An airborne sea observation millimeter wave radar imaging method and device based on space-time synchronous control

Abstract

The application discloses a kind of based on space-time synchronous control's airborne sea observation millimeter wave radar imaging method and device, to solve the problem of traditional software layer data alignment introducing timing deviation leading to image defocus.This application takes positioning navigation solution unit as core, constructs global synchronous time scale by GNSS second pulse, digital module sends physical trigger signal to solution unit at the starting moment of each frequency modulation cycle, FPGA in solution unit locks current time scale as time stamp in single clock cycle with pure hardware logic, and accurately matches platform motion data with the time stamp as index, generates motion compensation parameter;Imaging processing unit adopts FPGA and GPU heterogeneous architecture, completes motion compensation, synthetic aperture focusing and target detection.This application realizes the space-time synchronization of radar echo and navigation data, effectively eliminates phase error, significantly improves the imaging resolution and target detection accuracy in high dynamic environment at sea.

Description

Technical Field

[0001] This invention relates to the field of radar signal processing technology, and in particular to an airborne millimeter-wave radar imaging method and device for maritime observation based on spatiotemporal synchronization control. Background Technology

[0002] With the rapid development of airborne maritime observation technology, airborne platforms such as UAVs equipped with millimeter-wave radar (such as SAR / ISAR) have become an important means of monitoring sea surface targets and exploring the marine environment. They can achieve high-resolution imaging and accurate detection of targets such as ships and sea clutter on the sea surface, and have important application value in the fields of maritime supervision and national defense security.

[0003] In the high-altitude environment at sea, airborne platforms are susceptible to disturbances from high-altitude airflows and complex wind fields, resulting in complex movements such as roll, pitch, and yaw. These movements will cause serious phase errors in radar echoes, directly affecting imaging quality, leading to image defocus and reduced resolution, making it difficult to meet the requirements of high-precision target detection.

[0004] Currently, while existing airborne radar imaging methods cover the entire process from signal transmission, echo processing, motion compensation, to imaging detection, they suffer from significant shortcomings in the spatiotemporal coupling processing of motion and echo data. Specifically, in traditional imaging methods, the alignment of radar and navigation data is performed discretely at the software layer. Affected by factors such as clock drift in heterogeneous systems and software interrupt delays, motion compensation parameters exhibit microsecond-level timing deviations. In the millimeter-wave high-frequency band, these microsecond-level timing deviations translate into significant phase shifts, further exacerbating image defocusing and hindering resolution improvement. Furthermore, existing methods do not deeply integrate spatiotemporal synchronization control into the underlying hardware processing flow, lacking a precise hardware-level synchronization mechanism. This fails to address the imaging defects caused by timing deviations at their root, making it difficult to meet the high-definition, high-resolution imaging requirements of highly dynamic maritime environments.

[0005] In summary, the existing technologies have the following shortcomings: On the one hand, traditional airborne millimeter-wave radar imaging methods for maritime observation use software-layer discrete alignment of radar data and navigation data, which is susceptible to clock drift and interruption delays, resulting in timing deviations, which in turn lead to phase errors and image defocusing; on the other hand, existing methods lack a low-level hardware-level spatiotemporal synchronization control mechanism, failing to achieve precise spatiotemporal binding of motion data and echo data, and thus cannot meet the high-precision imaging and target detection requirements in highly dynamic maritime environments, leaving considerable room for improvement in system imaging performance. Summary of the Invention

[0006] In view of the above-mentioned deficiencies of the prior art, the present invention provides an airborne millimeter-wave radar imaging method and device for maritime observation based on spatiotemporal synchronization control. By using hardware-level closed-loop spatiotemporal synchronization control and multi-module technology collaboration, the method solves the problems of radar echo phase error and image defocus caused by the movement of airborne platforms in complex wind field environments at sea, and realizes high-definition, high-resolution real-time imaging and accurate target detection in high-dynamic environments at sea.

[0007] The present invention uses a Positioning and Navigation Solving Unit (PCS) as its core. By reverse-driving the underlying hardware through radar frequency sweep pulses, it constructs a cross-domain physical triggering and pure hardware single-cycle latching mechanism to achieve precise spatiotemporal binding between the radar's raw echo data stream and the continuous platform motion data sequence, thereby eliminating the timing deviation introduced by traditional software application layer alignment from the root. At the same time, through the collaboration of front-end hardware servo isolation and back-end temporal domain focusing algorithm, it fully adapts to nonlinear and irregular motion under complex high-altitude airflow, significantly improving the radar's imaging quality and sea surface target detection rate.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] In a first aspect, the present invention provides an airborne millimeter-wave radar imaging device for ocean observation based on spatiotemporal synchronization control, comprising a power module, an airborne terminal, and a ground terminal.

[0010] The power module is used to provide stable power distribution support for various functional hardware on the airborne end, and is adapted to the power supply environment of airborne platforms such as drones.

[0011] The airborne terminal preferably includes: a radar main unit, an antenna unit, a two-axis stabilization platform, and a positioning and navigation calculation unit (PCS); wherein, the radar main unit serves as the core control and processing center, and integrates an RF power amplifier module, a digital module, and a storage module. The hardware layers and their functions are defined as follows:

[0012] The antenna unit adopts an integrated transceiver design, operates in the millimeter-wave band, and is used to transmit frequency modulated continuous wave (FMCW) signals and receive reflected echo signals from sea surface targets and sea clutter.

[0013] The two-axis stabilization platform is electromechanically connected to the antenna unit and has a two-axis control function. It is used to dynamically isolate the roll and pitch attitude jitter of the airborne platform under complex wind field disturbances, and to perform yaw angle correction to maintain the stable pointing of the antenna beam line of sight to the sea observation. At the same time, it controls the stabilization platform to rotate at a constant speed to perform sea area scanning tasks.

[0014] The radio frequency power amplifier module is located inside the radar main unit and specifically includes a frequency source unit, a transmitting unit, and a receiving unit. The frequency source unit provides a phased-coherent frequency reference source for each radar sub-unit; the transmitting unit includes an up-converter, a frequency doubler, and a power amplifier, used to amplify the radio frequency signal and feed it to the antenna unit for outward radiation; the receiving unit includes a low-noise amplifier, a mixer, and a bandpass filter, used to amplify, filter, and down-convert the echo signal returned by the receiving antenna to output an intermediate frequency video signal.

[0015] The digital module is located inside the radar host and adopts a heterogeneous RFSoC chip architecture. It integrates an embedded microprocessor, on-chip programmable logic, and radio frequency analog-to-digital converter (ADC) and digital-to-analog converter (DAC) resources, supporting low-level hardware-level synchronous operation between multiple channels of ADC and DAC. The on-chip programmable logic integrates a two-stage direct digital frequency synthesis cascade module to generate a linear frequency modulation signal, which is output by the on-chip digital-to-analog converter and then sent to the radio frequency power amplifier module for up-conversion processing.

[0016] The embedded microprocessor in the digital module is used to receive remote control commands sent by the back-end GPU through the airborne network and to complete the global control of the receiving channel gain, AD and DA working modes; the programmable logic part is used to complete data packaging, efficient flow control processing and transmission.

[0017] Furthermore, status monitoring nodes are embedded within the power module, RF power amplifier module, and digital module. The programmable logic unit acquires information from each status monitoring node in real time and transmits it back to the processing software via the embedded microprocessor network, thereby realizing real-time automatic monitoring of the system's health status and rapid location of faulty units.

[0018] The storage module is located inside the radar host and is bidirectionally connected to the digital module. It is used to cache the radar's original two-dimensional echo data and full-dimensional auxiliary data. The full-dimensional auxiliary data includes, but is not limited to, measured spatiotemporal state parameters, radar sub-unit system parameters, antenna pointing angle parameters, and integrated navigation solution data used for heterogeneous joint matching.

[0019] The Positioning and Navigation Solving Unit (PCS) is the core module for achieving hardware-level spatiotemporal synchronization. It consists of a GNSS receiver module (with an external GNSS antenna), an inertial measurement unit (IMU), and a core solving control chip composed of an FPGA module and a DSP module.

[0020] The GNSS receiving module is used to receive satellite signals and output pulse-per-second (1PPS) and UTC time messages;

[0021] The IMU module is connected to the FPGA module and is used to acquire raw attitude data in real time;

[0022] The FPGA module constructs a global synchronization time stamp based on a 1PPS pulse in the underlying hardware and reads this time stamp as a timestamp to insert into the frame header when receiving raw data from the IMU. At the same time, the FPGA module receives the PRF physical trigger signal sent by the digital module through a physical hard connection. Using a pure hardware state machine, within the same clock cycle when the signal transition edge is captured, it directly latches the current global synchronization time stamp as the absolute physical timestamp anchor point of the radar transmission pulse, without the need for CPU or software interrupt intervention. The timed data is then uploaded to the DSP module through a high-speed interface.

[0023] The DSP module has high-speed floating-point operation capability and a dedicated data buffer area. It runs the Extended Kalman Filter (EKF) algorithm to fuse and solve GNSS and IMU data, compensate for platform attitude and position errors, and generate a continuous platform motion data sequence with high-precision timestamps. The DSP module forwards this continuous platform motion data sequence back to the FPGA module in real time and outputs it synchronously to the digital module of the radar host. The FPGA module acts as the control and storage center, controlling the internally integrated local storage module to perform real-time online transcription of the integrated navigation data.

[0024] Furthermore, the digital module includes an RFSoC core board and a GPU processing board connected to it via a PCIe bus. The on-chip programmable logic of the RFSoC core board is responsible for low-level data routing and uses the physical timestamp anchor point as an index to accurately match the corresponding platform motion data in the cache and convert it into fine motion compensation parameters in real time. The parallel accelerated GPU on the GPU processing board receives the original two-dimensional echo data stream and fine motion compensation parameters, and uses high-concurrency computing power to run the back-projection algorithm (BP) in real time to perform azimuth focusing imaging, generate high-resolution synthetic aperture radar (SAR) images, and executes the constant false alarm rate (CFAR) algorithm based on the images to complete adaptive detection and feature extraction of sea surface targets.

[0025] The ground terminal includes a ground integrated display and control terminal, which integrates a data receiving module, a display and control module, and a storage and playback module. The ground integrated display and control terminal communicates wirelessly with the airborne terminal via an airborne data link. It is used to receive real-time radar images, decoded target feature information, and equipment status transmitted from the airborne terminal and perform dynamic display and control, while simultaneously sending control commands to the airborne terminal.

[0026] Secondly, the present invention provides an airborne millimeter-wave radar imaging method for ocean observation based on spatiotemporal synchronization control, comprising the following steps:

[0027] S1, Radar signal transmission and echo preprocessing

[0028] The airborne RF power amplifier module generates a millimeter-wave linear frequency modulated signal and transmits it to the sea surface via the antenna unit. Simultaneously, it receives reflected echo signals from the target and sea clutter and down-converts them into intermediate frequency (IF) video signals. Subsequently, the digital module performs high-speed AD sampling on the IF video signal and runs hardware descrambling mixer and frequency domain pulse compression preprocessing in the underlying programmable logic to generate a raw two-dimensional radar echo data stream containing amplitude and phase information, which is then transmitted to the backend. Meanwhile, the system control unit, running on the embedded microprocessor of the digital module, receives remote control commands from the parallel-accelerated GPU via the airborne network to control various hardware units and collects real-time health status monitoring node information for each key module, transmitting this information back and locating faults via the network.

[0029] S2, Global Synchronization Timescale Construction and Navigation Data Acquisition

[0030] During the flight of the airborne platform, the Positioning and Navigation Calculation Unit (PCS) runs continuously in the background. The satellite navigation module in the PCS receives the second pulse signal and time message, and constructs a high-resolution global synchronization timescale in the underlying FPGA. At the same time, the PCS acquires high-frequency raw attitude data output by the inertial measurement unit (IMU) in real time. The FPGA inserts the global synchronization timescale as a timestamp into the header of the IMU data frame in real time, and uploads it to the DSP to run the extended Kalman filter (EKF) fusion calculation to generate a continuous platform motion data sequence containing the real-time attitude and position information of the airborne platform, and stores it in a designated buffer area. S3, Core Spatiotemporal Synchronization Implementation

[0031] At the start of each sweep pulse transmission by the control radar, the digital module sends a pulse repetition frequency (PRF) physical trigger signal unidirectionally to the underlying hardware pin of the Positioning and Navigation Calculation Unit (PCS) via a physical hardwire. The underlying FPGA of the PCS monitors the level transition edge of this signal in real time through a pure hardware state machine. Within the same clock cycle of capturing the transition edge, without CPU or software interrupt intervention, it directly and instantaneously latches the current global synchronization time stamp, uses it as the absolute physical timestamp anchor point of the radar transmission pulse, and records the corresponding radar pulse sequence number.

[0032] S4, data binding, motion compensation and imaging detection.

[0033] Preferably, in step S1, the radar signal transmission and echo preprocessing are implemented as follows: In the signal transmission stage, a two-stage direct digital frequency synthesis (DDS) cascaded architecture is adopted. The first-stage DDS, driven by a constant control word, outputs instantaneous phase data that increases linearly with time from its phase accumulator. The frequency control word of the second-stage DDS directly uses the instantaneous phase data output by the first-stage DDS, thereby generating a linearly frequency-modulated continuous wave (FMCW) or broadband FMCW signal. The embedded microprocessor (PS end) inside the radar signal preprocessing module calculates the phase and frequency control words of each DDS based on parameters such as center frequency, bandwidth, time width, and frequency modulation slope configured by the host computer. This information is then configured to the FMCW signal generation module in the on-chip programmable logic (PL end) by writing to registers. A high-speed digital-to-analog converter sampling rate is configured according to the system bandwidth requirements, and the throughput bandwidth of the digital-to-analog converter is matched through a multi-channel parallel data splicing bus. The transmission channel uses an "up-conversion + frequency doubling" scheme to generate radio frequency signals, which are then sent to the antenna unit for outward radiation. In the deskewing and pulse compression processing stages, hardware mixing is used for deskewing. The frequency domain pulse compression formula is:

[0034] The distance resolution after pulse compression is determined by the equivalent bandwidth B:

[0035] Preferably, in step S2, the specific implementation of the global synchronization time stamp construction and navigation data acquisition is as follows: the positioning and navigation calculation unit (PCS) establishes the absolute system time based on the received serial port UTC time message and 1PPS second pulse; in the pure hardware logic layer of the FPGA, the internal high-frequency counter is periodically cleared and calibrated with the 1PPS rising edge as the synchronization trigger to construct a global synchronization time stamp of "UTC whole second + microsecond-level high-frequency count"; every time the FPGA receives a frame of high-frequency IMU raw data, it immediately reads the current global synchronization time stamp as a timestamp, inserts it into the data frame header, and uploads it to the DSP; the DSP realizes the fusion calculation of GNSS data and IMU data through the extended Kalman filter (EKF) algorithm, generates a continuous platform motion data sequence with high-precision timestamps, and forwards it back to the FPGA module in real time for local transcription and designated area caching.

[0036] Preferably, in step S3, the key mechanism for achieving core spatiotemporal synchronization is as follows: Cross-domain physical triggering: Each time the digital module transmits a radar pulse, it sends the Pulse Repetition Frequency (PRF) signal unidirectionally through the driver chip via a physical hardwire and connects it to the FPGA port of the Positioning and Navigation Solving Unit (PCS). Pure hardware single-cycle latching: The FPGA of the PCS monitors the level transition edge of the PRF pin in real time through a pure hardware state machine. Within the same clock cycle of capturing the transition edge, without CPU or software interrupt intervention, it directly latches the global synchronization time stamp as the absolute physical timestamp anchor point of the radar transmission pulse.

[0037] Preferably, in step S4, the specific processes for data validity judgment, motion compensation, and target detection are as follows: Before extracting navigation data, IMU data validity is first judged to eliminate outliers caused by severe airflow disturbances on the airborne platform. If the outlier exceeds a threshold, the data is deemed invalid, and interpolation fitting compensation is performed using adjacent valid data. Subsequently, the parallel accelerated GPU integrated within the digital module receives the original two-dimensional echo data stream and fine motion compensation parameters from the radar. Under complex sea conditions, a back-projection time (BP) algorithm is used for azimuth synthetic aperture focusing, adaptively counteracting the nonlinear effects of high-altitude airflow on the trajectory, eliminating image defocus, and generating high-resolution SAR images in real time. The target detection uses a constant false alarm rate (CFAR) algorithm, and the clutter noise of the detected unit CUT is estimated as follows:

[0038]

[0039] Threshold calculation:

[0040] in Threshold coefficient:

[0041] Monitoring and Judgment: If the signal is clear, it is identified as a target. Otherwise, it is identified as clutter or noise. The high-resolution SAR image required by the constant false alarm rate (CFAR) algorithm is generated by the parallel accelerated GPU through a real-time runtime back projection (BP) algorithm before detection; at the same time, the system performs high-speed parallel storage of all raw radar echo data and the bound auxiliary data.

[0042] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0043] This invention employs a closed-loop synchronization design driven by radar pulses. When the radar sweeps the frequency, it sends out a physical trigger signal of the pulse repetition frequency (PRF) through a physical hardwire. The FPGA of the PCS uses a pure hardware state machine to latch the global synchronization time stamp within a single clock cycle, completely bypassing the intervention of the CPU and software interrupts, thus ensuring the spatiotemporal consistency of the data.

[0044] This invention isolates low-frequency, high-dynamic attitude sway at the physical layer through a two-axis stabilization platform, eliminates abnormal and abrupt values ​​at the data layer through data validity judgment by an inertial measurement unit (IMU), and adopts a time-domain back-projection (BP) algorithm to adapt to nonlinear flight trajectories, achieving multi-level fault tolerance and completely solving the problem of image defocusing in the millimeter-wave high-frequency band.

[0045] In summary, the millimeter-wave radar terminal of this invention does not require complex network timing or message parsing capabilities; it can achieve high-precision spatiotemporal coordination with the PCS unit simply by outputting physical pulses. Each module has a clear division of labor, and the computational burden is distributed through a high-speed parallel bus and heterogeneous platform, making it extremely valuable for real-time imaging deployment. Attached Figure Description

[0046] To more clearly illustrate the technical solution of the present invention, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the drawings described below are merely some exemplary embodiments of the present invention and are not intended to limit the entirety of the invention. For those skilled in the art, other drawings can be derived from these drawings without creative effort, and these drawings also fall within the protection scope of the present invention.

[0047] Figure 1 Overall block diagram of an airborne millimeter-wave radar system for maritime observation.

[0048] Figure 2 : Overall flowchart of spatiotemporal synchronization of airborne millimeter-wave radar imaging for maritime observation.

[0049] Figure 3 : Schematic diagram of the internal structure of the Positioning and Navigation Solving Unit (PCS). Detailed Implementation

[0050] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0051] It should be noted that the structures, proportions, sizes, etc., illustrated in the accompanying drawings of this specification are only used to complement the content disclosed in the specification for those skilled in the art to understand and read, and are not intended to limit the conditions under which the present invention can be implemented. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in the proportions, or adjustments to the size, without affecting the effects and objectives that the present invention can produce, should still fall within the scope of the technical content disclosed in the present invention.

[0052] Example 1

[0053] like Figure 1 As shown, this embodiment provides an airborne millimeter-wave radar imaging method for sea observation based on spatiotemporal synchronization control. This method operates based on an airborne millimeter-wave radar imaging system, which includes at least an airborne platform, a digital module, a positioning and navigation calculation unit (PCS), an imaging processing unit (GPU and FPGA cluster), and an antenna unit. The airborne platform serves as the carrier, carrying various functional modules to achieve full coverage observation of the sea surface area; the digital module generates and transmits millimeter-wave signals, receives echo signals from sea surface targets and sea clutter, and performs signal preprocessing; the PCS unit, as the core synchronization and navigation calculation module, consists of a GNSS receiving module, an IMU module, a DSP module, an FPGA module, and a storage module, realizing global timescale construction, navigation data acquisition and timing, data storage, and integrated navigation calculation; the imaging processing unit performs motion compensation of radar echoes, synthetic aperture focusing, and target detection; the antenna unit is responsible for transmitting millimeter-wave signals and receiving echo signals, employing an integrated transceiver design to adapt to the high-frequency transmission requirements of millimeter-wave bands.

[0054] The core improvement of this invention lies in constructing a hardware-level spatiotemporal synchronization mechanism with the PCS unit as the core, which solves the problems of timing deviation, phase error and image defocus caused by traditional software layer data alignment, and realizes high-definition and high-resolution imaging in a high-dynamic marine environment.

[0055] like Figure 2 As shown, the above-mentioned airborne millimeter-wave radar imaging method for maritime observation based on spatiotemporal synchronization control specifically includes the following steps:

[0056] S1, Radar signal transmission and echo preprocessing

[0057] The airborne millimeter-wave radar generates frequency-modulated signals and transmits them to the sea surface via an antenna. It receives echo signals from targets and sea clutter, and generates a raw two-dimensional echo data stream containing amplitude and phase information through high-speed sampling and pulse compression preprocessing.

[0058] The specific implementation further includes the following sub-steps:

[0059] S11. Millimeter-wave signal transmission implementation: In the signal transmission stage, a two-stage direct digital frequency synthesis (DDS) cascaded architecture is adopted. The first-stage DDS, driven by a constant control word, outputs instantaneous phase data that increases linearly with time from its phase accumulator. The frequency control word of the second-stage DDS directly uses the instantaneous phase data output by the first-stage DDS, thereby generating a linearly frequency-modulated continuous wave (FMCW) or broadband FMCW signal. In the embedded microprocessor inside the radar host, the phase and frequency control words of each DDS are calculated based on parameters such as center frequency, bandwidth, time width, and frequency modulation slope configured in the ground integrated display and control terminal. Then, these parameters are configured to the FMCW signal generation module by writing to registers.

[0060] To meet the high-frequency, high-bandwidth signal output requirements of millimeter waves, this embodiment configures the digital-to-analog converter (DAC) with a high-speed DAC sampling rate according to the system bandwidth requirements, and matches the DAC's throughput bandwidth through a multi-channel parallel data splicing bus to ensure the integrity and stability of the signal output. The transmission channel adopts an "upconversion + frequency doubling" scheme to generate radio frequency signals. The upconversion module converts the intermediate frequency signal generated by the DDS into a millimeter-wave intermediate frequency signal, and the frequency doubling module further boosts the signal frequency to the target millimeter-wave band, meeting the requirements of long-distance, high-resolution sea observation.

[0061] S12. Echo signal preprocessing: After the antenna unit receives the echo signals from sea surface targets and sea clutter, it transmits them to the radar preprocessing module. First, the echo signal is amplified by a low-noise amplifier (LNA) to suppress noise interference. Then, a bandpass filter is used to filter out clutter signals outside the frequency band to improve the signal-to-noise ratio.

[0062] The core steps of preprocessing are de-chewing and pulse compression: De-chewing uses hardware mixing to mix the received echo signal with the local reference signal, completing analog domain de-chewing mixing to obtain the difference frequency baseband signal containing target range information; the frequency domain pulse compression formula is:

[0063]

[0064] in, This is the output signal after pulse compression. This is the preprocessed echo signal. This is the complex conjugate of the local reference signal spectrum. The range resolution after pulse compression is determined by the equivalent bandwidth B, and the specific calculation formula is as follows:

[0065]

[0066] in, Let be the range resolution, c be the speed of light, and B be the equivalent bandwidth of the radar signal. Through pulse compression processing, the radar's range resolution can be improved to a high-resolution level, meeting the detection requirements for small targets on the sea surface. After preprocessing, a raw two-dimensional radar echo data stream containing amplitude and phase information is generated and transmitted to the imaging processing unit buffer, awaiting subsequent motion compensation processing.

[0067] S2, Global Synchronization Timescale Construction and Navigation Data Acquisition

[0068] The Positioning and Navigation Calculation Unit (PCS) receives the second pulse signal and time message output by the satellite navigation module and builds a high-precision global synchronization time stamp in the underlying hardware. At the same time, it collects inertial measurement unit (IMU) data in real time, inserts the global synchronization time stamp as a timestamp into the data frame, and generates a continuous platform motion data sequence through navigation calculation and stores it in the cache.

[0069] The specific implementation further includes the following sub-steps:

[0070] S21. High-precision global synchronization time scale construction: The core function of the PCS unit is to provide time synchronization for IMU data based on GNSS time information, complete the acquisition and storage of GNSS navigation data and IMU timing data, and realize real-time navigation calculation based on Kalman filtering. Specifically, the GNSS receiving module receives satellite signals in real time and outputs UTC time messages and 1PPS pulse signals. The PCS unit establishes the absolute system time based on the received serial UTC time messages and 1PPS pulses.

[0071] like Figure 3 As shown, in the FPGA pure hardware logic layer of the PCS unit, a 1PPS rising edge is used as the synchronous trigger signal to periodically reset and calibrate the internal high-frequency counter. The FPGA adopts a serial peripheral configuration bus to ensure the stability and convenience of the configuration process. The clock frequency of its internal high-frequency counter is consistent with the system clock to avoid time scale deviation caused by clock drift.

[0072] The S22 IMU module is connected to the FPGA of the PCS unit via a differential serial communication interface and a PRF signal interface. The differential serial communication interface uses a differential-to-single-ended interface circuit to access the FPGA and has open-circuit failure protection. The PRF signal is accessed to the FPGA via a low-latency driver chip to meet the timing requirements of high-precision system synchronization.

[0073] When the FPGA of the PCS unit captures the transition edge of the PRF signal, it latches the current global synchronization time stamp, packages the timestamp with the corresponding frame's IMU serial port data, and uploads it to the DSP for real-time high-speed interaction through the inter-chip parallel bus interface.

[0074] The DSP runs an extended Kalman filter (EKF) algorithm to fuse GNSS and IMU data to eliminate single navigation errors and generate a continuous platform motion data sequence containing the roll, pitch, yaw, velocity and position parameters of the airborne platform. This sequence is forwarded back to the FPGA module in real time for organization and storage in the storage module, and is also cached in a designated area for subsequent matching with radar echo data.

[0075] The raw GNSS data (including UTC time, satellite number, pseudorange, and carrier phase) and the IMU timing data (including timestamps, three-axis gyroscope and accelerometer measurements) are stored as separate files for subsequent data matching, aircraft attitude calculation, and phase error compensation for radar imaging.

[0076] S3, Core Spatiotemporal Synchronization Implementation

[0077] At the instant the digital module transmits the pulse, it sends a pulse repetition frequency (PRF) physical trigger signal to the underlying hardware of the PCS. The PCS monitors the transition edge of this signal through pure hardware logic and instantaneously latches the current global synchronization time stamp within a single clock cycle, using it as the physical timestamp anchor point of the radar's transmitted pulse.

[0078] The specific implementation further includes the following sub-steps:

[0079] S31. Cross-domain physical triggering implementation: When the digital module transmits a radar pulse each time, it sends the pulse repetition frequency (PRF) signal unidirectionally through the driver chip and connects it to the FPGA port of the PCS unit via a physical hardwire. The driver chip adopts the same model as the IMU PRF interface to ensure the consistency of signal transmission, while realizing the electrical isolation between the digital module and the PCS unit to avoid mutual interference and ensure interface safety.

[0080] The frequency of the PRF signal is consistent with the repetition frequency of the radar transmitted pulse, and its level transition edge is strictly synchronized with the start time of the radar transmitted pulse, ensuring that the FPGA of the PCS unit can accurately capture the instantaneous moment of the radar transmitted pulse and provide a precise trigger signal for the subsequent latching of the timestamp anchor point.

[0081] S32. Pure hardware single-cycle latch implementation: Within one clock cycle of capturing the transition edge, the FPGA directly latches the current global synchronization time stamp, uses it as the absolute physical timestamp anchor point of the radar transmission pulse, stores the timestamp anchor point in the cache, and marks the corresponding radar transmission pulse sequence number to facilitate subsequent index matching with radar echo data.

[0082] Because it is implemented with pure hardware logic, the latency of the latching process is negligible, ensuring that the physical timestamp anchor point is completely synchronized with the instantaneous moment of the radar transmission pulse. This achieves the underlying hardware-level spatiotemporal binding of navigation data and radar echo data, fundamentally solving the microsecond-level timing deviation problem caused by the discrete alignment of traditional software layers.

[0083] S4, Data Binding, Motion Compensation and Imaging Detection

[0084] Using the physical timestamp anchor point as an index, the corresponding motion data is accurately matched in the cache to generate precise motion compensation parameters; based on these parameters, fine motion compensation and synthetic aperture focusing processing are performed on the original two-dimensional radar echo data to generate a high-resolution radar image, and sea surface target detection and feature extraction are completed based on this image.

[0085] The specific implementation further includes the following sub-steps:

[0086] S41. Data Validity Judgment and Precise Matching: Before extracting navigation data, the validity of IMU data is first judged to eliminate abnormal data caused by severe disturbances of the airborne platform or sensor failure. The specific method of validity judgment is as follows: set the normal threshold range of each measurement parameter of IMU (such as gyroscope measurement value threshold, accelerometer measurement value threshold, temperature threshold). If any parameter of a frame of IMU data exceeds the threshold range, the data of that frame is determined to be invalid. Interpolation fitting compensation is performed using the valid data of the preceding and following frames (or extrapolation compensation is performed using the valid data of the previous frame) to ensure the continuity of the motion data sequence.

[0087] The imaging processing unit uses the physical timestamp anchor point latched in S3 as an index to search for the platform motion data (roll angle, pitch angle, yaw angle, etc.) corresponding to the timestamp in the cached motion data sequence. Since the timestamp anchor point is instantaneously synchronized with the radar transmitted pulse and the timestamp of the motion data sequence is consistent with the global time scale, it can achieve accurate matching between motion data and radar echo data with extremely low matching error, ensuring the accuracy of motion compensation parameters.

[0088] S42. Fine Motion Compensation and Synthetic Aperture Focusing: Based on the platform motion data obtained through matching, the PCS unit's calculation core generates precise motion compensation parameters. These parameters include the airborne platform's attitude error and position error, used to correct the phase error in the radar echo signal caused by the platform's motion. Motion compensation employs a hardware-level real-time processing method, collaboratively completed by the imaging processing unit's data stream FPGA and GPU. The FPGA is responsible for the real-time input and low-level preprocessing of the motion compensation parameters, while the GPU handles parallel focusing calculations on large-scale echo data, significantly improving compensation efficiency.

[0089] In complex sea conditions, the imaging processing unit employs a back-projection (BP) algorithm for azimuth synthetic aperture processing. This algorithm effectively suppresses sea clutter interference, improves image focusing, and ultimately generates high-resolution SAR images. The system utilizes the FPGA and GPU within the imaging processing unit to configure a high-speed data stream, storing all raw radar echo data and other auxiliary data (GNSS data, IMU data, motion compensation parameters), and working in conjunction with the high-performance GPU to achieve real-time SAR image imaging.

[0090] S43. Sea Surface Target Detection and Feature Extraction: Target detection employs the Constant False Alarm Rate (CFAR) algorithm. This algorithm is adaptive to the sea clutter environment, avoiding detection errors caused by changes in clutter intensity. The specific implementation process is as follows:

[0091] Select the cell under test (CUT) and select N reference cells around the cell under test (to estimate the clutter noise intensity).

[0092] The clutter noise intensity of the reference cell is estimated using the following formula:

[0093] The detection threshold is calculated using the following formula:

[0094] in, This is the threshold coefficient, used to adjust the false alarm rate of the detection. The calculation formula is:

[0095] Monitoring Decision: When the signal strength of the tested unit (CUT) is... If the detected unit is identified as a target, it is determined to be a sea surface target; otherwise, it is determined to be clutter or noise.

[0096] After target detection is completed, feature extraction is performed on the detected sea surface targets. The extracted features include the target's area, shape, position, speed, etc. The feature information is output together with the imaging image to provide accurate target data support for scenarios such as maritime supervision and national defense security.

Claims

1. An airborne millimeter-wave radar imaging device for maritime observation based on spatiotemporal synchronization control, comprising a power module, an airborne terminal, and a ground terminal; characterized in that, The airborne terminal includes a radar host, an antenna unit, a two-axis stabilizer, a positioning and navigation calculation unit, and a storage module; the radar host integrates an RF power amplifier module and a digital module. The positioning and navigation calculation unit includes a GNSS receiving module, an inertial measurement unit, a DSP module, an FPGA module, and an interface driving circuit. The FPGA module is configured to construct a global synchronization time scale, receive periodic trigger signals from the digital module, and latch the current global synchronization time scale as a timestamp within the same clock cycle when the transition edge of the trigger signal is detected. The digital module includes an RFSoC core board and a GPU processing board connected to it via a PCIe bus. The on-chip programmable logic of the RFSoC core board is configured to perform cycle-by-cycle loading of motion compensation parameters and low-level data flow control. The parallel accelerated GPU on the GPU processing board is configured to perform parallel synthetic aperture focusing operations of the back projection algorithm and constant false alarm rate target detection.

2. The airborne millimeter-wave radar imaging device for maritime observation based on spatiotemporal synchronization control according to claim 1, characterized in that, The radio frequency power amplifier module includes a frequency source unit, a transmitting unit, and a receiving unit; the frequency source unit is used to provide a phased frequency reference source for each radar sub-unit; the transmitting unit includes an up-converter, a frequency doubler, and a power amplifier; the receiving unit includes a low-noise amplifier, a mixer, and a bandpass filter.

3. The airborne millimeter-wave radar imaging device for maritime observation based on spatiotemporal synchronization control according to claim 1, characterized in that, The digital module adopts an RFSoC chip heterogeneous architecture, which integrates an embedded microprocessor, on-chip programmable logic, and radio frequency analog-to-digital converter and digital-to-analog converter. The on-chip programmable logic integrates a two-stage direct digital frequency synthesis cascade module to generate a linear frequency modulation signal, which is output by the on-chip digital-to-analog converter and then sent to the radio frequency power amplifier module for up-conversion processing.

4. The airborne millimeter-wave radar imaging device for maritime observation based on spatiotemporal synchronization control according to claim 1, characterized in that, The FPGA module of the positioning and navigation calculation unit is configured to: periodically reset and calibrate the high-frequency counter using the rising edge of the second pulse signal as the synchronization reference, and construct a global synchronization timescale characterized by the whole second time of the second pulse and the real-time count value of the high-frequency counter; immediately read the current global synchronization timescale as a timestamp and insert it into the data frame header and upload it to the DSP module after receiving each frame of raw data from the inertial measurement unit; the DSP module runs an extended Kalman filter algorithm to fuse GNSS data and inertial measurement unit data to generate a platform motion data sequence.

5. The airborne millimeter-wave radar imaging device for maritime observation based on spatiotemporal synchronization control according to claim 1, characterized in that, The FPGA module of the positioning and navigation calculation unit receives the periodic trigger signal sent by the digital module through physical hardwire, and monitors the level transition edge of the trigger signal in real time through a pure hardware state machine. Within the same clock cycle of capturing the transition edge, the current global synchronization time stamp is directly latched as the timestamp of the start time of the frequency modulation cycle.

6. The airborne millimeter-wave radar imaging device for maritime observation based on spatiotemporal synchronization control according to claim 1, characterized in that, The parallel acceleration GPU is configured to run a back-projection algorithm to complete azimuth synthetic aperture processing, generate high-resolution synthetic aperture radar (SAR) images, and run a constant false alarm rate (CFAR) algorithm to complete sea surface target detection.

7. The airborne millimeter-wave radar imaging device for maritime observation based on spatiotemporal synchronization control according to claim 1, characterized in that, The ground terminal includes a ground integrated display and control terminal, which integrates a data receiving module, a display and control module, and a storage and playback module. The ground integrated display and control terminal communicates wirelessly with the airborne terminal via an airborne data link. It is used to receive real-time radar images, target feature information, and equipment status transmitted from the airborne terminal and perform dynamic display and control, while also sending control commands to the airborne terminal.

8. An airborne millimeter-wave radar imaging method for maritime observation based on the device described in any one of claims 1-7, characterized in that, Includes the following steps: S1. Radar signal transmission and echo preprocessing: The radio frequency power amplifier module generates a millimeter-wave linear frequency modulated signal and transmits it to the sea surface through the antenna unit. At the same time, it receives the reflected echo signals of the target and sea clutter and down-converts them into intermediate frequency video signals. The digital module performs high-speed sampling of the intermediate frequency video signal and runs deskewing mixing and frequency domain pulse compression preprocessing in the underlying programmable logic to generate the original two-dimensional radar echo data stream. S2. Global synchronization time scale construction and navigation data acquisition: The positioning and navigation calculation unit receives the second pulse signal and time message through its internal GNSS receiving module, and constructs a global synchronization time scale in the underlying FPGA; it collects the raw attitude data output by the inertial measurement unit in real time, and the FPGA inserts the global synchronization time scale as a timestamp into the data frame header, and uploads it to the DSP to run extended Kalman filter fusion calculation, generating a continuous platform motion data sequence and storing it in the cache; S3. Core spatiotemporal synchronization implementation: At the start of each frequency sweep pulse transmission, the digital module sends a periodic trigger signal to the FPGA pin of the positioning and navigation calculation unit via a physical hardwired connection; the FPGA of the positioning and navigation calculation unit monitors the transition edge of the trigger signal in real time through a pure hardware state machine, and latches the current global synchronization time stamp within the same clock cycle of capturing the transition edge, as the physical timestamp anchor point of the radar transmission pulse; S4. Data Binding, Motion Compensation and Imaging Detection: The on-chip programmable logic of the RFSoC core board uses the physical timestamp anchor point as an index to match the corresponding platform motion data in the cache and generate motion compensation parameters; the parallel accelerated GPU receives the original two-dimensional echo data stream from the radar and the motion compensation parameters, runs the back projection algorithm to perform azimuth synthetic aperture focusing processing, generates a SAR image, and performs a constant false alarm rate algorithm based on the image to complete sea surface target detection and feature extraction.

9. The method according to claim 8, characterized in that, In step S1, the signal transmission adopts a two-stage direct digital frequency synthesis cascaded architecture, wherein the first-stage direct digital frequency synthesizer generates a phase control word, and the frequency control word of the second-stage direct digital frequency synthesizer is determined by the phase control word of the first-stage direct digital frequency synthesizer to generate a linear frequency modulated continuous wave signal. The transmission channel includes an upconverter and a frequency doubler cascaded together, which output millimeter-wave radio frequency signals after upconverting and frequency doublering the generated signals. The pulse compression in the echo preprocessing uses frequency domain matched filtering, and the distance resolution is determined by the equivalent bandwidth.

10. The method according to claim 8, characterized in that, The construction of the global synchronization time stamp in step S2 includes: using the rising edge of the second pulse signal as the synchronization reference, periodically clearing and calibrating the high-frequency counter inside the FPGA so that the count value of the high-frequency counter is traced back to the whole second of the second pulse, and the whole second time corresponding to the second pulse and the current count value of the high-frequency counter together constitute the global synchronization time stamp.

Images