On-orbit time synchronization system of low-orbit satellite

Abstract

The invention provides an on-orbit time synchronization system for a low-orbit satellite, and the system comprises a regional distributed on-orbit time synchronization subsystem which is used for dividing an overall coverage region of a low-orbit satellite constellation into a plurality of sub-regions which are partially overlapped at a current epoch; the GNSS product priority and receiving subsystem is used for selecting a corresponding GNSS product in the current epoch according to the service range of the GNSS satellite constellation and the sub-region where the low-orbit satellite is located; and the regional distributed on-orbit time synchronization subsystem is used for carrying out time synchronization on the low-orbit satellites of all the sub-regions in the regions and the intervals by utilizing a GNSS product in the current epoch. According to the system, a ground station does not need to participate, various GNSS products are introduced for on-orbit calculation, high operation and maintenance cost is avoided, meanwhile, time delay caused by signal transmission is greatly shortened, and the real-time performance is higher. The system can be used for uninterruptedly calibrating the low-orbit constellation atomic clock, and has higher time synchronization capability.

Description

Technical Field

[0001] The present invention belongs to the technical field of satellite positioning and timing, and in particular relates to an on-orbit time synchronization system for low-orbit satellites. Background Art

[0002] In recent years, the rapid development of low-orbit (LEO) constellations has become a significant trend in the aerospace industry. The US Starlink constellation, Europe's OneWeb constellation, and my country's GW constellation, among others, will deploy tens of thousands of LEO satellites in the future. Time synchronization of large-scale LEO constellations is a fundamental technical challenge in enabling global communications, navigation, and remote sensing services.

[0003] Existing time synchronization technologies for low-orbit constellations can be roughly divided into two types: satellite-ground coordinated time synchronization technology and inter-satellite autonomous time synchronization technology. Satellite-ground coordinated time synchronization technology usually refers to the ground station calculating the LEO satellite clock error through onboard GNSS observations combined with GNSS satellite products, and combining the ground station clock error generated by the same GNSS product to generate satellite-ground synchronization information and upload it to the low-orbit constellation, thereby achieving time synchronization of the low-orbit constellation; or refers to the use of Q / V frequency bands to achieve data transmission, including time information, between the ground and the constellation, and combined with technologies such as inter-satellite links to achieve time synchronization of the low-orbit constellation. Inter-satellite autonomous time synchronization technology usually refers to the use of inter-satellite lasers to achieve time transfer between low-orbit satellites, or refers to a distributed dynamic synchronization algorithm, that is, a synchronization method based on maximum likelihood estimation, to solve the problem of random delay under highly dynamic topologies.

[0004] Although satellite-ground coordinated time synchronization technology can achieve high-precision time synchronization for low-orbit constellations, its reliance on ground support also brings significant defects, such as dependence on low-orbit satellites and ground data uplink and downlink; due to the difficulty in deploying ground stations in polar / oceanic regions, satellites cannot be calibrated in time when passing overhead; in GNSS-denied environments, time synchronization systems that rely on ground stations have weak survivability; the long round-trip delay of satellite-ground signals restricts certain real-time applications; and the high cost of ground system operation and maintenance.

[0005] Intersatellite autonomous time synchronization technology, lacking an external clock source, also has significant drawbacks. For example, due to the limited long-term stability of satellite-borne atomic clocks, cumulative errors will occur without external calibration. Furthermore, satellites at different orbital altitudes experience clock rate differences due to gravitational potential differences, which are difficult to completely eliminate using intersatellite measurements alone. This leads to inconsistent time references within the constellation, thus impacting time synchronization performance. Intersatellite time transfer based on carrier phase common view of low-orbiting satellites imposes constraints on the relative distance between satellites. When there are too few satellites in common view, time transfer accuracy is compromised. Furthermore, it relies on the intersatellite transmission of raw GNSS observation data, placing high demands on intersatellite link data transmission. Summary of the Invention

[0006] In order to solve the above problems existing in the prior art, the present invention provides a low-orbit satellite on-orbit time synchronization system. The technical problem to be solved by the present invention is achieved through the following technical solutions: A low-orbit satellite on-orbit time synchronization system comprises: A regional distributed on-orbit time synchronization subsystem is used to divide the overall coverage area of ​​the low-orbit satellite constellation into a plurality of partially overlapping sub-areas at the current epoch; wherein each sub-area includes a plurality of low-orbit satellites; The GNSS product priority and reception subsystem is used to select the corresponding GNSS product from the GNSS products corresponding to multiple GNSS satellites in the current epoch based on the service range of the GNSS satellite constellation and the sub-region where the low-orbit satellite is located; The regional distributed on-orbit time synchronization subsystem is used to use the GNSS product to synchronize the time of low-orbit satellites in all sub-regions within the region and between regions at the current epoch.

[0007] Beneficial effects: 1) The present invention eliminates the need for ground station participation, thus avoiding high operation and maintenance costs. Furthermore, satellite-borne observations do not need to be transmitted to the ground, making the system suitable for scenarios where satellite-borne observations cannot be transmitted to the ground in real time. In situations of ground-based GNSS denial, the system has greater survivability, and all calculations in the system are performed on-orbit, significantly reducing the delay caused by signal transmission and achieving greater real-time performance.

[0008] 2) This invention incorporates the GNSST, a reference time for each navigation system, by leveraging various GNSS products received in orbit. Compared to the atomic clocks carried by low-orbit satellite constellations, GNSST typically has better long- and short-term stability and can be used to continuously calibrate the atomic clocks of these satellites. Therefore, compared to autonomous inter-satellite time synchronization, this system offers higher time synchronization performance.

[0009] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 It is a schematic diagram of an on-orbit time synchronization system for a low-orbit satellite provided by the present invention. DETAILED DESCRIPTION

[0011] The present invention will be further described in detail below with reference to specific examples, but the embodiments of the present invention are not limited thereto.

[0012] like Figure 1 As shown, the present invention provides a low-orbit satellite on-orbit time synchronization system comprising: A regional distributed on-orbit time synchronization subsystem is used to divide the overall coverage area of ​​the low-orbit satellite constellation into a plurality of partially overlapping sub-areas at the current epoch; wherein each sub-area includes a plurality of low-orbit satellites; Each sub-region is divided evenly, and the overlapping area of ​​adjacent sub-regions is 10%-15% of the entire sub-region.

[0013] The present invention divides the overall coverage of the low-orbit satellite constellation into N overlapping sub-areas based on the instantaneous coverage of the low-orbit satellites, and dynamically updates the grid structure at a certain frequency. When dividing the sub-areas, adjacent sub-areas need to maintain a coverage overlap of 10%-15% to ensure that when the low-orbit satellite crosses the regional boundary, the clock error can be smoothly transitioned through multiple base satellites to avoid synchronization jumps, while providing the possibility for inter-regional collaboration. Each sub-area tries to select a group of low-orbit satellites that can receive the same GNSS product. Under normal circumstances, the number of satellites in the area can be set to 5-50 (not limited) to balance the load and reliability. Each sub-area is configured with 2 reference satellites (hot backups of each other) and 1 cold backup satellite to form a "2+1" redundant structure. The reference satellites should carry high-performance atomic clocks and high-performance processing units.

[0014] The GNSS product priority and reception subsystem is used to select the corresponding GNSS product from the GNSS products corresponding to multiple GNSS satellites in the current epoch based on the service range of the GNSS satellite constellation and the sub-region where the low-orbit satellite is located; The GNSS products currently available in orbit include precise point positioning, such as Beidou PPP-B2b and Europe's Galileo HAS; satellite-based augmentation (SBAS), such as BDSBAS, GPS's WAAS and Galileo's EGNOS systems, and MADOCA products broadcast by Japan's QZSS; and broadcast ephemeris, such as the broadcast ephemeris of various GNSS systems.

[0015] The regional distributed on-orbit time synchronization subsystem is used to use the GNSS product to synchronize the time of low-orbit satellites in all sub-regions within the region and between regions at the current epoch.

[0016] In a specific embodiment of the present invention, the GNSS product priority and receiving subsystem is specifically configured to: S10: For any sub-region, determine the service range of most low-orbit satellites in the sub-region at the current epoch and the GNSS products corresponding to the service ranges, and select the determined GNSS product as the GNSS product corresponding to all low-orbit satellites in the sub-region; wherein at least two GNSS products correspond to low-orbit satellites in an overlapping area of ​​adjacent sub-regions; In a specific embodiment of the present invention, S10 includes: S101: for any sub-region, determine all low-orbit satellites in the sub-region at the current epoch, and preferentially select the GNSS product of the Galileo HAS with a global service range as the GNSS product for all low-orbit satellites in the sub-region; S102: For any sub-region, if the global Galileo HAS GNSS product in the current epoch has accuracy or availability issues, determine the service range of the low-orbit satellites in the sub-region and select the GNSS product corresponding to the service range with the largest number of low-orbit satellites as the GNSS product for all low-orbit satellites in the sub-region.

[0017] In this step, the GNSS satellite corresponding to the service range with the largest number of low-orbit satellites may be selected, and the GNSS product of the GNSS satellite corresponding to the GNSS signal with the highest pitch angle may be selected from the GNSS satellites as the GNSS product of all low-orbit satellites in the sub-area.

[0018] During in-orbit operation, LEO constellations must dynamically and adaptively select available GNSS augmentation products based on their coverage area. This selection strategy must take into account regional coverage adaptation, multi-system redundancy, and orbital altitude optimization. First, high-accuracy products with global coverage (such as Galileo HAS) should be prioritized to minimize product switching. If global coverage product accuracy or availability issues arise, regional coverage adaptation should be considered. For example, Beidou PPP-B2b and QZSS MADOCA products should be prioritized in the Asia-Pacific region, EGNOS products in Europe and Africa, and GPS WAAS products in the Americas. Broadcast ephemeris should be switched when no products are available. Regarding multi-system redundancy, adaptive fusion of multiple sources, such as Beidou BDSBAS and GPS SBAS, should be considered when satellites cross service area boundaries to avoid service interruptions. Furthermore, for LEO satellites at different altitudes, GNSS signal obstruction rates should be assessed. For very low orbits, QZSS or Galileo signals with higher elevation angles should be preferred.

[0019] S11: At the current epoch, all GNSS products are received and analyzed to obtain the clock bias of the GNSS satellites.

[0020] Currently, various products are broadcast using different frequencies and covering different areas. For example, Beidou PPP-B2b products are broadcast via the B2b signals of the three Beidou-3 GEO satellites, covering China and surrounding areas; Galileo uses the E6B signal; QZSS CLAS uses the L6D signal area to cover mainland Japan; BDSBAS uses Beidou GEO satellites to cover the Asia-Pacific region; WAAS / EGNOS uses the L1 band to cover the Americas and Europe; and QZSS MADOCA uses the QZSS L6E signal. Each GNSS system broadcasts its broadcast ephemeris globally using its own service frequency band. Therefore, it is necessary to receive and interpret the selected GNSS products.

[0021] In a specific embodiment of the present invention, the regional distributed on-orbit time synchronization subsystem is specifically used to: S20, in the current epoch, selecting a reference satellite from each sub-region, so that the reference satellite performs intra-region time synchronization based on the corresponding GNSS product and observation data of the observed GNSS satellite, so that the time of all low-orbit satellites in the same sub-region is the same; S21, in the current epoch, using the low-orbit satellites in the overlapping area of ​​the adjacent sub-areas that have completed the intra-area time synchronization, the low-orbit satellites in the adjacent sub-areas are synchronized between intervals, so that the time of the low-orbit satellites in all sub-areas is the same.

[0022] In a specific embodiment of the present invention, S20 includes: S201, for any sub-region in the current epoch, selecting a reference satellite within the sub-region, so that the reference satellite calculates a clock difference between the reference satellite and a reference of the GNSS product based on a corresponding GNSS product and observation data of GNSS satellites observed by itself, and transmits the clock difference to other low-orbit satellites in the same sub-region that have established communication links with the reference satellite; The clock difference between the reference satellite and the reference of the GNSS product is expressed as , calculated by the following formula: (1) (2) (3) (4) Where, is the identification number of the reference satellite; is the GNSS satellite number; is the corresponding observation epoch; is the speed of light in vacuum; The clock time of the reference satellite is used; The time of the GNSS satellite onboard clock; is the reference time of the reference satellite clock error, The reference time for the GNSS satellite clock error; 、 is the combined observation value of the corresponding reference satellite, GNSS satellite and epoch with the ionospheric effect eliminated; and 、 is its observation error; is the corresponding wavelength; is the geometric distance between the GNSS satellite position at the time of signal transmission and the reference satellite receiver position, is the reference satellite coordinate, is the GNSS satellite position.

[0023] From the above, we can see that the combined formulas (1)-(4) yield: (5); Other low-orbit satellites in the region are collectively referred to as node satellites. Node satellites can choose to establish inter-satellite links with reference satellites and exchange time and frequency information to achieve time synchronization with reference satellites.

[0024] S202: For other low-orbit satellites that have not established a communication link with the reference satellite, the low-orbit satellite is used as a reference satellite, so that the reference satellite calculates the clock difference between the reference satellite and the reference of the GNSS product based on the corresponding GNSS product and the observation data of the GNSS satellite observed by itself.

[0025] During the time synchronization process of the reference satellite, the GNSS product reference is introduced That is, the time reference GNSST of each GNSS system usually has better long- and short-term stability than the atomic clocks carried by the low-orbit constellation. It can be used to continuously calibrate the atomic clocks of the low-orbit constellation, avoid error accumulation during the autonomous time synchronization of the constellation, and enable the system to have higher time synchronization performance.

[0026] In a specific embodiment of the present invention, S202 includes: S2021, for other low-orbit satellites that have not established a communication link with the reference satellite, if the GNSS product corresponding to the other low-orbit satellite is the same as that corresponding to the reference satellite, calculate the clock difference between the other low-orbit satellite and the reference of the GNSS product using the same method as that used to calculate the clock difference of the reference satellite; The node satellite can also calculate the clock difference between the node satellite and the GNSS product benchmark by receiving GNSS products and onboard GNSS observation data using the PPP principle. There are two situations at this time, namely, receiving the same GNSS product as the benchmark satellite, or receiving different GNSS products. When the node satellite and the benchmark satellite receive the same product, the clock difference of the node satellite relative to the time benchmark of the received GNSS product can be known, as shown in formula (6). The principle and steps are the same as formulas (1)-(4). (6) S2022: For other low-orbit satellites that have not established a communication link with the reference satellite, if the GNSS products corresponding to the other low-orbit satellites are different from those of the reference satellite, the clock difference between the GNSS product and the reference is calculated using the GNSS products corresponding to the other low-orbit satellites and the observation data of the observed GNSS satellites.

[0027] When the node satellite and the reference satellite receive different GNSS products, the PPP principle shows that the clock difference between the reference GNSS product and the clock calculated by other low-orbit satellites can be expressed as: (7); in Indicates the time base of the GNSS products received by the node satellite.

[0028] At this time, the following method can be used to calculate : (8) Where N is the GNSS clock error received by the reference satellite in the current epoch GNSS clock difference received from other low-orbit satellites The number of common visible satellites in Indicates the time base of GNSS products received from other low-orbit satellites.

[0029] Combining formulas (1)-(8), we can get .

[0030] Node satellites prioritize time synchronization with reference satellites by establishing intersatellite links and exchanging time and frequency information to ensure system timeliness. Furthermore, corresponding time synchronization methods can be used for both situations where a communication link with the reference satellite is established and where no communication link is established. These two methods can be switched autonomously to ensure system robustness.

[0031] It is worth noting that the terms "first" and "second" in this disclosure are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly specify the number of the technical features indicated. Therefore, features defined as "first" or "second" may explicitly or implicitly include one or more of such features. In the description of this disclosure, "plurality" means two or more, unless otherwise specifically defined.

[0032] The above is a further detailed description of the present invention in conjunction with specific preferred embodiments, and the specific implementation of the present invention should not be considered to be limited to these descriptions. For those skilled in the art of the present invention, without departing from the concept of the present invention, several simple deductions or substitutions can be made, which should be considered to fall within the scope of protection of the present invention.

Claims

1. A low-orbit satellite on-orbit time synchronization system, characterized in that: include: A regional distributed on-orbit time synchronization subsystem is used to divide the overall coverage area of ​​the low-orbit satellite constellation into a plurality of partially overlapping sub-areas at the current epoch; wherein each sub-area includes a plurality of low-orbit satellites; The GNSS product priority and reception subsystem is used to select the corresponding GNSS product from the GNSS products corresponding to multiple GNSS satellites in the current epoch based on the service range of the GNSS satellite constellation and the sub-region where the low-orbit satellite is located; The regional distributed on-orbit time synchronization subsystem is used to use the GNSS product to synchronize the time of low-orbit satellites in all sub-regions within the region and between regions at the current epoch.

2. The low-orbit satellite on-orbit time synchronization system according to claim 1, characterized in that: The GNSS product priority and receiving subsystem is specifically used to: S10: For any sub-region, determine the service range of most low-orbit satellites in the sub-region at the current epoch and the GNSS products corresponding to the service ranges, and select the determined GNSS product as the GNSS product corresponding to all low-orbit satellites in the sub-region; wherein at least two GNSS products correspond to low-orbit satellites in an overlapping area of ​​adjacent sub-regions; S11: At the current epoch, all GNSS products are received and analyzed to obtain the clock bias of the GNSS satellites.

3. The low-orbit satellite on-orbit time synchronization system according to claim 2, characterized in that: The S10 includes: S101: for any sub-region, determine all low-orbit satellites in the sub-region at the current epoch, and preferentially select the GNSS product of the Galileo HAS with a global service range as the GNSS product for all low-orbit satellites in the sub-region; S102: For any sub-region, if the global Galileo HAS GNSS product in the current epoch has accuracy or availability issues, determine the service range of the low-orbit satellites in the sub-region and select the GNSS product corresponding to the service range with the largest number of low-orbit satellites as the GNSS product for all low-orbit satellites in the sub-region.

4. The low-orbit satellite on-orbit time synchronization system according to claim 3, characterized in that: The GNSS product corresponding to the service area with the largest number of low-orbit satellites is selected as the GNSS product for all low-orbit satellites in the sub-area, including: The GNSS satellite corresponding to the service range with the largest number of low-orbit satellites is selected, and the GNSS product of the GNSS satellite corresponding to the GNSS signal with the highest pitch angle is selected among the GNSS satellites as the GNSS product of all low-orbit satellites in the sub-area.

5. The low-orbit satellite on-orbit time synchronization system according to claim 1, characterized in that: Each sub-region is divided evenly, and the overlapping area of ​​adjacent sub-regions is 10%-15% of the entire sub-region.

6. The low-orbit satellite on-orbit time synchronization system according to claim 1, characterized in that: The regional distributed on-orbit time synchronization subsystem is specifically used for: S20, in the current epoch, selecting a reference satellite from each sub-region, so that the reference satellite performs intra-region time synchronization based on the corresponding GNSS product and observation data of the observed GNSS satellite, so that the time of all low-orbit satellites in the same sub-region is the same; S21, in the current epoch, using the low-orbit satellites in the overlapping area of ​​the adjacent sub-areas that have completed the intra-area time synchronization, the low-orbit satellites in the adjacent sub-areas are synchronized between intervals, so that the time of the low-orbit satellites in all sub-areas is the same.

7. The low-orbit satellite on-orbit time synchronization system according to claim 5, characterized in that: The S20 includes: S201, for any sub-region in the current epoch, selecting a reference satellite within the sub-region, so that the reference satellite calculates a clock difference between the reference satellite and a reference of the GNSS product based on a corresponding GNSS product and observation data of GNSS satellites observed by itself, and transmits the clock difference to other low-orbit satellites in the same sub-region that have established communication links with the reference satellite; S202: For other low-orbit satellites that have not established a communication link with the reference satellite, the low-orbit satellite is used as a reference satellite, so that the reference satellite calculates the clock difference between the reference satellite and the reference of the GNSS product based on the corresponding GNSS product and the observation data of the GNSS satellite observed by itself.

8. The low-orbit satellite on-orbit time synchronization system according to claim 7, characterized in that: The clock difference between the reference satellite and the reference of the GNSS product is expressed as , calculated by the following formula: Where, is the identification number of the reference satellite; is the GNSS satellite number; is the corresponding observation epoch; is the speed of light in vacuum; The clock time of the reference satellite is used; The time of the GNSS satellite onboard clock; is the reference time of the reference satellite clock error, The reference time for the GNSS satellite clock error; 、 is the combined observation value of the corresponding reference satellite, GNSS satellite and epoch with the ionospheric effect eliminated; and 、 is its observation error; is the corresponding wavelength; is the geometric distance between the GNSS satellite position at the time of signal transmission and the reference satellite receiver position, is the reference satellite coordinate, is the GNSS satellite position.

9. The low-orbit satellite on-orbit time synchronization system according to claim 7, characterized in that: S202 includes: S2021, for other low-orbit satellites that have not established a communication link with the reference satellite, if the GNSS product corresponding to the other low-orbit satellite is the same as that corresponding to the reference satellite, calculate the clock difference between the other low-orbit satellite and the reference of the GNSS product using the same method as that used to calculate the clock difference of the reference satellite; S2022: For other low-orbit satellites that have not established a communication link with the reference satellite, if the GNSS products corresponding to the other low-orbit satellites are different from those of the reference satellite, the clock difference between the GNSS product and the reference is calculated using the GNSS products corresponding to the other low-orbit satellites and the observation data of the observed GNSS satellites.

10. The low-orbit satellite on-orbit time synchronization system according to claim 9, characterized in that: The clock difference between the other low-orbit satellites in S2022 and the reference of the GNSS product is expressed as , calculated by the following formula: Where N is the GNSS clock error received by the reference satellite in the current epoch GNSS clock difference received from other low-orbit satellites The number of common visible satellites in Indicates the time base of GNSS products received from other low-orbit satellites.

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