CN113608427A - Centralized space-based time reference establishing method - Google Patents
Centralized space-based time reference establishing method Download PDFInfo
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- CN113608427A CN113608427A CN202110775862.1A CN202110775862A CN113608427A CN 113608427 A CN113608427 A CN 113608427A CN 202110775862 A CN202110775862 A CN 202110775862A CN 113608427 A CN113608427 A CN 113608427A
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- G—PHYSICS
- G04—HOROLOGY
- G04R—RADIO-CONTROLLED TIME-PIECES
- G04R20/00—Setting the time according to the time information carried or implied by the radio signal
- G04R20/02—Setting the time according to the time information carried or implied by the radio signal the radio signal being sent by a satellite, e.g. GPS
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- G—PHYSICS
- G04—HOROLOGY
- G04F—TIME-INTERVAL MEASURING
- G04F5/00—Apparatus for producing preselected time intervals for use as timing standards
- G04F5/14—Apparatus for producing preselected time intervals for use as timing standards using atomic clocks
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02D—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN INFORMATION AND COMMUNICATION TECHNOLOGIES [ICT], I.E. INFORMATION AND COMMUNICATION TECHNOLOGIES AIMING AT THE REDUCTION OF THEIR OWN ENERGY USE
- Y02D30/00—Reducing energy consumption in communication networks
- Y02D30/70—Reducing energy consumption in communication networks in wireless communication networks
Abstract
The invention provides a centralized space-based time reference establishing method, which comprises the steps of establishing an inter-satellite link between a low-orbit spacecraft and a Beidou satellite, and on one hand, obtaining clock error data between an atomic clock carried by a time frequency system on the low-orbit spacecraft and the Beidou satellite-carried clock through a time comparison link between the low-orbit spacecraft and the Beidou satellite; on the other hand, a time comparison link between the low-orbit spacecraft and the ground station is established through a microwave bidirectional link and a laser link which are arranged on the time frequency system of the low-orbit spacecraft, clock difference data between atomic clocks carried by the time frequency system of the low-orbit spacecraft and the atomic clocks of the ground station are obtained, and the centralized space-based time reference under the autonomous operation mode and the ground operation and control mode is calculated through a comprehensive time scale algorithm by utilizing the two kinds of clock difference data. The time reference has long-term timekeeping capability.
Description
Technical Field
The invention relates to a time reference establishing method.
Background
The Beidou navigation system time (BDT) independently established in China is mainly established and maintained by a time-frequency system of a ground operation and control station through a combined clock group mode. The time-frequency system can be divided into five major subsystems, namely a clock group, an internal measurement subsystem, an external comparison subsystem, a data processing subsystem and a signal generation subsystem. The internal measurement subsystem is used for carrying out cyclic comparison measurement in the clock group and providing a clock error measurement value for time scale calculation, the external comparison subsystem is used for obtaining the deviation of BDT relative to UTC (NTSC) and other time scales, the data processing subsystem is used for processing all the measurement values, a weighted average algorithm is adopted to generate a relatively stable time scale, namely BDT, and the BDT is used as the time reference of the whole Beidou system time.
The GPS system time is generated by the integrated operation of the ground segment and the space segment atomic clock. The GPS system time takes a high-precision atomic clock in a ground main control station as a reference clock, the internal time of the main control station internal clock is compared with the reference clock, the monitoring station and the satellite-borne atomic clock are compared with the reference clock for remote time, clock difference data of each atomic clock in the system relative to the reference clock of the main control station are obtained, and then a time scale algorithm is adopted to generate a freely running GPST. And finally, the freely running GPST is controlled by local coordination time UTC (USNO) maintained by the American navy astronomical table, so that high-precision time synchronization is realized.
The time references established by the two methods have the same point, namely, the time references are established on the ground, but a space system such as a navigation system needs to rely on the ground time reference and the space time reference to ensure the stable operation of the system under normal conditions, and the system needs to rely on the space time reference when the system is operated under abnormal conditions such as loss of ground connection. The ground time reference may be generated and maintained by a ground time-keeping laboratory based on established ground time-keeping theory. The Beidou satellite navigation system in China does not design a constellation timekeeping function, and cannot generate a space-based time reference with a unified constellation, namely the space-based time reference in China needs further tests and technical attack.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a centralized space-based time reference establishing method, which is based on a high-precision time frequency system carried by a low-orbit spacecraft, utilizes a high-precision time frequency signal continuously generated by a test system in an E-17 order as a comparison reference, establishes an inter-satellite link between the low-orbit spacecraft and a Beidou satellite, and brings the inter-satellite link into the Beidou system as a central node of the whole inter-satellite link, and on one hand, obtains clock difference data between a high-performance atomic clock carried by the high-precision time frequency system on the low-orbit spacecraft and the Beidou satellite-borne clock through the time comparison link between the low-orbit spacecraft and the Beidou satellite; on the other hand, a time comparison link between the low-orbit spacecraft and the ground station is established through a microwave bidirectional link and a laser link which are arranged on the high-precision time-frequency system of the low-orbit spacecraft, clock difference data between a high-performance atomic clock carried by the high-precision time-frequency system of the low-orbit spacecraft and an atomic clock of the ground station are obtained, and the centralized space-based time reference under an autonomous operation mode and a ground operation and control mode is calculated through a comprehensive time scale algorithm by utilizing the two clock difference data.
The technical scheme adopted by the invention for solving the technical problem comprises the following steps:
step 1, establishing an inter-satellite ranging and communication link between a low-orbit spacecraft and a Beidou satellite;
step 2, a time-division and agility working mode is adopted based on a phased array antenna system, the link is connected into a Beidou link network management control system, a high-precision time frequency system on a low-orbit spacecraft is completely incorporated into a Beidou satellite system, and the high-precision time frequency system is used as a central node for calculating the whole inter-satellite link and space-based time;
step 3, clock difference data between an atomic clock carried by a time frequency system on the low-orbit spacecraft and an atomic clock carried by the Beidou satellite are obtained through a time comparison link between the low-orbit spacecraft and the Beidou satellite, and the data are transmitted to a centralized space-based time computing node through a data transmission link;
step 4, generating a centralized space-based time reference through a comprehensive weighted atomic time algorithm,
wherein clockIs low inRepresents the atomic clock carried by the time-frequency system on the low-orbit spacecraft, TA (t) is the paper time, clock obtained by the atomic time algorithmIs low in-hi(t) refers to comparison data h 'of atomic clock and clock i carried by time-frequency system on low-orbit spacecraft'i(t) indicates the predicted value of clock i, ωi(t) refers to the weight of clock i,
the allen variance of clock i, i ═ 1,2 …, and N denotes the number of atomic clocks on the big dipper.
The method comprises the following steps that 1, time frequency signals continuously generated by a time frequency system on the low-orbit spacecraft are used as comparison reference, Ka inter-satellite link equipment is carried on the low-orbit spacecraft according to an inter-satellite link working mode between Beidou satellites to carry out link establishment planning, an inter-satellite link management center allocates time slots to the Beidou satellites and the low-orbit spacecraft inter-satellite link equipment, and an inter-satellite comparison link between the low-orbit spacecraft and the Beidou satellites is established in a specified time slot according to the requirement of a time slot table; in addition, the low-orbit spacecraft ground system injects a constellation configuration table, a time slot table, a routing table, a time slot/routing switching instruction, a Beidou satellite and low-orbit spacecraft orbit long-term prediction almanac and clock error parameters to the low-orbit spacecraft for the link establishment between the low-orbit spacecraft and the Beidou satellite.
And step 1, a time slot table is adopted for each hour, the time slot table comprises 20 time slots, and corresponding time slots and corresponding visible Beidou satellites are allocated to the low-orbit spacecraft according to the condition of visual entry to establish a link.
And 2, unifying the time references of the low-orbit spacecraft and the Beidou satellite through the GNSS receiver, synchronizing the time to the BDT, and exchanging ephemeris and ranging information of the two systems through a link.
Step 3, a microwave bidirectional link and a laser link which are arranged on a time frequency system on the low-orbit spacecraft are utilized to establish a time comparison link between the low-orbit spacecraft and the ground station, and clock difference data between an atomic clock carried by the time frequency system on the low-orbit spacecraft and an atomic clock of the ground station is obtained; and 4, establishing a bidirectional measurement link between the low-orbit spacecraft and the Beidou satellite, acquiring clock difference data between a high-performance atomic clock carried by the high-precision time-frequency system on the low-orbit spacecraft and the Beidou satellite-carried clock, realizing high-precision time comparison between the low-orbit spacecraft and the ground station through the microwave bidirectional link and the laser link, acquiring clock difference data between the high-performance atomic clock carried by the high-precision time-frequency system on the low-orbit spacecraft and the ground station atomic clock, synthesizing the two clock difference data, and generating centralized space-based time taking the atomic clock carried by the time-frequency system on the low-orbit spacecraft as a core through a comprehensive weighted atomic time algorithm.
And N represents the sum of the number of the Beidou satellite-borne atomic clocks and the number of the ground station atomic clocks.
The invention has the beneficial effects that:
(1) at present, a high-precision time comparison link between high and low tracks of a space base is not established, namely the space base time reference of a unified constellation cannot be generated. According to the invention, a high-precision time-frequency system on the low-orbit spacecraft is brought into the Beidou satellite system, and the high-precision time-frequency system on the low-orbit spacecraft is configured with a plurality of time-keeping atomic clocks, so that an inter-satellite link between the low-orbit spacecraft and the Beidou satellite is established, and a space-based time reference can be generated by using low-satellite precision clock error data, and has long-term time-keeping capability.
(2) Currently, most time references are established on the ground, or the navigation satellite is timed through the ground time reference. The invention establishes the time reference on the constellation, generates the centralized space-based time reference, and finally traces to UTC (UTC) (NTSC) through the space-ground laser link and the Ka microwave link to realize the backup of the ground time reference, thereby forming a unified space-ground mutual backup time reference system and ensuring the safety and reliability of the time reference in China.
(3) At present, the ultra-high-precision atomic clock is not carried on a navigation satellite for on-orbit testing, but the requirement of a future space system on a high-precision space-based time reference is more and more urgent, namely the requirements on the performance and the precision of the satellite-carried clock carried on a spacecraft are higher and higher. The method builds a test platform, establishes a set of low-orbit spacecraft and a ground station thereof, a high-precision time frequency system on the low-orbit spacecraft and a combined operation mechanism and a data interaction flow of the ground station of the low-orbit spacecraft and a Beidou satellite and the ground station of the Beidou satellite, and provides on-orbit verification data and technical support for the application of the ultrahigh-precision satellite-borne clock to the carrying of future space systems.
Drawings
FIG. 1 is a diagram of alignment links between systems;
FIG. 2 is a flow chart of a centralized time-of-day generation in an autonomous mode of operation;
FIG. 3 is a flow chart of a centralized space-based time generation in a combined space-ground mode of operation;
FIG. 4 is a schematic diagram of inter-satellite link calibration.
Detailed Description
The present invention will be further described with reference to the following drawings and examples, which include, but are not limited to, the following examples.
The invention builds a timekeeping platform on a low orbit spacecraft, integrates available space-based time-frequency resources, provides a centralized space-based time reference building method, builds a centralized space-based time reference generation test system, and finally generates a space-based time reference with a high-performance atomic clock carried by a high-precision time-frequency system on the low orbit spacecraft as a center.
The technical scheme adopted by the invention for solving the technical problem comprises the following steps:
step 1, taking high-precision time frequency signals continuously generated by a high-precision time frequency system on a low-orbit spacecraft as a comparison reference, carrying Ka inter-satellite link equipment on the low-orbit spacecraft according to an inter-satellite link working mode between Beidou satellites, carrying out link establishment planning according to requirements, allocating reasonable time slots to the Beidou satellites and the inter-satellite link equipment of the low-orbit spacecraft by an inter-satellite link management center, and establishing an inter-satellite comparison link between the low-orbit spacecraft and the Beidou satellites in a specified time slot according to the requirements of a time slot table. The low-orbit spacecraft ground system also can inject information such as a constellation configuration table, a time slot table, a routing table, a time slot/routing switching instruction, a Beidou satellite and low-orbit spacecraft orbit long-term forecast almanac, clock error parameters and the like to the low-orbit spacecraft for the link establishment between the low-orbit spacecraft and the Beidou satellite. The working complexity and the working process of the Beidou inter-satellite link management center are not increased as much as possible, one time slot table (20 time slots) is adopted every hour, and corresponding time slots and corresponding visible Beidou satellites are allocated to the low-orbit spacecraft according to the visual condition of entry to the low-orbit spacecraft to build a link. The low orbit spacecraft has a low orbit, and when the low orbit spacecraft establishes an inter-satellite link with the Beidou satellite, the visibility of the low orbit spacecraft and the Beidou satellite is deteriorated due to the influence of the orbit condition, a link needs to be established according to the number of visible arc segments, and the length of a proper link is determined so as to ensure the integrity of test data. And (3) carrying out simulation analysis on the visibility of the low-orbit spacecraft and the Beidou satellite, wherein the low-orbit spacecraft and 5 different Beidou satellites have more than 8 visible arc segments within a certain hour, planning the low-orbit spacecraft and the 5 Beidou satellites to circularly build chains, wherein the time length of each chain building is averagely 10 minutes, and each link is distributed with a time slot point every minute.
And 2, after an inter-satellite ranging and communication link is established between the low-orbit spacecraft and the Beidou satellite, the link is accessed to the existing Beidou link network management control system by adopting a time-division and agile working mode based on a phased array antenna system, and finally, the high-precision time frequency system on the low-orbit spacecraft is completely incorporated into the Beidou satellite system and is used as a central node for calculating the whole inter-satellite link and space-based time. The high-precision time frequency system on the low-orbit spacecraft is integrated into a Beidou satellite system, and the two key points are that the time references of the two systems are unified through a GNSS receiver, and the time is synchronized to a BDT; and secondly, ephemeris, ranging and other information of the two systems are exchanged through the link, so that the link can be established quickly.
Step 3, clock difference data between a high-performance atomic clock carried by a high-precision time frequency system on the low-orbit spacecraft and a Beidou satellite-carried atomic clock is obtained through a time comparison link between the low-orbit spacecraft and the Beidou satellite, and the data are transmitted to a centralized space-based time computing node through a data transmission link; and establishing a time comparison link between the low-orbit spacecraft and the ground station by utilizing a microwave bidirectional link and a laser link which are arranged on the high-precision time-frequency system on the low-orbit spacecraft, and obtaining clock difference data between a high-performance atomic clock carried by the high-precision time-frequency system on the low-orbit spacecraft and an atomic clock on the ground station. The acquisition of two precise clock offsets is the basis for calculating a centralized space-based time reference.
And 4, combining the two clock error data, and generating a centralized space-based time reference through a comprehensive weighted atomic time algorithm. The method for generating the centralized space-based time reference is divided into two methods, namely a space-based autonomous operation mode and a space-ground combined operation mode. Under a space-based autonomous operation mode, a high-performance atomic clock carried by a high-precision time frequency system on a low-orbit spacecraft is used as a comparison reference, the low-orbit spacecraft is used as a central node of a whole inter-satellite link, an inter-satellite link between the low-orbit spacecraft and a Beidou satellite is established, clock difference data between the high-performance atomic clock carried by the high-precision time frequency system on the low-orbit spacecraft and the Beidou satellite-carried clock is obtained through a time comparison link between the low-orbit spacecraft and the Beidou satellite, a centralized space-based time reference taking the high-performance atomic clock carried by the high-precision time frequency system on the low-orbit spacecraft as a core is calculated through a comprehensive weighted atomic time algorithm, comparison and source tracing are carried out on a ground time reference through a bidirectional comparison link between the low-orbit spacecraft and a ground station, and finally the source tracing to UTC (NTSC) is completed. Under a space-ground combined operation mode, a bidirectional measurement link is established between a low-orbit spacecraft and a Beidou satellite, clock difference data (low-satellite clock difference) between a high-performance atomic clock carried by a high-precision time-frequency system on the low-orbit spacecraft and the Beidou satellite-carried clock is obtained, high-precision time comparison between the low-orbit spacecraft and a ground station is realized through a microwave bidirectional link and a laser link, clock difference comparison data (low-ground clock difference) between the high-performance atomic clock carried by the high-precision time-frequency system on the low-orbit spacecraft and the ground station atomic clock is obtained, low-satellite and low-ground clock difference data are integrated, centralized space-based time taking the high-performance atomic clock carried by the high-precision time-frequency system on the low-orbit spacecraft as a core is generated through a comprehensive weighted atomic time algorithm, and the centralized space-based time is complementary with a ground time keeping system. The input requirement of the centralized space-based time generation test is precise clock error data, the low-satellite clock error acquired through an inter-satellite link between the low-orbit spacecraft and the Beidou satellite can be acquired only through error correction, and the low-ground clock error acquired through a microwave or laser link between the low-orbit spacecraft and the ground station is the precise clock error data and can be directly used. The specific calculation method of the centralized space-based time is that the time on the paper surface is not measurable, namely, the time is expressed as a clock difference form between a high-performance atomic clock carried by a high-precision time frequency system on the low-orbit spacecraft and the time on the paper surface, and further the calculation method can be expressed as follows through a comprehensive weighted atomic time algorithm:
wherein clockIs low in-hi(t) is precision clock error data (Low-Star precision clock error and Low-ground precision clock error), h'i(t) is a predicted value, ωi(t) the weight of the atomic clock.
An embodiment of the invention comprises the following steps:
step 1, establishing an inter-satellite comparison link
According to the working mode of the Beidou inter-satellite link, carrying an inter-satellite link device on the low earth orbit spacecraft to establish an inter-satellite comparison link between the low earth orbit spacecraft and the Beidou satellite, periodically providing information such as an orbit prediction almanac and clock error parameters of the low earth orbit spacecraft to a Beidou inter-satellite link control center by a ground station of the low earth orbit spacecraft so that the Beidou inter-satellite link control center can establish a time slot distribution table of the whole inter-satellite link including the low earth orbit spacecraft, and actively calling the low earth orbit spacecraft in a specified time slot according to the time slot table to establish the inter-satellite comparison link. In the whole process, the working mode of the Beidou inter-satellite link is not changed, and only the low-orbit spacecraft is incorporated into the time slot allocation of the whole inter-satellite link.
The specific working mode of the inter-satellite comparison link is that firstly, the inter-satellite link phased-array antenna of the low-orbit spacecraft is controlled to point at the inter-satellite link load of the Beidou satellite, the inter-satellite link load of the Beidou satellite sends a signal to the direction of the low-orbit spacecraft, the inter-satellite link equipment of the low-orbit spacecraft receives the signal to complete inter-satellite pseudo-range measurement and data analysis, meanwhile, the inter-satellite link load of the Beidou satellite receives the signal sent by the inter-satellite link equipment of the low-orbit spacecraft to complete the inter-satellite pseudo-range measurement and data analysis, and the inter-satellite link comparison measurement process adopts a bidirectional measurement mode, so that the method is the most commonly used method with the highest precision at present.
Step 2, bringing a high-precision time frequency system on the low-orbit spacecraft into the Beidou system
1) The time synchronization and time reference unification between the low-orbit spacecraft inter-satellite link equipment and the Beidou system inter-satellite link equipment are met by utilizing the L-frequency-band GNSS receiver carried by the high-precision time frequency system on the low-orbit spacecraft to synchronize the time of the low-orbit spacecraft inter-satellite link equipment to the Beidou time.
2) The system uses a microwave link and a laser link carried by a high-precision time frequency system on the low-orbit spacecraft to issue self-position telemetering information, and the ground station of the system comprehensively uses the telemetering information and a satellite-ground measurement result to calculate and generate orbit information, time information and the like of the low-orbit spacecraft and packages the orbit information, the time information and the like into an ephemeris format of a Beidou system.
3) And the ground station of the high-precision time frequency system on the low-orbit spacecraft and the Beidou operation and control system realize information exchange through ground communication, wherein the ground station comprises ephemeris of the Beidou system and ephemeris of the low-orbit spacecraft.
4) The operation and control system of the Beidou satellite utilizes an L-frequency band uplink to inject ephemeris information and the like to the in-orbit navigation satellite, and the ground station of the high-precision time frequency system on the low-orbit spacecraft utilizes an uplink microwave or laser link to inject the ephemeris information and the like to the system.
5) Planning and constructing a link configuration according to task arrangement, wherein a Beidou satellite measurement and control system sends a pre-generated link construction planning table to an on-orbit navigation satellite by using an S-frequency band link, and a ground station of a high-precision time frequency system on a low-orbit spacecraft sends the pre-generated link construction planning table to the system by using an uplink microwave or laser link and forwards and controls inter-satellite link equipment.
6) The Beidou satellite inter-satellite link equipment and the low-orbit spacecraft inter-satellite link equipment execute two-way communication according to task planning, exchange ranging information, complete one-time inter-satellite comparison measurement and bring a high-precision time frequency system on the low-orbit spacecraft into a Beidou system.
Step 3, obtaining precision clock error
1) Clock error (low-satellite clock error) between a high-performance atomic clock carried by a high-precision time-frequency system on the low-orbit spacecraft and a satellite-carried clock of the Beidou system is determined through an inter-satellite comparison link between the low-orbit spacecraft and the Beidou satellite, but error items such as the motion states of the low-orbit spacecraft and the Beidou satellite and the calibration of the comparison link need to be considered for obtaining the precision low-satellite clock error, and the specific correction mode of the error items is as follows:
a. accurate correction of low orbit spacecraft and Beidou satellite positions
The low-orbit spacecraft and the Beidou satellite have different running speeds and different gravitational potentials, so that a relativistic effect inevitably exists between the low-orbit spacecraft and the Beidou satellite. The relativistic effect belongs to an error which can be accurately modeled and can be completely deducted, so that the relativistic effect cannot influence clock error data obtained after preprocessing. However, the deviation of the position will have a certain influence on the comparison of the measured clock error, which requires that the calculation of the low-satellite clock error must be considered. Because the low-orbit spacecraft is carried with the multimode multi-frequency GNSS receiver, the observation data of the receiver can be transmitted back to the ground station through the data transmission link, and the ground station obtains a position resolving (realizing a cm level) and a clock error result with higher precision through subsequent data processing;
b. calibration of alignment chain (as shown in FIG. 4)
The low-orbit spacecraft and the Beidou satellite are provided with the same inter-satellite link equipment, so that when bidirectional comparison measurement is carried out between the low-orbit spacecraft and the Beidou satellite, the time delay of the inter-satellite link transceiver equipment exists in a measurement result in the form of system error. Therefore, the key point of the test is how to correct the time delay of the transceiver of the inter-satellite comparison link between the low-orbit spacecraft and the Beidou satellite so as to realize the calibration of the comparison link.
FIG. 4 is a schematic diagram of inter-satellite link system error calibration, without considering the effect of the error in the two-way comparison link between the Beidou satellite and the low-orbit spacecraft, and assuming inter-satellite link transceiver devicesThe extension remained unchanged during the alignment. The A star and the B star are any two of the Beidou satellites and are provided with THarvesting of A、TA hairTime delay of A star transceiver, TB harvesting、TB hairTime delay of B-star transceiver, TLow yield、TLow hair lossThe time delay of the transceiver of the low-orbit spacecraft is any different time at T1, T2, T3 and T4, and T1A、T2AT3 is the clock time of A star T1 and T2B、T4BT1 is the clock time of B star T3 and T4Is low in-T4Is low inRespectively, the time difference between the satellite A and the low-orbit spacecraft at the T1 moment measured by the satellite A is T1 when the low-orbit spacecraft is in the clock face at the T1-T4 momentsA is lowThe time difference between the low-orbit spacecraft and the satellite A at the time T2 measured by the low-orbit spacecraft is T2Low AThe time difference between the satellite B and the low-orbit spacecraft at the time T3 measured by the satellite B is T3B is lowThe time difference between the low-orbit spacecraft and the satellite B at the time T4 measured by the low-orbit spacecraft is T4Low BThen, there are:
T1a is low＝T1A-T1Is low in+DT1A is low+THarvesting of A+TLow hair loss (2)
T2Low A＝T2Is low in-T2A+DT2Low A+TA hair+TLow yield (3)
The low-orbit spacecraft can be subjected to high-precision interpolation calculation according to the measured values of the historical time and the T2 time to obtain the measured value T1 of the low-orbit spacecraft and the A star at the T1 timeLow A。
T1Low A＝T1Is low in-T1A+DT1Low A+TA hair+TLow yield (4)
The following equation (2) minus equation (4) is used:
T1a is low-T1Low A＝2(T1A-T1Is low in)+[(THarvesting of A-TA hair)-(TLow yield-TLow hair loss)] (5)
And similarly, the time difference data of the B-satellite and the low-orbit spacecraft can be calculated:
T3b is low-T3Low B＝2(T3B-T3Is low in)+[(TB harvesting-TB hair)-(TLow yield-TLow hair loss)] (6)
(5) The left side of the equation (6) is a known quantity, the first term on the right side of the equation is the clock error between the satellite and the low orbit spacecraft, the second term is the time delay difference of a link transceiving device between the satellite and the low orbit spacecraft, and the two terms on the right side are quantities to be solved.
Both terms are fixed deviations and therefore cannot be separated, and the clock error of the satellite and the low-orbit spacecraft provided by the satellite orbit determination test is considered. However, the data contains the internal time delay and random error of the low-orbit spacecraft receiver, the error items are represented by tau, and therefore processing is needed before use, and the time difference between the A star at the t1 time and the low-orbit spacecraft provided by the experiment is assumed to be MT1A is lowThe time difference between the B star and the low-orbit spacecraft at the time t3 is MT3B is lowThen, there are:
MT1a is low＝(T1A-T1Is low in)+τ (7)
MT3B is low＝(T3B-T3Is low in)+τ (8)
(7) The equation minus (8) is:
MT1a is low-MT3B is low＝(T1A-T1Is low in)-(T3B-T3Is low in) (9)
(5) The equation minus (6) is:
(T1a is low-T1Low A)-(T3B is low-T3Low B)
＝2[(T1A-T1Is low in)-(T3B-T3Is low in)]+[(THarvesting of A-TA hair)-(TB harvesting-TB hair)] (10)
Substituting equation (9) into equation (10) yields:
(Tharvesting of A-TA hair)-(TB harvesting-TB hair)
＝(T1A is low-T1Low A)-(T3B is low-T3Low B)-2MT1A is low-2MT3B is low (11)
(11) The right side of the equation is a known quantity, so that the equipment time delay difference of the inter-satellite link between the Beidou A satellite and the Beidou B satellite is obtained, the equipment time delay difference of the inter-satellite link between the low orbit spacecraft and the Beidou A satellite and the inter-satellite link between the low orbit spacecraft and the Beidou B satellite are obtained in the same way, and the calibration of the comparison link is finally realized. Through the correction processing, the precise low-star clock difference can be obtained.
2) Through a microwave or laser comparison link between the low-orbit spacecraft and the ground station, the precise clock difference (low-ground clock difference) between a high-performance atomic clock carried by a high-precision time-frequency system on the low-orbit spacecraft and an atomic clock of the ground station can be determined, and the two precise clock difference data are integrated to provide input for a centralized space-based time generation test.
Step 4, generating a centralized space-based time reference
And calculating a centralized space-based time reference by adopting a comprehensive weighted atomic time algorithm, and calculating according to clock difference comparison data among atomic clocks to obtain the paper surface of the centralized space-based time. If N atomic clocks have readings of hi(t), i ═ 1,2 …, N, ta (t) can be written in a time-scale general form as follows:
wherein, ω isi(t) represents the weighting factor of the atomic clock, and TA (t) is the paper time obtained by the atomic time algorithm. When each atomic clock is independent of the other, the weighted average (with the best weight) gives a more stable time scale than any watchdog clock alone.
The original purpose of equation (12) is to reduce fluctuation, and under this premise, the phase fluctuation of each atomic clock is extracted and averaged separately, and equation (12) can be written as:
since the paper surface is not measurable, the formula (13) is changed into a clock difference form:
wherein clockIs low in-hi(t) refers to comparison data h 'of high-performance atomic clock and clock i carried by high-precision time-frequency system on low-orbit spacecraft'i(t) indicates the predicted value of clock i, ωi(t) denotes the weight of clock i. In terms of the invention, the high-performance atomic clock carried by the high-precision time-frequency system on the low-orbit spacecraftIs low inCan be regarded as a reference signal of an atomic time algorithm, hi(t) can be regarded as the reading, omega, of the Beidou satellite-borne atomic clock or the ground station atomic clockiAnd (t) the weight of the Beidou satellite-borne atomic clock or the ground station atomic clock participating in calculation. Weight ωi(t) and prediction value h'iThe calculation of (t) can be realized by adopting different calculation modes.
Weight ωi(t) can be calculated by the following formula:
in the formula (15)
Allen variance of bell i.
H 'can be solved based on dynamic model of time frequency'iAnd (t), time is the superposition expansion along the time scale, namely the time is the cumulative quantity of frequency along the time, and the frequency is the cumulative quantity of frequency quadratic drift along the time. The time iteration of a clock can be represented by the following equation:
h′i(t)＝T0+Fre·Δt+0.5Fre_old·(Δt)2 (16)
wherein T is0Is the initial time difference of the reference time corresponding to time 0, Fre is the time interval 0, t]The frequency difference relative to the reference time, Fre _ old, is at the time interval [0, t]Δ t is the time difference that elapses from time 0 to time t with respect to the frequency quadratic drift coefficient of the reference time. Equation (16) can also be written as:
ΔTk＝Fre·Δt+0.5Fre_old·(Δt)2 (17)
ΔTkfor the time rate of change from time 0 to time t, continuing the simplification yields:
ΔTk+1＝Frek+1·Δtk+1 (18)
Frek+1＝Fre+0.5Fre_old·Δtk+1 (19)
the only direct measurement that can be obtained at present is the measurement of the time interval, i.e. the time variation, expressed as
T′k+1＝Tk+1+ω (20)
In formula (II)'k+1For measuring time, Tk+1For real time, ω is the measurement error.
Kalman filtering does not require that both signals and noise are stationary processes, and for system disturbance and observation errors (i.e., noise) at each moment, only some proper assumptions are made on the statistical properties of the system disturbance and the observation errors, the estimation value of a real signal with the minimum error can be obtained in an average sense by processing an observation signal containing noise, the estimation value can be analyzed from a time dynamic model and a measurement model, and the calculation of a time scale is a linear estimation problem of frequency and frequency quadratic drift estimation based on time measurement, and the Kalman filtering can be further used for solving. Based on the above model, the time difference, frequency difference and quadratic frequency drift coefficient of a certain time relative to the reference time are taken as the state vector of Kalman, i.e. the state vector
The state transition matrix is known as:
the optimal solution can be estimated by substituting equations (21) and (22) into the kalman filter for iteration. The obtained weight ωi(t) prediction value h'i(t) and precisionClock can be obtained by substituting clock error data into equation (14)Is low in-TA(t)。
The method is used for obtaining various indexes of the centralized space-based time reference through simulation calculation and comprises the following steps: the frequency stability of the centralized heaven-base time in the heaven-base autonomous operation mode is about 4E-15/day; the frequency stability of the concentrated heaven-earth base time in the heaven-earth combined operation mode is about 8E-16/day, and is improved by at least one order of magnitude compared with the BDT.
The method is based on a high-precision time frequency system on the low-orbit spacecraft, on one hand, based on the mature technology of the Beidou inter-satellite link, an inter-satellite bidirectional comparison link between the low-orbit spacecraft and the Beidou satellite is established, low-satellite clock difference comparison data is obtained, and a centralized space-based time reference under an autonomous operation mode with reference to a high-performance atomic clock carried by the high-precision time frequency system on the low-orbit spacecraft is generated through a comprehensive weighted time scale algorithm; on the other hand, a high-precision bidirectional time comparison link between the low-orbit spacecraft and the ground station is established through a microwave or laser link of the low-orbit spacecraft, a centralized space-based time reference in a space-ground combined operation mode is generated by combining ground station atomic clock data, space-ground combined time keeping is achieved, and the requirement of a future aerospace system on the high-precision time reference is met. The invention applies the ground timekeeping theory and method to the establishment of the centralized space-based time, strengthens the method theory of the autonomous establishment of the space-based time-frequency system through the research on the establishment and maintenance method of the centralized space-based time reference, and lays a foundation for establishing the high-precision centralized space-based time reference with mutual backup and cooperation between the heaven and the earth in the future.
Claims (6)
1. A centralized space-based time reference establishing method is characterized by comprising the following steps:
step 1, establishing an inter-satellite ranging and communication link between a low-orbit spacecraft and a Beidou satellite;
step 2, a time-division and agility working mode is adopted based on a phased array antenna system, the link is connected into a Beidou link network management control system, a high-precision time frequency system on a low-orbit spacecraft is completely incorporated into a Beidou satellite system, and the high-precision time frequency system is used as a central node for calculating the whole inter-satellite link and space-based time;
step 3, clock difference data between an atomic clock carried by a time frequency system on the low-orbit spacecraft and an atomic clock carried by the Beidou satellite are obtained through a time comparison link between the low-orbit spacecraft and the Beidou satellite, and the data are transmitted to a centralized space-based time computing node through a data transmission link;
step 4, generating a centralized space-based time reference through a comprehensive weighted atomic time algorithm,
wherein clockIs low inRepresents the atomic clock carried by the time-frequency system on the low-orbit spacecraft, TA (t) is the paper time, clock obtained by the atomic time algorithmIs low in-hi(t) refers to comparison data h 'of atomic clock and clock i carried by time-frequency system on low-orbit spacecraft'i(t) indicates the predicted value of clock i, ωi(t) refers to the weight of clock i,
the allen variance of clock i, i ═ 1,2 …, and N denotes the number of atomic clocks on the big dipper.
2. The method for establishing the centralized space-based time reference according to claim 1, wherein in the step 1, a time frequency signal continuously generated by a time frequency system on a low-earth spacecraft is used as a comparison reference, Ka inter-satellite link equipment is carried on the low-earth spacecraft according to an inter-satellite link working mode between Beidou satellites to perform link establishment planning, an inter-satellite link management center allocates time slots to the Beidou satellites and the inter-satellite link equipment of the low-earth spacecraft, and an inter-satellite comparison link between the low-earth spacecraft and the Beidou satellites is established in a specified time slot according to a time slot table requirement; in addition, the low-orbit spacecraft ground system injects a constellation configuration table, a time slot table, a routing table, a time slot/routing switching instruction, a Beidou satellite and low-orbit spacecraft orbit long-term prediction almanac and clock error parameters to the low-orbit spacecraft for the link establishment between the low-orbit spacecraft and the Beidou satellite.
3. The method according to claim 1, wherein step 1 employs a slot table per hour, comprising 20 slots, and allocates corresponding slots to the low-orbit spacecraft according to the inbound visibility condition to link with the corresponding visible Beidou satellite.
4. The method for establishing the centralized space-based time reference according to claim 1, wherein in the step 2, the time references of the two systems of the low-earth spacecraft and the Beidou satellite are unified through the GNSS receiver, the time is synchronized to the BDT, and ephemeris and ranging information of the two systems are exchanged through a link.
5. The method for establishing a centralized space-based time reference according to claim 1, wherein step 3 further utilizes a microwave bidirectional link and a laser link of a time frequency system on the low-orbit spacecraft to establish a time comparison link between the low-orbit spacecraft and the ground station, so as to obtain clock error data between an atomic clock carried by the time frequency system on the low-orbit spacecraft and an atomic clock carried by the ground station; and 4, establishing a bidirectional measurement link between the low-orbit spacecraft and the Beidou satellite, acquiring clock difference data between a high-performance atomic clock carried by the high-precision time-frequency system on the low-orbit spacecraft and the Beidou satellite-carried clock, realizing high-precision time comparison between the low-orbit spacecraft and the ground station through the microwave bidirectional link and the laser link, acquiring clock difference data between the high-performance atomic clock carried by the high-precision time-frequency system on the low-orbit spacecraft and the ground station atomic clock, synthesizing the two clock difference data, and generating centralized space-based time taking the atomic clock carried by the time-frequency system on the low-orbit spacecraft as a core through a comprehensive weighted atomic time algorithm.
6. The method of establishing a centralized space-based time reference as recited in claim 5, wherein N represents a sum of the number of Beidou satellite-borne atomic clocks and ground station atomic clocks.
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