parameters obtained by module M1, and uses the interpolated data for echo simulation. The imaging time is t0, and the time delay of the simulated echo data is t10 = t1. b +t1 s , where t1 b This refers to the delay corresponding to the gate opening time, i.e., the large bit delay. This delay can be set in the simulator interface. t1 s The fractional part of the delay, i.e., the micro-delay, is achieved by the echo.

[0028] Preferably, when module M3 performs imaging tests using a simulator, the echo is coupled to the receiving channel through the internal calibration network. In the actual test, the received echo signal delay, in addition to the theoretical delay t10 from the simulation, also includes the delay from the internal calibration network, the test cable delay, and the simulator hardware delay. This delay needs to be calibrated and compensated during testing. The formula for the delay t1 in the actual test data is: t1 = t10 + t1 d +t1 m +t1 l , where t1 d The time delay introduced for the calibration network, t1 m The latency introduced for the simulator, t1 l The time delay introduced for testing the cable.

[0029] Preferably, module M4 sets up overall latency and latency compensation settings in the simulator interface, wherein the overall simulator latency is set to t1. b The time delay compensation is set to dt, t1 b >dt.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] The method proposed in this invention solves the problems in ground SAR imaging quality testing, such as inconsistency between echo auxiliary data and simulation results, inconsistency between actual link delay and echo simulation, and inability to support on-board SAR data processing and imaging. It can greatly improve the efficiency of imaging quality testing and increase the test coverage. Attached Figure Description

[0032] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0033] Figure 1 This is a schematic diagram of the test connection for acquiring imaging parameters according to the present invention;

[0034] Figure 2 This is a schematic diagram of the imaging closed-loop test connection of the present invention;

[0035] Figure 3 This is an image showing the point target imaging results of the present invention;

[0036] Figure 4 This diagram illustrates the processing steps of the closed-loop testing method for spaceborne SAR satellite imaging performance according to the present invention. Detailed Implementation

[0037] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0038] Example 1

[0039] According to the present invention, a closed-loop test method for the imaging performance of a spaceborne SAR satellite is provided, the method comprising the following steps:

[0040] Step S1: Using a closed loop between the dynamics simulator and the GNSS simulator, record ephemeris, position, velocity, and attitude parameters; set up the closed loop of the dynamics simulator and the GNSS simulator. After setting the initial values ​​in the dynamics simulator, send the time, position, and velocity parameters to the GNSS simulator. After adding navigation information in the GNSS simulator, send the navigation message signal to the satellite GPS receiver. The GPS receiver generates position and velocity information in the WGS-84 system. The onboard attitude control subsystem generates ephemeris and attitude signals under the yaw guidance law, and extracts the ephemeris, position, velocity, and attitude parameters within one orbit through the bus monitor.

[0041] Step S2: Perform echo simulation using ephemeris, position, velocity, and attitude parameters; interpolate the ephemeris, position, velocity, and attitude parameters obtained in Step S1, and use the interpolated data for echo simulation. The imaging time is t0, and the time delay of the simulated echo data is t10 = t1. b +t1 s , where t1 b This refers to the delay corresponding to the gate opening time, i.e., the large bit delay. This delay can be set in the simulator interface. t1 s The fractional part of the delay, i.e., the micro-delay, is achieved by the echo.

[0042] Step S3: Measure the link delay using a fixed-delay standard linear frequency modulated signal. When performing imaging tests using a simulator, the echo is coupled to the receiving channel through the internal calibration network. In the actual test, the received echo signal delay, in addition to the theoretical delay t10 from the simulation, also includes the internal calibration network delay, the test cable delay, and the simulator hardware delay. This delay needs to be calibrated and compensated during testing. The formula for delay t1 in the actual test data is: t1 = t10 + t1 d +t1 m +t1 l , where t1 d The time delay introduced for the calibration network, t1 m The latency introduced for the simulator, t1 l The time delay introduced for testing the cable.

[0043] Step S4: Compensate for the time delay using the target echo simulator; in the simulator interface, set the overall time delay and time delay compensation settings, where the overall simulator time delay is set to t1. b The time delay compensation is set to dt, t1 b >dt.

[0044] Step S5: Conduct imaging tests using a dynamic simulator, GNSS simulator, and target echo simulator.

[0045] Example 2

[0046] Example 2 is a preferred example of Example 1, and is used to illustrate the present invention in more detail.

[0047] This invention also provides a closed-loop testing system for the imaging performance of spaceborne SAR satellites, the system comprising the following modules:

[0048] Module M1: Utilizes a closed loop between the dynamics simulator and the GNSS simulator to record ephemeris, position, velocity, and attitude parameters; sets up the closed loop of the dynamics simulator and the GNSS simulator; after initializing the dynamics simulator, it sends the time, position, and velocity parameters to the GNSS simulator; after adding navigation information, the GNSS simulator sends the navigation message signal to the satellite GPS receiver; the GPS receiver generates position and velocity information in the WGS-84 system; the onboard attitude control subsystem generates ephemeris and attitude signals under yaw guidance, and extracts the ephemeris, position, velocity, and attitude parameters within one orbit through the bus monitor.

[0049] Module M2: Performs echo simulation using ephemeris, position, velocity, and attitude parameters; interpolates the ephemeris, position, velocity, and attitude parameters obtained from the test of Module M1, and uses the interpolated data for echo simulation. Imaging time t0, and the time delay of the simulated echo data is t10 = t1. b +t1 s , where t1 b This refers to the delay corresponding to the gate opening time, i.e., the large bit delay. This delay can be set in the simulator interface. t1 s The fractional part of the delay, i.e., the micro-delay, is achieved by the echo.

[0050] Module M3: Measures link delay using a fixed-delay standard linear frequency modulated signal. When performing imaging tests using a simulator, the echo is coupled to the receiving channel through the internal calibration network. In actual testing, the received echo signal delay, in addition to the simulated theoretical delay t10, also includes the internal calibration network delay, test cable delay, and simulator hardware delay. This delay needs to be calibrated and compensated during testing. The formula for delay t1 in the actual test data is: t1 = t10 + t1 d +t1 m +t11, where t1 d The time delay introduced for the calibration network, t1 m t11 is the delay introduced by the simulator, and t11 is the delay introduced by the test cable.

[0051] Module M4: Compensates for time delay using a target echo simulator; it allows setting the overall time delay and time delay compensation settings in the simulator interface, where the overall simulator time delay is set to t1. b The time delay compensation is set to dt, t1 b >dt.

[0052] Module M5: Uses a dynamic simulator, GNSS simulator, and target echo simulator to perform imaging tests.

[0053] Example 3

[0054] Example 3 is a preferred example of Example 1, and is used to illustrate the present invention in more detail.

[0055] This invention provides a closed-loop testing method for the imaging performance of a spaceborne SAR satellite, comprising the following steps: Step 1, using a closed loop between a dynamic simulator and a GNSS simulator to record ephemeris, position, velocity, and attitude parameters; Step 2, using the imaging simulation parameters obtained from ground testing to perform echo simulation; Step 3, using a fixed-delay standard linear frequency modulated signal to measure the link delay; Step 4, using a target echo simulator to compensate for the delay; Step 5, using a closed loop between the dynamic simulator, GNSS simulator, and target echo simulator to perform imaging testing.

[0056] The specific implementation steps of this invention are as follows: Figure 4 As shown, it specifically includes:

[0057] Step 1: Use a closed loop between the dynamic simulator and the GNSS simulator to record ephemeris, position, velocity, and attitude parameters.

[0058] During platform testing, closed-loop configurations were established using a dynamics simulator and a GNSS simulator. After initial settings were configured in the dynamics simulator, time, position, and velocity parameters were sent to the GNSS simulator. After navigation information was added to the GNSS simulator, the navigation message signal was sent to the satellite GPS receiver. The GPS receiver generated position and velocity information in the WGS-84 system and sent it to the onboard attitude and orbit control computer. The attitude and orbit control computer generated periodic ephemeris data (including time, attitude, and orbit information) under the yaw guidance law based on the absolute positioning data. Absolute positioning data and ephemeris data were extracted via a bus monitor as echo simulation input parameters to ensure consistency between simulation and test parameters. The onboard attitude control subsystem generated ephemeris and attitude signals under the yaw guidance law, and parameters such as ephemeris, position, velocity, and attitude within one orbit were extracted via the bus monitor. A test connection diagram is attached. Figure 1 .

[0059] Step 2: Perform echo simulation using the imaging simulation parameters obtained from ground testing;

[0060] The ephemeris, position, velocity, and attitude parameters obtained from the above tests were interpolated. The interpolated data interval was better than 0.1 s. Echo simulation was performed using the interpolated data, with an imaging time of t0. The simulated echo data delay was t10 = t1. b +t1 s , where t1 b This refers to the delay corresponding to the gate opening time, i.e., the large bit delay. This delay can be set in the simulator interface. t1 s The fractional part of the delay, or micro-delay, is achieved by the echo.

[0061] Step 3: Measure the link delay using a fixed-delay standard linear frequency modulation signal;

[0062] When using a simulator for imaging tests, the echo signal delay received during actual testing is due to coupling to the receiving channel via the internal calibration network. In addition to the theoretical delay t10 from the simulation, the delay also includes delays from the internal calibration network, the test cable, and the simulator hardware. These additional delays require calibration compensation during testing. The formula for delay t1 in the actual test data is: t1 = t10 + t1 d +t1 m +t1 l , where t1 d The time delay introduced for the calibration network, t1 m The latency introduced for the simulator, t1 l The time delay introduced for testing the cable.

[0063] The imaging test setup is established, and the simulator sends a fixed-delay linear frequency modulated (LFM) signal. It has a delay deviation measurement function and uses a fixed-delay standard LFM signal to measure the link delay. Assume the fixed delay is t1. g SAR echoes use a start time of t1 c The received imaging echo signal is pulse-compressed, and the post-compressed time delay is t1. delay The time delay that needs to be compensated is dt = t1 c +t1 delay -t1 g .

[0064] It has a time delay calibration and compensation function, which uses the simulator to compensate for the time delay that needs to be compensated. The overall time delay of the simulator is set to t1. b If the time delay compensation is set to dt, then the time delay of the simulator hardware output signal after compensation is t1. b +t1 s -dt.

[0065] It can perform time delay calibration and compensation for multi-channel systems. The simulator interface allows setting different time delay compensation values ​​for different channels to compensate for the time delay of all receiving channels.

[0066] The onboard PRF signal is only output during imaging; it is not output during calibration, phase synchronization, or other signal transmissions. The simulator output has a pulse ignore function, which can delay the output of the simulated echo signal by n pulses, simulating the onboard echo delay reception function.

[0067] To establish the imaging test state, the simulator sends a linear frequency modulated signal with a fixed delay of 120µs. The fixed delay is t1. g =120us, SAR echo uses a start time of t1 c= 100 us, perform pulse compression processing on the received imaging echo signal. After pulse compression, the time delay is t1delay = 21.687 us, then the time delay to be compensated is dt = t1 c + t1 delay - t1 g = 1.687 us.

[0068] Step 4, use the target echo simulator to compensate for the time delay;

[0069] Set the overall time delay and the time delay compensation setting column in the simulator interface. The overall time delay of the simulator is set to t1 b = 4804.61299295653 us, and the time delay compensation is set to dt = 1.687 us. Since the simulator uses the PRF signal provided by the satellite as a reference to generate the echo time delay, the implementation of negative time delay is relatively complex. Therefore, it is necessary to ensure that t1 b > dt, such as t1 b < dt, then when generating the echo, t1 can be adjusted b , t1 s , to ensure that t1 b > dt. After setting the compensation value in the simulator interface, use a 120 us fixed time delay linear frequency modulation signal for testing. The difference between the actual time delay and the theoretical time delay after pulse compression is less than 1 ns, indicating that the time delay compensation is correct.

[0070] Step 5, perform imaging tests in a closed loop using a dynamics simulator, a GNSS simulator, and a target echo simulator;

[0071] The test connection block diagram is shown in Appendix Figure 2 , and the point target imaging results are shown in Appendix Figure 3 .

[0072] Those skilled in the art know that in addition to implementing the system and its various devices, modules, and units provided by the present invention in the form of pure computer-readable program code, the method steps can be logically programmed to enable the system and its various devices, modules, and units provided by the present invention to be implemented in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers, etc. to achieve the same function. Therefore, the system and its various devices, modules, and units provided by the present invention can be regarded as a hardware component, and the devices, modules, and units included therein for implementing various functions can also be regarded as the structure within the hardware component; the devices, modules, and units for implementing various functions can also be regarded as either software modules for implementing the method or the structure within the hardware component.

[0073] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

[0074] Those skilled in the art can understand this embodiment as a more specific description of Embodiment 1 and Embodiment 2.

[0075] Those skilled in the art will understand that, besides implementing the system and its various devices, modules, and units provided by this invention in the form of purely computer-readable program code, the same functions can be achieved entirely through logical programming of the method steps, making the system and its various devices, modules, and units of this invention function in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers. Therefore, the system and its various devices, modules, and units provided by this invention can be considered as a hardware component, and the devices, modules, and units included therein for implementing various functions can also be considered as structures within the hardware component; alternatively, the devices, modules, and units for implementing various functions can be considered as both software modules implementing the method and structures within the hardware component.

[0076] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A closed-loop testing method for the imaging performance of a spaceborne SAR satellite, characterized in that, The method includes the following steps: Step S1: Use a closed loop between the dynamic simulator and the GNSS simulator to record ephemeris, position, velocity, and attitude parameters; Step S2: Perform echo simulation using ephemeris, position, velocity, and attitude parameters; Step S3: Measure the link delay using a fixed-delay standard linear frequency modulation signal; Step S4: Compensate for time delay using the target echo simulator; This step has a time delay calibration and compensation function. The target echo simulator is used to compensate for the time delay that needs compensation. The overall time delay of the target echo simulator is set to... Delay compensation is set to The time delay of the compensated target echo simulator hardware output signal is then... ; Step S5: Conduct imaging tests using a closed-loop system employing a dynamics simulator, a GNSS simulator, and a target echo simulator; Step S2 interpolates the ephemeris, position, velocity, and attitude parameters obtained in step S1, and uses the interpolated data for echo simulation. The imaging time is t0, and the time delay of the simulated echo data is... ,in This refers to the time delay corresponding to the gate opening time, i.e., the large-position delay. This delay can be set in the target echo simulator interface. This is the fractional part of the time delay, i.e., the micro-delay, which is achieved by the echo. In step S3, an imaging test state is established. The target echo simulator sends a fixed-delay standard linear frequency modulated (LFM) signal, which has a time delay deviation measurement function. The link delay is measured using the fixed-delay standard LFM signal, where the fixed delay is... The SAR echo sampling start time is The received imaging echo signal is pulse-compressed, and the post-compressed time delay is... Then the delay needs to be compensated as follows: .

2. The closed-loop test method for spaceborne SAR satellite imaging performance according to claim 1, characterized in that, In step S1, the dynamic simulator and GNSS simulator are set up in a closed loop. After the dynamic simulator is set with initial values, the time, position, and velocity parameters are sent to the GNSS simulator. After the GNSS simulator adds navigation information, the navigation message signal is sent to the satellite GPS receiver. The GPS receiver generates position and velocity information under the WGS-84 system. The on-board attitude control subsystem generates ephemeris and attitude signals under the yaw guidance law, and extracts the ephemeris, position, velocity, and attitude parameters within one orbit through the bus monitor.

3. The closed-loop test method for spaceborne SAR satellite imaging performance according to claim 1, characterized in that, In step S3, when performing imaging tests using a target echo simulator, the echo is coupled to the receiving channel through the internal calibration network. Therefore, the actual received echo signal delay during the test is different from the theoretical delay simulated. In addition, internal calibration network latency, test cable latency, and target echo simulator hardware latency were introduced. These latency components require calibration compensation during testing. The actual test data uses the following formula for latency t1: ,in The latency introduced for the internal calibration network, The time delay introduced for the target echo simulator, The time delay introduced for testing the cable.

4. The closed-loop test method for spaceborne SAR satellite imaging performance according to claim 1, characterized in that, In step S4, the overall delay and delay compensation settings are configured in the target echo simulator interface, wherein the overall delay of the target echo simulator is set to... Delay compensation is set to , .

5. A closed-loop testing system for spaceborne SAR satellite imaging performance, characterized in that, The system includes the following modules: Module M1: Uses a closed loop between the dynamic simulator and the GNSS simulator to record ephemeris, position, velocity, and attitude parameters; Module M2: Performs echo simulation using ephemeris, position, velocity, and attitude parameters; Module M3: Measure link delay using a fixed-delay standard linear frequency modulation signal; Module M4: Compensates for time delay using the target echo simulator; it features time delay calibration and compensation functionality, using the target echo simulator to compensate for the required time delay. The overall time delay of the target echo simulator is set to... Delay compensation is set to The time delay of the compensated target echo simulator hardware output signal is then... ; Module M5: Imaging tests are conducted using a closed-loop system comprising a dynamics simulator, a GNSS simulator, and a target echo simulator; Module M2 interpolates the ephemeris, position, velocity, and attitude parameters obtained by module M1, and uses the interpolated data for echo simulation. The imaging time is t0, and the time delay of the simulated echo data is... ,in This refers to the time delay corresponding to the gate opening time, i.e., the large-position delay. This delay can be set in the target echo simulator interface. This is the fractional part of the time delay, i.e., the micro-delay, which is achieved by the echo. The module M3 establishes the imaging test state, and the target echo simulator sends a fixed-delay linear frequency modulated (LFM) signal, which has a time delay deviation measurement function. It uses the fixed-delay standard LFM signal to measure the link delay, where the fixed delay is... The SAR echo sampling start time is The received imaging echo signal is pulse-compressed, and the post-compressed time delay is... Then the delay needs to be compensated as follows: .

6. The closed-loop test system for spaceborne SAR satellite imaging performance according to claim 5, characterized in that, The module M1 sets up a closed loop for the dynamic simulator and the GNSS simulator. After the dynamic simulator is set with initial values, it sends the time, position, and velocity parameters to the GNSS simulator. After the GNSS simulator adds navigation information, it sends the navigation message signal to the satellite GPS receiver. The GPS receiver generates position and velocity information in the WGS-84 system. The onboard attitude control subsystem generates ephemeris and attitude signals under the yaw guidance law, and extracts the ephemeris, position, velocity, and attitude parameters within one orbit through the bus monitor.

7. The closed-loop test system for spaceborne SAR satellite imaging performance according to claim 5, characterized in that, When module M3 performs imaging tests using a target echo simulator, the echo is coupled to the receiving channel through the internal calibration network. Therefore, the actual delay of the received echo signal during the test is different from the theoretical delay simulated. In addition, internal calibration network latency, test cable latency, and target echo simulator hardware latency were introduced. These latency components require calibration compensation during testing. The actual test data uses the following formula for latency t1: ,in The latency introduced for the internal calibration network, The time delay introduced for the target echo simulator, The time delay introduced for testing the cable.

8. The closed-loop test system for spaceborne SAR satellite imaging performance according to claim 5, characterized in that, The module M4 has settings for overall latency and latency compensation in the target echo simulator interface, where the overall latency of the target echo simulator is set to... Delay compensation is set to , .

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