Single-photon non-vision-field target tracking system based on unmanned aerial vehicle platform

Abstract

The invention discloses a single-photon non-vision-field target tracking system based on an unmanned aerial vehicle platform, which belongs to the field of non-vision-field imaging and is characterized in that a laser is connected with a scanning galvanometer assembly through a first nanosecond optical path switch and an optical fiber circulator to form an emission optical path for emitting laser pulses to relay reflecting surfaces corresponding to four predetermined positioning points; the scanning galvanometer assembly is connected with the photon detector through the collimating optical fiber, the optical fiber circulator and the second nanosecond optical path switch to form a receiving optical path capable of receiving echo photons; the main control module is electrically connected with the laser, the first nanosecond light path switch, the second nanosecond light path switch, the scanning galvanometer assembly, the photon detector, the time-to-digital converter and the high-frequency inertial measurement unit respectively, and can control each component to perform laser ranging, double-window gating, single-photon detection, inertial measurement unit sampling and timing according to a unified time sequence; and the three-dimensional coordinates of the current non-vision target are calculated by using the distance measurement data through an ellipsoid intersection solution method, so that non-vision target tracking is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of non-visual imaging of unmanned aerial vehicle platforms, and particularly relates to a single-photon non-visual target tracking system based on an unmanned aerial vehicle platform. BACKGROUND

[0002] Three-dimensional imaging technology is a core technology in the fields of environmental perception, autonomous driving, surveying and mapping, and intelligent inspection. Laser radar (LiDAR) has become the mainstream solution for obtaining three-dimensional point cloud data due to its high resolution and high-precision ranging capability. However, when laser radar is mounted on a high-dynamic platform such as an unmanned aerial vehicle (UAV), its performance will be severely challenged. During flight, the unmanned aerial vehicle is inevitably disturbed by rotor vibration, wind gusts, and airflow turbulence, causing its attitude (pitch angle, roll angle, and yaw angle) to change frequently and unpredictably. The attitude of the aircraft is different each time a laser pulse is transmitted and received, which causes the point cloud data to be "distorted" and "deformed", seriously affecting the accuracy of three-dimensional reconstruction and becoming a major obstacle to high-precision surveying and navigation tasks.

[0003] In the prior art, some solutions use high-frequency laser radar or post-processing registration methods based on simultaneous localization and mapping (SLAM), but these methods are high in cost, complex in calculation, and have large delays, and cannot meet the real-time requirements. Another type of method relies on laser radar odometry or inertial navigation systems to provide frame-to-frame poses and performs interpolation compensation based on the assumption of uniform speed, but these methods cannot effectively cope with the high-frequency nonlinear vibration of the unmanned aerial vehicle platform. Therefore, how to achieve high-precision motion correction in a high-dynamic environment has become the primary technical problem faced by unmanned aerial vehicle-mounted laser radars.

[0004] In addition, single-photon imaging radar systems also face the problem of low signal-to-noise ratio. High-sensitivity single-photon detectors (SPADs) are extremely susceptible to external noise and device dark counts in strong background light environments such as daytime. More critically, internal fiber loop leakage and near-field strong backscattered light in the system also impact the detector, causing it to saturate and produce dead time, greatly limiting the system's detection robustness and effective range. Existing suppression methods, such as narrowband filtering or increasing laser power, are limited by the payload and power consumption of the unmanned aerial vehicle, and have limited effectiveness.

[0005] Unlike visual imaging, non-visual (NLOS) imaging aims to detect and track targets that are obscured by obstacles. Existing research has demonstrated short-range positioning and imaging capabilities for static hidden objects in laboratory environments. However, non-visual imaging relies on multiple diffuse reflections (usually three) of photon signals between the wall, target, and detector, which results in a sharp decay of echo signal energy, with its intensity inversely proportional to the fourth power of the distance Therefore, the existing non-visual imaging technology is difficult to realize remote detection of more than 50 meters, and mainly relies on static and stable laboratory platform.

[0006] The existing non-visual tracking method also has technical defects. For example, a non-visual target tracking patent technology based on a continuous frequency sweeping laser obtains target position and speed information by performing Fourier transform and peak value processing on the interference light signal. However, the system resolution of this scheme is limited, which leads to problems such as target confusion, misjudgment or loss in a multi-target tracking scene, and the tracking robustness is poor. Another type of non-visual imaging technology uses a large pixel array scanning, which will generate a large amount of data, and puts forward very high requirements for data caching and real-time processing capability.

[0007] In summary, there is no effective scheme in the prior art to integrate the non-visual imaging technology with the high dynamic platform of the unmanned aerial vehicle. How to overcome the comprehensive difficulties such as high-frequency jitter of the unmanned aerial vehicle platform, signal attenuation at a long distance, and low signal-to-noise ratio in a poor lighting environment, to realize remote, high-precision and real-time tracking of non-visual targets, has become a key problem to be solved in this technical field.

[0008] In view of this, the present application is proposed. SUMMARY

[0009] The purpose of the present application is to provide a single-photon non-visual target tracking system based on an unmanned aerial vehicle platform, which can realize remote, high-precision and real-time tracking of non-visual targets in a high dynamic environment of the unmanned aerial vehicle operation, thereby solving the above-mentioned problems existing in the prior art.

[0010] The purpose of the present application is achieved by the following technical solutions: A single-photon non-visual target tracking system based on an unmanned aerial vehicle platform, comprising: a laser, a first nanosecond-level optical path switch, a fiber ring, a fiber collimator, a scanning galvanometer assembly, a second nanosecond-level optical path switch, a photon detector, a time-to-digital converter, a master control module and a high-frequency inertial measurement unit; wherein, The laser is connected with the scanning galvanometer assembly in sequence through the first nanosecond-level optical path switch, the fiber ring and the fiber collimator, and constitutes a transmitting optical path capable of transmitting laser pulses to the corresponding relay reflection surface of the predetermined four positioning points in the forward scene according to the set angle; The scanning galvanometer assembly is connected with the photon detector in sequence through the collimating fiber, the fiber ring and the second nanosecond-level optical path switch, and constitutes a receiving optical path capable of receiving echo photons scattered by the corresponding relay reflection surface of each positioning point after being reflected by the non-visual target; The master module is electrically connected with the laser, the first nanosecond optical path switch, the second nanosecond optical path switch, the scanning galvanometer assembly, the photon detector, the time-to-digital converter and the high-frequency inertial measurement unit, can control each component according to a unified timing to perform laser ranging and target positioning in a transmission and reception cycle, in each transmission and reception cycle, a trigger signal is sent to the laser to generate a laser pulse, the first nanosecond optical path switch and the second nanosecond optical path switch are controlled to open and close according to a double-window gating timing, the transmission optical path and the receiving optical path are isolated in time in the transmission stage and the receiving stage, and according to four photon time-of-flight measurement values, corresponding four positioning point positions and UAV attitude data obtained by the high-frequency inertial measurement unit in each transmission and reception cycle, the four photon time-of-flight measurement values and the corresponding four positioning point positions are corrected by the UAV attitude data, the three-dimensional coordinates of the current non-visual target are calculated by using the corrected four photon time-of-flight measurement values and the corresponding four positioning point positions through an ellipsoid intersection solving method, and non-visual target tracking is realized.

[0011] Compared with the prior art, the single-photon non-visual target tracking system based on the UAV platform has the following beneficial effects: By arranging the laser, the first nanosecond optical path switch, the fiber ring, the scanning galvanometer assembly, the second nanosecond optical path switch, the photon detector, the time-to-digital converter, the master module, the high-frequency inertial measurement unit and the field programmable gate array, a single-photon non-visual target tracking system suitable for UAVs is formed, four positioning points are scanned on a relay reflection surface, the ellipsoid intersection solving method is used to solve the position of the non-visual target, the data amount is greatly reduced, the positioning efficiency is improved, and high-precision non-visual positioning is realized; the first nanosecond optical path switch and the second nanosecond optical path switch are controlled to open and close by the master module according to a double-window gating timing, the transmission optical path and the receiving optical path are isolated in time in the transmission stage and the receiving stage, the internal light leakage and near-field backscattering of the system are physically suppressed, and the signal-to-noise ratio of the echo signal is significantly improved; the four photon time-of-flight measurement values and the corresponding four positioning point positions are corrected by the UAV attitude data, the three-dimensional coordinates of the current non-visual target are calculated by using the corrected four photon time-of-flight measurement values and the corresponding four positioning point positions through the ellipsoid intersection solving method, single-photon event-level motion correction can be realized, the point cloud distortion caused by the high-dynamic attitude change of the UAV is accurately corrected, the distortion correction accuracy is improved from the centimeter level of the traditional method to the micrometer level, the master module controls each part through the field programmable gate array, high-precision timing synchronization can be realized, the hardware-level global timing synchronization can reach the picosecond level, and a physical basis is provided for seamless fusion and real-time processing of multi-sensor data. The present application solves the technical problems of limited detection distance, weak signal and serious point cloud distortion of non-visual targets in the high-dynamic flight environment of the UAV. Attached Figure Description

[0012] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0013] Figure 1 This is a schematic diagram illustrating the structure of a single-photon non-line-of-sight target tracking system based on an unmanned aerial vehicle (UAV) platform, as provided in an embodiment of the present invention.

[0014] Figure 2 The timing diagrams of various signals in the single-photon non-line-of-sight target tracking system based on an unmanned aerial vehicle platform provided in this embodiment of the invention are shown.

[0015] Figure 3 This is a schematic diagram illustrating the non-line-of-sight imaging principle of a single-photon non-line-of-sight target tracking system based on an unmanned aerial vehicle platform, and the method of using four positioning points, provided in an embodiment of the present invention.

[0016] Figure 4 The flowchart illustrates the algorithm for retrieving the distance and motion compensation of a non-line-of-sight target from the signals of four positioning points in a single-photon non-line-of-sight target tracking system based on an unmanned aerial vehicle platform, as provided in this embodiment of the invention. Detailed Implementation

[0017] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the specific content of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments, which do not constitute a limitation of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0018] First, the following explanations are provided for the terms that may be used in this article: The term "and / or" means that either or both can be achieved simultaneously. For example, X and / or Y means that it includes both "X" or "Y" as well as the three cases of "X and Y".

[0019] The terms "comprise", "comprising", "include", "including", "contain", "containing", "have", "having", or any other similar semantic descriptions, are to be construed as open-ended rather than limiting. For example, the inclusion of an element, such as a raw material, component, ingredient, carrier, dosage form, material, dimension, part, component, mechanism, device, step, process, method, reaction condition, processing condition, parameter, algorithm, signal, data, product, or article, is to be interpreted as including not only the explicitly recited element, but also any element known to one of ordinary skill in the art to be associated with or useful in conjunction with the explicitly recited element.

[0020] The term "consisting of" means excluding any element not specifically recited. If the term is used in the claims, the term "consisting of" shall be interpreted as a restriction against any element not specifically recited in the claims. If the term is used in a clause of the specification other than the claims, it shall be interpreted to mean the meaning ascribed in this paragraph.

[0021] The terms "mounting", "connected", "connecting", "fixed", and "fixing" should be construed broadly according to the context in which they are used, for example, connecting can be fixed connecting, detachable connecting, or integral connecting; can be mechanical connecting, or electrical connecting; can be direct connecting, or indirect connecting via an intermediate medium; can be internal connecting of two elements. The specific meaning of the above terms in the context of the present disclosure can be understood by one of ordinary skill in the art according to the specific circumstances.

[0022] When a concentration, temperature, pressure, dimension, or other parameter is expressed as a numerical range, the numerical range should be interpreted as specifically disclosing all ranges formed from any pair of an upper limit and a lower limit of the numerical range, whether or not the range is explicitly recited; for example, if a numerical range "2-8" is recited, the numerical range should be interpreted as including "2-7", "2-6", "5-7", "3-4 and 6-7", "3-5 and 7", "2 and 5-7", and the like. Unless otherwise specified, numerical ranges recited herein include all integers and fractions within the range.

[0023] The terms "central", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", and the like indicate the orientation or positional relationship shown in the drawings based on the orientation or positional relationship shown in the drawings, and are only for the convenience of description and simplification of description, and do not mean that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting herein.

[0024] The schemes provided by the present application will be described in detail below. The contents not described in detail in the embodiments of the present application belong to the prior art known to those skilled in the art. If the specific conditions are not specified in the embodiments of the present application, the conventional conditions or the conditions recommended by the manufacturer are used. If the reagents or instruments used in the embodiments of the present application are not specified by the manufacturer, they are all conventional products that can be purchased on the market.

[0025] As shown in Figure 1 , Figure 3 The present embodiment provides a single-photon non-visual target tracking system based on a UAV platform, which comprises: a laser, a first nanosecond optical switch, a fiber ring, a fiber collimator, a scanning galvanometer assembly, a second nanosecond optical switch, a photon detector, a time-to-digital converter, a master control module, and a high-frequency inertial measurement unit; wherein The laser is connected with the scanning galvanometer assembly in sequence through the first nanosecond optical switch, the fiber ring, and the fiber collimator, and constitutes a transmitting optical path capable of transmitting laser pulses to the corresponding relay reflection surface of the four predetermined positioning points in the forward scene according to the set angle; The scanning galvanometer assembly is connected with the photon detector in sequence through the collimating fiber, the fiber ring, and the second nanosecond optical switch, and constitutes a receiving optical path capable of receiving echo photons scattered by the corresponding relay reflection surface of each positioning point after being reflected by the non-visual target; The master module is electrically connected with the laser, the first nanosecond optical path switch, the second nanosecond optical path switch, the scanning galvanometer assembly, the photon detector, the time-to-digital converter and the high-frequency inertial measurement unit, can control each component in a unified time sequence to perform laser ranging and target positioning in a transmission and reception cycle, in each transmission and reception cycle, a trigger signal is sent to the laser to generate a laser pulse, the first nanosecond optical path switch and the second nanosecond optical path switch are controlled to open and close in a double-window gating time sequence, and the transmission optical path and the receiving optical path are isolated in time in the transmission stage and the receiving stage; and according to four photon time-of-flight measurement values collected in each transmission and reception cycle, corresponding four positioning point positions and unmanned aerial vehicle attitude data obtained through the high-frequency inertial measurement unit, the four photon time-of-flight measurement values and the corresponding four positioning point positions are corrected through the unmanned aerial vehicle attitude data, and the three-dimensional coordinates of the current non-visual target are calculated through the ellipsoid intersection solution method, so that the non-visual target tracking is realized.

[0026] The above-mentioned predetermined four positioning points can be manually selected within the range of the scanning galvanometer assembly. It can also be defaulted. For example, it can be manually selected within the 64*64 galvanometer scanning range.

[0027] The system of the application can solve the technical problems of limited detection distance of non-visual targets, weak signal and easy noise interference, and serious point cloud distortion in the high dynamic flight environment of the unmanned aerial vehicle.

[0028] The system transmits an ultra-short laser pulse to a visible reflecting surface (such as a wall) through the laser on the unmanned aerial vehicle, uses the reflecting surface as a relay, and indirectly illuminates the blocked target according to the three-time diffuse reflection non-visual imaging principle; the weak echo signal of the blocked target is scattered back to the airborne receiver through the reflecting surface and is captured by a high-sensitivity single-photon detector (such as an avalanche diode).

[0029] The master module precisely controls the double-window gating time sequence (as shown in Figure 2 In each transmission-reception cycle, the transmission optical path and the receiving optical path are strictly isolated in time: the first nanosecond optical path switch is opened and the second nanosecond optical path switch is closed during laser transmission, and the first nanosecond optical path switch is turned off immediately after the laser pulse is output; then the receiving second nanosecond optical path switch is opened in a very narrow time window expected to arrive the echo, and is turned off after a period of time.

[0030] Through this dynamic time gating mechanism, the interference of the detector caused by the leakage of the internal devices such as the fiber ring shaper and the near-field strong scattering light in the airborne optical path is physically isolated, and the signal-to-noise ratio and the effective dynamic range of the detector are greatly improved.

[0031] In the system workflow, the on-board galvanometer scanning component does not scan a large range of pixel arrays on the relay surface, but only selects predetermined four positioning points (as shown in Figure 3 The indirect echo signals of the target are obtained by sequentially emitting laser pulses to the four wall surface calibration positioning point positions. Each scanned positioning point corresponds to a time flight (ToF) measurement value of the non-visual light path formed by the hidden target and the detector, which limits the ellipsoidal distance constraint of the target relative to the scanning point.

[0032] The present application uses an ellipsoid intersection solving method to fuse the time sequence information from the four different calibration positioning points to solve the three-dimensional coordinate position of the hidden non-visual target. Compared with the existing scheme of densely scanning the entire wall surface or using low-resolution frequency modulation interference, the four-point scanning + ellipsoid positioning technology greatly reduces the data acquisition amount and calculation complexity, can efficiently and real-timely solve the position of the non-visual target, and provides higher resolution capability in a multi-target scene, avoids target confusion and loss.

[0033] In addition, the present application introduces a single-photon event level motion compensation algorithm to fuse high-frequency IMU inertial data to finely correct the motion of each returned photon event.

[0034] Specifically, the attitude change amount of the unmanned aerial vehicle within the nanosecond flight time from the laser emission moment to the echo photon detection moment is obtained by the IMU data pre-integration algorithm. The FPGA master control module associates each photon event with the unmanned aerial vehicle attitude at the emission time and the receiving time, and uses the attitude difference at the two time points to correct the ranging ray of the photon in space-time: the initial outgoing direction vector is determined according to the attitude at the emission time and the scanning galvanometer deflection angle, and the actual projection angle and distance of the light ray are corrected according to the change amount of the attitude at the receiving time relative to the geographic coordinate system. Thus, the point position offset caused by the unmanned aerial vehicle body shaking before correction can be accurately restored to a unified coordinate system, and point-by-point distortion elimination of each photon ranging point is realized.

[0035] Finally, the master control module relies on its powerful parallel computing capability to realize picosecond-level global time sequence synchronization, strictly synchronizes and aligns (error less than 1 ns) the laser emission, double-window gate switch, single-photon detection, IMU sampling and TDC timing, etc. multiple signals, and provides reliable time reference support for multi-sensor data fusion and real-time processing.

[0036] The present application proposes a new overall scheme in the aspects of non-visual imaging, target tracking, long-distance accurate positioning, high dynamic motion compensation, airborne system integration, etc., which can realize long-distance, high-precision and real-time tracking of the hidden target in the high dynamic environment of the unmanned aerial vehicle.

[0037] Referring to Figure 2, preferably, in the system, the master module controls the opening and closing of the first nanosecond optical switch and the second nanosecond optical switch in each transmission and reception cycle in the following manner: the master module sends a trigger signal to the laser to generate a laser pulse in each laser ranging phase, and controls the opening and closing of the first nanosecond optical switch and the second nanosecond optical switch in a double-window gating timing manner, so as to isolate the transmission optical path and the receiving optical path in time in the transmission phase and the receiving phase, comprising: At the beginning of each laser ranging phase in each transmission and reception cycle, the master module outputs a synchronization trigger signal to start the laser to emit a laser pulse with a predetermined time length and width; at the same time, the master module sends an opening instruction to the first nanosecond optical switch to switch the first nanosecond optical switch to a high-transmission state, so that the laser pulse passes through and enters the transmission optical path; and the second nanosecond optical switch is turned off to a low-transmission state to isolate the transmission optical path and the receiving optical path. Within a predetermined time length after the laser pulse leaves the laser emission port, the master module turns off the first nanosecond optical switch to a low-transmission state to block the residual light or stray light generated by the laser from entering the fiber ring.

[0038] Preferably, in the system, the master module controls the opening and closing of the second nanosecond optical switch in each transmission and reception cycle in the following manner: the master module controls the opening and closing of the second nanosecond optical switch in a double-window gating timing manner, comprising: In the echo receiving phase, the master module determines the receiving window according to the estimated distance or prior measurement value of the unmanned aerial vehicle and the non-visual target, and sends an instruction to open the second nanosecond optical switch according to the accurate delay time length determined by the receiving window, so that the second nanosecond optical switch becomes a high-transmission state within the preset receiving time window when the echo photons arrive, allowing the returned echo photons within the preset receiving time window to enter the photon detector; when the preset receiving time window ends, the master module immediately turns off the second nanosecond optical switch and reverts it to a low-transmission state.

[0039] Preferably, in the system, the master module adaptively adjusts the receiving window according to the change of the distance of the non-visual target during flight by using a sliding window strategy, comprising: The master module first fully opens the second nanosecond optical switch to find the approximate position of the suspected target signal, and then gradually adjusts the opening and closing time of the second nanosecond optical switch by a predetermined step length before and after the approximate position, until the strong target signal is tracked, and the opening value and the closing value of the second nanosecond optical switch are locked, that is, the adaptively adjusted receiving window is obtained.

[0040] By adaptively adjusting the delay and width of the receiving window through the sliding window strategy, it can be ensured that the echo photons from the target arrive exactly during the opening period of the photon detector, while most of the out-of-sync noise photons are rejected.

[0041] Preferably, in the system, the master module can time stamp the echo photons detected in each receiving window by a time-to-digital converter to record the time of flight of the echo photons relative to the time of laser triggering, and obtain the photon time of flight measurement value of the echo photons.

[0042] Preferably, in the system, the master module collects four photon time of flight measurement values in each transmission and receiving cycle in the following manner, including: At the beginning of each transmission and receiving cycle, the master module sends a trigger signal to the laser at a predetermined pulse repetition frequency, and simultaneously sends a synchronous start signal for the time-to-digital converter to record the time zero point; The laser emits a laser pulse every time it receives a trigger signal, which is coupled into the optical fiber via the opened first nanosecond optical path switch and is emitted in the direction of the predetermined positioning point of the first relay transmission surface by the scanning galvanometer assembly. After the pulse light is emitted, the master module immediately closes the first nanosecond optical path switch and opens the second nanosecond optical path switch after a set delay to open the receiving light path into the receiving window. If the echo photons reflected by the out-of-view target return during the receiving window period, they will be detected by the photon detector through the opened second nanosecond optical path switch. The electrical signal output by the photon detector is accurately timed by the time-to-digital converter to obtain the time of flight data corresponding to the echo photons as the photon time of flight measurement value. The master module adds a unified time label and a positioning point number to the photon time of flight measurement value. The positioning point number can be identified by the positioning point sequence number, such as the first positioning point identified as a positioning point number of 1, and so on.

[0043] The above process is repeated to obtain the photon time of flight measurement values of the echo photons corresponding to the other three predetermined positioning points.

[0044] In each transmission and receiving cycle, the master module obtains the UAV attitude data from the high-frequency inertial measurement unit in the following manner, including: In each transmission and receiving cycle, the high-frequency inertial measurement unit continuously outputs the three-axis angular velocity and acceleration data of the UAV at a high sampling rate of a predetermined frequency as IMU data. The master module performs real-time pre-integration operation on the IMU data obtained from the high-frequency inertial measurement unit to inverse the IMU data to the same nanosecond level as the laser. By aligning the time stamp recording the time of laser emission and the time of arrival of the corresponding echo photons provided by the time-to-digital converter with the attitude increment obtained by decomposing and solving the IMU data, the UAV attitude at the time of photon emission and the UAV attitude at the time of reception in each pulse light ranging are obtained.

[0045] Referring to Figure 4, preferably, in the system, the main control module corrects the four photon time-of-flight measurements by unmanned aerial vehicle attitude data in the following manner, comprising: regarding each photon ranging as an independent motion compensation unit, the ranging light of each photon is motion compensated, comprising: the initial emission direction of the pulsed light relative to the unmanned aerial vehicle coordinate system is determined by the unmanned aerial vehicle attitude at the emission time, and the precise position and direction of the light ray into the relay reflection surface are determined by combining the current deflection angle of the scanning galvanometer assembly; Then, the rotation and translation of the unmanned aerial vehicle coordinate system relative to the geographic coordinate system are determined by the unmanned aerial vehicle attitude at the receiving time, the spatial point coordinates obtained by the photon ranging are transformed from the unmanned aerial vehicle coordinate system to the geographic coordinate system, and the light path offset caused by the change of the unmanned aerial vehicle attitude between emission and reception is calculated according to the spatial point coordinates obtained by the photon ranging in the geographic coordinate system, and the real projection angle and length of the ranging light in the absolute coordinate system are obtained by inversion according to the light path offset, and the motion compensation of the ranging light is completed.

[0046] Preferably, in the system, the main control module uses the ellipsoid corresponding to the ranging of the four positioning points to accurately construct the intersection model in a unified coordinate system based on the data set of the corrected four photon time-of-flight measurements, and uses the ellipsoid intersection solving method to calculate the three-dimensional position of the non-visual area target; if multiple echo peaks of the detected echo photons are detected, the ellipsoid intersection solving method is used for multiple ellipsoid intersection solving, and the three-dimensional positions of multiple non-visual area targets are obtained to realize the positions of multiple non-visual area targets. Combined with the ranging and positioning results of each time, the continuous tracking of the motion of the non-visual area target is completed.

[0047] Preferably, in the system, the main control module is an FPGA main control module, which uses a high-speed digital logic device with internal clock synchronization accuracy reaching nanosecond level, and integrates high-speed ADC and DAC or high-speed serial interface; The first nanosecond-level optical path switch and the second nanosecond-level optical path switch use high-speed electro-optical modulators or optical fiber switches; The photon detector uses a single-photon detector or a face array detector composed of a multi-pixel single-photon detector array.

[0048] Preferably, in the system, the laser uses a single-pulse energy of several microjoules, a pulse width of 1.5 ns, a pulse repetition frequency of up to 1 MHz, and a wavelength of 1550 nm near-infrared pulsed fiber laser; The scanning galvanometer assembly uses any one of a two-dimensional scanning galvanometer assembly, a rotating polygon mirror assembly, a prism rotating optical scanner, and an optical phased array.

[0049] In summary, the target tracking system of the embodiment of the application has at least the following advantages: High-precision non-visual positioning and tracking: The ellipsoid intersection positioning method using four positioning points of the wall surface as a relay emitting surface significantly reduces the data sampling amount and improves the positioning resolution. The information of each positioning point cooperates with each other to achieve high-precision calculation of the hidden target position. Even in the case of multiple targets, different target positions can be distinguished through high-resolution time-series data, avoiding the problems of target confusion, misjudgment or loss in the existing continuous wave Fourier method. Experimental simulation shows that in a typical outdoor environment, the application can achieve sub-meter or even higher precision positioning of the hidden target, and the positioning precision is improved by more than one order of magnitude compared with the traditional method.

[0050] Signal-to-noise ratio and effective distance improvement: Through the dynamic light path gating of the double AOM controlled by the FPGA, the system physically suppresses the influence of direct light leakage and environmental stray light on the single-photon detector, and reduces the background noise count rate to close to the dark count level of the detector. Compared with the traditional suppression method which only relies on narrowband filtering or increases the laser power, the dynamic light leakage suppression mechanism of the application can still keep the detector unsaturated in strong sunlight background, and the signal-to-noise ratio is improved by several times to an order of magnitude, so that the effective detection distance is significantly expanded. In particular, the system still realizes a non-visual detection capability of more than 50 meters under the condition of weak laser power of 1550 nm, while the existing public technology generally cannot stably detect hidden targets more than 50 meters.

[0051] Motion distortion correction: The application adopts a single-photon event-level motion compensation algorithm based on high-frequency IMU to eliminate the point cloud distortion caused by the high-frequency and nonlinear jitter of the unmanned aerial vehicle platform at each photon event. The attitude information of each photon ranging point at the transmission and reception moment is fused through pre-integration to realize point-by-point coordinate correction, so that the point cloud accuracy is improved from the traditional uncorrected centimeter level to the micrometer level. The accuracy improvement is due to the accurate capture and compensation of the transient attitude change of the unmanned aerial vehicle, which ensures that the reconstructed point cloud is highly consistent with the real scene in the case of high-speed maneuvering, which is much better than the existing technology which relies on uniform speed interpolation correction.

[0052] Real-time performance and robustness: The FPGA main control module cooperates with the hardware-level global synchronization and parallel processing architecture of the field programmable gate array to ensure the strict timing cooperation of each module. Laser triggering, scanning, ranging and inertial data processing can run smoothly with sub-microsecond delay. Thanks to efficient data acquisition and fusion, the system can update the position of the hidden target in real time at several tens of hertz or even higher frame rate, realizing the immediate tracking of the moving target. This real-time performance is much better than the offline algorithm which needs a lot of post-processing, ensuring the detection and response ability of the unmanned aerial vehicle to the sudden target in the process of high-speed flight. At the same time, the multi-sensor fusion improves the robustness of the system in harsh conditions, and can still stably output high-quality positioning results in the face of factors such as severe vibration and wind disturbance.

[0053] High system integration and wide applicability: This invention ingeniously integrates single-photon lidar, inertial navigation, and non-line-of-sight imaging algorithms into a single unit, achieving a seamless combination of complex functions on a weight- and power-constrained UAV platform. Compared to solutions requiring large detection arrays or high-power lasers, this system boasts simple and efficient hardware: single-pixel SPAD detection combined with intelligent scanning strategies significantly reduces data processing pressure and equipment load, facilitating airborne deployment. Hardware-level synchronization and algorithm optimization ensure consistent system performance in various dynamic environments, providing strong technical support for UAVs to detect targets hiding behind obstacles in disaster search and rescue, urban security, and autonomous driving assistance applications, demonstrating broad-spectrum practical value.

[0054] To more clearly demonstrate the technical solution and its effects provided by the present invention, the following detailed description of the solution provided by the embodiments of the present invention is provided with reference to specific examples.

[0055] Example 1 This embodiment provides a single-photon non-line-of-sight target tracking system based on an unmanned aerial vehicle (UAV) platform. The system is installed on the UAV platform and is used to realize single-photon detection and tracking of long-distance non-line-of-sight targets.

[0056] like Figure 1 As shown, Figure 1 The schematic diagram of the optical path system and airborne processing unit of the airborne single-photon imaging system is shown. The system mainly includes: laser, first acousto-optic modulator (AOM_A) and second acousto-optic modulator (AOM_B), fiber optic circulator, fiber optic collimator, two-dimensional scanning galvanometer, single-photon detector (SPAD), time-to-digital converter (TDC), FPGA main control module and high-frequency inertial measurement unit (IMU). The connection relationship and working function of each component are as follows: The short laser pulse emitted by the laser is modulated by the first acousto-optic modulator and coupled into the fiber optic circulator. The fiber optic circulator guides the pulse to the two-dimensional scanning galvanometer, which then projects the pulse at a set angle onto the relay reflector surface (such as a building wall) in the scene in front of it.

[0057] The two-dimensional scanning galvanometer swings in the horizontal X-axis and vertical Y-axis directions, and sequentially projects laser pulses onto four pre-selected positioning points on the relay reflector surface to realize multi-positioning point scanning of the relay reflector surface to construct ellipsoidal positioning constraints. When the laser pulse irradiates the positioning point of the relay reflection surface, most of the light energy is diffusely reflected: a part of the light illuminates the non-visual target hidden behind the wall along the indirect path. The weak light signal reflected by the non-visual target returns to the relay reflection surface again and scatters, among which a small amount of photons as echo photons return along the original path into the two-dimensional scanning mirror and are coupled back into the fiber ring, guided to the second acousto-optic modulator of the receiving optical path; the second acousto-optic modulator is opened in a predetermined receiving time window, allowing the echo photons to pass through and eventually reach the single-photon detector. The single-photon detector works in Geiger mode, generates an electrical signal pulse for each incident photon event and outputs it, which is sent to the time-to-digital converter for time measurement to obtain the photon arrival timestamp data.

[0058] The FPGA master module is electrically connected with the above-mentioned components and can provide real-time control and data processing functions according to a unified time sequence: the FPGA master module sends a trigger signal to the laser to generate a laser pulse, controls the opening and closing of the first acousto-optic modulator and the second acousto-optic modulator according to a unified time sequence, acquires photon arrival timestamp data from the time-to-digital converter, and acquires high-frequency attitude measurement data from the high-frequency inertial measurement unit.

[0059] Through the above structure and connection, the modules work cooperatively under the control of the FPGA master module, realize the synchronous acquisition of single-photon laser ranging signals and the attitude of the unmanned aerial vehicle, and lay a foundation for subsequent non-visual positioning calculation and motion compensation.

[0060] As shown in Figure 2 The double-window dynamic light leakage suppression principle of the present application realizes time isolation of the transmitting optical path and the receiving optical path by strict time sequence control to shield the interference of strong background noise on detection. Figure 2 The jitter of the inertial navigation unit, the emission of the laser pulse, the nanosecond-level switching time sequence of the acousto-optic modulator A and the acousto-optic modulator B, and the echo photon receiving time sequence of the single-photon detector are shown in detail.

[0061] In the laser emission phase at the beginning of each laser ranging cycle, the FPGA master module first outputs a synchronization trigger pulse to start the laser to emit a laser pulse with a width of about 1.5 ns; almost at the same time, the FPGA master module sends an opening instruction to the first acousto-optic modulator to switch the first acousto-optic modulator to a high-transmission state, so that the laser pulse passes through smoothly and enters the transmitting optical path. Immediately after, within tens of nanoseconds after the pulse leaves the emission port, the FPGA master module quickly turns off the first acousto-optic modulator to a low-transmission (high-loss) state, thereby blocking the residual light or stray light that the laser may produce from entering the fiber ring. At the same time, the second acousto-optic modulator remains in the closed state to prevent the laser pulse from being directly coupled to the receiving end through the ring, causing the detector to be saturated.

[0062] Subsequently, the echo receiving phase is entered: according to the approximate distance of the unmanned aerial vehicle and the target or the prior measurement value, the receiving window is determined, and the master module sends a command to open the second acousto-optic modulator after a precise delay, so that it becomes a high-transmission state in the time window when the target echo is expected to arrive, allowing the photons returned in this period to enter the single-photon detector. Once the preset receiving window ends, the FPGA master module immediately turns off the second acousto-optic modulator and re-sets it to a low-transmission state. Through the above-mentioned "double window" gating mode, the direct return light at the moment of laser emission is effectively shielded, and the receiving light path remains closed for most of the time before and after the arrival of the echo signal, thereby significantly reducing the possibility of internal crosstalk and environmental stray light entering the single-photon detector.

[0063] The receiving window determined according to the approximate distance of the unmanned aerial vehicle and the target or the prior measurement value is determined in the following manner: the master module first fully opens the second acousto-optic modulator to find the approximate position of the suspected target signal, and then gradually adjusts the opening and closing time of the second acousto-optic modulator by a predetermined step size before and after the approximate position, until the strong target signal is tracked, the opening value and the closing value of the second nanosecond-level light path are locked, and the receiving window determined at this time is the receiving window determined according to the approximate distance of the unmanned aerial vehicle and the target or the prior measurement value.

[0064] The switching timing control precision of the FPGA master module on the first acousto-optic modulator and the second acousto-optic modulator can reach the nanosecond level, so that the receiving window only covers a very narrow time period (typically only a few tens of nanoseconds) when the expected signal arrives.

[0065] In addition, the system can also adaptively adjust the delay and width of the receiving window according to the dynamic changes of the target distance during flight, using the "sliding window" strategy to ensure that the signal photons from the target arrive exactly during the opening period of the detector, while most of the out-of-sync noise photons are rejected. The time-to-digital converter cooperates with the logic of the FPGA master module to time stamp the photon events detected in each receiving window, recording the time of flight of the photon relative to the reference trigger.

[0066] Thanks to the above-mentioned dynamic light leakage suppression design, even in a strong sunlight background or a complex reflection environment on board, the single-photon detector of the system of the present embodiment is still not easy to saturate, and can still maintain a very high signal-to-noise ratio and sensitivity. For example, the double-window gating can keep the single-photon detector in a closed state for hundreds of nanoseconds after the emission pulse, and cooperate with the narrowband filtering and time selection strategy to reduce the environmental noise count to the same order of magnitude as the dark count of the single-photon detector, thereby ensuring accurate time-of-flight measurement and reliable detection of weak signals at a long distance from the source.

[0067] As Figure 3 and Figure 4As shown, the single-photon event level motion compensation and non-visual target positioning algorithm proposed in the application fully combines high-frequency inertial measurement and space-time geometric constraint modeling to realize attitude correlation and spatial correction of each photon ranging data. Figure 3 It is shown how the laser pulse completes information transmission through three times of diffuse reflection between the reflecting wall, the non-visual target and the unmanned aerial vehicle platform, and the strategy of four-point scanning on the reflecting wall is illustrated. Figure 4 The complete algorithm chain from raw data acquisition to final target position output is elaborated in the form of a flowchart, including IMU pre-integration, positioning point position information correction, ToF data correction, non-visual target position solution based on ellipsoid intersection, and non-visual point cloud processing algorithm.

[0068] During system operation, the high-frequency inertial measurement unit continuously outputs three-axis angular velocity and acceleration data of the unmanned aerial vehicle at a high sampling rate of 1 kHz. The FPGA master control module or the processing unit connected thereto performs real-time pre-integration operation on the IMU data, inverts the IMU data to the same nanosecond level as the laser, and calculates the relative attitude change of the unmanned aerial vehicle between any two time points.

[0069] For each laser pulse ranging event, the timestamp data provided by the time-to-digital converter records the laser emission time and the corresponding single-photon echo arrival time. By aligning the attitude increment obtained by decomposing the IMU data with the two timestamp data, the unmanned aerial vehicle attitude at the emission time and the unmanned aerial vehicle attitude (heading angle, roll angle, pitch angle) at the reception time and the relative displacement of the photon event can be obtained. Since the laser flight time is only tens to hundreds of nanoseconds, even if the attitude of the unmanned aerial vehicle changes slightly during this period, the uncorrected reconstruction result may still have a slight deviation.

[0070] Therefore, the application regards each photon ranging as an independent motion compensation unit: first, the initial emission direction of the laser pulse relative to the body coordinate system is determined using the unmanned aerial vehicle attitude at the emission time, and the precise position and direction of the laser pulse entering the relay reflecting surface are determined in combination with the current deflection angle of the two-dimensional scanning galvanometer. Then, the rotation and translation of the unmanned aerial vehicle coordinate system relative to the geographical reference system are determined using the unmanned aerial vehicle attitude at the reception time, the spatial point coordinates obtained by the photon ranging are transformed from the body coordinate system to the geographical coordinate system, and the light path offset caused by the attitude change of the unmanned aerial vehicle between emission and reception is calculated accordingly.

[0071] In short, for each photon event, the real projection angle and length of the light ray in the absolute coordinate system are inversely calculated by the above motion compensation algorithm according to the attitude difference between the emission time and the receiving time, so as to completely remove the point distortion caused by the motion of the unmanned aerial vehicle. The point-by-point correction method effectively eliminates the cumulative error of nonlinear jitter, so that the onboard point cloud can still be accurately aligned even under severe maneuvering conditions.

[0072] In addition, in the indirect ranging scene unique to non-visual imaging, the position error of the relay reflection surface positioning point is calibrated and compensated by the above motion compensation algorithm, and the system time delay deviation calibrated in advance is combined to ensure that the ranging ellipsoid constraint corresponding to the four positioning points can accurately construct the intersection model in the unified coordinate system. Based on the corrected multi-point ToF data set, the system calculates the three-dimensional position of the hidden target by solving the ellipsoid intersection; if multiple photon echo peaks are detected, multiple ellipsoid intersection solving can be performed respectively to locate the positions of multiple hidden targets. Combined with the results of the previous scans, the filtering and trajectory calculation algorithm is further used to realize continuous tracking of the target motion.

[0073] The system time delay deviation is calibrated as follows: Emission photon time jitter: keep the emission light path unchanged, the emission time is a uniform period signal t1 of the field programmable gate array, a single photon detector is connected outside the optical fiber collimator, the receiving time t2 of the received photon is recorded, and the difference is the delay of the emission light path and the emission time jitter delay of the emission photon; Receiving photon receiving time jitter: keep the system receiving light path unchanged, connect a laser at the optical fiber collimator, the laser emission time has a time t3, and the single photon detector at the receiving place receives the photon time t4, and the time difference obtained is the delay of the receiving light path and the receiving photon time jitter delay.

[0074] The specific working process of the system of the embodiment includes the following stages. First, system initialization and synchronization: start the FPGA master module and each sensor module, reset the first acousto-optic modulator and the second acousto-optic modulator to the initial closed state, and the laser enters standby preparation; the FPGA master module generates a standard time pulse, aligns the time bases of all components of the system to a unified absolute time through time synchronization, and performs normalization reference calibration on the data time axis of the time-to-digital converter and the high-frequency inertial measurement unit.

[0075] Subsequently, a ranging operation cycle is entered: the FPGA master module sends a trigger signal to the laser at a predetermined pulse repetition frequency (for example: 1 MHz), while sending a synchronous start signal for the time-to-digital converter to record the time zero point. The laser emits a narrow laser pulse every time it receives a trigger, which is coupled into the optical fiber through the open first acousto-optic modulator and is emitted in the direction of the positioning point of the first relay emission surface (wall surface) by the two-dimensional scanning galvanometer. After the laser pulse is emitted, the FPGA master module immediately turns off the first acousto-optic modulator and turns on the second acousto-optic modulator after a set delay to enter the receiving window. If the photons reflected by the hidden target return within the window period, they will be detected through the second acousto-optic modulator that has been turned on and enter the single-photon detector; the electrical signal output by the single-photon detector is accurately timed by the time-to-digital converter, and the flight time data corresponding to the photons are obtained, and the FPGA master module simultaneously attaches a unified time label and positioning point number to the ranging event.

[0076] Throughout the process, the high-frequency inertial measurement unit continuously outputs high-frequency inertial data, and the FPGA master module synchronously receives and buffers the high-frequency inertial data stream. After one laser emission-reception process is completed, the FPGA master module calls the high-frequency inertial measurement unit IMU data in the corresponding time period to perform pre-integration calculation, and obtains the UAV attitude at the emission time and the UAV attitude at the reception time of the ranging, which are used for subsequent distortion correction and coordinate transformation.

[0077] Next, the two-dimensional scanning galvanometer quickly switches to the direction of the next positioning point of the relay reflection surface, and the above laser emission, gated reception and data recording process is repeated. After the measurement of the four positioning points is sequentially completed in this way, a complete scanning period is formed.

[0078] The FPGA master module collects the four ToF measurement values (i.e. photon flight time measurement values) in the period, and corresponding positioning point positions and UAV attitude data, and executes an ellipsoid intersection solving algorithm to calculate the three-dimensional coordinates of the current hidden non-visual target. The system continuously cycles the above scanning measurement and positioning calculation process to update the target position in real time.

[0079] Specifically, the processing flow of the ellipsoid intersection solving algorithm is as follows: Step 1, scan four positioning points, first extract the flight times one of the photons from the UAV to the four positioning points of the reflection relay surface in the photon histogram collected by the single-photon detector, respectively denoted as t11, t21, t31, t41; Step 2, then extract the flight times two of the photons from the four positioning points to the non-visual target from the photon histogram, respectively denoted as t10, t20, t30, t40; Step 3, according to the time of flight one extracted in step 1 and the time of flight two extracted in step 2, the distance from the reflection relay surface to the non-visual target is calculated: taking the distance of the first positioning point as an example, the calculation method is: (t10-t11x2) ÷2, recorded as T1; other positioning points are calculated in this way, respectively recorded as T2, T3, T4; Step 4, convert the time of flight into distance, L1=cxT1, c is the speed of light, get L1, L2, L3, L4; Respectively, take the four positioning points as the center, and L1, L2, L3, L4 as the radius to construct the ellipsoid intersection model, and the intersection point is the position of the non-visual target.

[0080] If the target moves, the continuous positioning results will form the target trajectory output; if no effective echo is detected in a certain scan, the system can predict the possible position of the target through the historical trajectory and adjust the subsequent scanning strategy to improve the probability of target recapture. During the whole running process, the FPGA master module coordinates the laser trigger, double window gating, galvanometer scanning, high frequency inertial measurement unit sampling and time digital converter timing, which strictly follows the unified timing reference, and the typical synchronization accuracy is better than 1 ns, which ensures that the multi-sensor data fusion error is reduced to be negligible.

[0081] In the data processing stage, first, basic geometric calculation and coordinate transformation are carried out: according to the time of flight recorded by the time digital converter, the total path length corresponding to each photon is calculated, and then combined with the known coordinates of the wall positioning point to which the photon belongs and the attitude of the unmanned aerial vehicle at the time of emission and reception, the preliminary position of the hidden target relative to the geographic coordinate system is solved through geometric relationship.

[0082] Then, point cloud distortion correction and registration are performed: the aforementioned single-photon event level motion compensation algorithm is applied to correct the spatial coordinates of all collected photon events, eliminate the position deviation caused by the motion of the unmanned aerial vehicle, and map each point to the same coordinate system.

[0083] For repeated points obtained in multiple scanning periods, graph optimization or Kalman filtering method is used to smooth estimate the unmanned aerial vehicle trajectory and target position, so as to improve the positioning accuracy and stability. Then, the echo data is filtered and denoised: due to the existence of noise trigger and multi-photon false image with a certain probability in single-photon detection, the system uses histogram statistics and clustering analysis method to screen effective signals, for example, the echo with the same distance repeatedly appearing at a certain spatial position in multiple scans is judged as a real target, and the isolated random trigger point is regarded as noise and removed.

[0084] Finally, the filtered non-visual three-dimensional measurement results are output: according to application requirements, the system can only output the real-time three-dimensional coordinates and motion trajectory of the target, or project the continuous point cloud into the existing map model for situation awareness; at the same time, if sufficient echo counts are counted, the reflection intensity information of the target can also be calculated, and the target reflection intensity is marked with pseudo-color to enhance the interpretation effect of the display.

[0085] Examples of key hardware parameters: In order to verify the feasibility of the present application, high-performance components are selected for each key device in this embodiment, and parameter optimization settings are made for single-photon detection requirements.

[0086] The laser preferably uses a 1550nm near-infrared pulsed fiber laser, with single-pulse energy on the order of several microjoules, pulse width of about 1.5ns, and pulse repetition frequency up to 1MHz, to ensure high sensitivity and sufficient sampling rate required for long-range detection.

[0087] AOM_A and AOM_B use high-speed acousto-optic modulators, with rising / falling edge response time not exceeding tens of nanoseconds, on-state insertion loss less than 3dB, and off-state extinction ratio higher than 50dB, to ensure that the gating window can be quickly switched and effectively isolate direct strong light.

[0088] The time-to-digital converter (TDC) has a picosecond-level time resolution, corresponding to a distance resolution capability of about millimeter level, providing accurate time-of-flight measurement in single-photon ranging.

[0089] The high-frequency IMU preferably combines a high-performance three-axis gyroscope and an accelerometer, with a sampling frequency not less than 1kHz, a small angle random walk coefficient, and a dynamic range covering the possible violent maneuvers of the UAV, to ensure that stable and accurate attitude data can still be output under high dynamic conditions.

[0090] The scanning field of view range of the two-dimensional scanning galvanometer assembly can be up to ±15° horizontally and ±10° vertically, for example, with a mechanical bandwidth of hundreds of hertz, capable of meeting the requirements of large-scale fast scanning of the forward sector of a small UAV.

[0091] The FPGA master control module uses high-speed digital logic devices, with internal clock synchronization accuracy reaching nanosecond level, and integrates high-speed ADC / DAC or high-speed serial interface for control and data acquisition of various sensors.

[0092] Through the above device configuration and parameter selection, the system of the present application can still ensure the accuracy, spatial resolution and overall reliability of single-photon laser ranging even in the environment of high-speed flight and violent shaking of the UAV, providing strong hardware support for non-visual long-range detection.

[0093] Optional solutions and variant examples: The main preferred embodiments of the application are described above in combination with the drawings, but the application is not limited to the specific structures and processes described above. In other variations, the modules and algorithms can be replaced or functionally extended according to actual application requirements.

[0094] The AOM_A and AOM_B can be replaced with high-speed electro-optical modulators or fiber switches as long as they have nanosecond-level optical switching capability; the single-photon detector is not limited to a single SPAD device, but can also use a multi-pixel SPAD array to form a surface array receiver to improve spatial coverage (at this time, the complexity of the scanning galvanometer can be correspondingly reduced); the scanning galvanometer assembly can be replaced with other types of scanning galvanometer assemblies, such as a rotating polygon mirror, a prism rotating optical scanner, or an optical phased array, to adapt to different platform sizes and power requirements.

[0095] In terms of motion compensation and positioning algorithms, different implementation methods can be selected according to real-time and accuracy requirements: for offline high-precision data processing, a factor graph global optimization method can be used to obtain the highest accuracy trajectory estimation and point cloud correction result; for real-time application environment, a tightly coupled solution based on an extended Kalman filter can be used to correct the ranging result of each laser point using IMU increments in real time to reduce calculation delay.

[0096] In addition, the system scheme of the application can also be extended to fuse SLAM (simultaneous localization and mapping) technology, relying only on IMU and laser point cloud features for autonomous positioning and map updating, which is also one of the potential applications of the application.

[0097] The application has at least the following advantages, including: (1) High-precision non-visual area positioning: four positioning points are scanned on the relay reflection surface, and the method of solving the non-visual area target position by the intersection of ellipsoids is used to greatly reduce the data volume and improve the positioning efficiency.

[0098] (2) Single-photon event-level motion correction: a single-photon event-level motion compensation algorithm based on high-frequency IMU data pre-integration is used to accurately correct the point cloud distortion caused by high dynamic attitude changes of the unmanned aerial vehicle, and the distortion correction accuracy is improved from centimeter level to micrometer level.

[0099] (3) Dynamic light leakage suppression: through the dynamic time-gating mechanism of the double acousto-optic modulator (AOM) controlled by the FPGA, the internal light leakage and near-field backscattering of the system are physically suppressed, and the signal-to-noise ratio of the echo signal is significantly improved.

[0100] (4) High-precision timing synchronization: using the parallel processing capability of the FPGA, picosecond-level hardware-level global timing synchronization can be achieved, providing a physical basis for seamless fusion and real-time processing of multi-sensor data.

[0101] Those skilled in the art can understand that all or part of the processes in the above-mentioned embodiment methods can be completed by programs instructing relevant hardware, and the programs can be stored in a computer readable storage medium. When the programs are executed, the processes of the above-mentioned embodiment methods can be included. The storage medium can be a magnetic disc, an optical disc, a Read-Only Memory (ROM) or a Random Access Memory (RAM) and the like.

[0102] The above describes only the preferred embodiments of the present application, but the protection scope of the present application is not limited to this. Any person skilled in the art can easily think of changes or replacements within the technical scope disclosed by the present application, which should be covered by the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims. The information disclosed in the background section of the present application is only intended to deepen the understanding of the general background of the present application, and should not be regarded as acknowledging or implying in any form that the information constitutes the prior art known by those skilled in the art.

Claims

1. A single-photon non-line-of-sight target tracking system based on an unmanned aerial vehicle (UAV) platform, characterized in that, include: The system includes a laser, a first nanosecond-level optical switch, a fiber optic circulator, a fiber optic collimator, a scanning galvanometer assembly, a second nanosecond-level optical switch, a photon detector, a time-to-digital converter, a main control module, a high-frequency inertial measurement unit, and a field-programmable gate array (FPGA). The laser is connected to the scanning galvanometer assembly in sequence via a first nanosecond-level optical path switch, an optical fiber circulator, and an optical fiber collimator, forming an emission optical path that can emit laser pulses at a set angle to the relay reflector surfaces corresponding to four predetermined positioning points in the scene ahead. The scanning galvanometer assembly is connected to the photon detector in sequence via a collimating fiber, a fiber optic circulator, and a second nanosecond-level optical path switch, forming a receiving optical path that can receive echo photons scattered by the relay reflector surfaces corresponding to each positioning point after being reflected by a non-line-of-sight target. The main control module is electrically connected to the laser, the first nanosecond-level optical path switch, the second nanosecond-level optical path switch, the scanning galvanometer assembly, the photon detector, the time-to-digital converter, and the high-frequency inertial measurement unit. It can control each component to perform laser ranging and target positioning in a unified timing sequence according to the transmission and reception cycle. In each transmission and reception cycle, it sends a trigger signal to the laser to generate a laser pulse. It controls the opening and closing of the first nanosecond-level optical path switch and the second nanosecond-level optical path switch with dual-window gating timing. It isolates the transmission optical path and the reception optical path according to time during the transmission and reception phases. Based on the four photon flight time measurements, the corresponding four positioning point positions, and the UAV attitude data obtained by the high-frequency inertial measurement unit in each transmission and reception cycle, it performs motion compensation correction on the four photon flight time measurements and the corresponding four positioning point positions using the UAV attitude data. Using the corrected four photon flight time measurements and the corresponding four positioning point positions, it calculates the three-dimensional coordinates of the current non-line-of-sight target using the ellipsoidal intersection solution method, thereby realizing non-line-of-sight target tracking.

2. The single-photon non-line-of-sight target tracking system based on an unmanned aerial vehicle platform according to claim 1, characterized in that, The main control module generates laser pulses by sending trigger signals to the laser during each transmission and reception cycle in the following manner: It controls the opening and closing of the first nanosecond-level optical path switch and the second nanosecond-level optical path switch using a dual-window gating timing sequence; and it isolates the transmission optical path and the reception optical path by time during the transmission and reception phases, including: At the beginning of each laser ranging phase within each transmission and reception cycle, the main control module outputs a synchronous trigger signal to start the laser to emit a laser pulse of a predetermined duration and width; simultaneously, the main control module sends an opening command to the first nanosecond-level optical path switch, switching the first nanosecond-level optical path switch to a high-transmission state, allowing the laser pulse to pass through and enter the transmission optical path; and turns off the second nanosecond-level optical path to a low-transmission state, isolating the transmission optical path from the reception optical path; Within a predetermined time after the laser pulse leaves the laser emitter port, the main control module turns off the first nanosecond-level optical path switch to a low-transmission state, blocking the residual light or stray light generated by the laser from entering the fiber optic circulator.

3. The single-photon non-line-of-sight target tracking system based on an unmanned aerial vehicle platform according to claim 2, characterized in that, The main control module controls the opening and closing of the second nanosecond-level optical path switch using a dual-window gating timing sequence during each transmit and receive cycle, including: During the echo reception phase, the main control module determines the reception window based on the estimated distance or prior measurement value between the UAV and the non-visual target. According to the precise delay duration determined by the reception window, it issues a command to activate the second nanosecond-level optical path switch, so that the second nanosecond-level optical path becomes a high-transmittance state within the preset reception time window when the echo photons arrive, allowing the echo photons returning within the preset reception time window to enter the photon detector; when the preset reception time window ends, the main control module immediately shuts off the second nanosecond-level optical path switch and resets it to a low-transmittance state.

4. The single-photon non-line-of-sight target tracking system based on an unmanned aerial vehicle platform according to claim 3, characterized in that, The main control module adaptively adjusts the receiving window using a sliding window strategy based on changes in the distance to non-line-of-sight targets during flight, including: The main control module first fully opens the second nanosecond-level optical path switch to find the approximate location of the suspected target signal. Then, it gradually slides and adjusts the opening and closing time of the second nanosecond-level optical path switch in predetermined steps before and after the approximate location until a strong target signal is tracked. Finally, it locks the opening and closing values ​​of the second nanosecond-level optical path switch to obtain the adaptively adjusted receiving window.

5. The single-photon non-line-of-sight target tracking system based on an unmanned aerial vehicle platform according to claim 3, characterized in that, The main control module can timestamp the echo photons detected within each receiving window using a time-to-digital converter, record the flight time of the echo photons relative to the laser triggering time, and obtain the photon flight time measurement value of the echo photons.

6. The single-photon non-line-of-sight target tracking system based on an unmanned aerial vehicle platform according to claim 3, characterized in that, The main control module acquires four photon time-of-flight measurements in each transmit and receive cycle in the following manner: At the start of each transmit and receive cycle, the main control module sends a trigger signal to the laser at a predetermined pulse repetition frequency, and at the same time sends a synchronization start signal for the time-to-digital converter to record the zero point of time. Each time the laser receives a trigger signal, it emits a laser pulse, which is coupled into the optical fiber through the first nanosecond-level optical path switch and emitted by the scanning galvanometer assembly towards the predetermined positioning point of the first relay transmitting surface. After the pulse light is emitted, the main control module immediately turns off the first nanosecond-level optical path switch and, after a set delay, turns on the second nanosecond-level optical path switch to open the receiving optical path and enter the receiving window. If an echo photon reflected from a non-visual target returns during the receiving window, it enters the photon detector through the activated second nanosecond-level optical path switch and is detected. The electrical signal output by the photon detector is precisely timed by a time-to-digital converter to obtain the flight time data corresponding to the echo photon as the photon flight time measurement value. The main control module adds a unified time tag and positioning point number to the photon flight time measurement value. Repeat the above process to obtain the photon flight time measurements of the echo photons corresponding to the other three predetermined positioning points in turn. The main control module acquires UAV attitude data via a high-frequency inertial measurement unit in each transmit and receive cycle in the following manner, including: During each transmit and receive cycle, the high-frequency inertial measurement unit continuously outputs the three-axis angular velocity and acceleration data of the UAV as IMU data at a high sampling rate at a predetermined frequency; The main control module performs real-time pre-integration calculations on the IMU data acquired by the high-frequency inertial measurement unit, and inverts the IMU data to the same nanosecond level as the laser. By aligning the attitude increment obtained by decomposing and solving the IMU data with the timestamps provided by the time-to-digital converter to record the laser emission time and the corresponding echo photon arrival time, the UAV attitude at the photon emission time and the UAV attitude at the reception time are obtained in each pulse light ranging.

7. The single-photon non-line-of-sight target tracking system based on an unmanned aerial vehicle platform according to claim 6, characterized in that, The main control module performs motion compensation correction on the four photon flight time measurements using UAV attitude data in the following manner: Each photon ranging is treated as an independent motion compensation unit. Motion compensation processing is performed on the ranging ray of each photon to obtain the corrected photon time-of-flight measurement value, including: The initial emission direction of the pulse light relative to the UAV coordinate system is determined by using the UAV attitude at the moment of launch, and then the precise position and direction of the light entering the relay reflector is determined by combining the current deflection angle of the scanning galvanometer assembly. Then, the rotation and translation of the UAV coordinate system relative to the geographic coordinate system are determined by the UAV attitude at the time of reception. The coordinates of the spatial point obtained by photon ranging are transformed from the UAV coordinate system to the geographic coordinate system. Based on the coordinates of the spatial point obtained by photon ranging in the geographic coordinate system, the optical path offset caused by the change in UAV attitude between transmission and reception is calculated. Based on the optical path offset, the true projection angle and length of the ranging ray in the absolute coordinate system are obtained, thus completing the motion compensation of the ranging ray.

8. The single-photon non-line-of-sight target tracking system based on an unmanned aerial vehicle platform according to claim 7, characterized in that, The main control module, based on the data set of the four corrected photon time-of-flight measurements, accurately constructs an intersection model in a unified coordinate system using ellipsoidal constraints corresponding to the distance measurements of the four positioning points. It then uses the ellipsoidal intersection solution method to calculate the three-dimensional position of the non-line-of-sight target. If the echo peaks of multiple echo photons are detected, the ellipsoidal intersection solution method is used to solve the problem multiple times to obtain the three-dimensional position of the multiple non-line-of-sight targets, thus realizing the position of the multiple non-line-of-sight targets. By combining the results of previous ranging and positioning measurements, continuous tracking of the movement of non-line-of-sight targets can be achieved.

9. The single-photon non-line-of-sight target tracking system based on an unmanned aerial vehicle platform according to any one of claims 1-8, characterized in that, The main control module adopts an FPGA main control module, which uses high-speed digital logic devices with internal clock synchronization accuracy at the nanosecond level, and integrates high-speed ADC and DAC or high-speed serial interface. The first nanosecond-level optical switch and the second nanosecond-level optical switch are high-speed electro-optic modulators or fiber optic switches; The photon detector is an area array detector composed of a single photon detector or a multi-pixel single photon detector array.

10. The single-photon non-line-of-sight target tracking system based on an unmanned aerial vehicle platform according to any one of claims 1-8, characterized in that, The laser is a near-infrared pulsed fiber laser with a single-pulse energy in the microjoule range, a pulse width of 1.5 ns, a maximum pulse repetition frequency of 1 MHz, and a wavelength of 1550 nm. The scanning galvanometer assembly can be any one of a two-dimensional scanning galvanometer assembly, a rotating multiprism assembly, a prism rotating optical scanner, or an optical phased array.

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