Satellite-borne SAR (Synthetic Aperture Radar) imaging device integrating multiple sensing modes

Abstract

The invention belongs to the technical field of imaging, and particularly relates to a satellite-borne SAR imaging device integrating multiple sensing modes, which is characterized by comprising central electronic equipment, a power supply and distribution subsystem, a measurement and control platform, a data transmission subsystem and an SAR antenna, the central electronic equipment comprises a power supply distribution module, a comprehensive digital processing module, a real-time imaging processing module and a comprehensive radio frequency module, and the comprehensive radio frequency module comprises a clock / transmitting module, a receiving module and an internal calibration module. The power supply distribution module, the comprehensive digital processing module, the real-time imaging processing module, the clock / transmitting module, the receiving module and the internal calibration module are electrically connected, and the central electronic equipment is externally connected with a power supply and distribution subsystem, a measurement and control platform, a data transmission subsystem and an SAR antenna. The light weight and high efficiency of the whole system are realized, and the harsh weight requirement is met.

Description

Technical Field

[0001] The present invention relates to the field of imaging technology, and in particular to a satellite-borne SAR imaging device integrating multiple sensing modes. Background Art

[0002] Radar satellite (SAR radar) is a general term for earth observation remote sensing satellites equipped with synthetic aperture radar (SAR). Defense and commercial aerospace have an increasing demand for small SAR remote sensing satellites and high-resolution SAR payloads.

[0003] At present, small SAR remote sensing satellites and high-resolution SAR payloads have not made breakthroughs in key technologies such as lightweight two-dimensional phased-scanned phased array antenna technology, on-board SAR real-time imaging, and detection. Summary of the invention

[0004] The purpose of the present invention is to provide a satellite-borne SAR imaging device that integrates multiple sensing modes, so as to solve the problem of the lack of breakthroughs in key technologies such as lightweight two-dimensional phase-scanned phased array antenna technology, on-board SAR real-time imaging, and detection proposed in the above-mentioned background technology, complete the overall design of lightweight and high-resolution SAR payload, develop SAR payload engineering prototypes, and carry out semi-physical simulation tests and field test tests, so as to provide strong support for building a lightweight and agile reconnaissance remote sensing system with high performance indicators and rapid response.

[0005] To achieve the above-mentioned object, the present invention provides the following technical solution: a spaceborne SAR imaging device integrating multiple sensing modes, characterized in that it comprises: a central electronic device, a power supply and distribution subsystem, a measurement and control platform, a data transmission subsystem and a SAR antenna;

[0006] Wherein, the central electronic device includes a power distribution module, an integrated digital processing module, a real-time imaging processing module and an integrated radio frequency module;

[0007] The integrated radio frequency module includes a clock / transmitter module, a receiver module and an internal calibration module;

[0008] The power distribution module, the integrated digital processing module, the real-time imaging processing module, the clock / transmitter module, the receiving module and the internal calibration module are electrically connected;

[0009] The central electronic equipment is externally connected to a power supply and distribution subsystem, a measurement and control platform, a data transmission subsystem and a SAR antenna.

[0010] Preferably, the SAR antenna array is designed to have a size of 4608mm*704mm, which can transmit SAR broadband signals and receive echo signals.

[0011] Preferably, the radio frequency module in the central electronic device integrates a clock / transmitter module, a radar receiving module, and an internal calibration, and is responsible for generating a radar reference frequency and a clock signal, and realizing amplification, filtering, and amplitude adjustment of the radar echo receiving signal.

[0012] Preferably, the integrated digital processing module in the central electronic device completes SAR payload data management, intermediate frequency transmission signal generation, receiving multi-channel data acquisition, timing control function, and beam control function. It is the time base of the SAR subsystem and is used to ensure the synchronization of the system's receiving and transmitting work, while performing ground remote control and telemetry tasks for the SAR subsystem.

[0013] Preferably, the real-time imaging processing module has the ability to complete SAR imaging, target detection, positioning and parameter extraction on-orbit.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] Advanced design and processing technologies are adopted for each unit of the system to improve system integration, integrate functions and reduce system weight. Through optimized design, the system is divided according to function, and similar functions are integrated and merged.

[0016] The integrated design of SAR real-time signal processing and signal acquisition simplifies the system's high-speed data interconnection and improves data transmission reliability. The integrated design of monitoring timer and beam controller simplifies the system composition by optimizing the combined system control functions, and uses large-scale highly integrated programmable devices to further improve system integration.

[0017] Achieve lightweight and high efficiency of the entire system and meet stringent weight requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a block diagram of the subsystem composition of the present invention;

[0019] Figure 2 It is a schematic diagram of SAR working mode;

[0020] Figure 3 Provide a connection diagram for the SAR subsystem system integration;

[0021] Figure 4 Form a block diagram for the target simulator;

[0022] Figure 5 It is the connection block diagram of the wired semi-physical simulation system;

[0023] Figure 6 It is the connection block diagram of the wireless semi-physical simulation system;

[0024] Figure 7 This is the overall scheme diagram of imaging processing and target detection;

[0025] Figure 8 This is a block diagram of the imaging processing architecture;

[0026] Fig. 9 This is the block diagram of the target detection and recognition module;

[0027] Fig.10 It is the block diagram of SAR antenna composition;

[0028] Fig.11 This is a schematic diagram of the antenna array feeding network;

[0029] Fig.12 is a schematic diagram of the unit layout;

[0030] Fig.13 Schematic diagram of antenna structure;

[0031] Fig.14 It is a schematic diagram of the antenna module structure layout;

[0032] Fig.15 It is a schematic diagram of the antenna page structure layout;

[0033] Fig.16 This is the flow chart of CS imaging processing in sliding beam mode;

[0034] Fig.17 It is the general flow chart of CFAR detection algorithm;

[0035] Fig.18 It is a schematic diagram of a hollow sliding window;

[0036] Fig.19 Positioning flow chart for RD. DETAILED DESCRIPTION

[0037] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] In the description of the present invention, it is necessary to understand that the terms "upper", "lower", "front", "back", "left", "right", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship are based on the orientation or position relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0039] Embodiment 1:

[0040] See also Figure 1-19 , the present invention provides a technical solution: a satellite-borne SAR imaging device integrating multiple sensing modes, including: a central electronic device, a power supply and distribution subsystem, a measurement and control platform, a data transmission subsystem and a SAR antenna;

[0041] Wherein, the central electronic device includes a power distribution module, an integrated digital processing module, a real-time imaging processing module and an integrated radio frequency module;

[0042] The integrated radio frequency module includes a clock / transmitter module, a receiver module and an internal calibration module;

[0043] The power distribution module, the integrated digital processing module, the real-time imaging processing module, the clock / transmitter module, the receiving module and the internal calibration module are electrically connected;

[0044] The central electronic equipment is externally connected to a power supply and distribution subsystem, a measurement and control platform, a data transmission subsystem and a SAR antenna. The specific connection method is shown in the figure.

[0045] The SAR antenna array is designed to be 4608mm*704mm in size, and can transmit SAR broadband signals and receive echo signals. The radio frequency module in the central electronic device integrates a clock / transmitter module, a radar receiving module, and an internal calibration, and is responsible for generating radar reference frequency and clock signals, and realizing amplification, filtering, and amplitude adjustment of radar echo receiving signals. The integrated digital processing module in the central electronic device completes SAR payload data management, intermediate frequency transmission signal generation, receiving multi-channel data acquisition, timing control functions, and beam control functions. It is the time reference of the SAR subsystem, and is used to ensure the synchronization of the system's receiving and transmitting work, while performing ground-based remote control and telemetry tasks for the SAR subsystem. The real-time imaging processing module has the ability to complete SAR imaging, target detection, positioning, and parameter extraction on orbit.

[0046] In order to achieve the lightweight and high efficiency of the entire system and meet the stringent weight requirements, it is necessary to adopt advanced design and processing technologies for each unit of the system to improve system integration, integrate functions, and reduce system weight. Through optimized design, the system is divided by function, and similar functions are merged; SAR real-time signal processing and signal acquisition are integrated to simplify the system's high-speed data interconnection and improve data transmission reliability; the monitoring timer and beam controller are integrated to simplify the system composition by optimizing the combined system control function, and large-scale high-integration programmable devices are used to further improve system integration.

[0047] Load engineering prototype development and ground verification

[0048] 1) Load engineering prototype development and system integration:

[0049] Carry out the development of payload engineering prototypes and ground semi-physical simulation verification, including lightweight two-dimensional scanning phased array antenna technology research and central electronic equipment research and design. For SAR antennas, comprehensively consider the imaging performance index requirements and antenna size, weight restrictions and other factors, break through the limitations of existing antenna scanning capabilities and power-to-area ratio, and complete the development of lightweight large-angle scanning antennas; for single units in the cabin, complete the development of digital, radio frequency and control integrated single units on the basis of high integration and lightweight; through semi-physical simulation and field test tests, test the main functions and indicators of key prototypes, and verify technologies such as high-resolution imaging processing and channel error compensation.

[0050] Complete the single-machine development, system integration, and joint debugging and testing of the small and light high-resolution SAR system engineering prototype, test the main functions and indicators of the SAR single machine and antenna, verify the correctness of the subsystem's internal control link, RF link, and data link, and verify that the engineering prototype has the ability to correctly perform tasks. Specifically include:

[0051] (a) Test the correctness of the internal / external functional indicators and performance indicators of the power supply module, integrated digital processing module and integrated radio frequency module of the central electronic equipment; such as single-machine interface, working parameter setting, system working sequence, software resource scheduling, etc.;

[0052] (b) Test the electrical performance of each component in the SAR antenna, verify the system functions of the small array, including control, power supply, link, software, scanning and other capabilities, and test the antenna transmission / reception state pattern;

[0053] (c) Verify the correctness and compatibility of the interface between the SAR engineering prototype subsystem and the entire satellite;

[0054] (d) Check whether the main functions and performance indicators of the subsystem meet the requirements of the guidelines;

[0055] (e) Verify the high reliability of engineering prototypes through environmental tests;

[0056] Connection diagram Figure 3 shown.

[0057] The ground test equipment required for system integration testing is mainly as follows:

[0058] (a) Whole-satellite ground measurement: It has the function of command compilation, can complete the working mode and parameter selection; it has the bus, 422, OC, LVDS, analog quantity, temperature quantity and other interfaces with the central electronic equipment;

[0059] (b) Data storage ground measurement: It has the function of storing two-way SAR echo data or calibration data simultaneously;

[0060] (c) Data processing and measurement: It has the imaging function of 2-channel SAR echo data and the analysis function of calibration data.

[0061] (d) Power supply: Provides DC regulated power to the central electronic equipment and SAR antenna.

[0062] The development of key prototypes follows the high reliability and equal life production assurance principles of satellite development. Environmental stress screening and environmental simulation tests are completed according to the environmental requirements of the satellite.

[0063] 2) Ground semi-physical simulation verification

[0064] In the engineering prototype stage, the semi-physical simulation method is used to achieve closed-loop simulation of the entire system from radio frequency to digital processing, and verify the correctness of the decomposition and design of subsystem indicators to SAR single-machine indicators. The semi-physical simulation system is designed based on the concept of universalization and modularization. By replacing or adjusting the simulator software settings, a simulation test scenario can be formed in a short time, and the rationality of the prototype indicators, the feasibility of the design scheme and some key performance indicators can be quickly verified.

[0065] ① Basic functions of the simulator

[0066] The basic functions that can be achieved by semi-physical simulation and verification include:

[0067] a. The semi-physical simulation system establishes common verification units such as radar microwave, intermediate frequency, and digital, and forms a complete simulated radar with engineering prototypes such as SAR antennas and central electronic equipment to meet the needs of rapid simulation verification of new systems and new algorithms of phased array radars;

[0068] b. The semi-physical simulation system can also be interconnected with the radar prototype to verify the performance of the radar subsystem and its compatibility with the entire system, and to verify the correctness of the working mode and index design of the low-orbit sub-meter ultra-wideband imaging system;

[0069] c. Conduct preliminary research and experimental verification of new radar systems, technologies and concepts, and verify the correctness of technologies such as sub-meter ultra-wideband spaceborne SAR full-link modeling research, multi-mode spaceborne SAR echo signal mathematical modeling, echo signal simulation, echo signal space-time two-dimensional distribution characteristics analysis, and azimuth non-uniform sampling compensation processing methods;

[0070] d. According to the characteristics of phased array radar, semi-physical verification of aperture resource management, beam control and other technologies is realized;

[0071] e. By using the radar multi-channel RF environment and simulation system to interconnect with the semi-physical radar, the comprehensive anti-interference performance of the phased array radar can be verified;

[0072] f. The following control parameters of the simulator are adjustable: platform parameters (time parameters, three-dimensional position of transmitting and receiving satellites, three-dimensional speed, three-dimensional attitude, etc.), radar parameters (working mode, wave position parameters, signal parameters, antenna parameters, quantization parameters, thermal noise parameters, etc.), scene parameters (scene type, scene size, etc.) and echo parameters (number of echo sampling points, number of echo bars, etc.). According to the above parameters, the scene echoes in each mode are output, with both semi-real-time and playback functions.

[0073] ②Simulator design scheme

[0074] The simulator uses the GPU computing unit of a high-performance server to calculate the scene echo data according to the given scene parameters, radar parameters and track parameters and downloads it to the baseband storage module. The simulator is set to the external coherent working mode and receives the external reference clock. When the synchronization signal comes, it reads the baseband echo data in the mass storage board and modulates the signal to the specified frequency band through DUC, interpolation, filtering, DA, analog up-conversion and other steps.

[0075] The simulator includes a display control unit, a baseband signal processing unit, and a microwave link unit. The simulator adopts a fully switched RF electronic system architecture, consisting of an 8U CPCI chassis and a 4U chassis with a front touch screen. The block diagram is shown in the figure. Figure 4 shown.

[0076] ③Hand-in-the-loop simulation workflow

[0077] Through wired semi-physical simulation, the target simulator and the central electronic equipment engineering prototype constitute a simulated radar link, and quickly simulate and verify the multi-mode spaceborne SAR echo signal mathematical modeling, echo signal simulation, echo signal space-time two-dimensional distribution characteristics analysis, azimuth non-uniform sampling compensation processing method and other technologies. Its workflow can be briefly divided into the following steps:

[0078] (a) Echo data generation: The GPU computing unit of a high-performance server is used to calculate the intermediate frequency echo data of the scene according to the given scene parameters, SAR working mode, SAR imaging parameters and orbit parameters, and the generated data is saved to the target simulator. The storage path is located in the storage module in the baseband signal processing unit; point target and surface target data with different imaging modes and parameters are generated.

[0079] (b) Wired simulation test: The two-channel RF simulation echo signals output by the echo simulator are sent to the receiving module of the central electronic device in turn, and the output PRT synchronization signal and 100MHz clock signal of the central electronic device are connected to the target simulator to ensure that the system works synchronously in a coherent state.

[0080] The simulator is set to the external coherent working mode. When the synchronization signal is received, the baseband echo data in the storage module is read, and the signal is modulated to 9.6GHz through DUC, interpolation, filtering, DA, analog up-conversion and other steps. The two-way RF signal is output to the SAR receiver after a specified time delay, and then the SAR data is stored through down-conversion, channel correction, AD and other steps. The system connection block diagram is shown in the figure below.

[0081] (c) Data verification: Analyze and generate data, complete imaging processing to obtain processing results, compare with ideal results or evaluate indicators. Figure 5 shown.

[0082] Wireless semi-physical simulation test: For multi-channel radar systems, it is difficult to complete a complete 2-channel spaceborne state semi-physical simulation using an air-feed link. It is planned to use the transceiver channel of the SAR antenna module of the engineering prototype with 256 channels, receive one RF echo signal from the 9 signals output by the simulator through the horn antenna, and air-feed it to the SAR antenna module. The signal returns to the down-conversion channel of the central electronics through the real antenna link; the remaining 8 echo data are connected to the central electronic equipment in turn, and the system channel data are made coherent through the simulator parameter settings such as time delay and MGC.

[0083] Use wireless semi-physical simulation test to verify the correctness of system beam control, multi-channel amplitude and phase correction technology, etc. Figure 6 shown.

[0084] 1. SAR real-time processing engineering implementation

[0085] 1) Overall plan

[0086] This project aims at miniaturization, low cost and high performance, focusing on the imaging and detection and recognition functions of SAR payloads on board. Design an integrated hardware and software architecture suitable for on-board, form a universal, scalable and flexibly configurable on-board computing platform, and achieve more accurate and comprehensive understanding of target types and target motions.

[0087] This project consists of one module for imaging and one module for detection. The modules are interconnected by high-speed interfaces. The SAR imaging module is mainly composed of DSP+FPGA, and the detection module is composed of FPGA+AI module. Figure 7 shown.

[0088] 2) Imaging Hardware Solution

[0089] The imaging processing module mainly completes SAR imaging. Currently, GPU, FPGA and DSP are widely used in radar digital signal processing. These three platforms have powerful signal processing capabilities and their own scope of application. Generally speaking, GPU has the strongest processing power. GPU can have thousands or even tens of thousands of stream processing cores, support floating-point operations, and excel in parallel computing. However, it has high power consumption and weak control capabilities. It has no advantage in executing algorithms with complex calculation steps and is widely used in machine learning. As a programmable logic device, FPGA implements user logic by changing internal logic blocks and wiring. In addition, current high-end FPGAs are embedded with a large number of dedicated DSP units and block storage units, and have strong processing capabilities. Although the system operating clock is relatively low, as long as there are sufficient resources, the FPGA processing capability can be enhanced by exchanging area for speed through pipelining and parallelization, thereby reducing processing time. However, its cost is relatively high and its development difficulty is relatively large. For example, when transplanting an algorithm, it is necessary to first perform fixed-point conversion (floating-point scatter consumes more resources), and then pay attention to scaling in the intermediate steps to take into account both calculation accuracy and dynamic range. The debugging cycle is long. If the project is large, it may take several days to recompile and generate files for download. The iteration cycle is long and debugging is inconvenient.

[0090] Digital signal processor (DSP) enhances the digital computing unit in the core on the basis of general-purpose processor, increases the number of multipliers, and adds hardware acceleration units for specific algorithms. Compared with general-purpose processors, the digital signal processing capability is greatly enhanced. Currently, DSP mainly uses C language for DSP development, which is relatively convenient. For key processing steps, assembly language can be used for optimization. This solution adopts the architecture of FPGA+DSP. Figure 8 shown.

[0091] This solution adopts a DSP-based, FPGA-based architecture. The main functions are described as follows:

[0092] Each DSP has two external DDR4s, totaling 16GB of storage. It is connected to the FPGA using the SRIO X4 interface and mainly performs tasks such as SAR imaging processing and auxiliary data calculation.

[0093] FPGA has a set of DDR externally, which mainly completes functions such as payload data reception, data preprocessing, parameter calculation acceleration, data post-processing and data forwarding, telemetry collection, remote control command analysis and execution, parameter annotation, and watchdog.

[0094] The imaged data is output to the image detection module through the GTX interface. Each module has independent output.

[0095] 3) Target detection and recognition hardware solution

[0096] The target detection and recognition module is mainly responsible for intelligent processing of input image data, including target detection and target recognition. A large amount of AI reasoning is required in the algorithm, so the FPGA+AI chip architecture is selected.

[0097] AI chips are divided into two categories according to their uses: cloud-based training acceleration chips and edge-side inference acceleration chips. Cloud-based training acceleration chips mainly focus on accelerating the training of deep learning networks. They have strong computing power, but huge power consumption. Generally, a single chip consumes more than 100W. Therefore, they are not suitable for deployment on satellite-borne embedded devices. The computing power of edge-side inference acceleration chips is about 4T to 22T, and the power consumption is about 5 to 10w. They are mainly used to run trained deep learning algorithms and perform result inference. They have a good effect on the first stage of network operation. At present, the deep learning algorithms running on satellite-borne embedded devices are all trained networks, which are suitable for acceleration with edge-side inference acceleration chips.

[0098] Spaceborne embedded devices have the characteristics of limited resources, complex operating environment, inability to expand hardware on orbit, and difficult software maintenance. In view of these characteristics, the project team mainly based the following criteria on the selection of AI chips:

[0099] (1) Good versatility. Supports various deep learning technologies and multi-modal intelligent processing

[0100] (2) High energy efficiency. The instruction set processor architecture used by the chip has a better energy efficiency ratio.

[0101] (3) Strong computing power. The peak computing power of a single chip reaches at least 12T@Int8, ensuring the inference speed.

[0102] (4) Providing high-performance, low-power deep learning solutions

[0103] According to the above criteria, the project team selected NVIDIA's AGX xavier module for design, and the AGXxavier module is used as an AI acceleration chip. It is a SOC chip specifically used for deep learning. It adopts the ARM+GPU architecture, and the chip integrates the main processor unit CPU and the neural network unit NPU. It supports integer operations in INT8 and INT16 formats, and the theoretical maximum computing power of INT8 can reach 32Tops. The chip supports 2-channel 64bit ECC LPDDR4 memory and supports multiple capacities to meet the computing and storage requirements of various reasoning scenarios. It also has a wealth of peripheral interfaces, such as PCIE, UART, SPI, SDIO, eMMC, GPIO and other interfaces. It is suitable for data analysis and reasoning applications of data nodes, and its application areas are mainly various occasions of image and video intelligent analysis.

[0104] The NPU module has the following four advantages:

[0105] (1) Brand-new architecture. The new architecture is built on an on-chip network, which improves the parallel processing efficiency of the chip and improves the effective cache capacity and bandwidth based on hardware-based on-chip data compression. The new architecture provides comprehensive AI precision support for INT16, INT8, INT4, FP32, and FP16, meeting the computing power requirements of diverse neural networks and having better adaptability for the development of high-precision intelligent models and subsequent model updates;

[0106] (2) Enhanced inference performance. When the new architecture uses INT8 precision for AI inference calculations, the non-sparse network performance is up to 10 times higher than the first generation. Its built-in hardware video and image codecs can effectively reduce the CPU pre-processing load and PCIe bandwidth usage when the system processes such applications, further improving the performance of applications.

[0107] (4) Support for multiple neural network inference acceleration operators: Support for deep learning networks: AlexNet, GoogleNet, VGG, ResNet, YOLO, SSD, etc., support for frameworks: Caffe, TensorFlow, Pytorch, etc.

[0108] The target detection and recognition module implementation block diagram is as follows Fig. 9 shown.

[0109] Among them, FPGA uses Xilinx's ultral+ series, which is responsible for image preprocessing and data routing. The AI ​​chip uses NVIDIA's AGX xavier module, which exchanges image data with FPGA through the PCIE interface. The total computing power can reach 30TOPS.

[0110] 2. Main technical performance indicators

[0111] The main technical indicators required to be achieved are as follows:

[0112] The main functions and performance technical indicators of the lightweight high-resolution SAR payload system are as follows:

[0113] 1) SAR central operating frequency: 9.6 GHz;

[0114] 2) Polarization mode: single polarization;

[0115] 3) SAR maximum operating bandwidth: 600MHz;

[0116] 4) Normal viewing angle of on-orbit SAR antenna: 0°;

[0117] 5) SAR visual range: not less than ±8° in azimuth and not less than ±20° in elevation;

[0118] 6) 2-channel transmission and reception in azimuth and single-channel transmission and reception in elevation;

[0119] 7) Working mode: strip, TOPS, bunching;

[0120] 8) Transmitting duty cycle: 8%~12% adjustable (non-continuous)

[0121] 9) Imaging resolution and width:

[0122] Strip: 2m / 20km;

[0123] Beaming mode: 0.5m / 5km, 1m / 510km;

[0124] TopSAR: 10m / 100km, 15m / 150km;

[0125] 10) Target positioning accuracy:

[0126] Better than 10m (1σ) @ elevation error ≤ 3m;

[0127] The final mark is better than 20m (1σ) @ elevation error ≤ 30m;

[0128] 11) Gaze focusing duration: 53s@40° visual angle;

[0129] 12) Image quality indicators:

[0130] ①Peak sidelobe ratio: ≤-20dB;

[0131] ②Integrated sidelobe ratio: ≤-13dB;

[0132] ③ Azimuth ambiguity: ≤-20dB (worst value, based on actual ground direction map);

[0133] ④ Range ambiguity: ≤-20dB (worst value, based on actual ground direction map);

[0134] ⑤Equivalent noise coefficient: ≤-20dB (worst value);

[0135] 13) Antenna array size: no larger than 4.6m (A) × 0.8m (R);

[0136] 14) Weight: <100kg;

[0137] 3. Key core technologies

[0138] (1) Lightweight 2D scanning phased array antenna technology

[0139] Since the satellite platform has strict volume and weight restrictions during launch, the antenna must be light and can be folded during launch and then deployed after launch. At the same time, the space environment is very different from the ground in terms of temperature, radiation, gravity characteristics and technical support. In addition to the study of electrical performance characteristics, the design of space-based SAR antennas also requires research on the selection of lightweight materials, antenna deployment mechanism, and antenna adaptation to the space environment.

[0140] Develop new design technologies, combine new materials and new processes, and use integrated, lightweight low-profile antenna technology and miniaturized T / R component technology to achieve lightweight design of broadband, large-scan phased array antennas.

[0141] 1) Function

[0142] The SAR antenna in the lightweight high-resolution SAR payload has the following functions:

[0143] ①Use solid-state active phased array technology to achieve two-dimensional phase scanning;

[0144] ② A common receive / transmit beam can be formed through the feed network;

[0145] ③The SAR array has an internal correction network that can collaborate to achieve on-orbit calibration;

[0146] ④The SAR array exciter receives control instructions, calculates and controls the amplitude, phase, and delay of the antenna, and finally realizes beam forming, scanning, and control;

[0147] There are three working modes:

[0148] A. Transmitting Mode

[0149] In the transmitting mode, the transmitting array operates in saturation transmission to achieve maximum power radiation, and can also complete power radiation in different beam directions according to system requirements.

[0150] B. Receiving Mode

[0151] In receiving mode, the receiving array works with equal amplitude and phase in the radiation direction to achieve maximum power reception. At the same time, it can complete echo reception in different beam directions according to system requirements.

[0152] C. Internal calibration mode

[0153] a) Measure the receiving and transmitting channels one by one to achieve amplitude and phase correction of all receiving and transmitting channels;

[0154] b) Calibrate the entire transmitting array and the entire receiving array to achieve gain calibration of the entire transmitting and receiving link.

[0155] 2) Composition

[0156] The light and small SAR antenna mainly includes: antenna array, four-channel T / R components, small position delay components, large position delay components, bidirectional amplifiers, secondary power supply and exciter modules, active mounting plates (including heat pipes), array frames, array thermal control, and cable components for brackets of various components. Fig.10 shown.

[0157] 3) Overall design of the array

[0158] A. Effective size of the array

[0159] According to the system requirements for SAR antennas, the SAR antenna center frequency is required to be 9.6GHz and the bandwidth is 600MHz; the center frequency beam width requirements are: azimuth: 0.4°, range: 2.1°.

[0160] Taking into account the inconsistency of amplitude and phase of each channel of the array, the effective size of the array is designed according to the beam width requirements of 0.4° in azimuth and 2.1° in range. It can be known from the requirements allocated to the SAR antenna array indicators above:

[0161] Direction: (Calculated based on 0.4°)

[0162] (Calculated based on 0.34°)

[0163] Distance direction: (Calculated based on 2.1°)

[0164] (Calculated based on 2.3°)

[0165] Where D az Represents the aperture size of the antenna in azimuth, D el Represents the aperture size of the antenna in the distance direction.

[0166] From the above calculation process, the range of the aperture size of the antenna in azimuth is 3984.38mm~4687.5mm, and the range of the aperture size of the antenna in distance is 692.93mm~758.93mm.

[0167] In this scheme, the effective length of the antenna in azimuth is 4607mm and the effective length in range is 704mm (corresponding to the antenna beam width in azimuth is 0.34° and the beam width in range is 2.25°), and its design indicators meet the design requirements.

[0168] B. Scanning Capability

[0169] a) Scanning range

[0170] According to the system's special requirements for the scanning capability of the SAR antenna: ±0.34° in azimuth and ±2.25° in range, it is planned to use T / R components to independently feed each unit in azimuth and range to achieve two-dimensional scanning capability. Fig.11 shown.

[0171] The radiation units are arranged side by side to ensure uniform arrangement of the units; the ±8° scanning is satisfied in the azimuth direction, and the unit spacing is estimated by the following formula:

[0172]

[0173] In the azimuth direction, the scanning angle is ±20°, and the unit spacing is estimated by the following formula:

[0174]

[0175] Taking into account the correction of beam width and design margin, the unit spacing in azimuth is 24mm and the spacing in range is 22mm.

[0176] b) Scanning step

[0177] The use of phased array antennas brings the advantages of large spatial synthetic power, small feeding loss, and flexible and controllable beams. However, since the beam pointing of phased array antennas is discontinuous, the jump problem between adjacent beam pointing must be considered.

[0178] A common method to reduce beam jumps is to use virtual bit technology. This method uses virtual bit calculation to reduce beam jumps. The number of virtual bit calculations is related to the number of units in the array. The T / R component of this solution uses a 6-phase shifter (i.e. k1=6), and the maximum number of virtual bit digits of the virtual bit technology is: k2<log2(N-1);

[0179] The minimum beam jump that can be achieved by the phased array antenna after using the virtual position technology is

[0180]

[0181] 1) For the distance direction, N=32, d=22mm, and the scanning angle θ is 20°. It is calculated that the scanning step in the distance direction is not less than 0.0015°, which meets the system index requirements.

[0182] 2) For the azimuth direction, N=192, d=24 mm, and the scanning angle θ is 8°. It is calculated that the azimuth scanning step is not less than 0.00018°, which meets the system index requirements.

[0183] 4) Array division

[0184] The effective design size of the entire antenna array is approximately 4608mm×704mm. Each microstrip patch serves as an antenna unit with a design size of 24mm (azimuth)×22mm (range). The unit arrangement is 32 (azimuth)×192 (range), for a total of 6144 units.

[0185] The antenna array adopts a modular design. The antenna is divided into 4 pages along the azimuth direction, and the whole array is divided into 4×1 pages. Each page is divided into 3×2 microstrip sub-array modules, that is, the whole array has a total of 12×2 microstrip sub-array modules, each module is 384mm×352mm in size. The size of the whole array is 4608mm×704mm.

[0186] The whole array, page, and module configuration are as follows Fig.12 shown.

[0187] SAR antenna dimensions: 4608mm*704mm;

[0188] Antenna mass: ≤90kg.

[0189] For details on the array shape and installation structure, see Fig.13 .

[0190] The SAR antenna array consists of 4 antenna panels, each of which has 2*3 modules, each of which has 4*4 antenna subarrays, and each subarray has 4*4 radiating units (TR channels). The entire array provides 6144 radiating units, corresponding to 6144 TR channels, and each subarray corresponds to a delay channel, for a total of 384 delay channels. Fig.14 , 15 shown.

[0191] (2) SAR real-time imaging, detection, and positioning technology

[0192] 1) On-board real-time SAR imaging algorithm

[0193] On-board real-time imaging processing is mainly for 2m strip mode and 0.5m sliding beam. In azimuth, SAR mainly obtains azimuth resolution and imaging width by controlling the antenna beam pointing. For the sliding beam mode, its antenna beam points to a point far away from the center of the imaging scene. For the strip mode, its antenna beam points to an infinite point. Therefore, the strip mode can be regarded as a special case of the sliding beam mode.

[0194] Next, we will take the sliding beam mode as an example to introduce the SAR imaging algorithm in detail. In this project, the CS imaging algorithm is used, and its processing flow is as follows: Fig.16 shown.

[0195] For the sliding beamforming mode, ignoring the amplitude and phase error terms, the echo signal can be expressed as:

[0196]

[0197] Among them, t r and t a Respectively represent the time in the range and azimuth directions, w r and w a They represent the range and azimuth window functions respectively, which are determined by the bidirectional antenna pattern. t0 represents the target’s illumination center time, T a represents the synthesis time, T p represents the pulse width, λ represents the wavelength, and R represents the slant range of the radar to the target.

[0198] Since the azimuth spectrum is ambiguous in this mode, the echo needs to be preprocessed. The reference function is:

[0199]

[0200] Among them, v represents the satellite speed. After the preprocessing convolution operation, the signal S2 can be further obtained, which is expressed as

[0201]

[0202] To obtain We need to compensate for the second phase term on the right side of the equation, and the compensation function is

[0203]

[0204] After the above series of azimuth preprocessing, we get an unambiguous signal in the azimuth frequency domain. Next, we can use the efficient and accurate CS algorithm to correct the data for range pulse pressure and migration. The scaling equation is:

[0205]

[0206] In order to achieve range processing, the range compensation factor is

[0207]

[0208] After the range processing is completed, the azimuth signal is processed, and the azimuth phase is corrected and scaled to

[0209]

[0210] After the above operations, we can get the azimuth time domain unambiguous signal S3. We can use the de-ramping operation to compensate the quadratic phase in the S3 signal. The compensation function is:

[0211]

[0212] After the de-ramping operation is compensated, the final imaging result can be obtained.

[0213] 2) Onboard ship detection algorithm

[0214] Radar image ship detection has been widely used in the fields of fishing boat monitoring, marine resource protection, and sea condition monitoring. The most commonly used algorithm in SAR image target detection is the constant false alarm (CFAR) algorithm. The general algorithm flow chart is shown in the figure below. The general steps are: for each pixel in the image, the local background window position of the pixel is determined with the pixel as the center pixel. After the clutter background distribution is determined in advance, its distribution parameters are calculated and estimated through some or all of the clutter pixels in the background window. It is considered that the false alarm probability set for detection is solved through its probability density function to solve the CFAR threshold corresponding to the pixel. The pixel value of the pixel is then compared with the solved CFAR threshold to determine whether the pixel is a target pixel. In order to solve the target pixel position of the entire SAR image, the local background window statistics of each pixel can be regarded as a sliding window. Therefore, during the sliding process of the local window, each pixel participates in the parameter estimation calculation of the distribution function many times. As Fig.17 shown.

[0215] When calculating the distribution parameters within the local window of each pixel, its value is mainly determined by the size of the local window and the shape of the construction. In general, the shape is constructed in two ways: solid and hollow. The design of a solid window will cause the target pixel values ​​around the pixel to be detected to leak into the clutter data window, so that the statistical distribution parameters are not accurate enough. Therefore, the CFAR algorithm generally uses a hollow window as its sliding window. The design idea of ​​the hollow sliding window is: with the pixel to be detected as the center, according to the size of the target to be detected and the size of the image resolution, the size of the protection window and the clutter background window are artificially set, as shown in the figure below. When estimating the distribution parameters of each pixel, the pixels within the protection window are not included in the calculation. This statistical method can alleviate the influence of the target pixel on the clutter pixel to a certain extent. Fig.18 shown.

[0216] After completing the distribution parameter estimation calculation in each pixel sliding window, it is necessary to calculate its detection threshold. The detection threshold of each pixel in the SAR image is obtained by solving the following formula:

[0217]

[0218] Among them, P fa represents the false alarm rate, and p(x) represents the probability density function.

[0219] The false alarm rate is mainly set by human experience. The p(x) function is mainly determined by the image scene. When the image scene distribution is relatively uniform, Rayleigh distribution, Gaussian distribution and lognormal distribution functions are generally used as the probability density function of detection; when the image scene distribution is not uniform, the distribution functions with more complex probability density functions such as Weibull, K and G are generally used.

[0220] 3) SAR positioning algorithm

[0221] The target can be detected by the CFAR algorithm, and the range Doppler data s r (f a ,τ;R0) into the three equations of the RD model, as shown below. Using the ellipsoid model or DEM data, the position of the target in the inertial geocentric coordinate system is calculated through iteration as (X t ,Y t ,Z t ), thereby achieving the positioning of any pixel.

[0222]

[0223] According to f a Convert the time of the azimuth in 32 pulses (N=32, n=0, 1, 2, ..., N-1), t=t0+n·1 / PRF According to the time t, the position of the satellite can be obtained according to the orbit ephemeris (X s ,Y s ,Z s ) and speed (v x ,v y ,v z ), the slant distance R0 is known, and the distance direction τ is converted into pixel coordinates x according to the sampling rate, M slant The slant distance sampling interval can be used to calculate the slant distance of any pixel point. e and R p are the equatorial radius and polar radius of the earth respectively, and h is the elevation provided by DEM. Currently available global DEM data include NASA's ASTER GDEM 30m data or SRTM 30m data (this data only covers -60 degrees south latitude to 60 degrees north latitude). The above RD mathematical model is used to iterate the forward model to realize the positioning of any pixel in the inertial geocentric coordinate system (X t ,Y t ,Z t ). This coordinate can be converted into a position in any coordinate system for subsequent target guidance, identification and tracking. In summary, the positioning process is as follows Fig.19 shown.

[0224] The above shows and describes the basic principles and main features of the present invention and the advantages of the present invention. For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic features of the present invention; therefore, no matter from which point of view, the embodiments should be regarded as exemplary and non-restrictive. The scope of the present invention is limited by the attached claims rather than the above description. Therefore, it is intended to include all changes within the meaning and scope of the equivalent elements of the claims in the present invention, and any figure marks in the claims should not be regarded as limiting the claims involved.

[0225] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A spaceborne SAR imaging device that integrates multiple sensing modes, characterized in that: include: Central electronic equipment, power supply and distribution subsystem, measurement and control platform, data transmission subsystem and SAR antenna; Wherein, the central electronic device includes a power distribution module, an integrated digital processing module, a real-time imaging processing module and an integrated radio frequency module; The integrated radio frequency module includes a clock / transmitter module, a receiver module and an internal calibration module; The power distribution module, the integrated digital processing module, the real-time imaging processing module, the clock / transmitter module, the receiving module and the internal calibration module are electrically connected; The central electronic equipment is externally connected to a power supply and distribution subsystem, a measurement and control platform, a data transmission subsystem and a SAR antenna.

2. The spaceborne SAR imaging device integrating multiple sensing modes according to claim 1, characterized in that: The SAR antenna array is designed to have a size of 4608mm*704mm, which can transmit SAR broadband signals and receive echo signals.

3. The spaceborne SAR imaging device integrating multiple sensing modes according to claim 1, characterized in that: The radio frequency module in the central electronic device integrates a clock / transmitter module, a radar receiving module, and an internal calibration, and is responsible for generating a radar reference frequency and a clock signal, and realizing amplification, filtering, and amplitude adjustment of radar echo receiving signals.

4. The spaceborne SAR imaging device integrating multiple sensing modes according to claim 1, characterized in that: The integrated digital processing module in the central electronic device completes SAR payload data management, intermediate frequency transmission signal generation, receiving multi-channel data acquisition, timing control function, and beam control function. It is the time base of the SAR subsystem and is used to ensure the synchronization of the system's receiving and transmitting work, while performing ground remote control and telemetry tasks for the SAR subsystem.

5. The spaceborne SAR imaging device integrating multiple sensing modes according to claim 1, characterized in that: The real-time imaging processing module has the ability to complete SAR imaging, target detection, positioning and parameter extraction on-orbit.

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