VLBI world time measurement method based on inter-station time synchronization constraint

Abstract

The invention provides an inter-station time synchronization constrained VLBI world time measurement method, which comprises the following steps: determining at least two observation stations for observing a radio power source, and carrying out precise clock synchronization to obtain a precise inter-station clock difference; using the at least two observation stations to simultaneously observe the same radio power source to obtain observation time delays from the same radio power source to the at least two observation stations; separating the clock error in the observation time delay by using the precise inter-station clock error to obtain the observation time delay of the separated clock error; and removing ionosphere time delay, troposphere time delay and equipment time delay in the observation time delay of the separated clock error to obtain final observation time delay, and resolving a residual error between the final observation time delay and theoretical time delay to obtain world time. According to the invention, precise time synchronization between observation stations is carried out in advance, and inter-station errors are effectively separated. The problem that VLBI observation time delay and inter-station clock correction are difficult to separate is solved, the UT1 resolving precision can be effectively improved, and the requirement for high-precision UT1 in the fields of deep space exploration and the like is expected to be met.

Description

Technical Field

[0001] The invention belongs to the technical field of universal time UT1, and in particular relates to a VLBI method for measuring universal time with time synchronization constraints between stations. Background Art

[0002] Universal Time (UT1) is a time scale defined by the rotation of the Earth and is an important component of national standard time. UT1 is the core parameter for coordinate transformation between the celestial sphere and the Earth, and is an indispensable parameter in the fields of deep space exploration, satellite navigation, and spacecraft measurement and control.

[0003] VLBI (Very Long Baseline Interferometry) technology is the only space geodetic technology that can measure UT1, the universal time. Because of its extremely high angular measurement accuracy and long-term stability, it plays an irreplaceable role in solving EOP (Earth Orbit Plane) and maintaining the Earth and celestial reference frames. According to the principle of VLBI observation technology, its spatial resolution is approximately equal to the observed wavelength divided by the baseline length (that is, the distance between the two VLBI radio telescopes). When the observation wavelength is constant, the baseline length is the main factor affecting the accuracy of UT1. When the baseline length and the observation wavelength are constant, it is possible to consider taking certain technical means to suppress the error terms in the VLBI observation of UT1, so as to achieve the effect of improving the accuracy.

[0004] In the VLBI measurement model, the original "delay" observation includes not only the geometric delay, but also the inter-station clock error, so the inter-station clock error is an important systematic error in VLBI observations. However, since VLBI measures the time difference between the signal arriving at the observation station, and the inter-station clock error and the measured delay have no special rules, the two are difficult to separate.

[0005] The existing VLBI measurement method for solving UT1 is to set the clock difference between VLBI observation stations as an unknown number, and then use parameter estimation to solve it. However, since VLBI measures the time difference between the signal reaching the observation station, and the clock difference between stations and the measurement delay have no special rules, the existing parameter estimation method is difficult to separate the two, resulting in the clock difference between stations becoming one of the main error sources in VLBI measurement of UT1. Summary of the invention

[0006] In order to solve the above problems existing in the prior art, the present invention provides a VLBI method for measuring universal time with time synchronization constraints between stations. The technical problem to be solved by the present invention is achieved by the following technical solutions: A method for measuring universal time with VLBI constrained by time synchronization between stations includes: S100, determining at least two observation stations for observing radio sources, and performing precise clock synchronization on the at least two observation stations to obtain a precise inter-station clock difference; S200, using the at least two observation stations to simultaneously observe the same radio source, and obtain an observation delay between the same radio source and the at least two observation stations; wherein the observation delay includes geometric delay, ionospheric delay, tropospheric delay, antenna deformation delay, clock error, equipment delay and other delays; S300, using the precise inter-station clock difference to separate the clock difference in the observed time delay, to obtain the observed time delay with the separated clock difference; S400, removing the ionospheric delay, tropospheric delay and equipment delay in the observed delay of the separated clock difference to obtain a final observed delay, and solving the residual between the final observed delay and the theoretical delay to obtain the universal time.

[0007] Beneficial effects: The present invention proposes a method for measuring world time in VLBI with time synchronization constraints between stations, including: determining at least two observation stations for observing radio sources, and performing precise clock synchronization to obtain precise clock differences between stations; using the at least two observation stations to simultaneously observe the same radio source to obtain the observation delay from the same radio source to at least two observation stations; using the precise clock differences between stations to separate the clock differences in the observation delay to obtain the observation delay with separated clock differences; removing the ionospheric delay, tropospheric delay and equipment delay in the observation delay with separated clock differences to obtain the final observation delay, and solving the residual between the final observation delay and the theoretical delay to obtain world time. The present application performs precise time synchronization between observation stations in advance, and effectively separates the errors between stations. The present application solves the problem that it is difficult to separate the VLBI observation delay from the clock differences between stations, and can effectively improve the solution accuracy of UT1, which is expected to meet the needs of deep space exploration and other fields for high-precision UT1.

[0008] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 It is a flow chart of a method for measuring universal time with VLBI with time synchronization constraints between stations provided by the present invention; Figure 2 It is a schematic diagram of the process of solving UT1 of VLBI measurement with precise inter-station time synchronization constraints provided by the present invention; Figure 3 It is a schematic diagram of the influence of the delay measurement error provided by the present invention on UT1; Figure 4 It is a flow chart of the influence of the simulation station coordinate error on the UT1 solution accuracy provided by the present invention; Figure 5It is a schematic diagram of the influence of the station coordinate error provided by the present invention on the accuracy of UT1. DETAILED DESCRIPTION

[0010] 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.

[0011] like Figure 1 As shown, the present invention provides a method for measuring world time with VLBI constrained by time synchronization between stations, including: S100, determining at least two observation stations for observing radio sources, and performing precise clock synchronization on the at least two observation stations to obtain a precise inter-station clock difference; During VLBI observations, the VLBI telescopes at two observatories need to observe the same radio source together. The time it takes for the signal from the same radio source to reach the two VLBI telescopes is different. Since the radio source is too far away from the two VLBI telescopes, the propagation path of the signal from the radio source to the two VLBI telescopes can be regarded as parallel lines. According to the geometric relationship, the geometric delay of the same signal reaching the two telescopes is , expressed as: (1); in, are the times when the same wavefront signal arrives at the two telescopes, and subscript 1 indicates the telescope that receives the signal first. The coordinate vectors of the two observatories in the Geocentric Celestial Reference Frame (GCRS) and Subtracting them, we get is the unit direction vector of the radio source, which can be obtained from the right ascension and declination of the radio source in the celestial reference frame.

[0012] S200, using the at least two observation stations to simultaneously observe the same radio source, and obtain an observation delay between the same radio source and the at least two observation stations; Due to the calculation of the time delay difference between the two telescopes, the telescope coordinates at the time of observation need to be converted from the International Terrestrial Reference System (ITRS) to the International Celestial Reference System (ICRS).

[0013] In a specific implementation of the present invention, S200 includes: S210, using a conversion matrix to convert the coordinates of the at least two observation stations from an original international terrestrial reference system to an international celestial reference system; Among them, the transformation matrix is ​​expressed as: (2); In the formula, , and They represent the precession-nutation rotation matrix formed by the movement of the celestial pole in ICRS, the earth rotation angle rotation matrix formed by the rotation of the earth around the polar axis, and the polar motion rotation matrix formed by the movement of the pole in ITRS. is the international terrestrial reference system, It is the International Celestial Reference System.

[0014] S220, using the at least two observation stations to simultaneously observe the same radio source to obtain an observation delay; The observation delay includes geometric delay, ionospheric delay, tropospheric delay, antenna deformation delay, clock error, equipment delay and other delays; S230, calculating the geometric delay based on the coordinates of the at least two observation stations and the coordinates of the radio source in the international celestial reference system.

[0015] Therefore, after the coordinates are converted, the geometric delay between the same radio source and at least two observation stations is expressed as: (3); In the formula, represents the geometric delay between two observation stations, represents the speed of light, represents the source vector, Represents the baseline vector.

[0016] The geometric delay has a dependency relationship with the world time, which can be expressed as: (4); in, ; , ; In the formula, It is world time. is the Julian day count starting from J2000.0 Universal Time, .

[0017] Since the signal from the radio source to the telescope is affected by the gravity of the planet, the ionosphere, the troposphere, the clock error between the two telescopes, and the influence of the instruments and equipment, the observation delay between the signal from the radio source and the two telescopes is expressed by the formula: (5); In the formula, is the geometric delay, which is mainly related to the relative position of the VLBI telescope, the position of the radio source and the direction of the Earth; The ionospheric delay is mainly due to the different propagation speed of electromagnetic waves in the medium and in a vacuum, and the bending effect will increase the propagation distance of the signal when it propagates in the medium. is the tropospheric delay, which is mainly related to the temperature, pressure and humidity of the propagation path, and varies with the direction and time of the observation location; is the clock difference between the at least two observation stations, which is mainly related to the different oscillation rates of the hydrogen clocks between the telescopes. It is the antenna deformation delay, which is mainly related to the structural deformation caused by temperature, wind load and gravity. The device delay is mainly the delay introduced by the device between the signal being received from the feed source and the data being recorded. Generally, corresponding signal calibration is required. For other delays.

[0018] S300, using the precise inter-station clock difference to separate the clock difference in the observed time delay to obtain the observed time delay with the separated clock difference; This step uses the precise inter-station clock difference to separate the clock difference in the original observation delay. In the subsequent solution, there is no need to estimate the clock difference parameters, which can effectively improve the solution accuracy of UT1.

[0019] S400, removing the ionospheric delay, tropospheric delay and equipment delay in the observed delay of the separated clock difference to obtain a final observed delay, and solving the residual between the final observed delay and the theoretical delay to obtain the universal time.

[0020] In a specific implementation of the present invention, S400 includes: S410, deducting the device delay, ionospheric delay and tropospheric delay from the observation delay of the separation clock difference to obtain a final observation delay; S420, obtaining a residual between the final observed delay and the theoretical delay; S430, using the least square method to solve the residual to obtain the universal time.

[0021] The observation delay is obtained by measuring multiple groups of radio sources with known precise positions, and then the delay model and error equation are established, in which the geometric delay is calculated by the coordinates of the observation station and the position of the radio source, while the equipment delay, ionospheric delay, tropospheric delay, etc. are deducted in the post-processing process. The residual is then subtracted from the calculated theoretical delay and the observed delay obtained by post-processing, and the required parameter estimation value can be solved by performing the least square method.

[0022] The residual formula is as follows: (6); In the formula, is the final observation delay, is the theoretical delay, is the change of UT1 in a time period. It is the precise station clock difference between two observation stations and is a known quantity.

[0023] Since the equipment delay, ionosphere delay, and troposphere delay have been deducted, there is no need to consider them. In the enhanced observation of UT1, because the number of radio sources observed during the observation period is small, it is impossible to solve multiple parameters, and the antenna deformation delay and other delays have little effect, so they can be ignored when calculating the residual in the enhanced observation of UT1.

[0024] In this application, it is proposed to use the precise inter-station clock difference to separate the clock difference in the observation delay, then remove the equipment delay, ionospheric delay and tropospheric delay, and finally calculate the residual between the final observation delay and the theoretical delay. The residual is expressed by the formula: (7); In the formula, is a constant.

[0025] The traditional solution has two unknown parameters during calculation, namely UT1 and the precise station clock difference between the two observation stations. The precise station clock difference of the present application is also known, so only the UT1 parameter is calculated.

[0026] When conducting VLBI observations, this application uses precise time synchronization technology such as fiber optic time synchronization to synchronize the two stations with high precision. Compared with traditional satellite common view, full view, satellite two-way and other time transfer methods, fiber optic time synchronization technology has higher stability and accuracy, and the technology has now entered the practical stage. This application proposes to use external precision station-to-station time synchronization technology to constrain the clock difference, thereby improving the measurement accuracy of UT1. For example, the station-to-station clock difference with an accuracy of 0.1ns obtained in advance through fiber optic time transfer can be used as prior information for VLBI measurement, effectively separating system errors (station-to-station clock differences), thereby improving the accuracy of observation delays, and ultimately improving the accuracy of world time measurements. Using this technology, it is no longer necessary to estimate the clock difference parameters during parameter estimation, which can effectively improve the measurement accuracy of UT1.

[0027] like Figure 3 As shown, it is assumed that the VLBI stations A and B receiving the radio source signal are located on the equatorial plane. Since the present invention adopts a precise inter-station time synchronization technology to synchronize the time of the two VLBI stations, taking the optical fiber time transfer as an example, the current time synchronization accuracy can reach 50ps, which is equivalent to a VLBI measurement delay error of 0.05ns and a VLBI measurement distance error of 0.05ns. 299792458m / s=1.5cm, which is equivalent to the actual receiving position of station A in the figure as A1, X=1.5cm, then the error caused by the delay error on UT1 measurement is the UT1 error corresponding to arc segment AA1, which is given by Figure 3It can be seen that the specific length of arc segment AA1 needs to be calculated based on the actual station location and the radio source location. Assuming that the incident angle of the radio source is 45°, AA1 is about 2 cm, and its corresponding deviation angle is Approximately: (8); Corresponding UT1 error for: (9); According to the above calculation, the error brought by the measurement of UT1 is about 43.1μs. That is, from a geometric point of view, there is a definite relationship between the estimation result of UT1 and the coordinate error of the VLBI station.

[0028] However, in actual VLBI observations, the measurement error of UT1 is also affected by the specific baseline position, length, station distribution, etc. Due to the use of precise inter-station time synchronization technology, there is no need to estimate the clock error parameters in the final parameter estimation, and at the same time, the impact of the inter-station clock error on the VLBI observation delay is reduced. The VLBI measurement and UT1 solution technology using inter-station time synchronization constraints will effectively improve the solution accuracy of UT1.

[0029] The following experiments are used to illustrate the technical effects of this application.

[0030] Since the VLBI method for measuring universal time UT1 constrained by inter-station time synchronization is currently in the pre-research stage, precise time transfer cannot be used directly to constrain VLBI. However, according to the above analysis, the delay error caused by time synchronization can be converted into an error in the measured distance, which can then be characterized by the coordinate error of the observation station. For example, the accuracy of optical fiber time transfer can reach 50ps, corresponding to a VLBI measurement distance error of 1.5cm. When the incident angle of the radio source is 45°, the corresponding observation station coordinate error is about 2cm. Therefore, the influence of inter-station time synchronization on the UT1 solution accuracy can be reflected by analyzing the influence of the station coordinate error of the observation station on the UT1 measurement accuracy.

[0031] When observing UT1, in order to reduce the influence of the troposphere, the sky distribution of the radio source should be as uniform as possible. Therefore, the incident angle of the radio source is random, so the error of the observation station coordinates changes with the angle of the incident angle. For VLBI observations, the incident angle of the radio source should vary between 0-90°, corresponding to the observation station coordinate error varying between 1.5cm / cos0°-1.5cm / cos90°. Therefore, this implementation case calculates the UT1 solution accuracy when the observation station coordinate changes by 1.5cm in the X direction, and the UT1 solution accuracy when the observation station coordinate changes by 1.5cm in the Y direction, and then quantitatively explains the impact of the station coordinate error on the UT1 solution accuracy, thereby verifying the impact of inter-station time synchronization on the UT1 solution accuracy.

[0032] The method is verified using UT1 observations made by the National Time Service Center Kashgar and Jilin VLBI Observatories during November-December 2024. Figure 4 As shown, first, the VLBI observation data from November to December 2024 are processed and post-processed, and then the post-processed NGS files are used for UT1 solution through the solution software, and finally the UT1 solution results under normal station coordinates are obtained, and this is used as the control group.

[0033] The station coordinates of the Kashgar Observatory are fixed, and the error of the X-direction station coordinates of the Jilin Station is increased by 1.5 cm. Without changing other parameters, the VLBI observation data from November to December 2024 are processed and post-processed. Then, the post-processed NGS files are used for UT1 solution through the solution software, and finally the UT1 solution result is obtained by changing only the X-direction station coordinates of the Jilin Station.

[0034] The station coordinates of the Kashgar Observatory are fixed, and the error of the Y-direction station coordinates of the Jilin Station is increased by 1.5 cm. Without changing other parameters, the VLBI observation data from November to December 2024 are processed and post-processed. Then, the post-processed NGS files are used for UT1 solution through the solution software, and finally the UT1 solution result is obtained by changing only the Y-direction station coordinates of the Jilin Station.

[0035] The UT1 solution results obtained from the two groups of experiments were subtracted from those of the control group, and the average value of the deviation was calculated. Figure 5 As shown, Figure 5 The influence of station coordinates on the accuracy of UT1 solution is demonstrated. Figure 5 The vertical axis DIFF-UT1 is the error of universal time, and the horizontal axis MJD / d represents the simplified Julian day in days. MJD represents the simplified Julian day, and d represents every day. Figure 5 The black dots in the middle left picture are the difference between the UT1 results obtained by the National Time Service Center using the Jilin and Kashgar Observatory during November-December 2024 and the UT1 results calculated by adding 1.5cm error to the X-direction station coordinates of the Jilin Station; the red dots in the right picture are the difference between the UT1 results obtained by the National Time Service Center using the Jilin and Kashgar Observatory during November-December 2024 and the UT1 results calculated by adding 1.5cm error to the Y-direction station coordinates of the Jilin Station. Figure 5 It can be seen that the influence of the station coordinate error in the X direction on UT1 is smaller than that in the Y direction. Table 1 shows in detail the influence of the station coordinate error on the solution accuracy of UT1.

[0036] Table 1 The influence of station coordinate error on UT1 solution accuracy

[0037] 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.

[0038] 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 protection scope of the present invention.

Claims

1. A method for measuring universal time using VLBI with time synchronization constraints between stations, characterized in that: include: S100, determining at least two observation stations for observing radio sources, and performing precise clock synchronization on the at least two observation stations to obtain a precise inter-station clock difference; S200, using the at least two observation stations to simultaneously observe the same radio source, and obtain an observation delay between the same radio source and the at least two observation stations; wherein the observation delay includes geometric delay, ionospheric delay, tropospheric delay, antenna deformation delay, clock error, equipment delay and other delays; S300, using the precise inter-station clock difference to separate the clock difference in the observed time delay, to obtain the observed time delay with the separated clock difference; S400, removing the ionospheric delay, tropospheric delay and equipment delay in the observed delay of the separated clock difference to obtain a final observed delay, and solving the residual between the final observed delay and the theoretical delay to obtain the universal time.

2. The method for measuring universal time with VLBI under inter-station time synchronization constraints according to claim 1, characterized in that: S200 includes: S210, using a conversion matrix to convert the coordinates of the at least two observation stations from an original international terrestrial reference system to an international celestial reference system; S220, using the at least two observation stations to simultaneously observe the same radio source to obtain an observation delay; S230, calculating the geometric delay based on the coordinates of the at least two observation stations and the coordinates of the radio source in the international celestial reference system.

3. The method for measuring universal time with VLBI under inter-station time synchronization constraints according to claim 2, characterized in that: The conversion matrix is ​​expressed as: ； In the formula, , and They represent the precession-nutation rotation matrix formed by the movement of the celestial pole in ICRS, the earth rotation angle rotation matrix formed by the rotation of the earth around the polar axis, and the polar motion rotation matrix formed by the movement of the pole in ITRS. is the international terrestrial reference system, It is the International Celestial Reference System.

4. The method for measuring universal time with VLBI under inter-station time synchronization constraints according to claim 3, characterized in that: The observation delay in S200 is expressed as: ； ； In the formula, is the geometric delay, is the ionospheric delay, is the tropospheric delay, is the clock difference between the at least two observation stations, is the antenna deformation delay, is the device delay, For other delays, represents the speed of light, represents the source vector, Represents the baseline vector.

5. The method for measuring universal time with VLBI under inter-station time synchronization constraints according to claim 4, characterized in that: The geometric delay has a dependency relationship with the world time, which can be expressed as: ； in, ; , ; In the formula, It is world time. The Julian day count starting from J2000.0 Universal Time.

6. The method for measuring universal time with VLBI under inter-station time synchronization constraints according to claim 1, characterized in that: S400 includes: S410, deducting the device delay, ionospheric delay and tropospheric delay from the observation delay of the separation clock difference to obtain a final observation delay; S420, obtaining a residual between the final observed delay and the theoretical delay; S430, using the least square method to solve the residual to obtain the universal time.

7. The method for measuring universal time with VLBI under inter-station time synchronization constraints according to claim 6, characterized in that: The residual is expressed by the formula: ； In the formula, is the final observation delay, is the theoretical delay, is the change of UT1 in a time period, is the precise station clock difference between the two observation stations, which is a known quantity. is the clock difference between the at least two observation stations, and UT1 is the universal time.

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