Intelligent dynamic switching method and system for communication link based on beidou and low earth orbit satellite

Abstract

The embodiment of the application provides a communication link intelligent dynamic switching method and system based on Beidou and low-orbit satellites, relates to the technical field of satellite communication, and first receives a Beidou satellite signal frame structure set and a low-orbit beacon beam scanning sequence, carries out cross-system time reference unification processing to generate a time synchronization offset set, then constructs an interactive trigger point positioning mapping table, which contains an interactive satellite combination identifier and a beam coverage overlapping area coordinate range, when a user terminal link switching request instruction is received, the mapping table is called to match trigger point coordinates, a link dynamic switching relay instruction is generated and sent to a user terminal baseband processing unit, and cross-satellite system signal acquisition and synchronization establishment operations are triggered to be performed. The application improves the accuracy and reliability of satellite communication link switching.

Description

Technical Field

[0001] This invention relates to the field of satellite communication technology, and more specifically, to a method and system for intelligent dynamic switching of communication links between BeiDou and low-orbit satellites. Background Technology

[0002] In the field of satellite communications, with the increasing diversification and complexity of service demands, a single satellite system often struggles to meet users' requirements in terms of communication coverage, communication quality, data transmission rate, and other aspects. Therefore, cross-satellite system communication link switching has become one of the key technologies for improving the performance of satellite communication services.

[0003] Currently, common satellite communication systems mainly include the BeiDou Navigation Satellite System and low-Earth orbit (LEO) satellite communication systems. The BeiDou Navigation Satellite System boasts advantages such as global coverage, high positioning accuracy, and precise timing, playing a crucial role in navigation, positioning, and time synchronization. LEO satellite communication systems, on the other hand, feature low propagation delay and low link loss, making them suitable for providing high-speed data communication services. However, existing methods for switching communication links across satellite systems have several shortcomings. Firstly, the time bases of different satellite systems differ, making precise time synchronization difficult and prone to signal loss or communication interruptions during link switching. Secondly, the lack of an effective interactive trigger point positioning mechanism makes it impossible to accurately determine when and where to switch between satellite systems, resulting in a lack of flexibility and accuracy in the switching process, failing to meet users' needs for real-time and reliable communication. Summary of the Invention

[0004] In view of this, the purpose of this invention is to provide a method and system for intelligent dynamic switching of communication links between BeiDou and low-orbit satellites.

[0005] According to one aspect of the present invention, a method for intelligent dynamic switching of communication links based on BeiDou and low-Earth orbit satellites is provided, the method comprising: Receive a set of BeiDou signal frame structures continuously broadcast by the BeiDou satellite navigation system. The set of BeiDou signal frame structures includes navigation message data units with frame count sequence numbers and synchronization header timing distribution parameters corresponding to the navigation message data units. Synchronously capture the low-Earth orbit beacon beam scanning sequence periodically broadcast by the low-Earth orbit satellite communication system. The low-Earth orbit beacon beam scanning sequence includes a set of arrival timestamps of beacon signals arriving at the ground terminal and a beam pointing angle mapping relationship corresponding to the set of arrival timestamps. The synchronization header timing distribution parameters of the BeiDou signal frame structure set and the arrival timestamp set of the low-orbit beacon beam scanning sequence are subjected to cross-system time reference unification processing to generate a time synchronization offset set with the local clock of the ground terminal as the reference system. Based on the time synchronization offset set and the beam pointing angle mapping relationship, an interaction trigger point positioning mapping table between Beidou satellites and low-orbit satellites is constructed. The interaction trigger point positioning mapping table includes the interactive satellite combination identifier corresponding to each time synchronization offset and the coordinate range of the beam coverage overlap area associated with the interactive satellite combination identifier. When a link switching request command containing the current serving satellite type identifier and the target satellite type identifier is received from a user terminal, the interactive trigger point positioning mapping table is retrieved. Based on the current serving satellite type identifier and the target satellite type identifier, the corresponding interactive satellite combination identifier is matched. The trigger point coordinates matching the current location coordinates of the user terminal are extracted from the beam coverage overlap area coordinate range corresponding to the interactive satellite combination identifier. A link dynamic switching relay command containing the trigger point coordinates and the corresponding time parameter in the time synchronization offset set is generated. The link dynamic switching relay command is sent to the user terminal baseband processing unit, triggering the user terminal baseband processing unit to perform cross-satellite system signal acquisition and synchronization establishment operations based on the trigger point coordinates and the time parameter.

[0006] According to another aspect of the present invention, a smart dynamic switching system for communication links based on BeiDou and low-Earth orbit satellites is provided, comprising: a processor, a communication interface, a memory, and a communication bus, wherein the processor, the communication interface, and the memory communicate with each other through the communication bus; the memory is used to store a computer program; and the processor is used to execute the computer program to implement the steps of the smart dynamic switching method for communication links based on BeiDou and low-Earth orbit satellites as described above.

[0007] According to another aspect of the present invention, a readable storage medium is provided, on which a computer program is stored, which, when executed by a processor, can perform the steps of the above-described method for intelligent dynamic switching of communication links based on BeiDou and low-orbit satellites.

[0008] By receiving the signal frame structure set of the BeiDou Navigation Satellite System and the beacon beam scanning sequence of the Low Earth Orbit (LEO) satellite communication system through any of the above methods, and performing cross-system time reference unification processing, a set of time synchronization offsets with the local clock of the ground terminal as the reference system can be accurately generated. This effectively solves the problem of inconsistent time references between different satellite systems. Based on the time synchronization offset set and the beam pointing angle mapping relationship, an interactive trigger point positioning mapping table is constructed, achieving precise positioning of the interactive trigger point between BeiDou satellites and LEO satellites. It can quickly match suitable interactive satellite combination identifiers and trigger point coordinates according to the service satellite type and target satellite type of the user terminal, improving the flexibility and accuracy of link switching. When a link switching request command is received from the user terminal, a dynamic link switching relay command containing trigger point coordinates and time parameters can be generated, triggering the user terminal baseband processing unit to perform cross-satellite system signal acquisition and synchronization establishment operations. This ensures the smoothness and reliability of the link switching process, greatly improving the quality of satellite communication services and user experience.

[0009] To make the above-mentioned objects, features and advantages of the embodiments of the present invention more apparent and understandable, a detailed description will be given below in conjunction with the embodiments and the accompanying drawings. Attached Figure Description

[0010] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0011] Figure 1 A schematic diagram of the components of the intelligent dynamic switching system for communication links based on BeiDou and low-orbit satellites provided in an embodiment of the present invention is shown. Figure 2 The diagram shows a flowchart of the intelligent dynamic switching method for communication links based on BeiDou and low-orbit satellites provided in an embodiment of the present invention. Detailed Implementation

[0012] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0013] The terms “first,” “second,” “third,” etc. (if present) in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a particular order or sequence. It should be understood that such data can be interchanged where appropriate so that embodiments of the invention described herein can be implemented, for example, in orders other than those illustrated or described herein. Furthermore, the terms “comprising” and “having,” and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0014] Figure 1 A schematic diagram of exemplary components of a BeiDou-based and low-Earth orbit satellite-based intelligent dynamic handover system 100 is shown. The BeiDou-based and low-Earth orbit satellite-based intelligent dynamic handover system 100 may include one or more processors 104, such as one or more central processing units (CPUs), each of which may implement one or more hardware threads. The BeiDou-based and low-Earth orbit satellite-based intelligent dynamic handover system 100 may also include any storage medium 106 for storing any kind of information such as code, settings, data, etc. Non-limitingly, for example, the storage medium 106 may include any type of RAM, any type of ROM, flash memory device, hard disk, optical disk, etc. More generally, any storage medium can use any technology to store information. Furthermore, any storage medium can provide volatile or non-volatile retention of information. Furthermore, any storage medium may represent a fixed or removable component of the BeiDou-based and low-Earth orbit satellite-based intelligent dynamic handover system 100. In one scenario, when processor 104 executes dependent instructions stored in any storage medium or combination of storage media, the BeiDou-Low Earth Orbit (LEO) satellite-based intelligent dynamic switching system 100 can execute any operation of the associated instructions. The BeiDou-LEO satellite-based intelligent dynamic switching system 100 also includes one or more drive units 108 for interacting with any storage medium, such as hard disk drive units, optical disk drive units, etc.

[0015] The intelligent dynamic switching system 100 for communication links between BeiDou and low-Earth orbit satellites also includes input / output (I / O) 110 for receiving various inputs (via input unit 112) and providing various outputs (via output unit 114). A specific output mechanism may include a presentation device 116 and a dependent graphical user interface (GUI) 118. The intelligent dynamic switching system 100 for communication links between BeiDou and low-Earth orbit satellites may also include one or more network interfaces 120 for exchanging data with other devices via one or more communication units 122. One or more communication buses 124 couple the components described above together.

[0016] The communication unit 122 can be implemented in any way, such as via a local area network, a wide area network (e.g., the Internet), a point-to-point connection, or any combination thereof. The communication unit 122 may include any combination of hardwired links, wireless links, routers, gateway functions, or a communication link intelligent dynamic switching system 100 based on BeiDou and low-Earth orbit satellites, governed by any protocol or combination of protocols.

[0017] Figure 2 This diagram illustrates a flowchart of a method and system for intelligent dynamic switching of communication links based on BeiDou and low-Earth orbit satellites, provided by an embodiment of the present invention. This method and system for intelligent dynamic switching of communication links based on BeiDou and low-Earth orbit satellites can be derived from... Figure 1 The intelligent dynamic switching system 100 for communication links based on BeiDou and low-Earth orbit satellites shown in the figure is executed. The detailed steps of the intelligent dynamic switching method for communication links based on BeiDou and low-Earth orbit satellites are described below.

[0018] This embodiment provides a method for intelligent dynamic switching of communication links based on BeiDou and low-Earth orbit (LEO) satellites. It aims to solve the technical problem of how ground terminals can accurately, efficiently, and seamlessly switch between communication links of different satellite systems in an environment where multiple satellite navigation and communication systems coexist. The core of this method lies in unifying the time reference of different satellite systems and accurately calculating the overlapping areas of their beam coverage, thereby providing an accurate spatiotemporal trigger point for link switching of terminal devices and realizing cross-system link relay. In this embodiment, a UAV performing remote logistics delivery is used as an example. This UAV carries both a BeiDou navigation signal receiving module and a LEO satellite communication module, and needs to dynamically switch between the BeiDou short message link and the LEO satellite broadband data link during flight according to mission requirements and signal coverage.

[0019] Step S110: Receive the BeiDou signal frame structure set continuously broadcast by the BeiDou satellite navigation system. The BeiDou signal frame structure set includes navigation message data units with frame count sequence numbers and synchronization header timing distribution parameters corresponding to the navigation message data units.

[0020] First, the UAV continuously receives navigation signals broadcast from visible satellites in the BeiDou Navigation Satellite System constellation via the radio frequency front-end antenna of its BeiDou navigation receiver module. These signals are organized according to a predetermined frame structure, forming a set of BeiDou signal frame structures. Within a flight period, a set of multiple navigation message data units with consecutive frame count sequence numbers, received from multiple BeiDou satellites such as BDS-01 and BDS-02, is received. Each navigation message data unit encapsulates information such as satellite ephemeris, clock bias, and ionospheric model parameters. Simultaneously, each frame structure carries a synchronization header for symbol and frame synchronization at the receiver during physical layer transmission. Therefore, the timing distribution parameters of the synchronization header corresponding to the navigation message data unit are also included in the set. This parameter precisely describes the occurrence pattern of the synchronization header on the time axis, such as the precise value of the frame start time. By parsing these synchronization headers, the UAV can obtain a precise time stamp based on BeiDou system time. For example, when the UAV receives navigation message data unit number 1024 broadcast by the BDS-01 satellite, the timing distribution parameters of the synchronization header indicate that the start time of this frame corresponds to the 345678.9th second of the 1024th cycle of BeiDou time.

[0021] Step S120: Synchronously capture the low-Earth orbit beacon beam scanning sequence periodically broadcast by the low-Earth orbit satellite communication system. The low-Earth orbit beacon beam scanning sequence includes a set of arrival timestamps of beacon signals arriving at the ground terminal and a beam pointing angle mapping relationship corresponding to the set of arrival timestamps.

[0022] Meanwhile, the UAV also needs to process signals from the low-Earth orbit (LEO) satellite communication system via its LEO satellite communication module. Taking the LEO-A-01 satellite as an example, due to its orbital motion and beamforming technology, its coverage area on the ground is dynamically scanned. Therefore, the satellite periodically broadcasts beacon signals, organized as a LEO beacon beam scanning sequence. Each beacon signal in this sequence carries information such as the satellite's real-time position and beam pointing. When a beacon signal reaches the UAV, the communication module records the precise arrival time; these time points together constitute an arrival timestamp set. Furthermore, this sequence implicitly contains a beam pointing angle mapping relationship; that is, the transmission of each beacon signal corresponds to a specific beam pointing angle of the satellite at that moment, including the azimuth and elevation components of the beam center axis in the satellite's coordinate system. By analyzing the ephemeris and beam control information in the sequence, the UAV can establish a mapping relationship between "arrival timestamp - beam pointing angle". For example, a beacon signal transmitted by the LEO-A-01 satellite has an arrival timestamp of local counter value 876543210, and the resolved beam pointing angle is azimuth 120 degrees and elevation 45 degrees.

[0023] Step S130: Perform cross-system time reference unification processing on the synchronization header timing distribution parameters of the BeiDou signal frame structure set and the arrival timestamp set of the low-orbit beacon beam scanning sequence to generate a time synchronization offset set with the local clock of the ground terminal as the reference system.

[0024] Because the BeiDou system and the low-Earth orbit (LEO) satellite system operate on different time bases, such as BeiDou time and the LEO satellite system's internal time, a unified time base processing is necessary to coordinate the use of information from both systems. This processing uses the local clock driven by a high-stability temperature-compensated crystal oscillator inside the UAV as a common reference frame, generating a core data product—a set of time synchronization offsets. This set quantifies the time relationship between signal events from the two systems at the same local clock scale. The specific implementation details are elaborated through the following sub-steps.

[0025] Step S131: Extract the synchronization header timing distribution parameters corresponding to each navigation message data unit in the BeiDou signal frame structure set. The synchronization header timing distribution parameters include the precise value of the frame start time and the frame period duration value under the BeiDou system time reference.

[0026] The BeiDou signal tracking loop in the UAV baseband processing unit, after locking the synchronization header of each BeiDou signal frame, parses the precise transmission time corresponding to that frame from the demodulated navigation message. This time is the precise frame start time value, which is based on the BeiDou system time reference and consists of two parts: week count and second count within the week. Meanwhile, according to the system protocol, the frame period duration is a known fixed constant; for example, the duration of a subframe in the BeiDou-3 system is fixed at a specific number of milliseconds. These two parameters form the basis for subsequent time deviation calculations. From navigation message data unit number 1024 of the BDS-01 satellite, the precise frame start time value is extracted as second 345678.9 of the 1024th week in BeiDou time, and the frame period duration value is a specific number of milliseconds.

[0027] Step S132: Extract the set of arrival timestamps corresponding to each beacon signal in the low-Earth orbit beacon beam scanning sequence. The set of arrival timestamps includes the precise value of the beacon signal transmission time under the time reference of the low-Earth orbit satellite system and the pre-compensation value of the beacon signal propagation delay.

[0028] When a low-Earth orbit (LEO) satellite transmits a beacon signal, it embeds its transmission time in a specific field of the signal frame. This time is based on the LEO satellite system's internal time and is represented by a count value from the onboard clock. Simultaneously, to simplify terminal calculations, the satellite sometimes pre-estimates the signal propagation delay based on its known orbital position and includes this pre-compensation value in the specific field of the signal frame. After successfully acquiring and demodulating the beacon signal, the UAV can extract the precise transmission time and the propagation delay pre-compensation value. These data form the basis for unifying the LEO system's time reference. From the beacon signal transmitted by the LEO-A-01 satellite, the extracted precise transmission time value is a specific count value under the LEO system's time reference, and the propagation delay pre-compensation value is a specific number of milliseconds.

[0029] Step S133: Read the first local timestamp sequence recorded by the local clock of the ground terminal when receiving the BeiDou signal frame structure set. The first local timestamp sequence corresponds one-to-one with the precise value of the frame start time of the BeiDou signal frame structure set.

[0030] Within the baseband processing unit, there is a high-precision free-running counter whose output serves as the time scale for the local clock. Whenever the synchronization header of a BeiDou signal frame is successfully detected, the current value of this counter is immediately captured and latched, forming a first local timestamp. By continuously capturing multiple frames, a first local timestamp sequence is formed that is strictly aligned in time with the precise start time of each BeiDou frame. This sequence represents the occurrence time of BeiDou signal events from the perspective of the local clock. For example, when the synchronization header of navigation message data unit 1024 of the BDS-01 satellite is detected, the value of the free-running counter, 876540000, is latched as the first local timestamp corresponding to that frame.

[0031] Step S1331: During the process of receiving the BeiDou signal frame structure set, the arrival time of each BeiDou signal frame structure set to the radio frequency front end of the ground terminal is recorded synchronously as the original value of the original local timestamp.

[0032] The specific operations during the generation of the first local timestamp are as follows: After the BeiDou signal undergoes down-conversion and analog-to-digital conversion at the RF front-end, it becomes a digital baseband signal. The matched filter in the baseband processing unit continuously performs correlation operations on the synchronization header sequence. When the correlation peak exceeds a preset threshold, it is determined that a frame synchronization header has been detected. At this detection moment, a latch signal is immediately triggered, latching the current value of the free-running counter into a dedicated register. This value is the original value of the local timestamp. This original value strictly corresponds to the actual arrival time of the signal at the hardware level.

[0033] Step S1332: Perform analog-to-digital conversion on the original local timestamp values ​​to generate a digitized first local timestamp original sequence.

[0034] Since the free-running counter is itself part of the digital logic circuit, its output raw local timestamp value is already in digital form. The analog-to-digital conversion process described here actually refers to reading it from the latch register and encapsulating it into a digitized value that can be read by a subsequent digital signal processor according to a specific data format, thus forming a sequence of multiple such values, namely the first raw local timestamp sequence. Each element in this sequence is a local clock count value corresponding to the arrival time of a BeiDou signal frame synchronization header.

[0035] Step S1333: Input the first local timestamp original sequence into the local clock discipline loop. The local clock discipline loop includes a digital phase-locked loop and a voltage-controlled crystal oscillator. Through a feedback control mechanism, the frequency and phase deviations between the local clock and the BeiDou system time reference are gradually eliminated.

[0036] The first local timestamp sequence reflects the local clock's count value, while the precise frame start time extracted in step S131 reflects the BeiDou system time. There is a discrepancy between these two. The first local timestamp sequence is used as a feedback signal and compared in phase with the BeiDou time reference represented by the precise frame start time value in a digital phase-locked loop (PLL). The error signal output from the PLL passes through a loop filter to generate a control voltage, which acts on a voltage-controlled crystal oscillator (VCO) to fine-tune its oscillation frequency. Through this closed-loop negative feedback mechanism, the output frequency and phase of the local clock are gradually pulled and locked to the BeiDou system time reference, thus achieving local clock taming.

[0037] Step S1334: After the local clock discipline loop is locked, extract the stable first local timestamp sequence that is aligned with the precise value of the start time of each BeiDou signal frame structure set frame.

[0038] Once the local clock discipline loop reaches the locked state, the local clock is synchronized with the BeiDou system time reference. At this point, the output of the local free-running counter is no longer an independent timeline with random drift from BeiDou time, but rather a precise local reproduction of BeiDou time. The first local timestamp recorded at this time is a stable version strictly aligned with the precise value of the frame start time. For example, after discipline, when a new BeiDou signal frame is received, the first recorded local timestamp will be directly equal to the BeiDou system time value of that frame multiplied by the clock cycle conversion coefficient of the local counter.

[0039] Step S1335: Compare the stabilized first local timestamp sequence with the precise value of the corresponding frame start time, and calculate the residual time error sequence.

[0040] Even after loop taming and locking, a small residual error still exists between the two due to loop noise and the short-term stability limitations of the crystal oscillator. Subtracting each value in the stabilized first local timestamp sequence point by point from the precise frame start time value parsed from the navigation message yields a series of small differences, which constitute the residual time error sequence. This sequence quantifies the remaining dynamic tracking error after loop taming.

[0041] Step S1336: The residual time error sequence is superimposed as a correction amount into the subsequent generation process of the first local timestamp sequence to form the first local timestamp sequence after closed-loop correction.

[0042] To further improve accuracy, the residual time error sequence calculated in step S1335 is statistically modeled or filtered to obtain a real-time error prediction value. When generating a new first local timestamp, the original latch value is first recorded, then the predicted error correction is subtracted or added, and the final output value is the first local timestamp after closed-loop correction. This final timestamp sequence has a higher degree of consistency with the BeiDou system time reference and less jitter.

[0043] Step S134: Read the second local timestamp sequence recorded by the local clock of the ground terminal when capturing the low-Earth orbit beacon beam scanning sequence. The second local timestamp sequence corresponds one-to-one with the precise value of the beacon signal transmission time of the low-Earth orbit beacon beam scanning sequence.

[0044] Similar to recording BeiDou signals, whenever the UAV successfully detects the preamble of a LEO beacon signal and completes synchronization, the current value of the local free-running counter is immediately captured, forming a second local timestamp. In this way, a second local timestamp sequence corresponding to the precise transmission time of each LEO beacon signal (provided by the satellite) can be obtained. This sequence represents the occurrence time of LEO satellite signal events from the perspective of the local clock.

[0045] Step S1341: During the acquisition of the LEO beacon beam scanning sequence, the occurrence time of the correlation peak of the preamble in each LEO beacon beam scanning sequence is detected synchronously and used as the original value of the second local timestamp.

[0046] The specific operation is as follows: The baseband processing unit of the low-Earth orbit (LEO) satellite communication module contains a dedicated correlator that performs a sliding correlation between its local sequence and the preamble sequence in the received signal. When the correlation value reaches a peak and exceeds a detection threshold, it is determined that the start of a beacon signal has been detected. At the precise moment this peak occurs, a latch signal is triggered, latching the current value of the free-running counter; this value is the original value of the second local timestamp. This original value corresponds strictly to the arrival time of the LEO beacon signal at the hardware level.

[0047] Step S1342: Perform a de-jitter filtering process on the original value of the second local timestamp to eliminate the random jitter of the relevant peak position caused by the multipath effect and generate a smoothed intermediate sequence of the second local timestamp.

[0048] Due to the multipath effect of wireless channels, the position of the preamble correlation peak may randomly advance or lag within one symbol period; this phenomenon is called jitter. To eliminate the impact of jitter on timestamp accuracy, the original values ​​of the second local timestamps generated from multiple consecutive beacon signals are filtered. For example, a low-pass filter or a sliding median filter can be used to smooth the original timestamp sequence, obtaining a smoothed intermediate sequence of second local timestamps. This sequence more accurately reflects the average arrival time of the signal.

[0049] Step S1343: Associate and match the smoothed second local timestamp intermediate sequence with the frame number in the low-orbit beacon beam scanning sequence to establish a mapping table between the frame number and the second local timestamp.

[0050] Each LEO beacon signal contains its unique frame number. Each timestamp in the smoothed second local timestamp intermediate sequence generated in step S1342 is paired with the corresponding beacon signal's frame number. A two-dimensional lookup table is constructed in memory, indexed by the frame number, to quickly retrieve the smoothed second local timestamp corresponding to that beacon signal. This mapping relationship is a crucial bridge for subsequent alignment of satellite time with local time.

[0051] Step S1344: Analyze the ephemeris parameters contained in the low-Earth orbit beacon beam scanning sequence and extract the conversion relationship between the low-Earth orbit satellite system time reference and Coordinated Universal Time.

[0052] The ephemeris parameters of low-Earth orbit (LEO) satellites contain not only orbital information but also conversion parameters between the satellite system's internal time base and Coordinated Universal Time (UTC). For example, it might include the initial value of a system time counter and its correspondence to UTC seconds. By analyzing these parameters, a precise conversion function between the LEO satellite system time and UTC can be established.

[0053] Step S1345: Convert the precise value of the low-orbit beacon signal transmission time into a unified time representation aligned with Coordinated Universal Time according to the conversion relationship, and generate an alignment reference for the second local timestamp sequence.

[0054] Using the conversion relationship obtained in step S1344, the precise transmission time value of each LEO beacon signal extracted in step S132 (based on LEO system time) is converted into the corresponding Coordinated Universal Time (UTC). This converted UTC becomes the reference for aligning the second local timestamp with it. Since the UAV's locally tamed clock ultimately traces back to UTC, a common reference anchor point is found for the two time systems.

[0055] Step S1346: The smoothed second local timestamp intermediate sequence is time-axis calibrated according to the alignment reference to generate a second local timestamp sequence that has a definite correspondence with the precise value of the low-orbit beacon signal transmission time.

[0056] The final step involves comparing the timestamps in the "Frame Number - Second Local Timestamp" mapping table established in step S1343 with the "Frame Number - Transmission Time" calculated in step S1345 based on Coordinated Universal Time (UTC). Using methods such as least squares or linear regression, a linear relationship model between the second local timestamp and UTC is calculated, i.e., how the local counter value maps to UTC. Using this calibrated model, the smoothed intermediate sequence of second local timestamps is recalculated to generate the final second local timestamp sequence. Each value in this sequence has a deterministic functional relationship with the precise value of the corresponding LEO beacon signal transmission time (based on UTC).

[0057] Step S135: Calculate the difference between the precise value of the frame start time of each BeiDou signal frame structure set and the corresponding first local timestamp sequence to obtain the BeiDou time drift sequence of the BeiDou system time reference relative to the local clock of the ground terminal.

[0058] For each received BeiDou signal frame, the precise frame start time value extracted in step S131 under the BeiDou system time reference is denoted as T_bds_i. This value is then compared with the first local timestamp of the corresponding frame after closed-loop correction generated in step S133, denoted as L_bds_i. The difference between the two is calculated as D_bds_i = T_bds_i - L_bds_i. Since the BeiDou system time and local clock may still have slight integer count deviations after frequency synchronization following discipline loop locking, this difference sequence D_bds_i reflects the aforementioned residual, stable, or slowly changing inter-system time offset. Performing this operation on a series of consecutive frames forms a time series consisting of differences D_bds_1, D_bds_2, ..., D_bds_n, i.e., the BeiDou time drift sequence.

[0059] Step S136: Calculate the difference between the precise value of the beacon signal transmission time of each low-Earth orbit beacon beam scanning sequence and the corresponding second local timestamp sequence to obtain the low-Earth orbit time drift sequence of the low-Earth orbit satellite system time reference relative to the local clock of the ground terminal.

[0060] The LEO beacon signals are processed using the same logic as in step S135. The precise transmission time of each beacon, converted to Coordinated Universal Time (UTC) (based on LEO system time), is denoted as T_leo_j, and its corresponding calibrated second local timestamp is denoted as L_leo_j. The difference D_leo_j = T_leo_j - L_leo_j is calculated. This calculation is performed on all received beacon signals to obtain a time series consisting of the differences D_leo_1, D_leo_2, ..., D_leo_m, i.e., the LEO time drift sequence. This sequence quantifies the dynamic offset of the LEO system time base (mediated by UTC) relative to the local clock.

[0061] Step S137: Perform sliding window mean filtering on the BeiDou time drift sequence and the LEO time drift sequence to eliminate instantaneous jitter noise and generate smoothed BeiDou time drift correction values ​​and LEO time drift correction values.

[0062] The original drift sequences D_bds_i and D_leo_j contain instantaneous jitter introduced by factors such as signal reception noise and multipath effects. To obtain a more stable time reference relationship, filtering is required. A sliding window mean filter is used for processing. A fixed-length window is set, for example, the window size is set to N sample points. For the current BeiDou time drift sequence, the most recent N D_bds_i values ​​are taken, and their arithmetic mean is calculated as the smoothed BeiDou time drift correction value for the current time, denoted as D_bds_filt. Similarly, the same sliding window mean is applied to the LEO time drift sequence to obtain the smoothed LEO time drift correction value for the current time, denoted as D_leo_filt. As new samples arrive, the window slides, and these two correction values ​​are updated in real time.

[0063] Step S138: Input the BeiDou time drift correction value and the LOR time drift correction value into the same time coordinate system conversion function. The time coordinate system conversion function uses the local clock of the ground terminal as a unified output reference and outputs the BeiDou unified time parameter corresponding to the precise value of the start time of each BeiDou signal frame.

[0064] A time coordinate system transformation function F(X, Y) is defined, which converts time values ​​based on their respective satellite system times into a unified time value based on the local clock. Specifically, for any precise value T_bds_i of the start time of a BeiDou signal frame, its corresponding unified BeiDou time parameter U_bds_i is calculated by the function F_bds: U_bds_i = T_bds_i - D_bds_filt. The physical meaning of this calculation process is: subtracting the drift of BeiDou time relative to the local clock from the BeiDou system time, the result is the time value corresponding to the "projection" of BeiDou time onto the local clock time axis. Since D_bds_filt is updated smoothly in real time, each T_bds_i can be accurately converted into a corresponding U_bds_i.

[0065] Step S139: Synchronously process the precise value of the transmission time of each LOR beacon signal in the time coordinate system transformation function, and output the LOR unified time parameter corresponding to the precise value of the transmission time of each LOR beacon signal.

[0066] Parallel to and logically consistent with step S138, for any precise value T_leo_j of the LEO beacon signal transmission time that has been converted to UTC, it is input into the time coordinate system transformation function F_leo for the LEO system to obtain the corresponding unified LEO time parameter U_leo_j: U_leo_j = T_leo_j - D_leo_filt. Through this calculation, the time of all LEO satellite signal events is also unified to the same reference frame as BeiDou signal events—the local clock time axis. At this point, the event times of both BeiDou signals and LEO satellite signals are unified to the time values ​​U_bds_i and U_leo_j from the local clock perspective, clearing the time reference obstacle for subsequent comparison and correlation.

[0067] Step S1310: Extract the difference between the BeiDou unified time parameter and the LEO unified time parameter under the same local clock scale, and generate a time synchronization offset set with the local clock of the ground terminal as the reference system. Each time synchronization offset in the time synchronization offset set corresponds to a pairable combination of BeiDou signal frames and LEO beacon signals with the same local clock scale.

[0068] After obtaining the unified time parameter sequences U_bds and U_leo, the terminal continuously monitors these two sequences. When a pair (or more pairs) of U_bds_i and U_leo_j are found on the local clock axis, and their values ​​are very close, with the absolute value of their difference being less than a preset minimum time threshold (e.g., one symbol period), it is considered that near that moment of the local clock, there exists a signal frame from the BeiDou system and a beacon signal from the LEO system, which are temporally pairable. The difference between this pair of signals is calculated, i.e., ΔT_k = U_leo_j - U_bds_i. This ΔT_k is a time synchronization offset, which precisely represents the time offset of an LEO beacon signal relative to a BeiDou signal frame when the local clock is used as a reference frame. Calculating all the above-mentioned pairable signal combinations within the observation period forms the final set of time synchronization offsets, denoted as {ΔT_1, ΔT_2, ..., ΔT_K}. Each offset in the set indexes a specific set of BeiDou signal frames and low-orbit beacon signals.

[0069] Step S140: Construct an interaction trigger point positioning mapping table between BeiDou satellites and low-orbit satellites based on the time synchronization offset set and the beam pointing angle mapping relationship. The interaction trigger point positioning mapping table includes an interactive satellite combination identifier corresponding to each time synchronization offset and the coordinate range of the beam coverage overlap area associated with the interactive satellite combination identifier.

[0070] Having established a unified time reference, the next step is to fuse time and spatial information. Each time synchronization offset ΔT_k obtained in step S130 is associated with a specific BeiDou satellite and a low-Earth orbit satellite. The purpose of this step is to pair each of these satellites, calculate whether there is a ground overlap area covered by beam coverage at a specific time (the time point implied by ΔT_k), and record the coordinate range of this overlap area to form a mapping table that can be quickly queried. The specific construction process is implemented through the following sub-steps.

[0071] Step S141: Parse the BeiDou satellite number information of the BeiDou signal frame and the low-orbit satellite number information of the low-orbit beacon signal associated with each time synchronization offset in the time synchronization offset set, and form a satellite pairing tuple with the time synchronization offset as the index key.

[0072] Each element ΔT_k in the time synchronization offset set is calculated from a specific pair of BeiDou signal frames and LEO beacon signals. Therefore, the source satellite of this BeiDou signal frame can be traced, and its satellite ID, denoted as BDS_ID_k, can be extracted from the navigation message. Similarly, the source satellite ID, denoted as LEO_ID_k, can be extracted from the LEO beacon signal. Using ΔT_k as a unique index key, it is combined with the satellite ID pair (BDS_ID_k, LEO_ID_k) to form a satellite pairing tuple, i.e., {ΔT_k: (BDS_ID_k, LEO_ID_k)}. This tuple is the core index structure for constructing the mapping table.

[0073] Step S142: Extract the instantaneous orbital position parameters of the BeiDou satellite corresponding to the BeiDou satellite number information in each satellite pairing tuple. The instantaneous orbital position parameters of the BeiDou satellite include the three-dimensional coordinate components of the BeiDou satellite in the protocol Earth coordinate system.

[0074] Based on the BeiDou satellite ID BDS_ID_k parsed from step S141, the UAV obtains the precise orbital position of the satellite at the time it generated the aforementioned BeiDou signal frame (i.e., the time corresponding to ΔT_k) from its locally stored ephemeris data or by real-time parsing of the navigation message data unit corresponding to the satellite. This position is typically represented by three-dimensional coordinate components in the protocol Earth coordinate system, denoted as (X_bds_k, Y_bds_k, Z_bds_k), which serve as the starting point for subsequent beam coverage geometry calculations. For the BDS-01 satellite, its position coordinates at a specific time are obtained by interpolation from the ephemeris as (X_bds_k, Y_bds_k, Z_bds_k).

[0075] Step S143: Extract the instantaneous orbital position parameters of the low-Earth orbit satellite corresponding to the low-Earth orbit satellite number information in each satellite pairing tuple. The instantaneous orbital position parameters of the low-Earth orbit satellite include the three-dimensional coordinate components of the low-Earth orbit satellite in the protocol Earth coordinate system.

[0076] Parallel to step S142, based on the LEO-ID_k satellite number, the UAV extracts the instantaneous orbital position parameters of the LEO-A-01 satellite at the time of transmitting the corresponding beacon signal from the ephemeris parameters parsed from the LEO-A-ID signal. For consistency in spatial calculations, these position coordinates need to be transformed to the same protocol Earth coordinate system as the BeiDou system, yielding three-dimensional coordinate components denoted as (X_leo_k, Y_leo_k, Z_leo_k). In this way, the positions of the BeiDou satellite and the LEO-A-01 satellite are defined within the same spatial reference frame. For the LEO-A-01 satellite, its position coordinates at a specific moment are obtained by interpolation from the ephemeris as (X_leo_k, Y_leo_k, Z_leo_k).

[0077] Step S144: Calculate the boundary curve of the beam coverage area pointing from the center axis of the BeiDou satellite beam to the ground based on the instantaneous orbital position parameters of the BeiDou satellite, and generate a polygon of the BeiDou beam coverage area. The polygon of the BeiDou beam coverage area contains the boundary latitude and longitude coordinate sequence of the projection of the main lobe of the BeiDou satellite beam onto the ground.

[0078] Based on the spatial position (X_bds_k, Y_bds_k, Z_bds_k) of the BeiDou satellite and the known beam pointing parameters of the satellite antenna, the coverage area of ​​its beam on the ground is calculated. For BeiDou satellites, the beam pointing is usually fixed, such as being oriented towards the Earth. First, the direction vector from the satellite position to the Earth's center is determined. The beam center axis usually coincides with this vector or has a small fixed angle. Then, combined with the known beamwidth angle parameters, the intersection point of the beam edge and the Earth's surface can be calculated. Specifically, a cone is formed with the satellite position as the vertex, the beam center axis as the axis, and half the beamwidth angle as the semi-cone angle. The intersection of this cone with the surface of the Earth ellipsoid model is the boundary curve of the beam coverage area. Dense sampling is performed on this boundary curve to obtain a series of latitude and longitude coordinate pairs of ground points, such as (Lon_b_1, Lat_b_1), (Lon_b_2, Lat_b_2), etc. Connecting the coordinate pairs in sequence will form a closed polygon of the BeiDou beam coverage area, and the area inside it is the range within which the BeiDou satellite can provide services at the current moment.

[0079] Step S1441: Read the three-dimensional coordinate components in the instantaneous orbital position parameters of the BeiDou satellite to determine the spatial position point of the BeiDou satellite in the protocol Earth coordinate system.

[0080] First, the three-dimensional coordinate components (X_bds_k, Y_bds_k, Z_bds_k) obtained from step S142 directly determine the spatial position point P_bds of the Beidou satellite in the protocol Earth coordinate system, and its coordinates are the combination of these three components.

[0081] Step S1442: Obtain the pointing angle of the BeiDou satellite beam center axis relative to the satellite body coordinate system. The pointing angle includes azimuth offset and elevation offset, which are used to determine the direction vector of the beam center axis in inertial space.

[0082] From the antenna pattern parameters section of the BeiDou satellite's ephemeris or configuration file, read the pointing angle of the beam center axis in the satellite's body coordinate system. For example, assuming the BeiDou satellite's antenna is fixed to the ground, the azimuth and elevation offsets may both be zero, indicating that the beam center axis coincides with the -Z axis (pointing towards the Earth's center) of the satellite's body coordinate system. If an offset exists, these two angles can be used to determine the representation of a unit vector pointing from the satellite to the ground in the body coordinate system.

[0083] Step S1443: Starting from the BeiDou satellite's spatial position point, draw a ray along the direction vector, and calculate the coordinates of the intersection point of the ray with the surface of the Earth ellipsoid model. The coordinates of the intersection point are used as the coordinates of the landing point of the beam center axis on the Earth's surface.

[0084] The direction vector obtained in step S1442 is transformed from the satellite body coordinate system to the conventional Earth coordinate system, resulting in vector V_bds. Starting from the satellite position P_bds, a ray is drawn along the V_bds direction, and the intersection point of this ray with the surface of the Earth ellipsoid model (e.g., the WGS-84 ellipsoid) is determined. This is a classic geometric problem, typically solved using iterative or direct methods to obtain a quadratic equation in one variable concerning the ray parameters. The coordinates of the obtained intersection point P_center_bds represent the point where the beam center axis falls on the Earth's surface, usually expressed in latitude and longitude.

[0085] Step S1444: Based on the beamwidth angle parameter of the main lobe of the BeiDou satellite beam and the orbital altitude of the BeiDou satellite, calculate the coverage radius of the beam on the ground through geometric relationships, and generate a circular beam coverage area centered on the landing point coordinates.

[0086] The main lobe of a BeiDou satellite beam is typically approximated as a cone, and its half-power beamwidth angle is a known parameter, denoted as θ_3dB. The satellite's orbital altitude H can be approximated by subtracting the Earth's radius from its position vector magnitude. The beam's coverage radius on the ground, R_cover, can be estimated using a simple geometric relationship: R_cover ≈ H tan(θ_3dB / 2). With the landing point coordinates P_center_bds as the center and R_cover as the radius, draw a circle on the Earth's surface (approximately a local spherical surface) to obtain an approximate circular beam coverage area.

[0087] Step S1445: Sample the circumference of the circular beam coverage area at equal angular intervals to generate a sequence of latitude and longitude coordinates of multiple sampling points on the circumference.

[0088] To digitize a circular area into a polygon, the circumference needs to be sampled. A sampling step size is set, for example, taking a point every 1 degree of the central angle. Based on the coordinates of the center and the radius, the latitude and longitude coordinates corresponding to each sampling point on the circumference are calculated using geodetic formulas based on spherical geometry or local plane approximation, forming a sequence of latitude and longitude coordinates of the sampling points.

[0089] Step S1446: Perform curve fitting processing on the latitude and longitude coordinate sequence of multiple sampling points on the circumference to generate a smooth boundary curve of the BeiDou beam coverage area.

[0090] For the discrete sampling points generated in step S1445, spline interpolation or polynomial fitting can be used to generate a smooth curve, which serves as the final beam coverage area boundary curve. This step can eliminate the jaggedness caused by the limited number of sampling points, making the boundary closer to a true circle.

[0091] Step S1447: Convert the area enclosed by the boundary curve of the BeiDou beam coverage area into a BeiDou beam coverage area polygon, which is formed by connecting the latitude and longitude coordinates of the feature points on the boundary curve in sequence.

[0092] After smoothing the boundary curve, key feature points are extracted according to certain simplification rules (such as the Douglas-Peucker algorithm). These feature points are then connected sequentially with line segments to form a closed polygon. This polygon is the final BeiDou beam coverage area polygon used for spatial calculations, and its vertices are composed of a series of latitude and longitude coordinate pairs.

[0093] Step S145: Based on the instantaneous orbital position parameters of the low-orbit satellite and the beam pointing angle associated with the low-orbit satellite number information in the beam pointing angle mapping relationship, calculate the boundary curve of the beam coverage area pointing from the center axis of the low-orbit satellite beam to the ground, and generate a low-orbit beam coverage area polygon. The low-orbit beam coverage area polygon contains the boundary latitude and longitude coordinate sequence of the main lobe of the low-orbit satellite beam projected on the ground.

[0094] The beams of low-Earth orbit (LEO) satellites are typically scanned, making the calculation of their coverage area more complex. First, based on the LEO satellite's identification number LEO_ID_k, the beam pointing angle associated with the beacon signal corresponding to the current ΔT_k is found from the previously captured LEO beacon beam scan sequence. This angle parameter contains the precise pointing of the beam's central axis in the satellite's body coordinate system. Combining the satellite's instantaneous spatial position and attitude information, this beam pointing vector can be transformed into the conventional Earth coordinate system. Then, again starting from the satellite's position, a ray is drawn along this pointing vector; the intersection of this ray with the Earth's ellipsoidal surface is the beam center's location. Due to the low altitude of LEO satellites, the beam's projection on the ground is typically elliptical. Its shape is determined by the beamwidth, satellite altitude, and the incident angle of the beam pointing direction. Through geometric relationships, the major and minor axes of this ellipse can be calculated, thus generating the elliptical boundary curve. By parametrically sampling the boundary curve, a series of latitude and longitude coordinate pairs of ground points are obtained. Connecting these coordinate pairs in sequence will form a polygon of the low-orbit beam coverage area.

[0095] Step S1451: Read the three-dimensional coordinate components in the instantaneous orbital position parameters of the low-Earth orbit satellite to determine the spatial position point of the low-Earth orbit satellite in the protocol Earth coordinate system.

[0096] First, the three-dimensional coordinate components (X_leo_k, Y_leo_k, Z_leo_k) obtained from step S143 directly determine the spatial position point P_leo of the low-orbit satellite in the protocol Earth coordinate system, and its coordinates are the combination of these three components.

[0097] Step S1452: Extract the beam pointing angle associated with the low-orbit satellite number information from the beam pointing angle mapping relationship. The beam pointing angle includes the start angle, end angle and scan step angle of the beam scan, which is used to describe the change law of the beam center axis over time.

[0098] Based on LEO_ID_k, query the information corresponding to the satellite in the LEO beacon beam scan sequence. The beam pointing angle mapping relationship not only provides the pointing at a single moment but may also describe the pattern of beam scanning. For example, it may give the scanning range of the beam center axis in the azimuth angle within a scan cycle, from the starting angle to the ending angle, with a specific step angle. Based on the specific moment corresponding to the current beacon signal, the precise instantaneous pointing angle at that moment can be interpolated.

[0099] Step S1453: Based on the orbital velocity of the low-orbit satellite and the beam scanning step angle, calculate the instantaneous pointing direction of the beam center axis relative to the satellite body coordinate system at the current moment.

[0100] Combining the scanning pattern from step S1452 with the satellite's current orbital velocity (obtainable from continuous position differences in ephemeris), the additional change in beam pointing caused by satellite motion can be calculated. Finally, by integrating the scanning pattern and satellite motion, the precise instantaneous pointing direction of the beam center axis in the satellite's body coordinate system at the current specific moment is determined, and this direction is represented by a unit vector.

[0101] Step S1454: Starting from the low-orbit satellite's spatial position point, draw a ray along the instantaneous pointing direction, calculate the coordinates of the intersection point of the ray and the surface of the Earth ellipsoid model, and use the intersection point coordinates as the landing point coordinates of the beam center axis on the Earth's surface at the current moment.

[0102] The instantaneous pointing direction vector obtained in step S1453 is transformed from the satellite body coordinate system to the protocol Earth coordinate system to obtain the vector V_leo. Starting from the satellite position P_leo, a ray is drawn along the V_leo direction. Using the same method as in step S1443, the intersection point of this ray with the surface of the Earth ellipsoid model is solved to obtain the coordinates P_center_leo of the current moment where the beam center axis falls on the Earth's surface.

[0103] Step S1455: Calculate the coverage area of ​​the beam on the ground based on the beamwidth angle parameter of the main lobe of the low-orbit satellite beam and the slant distance between the low-orbit satellite and the ground, and generate an elliptical beam coverage area centered on the landing point coordinates, the major axis direction and the minor axis direction being related to the beam scanning direction and the orbital motion direction.

[0104] The beams of low-Earth orbit (LEO) satellites are typically conical, but because their direction is not perpendicular to the ground, their projection onto the ground is distorted into an ellipse. First, the slant range R_slant from the satellite to the landing point P_center_leo is calculated. Then, based on the conical beam and slant range geometry, the major and minor axes of the ellipse can be calculated. The major axis roughly follows the direction of the beam pointing vector's projection onto the ground, while the minor axis is perpendicular to it. The center of the ellipse is the landing point coordinate P_center_leo. This generates an elliptical beam coverage area centered at P_center_leo, with a major axis of a, a minor axis of b, and a specific orientation.

[0105] Step S1456: Parametrically sample the boundary of the elliptical beam coverage area to generate a sequence of latitude and longitude coordinates of multiple sampling points on the boundary.

[0106] For the generated ellipse, an ellipse parametric equation is introduced, with parameters typically being angle variables ranging from 0 to 2π. This parameter is sampled at certain angular steps (e.g., 1 degree). Using the ellipse equation and the coordinates of the center point, combined with a local approximation of the Earth's curvature, the latitude and longitude coordinates corresponding to each sampling point on the ellipse boundary are calculated, forming a sequence of latitude and longitude coordinates of the sampling points.

[0107] Step S1457: Perform curve fitting processing on the latitude and longitude coordinate sequence of multiple sampling points on the boundary to generate a smooth boundary curve of the low-orbit beam coverage area.

[0108] Similar to step S1446, spline interpolation or polynomial fitting is performed on the discrete sampling points generated in step S1456 to generate a smooth elliptical boundary curve and eliminate the jaggedness.

[0109] Step S1458: Convert the area enclosed by the boundary curve of the low-orbit beam coverage area into a low-orbit beam coverage area polygon, which is formed by connecting the latitude and longitude coordinates of feature points on the boundary curve in sequence.

[0110] The smoothed elliptical boundary curve is then processed by extracting key feature points according to certain simplification rules. These feature points are then connected sequentially with line segments to form a closed polygon. This polygon is the final low-orbit beam coverage area polygon used for spatial calculations, and its vertices are composed of a series of latitude and longitude coordinate pairs.

[0111] Step S146: Perform spatial superposition and intersection calculation on the polygon of the BeiDou beam coverage area and the polygon of the low-orbit beam coverage area, extract the boundary coordinate points of the intersection of the two polygons, and form the coordinate range of the beam coverage overlap area. The coordinate range of the beam coverage overlap area includes the latitude and longitude coordinate pairs of all vertices on the boundary of the intersection area.

[0112] The BeiDou beam coverage area polygon generated in step S144 and the corresponding LEO beam coverage area polygon generated in step S145 are overlaid and analyzed in two-dimensional geographic space. A polygon intersection algorithm, such as one based on scan lines or plane segmentation, is used to calculate the intersection of the two polygons. The intersection may be a new polygon or it may be empty. If the intersection is not empty, the boundary of the intersection polygon is extracted to obtain a sequence of latitude and longitude coordinate pairs of all vertices constituting its shape, denoted as {(Lon_ov_1, Lat_ov_1), (Lon_ov_2, Lat_ov_2), ...}. This sequence of coordinate points precisely defines the boundary of the ground area that can be simultaneously covered by BeiDou satellites and LEO satellites at the current moment, i.e., the coordinate range of the beam coverage overlap area.

[0113] Step S147: Combine and splice the BeiDou satellite number information and the low-orbit satellite number information in the satellite pairing tuple to generate an interactive satellite combination identifier. The interactive satellite combination identifier is arranged in the order of BeiDou satellite number first and low-orbit satellite number last.

[0114] The BeiDou satellite ID BDS_ID_k and the LEO satellite ID LEO_ID_k parsed in step S141 are concatenated into a unique string according to a fixed format to generate the pair satellite combination identifier, denoted as Pair_ID_k. For example, it can be concatenated as "BDS-01_LEO-A-01". This identifier is used to uniquely identify the pair of satellites, "BeiDou satellite BDS-01" and "LEO satellite LEO-A-01", which can interact in a specific time and space.

[0115] Step S148: Use the time synchronization offset as the primary key of the mapping table, and use the interactive satellite combination identifier and the coordinate range of the beam coverage overlap area as field values ​​associated with the primary key, and store them in the record row of the interactive trigger point positioning mapping table.

[0116] Now, all the information needed to build the mapping table is obtained. Create a new record in the database or in-memory data structure. The primary key of this record is the time synchronization offset ΔT_k. The field value associated with this primary key is a composite data structure containing two parts: first, the interactive satellite combination identifier Pair_ID_k; and second, the coordinate range of the beam coverage overlap area, i.e., the sequence of vertex coordinate pairs of the aforementioned intersection polygon. This key-value pair approach enables fast indexing from the time offset to specific satellite combinations and their spatial overlap areas.

[0117] Step S149: Traverse all time synchronization offsets in the time synchronization offset set, and repeatedly execute the above steps of satellite pairing tuple parsing, BeiDou beam coverage area polygon generation, low-orbit beam coverage area polygon generation, beam coverage overlap area coordinate range extraction, and interactive satellite combination identifier generation until all elements in the time synchronization offset set have been processed to form a complete interactive trigger point positioning mapping table.

[0118] For each element in the time synchronization offset set {ΔT_1, ΔT_2, ..., ΔT_K} generated in step S1310, the operations from steps S141 to S148 are performed once. After a complete round of iterative calculation, for each possible cross-system satellite pairing opportunity, a record containing precise time and space information is generated in the interaction trigger point positioning mapping table. The final mapping table is the core basis for the UAV to make all subsequent link switching decisions, tightly coupling the abstract time and frequency synchronization information with the specific spatial geographical location information.

[0119] Step S150: When a link switching request instruction containing the current serving satellite type identifier and the target satellite type identifier is received from the user terminal, the interactive trigger point positioning mapping table is retrieved. Based on the current serving satellite type identifier and the target satellite type identifier, the corresponding interactive satellite combination identifier is matched. The trigger point coordinates matching the current location coordinates of the user terminal are extracted from the beam coverage overlap area coordinate range corresponding to the interactive satellite combination identifier. A link dynamic switching relay instruction containing the trigger point coordinates and the corresponding time parameter in the time synchronization offset set is generated. The link dynamic switching relay instruction is sent to the user terminal baseband processing unit, triggering the user terminal baseband processing unit to perform cross-satellite system signal acquisition and synchronization establishment operations based on the trigger point coordinates and the time parameter.

[0120] During drone flight, its flight control computer or communication protocol stack may initiate a link switching request due to factors such as deteriorating link quality, changes in service requirements, or network planning. For example, when a drone is about to leave the coverage area of ​​the LEO-A-01 low-Earth orbit satellite and needs to switch to the BeiDou BDS-01 satellite for short message communication, it can generate a switching request command. This command is a data structure containing the currently used service satellite type identifier "LEO" and the target satellite type identifier "BDS". Upon receiving this request, the drone's main control processor immediately retrieves the locally constructed interactive trigger point positioning mapping table. Based on the "LEO" and "BDS" type identifiers, it performs pattern matching in the "Interactive Satellite Combination Identifier" field of the mapping table to find all combinations that begin with "BDS" and contain "LEO", thus identifying the pairing of BeiDou satellites and low-Earth orbit satellites. Then, combining the precise geographic coordinates (Lon_ue, Lat_ue) output by the UAV's current GPS positioning module, within the coordinate range of the beam overlap area corresponding to these selected combinations, an algorithm based on the relationship between points and polygons is used to determine whether (Lon_ue, Lat_ue) is located inside a certain overlapping polygon. Once such an overlapping area is found, it indicates that the UAV is currently within the beam overlap area of ​​a specific pair of BeiDou satellites and low-Earth orbit satellites. At this point, two core parameters are extracted from the corresponding record in the mapping table: one is the coordinates of the geometric center point of the overlapping area, or the UAV's current position coordinates themselves, as the trigger point coordinates; the other is the primary key corresponding to this record—the time synchronization offset ΔT. The main control processor encapsulates these two parameters into a link dynamic switching relay command and sends it to the baseband processing unit. After receiving the command, the baseband processing unit parses the trigger point coordinates and ΔT. The trigger point coordinates are used to guide the antenna array to perform coarse beam pointing adjustments to align with the target BeiDou satellite for signal acquisition. The time parameter ΔT is used to precisely set the local time reference of the baseband processing unit, enabling it to accurately initiate the acquisition and synchronization establishment operation of the target BeiDou satellite signal at the unified local clock time indicated by ΔT, thereby achieving seamless and precise switching from the low-orbit satellite link to the BeiDou link.

[0121] For example, after a link switch is completed in step S150 above, in order to further ensure communication continuity, the UAV also has the function of predicting and preparing for possible future switches. This step, based on the prediction of satellite motion trajectory and the UAV's own flight path, calculates the timing and parameters of the next switch in advance and caches them for later use, thereby minimizing switch delay. This is specifically implemented through the following sub-steps.

[0122] Step S211: Based on the ephemeris parameters of the current serving satellite and the motion trajectory prediction model of the user terminal, calculate the remaining service duration of the current serving satellite beam coverage area at the current location of the user terminal. This remaining service duration represents the length of the time window before the central axis of the current serving satellite beam leaves the current location of the user terminal.

[0123] After successfully switching to a new link, such as the current BeiDou BDS-01 satellite communication, the UAV will continuously track the satellite's ephemeris parameters to update its beam coverage area polygon in real time. Simultaneously, the UAV's flight control system, combining its preset flight path, current speed, and wind resistance model, runs a trajectory prediction model (e.g., a Kalman filter-based state estimator) to output a predicted sequence of the UAV's position over a future period. By combining the satellite beam's motion (primarily due to relative motion caused by the Earth's rotation and satellite motion) and the UAV's own motion, the estimated time when the edge of the satellite beam's coverage area will "sweep" across the UAV's current position can be calculated. The duration from the current moment to this estimated time is the remaining service duration T_remain of the currently serving satellite. This duration is a dynamically changing value, representing the time window during which the current link can be stably maintained.

[0124] Step S212: Based on the ephemeris parameters of the target satellite and the motion trajectory prediction model of the user terminal, calculate the expected entry time when the target satellite beam coverage area enters the current position of the user terminal. This expected entry time represents the time when the central axis of the target satellite beam will enter the current position of the user terminal.

[0125] In parallel with calculating the remaining service time, the drone also needs to monitor the next available target satellite, such as the soon-to-be-launched low-Earth orbit satellite LEO-B-02. Based on the ephemeris parameters of this target satellite, the future trajectory of its beam coverage area can be predicted. Combining this with the drone's own trajectory prediction model, the estimated time when the edge of the target satellite's beam coverage area first contacts the drone's current position can be calculated. This time is precisely denoted as the estimated entry time T_entry, which marks the starting point of a potential future handover opportunity.

[0126] Step S213: Compare the remaining service duration with the preset handover preparation lead time. If the remaining service duration is less than or equal to the handover preparation lead time, the pre-handover preparation procedure is automatically triggered.

[0127] The drone internally has a fixed time lead, such as T_advance, representing the time required to execute a complete pre-handover preparation process, including querying the mapping table, calculating parameters, and caching data. The main control processor continuously compares the values ​​of T_remain and T_advance. Once it finds that T_remain ≤ T_advance, meaning that the remaining service time of the current link is insufficient to support a smooth handover preparation, the system immediately and automatically triggers the pre-handover preparation program to prepare for the upcoming handover.

[0128] Step S214: In the pre-handover preparation procedure, a pre-handover query instruction is sent to the main control processor of the ground terminal. The pre-handover query instruction includes the current service satellite type identifier and the expected entry time, and requests to obtain the coordinates of the pre-handover trigger point that matches the expected entry time.

[0129] After the pre-handover preparation procedure is initiated, the baseband processing unit or a dedicated task module generates a pre-handover query instruction and sends it to the main control processor. This instruction contains the identifier of the currently used service satellite type (e.g., "BDS") and the expected entry time T_entry calculated in step S212. The core purpose of this instruction is to request the main control processor to answer: at the future time T_entry, is there a switchable, interactive satellite combination whose corresponding overlapping area can cover the UAV's location?

[0130] Step S215: After receiving the pre-switching query command, the main control processor of the ground terminal retrieves the interactive trigger point positioning mapping table, matches the corresponding time synchronization offset according to the expected entry time, and extracts the interactive satellite combination identifier and beam coverage overlap area coordinate range associated with the time synchronization offset.

[0131] After receiving the query command, the main control processor retrieves the interaction trigger point location mapping table again. The challenge here is that the primary key of the mapping table is the time synchronization offset ΔT, while the query command provides the expected future entry time T_entry. Therefore, the main control processor needs to use the local clock and the real-time updated time drift correction value to "reverse calculate" T_entry into the corresponding predicted future time synchronization offset value ΔT_pred. Specifically, ΔT_pred can be calculated based on currently known parameters such as D_leo_filt, through model extrapolation. Its core logic is to convert the future local clock time into a prediction of the time offset between a pair of future satellite signals. Then, using ΔT_pred as an index, it searches for the closest one or more records in the mapping table. From the matching records, it extracts the corresponding interactive satellite combination identifier and the coordinate range of the beam coverage overlap area.

[0132] Step S216: Extract the coordinates of the pre-switching trigger point that match the current location coordinates of the user terminal from the coordinate range of the beam coverage overlap area, and return the coordinates of the pre-switching trigger point and the associated time synchronization offset to the user terminal baseband processing unit.

[0133] Within the beam coverage overlap area coordinates extracted in step S215, combined with the UAV's current precise position coordinates (Lon_ue, Lat_ue), a specific pre-switching trigger point coordinate is determined through geometric calculations, such as finding the point closest to the UAV within the overlap area, or directly using the UAV's current position coordinates (if it is already within the predicted overlap area), denoted as (Lon_pre, Lat_pre). Simultaneously, the time synchronization offset ΔT (or the predicted value ΔT_pred) used to index this record is also retrieved. The main control processor returns the two parameters (Lon_pre, Lat_pre) and ΔT as the query results to the baseband processing unit that initiated the query.

[0134] Step S217: Receive the coordinates of the pre-switching trigger point and the time synchronization offset, and cache them as pre-switching parameters in the temporary register of the baseband processing unit, ready to be directly called when the expected entry time is reached.

[0135] After receiving the pre-switching parameters returned by the main control processor, the baseband processing unit does not immediately execute the switch. Instead, it stores the aforementioned parameters—the coordinates of the pre-switching trigger point (Lon_pre, Lat_pre) and the time synchronization offset ΔT—into its internal high-speed temporary register or dedicated cache. Simultaneously, it can start a hardware timer, configured to generate an interrupt signal when the expected entry time T_entry arrives.

[0136] Step S218: When the expected entry time is reached, read the coordinates of the pre-switching trigger point and the time synchronization offset from the temporary register, generate a pre-switching execution instruction containing the coordinates of the pre-switching trigger point and the time synchronization offset, and start the pre-acquisition and pre-synchronization operation of the target satellite signal according to the pre-switching execution instruction.

[0137] When a timer interrupt is triggered, indicating that the expected entry time T_entry has been reached, the baseband processing unit immediately reads the pre-buffered pre-switching trigger point coordinates (Lon_pre, Lat_pre) and time synchronization offset ΔT from a temporary register. Based on these two parameters, a pre-switching execution instruction is generated. Subsequently, the acquisition and synchronization module in the baseband processing unit begins operation. It uses (Lon_pre, Lat_pre) to guide the direction of the RF front-end's sky beam and adjusts the phase shifter of the phased array antenna to roughly align the beam with the target satellite (LEO-B-02) that is about to enter the coverage area. At the same time, it precisely adjusts the phase of the local numerically controlled oscillator according to ΔT, so that the locally generated pseudocode and carrier frequency can be quickly aligned with the expected target satellite signal. Thus, as soon as the signal enters the coverage area, the pre-acquisition and pre-synchronization operations of the target satellite signal can be completed in a very short time, establishing a preliminary link connection.

[0138] Step S219: After completing the pre-acquisition and pre-synchronization operations, the pre-handover success flag and the pre-handover completion timestamp are reported to the main control processor of the ground terminal. The main control processor updates the locally maintained satellite link status table according to the pre-handover success flag and marks the target satellite link as a hot backup status for direct use in subsequent emergency handovers.

[0139] After successful pre-acquisition and pre-synchronization, the baseband processing unit generates a pre-handover success flag and a precise pre-handover completion timestamp, and reports these two pieces of information to the main control processor. Upon receiving this report, the main control processor updates the status field of the target satellite link (LEO-B-02) corresponding to this pre-handover from "idle" or "unavailable" to "hot backup" in its maintained global satellite link status table. Simultaneously, it records the current key parameters of the link, such as the synchronized code phase, carrier frequency, and automatic gain control value. This means that the link is ready and can take over data communication at any time. If the currently primary BeiDou link suddenly fails or its quality deteriorates sharply, the main control processor can directly instruct the baseband processing unit to seamlessly switch the communication service flow to this "hot backup" link without going through a time-consuming query and preparation process, thus achieving a truly seamless, zero-interruption link handover.

[0140] Furthermore, in this embodiment, the link quality of the target satellite communication link after switching can be dynamically monitored and adjusted.

[0141] To ensure stable and high-quality operation of the communication link after the switchover, this invention also includes a step of continuous dynamic monitoring and closed-loop feedback adjustment of the target satellite communication link. This step is initiated after each switchover and continues throughout the entire communication process. Specifically, it is implemented through the following sub-steps.

[0142] Step S311: After switching to the target satellite communication link, continuously collect the received signal-to-noise ratio sequence of the target satellite signal, wherein the received signal-to-noise ratio sequence contains multiple signal-to-noise ratio sampling points with frame period as the interval.

[0143] After successfully switching to the target satellite (e.g., LEO-B-02) and establishing stable communication, the signal quality monitoring module in the baseband processing unit begins continuously acquiring the received signal-to-noise ratio (SNR) of the satellite signal. During the pilot symbol period of each data frame, an instantaneous SNR estimate is obtained by calculating the ratio of signal power to noise power. Arranging these estimates in time sequence with frame periods constitutes a received SNR sequence, denoted as {SNR_1, SNR_2, ..., SNR_p}. This sequence forms the basis for evaluating the overall signal strength of the link.

[0144] Step S312: Analyze the pilot symbol sequence in the target satellite signal, calculate the phase rotation angle of the pilot symbol sequence, and generate a carrier phase tracking error sequence, which reflects the phase difference between the local carrier and the target satellite carrier.

[0145] The carrier tracking loop (such as a Costas loop) in the receiver operates continuously. Within the loop, the received complex baseband signal containing pilot symbols is multiplied conjugately with the carrier signal reproduced by the local numerically controlled oscillator. The complex argument of the multiplication result is the instantaneous carrier phase error at the current symbol moment. This error value can be positive or negative, representing whether the local carrier phase leads or lags the received signal carrier. Repeating this operation for multiple consecutive pilot symbols forms the carrier phase tracking error sequence, denoted as {φ_err_1, φ_err_2, ..., φ_err_q}.

[0146] Step S313: Analyze the frame synchronization header in the target satellite signal, detect the offset of the correlation peak position of the frame synchronization header, and generate a symbol timing tracking error sequence, which reflects the timing deviation between the local symbol clock and the target satellite symbol clock.

[0147] The timing synchronization loop in the receiver (such as an early-late gate synchronizer) operates continuously. This loop evaluates the accuracy of the local symbol clock sampling time by comparing the energy of the early and late sampling points within each symbol period. When the frame synchronization header arrives, the loop detects the precise location of its correlation peak. If the correlation peak appears slightly before or after the ideal sampling point, it indicates a timing error; this offset is the symbol timing tracking error. Detecting the synchronization headers of multiple consecutive frames forms a symbol timing tracking error sequence, denoted as {τ_err_1, τ_err_2, ..., τ_err_r}.

[0148] Step S314: Input the received signal-to-noise ratio sequence, carrier phase tracking error sequence, and symbol timing tracking error sequence into the link quality evaluation function. The link quality evaluation function standardizes various errors and outputs a comprehensive link quality score. The comprehensive link quality score is positively correlated with the signal-to-noise ratio and negatively correlated with the standardized phase error and timing error.

[0149] To comprehensively evaluate link quality, a multi-input, single-output link quality evaluation function Q=f(SNR_seq, φ_seq, τ_seq) is defined. This function first extracts features from the three input sequences, and then calculates a single score Q through a specific mathematical combination to quantify the overall stability and quality of the link. The specific implementation details are elaborated in the following sub-steps.

[0150] Step S315: Read the preset set of link quality evaluation function coefficients from the internal register. The set of link quality evaluation function coefficients includes signal-to-noise ratio weight coefficients, phase error weight coefficients, and timing error weight coefficients.

[0151] Before evaluation, a set of pre-defined weighting coefficients is first read from the UAV's non-volatile memory or configuration registers. These coefficients reflect the relative importance of signal-to-noise ratio (SNR), phase error, and timing error in the overall score. For example, the SNR weighting coefficient is set to W_snr, the phase error weighting coefficient to W_phi, and the timing error weighting coefficient to W_tau. These coefficients are fixed values ​​obtained through extensive flight experiments or link simulation optimization and are stored in read-only memory.

[0152] Step S316: Perform moving average filtering on the received signal-to-noise ratio sequence to eliminate signal-to-noise ratio spike noise caused by instantaneous deep fading and generate a smoothed signal-to-noise ratio mean sequence.

[0153] The original SNR sequence {SNR_p} may contain severe fluctuations caused by transient interference, multipath fading, or blockage. To obtain a more stable signal strength representation, a sliding window averaging filter is applied. A window of length L is defined, and for each point in the sequence, the arithmetic mean of the L / 2 points before and after it (a total of L points) is taken as the smoothed SNR for that point. After processing the entire sequence, the smoothed SNR mean sequence {SNR_filt_p} is obtained. The last value of this sequence, SNR_filt_last, is used as the current representative SNR value.

[0154] Step S317: Perform absolute value transformation processing on the carrier phase tracking error sequence to convert the phase error with positive and negative signs into a phase error absolute amplitude sequence, and calculate the root mean square value of the phase error absolute amplitude sequence to generate the carrier phase error root mean square parameter.

[0155] The original phase error sequence {φ_err_q} has positive and negative values, and its long-term mean may be zero, but its fluctuations reflect the severity of phase jitter. First, the absolute value of each element in the sequence is taken to obtain the absolute amplitude sequence {|φ_err_q|}. Then, the root mean square (RMS) value of this absolute amplitude sequence is calculated. The RMS value is calculated by first summing the squares of all elements in the sequence, then dividing by the sequence length, and finally taking the square root of the result. The result is denoted as Φ_rms. Φ_rms is a non-negative value that quantifies the average amplitude of carrier phase jitter.

[0156] Step S318: Perform absolute value transformation processing on the symbol timing tracking error sequence to convert the timing error with positive and negative signs into a timing error absolute amplitude sequence, and calculate the root mean square value of the timing error absolute amplitude sequence to generate the symbol timing error root mean square parameter.

[0157] The processing logic is exactly the same as in step S317. For the timing error sequence {τ_err_r}, first, the absolute value is taken to obtain {|τ_err_r|}, and then the root mean square value of the absolute value sequence is calculated to obtain the root mean square parameter of the symbol timing error, denoted as T_rms. T_rms quantifies the average amplitude of the symbol timing jitter.

[0158] Step S319: Take the last signal-to-noise ratio mean point in the smoothed signal-to-noise ratio mean sequence as the current signal-to-noise ratio representative value.

[0159] From the smoothed signal-to-noise ratio mean sequence {SNR_filt_p} generated in step S316, the most recently calculated value is extracted and denoted as SNR_cur. This value represents the most stable signal strength level at the current moment.

[0160] Step S3110: Multiply the signal-to-noise ratio weighting coefficient by the standardized current signal-to-noise ratio representative value to generate the signal-to-noise ratio contribution component.

[0161] To combine physical quantities with different dimensions, standardization is required. First, SNR_cur is divided by a preset signal-to-noise ratio (SNR) reference value, SNR_ref, which can be the typical SNR of the receiver under ideal conditions, resulting in the standardized SNR value SNR_norm = SNR_cur / SNR_ref. Then, the SNR weighting coefficient W_snr is multiplied by SNR_norm to obtain the SNR contribution component C_snr = W_snr. SNR_norm.

[0162] Step S3111: Multiply the phase error weighting coefficient by the negative value of the standardized carrier phase error root mean square parameter to generate a phase error penalty component. The phase error penalty component decreases as the standardized carrier phase error root mean square parameter increases.

[0163] Divide the calculated Φ_rms in step S317 by a preset phase error tolerance value Φ_tol. This tolerance value can be the maximum phase error that the receiver synchronization loop can stably track, resulting in the standardized phase error value Φ_norm = Φ_rms / Φ_tol. Since a larger phase error results in poorer link quality, its contribution to the overall score should be a penalty term, i.e., a negative value. Calculate the phase error penalty component C_phi = -(W_phi) Φ_norm). This value is always less than or equal to zero.

[0164] Step S3112: Multiply the timing error weighting coefficient by the negative value of the standardized root mean square parameter of the symbolic timing error to generate a timing error penalty component. The timing error penalty component decreases as the standardized root mean square parameter of the symbolic timing error increases.

[0165] Using the same logic as step S3111, T_rms is divided by the preset timing error tolerance value T_tol to obtain the standardized timing error value T_norm = T_rms / T_tol. The timing error penalty component C_tau = -(W_tau) is then calculated. This value is always less than or equal to zero.

[0166] Step S3113: Perform an algebraic summation operation on the signal-to-noise ratio contribution component, phase error penalty component, and timing error penalty component to generate an initial comprehensive link quality score.

[0167] A simple algebraic addition is performed on the above three components to obtain an initial comprehensive link quality score Q_init. The calculation formula is: Q_init=C_snr+C_phi+C_tau. Said Q_init is a dimensionless value, and its magnitude is jointly determined by the weight coefficients and the normalized errors of each item.

[0168] Step S3114: inputting the initial comprehensive link quality score into a normalization mapping function, mapping the initial comprehensive link quality score to a preset score interval, and outputting a final comprehensive link quality score, wherein the final comprehensive link quality score is used to characterize the stability of the current target satellite communication link.

[0169] In order to make the scoring result easy to understand and compare, Q_init needs to be mapped to a fixed and intuitive interval, for example, from 0 to 100. A normalization mapping function is defined, for example, a simple linear truncation mapping. It is determined based on experience or simulation that when Q_init reaches a certain upper limit value Q_high, the score is 100; when Q_init is lower than a certain lower limit value Q_low, the score is 0. For Q_init between [Q_low, Q_high], the value is obtained by the linear interpolation formula Q_final=100 (Q_init-Q_low) / (Q_high-Q_low) to calculate the final comprehensive link quality score Q_final. Q_final is a value between 0 and 100. A higher value indicates a more stable link and better communication quality. This score can be used to trigger subsequent adjustments or alarms.

[0170] Step S3115: comparing the comprehensive link quality score with a preset link quality stability threshold, and triggering a link quality fine-tuning program if the comprehensive link quality score is lower than the link quality stability threshold.

[0171] The calculated Q_final is compared with a preset link quality stability threshold (e.g., TH_stable, set to 60). If Q_final≥TH_stable, the link quality is considered qualified, and only continuous monitoring is required. If Q_final<TH_stable, the link quality is considered degraded, and a more detailed adjustment program needs to be started.

[0172] Step S3116: in the link quality fine-tuning program, calculating a carrier phase correction amount according to the mean value of the carrier phase tracking error sequence, superimposing the carrier phase correction amount onto the control word of the local carrier numerically controlled oscillator, and gradually reducing the carrier phase tracking error.

[0173] After the fine-tuning procedure is triggered, the carrier phase is processed first. For the original carrier phase tracking error sequence {φ_err_q} generated in step S312, the values ​​within the most recent window, such as the errors of the most recent 100 symbols, are taken, and their arithmetic mean is calculated to obtain the average phase error φ_avg. This average value represents the static phase offset caused by Doppler shift estimation bias or local crystal oscillator drift. φ_avg is multiplied by a loop filter coefficient Kp (usually less than 1) to obtain the carrier phase correction amount Δφ_corr=Kp. φ_avg. Then, Δφ_corr is superimposed on the frequency control word controlling the local carrier numerically controlled oscillator. The numerically controlled oscillator adjusts its output frequency and phase according to the updated control word, thereby gradually pulling the average phase error φ_avg to near zero.

[0174] Step S3117: Simultaneously calculate the symbol timing correction amount based on the mean of the symbol timing tracking error sequence, and superimpose the symbol timing correction amount onto the control word of the local symbol clock numerically controlled oscillator to gradually reduce the symbol timing tracking error.

[0175] In parallel with carrier correction, symbol timing is processed. For the original symbol timing tracking error sequence {τ_err_r} generated in step S313, the values ​​within the most recent window are taken, and their arithmetic mean is calculated to obtain the average timing error τ_avg. This average value represents the cumulative timing offset caused by the symbol clock frequency deviation. τ_avg is multiplied by a loop filter coefficient Kt to obtain the symbol timing correction amount Δτ_corr=Kt. τ_avg. Then, Δτ_corr is superimposed on the frequency control word that controls the local symbol clock numerically controlled oscillator, and the sampling frequency and phase of the symbol clock are finely adjusted, thereby gradually pulling the average timing error τ_avg to near zero.

[0176] Step S3118: After completing carrier phase correction and symbol timing correction, reacquire the received signal-to-noise ratio sequence of the target satellite signal, calculate the new comprehensive link quality score, and compare the new comprehensive link quality score with the preset link quality stability threshold until the new comprehensive link quality score remains stable above the link quality stability threshold.

[0177] After completing one calibration, the system does not stop immediately. It can execute steps S311 to S3114 again to re-collect data and calculate a new comprehensive link quality score Q_final_new. Then, Q_final_new is compared with the threshold TH_stable again. If Q_final_new ≥ TH_stable, it indicates that the calibration is effective and the link has stabilized. If Q_final_new is still below the threshold, it may be necessary to repeat steps S3116 and S3117 for multiple iterative adjustments, or determine that the link cannot be recovered through fine-tuning, thereby triggering a higher-level link reselection or switching process. The entire fine-tuning process constitutes a closed-loop negative feedback control system, which can adaptively eliminate the slow deterioration of link quality caused by environmental changes or device aging, ensuring the long-term stability and reliability of the communication link.

[0178] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working processes of the systems, devices, and units described above can be referred to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0179] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

Claims

1. A method for intelligent dynamic switching of communication links based on BeiDou and low-orbit satellites, characterized in that, The method includes: Receive a set of BeiDou signal frame structures continuously broadcast by the BeiDou satellite navigation system. The set of BeiDou signal frame structures includes navigation message data units with frame count sequence numbers and synchronization header timing distribution parameters corresponding to the navigation message data units. Synchronously capture the low-Earth orbit beacon beam scanning sequence periodically broadcast by the low-Earth orbit satellite communication system. The low-Earth orbit beacon beam scanning sequence includes a set of arrival timestamps of beacon signals arriving at the ground terminal and a beam pointing angle mapping relationship corresponding to the set of arrival timestamps. The synchronization header timing distribution parameters of the BeiDou signal frame structure set and the arrival timestamp set of the low-orbit beacon beam scanning sequence are subjected to cross-system time reference unification processing to generate a time synchronization offset set with the local clock of the ground terminal as the reference system. Based on the time synchronization offset set and the beam pointing angle mapping relationship, an interaction trigger point positioning mapping table between Beidou satellites and low-orbit satellites is constructed. The interaction trigger point positioning mapping table includes the interactive satellite combination identifier corresponding to each time synchronization offset and the coordinate range of the beam coverage overlap area associated with the interactive satellite combination identifier. When a link switching request command containing the current serving satellite type identifier and the target satellite type identifier is received from a user terminal, the interactive trigger point positioning mapping table is retrieved. Based on the current serving satellite type identifier and the target satellite type identifier, the corresponding interactive satellite combination identifier is matched. The trigger point coordinates matching the current location coordinates of the user terminal are extracted from the beam coverage overlap area coordinate range corresponding to the interactive satellite combination identifier. A link dynamic switching relay command containing the trigger point coordinates and the corresponding time parameter in the time synchronization offset set is generated. The link dynamic switching relay command is sent to the user terminal baseband processing unit, triggering the user terminal baseband processing unit to perform cross-satellite system signal acquisition and synchronization establishment operations based on the trigger point coordinates and the time parameter.

2. The intelligent dynamic switching method for communication links based on BeiDou and low-orbit satellites according to claim 1, characterized in that, The synchronization header timing distribution parameters of the BeiDou signal frame structure set and the arrival timestamp set of the low-orbit beacon beam scanning sequence are subjected to cross-system time reference unification processing to generate a time synchronization offset set with the local clock of the ground terminal as the reference system, including: Extract the synchronization header timing distribution parameters corresponding to each navigation message data unit in the BeiDou signal frame structure set. The synchronization header timing distribution parameters include the precise value of the frame start time and the frame period duration value under the BeiDou system time reference. Extract the set of arrival timestamps corresponding to each beacon signal in the low-Earth orbit beacon beam scanning sequence. The set of arrival timestamps includes the precise value of the beacon signal transmission time under the time reference of the low-Earth orbit satellite system and the pre-compensation value of the beacon signal propagation delay. Read the first local timestamp sequence recorded by the local clock of the ground terminal when receiving the BeiDou signal frame structure set. The first local timestamp sequence corresponds one-to-one with the precise value of the frame start time of the BeiDou signal frame structure set. The second local timestamp sequence recorded by the local clock of the ground terminal when capturing the low-Earth orbit beacon beam scanning sequence is read. The second local timestamp sequence corresponds one-to-one with the precise value of the beacon signal transmission time of the low-Earth orbit beacon beam scanning sequence. The difference between the precise value of the frame start time of each BeiDou signal frame structure set and the corresponding first local timestamp sequence is calculated to obtain the BeiDou time drift sequence relative to the local clock of the ground terminal. The difference between the precise value of the beacon signal transmission time of each low-Earth orbit beacon beam scanning sequence and the corresponding second local timestamp sequence is calculated to obtain the low-Earth orbit time drift sequence of the low-Earth orbit satellite system time reference relative to the local clock of the ground terminal. The BeiDou time drift sequence and the LEO time drift sequence are subjected to sliding window mean filtering to eliminate instantaneous jitter noise and generate smoothed BeiDou time drift correction values ​​and LEO time drift correction values. The BeiDou time drift correction value and the LEO time drift correction value are input into the same time coordinate system conversion function. The time coordinate system conversion function uses the local clock of the ground terminal as a unified output reference and outputs the BeiDou unified time parameter corresponding to the precise value of the start time of each BeiDou signal frame. The precise value of the transmission time of each LOR beacon signal is processed synchronously in the time coordinate system transformation function, and the LOR unified time parameter corresponding to the precise value of the transmission time of each LOR beacon signal is output. Extract the difference between the BeiDou unified time parameter and the LEO unified time parameter at the same local clock scale, and generate a time synchronization offset set with the local clock of the ground terminal as the reference system. Each time synchronization offset in the time synchronization offset set corresponds to a pairable combination of BeiDou signal frames and LEO beacon signals with the same local clock scale.

3. The intelligent dynamic switching method for communication links based on BeiDou and low-orbit satellites according to claim 1, characterized in that, The step involves constructing an interaction trigger point positioning mapping table between BeiDou satellites and low-Earth orbit satellites based on the time synchronization offset set and the beam pointing angle mapping relationship. This interaction trigger point positioning mapping table includes an identifier for each interactive satellite combination corresponding to the time synchronization offset and the coordinate range of the beam coverage overlap area associated with the interactive satellite combination identifier, including: The BeiDou satellite number information of the BeiDou signal frame and the low-orbit satellite number information of the low-orbit beacon signal associated with each time synchronization offset in the set of time synchronization offsets are analyzed to form a satellite pairing tuple with the time synchronization offset as the index key. Extract the instantaneous orbital position parameters of the BeiDou satellite corresponding to the BeiDou satellite number information in each satellite pairing tuple. The instantaneous orbital position parameters of the BeiDou satellite include the three-dimensional coordinate components of the BeiDou satellite in the protocol Earth coordinate system. Extract the instantaneous orbital position parameters of the low-Earth orbit satellite corresponding to the low-Earth orbit satellite number information in each satellite pairing tuple. The instantaneous orbital position parameters of the low-Earth orbit satellite include the three-dimensional coordinate components of the low-Earth orbit satellite in the protocol Earth coordinate system. Based on the instantaneous orbital position parameters of the BeiDou satellite, the boundary curve of the beam coverage area pointing from the center axis of the BeiDou satellite beam to the ground is calculated, and a polygon of the BeiDou beam coverage area is generated. The polygon of the BeiDou beam coverage area contains the boundary latitude and longitude coordinate sequence of the projection of the main lobe of the BeiDou satellite beam on the ground. Based on the instantaneous orbital position parameters of the low-orbit satellite and the beam pointing angle associated with the low-orbit satellite number information in the beam pointing angle mapping relationship, calculate the boundary curve of the beam coverage area pointing from the center axis of the low-orbit satellite beam to the ground, and generate a low-orbit beam coverage area polygon. The low-orbit beam coverage area polygon contains the boundary latitude and longitude coordinate sequence of the low-orbit satellite beam main lobe projected on the ground. Spatial superposition and intersection calculation are performed on the polygon of the BeiDou beam coverage area and the polygon of the low-orbit beam coverage area. The boundary coordinate points of the intersection of the two polygons are extracted to form the coordinate range of the beam coverage overlap area. The coordinate range of the beam coverage overlap area includes the latitude and longitude coordinate pairs of all vertices on the boundary of the intersection area. The BeiDou satellite number information and the low-orbit satellite number information in the satellite pairing tuple are combined and spliced ​​to generate an interactive satellite combination identifier. The interactive satellite combination identifier is arranged in the order of BeiDou satellite number first and low-orbit satellite number last. The time synchronization offset is used as the primary key of the mapping table, and the interactive satellite combination identifier and the coordinate range of the beam coverage overlap area are used as field values ​​associated with the primary key and stored in the record row of the interactive trigger point positioning mapping table. Traverse all time synchronization offsets in the time synchronization offset set, and repeatedly execute the above steps of satellite pairing tuple parsing, BeiDou beam coverage area polygon generation, low-orbit beam coverage area polygon generation, beam coverage overlap area coordinate range extraction, and interactive satellite combination identifier generation until all elements in the time synchronization offset set have been processed to form a complete interactive trigger point positioning mapping table.

4. The intelligent dynamic switching method for communication links based on BeiDou and low-orbit satellites according to claim 2, characterized in that, The step of reading the first local timestamp sequence recorded by the local clock of the ground terminal when receiving the BeiDou signal frame structure set, wherein the first local timestamp sequence corresponds one-to-one with the precise value of the frame start time of the BeiDou signal frame structure set, further includes: During the process of receiving the BeiDou signal frame structure set, the arrival time of each BeiDou signal frame structure set to the radio frequency front end of the ground terminal is recorded synchronously as the original value of the original local timestamp. The original local timestamp values ​​are subjected to analog-to-digital conversion to generate a digitized first local timestamp original sequence; The original sequence of the first local timestamp is input into the local clock discipline loop, which includes a digital phase-locked loop and a voltage-controlled crystal oscillator. The frequency and phase deviations between the local clock and the BeiDou system time reference are gradually eliminated through a feedback control mechanism. After the local clock discipline loop is locked, a stable first local timestamp sequence aligned with the precise value of the start time of each BeiDou signal frame structure set frame is extracted. The stabilized first local timestamp sequence is compared with the precise value of the corresponding frame start time to calculate the residual time error sequence; The residual time error sequence is superimposed as a correction factor into the generation process of the subsequent first local timestamp sequence to form the first local timestamp sequence after closed-loop correction.

5. The intelligent dynamic switching method for communication links based on BeiDou and low-orbit satellites according to claim 2, characterized in that, The step of reading the second local timestamp sequence recorded by the ground terminal's local clock when capturing the low-Earth orbit beacon beam scanning sequence, wherein the second local timestamp sequence corresponds one-to-one with the precise value of the beacon signal transmission time of the low-Earth orbit beacon beam scanning sequence, further includes: During the capture of the LEO beacon beam scanning sequence, the occurrence time of the correlation peak of the preamble in each LEO beacon beam scanning sequence is detected simultaneously and used as the original value of the second local timestamp. The original value of the second local timestamp is subjected to de-jitter filtering to eliminate the random jitter of the relevant peak position caused by multipath effect, and a smoothed intermediate sequence of the second local timestamp is generated. The smoothed second local timestamp intermediate sequence is associated and matched with the frame number in the low-orbit beacon beam scanning sequence to establish a mapping table between the frame number and the second local timestamp. The ephemeris parameters contained in the low-Earth orbit beacon beam scanning sequence were analyzed to extract the conversion relationship between the low-Earth orbit satellite system time reference and Coordinated Universal Time. Based on the aforementioned conversion relationship, the precise value of the low-orbit beacon signal transmission time is converted into a unified time representation aligned with Coordinated Universal Time, generating an alignment reference for the second local timestamp sequence; The smoothed second local timestamp intermediate sequence is time-axis calibrated according to the alignment reference to generate a second local timestamp sequence that has a definite correspondence with the precise value of the low-orbit beacon signal transmission time.

6. The intelligent dynamic switching method for communication links based on BeiDou and low-orbit satellites according to claim 3, characterized in that, The process involves calculating the boundary curve of the beam coverage area pointing from the center axis of the BeiDou satellite beam to the ground based on the instantaneous orbital position parameters of the BeiDou satellite, generating a polygon of the BeiDou beam coverage area. This polygon contains the latitude and longitude coordinate sequence of the boundary of the BeiDou satellite beam main lobe projected onto the ground, including: Read the three-dimensional coordinate components in the instantaneous orbital position parameters of the BeiDou satellite to determine the spatial position point of the BeiDou satellite in the protocol Earth coordinate system; Obtain the pointing angle of the center axis of the Beidou satellite beam relative to the satellite body coordinate system. The pointing angle includes azimuth offset and elevation offset, which are used to determine the direction vector of the beam center axis in inertial space. Starting from the spatial position point of the Beidou satellite, a ray is drawn along the direction vector, and the coordinates of the intersection point of the ray and the surface of the Earth ellipsoid model are calculated. The coordinates of the intersection point are used as the coordinates of the landing point of the beam center axis on the Earth's surface. Based on the beamwidth angle parameter of the main lobe of the BeiDou satellite beam and the orbital altitude of the BeiDou satellite, the coverage radius of the beam on the ground is calculated through geometric relationships, and a circular beam coverage area is generated with the landing point coordinates as the center. The circumference of the circular beam coverage area is sampled at equal angular intervals to generate a sequence of latitude and longitude coordinates of multiple sampling points on the circumference. Curve fitting is performed on the latitude and longitude coordinate sequences of multiple sampling points on the circumference to generate a smooth boundary curve of the BeiDou beam coverage area; The area enclosed by the boundary curve of the BeiDou beam coverage area is converted into a BeiDou beam coverage area polygon, which is formed by connecting the latitude and longitude coordinates of the feature points on the boundary curve in sequence.

7. The intelligent dynamic switching method for communication links based on BeiDou and low-orbit satellites according to claim 3, characterized in that, The method involves calculating the boundary curve of the beam coverage area pointing from the center axis of the low-Earth orbit satellite to the ground, based on the instantaneous orbital position parameters of the low-Earth orbit satellite and the beam pointing angle mapping relationship associated with the low-Earth orbit satellite's identification number. This generates a low-Earth orbit beam coverage area polygon, which contains the latitude and longitude coordinate sequence of the boundary of the low-Earth orbit satellite beam main lobe projected onto the ground, including: Read the three-dimensional coordinate components in the instantaneous orbital position parameters of the low-Earth orbit satellite to determine the spatial position point of the low-Earth orbit satellite in the agreed Earth coordinate system; The beam pointing angle associated with the low-orbit satellite number information is extracted from the beam pointing angle mapping relationship. The beam pointing angle includes the start angle, end angle and scan step angle of the beam scan, which is used to describe the change law of the beam center axis over time. Based on the orbital velocity of the low-orbit satellite and the beam scanning step angle, calculate the instantaneous pointing direction of the beam center axis relative to the satellite's coordinate system at the current moment; Starting from the low-orbit satellite's spatial position point, a ray is drawn along the instantaneous pointing direction. The coordinates of the intersection point of the ray and the surface of the Earth ellipsoid model are calculated. The coordinates of the intersection point are used as the coordinates of the landing point of the beam center axis on the Earth's surface at the current moment. Based on the beamwidth angle parameter of the main lobe of the low-Earth orbit satellite beam and the slant distance between the low-Earth orbit satellite and the ground, the coverage area of ​​the beam on the ground is calculated, and an elliptical beam coverage area is generated with the landing point coordinates as the center. Its major axis direction and minor axis direction are related to the beam scanning direction and the orbital motion direction. The boundary of the elliptical beam coverage area is parameterized and sampled to generate a sequence of latitude and longitude coordinates of multiple sampling points on the boundary. Curve fitting is performed on the latitude and longitude coordinate sequences of multiple sampling points on the boundary to generate a smooth boundary curve of the low-orbit beam coverage area. The area enclosed by the boundary curve of the low-orbit beam coverage area is converted into a low-orbit beam coverage area polygon, which is formed by connecting the latitude and longitude coordinates of feature points on the boundary curve in sequence.

8. The intelligent dynamic switching method for communication links based on BeiDou and low-orbit satellites according to claim 1, characterized in that, After sending the link dynamic switching relay command to the user terminal baseband processing unit, triggering the user terminal baseband processing unit to perform cross-satellite system signal acquisition and synchronization establishment operations based on the trigger point coordinates and the time parameters, the method further includes a step of dynamically monitoring and adjusting the link quality of the target satellite communication link after switching, specifically including: After switching to the target satellite communication link, the received signal-to-noise ratio (SNR) sequence of the target satellite signal is continuously collected. The received SNR sequence contains multiple SNR sampling points spaced at frame periods. The pilot symbol sequence in the target satellite signal is analyzed, the phase rotation angle of the pilot symbol sequence is calculated, and a carrier phase tracking error sequence is generated. The carrier phase tracking error sequence reflects the phase difference between the local carrier and the target satellite carrier. The frame synchronization header in the target satellite signal is analyzed, the position offset of the correlation peak in the frame synchronization header is detected, and a symbol timing tracking error sequence is generated. The symbol timing tracking error sequence reflects the timing deviation between the local symbol clock and the target satellite symbol clock. The received signal-to-noise ratio sequence, carrier phase tracking error sequence, and symbol timing tracking error sequence are input into the link quality evaluation function. The link quality evaluation function standardizes the various errors and outputs a comprehensive link quality score. The comprehensive link quality score is positively correlated with the signal-to-noise ratio and negatively correlated with the standardized phase error and timing error. The overall link quality score is compared with a preset link quality stability threshold. If the overall link quality score is lower than the link quality stability threshold, the link quality fine-tuning procedure is triggered. In the link quality fine-tuning program, the carrier phase correction amount is calculated based on the mean of the carrier phase tracking error sequence, and the carrier phase correction amount is superimposed on the control word of the local carrier numerically controlled oscillator to gradually reduce the carrier phase tracking error. Simultaneously, the symbol timing correction amount is calculated based on the mean of the symbol timing tracking error sequence, and the symbol timing correction amount is superimposed on the control word of the local symbol clock numerically controlled oscillator to gradually reduce the symbol timing tracking error; After completing carrier phase correction and symbol timing correction, the received signal-to-noise ratio sequence of the target satellite signal is reacquired, a new comprehensive link quality score is calculated, and the new comprehensive link quality score is compared with the preset link quality stability threshold until the new comprehensive link quality score remains stable above the link quality stability threshold.

9. The intelligent dynamic switching method for communication links based on BeiDou and low-orbit satellites according to claim 8, characterized in that, The process involves inputting the received signal-to-noise ratio sequence, carrier phase tracking error sequence, and symbol timing tracking error sequence into a link quality evaluation function, and outputting a comprehensive link quality score, including: Read a preset set of link quality evaluation function coefficients from an internal register. The set of link quality evaluation function coefficients includes signal-to-noise ratio weight coefficients, phase error weight coefficients, and timing error weight coefficients. The received signal-to-noise ratio sequence is subjected to moving average filtering to eliminate signal-to-noise ratio spike noise caused by instantaneous deep fading, and a smoothed signal-to-noise ratio mean sequence is generated. The carrier phase tracking error sequence is subjected to absolute value transformation processing to convert the phase error with positive and negative signs into a sequence of absolute amplitude of phase error, and the root mean square value of the absolute amplitude sequence of phase error is calculated to generate the root mean square parameter of carrier phase error. The symbol timing tracking error sequence is subjected to absolute value transformation processing to convert the timing error with positive and negative signs into a timing error absolute amplitude sequence, and the root mean square value of the timing error absolute amplitude sequence is calculated to generate the symbol timing error root mean square parameter. The last signal-to-noise ratio mean point in the smoothed signal-to-noise ratio mean sequence is taken as the current signal-to-noise ratio representative value; The signal-to-noise ratio weighting coefficient is multiplied by the standardized current signal-to-noise ratio representative value to generate the signal-to-noise ratio contribution component; The phase error weighting coefficient is multiplied by the negative value of the standardized carrier phase error root mean square parameter to generate a phase error penalty component. The phase error penalty component decreases as the standardized carrier phase error root mean square parameter increases. The timing error weighting coefficient is multiplied by the negative value of the standardized root mean square parameter of the symbolic timing error to generate a timing error penalty component. The timing error penalty component decreases as the standardized root mean square parameter of the symbolic timing error increases. The signal-to-noise ratio contribution component, phase error penalty component, and timing error penalty component are summed algebraically to generate an initial comprehensive link quality score. The initial link quality comprehensive score is input into a normalized mapping function, which maps the initial link quality comprehensive score to a preset scoring range, and outputs the final link quality comprehensive score. The final link quality comprehensive score is used to characterize the stability of the current target satellite communication link.

10. A smart dynamic switching system for communication links based on BeiDou and low-orbit satellites, characterized in that, include: The processor, communication interface, memory, and communication bus are provided, wherein the processor, communication interface, and memory communicate with each other via the communication bus. The memory is used to store computer programs; the processor is used to execute the computer programs to implement the steps of the intelligent dynamic switching method for communication links based on BeiDou and low-orbit satellites as described in any one of claims 1-9.

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