A method for generating time reference considering low-orbit satellite clocks

Abstract

The present invention provides a method for generating a time base that takes into account the clocks on low-orbit satellites. After obtaining a comprehensive time scale relative to GPST by acquiring multiple groups of clock data and then preprocessing, the comprehensive time scale is synchronized with UTC using a time scale algorithm, and then the time difference between the ground station, GNSS satellite, and LEO satellite and the comprehensive time scale is calculated in combination with the preprocessed clock data; the time difference is broadcast to users through a broadcast message. The present invention uses clocks with superior performance in ground stations, combined with clocks of a huge number of LEO satellites and GNSS satellites, to expand the scale of the clock group, increase the redundancy of the clock group and the robustness of the time base, and can generate a comprehensive time scale with high stability, continuity, and reliability. It can not only provide a time base with better performance for LEO satellites, but also evaluate the performance and operating status of LEO satellites, and can provide a relatively long autonomous and stable time base for autonomous LEO satellites.

Description

Technical Field

[0001] The invention belongs to the technical field of satellite timing, and in particular relates to a time reference generation method taking into account a low-orbit satellite-borne clock. Background Art

[0002] In satellite navigation systems, the time reference is the basis for system positioning, navigation and time comparison. It needs to be synchronized with UTC (Cordinated Universal Time) within 100ns and remain stable, continuous and reliable.

[0003] Currently, widely used time standards include GPST (Global Positioning System Time). GPST is the system time of GPS (Global Positioning System) established by combining the atomic clocks of the ground segment and the space segment in the combined clock mode. The GPS system uses a high-precision atomic clock in the master control station as the reference clock. The internal clock of the master control station is compared with the reference clock on the timeline. The monitoring station and the satellite-borne atomic clock are compared with the reference clock remotely to obtain the clock difference data of each atomic clock in the system relative to the reference clock of the master control station, and then use the time scale algorithm to generate the free-running GPST. Finally, the local coordinated time maintained by the United States Naval Observatory (United Stated Naval Observatory, USNO) is used. Control the free-running GPST to achieve time synchronization accuracy.

[0004] However, low-orbit satellites have the advantages of low orbit altitude and high speed, and are often used as low-orbit enhanced navigation systems. They can also be used as independent satellite navigation systems. When the GPST system was established, it used high-precision ground atomic clocks and GNSS (Global Navigation Satellite System) onboard clocks, ignoring the onboard clock resources of low-orbit satellites. As a result, the time base of the existing timing scheme has limited the independent operation and autonomous time maintenance of low-orbit satellites to a certain extent; and the stability, accuracy and robustness of the existing time base still have room for improvement. Summary of the invention

[0005] In order to solve the above problems existing in the prior art, the present invention provides a method for generating a time reference taking into account the low-orbit satellite-borne clock. The technical problem to be solved by the present invention is achieved through the following technical solutions:

[0006] A method for generating a time reference taking into account a low-orbit satellite-borne clock comprises:

[0007] S100, acquiring multiple sets of clock data, the multiple sets of clock data including first clock data of a ground station, second clock data of a GNSS satellite, and third clock data of a LEO (Low Earth Orbit) satellite;

[0008] S200, preprocessing the multiple groups of clock data to obtain preprocessed clock data; the preprocessed clock data includes preprocessed first clock difference data, preprocessed second clock difference data and preprocessed third clock difference data;

[0009] S300, using a time scale algorithm to obtain a comprehensive time scale relative to GPST and synchronize it with UTC; using the preprocessed clock data and the comprehensive time scale, calculating the time difference between the ground station, the GNSS satellite, and the LEO satellite and the comprehensive time scale respectively;

[0010] S400: broadcast the time difference to the user via a broadcast message.

[0011] Beneficial effects:

[0012] The present invention provides a method for generating a time base that takes into account the clocks on low-orbit satellites, by acquiring multiple groups of clock data; preprocessing the multiple groups of clock data to obtain preprocessed clock data; using a time scale algorithm to obtain a comprehensive time scale relative to GPST, and synchronizing the comprehensive time scale with UTC; using the comprehensive time scale and the preprocessed clock data to calculate the time difference between the ground station, GNSS satellite, and LEO satellite and the comprehensive time scale respectively; and broadcasting the time difference to users through broadcast messages. On the one hand, the present invention uses atomic clocks with superior ground performance, and on the other hand, it considers the clocks of a huge number of LEO satellites and GNSS satellites, expands the scale of the clock group, increases the redundancy of the clock group and the robustness of the time base, and can generate a comprehensive time scale with high stability, continuity, and reliability. For LEO satellites, this comprehensive time scale can not only provide a time base with better performance for low-orbit satellite clock error products, but also evaluate the performance and operating status of the current low-orbit satellite-borne atomic clocks, and can provide a relatively long autonomous and stable time base for autonomous LEO satellites.

[0013] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a flow chart of a method for generating a time reference taking into account a low-orbit satellite-borne clock provided by the present invention. DETAILED DESCRIPTION

[0015] The present invention is further described in detail below with reference to specific embodiments, but the embodiments of the present invention are not limited thereto.

[0016] like Figure 1 As shown, the present invention provides a method for generating a time reference taking into account a low-orbit satellite-borne clock, comprising:

[0017] S100, acquiring multiple sets of clock data, where the multiple sets of clock data include first clock data of a ground station, second clock data of a GNSS satellite, and third clock data of a LEO satellite;

[0018] This step combines the clocks of ground stations, GNSS satellites and LEO satellites.

[0019] In a specific embodiment of the present invention, S100 includes:

[0020] S110, obtaining the clock deviation between the ground station clock and UTC in real time through the local measurement system of the ground station, and calculating the ground station clock data relative to GPST in combination with the deviation between GPST and UTC, and using it as the first clock data;

[0021] It is worth noting that the clock of the ground station usually used to establish the time reference is connected to signal, so that the clock deviation between the ground station clock and UTC can be obtained, and then the deviation between the ground clock and GPST can be calculated by combining the deviation between GPST and UTC. signal, the deviation from GPST can be directly obtained.

[0022] As an optional implementation of the present invention, the ground station is connected In the case of signal, S110 includes:

[0023] S111, obtaining in real time a first clock deviation between a clock of the ground station and UTC through a local measurement system of the ground station;

[0024] S112, obtaining a second clock deviation between UTC and GPST by using a time difference monitoring technology;

[0025] S113, calculating a third clock deviation between the GPST and the ground station clock using the first clock deviation and the second clock deviation;

[0026] S114: Use the third clock deviation as first clock data.

[0027] This implementation method can obtain some or all of the high-precision atomic clock data participating in the calculation of the International Atomic Time from global timing laboratories to form the clock of the ground station. UTC is a delayed paper time, usually 40-45 days behind. Its physical realization in timing laboratories around the world is called Since UTC acquisition is delayed by 40-45 days, it can be obtained in real time through the local measurement system of the ground station. The deviation between the clock of the ground station and the ground station is obtained by time difference monitoring technology, as shown in formula (1). and The deviation of The deviation from the local measurement clock group is shown in formula (3). Of course, the clock of the ground station can also be directly obtained through GNSS precise single-point positioning technology. The deviation is shown in formula (3).

[0028] (1);

[0029] (2);

[0030] (3);

[0031] In the formula, is the clock of the ground station;

[0032] S120, acquiring GNSS satellite clock data relative to GPST from each analysis center of IGS (International GPS service for Geodynamics), and using the data as second clock data;

[0033] In this step, the GNSS satellite clock data relative to GPST can be obtained from each IGS analysis center, as shown in formula (4):

[0034] (4);

[0035] In the formula, It is the clock of the GNSS satellite;

[0036] S130, acquiring LEO clock data relative to GPST from the clock of the LEO satellite, and using it as third clock data.

[0037] In this step, the third clock data can be obtained by kinematic or dynamic methods based on the clock error orbit product of the GNSS satellite and the onboard GNSS data of the LEO satellite, as shown in formula (5).

[0038] (5);

[0039] In the formula, is the clock of the LEO satellite;

[0040] The measurement interval of the ground station clock is generally 1s, and the uncertainty of the measurement equipment is about tens of picoseconds, or even lower. The clocks of both GNSS satellites and LEO satellites are measured using a two-way radio method between the satellite and the ground, with a measurement uncertainty of about 1ns, and the measurement interval is usually 5s or more. When fusing data, the sampling intervals of the two need to be consistent to facilitate timing calculations.

[0041] Considering that GNSS satellites and LEO satellites are distributed in orbits at different altitudes, clock error measurements will be affected by the relativistic effect. The relativistic effect has an impact on the onboard clock at the nanosecond level, which will cause anomalies when evaluating the stability of the onboard clock. Therefore, the impact of relativity needs to be considered and deducted in advance when fusion data is used.

[0042] S200, preprocessing the multiple groups of clock data to obtain preprocessed clock data;

[0043] Wherein, the preprocessed clock data includes preprocessed first clock difference data, preprocessed second clock difference data and preprocessed third clock difference data;

[0044] In a specific implementation of the present invention, S200 includes:

[0045] S210, detecting gross errors in the plurality of sets of clock data using a gross error detection method, and performing corrections based on the gross errors to obtain gross error-corrected clock data;

[0046] S220, using a clock difference frequency hopping and phase hopping method to detect frequency hopping and phase hopping in the gross error corrected clock data, and based on the frequency hopping and phase hopping, correcting the gross error corrected clock data to obtain preprocessed clock data.

[0047] Whether it is the clock data of the ground station, GNSS or LEO satellite, it is difficult to avoid the existence of gross errors, phase and frequency changes, especially the clock data on board. Before calculating the atomic time, these abnormal data need to be processed. Commonly used gross error detection methods include the median method (not limited to these). Clock error frequency hopping and phase hopping can be corrected by methods such as the detection method based on Kalman filtering (not limited to these).

[0048] S300, using a time scale algorithm to obtain a comprehensive time scale relative to GPST and synchronize it with UTC; using the preprocessed clock data and the comprehensive time scale, calculating the time difference between the ground station, the GNSS satellite, and the LEO satellite and the comprehensive time scale respectively;

[0049] In a specific implementation of the present invention, S300 includes:

[0050] S310, using a clock error prediction method, respectively predicting the predicted clock errors of the multiple sets of clock data at any time;

[0051] Among them, the clock error prediction method can adopt polynomial prediction or machine learning prediction method, etc., and the present invention is not limited here.

[0052] The time scale algorithm is a core element in the generation of time standards. Currently, the commonly used time scale algorithms include the weighted average algorithm, the ALGOS algorithm (Algorithm for Global Optimization of Time Scales), the AT1 algorithm (Atomic Time Algorithm 1), and the Kalman algorithm. Both the ALGOS algorithm and the AT1 algorithm can only assign a weight to each atomic clock and cannot suppress all atomic clock noises, but the principle is simple and has good long-term stability. The Kalman algorithm can model and suppress a variety of atomic clock noises, but it is necessary to guard against the problem of divergence of estimation errors. When implementing, the comprehensive time scales generated under a variety of timing algorithms can be compared. The algorithm is optimized based on its stability.

[0053] The key steps of the time scale algorithm can be divided into: 1) atomic clock prediction; 2) weight assignment.

[0054] In time scale calculations, atomic clock prediction plays a vital role. Since various factors (such as temperature changes, electromagnetic interference, system aging, etc.) will cause the deviation between atomic clock data and the ideal reference to change over time, the purpose of atomic clock prediction is to predict and compensate for these deviations through mathematical modeling methods to ensure the accuracy of the time scale. Real-time atomic clock prediction is performed using the state characteristics (clock error and frequency deviation) and noise characteristics (such as frequency drift noise, measurement noise, etc.) of the atomic clock and atomic clock data. There are currently many atomic clock prediction methods, such as least squares method, Kalman filtering, machine learning, etc., and different prediction methods can be selected according to different clock group data.

[0055] Taking the least squares method as an example, The time of the clock can be expressed as:

[0056] (6);

[0057] in, It can be a ground station, LEO satellite clock or GNSS satellite clock. , . , and The time Relative to Phase difference, clock speed and clock drift, is the initial moment.

[0058] According to the least squares method, let:

[0059] ;

[0060] make:

[0061] , ,

[0062] but:

[0063] (7);

[0064] In the formula, the superscript Indicates transpose.

[0065] According to formula (7), the phase, clock speed and clock drift of the clock error atomic clock can be obtained. At the same time, the predicted value of the clock error at any time can be obtained, that is, the predicted clock error can be expressed as:

[0066] (8);

[0067] In the formula, Indicates the time between ground station, GNSS satellite and LEO satellite The forecast clock error is , and The time Relative to Phase difference, clock speed and clock drift, is the initial moment.

[0068] S320, setting a weight according to the absolute deviation between the predicted clock error and the actual clock error;

[0069] The weight allocation in the time scale algorithm is a crucial step in the time synchronization and maintenance process. The weight allocation directly determines the influence of each clock involved in the calculation on the final time scale. Through reasonable weight allocation, the stability and robustness of the time base can be maximized while ensuring accuracy and stability. At present, the hydrogen atomic clocks commonly used in ground timing laboratories have a thousand-second stability of 1e-15, the thousand-second stability of GNSS satellite-borne clocks is distributed in the order of 1e-13 to 1e-15, and the thousand-second stability of LEO satellite clocks is in the order of 1e-13. Atomic clocks with different stabilities need to be assigned different weights to integrate the time and frequency information of these atomic clocks. The weight allocation method can be any one or a combination of methods such as Gaussian weighting, optimization weighting or dynamic weighting.

[0070] Taking the commonly used Gaussian weighting as an example, according to the absolute deviation between the predicted clock error and the actual clock error Setting weights , Indicates Atomic clocks, absolute deviation It is expressed as:

[0071] (9);

[0072] The weight is expressed as:

[0073] (10);

[0074] In the formula, Indicates The absolute deviation between the predicted clock error and the actual clock error under the atomic clock, Indicates The weight under the atomic clock, Indicates Atomic clocks, is the total number of atomic clocks.

[0075] S330, calculating a comprehensive time scale relative to GPST using a time scale algorithm according to the set weights;

[0076] The integrated time scale has sufficient redundancy and self-repair capabilities, that is, if some data sources (such as LEO satellites, GNSS satellites or ground station clocks) are lost, the algorithm can still automatically adjust the weighting coefficients according to the accuracy and reliability of each data source, so that the time base can operate stably and ensure that no significant errors occur. When the LEO satellite is out of the clock support of the GNSS satellite or ground station, the integrated time scale can still be calculated based on the LEO satellite clock, which can provide time base protection for the autonomous operation of low-orbit satellites.

[0077] This step uses the time scale algorithm and the set weights to obtain the deviation between the free-running integrated time scale and GPST. The time scale algorithm in this step can use the ALGOS algorithm to calculate the integrated time scale relative to GPST, which can be expressed as:

[0078] (11);

[0079] In the formula, For time The combined time scale, For time GPST, for The weight of The time between ground stations, GNSS satellites and LEO satellites The clock, is the correction value of the clock error forecast, .

[0080] S340, synchronizing the integrated time scale with UTC, and calculating the time differences between the clocks of the ground station, GNSS satellite, and LEO satellite and the integrated time scale respectively according to the preprocessed clock data and the integrated time scale.

[0081] Combining formula (11) and formula (2), the integrated time scale can be synchronized with UTC to meet the time reference requirements of the satellite navigation system. Combining formula (11), formula (3)-(5) and the preprocessing process of clock data, the time difference between the clocks of the ground station, GNSS satellite, and LEO satellite involved in the calculation and the integrated time scale can be obtained, which is expressed as:

[0082] (12);

[0083] (13);

[0084] (14);

[0085] In the formula, Indicates at time Ground station clocks and integrated time scales time difference, Indicates at time GNSS satellite clocks and integrated time scale time difference, Indicates at time Clocks and integrated time scales of LEO satellites time difference.

[0086] S400: broadcast the time difference to the user via a broadcast message.

[0087] The time differences between the clocks of the GNSS satellite and the LEO satellite and the integrated time scale are fitted respectively, and the fitting coefficients are sent to the LEO satellite and the GNSS satellite, so that the GNSS satellite and the LEO satellite can broadcast their own fitting coefficients to users through broadcast messages at certain time intervals.

[0088] The present invention provides a method for generating a time base that takes into account the clocks on low-orbit satellites, by acquiring multiple groups of clock data; preprocessing the multiple groups of clock data to obtain preprocessed clock data; using a time scale algorithm to obtain a comprehensive time scale relative to GPST and synchronize it with UTC; using the comprehensive time scale to calculate the time difference between the ground station, GNSS satellite, and LEO satellite and the comprehensive time scale respectively; and broadcasting the time difference to users through a broadcast message. On the one hand, the present invention uses atomic clocks with superior ground performance, and on the other hand, it takes into account the clocks of a huge number of LEO satellites and GNSS satellites, expands the scale of the clock group, increases the redundancy of the clock group and the robustness of the time base, and can generate a comprehensive time scale with high stability, continuity, and reliability. For LEO satellites, this comprehensive time scale can not only provide a time base with better performance for low-orbit satellite clock error products, but also evaluate the performance and operating status of the current low-orbit satellite-borne atomic clocks, and can provide a relatively long autonomous and stable time base for autonomous LEO satellites.

[0089] It is worth noting that the terms "first" and "second" in the present invention are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0090] Although the present application is described herein in conjunction with various embodiments, in the process of implementing the claimed application, those skilled in the art may understand and implement other variations of the disclosed embodiments by reviewing the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other components or steps, and "a" or "an" does not exclude a plurality of components or steps.

[0091] The above contents are further detailed descriptions of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, which should be regarded as falling within the scope of protection of the present invention.

Claims

1. A method for generating a time reference taking into account a low-orbit satellite-borne clock, characterized in that: include: S100, acquiring multiple sets of clock data, where the multiple sets of clock data include first clock data of a ground station, second clock data of a GNSS satellite, and third clock data of a LEO satellite; S200, preprocessing the multiple groups of clock data to obtain preprocessed clock data; S300, using a time scale algorithm to obtain a comprehensive time scale relative to GPST and synchronize it with UTC; using the preprocessed clock data and the comprehensive time scale, calculating the time difference between the ground station, the GNSS satellite, and the LEO satellite and the comprehensive time scale respectively; S400, broadcasting the time difference to the user via a broadcast message; S300 includes: S310, using a clock error prediction method, respectively predicting the predicted clock errors of the multiple sets of clock data at any time; S320, setting a weight according to the absolute deviation between the predicted clock error and the actual clock error; S330, calculating a comprehensive time scale relative to GPST using a time scale algorithm according to the set weights; S340, synchronizing the integrated time scale with UTC, and calculating the time differences between the clocks of the ground station, GNSS satellite, and LEO satellite and the integrated time scale respectively according to the preprocessed clock data and the integrated time scale.

2. The method for generating a time reference taking into account a low-orbit satellite-borne clock according to claim 1, characterized in that: S100 includes: S110, obtaining the clock deviation between the ground station clock and UTC in real time through the local measurement system of the ground station, and calculating the ground station clock data relative to GPST in combination with the deviation between GPST and UTC, and using it as the first clock data; S120, acquiring GNSS satellite clock data relative to GPST from each analysis center of the IGS, and using the data as second clock data; S130, acquiring LEO clock data relative to GPST from the clock of the LEO satellite, and using it as third clock data.

3. The method for generating a time reference taking into account a low-orbit satellite-borne clock according to claim 2, characterized in that: S110 includes: S111, obtaining in real time a first clock deviation between a clock of the ground station and UTC through a local measurement system of the ground station; S112, obtaining a second clock deviation between UTC and GPST by using a time difference monitoring technology; S113, calculating a third clock deviation between the GPST and the ground station clock using the first clock deviation and the second clock deviation; S114: Use the third clock deviation as first clock data.

4. The method for generating a time reference taking into account a low-orbit satellite-borne clock according to claim 1, characterized in that: S200 includes: S210, detecting gross errors in the plurality of sets of clock data using a gross error detection method, and performing corrections based on the gross errors to obtain gross error-corrected clock data; S220, using a clock difference frequency hopping and phase hopping method to detect frequency hopping and phase hopping in the gross error corrected clock data, and based on the frequency hopping and phase hopping, correcting the gross error corrected clock data to obtain preprocessed clock data.

5. The method for generating a time reference taking into account a low-orbit satellite-borne clock according to claim 1, characterized in that: The predicted clock error in S310 is expressed by the formula: clock'(t)=a0+a1(t-t0)+a2(t-t0) 2 ; Where clock'(t) represents the predicted clock error of the ground station, GNSS satellite and LEO satellite at time t, a0, a1 and a2 are the phase difference, clock speed and clock drift of time t relative to t0, respectively, and t0 is the initial time.

6. The method for generating a time reference taking into account a low-orbit satellite-borne clock according to claim 1, characterized in that: The weight in S320 is expressed by the formula: In the formula, ε i represents the absolute deviation between the predicted clock error and the actual clock error under the i-th atomic clock, ω i represents the weight under the i-th atomic clock, i represents the i-th atomic clock, and N is the total number of atomic clocks.

7. The method for generating a time reference taking into account a low-orbit satellite-borne clock according to claim 1, characterized in that: The calculation in S330 is expressed in the formula relative to the integrated time scale of GPST: Where TA(t) is the integrated time scale at time t, GPST(t) is the GPST at time t, ω i (t) is clock i The weight of (t), clock i (t) is the clock of the ground station, GNSS satellite and LEO satellite at time t, is the correction value of the clock error forecast, x i ,GPST(t)=GPST(t)-clock i (t).

8. The method for generating a time reference taking into account a low-orbit satellite-borne clock according to claim 1, characterized in that: The time difference between the clocks of the ground station, GNSS satellite, and LEO satellite and the integrated time scale in S340 is expressed as: TA(t)-clock_e(t); TA(t)-clock_g(t); TA(t)-clock_l(t); Wherein, TA(t)-clock_e(t) represents the time difference between the ground station clock at time t and the integrated time scale TA(t), TA(t)-clock_g(t) represents the time difference between the GNSS satellite clock at time t and the integrated time scale TA(t), and TA(t)-clock_l(t) represents the time difference between the LEO satellite clock at time t and the integrated time scale TA(t).

9. The method for generating a time reference taking into account a low-orbit satellite-borne clock according to claim 1, characterized in that: S400 includes: The time differences between the clocks of the GNSS satellite and the LEO satellite and the integrated time scale are fitted respectively, and the fitting coefficients are sent to the LEO satellite and the GNSS satellite, so that the GNSS satellite and the LEO satellite can broadcast their own fitting coefficients to users through broadcast messages at certain time intervals.

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