Method for autonomously establishing low-orbit constellation time reference

Abstract

The invention relates to a low-orbit constellation time reference autonomous establishment method, and belongs to the field of satellite time synchronization. When the time reference is established, the low-orbit satellites are required to observe own visible Beidou satellites at the same time, time comparison is performed by using laser inter-satellite links, then comparison results are summarized to the specified low-orbit satellites through low-orbit laser links, and finally comprehensive time difference calculation is completed on the specified low-orbit satellites. And finally realizing the establishment of the high-precision time reference of the low-orbit constellation. The method has autonomy and high precision, and has the significance that even under the condition that ground facilities are unavailable in special periods, the autonomous timing and time keeping capabilities of low constellations can still be realized through the method, so that time services are provided for space and ground related important equipment.

Description

Technical Field

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

[0002] At present, low-orbit satellite constellation systems such as navigation enhancement, communication, meteorology, and remote sensing are developing rapidly, such as Starlink, OneNet, and Kuiper Constellation in foreign countries, and Microcentispace, Qianfan, Tianqi, Tianmu-1, and Jilin-1 in China. The realization of these constellation system functions and multi-task collaboration between users all require the support of high-precision time references. Compared with medium and high-orbit satellites, low-orbit constellation systems have a large number of satellites, and the capabilities and funds of single-satellite platforms are limited. Due to the high price of high-performance satellite-borne atomic clocks, in order to control costs, low-orbit constellations usually choose to install lower-priced, lower-performance atomic clocks or crystal oscillators.

[0003] There are usually several technical approaches to establish the time base of a low-orbit satellite constellation. The first is to use a high-precision ground clock group, a satellite-to-ground link between a ground station and a low-orbit satellite, but this method is heavily dependent on the ground time base. When the ground clock group fails or is invisible, it cannot meet the needs of establishing the time base of the low-orbit constellation. In addition, due to the huge number of satellites, relying on ground stations to establish the time base will overwhelm the ground system. The second is to use the Beidou satellite high-precision time base and downlink navigation signal link. Since the downlink navigation signal is weak and susceptible to interference, and is affected by factors such as the ionosphere, the availability and accuracy are limited under complex electromagnetic environment conditions. The third is to use the inter-satellite link measurement and comparison between low-orbit satellites to achieve mutual measurement and comparison between low-orbit satellites through inter-satellite links, and comprehensively generate the time base of the low-orbit constellation. Due to the poor performance of atomic clocks or crystal oscillators, it is difficult to achieve high accuracy.

[0004] The onboard atomic clock is called the "heart" of the navigation satellite. Its function is to provide a highly stable time and frequency reference signal for the satellite system. It determines the accuracy of the navigation system's navigation positioning, speed measurement and timing, and is one of the core technologies that determine whether a country can independently develop a navigation system.

[0005] The BeiDou global navigation constellation currently has a high-precision time reference and autonomous maintenance capability. First, the BeiDou system is equipped with high-precision, high-stability onboard hydrogen atomic clocks (with a daily stability better than 10E-14) on satellites; second, BeiDou can use laser links to continuously perform inter-satellite precision measurements of the entire constellation and provide data transmission to achieve high-precision time synchronization for the entire constellation. Therefore, the BeiDou global navigation constellation has the ability to autonomously maintain highly stable time for the entire constellation.

[0006] Considering the payload cost of satellite-borne atomic clocks, low-orbit satellites currently generally use satellite-borne atomic clock devices with suboptimal performance, which have high short-term stability but require frequent time calibration. For example, when Iridium satellites use constant temperature crystal oscillators to generate communication signals and maintain systems, their performance is very stable in the short term. After 100 seconds, the clock error and clock drift stability are lower than those of satellite-borne atomic clocks. The frequency stability of Iridium satellite clocks is generally better than 10 -11 , and flicker noise and random walk noise appear before and after 1000 seconds, respectively, so at least 2 frequency adjustments are required every day to obtain time synchronization. Therefore, a highly stable clock source and high-precision comparison technology are the key to establishing and maintaining the time of the low-orbit constellation.

[0007] As a key technology, 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-orbit satellite mobile communication systems on ground networks, thereby enhancing the system's routing flexibility and network management convenience. Research results show that when using laser links to perform time synchronization measurements between satellites and ground clock sources, the error standard deviation that can be achieved is less than 0.05 nanoseconds, demonstrating extremely high accuracy. Furthermore, since intersatellite links are almost unaffected by atmospheric interference, their time comparison accuracy is superior to satellite-to-ground comparisons. This feature provides strong support for the establishment and maintenance of high-precision time synchronization in low-orbit constellation systems.

[0008] At present, the time synchronization system of low-orbit satellite constellations is mainly built on the basis of ground clock groups. Although this traditional approach has met the basic needs to a certain extent, it also faces many limitations.

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

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

[0011] Third, if low-orbit constellations rely on ground infrastructure for time calibration for a long time, their autonomous operation and independent service capabilities will be greatly reduced. When ground facilities fail, undergo maintenance, or are subject to external interference, the constellation may face the risk of time synchronization interruption, which in turn affects the robust operation and global service capabilities of the entire constellation, posing a major challenge to improving the autonomy and resilience of satellite constellations.

[0012] Therefore, it is urgent and necessary to explore a time synchronization solution that is independent of the ground, has high stability and autonomy, and to establish a low-orbit constellation time reference for a space-based navigation constellation with high-precision time autonomous maintenance capabilities. Summary of the invention

[0013] 1. Technical issues to be resolved

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

[0015] (II) Technical solution

[0016] In order to solve the above technical problems, the present invention proposes a method for autonomously establishing a low-orbit constellation time reference, which comprises 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 low-orbit satellite system are equipped with communication payloads that are compatible with the Beidou laser link system, and the entire low-orbit constellation has efficient information synchronization capabilities and computing nodes;

[0019] Step S3, information synchronization: the low-orbit constellation independently formulates a link establishment planning table, and synchronizes the Beidou ephemeris, low-orbit satellite ephemeris information, and the link establishment planning table to each low-orbit satellite through a laser link;

[0020] Step S4: According to the link establishment planning table, the low-orbit satellite G1 and the visible Beidou satellite Perform bidirectional beam steering;

[0021] Step S5: After beam orientation is completed, intersatellite laser link establishment is performed and two-way distance observation is carried out;

[0022] Step S6: Calculate the distance between the low-orbit satellite G1 and the Beidou satellite through the distance observation Time difference

[0023] Step S7, other low-orbit satellites repeat steps S4 to S6 and complete the time comparison measurement, and send all measurement results to the designated low-orbit satellite;

[0024] Step S8: On the designated low-orbit satellite, the satellite resource management system starts the calculation task and performs average time difference calculation to obtain the reference time of the low-orbit constellation, and then the low-orbit satellite system synchronizes the results through the laser link.

[0025] (III) Beneficial effects

[0026] The present invention proposes a method for autonomously establishing a time reference for a low-orbit constellation. The present invention proposes a method based on a Beidou satellite high-precision time reference and a laser link. First, the Beidou satellite and the low-orbit satellite are used for time comparison, and then the comparison information is transmitted through the link between the low-orbit satellites. Then, the low-orbit constellation's own resources are used to complete the calculation of the average time difference, and finally the establishment of a high-precision time reference for the low-orbit constellation is achieved. The present invention is a method for establishing and maintaining a time reference based on the autonomous capability of the constellation and the current higher measurement technology of laser. It is characterized by autonomy and high precision. 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. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

[0029] Figure 3 It is a schematic diagram of the time difference calculation method of the present invention. DETAILED DESCRIPTION

[0030] In order to make the purpose, content and advantages of the present invention more clear, the specific implementation methods of the present invention are further described in detail below in conjunction with the drawings and examples.

[0031] 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 the comparison results are summarized to the designated low-orbit satellite through the low-orbit laser link (the designated low-orbit satellite is designated by the autonomous decision-making system on the low-orbit satellite according to the satellite mission arrangement, calculation energy consumption and other specifications), and finally the comprehensive time difference calculation is completed on the designated low-orbit satellite. The overall link topology is as follows Figure 1 As shown:

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

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

[0034] Step S2: All satellites in the low-orbit satellite system are equipped with communication payloads that are compatible with the Beidou laser link system, and the entire low-orbit constellation has efficient information synchronization capabilities and computing nodes;

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

[0036] Step S4: According to the link establishment planning table, the low-orbit satellite G1 and the visible Beidou satellite Perform bidirectional beam steering;

[0037] Step S5: After beam orientation is completed, intersatellite laser link establishment is performed and two-way distance observation is carried out;

[0038] Step S6: Calculate the distance between the low-orbit satellite G1 and the Beidou satellite through the distance observation Time difference

[0039] like Figure 3 As shown in the figure, it is assumed that the low-orbit satellite G1 is moving towards the Beidou satellite at local time t0 The measurement signal is transmitted, and the signal reaches the Beidou satellite at time t1 and is reflected. G1 receives the echo signal at time t2, and the Beidou satellite is obtained by combining the information at time t1 transmitted by the Beidou satellite. The clock difference between the low-orbit satellite clock and the Beidou satellite clock is assumed to be The time interval between the low-orbit satellite measurement signal sending time t0 and its second pulse is T G , the time interval between Beidou satellite time t1 and Beidou satellite second pulse is T s , when the uplink signal light is transmitted, τ up Indicates that the downlink signal light is τ down The time from the measurement signal being sent out to its return is represented by τ, and the time difference between the low-orbit satellite clock and the Beidou clock can be calculated by the following formula.

[0040]

[0041] Step S7: At the same time, other low-orbit satellites repeat steps S4 to S6 and complete the time comparison measurement, and send all measurement results to the designated low-orbit satellite;

[0042] Each low-orbit satellite only measures the Beidou satellites that are visible at the time of measurement. The specific one will be determined in advance in the link planning table. Each low-orbit satellite reports its own and received measurement information, and the redundant information involved will be processed by the computing unit on the designated low-orbit satellite.

[0043] A designated low-orbit satellite refers to a satellite that has been agreed upon in advance on the ground and has an efficient computing unit and autonomous operating software, and mainly emphasizes satellites with relevant capabilities. No specific satellite is mentioned here.

[0044] Step S8: On the designated low-orbit satellite, the satellite resource management system starts the calculation task and performs average time difference calculation to obtain the reference time of the low-orbit constellation, and then the low-orbit satellite system synchronizes the results through the laser link.

[0045] The average time difference is obtained by weighted averaging the comparison measurement results. Assume that the time base of the BeiDou satellite system is T BDS , which is the ground time reference and is used to support the time system of ground applications; the reference time of the low-orbit constellation is T LEO It can be obtained using the following formula;

[0046]

[0047] in, It represents the average time difference between the low-orbit constellation time base and the BeiDou satellite system time base. It represents the clock difference between low-orbit satellite i and Beidou satellite j, i represents low-orbit satellite, j represents Beidou satellite, represents the weight of BeiDou satellite j observed by low-orbit satellite i, in is the standard deviation of the output frequency of the onboard atomic clock on BeiDou satellite j, which is generated by the BeiDou ground operation and control center based on the long-term satellite clock monitoring data. After the calculation is completed, the low-orbit satellite system synchronizes the results through the laser link.

[0048] Step S9, regularly repeating steps S3-S8 can achieve the maintenance of the low-orbit constellation time reference.

[0049] It should be noted that intersatellite links generally include microwave links and the laser links mentioned in the present invention. The satellite ground management and control center can establish the time reference of the low-orbit constellation by collecting the time comparison results of the microwave links between satellites. This method has a large dependence on the ground infrastructure. However, the innovation of the present invention proposes a method for establishing and maintaining the time reference based on the autonomous capability of the constellation and the laser, which is currently a high-precision measurement technology. Its characteristics are autonomy and high precision. 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.

[0050] Key Points:

[0051] The present invention innovatively proposes a low-orbit constellation time reference establishment technology based on the Beidou global constellation's high-performance atomic clock, high-precision laser link time comparison technology and the low-orbit constellation autonomous operation system.

[0052] Protection point:

[0053] The present invention protects not only the application of the high-performance atomic clock of the Beidou constellation, but also describes in detail the specific implementation of the high-precision laser link time comparison technology. The protection point is to propose a low-orbit constellation time base establishment technology based on the Beidou global constellation high-performance atomic clock, high-precision laser link time comparison technology and low-orbit constellation autonomous operation system. The technical solutions, implementation methods and specific details involved in the present invention should be protected by intellectual property laws. Any unauthorized copying, imitation or use of the technical solutions of the present invention for commercial activities will be deemed to infringe the intellectual property rights of the present invention.

[0054] Beneficial effects:

[0055] The present invention proposes a method based on Beidou satellite high-precision time reference and laser link, firstly compares time between Beidou satellite and low-orbit satellite, then transmits comparison information through the link between low-orbit satellites, and then uses the low-orbit constellation's own resources to complete the calculation of average time difference, and finally realizes the establishment of high-precision time reference of low-orbit constellation. The present invention is a method for establishing and maintaining time reference based on the autonomous capability of constellation and laser, which is currently a relatively high measurement accuracy technology. It is characterized by autonomy and high precision. Its significance is that even if ground facilities are unavailable during special periods, the autonomous timing and punctuality of low-orbit constellation can still be realized through the method of the present invention, thereby providing time services for important space and ground-related equipment.

[0056] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for autonomously establishing a time reference for a low-orbit constellation, characterized in that: The method comprises 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-orbit satellite system are equipped with communication payloads that are compatible with the Beidou laser link system, and the entire low-orbit constellation has efficient information synchronization capabilities and computing nodes; Step S3, information synchronization: the low-orbit constellation independently formulates a link establishment planning table, and synchronizes the Beidou ephemeris, low-orbit satellite ephemeris information, and the link establishment planning table to each low-orbit satellite through a laser link; Step S4: According to the link establishment planning table, the low-orbit satellite G1 and the visible Beidou satellite Perform bidirectional beam steering; Step S5: After beam orientation is completed, intersatellite laser link establishment is performed and two-way distance observation is carried out; Step S6: Calculate the distance between the low-orbit satellite G1 and the Beidou satellite through the distance observation Time difference Step S7, other low-orbit satellites repeat steps S4 to S6 and complete the time comparison measurement, and send all measurement results to the designated low-orbit satellite; Step S8: On the designated low-orbit satellite, the satellite resource management system starts the calculation task and performs average time difference calculation to obtain the reference time of the low-orbit constellation, and then the low-orbit satellite system synchronizes the results through the laser link.

2. The method for autonomously establishing a low-orbit constellation time reference according to claim 1, characterized in that: The autonomous timing accuracy of the Beidou satellite navigation constellation is higher than 10 -14 above.

3. The method for autonomously establishing a low-orbit constellation time reference according to claim 1, characterized in that: In step S6, it is assumed that the low-orbit satellite G1 is moving towards the Beidou satellite at local time t0. The measurement signal is transmitted, and the signal reaches the Beidou satellite at time t1 and is reflected. G1 receives the echo signal at time t2, and the Beidou satellite is obtained by combining the information at time t1 transmitted by the Beidou satellite. The clock difference with the low-orbit satellite G1.

4. The method for autonomously establishing a low-orbit constellation time reference according to claim 3, characterized in that: Assume that the time difference between the low-orbit satellite clock and the Beidou satellite clock is The time interval between the low-orbit satellite measurement signal sending time t0 and its second pulse is T G , the time interval between Beidou satellite time t1 and Beidou satellite second pulse is T s , when the uplink signal light is transmitted, τ up Indicates that the downlink signal light is τ down The time from the measurement signal being sent out to the return is represented by τ, and the time difference between the low-orbit satellite clock and the Beidou clock is calculated by the following formula:

5. The method for autonomously establishing a low-orbit constellation time reference according to claim 4, characterized in that: In step S7, each low-orbit satellite only measures the Beidou satellites that are visible at the measurement time, and the specific one will be determined in advance in the link establishment planning table.

6. The method for autonomously establishing a low-orbit constellation time reference according to claim 4, characterized in that: In step S7, the low-orbit satellite is designated by an autonomous decision-making system on the low-orbit satellite based on satellite mission arrangements and calculated energy consumption.

7. The method for autonomously establishing a low-orbit constellation time reference according to claim 4, characterized in that: In step S7, the designated low-orbit satellite refers to a satellite previously agreed upon on the ground that has a high-efficiency computing unit and autonomous operating software.

8. The method for autonomously establishing a low-orbit constellation time reference according to claim 4, characterized in that: In step S8, the weighted average of the comparison measurement results is performed to obtain the average time difference Assume that the time base of the BeiDou satellite system is T BDS , this time reference is the ground time reference, used to support the time system of ground applications; The reference time T of the low-orbit constellation LEO Use the following formula to obtain; in, It represents the average time difference between the low-orbit constellation time base and the BeiDou satellite system time base. It represents the clock difference between low-orbit satellite i and Beidou satellite j, i represents low-orbit satellite, j represents Beidou satellite, Represents the weight of BeiDou satellite j observed by low-orbit satellite i.

9. The method for autonomously establishing a low-orbit constellation time reference according to claim 8, characterized in that: in is the standard deviation of the output frequency of the onboard atomic clock of BeiDou satellite j, which is evaluated and generated by the BeiDou Ground Operation and Control Center based on long-term satellite clock monitoring data.

10. The method for autonomously establishing a low-orbit constellation time reference according to claim 8, characterized in that: After step S8, the method further includes: regularly repeating steps S3-S8 to maintain the time reference of the low-orbit constellation.

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