Highly reliable satellite time correction method based on gnss

Abstract

The application provides a high-reliability satellite time correction method based on GNSS, which is applied to a satellite system comprising a GNSS receiver and a central management unit (CMU), and comprises the following steps: when the GNSS receiver is switched from a non-positioning state to a positioning state, if the calculated clock difference and clock drift satisfy a preset fine adjustment threshold, the output of the second pulse signal period is allowed to be adjusted; when the GNSS receiver is performing positioning calculation, the minimum number of satellites required to start the positioning calculation is increased and the time initialization strategy is optimized; on the side of the central management unit (CMU), when the GNSS receiver is in an enabled state, receives a valid GNSS second pulse and acquires a GNSS telemetry value of 0xA5A5 representing an extrapolation mode or 0xFFFF representing a real-time mode, the GNSS time is used to correct the CMU time; otherwise, the CMU local time is used to keep time. In this way, the application can guarantee the correctness of the whole satellite time system and meet the time use requirements of the satellite.

Description

Technical Field

[0001] This invention relates to the field of spacecraft technology, and specifically to a highly reliable satellite time synchronization method based on GNSS. Background Technology

[0002] As high-orbit satellite missions become increasingly complex, various payload units require high-precision time information to complete their tasks. Therefore, satellites need to be equipped with high-orbit GNSS navigation systems as important units in the time unification system. These systems, combined with the CMU's time management and distribution functions, provide stable and reliable time and orbit information to various subsystems of the high-orbit satellite.

[0003] The current GNSS system faces the following problems: Firstly, due to the special nature of high-orbit satellite orbits, there are several hours each day when the number of GNSS satellite observations is insufficient to switch to non-positioning mode. During the transition from non-positioning to positioning mode, the GNSS receiver experiences a process where the number of available satellites increases. When there are few available satellites, the initialization error of the positioning solution time is large, resulting in an excessively large second pulse adjustment, even reaching the second level, which exceeds the variation range allowed by the CMU and causes the entire satellite to fail to introduce the correct time. Secondly, navigation signals are currently susceptible to deceptive interference from ground broadcasts, which can affect GNSS receivers and cause them to calculate time using deceptive signals, resulting in a discrepancy between the actual time and the time of the entire satellite. Third, current GNSS time design schemes only consider the situation where GNSS is effective. The central management unit introduces second pulses and time information for time synchronization. Once the number of GNSS observations is insufficient or the system is affected by interference signals, resulting in the inability to locate for a long time, it will switch to extrapolation mode or invalid mode. In this case, the central management unit will no longer introduce time information and will only rely on local time for timekeeping. The crystal oscillator of the central management unit itself is not very accurate due to the influence of the space environment, which cannot meet the functional requirements of the satellite. Summary of the Invention

[0004] To address the issues of second pulse jumps, weak anti-interference capabilities, and low timekeeping accuracy in extrapolation mode in existing high-orbit satellite GNSS time synchronization technologies, this invention provides a highly reliable satellite time synchronization method based on GNSS. By increasing the fine adjustment threshold, raising the threshold for calculating the number of satellites, optimizing the time initialization strategy, and extending the CMU to introduce extrapolation mode time, the continuity and reliability of time synchronization are improved.

[0005] To achieve the above-mentioned technical effects, this invention provides a highly reliable satellite time synchronization method based on GNSS, applicable to a satellite system including a GNSS receiver and a central management unit (CMU), comprising: When the GNSS receiver switches from a non-positioning state to a positioning state, if the clock error and clock drift calculated using the least squares method meet the preset fine adjustment threshold, then the period of the output second pulse signal is allowed to be adjusted. When performing positioning calculations on the GNSS receiver side, the minimum number of satellites required to initiate positioning calculations is increased and the time initialization strategy is optimized. On the CMU side, when the GNSS receiver is enabled, receives a valid GNSS second pulse, and the acquired GNSS telemetry value is 0xA5A5 for extrapolation mode or 0xFFFF for real-time mode, the GNSS time is used to synchronize the CMU time; otherwise, the CMU local time is used for timekeeping.

[0006] The preset fine adjustment threshold is specifically as follows: The clock error is no more than 2000 meters and the clock drift rate is less than 2; and after 20 seconds of continuous stable positioning, the clock error threshold is tightened to no more than 200 meters, and the clock drift rate remains less than 2.

[0007] After meeting the preset fine adjustment threshold, when the period of the output second pulse signal is allowed to be adjusted, the adjustment range of the period of the second pulse signal is limited to within 50 microseconds.

[0008] The increase in the minimum number of satellites required to initiate positioning calculations specifically refers to: When using a single-satellite navigation system, least squares positioning calculation is initiated only when there are at least 5 available satellites; When using a dual-satellite navigation system, least squares positioning is only initiated when there are at least 6 available satellites.

[0009] The optimized time initialization strategy is as follows: When the GNSS receiver initializes its time, it uses the BeiDou time of the BeiDou satellite navigation system, and only uses GPS time when there are no BeiDou satellites. When BeiDou time is used first, the acquired BeiDou time is converted into GPS time and then assigned to the GPS time system to correct its time reference.

[0010] The triggering conditions for using CMU local time for timekeeping specifically include: The GNSS receiver access is disabled; or If no GNSS second pulse is received for more than 20 seconds, the second pulse is considered invalid; or The acquired GNSS telemetry value is 0x0000, indicating that the time is invalid.

[0011] The central management unit (CMU) synchronizes its time with GNSS time and then sets the second pulse synchronization success flag to 0xFF; when using the CMU local time for timekeeping, it sets the second pulse synchronization success flag to 0x00.

[0012] Furthermore, it also includes: if the satellite fails to synchronize time continuously for more than 2 hours, the GNSS receiver access status will be automatically disabled, and the central management unit (CMU) will use local time to keep time until it receives a GNSS access enable command sent from the ground, at which point it will resume using GNSS time to synchronize the CMU time.

[0013] This invention provides a highly reliable satellite time synchronization method based on GNSS. On the GNSS receiver side, when switching from a non-positioning state to a positioning state, a fine adjustment threshold is added. Adjustment of the second pulse signal period is only permitted if the clock error calculated using the least squares method is no greater than 2000 meters and the clock drift rate is less than 2, and the clock error threshold is tightened to no greater than 200 meters after 20 seconds of continuous stable positioning. This limits the adjustment range to within 50 microseconds. Simultaneously, the minimum number of satellites required to initiate positioning calculation is increased to 5 for a single system and 6 for a dual system, with BeiDou time being prioritized for initialization to enhance anti-interference capabilities. On the central management unit (CMU) side, the range of time sources is expanded. GNSS time synchronization is used when GNSS access is enabled, the second pulse is valid, and telemetry is in extrapolation or real-time mode; otherwise, local time is used. GNSS access is automatically disabled after more than 2 hours of continuous time synchronization failures, requiring a ground-based enable command to restore access. Thus, through the synergistic improvement of the above three aspects, the present invention effectively eliminates time jumps during positioning mode switching, reduces the probability of positioning calculation anomalies, enhances the system's ability to resist deceptive interference, and can still introduce a high-precision time source in GNSS extrapolation mode, significantly improving the continuity, reliability and accuracy of the entire satellite time system, and meeting the stringent time management requirements of high-orbit satellites. Attached Figure Description

[0014] Figure 1 A flowchart illustrating the steps of the GNSS-based high-reliability satellite time synchronization method provided in an embodiment of the present invention; Figure 2 A flowchart for time adjustment of existing satellite systems; Figure 3 The flowchart shows the time adjustment process using the GNSS-based high-reliability satellite time synchronization method described in this invention. Figure 4 This is a schematic diagram of test data results using existing technology; Figure 5This is a schematic diagram of the test data results using the GNSS-based high-reliability satellite time synchronization method described in this invention. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0016] It should be noted that references to "an embodiment," "embodiment," "example embodiment," etc., in this specification refer to the described embodiment including specific features, structures, or characteristics, but not every embodiment must include these specific features, structures, or characteristics. Furthermore, such expressions do not refer to the same embodiment. Moreover, when describing specific features, structures, or characteristics in conjunction with embodiments, whether or not explicitly described, it is indicated that incorporating such features, structures, or characteristics into other embodiments is within the knowledge of those skilled in the art.

[0017] Furthermore, certain terms are used in the specification and subsequent claims to refer to specific components or parts. Those skilled in the art will understand that manufacturers may use different names or terms to refer to the same component or part. This specification and subsequent claims do not distinguish components or parts by differences in name, but rather by differences in function.

[0018] GNSS equipment receives satellite navigation signals (BDS, GPS, or BDS+GPS) to achieve navigation, positioning, and timing functions. As an important unit in the time unification system, it completes the comprehensive service tasks of time and navigation information, providing stable and reliable time, position, and velocity information for satellites.

[0019] The satellite system is provided with time and orbital position information by GNSS. The satellite time maintenance of the whole satellite is mainly the responsibility of CMU. The satellite time of other individual units is mainly realized through CMU bus time synchronization + GNSS second pulse. For individual units without second pulse signal, their satellite time is mainly executed through 1553B bus time transmission. Therefore, the reliability and continuity of the satellite time system depends on two aspects. On the one hand, GNSS needs to provide accurate time information and second pulse. On the other hand, CMU needs to continuously receive time information and second pulse from GNSS to calibrate its own satellite time and then send it to each lower unit. In view of the current problems, based on these two reasons, the following three aspects of the design scheme are proposed: (1) Optimization of GNSS time processing flow; (2) Change of reliability measures; (3) Optimization of CMU's use of GNSS time range.

[0020] To enable those skilled in the art to reproduce and implement the present invention, the following detailed description of each step of the GNSS-based highly reliable satellite time synchronization method will be provided with reference to the accompanying drawings and specific embodiments.

[0021] Figure 1 This invention illustrates a highly reliable satellite time synchronization method based on GNSS, applied to a satellite system including a GNSS receiver and a central management unit (CMU). The method includes steps S101 (GNSS time processing flow optimization), S102 (reliability measure modification), and S103 (CMU GNSS time range optimization). Specifically: S101: When switching from a non-positioning state to a positioning state on the GNSS receiver side, if the clock error and clock drift calculated using the least squares method meet a preset fine adjustment threshold, then the period of the output second pulse signal is allowed to be adjusted; wherein, the preset fine adjustment threshold is specifically: the clock error is no greater than 2000 meters and the clock drift change rate is less than 2; and after 20 seconds of continuous stable positioning, the clock error threshold is tightened to no greater than 200 meters, and the clock drift change rate is kept less than 2; when the period of the output second pulse signal is allowed to be adjusted after meeting the preset fine adjustment threshold, the adjustment range of the period of the second pulse signal is limited to within 50 microseconds.

[0022] To address the issue of time jumps during the switching between positioning and non-positioning when the number of GNSS observations is insufficient, this embodiment optimizes the GNSS time processing flow in step S102.

[0023] In this embodiment, the SOC navigation information processing software adds a reasonableness judgment for time adjustment during the non-positioning to positioning process. Specifically, a reasonableness judgment of a "fine adjustment threshold" is added when adjusting the second pulse signal period. The preset fine adjustment threshold is as follows: clock error no greater than 2000 meters and clock drift rate of change less than 2; and after 20 seconds of continuous stable positioning, the clock error threshold is tightened to no greater than 200 meters, and the clock drift rate of change remains less than 2. The above measures ensure that the receiver's least squares positioning result meets the reasonableness judgment condition, and the clock error and clock drift converge and decrease before the second pulse signal period is adjusted, ensuring that the second pulse signal period output by the receiver before the output is in positioning state will not exceed the range limited by the fine adjustment threshold. By judging the reasonableness of clock error meeting the fine adjustment threshold, after meeting the preset fine adjustment threshold, when the output second pulse signal period is allowed to be adjusted, the adjustment range of the second pulse signal period is limited to within 50 microseconds, and the second pulse signal period no longer exhibits unacceptable deviations of the CMU.

[0024] The process for adjusting the time before the optimization measures are changed is as follows: Figure 2 As shown, the time adjustment process after the change is as follows: Figure 3 As shown.

[0025] S102: When performing positioning calculations on the GNSS receiver side, increase the minimum number of satellites required to initiate the positioning calculation and optimize the time initialization strategy. Step S102 involves reliability and security design for navigation information processing to address the impact of interference or spoofing signals on GNSS positioning and orbit determination.

[0026] The SOC navigation information processing software sets the minimum number of satellites for positioning to 4 for a single system and 5 for a dual system. Under this condition, the objective factors of poor high-orbit signal quality and poor geometric configuration, when incorporated into the least squares solution, can lead to calculation anomalies and deviations. To improve the overall positioning performance and reduce the probability of positioning calculation anomalies, this embodiment adopts an optimization measure that increases the minimum number of satellites required to initiate the positioning calculation. Specifically, when using a single-satellite navigation system, the least squares positioning calculation is only initiated when at least 5 satellites are available; when using a dual-satellite navigation system, the least squares positioning calculation is only initiated when at least 6 satellites are available. This measure is to avoid the occurrence of calculation anomalies and deviations after incorporating the original minimum number of satellites (4 for a single system and 5 for a dual system) into the least squares calculation under the objective factors of poor high-orbit signal quality and poor geometric configuration, thereby improving the overall positioning performance and reducing the probability of positioning calculation anomalies.

[0027] Meanwhile, this embodiment optimizes the time initialization strategy. Specifically, when the GNSS receiver initializes its time, it prioritizes using the BeiDou time of the BeiDou satellite navigation system, and only uses GPS time when there are no BeiDou satellites available. Furthermore, when prioritizing BeiDou time, the acquired BeiDou time is converted to GPS time and then assigned to the GPS time system to correct its time reference. That is, the original receiver time initialization prioritized GPS time; after the change, it prioritizes using BD time (except for GPS satellite time), and converts the BD time to GPS time and assigns it to the GPS time system to help correct its time reference and prevent GPS time from being affected by interference signals.

[0028] Currently, most satellite time is only introduced during GNSS real-time positioning and orbit determination. However, in reality, GNSS time accuracy remains very high even in extrapolation mode, exceeding the time accuracy of the CMU itself. Therefore, the CMU can continue to incorporate time information and second pulses from GNSS extrapolation mode for system-level time calibration and transmit reliable time information to each lower-level machine via the 1553B bus. Specific optimization measures are detailed in step S103 below.

[0029] S103: On the CMU side, when the GNSS receiver access is enabled, a valid GNSS second pulse is received, and the acquired GNSS telemetry value is 0xA5A5 for extrapolation mode or 0xFFFF for real-time mode, the GNSS time is used to synchronize the CMU time; otherwise, the CMU local time is used for timekeeping.

[0030] The triggering conditions for using CMU local time for timekeeping specifically include: the GNSS receiver access is disabled; or if no GNSS second pulse is received for more than 20 seconds, the second pulse is determined to be invalid; or the acquired GNSS telemetry value is 0x0000, indicating invalid time.

[0031] In this embodiment, after the central management unit (CMU) synchronizes its time with GNSS time, it sets the second pulse time synchronization success flag to 0xFF; when using the CMU local time for timekeeping, it sets the second pulse time synchronization success flag to 0x00.

[0032] Furthermore, to ensure the reliability of the entire satellite time system, this embodiment also includes a failure protection mechanism: if on-board time synchronization fails continuously for more than 2 hours, the GNSS receiver access status is automatically disabled. Even if the time synchronization conditions are met thereafter, the Central Management Unit (CMU) continues to use local time for timekeeping until it receives a GNSS access enable command from the ground, at which point it resumes using GNSS time to synchronize with the CMU time. This mechanism prevents time instability caused by repeated switching between abnormal and normal states, requiring ground intervention for confirmation before recovery.

[0033] Definitions of telemetry and remote control information related to time synchronization: (1) GNSS access enable / disable command GNSS access is divided into enabled and disabled. It is disabled by default and can be changed via commands.

[0034] GNSS access enabled: CMU uses GNSS for time synchronization; GNSS access prohibited: CMU does not use GNSS for time synchronization.

[0035] (2) Definition of satellite time attributes (time information sent to the lower-level machine via the 1553B bus) GNSS access enablement is shown in Table 1 below: Table 1:

[0036] GNSS access is prohibited as shown in Table 2 below: Table 2:

[0037] ①: When the CMU can receive the second pulse from the GNSS normally and the telemetry is "0x5A5A or 0xA5A5 or 0xFFFF", the CMU time is synchronized with the GNSS time. This means that the CMU uses the GNSS time for synchronization and sets the PPS time synchronization success flag to "0xFF" (time synchronization successful). ②: If the CMU does not receive a second pulse from GNSS for more than 20 seconds, or if the GNSS telemetry indicates invalid time, or if GNSS access is disabled, the CMU will use its local crystal oscillator to accumulate time, indicating that the CMU is using its own time, and will mark the PPS time synchronization success flag as "0x00" (time synchronization failed). If time synchronization fails continuously for more than 2 hours, GNSS access will be automatically disabled. Even if the time synchronization conditions are met, the CMU will not be able to use GNSS time. The ground must resend the GNSS access enable signal before the CMU can use GNSS time again.

[0038] Figure 4 and Figure 5 The data comparison before and after the time synchronization scheme change is shown. The comparison data is the PPS time synchronization success flag. When the value is "0xFF", it means that the time synchronization was successful, and when the value is "0x00", it means that the time synchronization failed.

[0039] The comparison results show that before the optimization, there were about 8 time synchronization failures in about 5 days, and 4 of them were long-term (greater than 2 hours) time synchronization failures. At this time, only ground intervention was required to successfully resynchronize the time. After the optimization, there was only 1 time synchronization failure in about 5 days, and the duration was less than 2 hours. The satellite could automatically synchronize the time and no ground intervention was required.

[0040] In summary, through the synergistic improvements in the above three aspects, this invention effectively eliminates time jumps during positioning mode switching, reduces the probability of positioning calculation anomalies, enhances the system's ability to resist deceptive interference, and can still introduce a high-precision time source in GNSS extrapolation mode, significantly improving the continuity, reliability, and accuracy of the whole satellite time system.

[0041] Numerous specific details are set forth in the specification provided herein. However, it will be understood that embodiments of the invention may be practiced without these specific details. In some instances, well-known methods, structures, and techniques have not been shown in detail so as not to obscure the understanding of this specification.

[0042] It should be noted that the scope of the methods and apparatus in the embodiments of the present invention is not limited to performing functions in the order shown or discussed, but may also include performing functions substantially simultaneously or in the reverse order, depending on the functions involved. For example, the described methods may be performed in a different order than described, and various steps may be added, omitted, or combined. In addition, features described with reference to certain examples may be combined in other examples.

[0043] Of course, the present invention may have other various embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and modifications according to the present invention, but these corresponding changes and modifications should all fall within the protection scope of the appended claims.

Claims

1. A highly reliable satellite time synchronization method based on GNSS, applied to a satellite system including a GNSS receiver and a central management unit (CMU), characterized in that, include: When the GNSS receiver switches from a non-positioning state to a positioning state, if the clock error and clock drift calculated using the least squares method meet the preset fine adjustment threshold, then the period of the output second pulse signal is allowed to be adjusted. When performing positioning calculations on the GNSS receiver side, the minimum number of satellites required to initiate positioning calculations is increased and the time initialization strategy is optimized. On the CMU side, when the GNSS receiver is enabled, receives a valid GNSS second pulse, and the acquired GNSS telemetry value is 0xA5A5 for extrapolation mode or 0xFFFF for real-time mode, the GNSS time is used to synchronize the CMU time; otherwise, the CMU local time is used for timekeeping.

2. The method according to claim 1, characterized in that, The preset fine adjustment threshold is specifically as follows: The clock error is no more than 2000 meters and the clock drift rate is less than 2; and after 20 seconds of continuous stable positioning, the clock error threshold is tightened to no more than 200 meters, and the clock drift rate remains less than 2.

3. The method according to claim 2, characterized in that, After meeting the preset fine adjustment threshold, when the period of the output second pulse signal is allowed to be adjusted, the adjustment range of the period of the second pulse signal is limited to within 50 microseconds.

4. The method according to claim 1, characterized in that, The increase in the minimum number of satellites required to initiate positioning calculations specifically refers to: When using a single-satellite navigation system, least squares positioning calculation is initiated only when there are at least 5 available satellites; When using a dual-satellite navigation system, least squares positioning is only initiated when there are at least 6 available satellites.

5. The method according to claim 1, characterized in that, The optimized time initialization strategy is as follows: When the GNSS receiver initializes its time, it uses the BeiDou time of the BeiDou satellite navigation system, and only uses GPS time when there are no BeiDou satellites. When BeiDou time is used first, the acquired BeiDou time is converted into GPS time and then assigned to the GPS time system to correct its time reference.

6. The method according to claim 1, characterized in that, The triggering conditions for using CMU local time for timekeeping specifically include: The GNSS receiver access is disabled; or If no GNSS second pulse is received for more than 20 seconds, the second pulse is considered invalid; or The acquired GNSS telemetry value is 0x0000, indicating that the time is invalid.

7. The method according to claim 1, characterized in that, The central management unit (CMU) synchronizes its time with GNSS time and then sets the second pulse synchronization success flag to 0xFF; when using the CMU local time for timekeeping, it sets the second pulse synchronization success flag to 0x00.

8. The method according to claim 1, characterized in that, Also includes: If the satellite fails to synchronize time continuously for more than 2 hours, the GNSS receiver access status will be automatically disabled. The central management unit (CMU) will use local time to keep time until it receives a GNSS access enable command from the ground, at which point it will resume using GNSS time to synchronize the CMU time.

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