Low earth orbit satellite on-orbit time synchronization system

Abstract

The application provides a low-orbit satellite on-orbit time synchronization system, which comprises a regionally distributed on-orbit time synchronization subsystem, which is used for dividing the overall coverage area of a low-orbit satellite constellation into a plurality of sub-areas with partial overlaps at a current epoch; a GNSS product priority and receiving subsystem, which is used for selecting corresponding GNSS products according to the service range of a GNSS satellite constellation and the sub-area where the low-orbit satellite is located at the current epoch; and the regionally distributed on-orbit time synchronization subsystem, which is used for performing time synchronization of the low-orbit satellite in all sub-areas in the region and between regions by using the GNSS products at the current epoch. The system of the application does not need to involve ground stations and introduces various GNSS products for on-orbit calculation, thereby avoiding high operation and maintenance costs, greatly reducing the time delay caused by signal transmission, and being more real-time. The system can be used for uninterrupted calibration of low-orbit constellation atomic clocks and has higher time synchronization capability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of satellite positioning and timing, and particularly relates to an on-orbit time synchronization system of a low-orbit satellite. BACKGROUND

[0002] In recent years, low-orbit constellation has developed rapidly and has become an important development trend in the field of aerospace. The Starlink constellation of the United States, the OneWeb constellation of Europe and the GW constellation of China will deploy tens of thousands of low-orbit satellites in the future. Time synchronization of large-scale low-orbit constellation is a basic technical challenge to realize global communication, navigation and remote sensing services.

[0003] The existing low-orbit constellation time synchronization technology can be roughly divided into two types, i.e., star-ground cooperative time synchronization technology and inter-satellite autonomous time synchronization technology. The star-ground cooperative time synchronization technology usually refers to that a ground station calculates the clock difference of a LEO satellite through satellite-borne GNSS observation combined with GNSS satellite products, generates a ground station clock difference combined with the same GNSS product, generates star-ground synchronization information and uploads it to the low-orbit constellation, and then realizes the time synchronization of the low-orbit constellation; or refers to that the data transmission including time information between the ground and the constellation is realized through Q / V frequency band, and the time synchronization of the low-orbit constellation is realized combined with inter-satellite link technology. The inter-satellite autonomous time synchronization technology usually refers to that the time transfer between each low-orbit satellite is realized by inter-satellite laser, or refers to a distributed dynamic synchronization algorithm, i.e., a synchronization method based on maximum likelihood estimation, which solves the time delay randomness problem under high dynamic topology.

[0004] Although the star-ground cooperative time synchronization technology can realize high-precision time synchronization of the low-orbit constellation, its dependence on the ground support also brings significant defects, such as dependence on the uplink and downlink of low-orbit satellites and ground data; due to the difficulty in deploying ground stations in polar regions / ocean areas, the satellite cannot be calibrated in time when it passes over; in the GNSS denial environment, the time synchronization system relying on the ground station has weak survivability; the long round-trip time delay of star-ground signal restricts certain real-time applications; and the operation and maintenance cost of the ground system is high.

[0005] The inter-satellite autonomous time synchronization technology also has obvious defects due to the absence of external clock source, for example, due to the limited long-term stability of satellite-borne atomic clock, cumulative error will be generated without external calibration; at the same time, satellites at different altitudes have clock rate difference due to gravity potential difference, which is difficult to completely eliminate by inter-satellite measurement, and will lead to non-uniformity of time reference within the constellation, and then affect the time synchronization performance. The inter-satellite time transfer of low-orbit satellites based on carrier phase common view has constraint requirements on the relative distance between satellites, and when the number of common view satellites is too small, the time transfer accuracy will be affected, and at the same time, it relies on the inter-satellite transmission of original GNSS observation data, which has high requirements on inter-satellite link data transmission. SUMMARY

[0006] In order to solve the above problems existing in the prior art, the application provides a low-orbit satellite on-orbit time synchronization system.

[0007] The low-orbit satellite on-orbit time synchronization system comprises:

[0008] A regional distributed on-orbit time synchronization subsystem is configured to divide a whole coverage area of a low-orbit satellite constellation into a plurality of sub-areas with partial overlaps at a current epoch, wherein each sub-area comprises a plurality of low-orbit satellites.

[0009] A GNSS product priority and receiving subsystem is configured to select a corresponding GNSS product from a plurality of GNSS products of GNSS satellites according to a service range of the GNSS satellite constellation and a sub-area where the low-orbit satellite is located at the current epoch.

[0010] The regional distributed on-orbit time synchronization subsystem is configured to perform time synchronization of the low-orbit satellites in each sub-area and between sub-areas by using the GNSS product at the current epoch.

[0011] Advantages:

[0012] 1) The application does not need to involve ground stations to avoid high operation and maintenance costs, and the on-board observation does not need to be transmitted to the ground, so the system of the application is suitable for scenarios where on-board observation cannot be transmitted to the ground in real time. In the case of ground GNSS denial, the system of the application has stronger survivability, and all calculations in the system are performed on-orbit, which greatly reduces the time delay caused by signal transmission and has stronger real-time performance.

[0013] 2) The application introduces the reference time GNSST of each navigation system by participating in various GNSS products received on-orbit, and compared with the atomic clock carried by the low-orbit satellite constellation, the GNSST usually has better long-term and short-term stability, which can be used to continuously calibrate the atomic clock of the low-orbit satellite constellation. Therefore, compared with the inter-satellite autonomous time synchronization, the system has higher time synchronization performance.

[0014] The application will be further described in detail below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 FIG. 1 is a schematic diagram of a low-orbit satellite on-orbit time synchronization system provided by the application. DETAILED DESCRIPTION

[0016] The application will be further described in detail below with reference to the accompanying drawings and embodiments.

[0017] As Figure 1As shown, the present application provides a low-orbit satellite on-orbit time synchronization system comprising:

[0018] The regional distributed on-orbit time synchronization subsystem is used to divide the overall coverage area of the low-orbit satellite constellation into multiple sub-areas with partial overlap at the current epoch, wherein each sub-area includes multiple low-orbit satellites.

[0019] In the process of dividing each sub-area, a uniform manner is adopted, and the overlapping area of adjacent sub-areas is 10%-15% of the entire sub-area.

[0020] The present application divides the overall coverage area of the low-orbit satellite constellation into N sub-areas with overlap according to the instantaneous coverage range of the low-orbit satellite, and dynamically updates the grid structure at a certain frequency; in the process of dividing the sub-area, the adjacent sub-areas need to maintain 10%-15% of the coverage overlap, so as to ensure that the clock difference can be smoothly transitioned through multiple reference satellites when the low-orbit satellite crosses the regional boundary, avoid synchronization jump, and provide the possibility for inter-regional cooperation; each sub-area selects a group of low-orbit satellites that can receive the same GNSS product as much as possible, and the number of satellites in the region is usually set to 5-50 (not limited) to balance the load and reliability; each sub-area is configured with 2 reference satellites (hot backup) and 1 cold backup satellite, forming a "2+1" redundant structure; the reference satellite should carry a high-performance atomic clock and a high-performance processing unit.

[0021] The GNSS product priority and receiving subsystem is used to select the corresponding GNSS product from the GNSS products corresponding to multiple GNSS satellites according to the service range of the GNSS satellite constellation and the sub-area where the low-orbit satellite is located at the current epoch.

[0022] The currently available GNSS products on-orbit include precise point positioning (PPP) such as Beidou PPP-B2b, European Galileo HAS, etc.; satellite-based augmentation (SBAS) such as BDS BAS, WAAS of GPS and EGNOS system of Galileo, MADOCA product broadcast by Japanese QZSS, etc.; broadcast ephemeris such as broadcast ephemeris of each GNSS system.

[0023] The regional distributed on-orbit time synchronization subsystem is used to synchronize the time of the low-orbit satellites in all sub-areas within and between regions at the current epoch by using the GNSS products.

[0024] In a specific embodiment of the present application, the GNSS product priority and receiving subsystem is specifically used for:

[0025] S10, for any sub-region, determining the service range of the majority of low-orbit satellites in the sub-region at the current epoch and the GNSS product corresponding to the service range, and selecting the determined GNSS product as the GNSS product corresponding to all low-orbit satellites in the sub-region; wherein there are at least two corresponding GNSS products for the low-orbit satellites in the overlapping region of adjacent sub-regions;

[0026] In a specific embodiment of the application, S10 comprises:

[0027] S101, for any sub-region, determining all low-orbit satellites in the sub-region at the current epoch, and preferentially selecting the GNSS product of the Galileo HAS with global service range as the GNSS product of all low-orbit satellites in the sub-region;

[0028] S102, for any sub-region, if the GNSS product of the global Galileo HAS has accuracy or availability problems at the current epoch, determining the service range of the low-orbit satellites in the sub-region, and selecting the GNSS product corresponding to the service range with the largest number of low-orbit satellites as the GNSS product of all low-orbit satellites in the sub-region.

[0029] This step can select the GNSS satellite corresponding to the service range with the largest number of low-orbit satellites, and select the GNSS product of the GNSS satellite corresponding to the GNSS signal with the highest elevation angle as the GNSS product of all low-orbit satellites in the sub-region.

[0030] When the low-orbit constellation is in orbit, it needs to dynamically and adaptively select available GNSS enhancement products according to its flight coverage area. The selection strategy needs to consider regional coverage adaptation, multi-system redundancy backup, and orbit height optimization. First, give priority to high-precision and global coverage products (such as Galileo HAS) to minimize product switching frequency. When the accuracy or availability of the global coverage product is problematic, regional coverage adaptation needs to be considered, such as preferentially receiving Beidou PPP-B2b and QZSS MADOCA products in the Asia-Pacific region, preferentially receiving EGNOS products in the European and African regions, and preferentially receiving GPS WAAS products in the American region. When there is no available product, switch to broadcast ephemeris. In terms of multi-system redundancy backup, when the satellite crosses the service area boundary, it needs to adaptively fuse multi-source data such as Beidou BDS BAS and GPS SBAS to avoid service interruption. At the same time, for low-orbit satellites of different altitudes, the GNSS signal blocking rate needs to be evaluated, and the low-orbit satellites tend to select QZSS or Galileo signals with higher elevation angles.

[0031] S11, at the current epoch, receiving all GNSS products and parsing them to obtain the clock error of the GNSS satellite.

[0032] Currently, various products broadcast via different frequencies and cover different areas. For example, the BeiDou PPP-B2b product broadcasts via the B2b signals of the three BeiDou-3 GEO satellites, covering China and surrounding areas; Galileo broadcasts via the E6B signal; QZSS CLAS broadcasts via the L6D signal, covering mainland Japan; BDSBAS broadcasts via BeiDou GEO satellites, covering the Asia-Pacific region; WAAS / EGNOS covers the Americas and Europe via the L1 band; and QZSS MADOCA broadcasts via the QZSS L6E signal. Each GNSS system broadcasts ephemeris signals globally via its respective service frequency band. Therefore, it is necessary to receive and analyze the selected GNSS products.

[0033] In one specific embodiment of the present invention, the regionally distributed on-orbit time synchronization subsystem is specifically used for:

[0034] S20, at the current epoch, select a reference satellite from each sub-region so that the reference satellite performs intra-region time synchronization based on the corresponding GNSS product and the observation data of the observed GNSS satellite, thereby making the time of all low-orbit satellites in the same sub-region the same;

[0035] S21, at the current epoch, using the low-orbit satellites that have already completed intra-regional time synchronization within the overlapping areas of adjacent sub-regions, perform inter-regional time synchronization on the low-orbit satellites within the adjacent sub-regions, thereby making the time of low-orbit satellites in all sub-regions the same.

[0036] In one specific embodiment of the present invention, S20 includes:

[0037] S201, at the current epoch, for any sub-region, select a reference satellite from that sub-region, 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 its own observation data of the GNSS satellite, and sends the clock difference to other low-orbit satellites in the same sub-region that have established a communication link with the reference satellite;

[0038] The clock difference between the reference satellite and the reference of the GNSS product is expressed as: It is calculated using the following formula:

[0039] (1)

[0040] (2)

[0041] (3)

[0042] (4)

[0043] In the formula, The identifier for the reference satellite; GNSS satellite number; For the corresponding observation epoch; The speed of light in a vacuum; Use the clock face time of the reference satellite; The clock face time of the GNSS satellite's onboard clock; Reference time for the clock bias of the benchmark satellite. Reference time for the clock bias of GNSS satellites; , This refers to combined observations from the corresponding reference satellite, GNSS satellite, and epoch, after the elimination of ionospheric effects; while , Its observation error; For the corresponding wavelength; This represents the geometric distance between the GNSS satellite position and the reference satellite receiver position at the time of signal transmission. Based on the reference satellite coordinates, This indicates the location of the GNSS satellite.

[0044] From the above, we can see that by combining formulas (1) and (4), we can obtain:

[0045] (5);

[0046] 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.

[0047] S202, for other low-orbit satellites that have not established a communication link with the reference satellite, the low-orbit satellite is used as the reference satellite, so that the reference satellite can calculate 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 it observes.

[0048] During the time synchronization process with the reference satellite, a GNSS product reference was introduced. In other words, the GNSST, the time reference of each GNSS system, generally has better long-term and short-term stability than the atomic clocks carried by the low-Earth orbit constellation. It can be used to continuously calibrate the atomic clocks of the low-Earth orbit constellation, avoid the accumulation of errors during the autonomous time synchronization of the constellation, and enable the system to have higher time synchronization performance.

[0049] In one specific embodiment of the present invention, S202 includes:

[0050] S2021, For other low-orbit satellites that have not established a communication link with the reference satellite, if the other low-orbit satellite has the same GNSS product as the reference satellite, the clock difference between the reference satellite and the reference of the GNSS product is calculated using the same method as the clock difference calculation for the reference satellite.

[0051] Node satellites can also receive GNSS products and onboard GNSS observation data, and use the PPP principle to calculate the clock difference between the node satellite and the GNSS product reference. In this case, there are two situations: receiving the same GNSS product as the reference satellite, or receiving different GNSS products. When the node satellite and the reference satellite receive the same product, the clock difference of the node satellite relative to the time reference of the received GNSS product can be known, as shown in formula (6). The principle and steps are the same as formulas (1)-(4).

[0052] (6)

[0053] S2022, 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 different from that of the reference satellite, the clock difference between the other low-orbit satellite and the reference of the GNSS product is calculated using the GNSS product corresponding to the other low-orbit satellite and the observation data of the observed GNSS satellite.

[0054] When a node satellite and a reference satellite receive different GNSS products, the clock difference calculated by other low-Earth orbit satellites relative to the reference GNSS product can be expressed as follows, using the PPP principle:

[0055] (7);

[0056] in This indicates the time reference for GNSS products received by the node satellite.

[0057] At this point, it can be calculated using the following method. :

[0058] (8)

[0059] Where N is the GNSS clock bias received by the reference satellite at the current epoch. GNSS clock bias received by other low-Earth orbit satellites The common number of visible satellites in the region, This indicates the time reference for GNSS products received by other low-Earth orbit satellites.

[0060] By combining formulas (1) and (8), we can obtain .

[0061] Node satellites prioritize establishing inter-satellite links and exchanging time and frequency information to achieve time synchronization with the reference satellite, ensuring system timeliness. Simultaneously, for both scenarios with and without established communication links with the reference satellite, corresponding time synchronization methods can be used. These two methods can be switched autonomously to ensure system robustness.

[0062] It is worth noting that the terms "first" and "second" in this invention are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0063] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A low-Earth orbit satellite on-orbit time synchronization system, characterized in that, include: The regional distributed on-orbit time synchronization subsystem is used to divide the overall coverage area of ​​the low-Earth orbit satellite constellation into multiple sub-regions with partial overlap at the current epoch; each sub-region includes multiple low-Earth orbit satellites. The GNSS product prioritization and reception subsystem is used to select the corresponding GNSS product from multiple GNSS products at the current epoch, based on the service range of the GNSS satellite constellation and the sub-region where the low-Earth orbit satellite is located. The regional distributed on-orbit time synchronization subsystem is used to synchronize the time of all low-orbit satellites in all sub-regions within and between regions using the GNSS products at the current epoch. The GNSS product prioritization and reception subsystem is specifically used for: S10, for any sub-region, determine the service range of most low-Earth orbit satellites in the sub-region at the current epoch and the GNSS products corresponding to the service range, and select the determined GNSS products as the GNSS products corresponding to all low-Earth orbit satellites in the sub-region; wherein, low-Earth orbit satellites in the overlapping areas of adjacent sub-regions have at least two corresponding GNSS products. S11, at the current epoch, receive all GNSS products and analyze them to obtain the clock bias of the GNSS satellites; The regional distributed on-orbit time synchronization subsystem is specifically used for: S20, at the current epoch, select a reference satellite from each sub-region so that the reference satellite performs intra-region time synchronization based on the corresponding GNSS product and the observation data of the observed GNSS satellite, thereby making the time of all low-orbit satellites in the same sub-region the same; S21, at the current epoch, using the low-orbit satellites that have already completed intra-regional time synchronization within the overlapping areas of adjacent sub-regions, perform inter-regional time synchronization on the low-orbit satellites within the adjacent sub-regions, thereby making the time of low-orbit satellites in all sub-regions the same.

2. The low-Earth orbit satellite on-orbit time synchronization system according to claim 1, characterized in that, S10 includes: S101, For any sub-region, identify all low-Earth orbit satellites in that sub-region at the current epoch, and select the GNSS product of Galileo HAS, which has a global service range, as the GNSS product for all low-Earth orbit satellites in that sub-region. S102, for any sub-region, if there is an accuracy or availability problem with the Galileo HAS GNSS product in the current epoch, then determine the service range of the low-Earth orbit satellites in the sub-region, and select the GNSS product corresponding to the service range with the largest number of low-Earth orbit satellites as the GNSS product for all low-Earth orbit satellites in that sub-region.

3. The low-orbit satellite on-orbit time synchronization system according to claim 2, characterized in that, The GNSS product corresponding to the service area with the largest number of low-Earth orbit satellites selected as the GNSS product for all low-Earth orbit satellites in that sub-region includes: Select the GNSS satellite corresponding to the service area with the largest number of low-Earth orbit satellites, and then select the GNSS product of the GNSS satellite corresponding to the GNSS signal with the highest elevation angle from the GNSS satellites to serve as the GNSS product of all low-Earth orbit satellites in that sub-area.

4. The low-Earth orbit satellite on-orbit time synchronization system according to claim 1, characterized in that, When dividing each sub-region, the division should be uniform, with the overlap between adjacent sub-regions being 10%-15% of the entire sub-region.

5. The low-orbit satellite on-orbit time synchronization system according to claim 3, characterized in that, S20 includes: S201, at the current epoch, for any sub-region, select a reference satellite from that sub-region, 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 its own observation data of the GNSS satellite, and sends the clock difference to other low-orbit satellites in the same sub-region that have established a communication link 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 the reference satellite, so that the reference satellite can calculate 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 it observes.

6. The low-Earth orbit satellite on-orbit time synchronization system according to claim 5, characterized in that, The clock difference between the reference satellite and the reference of the GNSS product is expressed as: It is calculated using the following formula: In the formula, The identifier for the reference satellite; GNSS satellite number; For the corresponding observation epoch; The speed of light in a vacuum; Use the clock face time of the reference satellite; The clock face time of the GNSS satellite's onboard clock; Reference time for the clock bias of the benchmark satellite. Reference time for the clock bias of GNSS satellites; , This refers to combined observations from the corresponding reference satellite, GNSS satellite, and epoch, after the elimination of ionospheric effects; while , Its observation error; For the corresponding wavelength; This represents the geometric distance between the GNSS satellite position and the reference satellite receiver position at the time of signal transmission. Based on the reference satellite coordinates, This indicates the location of the GNSS satellite.

7. The low-Earth orbit satellite on-orbit time synchronization system according to claim 5, characterized in that, S202 includes: S2021, For other low-orbit satellites that have not established a communication link with the reference satellite, if the other low-orbit satellite has the same GNSS product as the reference satellite, the clock difference between the reference satellite and the reference of the GNSS product is calculated using the same method as the clock difference calculation for the reference satellite. S2022, 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 different from that of the reference satellite, the clock difference between the other low-orbit satellite and the reference of the GNSS product is calculated using the GNSS product corresponding to the other low-orbit satellite and the observation data of the observed GNSS satellite.

8. The low-Earth orbit satellite on-orbit time synchronization system according to claim 7, characterized in that, The clock difference between other low-Earth orbit satellites in S2022 and the reference GNSS product is expressed as follows: It is calculated using the following formula: Where N is the GNSS clock bias received by the reference satellite at the current epoch. GNSS clock bias received by other low-Earth orbit satellites The common number of visible satellites in the region, This indicates the time reference for GNSS products received by other low-Earth orbit satellites.

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