MF-TDMA low earth orbit satellite time synchronization method based on feedback mechanism

Abstract

The application discloses a MF-TDMA low-orbit satellite time synchronization method based on a feedback mechanism, which comprises the following steps: a two-way link communication model is established, a satellite sends signals to a ground station terminal through a downlink, and receives signals from the ground station terminal through an uplink; the ground station terminal establishes a ground time scale table through downlink acquisition synchronization, performs coarse time compensation according to a time delay, and sends an uplink signal according to the ground time scale table; the satellite receives the uplink signal, establishes a time error function at a synchronous time, and the ground station terminal dynamically adjusts the ground time scale table with the time error function as a feedback value, performs fine time compensation, until the time synchronization error is eliminated, and fine time synchronization is completed. The application can support time synchronization of multiple ground station terminals and a satellite at the same time, realizes efficient utilization of spectrum resources, and can realize real-time optimization of resource allocation through the feedback mechanism, and has high real-time performance and accuracy.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of satellite communication, and relates to a MF-TDMA low-orbit satellite time synchronization method based on a feedback mechanism. BACKGROUND

[0002] With the rapid development of satellite communication technology, low-orbit satellite communication technology is widely used in data transmission, remote sensing monitoring, navigation positioning and other fields. And due to the high-speed movement and complex environment of low-orbit satellites, the communication is affected by low signal-to-noise ratio and high dynamic factors. There is a large relative radial motion between satellite payload and ground terminal, and the time synchronization between satellite and terminal becomes a problem to be solved. The error of time synchronization may cause confusion of data transmission, decrease of positioning accuracy and reduction of network efficiency.

[0003] The orbit height and speed of low-orbit satellites change, which changes the transmission time delay between satellites and ground terminals, increasing the difficulty of time synchronization. Some high-precision time synchronization algorithms may have high complexity, requiring a large amount of computing resources and time. This may result in the inability to achieve real-time synchronization in practical applications, or the need for additional hardware support to speed up the execution of the algorithm.

[0004] Multi-frequency time division multiple access (MF-TDMA) system is based on the demand for efficient spectrum utilization and multi-user access in the field of wireless communication. MF-TDMA is a hybrid multiple access technology combining time division multiple access (TDMA) and frequency division multiple access (FDMA), which allows multiple users to use allocated frequency resources in different time slots, thereby achieving efficient multi-user communication. In satellite communication, due to the large signal propagation delay, the difficulty of time synchronization is further increased. The synchronization algorithm using feedback mechanism dynamically adjusts the time slot allocation and clock bias. The core idea of feedback mechanism is to dynamically adjust the clock frequency and phase by real-time monitoring of clock state, signal propagation delay and network topology changes, so as to realize real-time compensation of time error.

[0005] Common methods include ranging method, pseudo-code high-precision distance measurement method and two-way time comparison method. These methods achieve time synchronization of satellites to some extent, but still have shortcomings. Among them, the ranging method is limited by factors such as atmospheric delay and multipath effect; the pseudo-code high-precision distance measurement method relies on more complex signal processing and hardware system support; the two-way time comparison method requires high coordination between satellite and ground station. For low-orbit satellite communication in high dynamic environment, it is particularly important to develop an efficient and accurate low-orbit satellite time synchronization method. SUMMARY

[0006] The present application aims to solve the technical problems of real-time synchronization difficulty and large error between satellite and ground terminal. The present application provides a MF-TDMA low-orbit satellite time synchronization method based on feedback mechanism, and the technical scheme is as follows:

[0007] A MF-TDMA low-orbit satellite time synchronization method based on feedback mechanism, comprising the steps of:

[0008] S1, establishing a bidirectional link communication model, the bidirectional link communication model comprising a downlink and an uplink, the satellite sending signals to the ground station terminal through the downlink and receiving signals from the ground station terminal through the uplink;

[0009] S2, the ground station terminal establishing a ground time scale table through downlink acquisition synchronization;

[0010] S3, the ground station terminal performing coarse time compensation according to time delay;

[0011] S4, the ground station terminal sending uplink signals according to the ground time scale table, the satellite receiving the uplink signals, synchronizing time, and establishing a time error function;

[0012] S5, the ground station terminal taking the time error function as a feedback value, dynamically adjusting the ground time scale table, performing fine time compensation, until the time synchronization error is eliminated, and fine time synchronization is completed.

[0013] In an embodiment of the present application, the S1 comprises:

[0014] S11, designing a signal frame format of the downlink, the data of a synchronization time slot being a pseudo-random sequence and a time stamp, the length of the synchronization time slot being T1, the number of service information being n, i.e. n service time slots, the length of the service time slot being T2, and the time length of a frame of data being T1+nT2;

[0015] S12, designing a signal frame format of the uplink, the data of a synchronization header carrying ID address information, the length of a time slot being Tu1, the number of service information being m, i.e. m service time slots, the length of a time slot being Tu2, and the time length of a frame of data being Tu1+mTu2.

[0016] In an embodiment of the present application, the S2 comprises:

[0017] S21, establishing a time scale table t s with a clock on the satellite as a reference, one scale time being Δt, the dimension being microsecond level, the time of a frame of data being l times of Δt, the satellite uninterruptedly emitting downlink signals at integer multiple of lΔt, i.e. each time stamp information being an integer multiple of lΔt;

[0018] S22, the ground station terminal obtains a preliminary estimation of Doppler frequency offset by calculating real-time position and speed of the satellite, performs coarse frequency offset compensation, demodulates the downlink signal after frequency down conversion processing, correlates with the local sequence, and performs frequency offset estimation, performs fine frequency offset compensation according to the estimation value, and realizes downlink acquisition synchronization;

[0019] S23, the ground time scale is established.

[0020] In an embodiment of the present application, the S23 comprises:

[0021] S231, the ground station terminal adopts CRC check on the timestamp information, and judges whether the received downlink signal conforms to the satellite motion law or is disturbed by a complex electromagnetic environment according to the change rate of the historical frequency offset estimation value and the signal quality factor;

[0022] S232, the ground station terminal sets the ground time scale t g , accumulates timing with a Δt time scale, and periodically maintains and updates the time scale with the completely passed timestamp information.

[0023] In an embodiment of the present application, the S3 comprises:

[0024] The coarse time compensation is implemented according to the time delay between the satellite and the ground and the local hardware and software processing time delay;

[0025] The ground station terminal estimates the time delay between the satellite and the ground according to the ephemeris, and performs coarse time compensation, and the compensation amount is t coarse .

[0026] The ground station terminal tests and calculates the inherent time offset of the hardware and software of the ground system, and the compensation amount is t co .

[0027] In an embodiment of the present application, the S4 comprises:

[0028] S41, the ground station terminal sends an uplink signal according to the ground time scale, and the satellite receives the uplink signal;

[0029] S42, the satellite calculates a time error function for the ground station terminal according to the ID address extracted from the uplink synchronization header, and configures the time error function in the first service information time slot, i.e. the state time slot, of the downlink signal.

[0030] In an embodiment of the present application, the representation method of the time error function is:

[0031] Given the ground station terminal time scale t g , the accurate time scale t sOnly in satellite record, using master-slave time synchronization, taking satellite as master, the clock of each ground station terminal is in tracking reference clock state, satellite receives the uplink signal sent by the ground station terminal, the synchronization time is t', the satellite expects synchronization time is t e , the time error function e is established, the expression is e=t'-t e .

[0032] In one embodiment of the present application, the S4 further comprises:

[0033] S43, by analyzing the rate of change of the time error function of each ID address, the channel quality is evaluated, and the frequency and time slot allocation are dynamically adjusted to avoid the frequency band with serious interference.

[0034] In one embodiment of the present application, the S5 comprises:

[0035] The ground station terminal solves the corresponding time error function according to the ID address contained in the state time slot, if the corresponding time error function cannot be solved, it means that the ground station terminal has not realized uplink synchronization, at this time, the search type adjustment of the time compensation amount is needed until the uplink synchronization, and the time error function is used as the feedback value to realize the fine time compensation.

[0036] In one embodiment of the present application, the time compensation amount is represented as: Delta T=t coarse +t co -e, wherein t coarse is the coarse time compensation amount of the inter-satellite time delay, t co is the compensation amount of the hardware and software, and e is the time error function.

[0037] The beneficial effects of the present application are:

[0038] 1. The MF-TDMA low-orbit satellite time synchronization method based on the feedback mechanism of the present application establishes the time error function by the synchronization time of the uplink and the expected synchronization time set by the local time reference of the satellite, and builds the feedback mechanism, the ground station terminal can dynamically adjust the time compensation amount according to the feedback value, and follow and track the time of the satellite, even if the uplink is out of step at a certain beat, the ground station terminal can scan and adjust the time compensation amount within a reasonable time interval, realize the coarse synchronization of the uplink, and realize the fine synchronization by the feedback mechanism, which has high real-time and accuracy, and is easy to realize.

[0039] 2. The MF-TDMA low-orbit satellite time synchronization method based on a feedback mechanism according to the present application divides a frequency band into multiple sub-frequency bands (FDMA) in an MF-TDMA system, and further divides time slots (TDMA) in each sub-frequency band, thereby achieving efficient use of frequency spectrum resources, and optimizing resource allocation in real time through a feedback mechanism to ensure communication quality, that is, by analyzing the rate of change of the time error function of each ID address, the evaluation of channel quality is realized, and the frequency and time slot allocation is dynamically adjusted to avoid frequency bands with serious interference. Compared with the traditional time synchronization method, the present application can simultaneously support multiple ground station terminals to perform time synchronization with a satellite, and in the case of a poor channel, the time synchronization can be further optimized through the allocation of time slot resources and frequency domain resources. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 is a flowchart of the MF-TDMA low-orbit satellite time synchronization method based on a feedback mechanism according to an embodiment of the present application;

[0041] Figure 2 is a schematic diagram of a one-satellite and multiple-grounds model according to an embodiment of the present application;

[0042] Figure 3 is a schematic diagram of a signal frame format of a downlink according to an embodiment of the present application;

[0043] Figure 4 is a schematic diagram of a signal frame format of an uplink according to an embodiment of the present application. DETAILED DESCRIPTION

[0044] The present application will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0045] The present application provides an MF-TDMA low-orbit satellite time synchronization method based on a feedback mechanism, which establishes a bidirectional communication link with high bandwidth, low delay and anti-interference capability to ensure the transmission and reception of synchronization signals. Based on the bidirectional link, a feedback mechanism is designed, and error processing, i.e., time compensation, is performed to finally achieve precise time synchronization.

[0046] Referring to the accompanying drawings, Figure 1 The MF-TDMA low-orbit satellite time synchronization method based on a feedback mechanism includes the following steps:

[0047] S1, a bidirectional link communication model is established, which includes a downlink and an uplink. The satellite sends signals to the ground station terminal through the downlink, and receives signals from the ground station terminal through the uplink;

[0048] S2, the ground station terminal establishes a ground time scale table through downlink acquisition synchronization;

[0049] S3, the ground station terminal performs coarse time compensation according to the time delay;

[0050] S4, the ground station terminal sends an uplink signal according to a ground time scale, the satellite receives the uplink signal, synchronizes the time, and establishes a time error function;

[0051] S5, the ground station terminal dynamically adjusts the ground time scale by taking the time error function as a feedback value, performs fine time compensation, until the time synchronization error is eliminated, and fine time synchronization is completed.

[0052] The ground station terminal is assigned an ID address, a satellite can establish a link communication with multiple ground terminal stations, and a multiple access technology is used to distinguish the signals of different ground stations. The uplink signal has multiple frequency points that can be selected to send, and multiple time slots that can be configured to send business information, which can be sent in full time slots or in single time slots.

[0053] In the application, S1 includes downlink design and uplink design.

[0054] S11, the signal frame format of the downlink is designed, the data of the synchronization time slot is a pseudo-random sequence and a time stamp, the synchronization time slot length is T1, the business information has n, that is, there are n business time slots, the business time slot length is T2, and the time length of a frame of data is T1+nT2.

[0055] In order to separate the data of each time slot, the length of a frame of data needs to be known, and the time point of the synchronization time slot can be found by capturing the synchronization, and the data of which time slot can be known by calculating how much time length backward. In order to carry more information, the business information adopts a higher symbol rate. And x frames constitute a super frame for sending data.

[0056] S12, the signal frame format of the uplink is designed, the data of the synchronization header carries ID address information, the time slot length is Tu1, the business information has m, that is, there are m business time slots, the time slot length is Tu2, and the time length of a frame of data is Tu1+mTu2. And x frames constitute a super frame for sending data.

[0057] There are multiple carrier modes to select for up-conversion to realize frequency division multiple access. Time division multiple access is reflected by the frame format, that is, different time slots have different data, which are separated in time. The downlink signal and the uplink signal are both time division multiple access. The downlink is a signal sent by the satellite, and each terminal receives the same signal, and the data in different business time slots is allocated to different terminals. Frequency division multiple access means that the uplink signals sent by different terminals are at different frequency points, so that multiple terminals can send uplink signals to the satellite at the same time. If all the uplink signals are at the same frequency point, the uplink signals will conflict.

[0058] The downlink frame has synchronization time slot data in each frame, and the uplink only has a synchronization header in the first frame of the superframe, so as to reduce the processing pressure of the satellite on the uplink synchronization, because the satellite needs to process the uplink signals from multiple terminals.

[0059] In one embodiment of the present application, S2 comprises:

[0060] S21, the satellite-ground system is started, and a time scale t is established based on the clock on the satellite s A scale time is Δt, and the time of one frame of data is l times of Δt. The satellite continuously transmits the downlink signal at the integer multiple of lΔt, that is, each time stamp information is an integer multiple of lΔt.

[0061] S22, the ground station terminal calculates the real-time position and speed of the satellite for a period of time, and then obtains a preliminary estimation of the Doppler frequency offset, performs coarse compensation of the frequency offset, demodulates the downlink signal processed by frequency down-conversion, and correlates with the local sequence, and performs frequency offset estimation, then performs fine compensation of the frequency offset according to the estimation value, and realizes downlink acquisition synchronization. The pseudo-random sequence of the synchronization time slot is used to correlate the signal with the local pseudo-random sequence, and it is detected whether the correlation peak value is greater than the set decision threshold, so as to realize acquisition synchronization.

[0062] The pseudo-random sequence of the synchronization time slot is used to correlate the signal with the local pseudo-random sequence, and it is detected whether the correlation peak value is greater than the set decision threshold, so as to realize acquisition synchronization. A carrier synchronization algorithm PMF-FFT (segmented matched filter- fast Fourier transform combination) is used to estimate the frequency offset.

[0063] Finally, the time stamp information and the service information are successfully differentially demodulated. The time stamp signal is differentially BPSK (Binary Phase Shift Keying) modulated, and the rest is differentially QPSK (Quadrature Phase Shift Keying) modulated.

[0064] S23, a ground time scale is established. In order to avoid that the error of the time stamp information directly affects the system work, the ground station terminal adopts CRC (Cyclic Redundancy Check) verification on the time stamp information, and according to the change rate of the historical frequency offset estimation value and the signal quality factor, it is judged whether the received downlink signal conforms to the satellite motion law or is disturbed by the complex electromagnetic environment. The remote movement of the satellite is from far to near, and then from near to far. The low-orbit satellite revolves around the earth, and the frequency offset is linearly changed. If the difference between two consecutive frequency offset estimation values is abnormal, it is possible that the data in this section is abnormal, so it can be judged that the downlink signal is disturbed, for example, in rainy days, the communication is poor, and the change rate fluctuation is large.

[0065] The specific method of the ground station terminal for checking the time stamp information is that the sender performs modulo-2 division on the original data and a generating polynomial to obtain a remainder (i.e. a CRC check code), and appends the remainder to the original data and sends the appended data. The receiver performs modulo-2 division on the received data using the same generating polynomial, and if the remainder is 0, the data is correct.

[0066] Thus, the ground station terminal sets a ground time scale t g , accumulates timing in a unified manner with a time scale of Δt, and periodically maintains and updates the time stamp information with complete check pass. Therefore, t g The difference in actual ground 1-second time is dynamically scalable.

[0067] In one embodiment of the application, S3 includes performing coarse time compensation according to the inter-satellite time delay and the local hardware and software processing time delay. The ground station terminal estimates the inter-satellite time delay in combination with the ephemeris to perform coarse time compensation, and the compensation amount is t coarse ; and the ground station terminal tests and calculates the inherent time offset of the hardware and software of the ground system, and the compensation amount is t co .

[0068] In one embodiment of the application, S4 includes:

[0069] S41, the ground station terminal sends an uplink signal according to the ground time scale, and the satellite receives the uplink signal.

[0070] After the coarse time compensation, the uplink synchronization probe head can be sent according to the local time scale. Compared with the downlink which has synchronization information in each frame, the uplink has a synchronization head in every x frames, and the synchronization head time slots of the remaining frames are filled with 0. Assuming that the time error function is 0, the synchronization probe head is sent at an integer multiple of xlΔt. However, in the actual environment, due to factors such as the inter-satellite time delay and satellite movement, the uplink transmission needs to be adjusted for early transmission or delayed transmission according to the time compensation amount. When the transmission is delayed, only the next x frames of data need to be delayed as a whole, and the previous frame of data will not be affected. If the transmission is early, the previous frame of data needs to be truncated.

[0071] S42, after the synchronization of the satellite, the satellite calculates the time error function for the corresponding ground station terminal according to the ID address extracted from the uplink synchronization head, and configures the time error function in the first service information time slot of the downlink signal, i.e. the state time slot. One ID address is followed by one time error function value, and multiple ID addresses are included in one state time slot.

[0072] The ground station terminal time scale t g is known, and the accurate time scale t sOnly in satellite record, using master-slave time synchronization, satellite as the main, each ground station terminal clock in tracking reference clock state, satellite received ground station terminal sent uplink signal, synchronization time for t', satellite synchronization time for t e , establish time error function e, expression is: e = t'-t e . For e, for positive value indicates that the ground terminal station sent uplink signal lag, for negative value indicates that the ground terminal station sent uplink signal ahead.

[0073] S43, through the analysis of each ID address time error function rate of change, realize the evaluation of channel quality, dynamic adjustment of frequency and time slot allocation to avoid serious interference frequency band.

[0074] Ground station terminal also according to state information contains the ID address to solve the corresponding time error function, if failed to solve the corresponding time error function, it represents that the ground station terminal has not realized the uplink synchronization, at this time, need to search for the time compensation in a reasonable interval adjustment, until the uplink synchronization, with time error function as feedback value to realize the precise time compensation.

[0075] The length of superframe is fixed, time compensation will make frame move in time, the moving amount is too large will exceed the range, superframe will move to more than the last superframe position, or more than the next superframe position.

[0076] Satellite received each terminal sent uplink signal, will be synchronized, satellite will set a standard synchronization time range, in the range, synchronization operation, the range is generally set to a superframe length, if the synchronization time t' exceeds the range, it is not considered uplink synchronization, only when the uplink synchronization is considered successful, there will be time error function configuration. If the ground terminal sent uplink signal, but the downlink state time slot does not solve the time error function, it represents that the uplink is not synchronized with the satellite, that is, out of step.

[0077] The expression of time compensation is: ΔT = t coarse +t co -e, wherein, t coarse is the coarse time compensation of the time delay between satellite and ground, t co is the compensation of hardware and software, e is the time error function. For ΔT, t coarse , t co , the time compensation is positive value indicating the adjustment amount of time delay, negative value indicating the adjustment amount of time advance.

[0078] A complete feedback mechanism is thus established, the ground station terminal iteratively adjusts the time compensation in a tracking form, and finally realizes the state that the time error function tends to 0, that is, the time error function is less than the set value of the accuracy requirement, and is regarded as successful completion of time precision synchronization. The uplink signal can be sent at different frequencies, and can also be sent at different time slots. The application optimizes resource allocation in real time through the feedback mechanism, ensures communication quality, and stabilizes the rate of change of the time error function of each ID address at the satellite end, determines that the channel quality is good, and the fluctuation is large, and the channel quality is relatively poor.

[0079] In summary, the MF-TDMA low-orbit satellite time synchronization method based on the feedback mechanism of the application establishes a two-way link communication model, designs an MF-TDMA system, and the frame format of the downlink and uplink is embodied. The time clock of the satellite is the reference of the entire satellite-ground system, the downlink signal is sent by the satellite to each ground station terminal, the ground station terminal acquires timestamp information through acquisition synchronization, establishes its own local time scale, performs coarse time synchronization according to the satellite-ground time delay and local hardware and software processing time delay, and then sends the uplink signal corresponding to the ID. The satellite receives the uplink signal, evaluates it synchronously, and issues the feedback value. The ground station terminal dynamically adjusts the local clock with the feedback value, tracks the satellite time clock, and finally eliminates the time synchronization error to realize precise time synchronization.

[0080] Unlike the traditional time synchronization that requires two-way time synchronization adjustment, the application is a master-slave time synchronization method, which has a clear architecture. The satellite is responsible for generating and distributing the time reference signal, and the ground station terminal only needs to receive and adjust the local clock, and then send the uplink synchronization signal for the satellite to evaluate the corresponding ground station terminal to generate the time error function, without adjusting the time clock of the satellite. And the satellite as the center of the network can efficiently manage and synchronize the time of all ground station terminals, ensuring the time consistency of the entire network. It is not necessary to adjust the time signal in real time according to the motion state and Doppler effect in high-precision mode, but only to ensure a certain accuracy of the time signal adjustment, and to realize the effective improvement of the accuracy with the aid of the feedback mechanism, greatly enhancing the adaptability of low-orbit satellite communication to high-dynamic change environment.

[0081] The above is only a specific embodiment of the application, but the protection scope of the application is not limited thereto. Any modification, equivalent replacement and improvement made by any person skilled in the art within the technical scope disclosed by the application, as long as it is within the spirit and principles of the application, should be covered within the protection scope of the application.

Claims

1. A time synchronization method for low-Earth orbit satellites based on a feedback mechanism, characterized in that, Including the following steps: S1. Establish a bidirectional link communication model, which includes a downlink and an uplink. The satellite sends signals to the ground station terminal through the downlink and receives signals from the ground station terminal through the uplink. S2. The ground station terminal establishes a ground time scale table through downlink capture synchronization. S3. The ground station terminal performs coarse time compensation based on the delay. S4. The ground station terminal sends uplink signals according to the ground time scale, the satellite receives the uplink signals, synchronizes the time, and establishes a time error function. S5. The ground station terminal uses the time error function as feedback value to dynamically adjust the ground time scale, perform fine time compensation, until the time synchronization error is eliminated and fine time synchronization is completed. Wherein, S1 includes: S11. Design the signal frame format of the downlink, where the data in the synchronization time slot is a pseudo-random sequence and a timestamp, and the synchronization time slot length is... The business information is total There are a total of [number], that is, a total of [number] Each service time slot, with a service time slot length of [number] times. The time length of one frame of data is ; S12. Design the signal frame format of the uplink, wherein the synchronization header data carries ID address information, and the time slot length is [missing information]. The business information is total There are one, that is, there are Each service time slot, with a time slot length of [number] times. The time length of one frame of data is ; S2 includes: S21. Establish a time scale based on the clock on the satellite. One scale unit is The unit is microseconds, and the time of one frame of data is... of Times, satellite in Downlink signals are transmitted continuously at integer multiples of each other, meaning each timestamp is... Integer multiples of; S22. The ground station terminal calculates the real-time position and velocity of the satellite to obtain a preliminary estimate of the Doppler frequency offset, performs coarse frequency offset compensation, demodulates the downlink signal after downconversion processing, correlates it with the local sequence, and performs frequency offset estimation. Based on the estimated value, it performs fine frequency offset compensation to achieve downlink acquisition synchronization. S23. Establish the ground time scale table; S23 includes: S231. The ground station terminal performs CRC verification on the timestamp information and determines whether the received downlink signal conforms to the satellite motion law or is affected by complex electromagnetic environment interference based on the rate of change of the historical frequency offset estimate and signal quality factors. S232, The ground station terminal is equipped with the ground time scale. Unified The time scale is accumulated and periodically maintained and updated using fully verified timestamp information; S4 includes: S41. The ground station terminal sends uplink signals according to the ground time scale, and the satellite receives the uplink signals. S42. The satellite calculates a time error function for the ground station terminal based on the ID address obtained from the uplink synchronization header, and configures the time error function in the first service information time slot of the downlink signal, i.e., the status time slot. The time error function is represented as follows: The ground station terminal time scale table is known. The satellite terminal has an accurate time scale. Recording is done only on satellite, using a master-slave time synchronization mechanism, with the satellite as the master. The clocks of all ground station terminals are tracking the reference clock. The satellite receives the uplink signal from the ground station terminals, and the synchronization time is... The expected synchronization time of the satellite is Establish time error function The expression is: .

2. The MF-TDMA low-Earth orbit satellite time synchronization method based on a feedback mechanism according to claim 1, characterized in that, S3 includes: Coarse time compensation is implemented based on the time delay between satellite and ground stations and the local hardware and software processing delay. The ground station terminal, using ephemeris data, estimates the time delay between the satellite and the ground station and performs coarse time compensation. The compensation amount is... ; The ground station terminal tests and calculates the inherent time offset caused by the hardware and software of the ground system, and the compensation amount is... .

3. The MF-TDMA low-Earth orbit satellite time synchronization method based on a feedback mechanism according to claim 1, characterized in that, S4 further includes: S43. By analyzing the rate of change of the time error function of each ID address, the channel quality can be evaluated, and the frequency and time slot allocation can be dynamically adjusted to avoid frequency bands with severe interference.

4. The MF-TDMA low-Earth orbit satellite time synchronization method based on a feedback mechanism according to claim 1, characterized in that, S5 includes: The ground station terminal solves the corresponding time error function based on the ID address contained in the status time slot. If the corresponding time error function cannot be solved, it means that the ground station terminal has not achieved uplink synchronization. At this time, the time compensation amount needs to be adjusted in a search manner until uplink synchronization is achieved, and the time error function is used as a feedback value to achieve precise time compensation.

5. The MF-TDMA low-Earth orbit satellite time synchronization method based on a feedback mechanism according to claim 4, characterized in that, The time compensation amount is expressed as: ,in, This is a coarse time compensation amount for the time delay between satellite and ground. This refers to the compensation amount for both hardware and software. This is the time error function.

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