A Method for Autonomous Establishment of Time Reference for Low Earth Orbit Constellations

Abstract

The present invention relates to a method for autonomously establishing a time reference for a low-orbit constellation, and belongs to the field of satellite time synchronization. When establishing the time reference of the present invention, the low-orbit satellite is required to simultaneously observe the Beidou satellite visible to itself, and use the laser inter-satellite link to perform time comparison, and then summarize the comparison results to the designated low-orbit satellite through the low-orbit laser link, and finally complete the comprehensive time difference calculation on the designated low-orbit satellite, and finally realize the establishment of a high-precision time reference for the low-orbit constellation. The present invention has autonomy and high precision, and its significance is that even if the ground facilities are unavailable during special periods, the autonomous timing and punctuality capabilities of the low-orbit constellation can still be achieved through the method of the present invention, thereby providing time services for important space and ground-related equipment.

Description

Technical Field

[0001] The present invention belongs to the field of satellite time synchronization, and particularly relates to a method for autonomously establishing a time reference for a low-earth orbit constellation. Background Art

[0002] Currently, low-earth orbit satellite constellation systems such as navigation augmentation, communication, meteorology, and remote sensing are developing rapidly, such as Starlink, OneWeb, and Kuiper Constellation abroad, and Weili Space, Qianfan, Tianqi, Tianmu-1, and Jilin-1 in China. The realization of the functions of these constellation systems and the multi-task collaboration among users all require high-precision time reference for support. Compared with medium and high-earth orbit satellites, the number of satellites in the low-earth orbit constellation system is huge, and the capabilities and funds of single-satellite platforms are limited. Due to the high price of high-performance spaceborne atomic clocks, in order to control costs, low-earth orbit constellations usually choose to install atomic clocks or crystal oscillators with lower prices and poorer performance.

[0003] There are usually the following technical approaches for the method of establishing the time reference of the low-earth orbit satellite constellation. One is to realize it based on a high-precision ground clock group, the space-ground link between the ground station and the low-earth orbit satellite. However, this method relies heavily on the ground time reference and cannot meet the requirements for establishing the time reference of the low-earth orbit constellation when the ground clock group fails or is invisible. In addition, due to the huge number of satellites, relying on the ground station to establish the time reference will overload the ground system. The second is to realize it based on the high-precision time reference of the Beidou satellite and the downlink navigation signal link. Since the downlink navigation signal is weak and vulnerable to interference, and is affected by factors such as the ionosphere, its availability and accuracy are limited under complex electromagnetic environment conditions. The third is to realize it based on the inter-satellite link measurement and comparison between low-earth orbit satellites themselves. Through the inter-satellite link, mutual measurement and comparison are carried out between low-earth orbit satellites, and the time reference of the low-earth orbit constellation is comprehensively generated. Due to the poor performance of atomic clocks or crystal oscillators, it is difficult to achieve high precision.

[0004] The spaceborne atomic clock is called the "heart" of the navigation satellite, and its role is to provide a highly stable time and frequency reference signal for the satellite system. It determines the navigation positioning, speed measurement, and timing accuracy of the navigation system, and is one of the core technologies for a country to independently develop a navigation system.

[0005] The Beidou global navigation constellation currently has high-precision time reference and autonomous maintenance capabilities. First, the Beidou system is equipped with high-precision and high-stability spaceborne hydrogen atomic clocks (the day stability is better than 10E-14) on the satellites. Second, Beidou can use the laser link to continuously carry out precise inter-satellite measurements of the entire constellation and provide data transmission to achieve high-precision time synchronization of the entire constellation. Therefore, the Beidou global navigation constellation has the ability to autonomously maintain high stability of time for the entire constellation.

[0006] Considering the payload cost of spaceborne atomic clocks, currently, low-Earth orbit satellites generally use spaceborne atomic clock devices with sub-optimal performance. Their short-term stability performance is relatively high, but they require frequent time calibration. For example, when Iridium satellites use a temperature-controlled crystal oscillator to generate communication signals and maintain the system time, the performance is very stable in the short term. However, after 100 seconds, the clock error and clock drift stability are lower than those of spaceborne atomic clocks. The frequency stability of Iridium satellite clocks is generally better than 10 -11 , showing flicker noise and random walk noise before and after 1000 seconds respectively. Therefore, at least 2 frequency adjustments are required every day to achieve time synchronization. Therefore, a highly stable clock source and high-precision comparison technology are the keys to establishing and maintaining the time of low-Earth orbit constellations.

[0007] As a key technology, the laser link plays an important role in the measurement and information exchange between spacecraft in the outer space of the Earth. Its application significantly reduces the dependence of low-Earth orbit satellite mobile communication systems on ground networks, thus enhancing the routing flexibility of the system and the convenience of network management. Research results show that when using a laser link to perform time synchronization measurement between a satellite and a ground clock source, the achievable error standard deviation is less than 0.05 nanoseconds, demonstrating extremely high precision. Furthermore, since the inter-satellite link is almost not affected by the atmosphere, its time comparison accuracy is superior to that of satellite-ground comparison. This characteristic provides strong support for the establishment and maintenance of high-precision time synchronization in low-Earth orbit constellation systems.

[0008] Currently, the time synchronization system of low-Earth orbit satellite constellations is mainly built on the basis of ground clock groups. Although this traditional approach meets the basic requirements to a certain extent, it also faces various limitations.

[0009] First of all, for China, the uneven distribution of ground monitoring stations makes it difficult to achieve global coverage. This shortcoming directly restricts the comprehensive monitoring and efficient time synchronization of low-Earth orbit satellite groups over foreign regions, resulting in obvious deficiencies in satellite time calibration in some remote or high-latitude regions.

[0010] Second, due to being close to the Earth's atmosphere, low-Earth orbit satellites are frequently affected by atmospheric drag. This dynamic change not only accelerates the drift of satellite orbit parameters but also poses a severe test to the time reference system carried by the satellite. Minor changes in orbit parameters can cause error accumulation in time measurement, thereby damaging the accuracy and long-term stability of the time reference, which constitutes a major obstacle to application scenarios that rely on high-precision time services.

[0011] Thirdly, if the LEO constellation relies on ground infrastructure for time calibration in the long term, its autonomous operation and independent service capabilities will be greatly reduced. When ground facilities fail, are under maintenance, or are affected by external interference, the constellation may face the risk of time synchronization interruption, which will in turn affect the stable operation of the entire constellation and its global service capabilities. This poses a major challenge to enhancing the autonomy and resilience of satellite constellations.

[0012] Therefore, it is urgent and necessary to explore a time synchronization scheme that does not rely on the ground, has high stability and autonomy, and establish a time reference for the LEO constellation based on space with high-precision autonomous time maintenance capabilities for the navigation constellation. Summary of the Invention

[0013] (1) Technical Problems to be Solved

[0014] The technical problem to be solved by the present invention is how to provide a method for autonomously establishing a time reference for a LEO constellation to solve the problem of establishing a time reference for a LEO constellation.

[0015] (2) Technical Solutions

[0016] To solve the above technical problems, the present invention proposes a method for autonomously establishing a time reference for a LEO constellation, which includes the following steps:

[0017] Step S1: The Beidou satellite navigation constellation is equipped with a laser link communication payload;

[0018] Step S2: All satellites in the LEO satellite system are equipped with communication payloads that can be compatible with the Beidou laser link system at the same time, and the entire LEO constellation has efficient information synchronization capabilities and computing nodes;

[0019] Step S3: Information synchronization: The LEO constellation autonomously formulates a link establishment schedule, and synchronizes the Beidou ephemeris, LEO satellite ephemeris information, and link establishment schedule to each LEO satellite through the laser link;

[0020] Step S4: According to the link establishment schedule, the LEO satellite G1 and the visible Beidou satellites perform beam bidirectional orientation;

[0021] Step S5: After the beam orientation is completed, establish an inter-satellite laser link and carry out two-way distance observation;

[0022] Step S6: Through the distance measurement, calculate the time difference between the LEO satellite G1 and the Beidou satellite

[0023] Step S7: Other LEO satellites repeat steps S4 to S6 and complete time comparison measurements, and send all measurement results to a designated LEO satellite;

[0024] Step S8: On the specified low-earth orbit satellite, the satellite resource management system starts the calculation task and calculates the mean time difference, thereby obtaining the reference time of the low-earth orbit constellation. Then, the low-earth orbit satellite system synchronizes the results through the laser link.

[0025] (III) Beneficial Effects

[0026] The present invention provides a method for independently establishing the time reference of a low-earth orbit constellation. Based on the high-precision time reference of Beidou satellites and the laser link, the present invention first compares the time between Beidou satellites and low-earth orbit satellites, then transmits the comparison information through the inter-satellite link between low-earth orbit satellites, and then uses the resources of the low-earth orbit constellation itself to complete the calculation of the mean time difference, and finally realizes the establishment of the high-precision time reference of the low-earth orbit constellation. The present invention is a method for establishing and maintaining the time reference based on the autonomous ability of the constellation and the current high-precision measurement technology of lasers. Its characteristics are autonomy and high precision. Its significance is that even in special periods when ground facilities are unavailable, the autonomous timing and timekeeping capabilities of the low constellation can still be realized through the method of the present invention, thereby providing time services for important equipment related to space and ground. Description of the Drawings

[0027] Figure 1 It is the overall link topology diagram of the present invention;

[0028] Figure 2 It is the technical flow chart of the present invention;

[0029] Figure 3 It is the schematic diagram of the time difference calculation method of the present invention. Detailed Embodiments

[0030] To make the objectives, contents, and advantages of the present invention clearer, the following further describes in detail the specific embodiments of the present invention with reference to the drawings and embodiments.

[0031] When establishing the time reference of the present invention, it is required that the low-earth orbit satellites simultaneously observe the visible Beidou satellites by themselves, and use the laser inter-satellite link for time comparison. Then, the comparison results are summarized to the specified low-earth orbit satellite through the low-earth orbit laser link (the specified low-earth orbit satellite is specified by the autonomous decision-making system on the low-earth orbit satellite according to satellite mission arrangements, calculation energy consumption, etc.). Finally, the comprehensive time difference calculation is completed on the specified low-earth orbit satellite. The overall link topology is as Figure 1 shown:

[0032] The technical process of the present invention is as Figure 2 shown:

[0033] Step S1: The autonomous timekeeping accuracy of the Beidou satellite navigation constellation is higher than 10 -14 above and is equipped with a laser link communication payload;

[0034] Step S2: Communication payloads compatible with the Beidou laser link system are carried on all satellites of the low-earth orbit satellite system, and the entire low-earth orbit constellation has efficient information synchronization capabilities and computing nodes;

[0035] Step S3: Information synchronization: The low-earth orbit constellation independently formulates a link establishment planning table and synchronizes the Beidou ephemeris, low-earth orbit satellite ephemeris information, and link establishment planning table to each low-earth orbit satellite through the laser link;

[0036] Step S4: According to the link establishment planning table, the low-earth orbit satellite G1 and the visible Beidou satellites perform two-way beam orientation;

[0037] Step S5: After the beam orientation is completed, an inter-satellite laser link is established, and two-way distance observations are carried out;

[0038] Step S6: Through the distance measurement, calculate the time difference between the low-earth orbit satellite G1 and the Beidou satellite As shown in the figure, assume that the low-earth orbit satellite G1 transmits a measurement signal to the Beidou satellite at the local time t0. This signal is reflected after reaching the Beidou satellite at t1, and G1 receives the echo signal at t2. By integrating the information of the t1 moment transmitted by the Beidou satellite, the clock difference between the Beidou satellite

[0039] and the low-earth orbit satellite G1 is solved. Assume that the time difference between the low-earth orbit satellite clock and the Beidou satellite clock is Figure 3 The time interval between the moment t0 when the low-earth orbit satellite measurement signal is sent and its second pulse is T G , the time interval between the Beidou satellite time t1 and the Beidou satellite second pulse is T s , the light travel time of the uplink signal is represented by τ up , the light travel time of the downlink signal is represented by τ down , and the time elapsed from the transmission to the return of the measurement signal is represented by τ. Then, the time difference between the low-earth orbit satellite clock and the Beidou satellite clock can be calculated through the following formula.

[0040]

[0041]

[0042]

[0043]

[0044] Step S7: At the same time, other low-earth orbit satellites repeat steps S4 to S6 and complete time comparison measurements, and send all measurement results to the designated low-earth orbit satellite;

[0042] Each low-earth orbit satellite only measures the visible Beidou satellites at the measurement moment, and which ones specifically will be determined in advance in the link establishment planning table. Each low-earth orbit satellite reports its own and the received measurement information, and the redundant information involved will be processed by the computing unit on the designated low-earth orbit satellite.

[0043] A specified low-earth orbit satellite refers to a certain satellite agreed upon in advance on the ground that has an efficient computing unit and autonomous operating software, mainly emphasizing a satellite with relevant capabilities. It does not specifically refer to a certain one.

[0044] Step S8: On the specified low-earth orbit satellite, the satellite resource management system starts a computing task and calculates the mean time difference, thereby obtaining the reference time of the low-earth orbit constellation. Then, the low-earth orbit satellite system synchronizes the results through a laser link.

[0045] The comparison measurement results are weighted and averaged to obtain the mean time difference. Assume that the time reference of the Beidou satellite system is T. BDS , this time reference is the time reference on the ground and is used to support the time system for ground applications; the reference time T of the low-earth orbit constellation. LEO can be obtained using the following formula;

[0046]

[0047] Among them, represents the mean time difference between the time reference of the low-earth orbit constellation and the time reference of the Beidou satellite system. represents the clock difference between the low-earth orbit satellite i and the Beidou satellite j. i represents the low-earth orbit satellite, and j represents the Beidou satellite. represents the weight of the Beidou satellite j observed by the low-earth orbit satellite i. Among them is the standard deviation of the output frequency of the on-board atomic clock on the Beidou satellite j, which is evaluated and generated by the Beidou ground operation and control center based on long-term satellite clock monitoring data. After the calculation is completed, the low-earth orbit satellite system synchronizes the results through a laser link.

[0048] Step S9: Regularly repeating steps S3 - S8 can maintain the time reference of the low-earth orbit constellation.

[0049] It should be noted that the inter-satellite link generally includes a microwave link and the laser link mentioned in the present invention. The satellite ground management and control center can establish the time reference of the low-earth orbit constellation by collecting the time comparison results of the microwave link between satellites. This method has a large dependence on ground infrastructure. However, the innovation point of the present invention proposes a method for establishing and maintaining the time reference based on the autonomous capabilities of the constellation and the currently high-precision measurement technology of lasers. Its characteristics are autonomy and high precision. Its significance is that even in special periods when ground facilities are unavailable, the autonomous timing and timekeeping capabilities of the low constellation can still be achieved through the method of the present invention, thereby providing time services for important equipment related to space and the ground.

[0050] Key points:

[0051] The present invention innovatively proposes a technique for establishing a time reference for a low-Earth orbit (LEO) constellation based on high-performance atomic clocks of the Beidou global constellation, high-precision laser link time comparison technology, and an autonomous operation system for the LEO constellation.

[0052] Points of protection:

[0053] What the present invention protects is not only the application of high-performance atomic clocks of the Beidou constellation. The present invention details the specific implementation of high-precision laser link time comparison technology. The point of protection lies in proposing a technique for establishing a time reference for a LEO constellation based on high-performance atomic clocks of the Beidou global constellation, high-precision laser link time comparison technology, and an autonomous operation system for the LEO constellation. The technical solutions, implementation manners, specific details, etc. involved in the present invention should all be protected by intellectual property law. Any act of unauthorized copying, imitation, or commercial use of the technical solutions of the present invention will be regarded as an infringement of the intellectual property rights of the present invention.

[0054] Advantageous effects:

[0055] The present invention proposes a method based on the high-precision time reference of Beidou satellites and laser links. First, time comparison is performed between Beidou satellites and LEO satellites, then the comparison information is transmitted through the links between LEO satellites, and then the average time difference is calculated using the resources of the LEO constellation itself, and finally, the establishment of a high-precision time reference for the LEO constellation is achieved. The present invention is a method for establishing and maintaining a time reference based on the autonomous capabilities of the constellation and laser, a technology with relatively high current measurement accuracy. Its characteristics are autonomy and high precision. Its significance is that even in special periods when ground facilities are unavailable, the autonomous timing and timekeeping capabilities of the low constellation can still be achieved through the method of the present invention, thereby providing time services for important equipment in space and on the ground.

[0056] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the technical principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for autonomously establishing a time reference for a low-earth orbit constellation, characterized in that The method includes the following steps: Step S1: The Beidou satellite navigation constellation is equipped with a laser link communication payload; Step S2: All satellites in the low-Earth orbit satellite system are equipped with communication payloads that can be compatible with the Beidou laser link system at the same time, and the entire low-Earth orbit constellation has efficient information synchronization capabilities and computing nodes; Step S3: Information synchronization: The low-Earth orbit constellation independently formulates a link establishment plan table, and synchronizes the Beidou ephemeris, low-Earth orbit satellite ephemeris information, and link establishment plan table to each low-Earth orbit satellite through the laser link; Step S4: According to the link establishment plan table, the low-earth orbit satellite G1 and the visible Beidou satellites perform two-way beam orientation; Step S5: After the beam orientation is completed, an inter-satellite laser link is established, and two-way distance observation is carried out; Step S6: Calculate the time difference between the clock of the low-earth orbit satellite G1 and the clock of the Beidou satellite through the distance measurement ​ Step S7: Other low-Earth orbit satellites repeat Steps S4 to S6 and complete the time comparison measurement, and send all measurement results to the designated low-Earth orbit satellite; Step S8: On the designated low-Earth orbit satellite, the satellite resource management system starts a calculation task and calculates the average time difference, thereby obtaining the reference time of the low-Earth orbit constellation, and then the low-Earth orbit satellite system realizes the synchronization of the results through the laser link; Wherein, In step S6, it is assumed that the low-earth orbit satellite G1 transmits a measurement signal to the Beidou satellite at the local time t0 The signal is reflected after reaching the Beidou satellite at time t1, and G1 receives the echo signal at time t2. By synthesizing the information of the Beidou satellite transmitted at time t1, the time difference between the clock of the Beidou satellite and the clock of the low-earth orbit satellite G1 is calculated; Assume that the time difference between the LEO satellite clock and the Beidou satellite clock is The time interval between the time t0 when the LEO satellite measures the signal is sent and its second pulse is T G , the time interval between the Beidou satellite time t1 and the Beidou satellite second pulse is T s , the light travel time of the uplink signal is represented by τ up , the light travel time of the downlink signal is represented by τ down represented, and the time elapsed from when the measurement signal is sent to when it returns is represented by τ. Then, the time difference between the LEO satellite clock and the Beidou satellite clock is calculated by the following formula: In the step S8, the comparison measurement results are weighted and averaged to obtain the average time difference Assume that the time reference of the Beidou satellite system is T BDS , which is the time reference on the ground and is used to support the time system for ground applications; the reference time T of the low-earth orbit constellation LEO is obtained using the following formula; Among them, represents the average time difference between the time reference of the low-earth orbit constellation and the time reference of the Beidou satellite system, represents the time difference between the clock of the low-earth orbit satellite i and the clock of the observable Beidou satellite j, where i represents the low-earth orbit satellite and j represents the Beidou satellite observable by the low-earth orbit satellite i, represents the weight of the Beidou satellite j observed by the low-earth orbit satellite i.

2. The method for autonomously establishing a time reference for a low-earth orbit constellation according to claim 1, wherein The autonomous timekeeping accuracy of the Beidou satellite navigation constellation is higher than 10 -14 .

3. The method for autonomously establishing a time reference for a low-earth orbit constellation according to claim 1, wherein In Step S7, each low-Earth orbit satellite only measures the visible Beidou satellites at the measurement moment, and the visible Beidou satellites at the measurement moment are pre-determined by the link establishment plan table.

4. The method for autonomously establishing the time reference of a low-earth orbit constellation according to claim 1, characterized in that, In Step S7, the designated low-Earth orbit satellite is designated by the autonomous decision-making system on the low-Earth orbit satellite according to the satellite mission arrangement and calculation energy consumption.

5. The method for autonomously establishing the time reference of a low-earth orbit constellation according to claim 1, wherein In Step S7, the designated low-Earth orbit satellite refers to a low-Earth orbit satellite that has been agreed upon in advance on the ground and has an efficient computing unit and autonomous operating software.

6. The method for autonomously establishing the time reference of a low-earth orbit constellation according to claim 1, characterized in that, Among them is the standard deviation of the output frequency of the on-board atomic clock on the Beidou satellite, which is generated by the Beidou ground operation and control center based on the long-term satellite clock monitoring data evaluation.

7. The method for autonomously establishing the time reference of a low-earth orbit constellation according to claim 1, characterized in that, After Step S8, it further includes: regularly repeating Steps S3 - S8 to maintain the time reference of the low-Earth orbit constellation.

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