Time reference system and time synchronization control method for same
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Filing Date
- 2023-06-26
- Publication Date
- 2026-03-26
AI Technical Summary
Telecommunications carriers face challenges in maintaining accurate time synchronization during interruptions in Global Navigation Satellite System (GNSS) signals, which can lead to increased time errors in high-precision time reference systems, and the continuous operation of optical clocks is difficult due to their sensitivity to vibrations and long-term stability issues.
A time reference system that employs a first frequency signal generated by a cesium atomic clock for continuous operation and a second frequency signal from an optical clock for high accuracy, with priority selection based on the stability of each signal to minimize time errors during GNSS signal interruptions.
This approach allows for continuous and accurate time synchronization, maintaining time errors within a small range (e.g., 100 ns) even during GNSS signal interruptions, and enables the practical use of optical clocks by reducing their operational burden and vulnerability to disturbances.
Abstract
Description
Time reference system and time synchronization control method
[0001] The present invention relates to a time reference system and a time synchronization control method thereof.
[0002] Telecommunications carriers need to provide accurate time information to the terminals of many users located in various locations as part of their communication services. Therefore, they have time synchronization networks and frequency synchronization networks as communication facilities required to deliver accurate time information to each user.
[0003] For example, Non-Patent Document 1 discloses the standard (ITU-T G.8272.1) for enhanced primary reference time clocks (ePRTC) required for time synchronization networks.
[0004] "Timing characteristics of enhanced primary reference time clocks", Recommendation ITU-T G.8272.1 / Y.1367.1, ITU-T.
[0005] The ITU-T standard (ePRTC (G.8272.1)) shown in Non-Patent Document 1 requires that the time error of the reference time be maintained within 100 ns over a 14-day period. Telecommunications carriers that provide reference time information to various users generally use an antenna to receive radio signals transmitted from satellites of the Global Navigation Satellite System (GNSS) 32, as in the communication system shown in Figure 1, and constantly keep track of the reference time using a High Precision Time Reference Controller (ePRTC) 31.
[0006] However, due to various factors, there may be a temporary interruption of the GNSS 32 signal arriving at the high precision time reference device 31. When the GNSS 32 signal is interrupted, the high precision time reference device 31 is no longer able to supply reference time information to the downstream high precision time synchronization device (T-BC: Telecom Boundary Clock) 34.
[0007] Therefore, when the signal from the GNSS 32 is interrupted, the high-precision time reference device 31 performs holdover of the reference time by using the highly stable clock frequency output by the high-precision frequency reference device 33. In other words, by supplying the frequency output by the high-precision frequency reference device 33, an increase in the time error of the reference time (local time) grasped by the high-precision time reference device 31 during the period when the signal from the GNSS 32 is interrupted is prevented.
[0008] Furthermore, before carrying out a holdover of the reference time, the high-precision time reference device 31 needs to take a sufficient amount of time to learn the clock frequency output by the high-precision frequency reference device 33. Therefore, as shown in Fig. 2 , the clock frequency needs to be supplied from the high-precision frequency reference device 33 to the high-precision time reference device 31 throughout the entire "clock supply period" T0 from time tn, which is 14 days before time t0, when the signal from GNSS 32 is interrupted, to time tp, which is 14 days after time t0, when the signal from GNSS 32 is interrupted.
[0009] However, it is impossible to predict the time t0 when the signal from the GNSS 32 is lost. Therefore, in practice, the high precision frequency reference unit 33 must constantly supply the clock frequency to the high precision time reference unit 31.
[0010] On the other hand, it is expected that time keeping requirements will become stricter in the future. Therefore, telecommunications carriers will need to consider how to constantly supply the high precision time reference device 31 with a clock frequency that is even more accurate than the current high precision frequency reference device 33.
[0011] Therefore, when a telecommunications carrier distributes information about the reference time to users, it is assumed that the carrier will use a communication system having the configuration shown in Fig. 3. The communication system shown in Fig. 3 includes an optical clock 41, a synchronization network 42, a frequency distribution system 43, a time reference device 44, and a GNSS 45.
[0012] 3, a highly accurate frequency signal output by an optical clock 41 is input to a time reference device 44 via a synchronization network 42 and a frequency distribution system 43. Therefore, if the time information supplied from the GNSS 45 to the time reference device 44 is interrupted, the time reference device 44 can prevent an increase in time error by using the frequency supplied from the optical clock 41.
[0013] Figure 4 shows the time errors A1 to A4, which are representative examples of four types of highly accurate clocks. As shown in Figure 4, the time error (frequency accuracy) of the optical clock is －１４ A4 is the time error of the ePRTC (when using a clock combiner) A3, and the time error of the high-precision cesium atomic clock (frequency accuracy 5 x 10 －１３ ) A2, time error of standard cesium atomic clock (frequency accuracy 10 －１２ 3, the time error is smaller than that of A1. In other words, by using the optical clock 41 as in the communication system shown in FIG. 3, it is possible for the time reference device 44 to perform time holdover for a longer period than the current time, and it is possible to maintain the time error within a small range even during periods when time information from the GNSS 45 is interrupted.
[0014] 5 and 6 show examples of communication system configurations that can be envisioned when using optical clocks. In the communication system shown in Fig. 5, the output of an optical clock 51 is connected to the input of a frequency distribution device 52, and the output of the frequency distribution device 52 is connected to the input of a time reference device 53. In the communication system shown in Fig. 6, the signal is input to the frequency distribution system via an optical network 54, and the output is connected to the input of the time reference device 53.
[0015] 5 and 6, the frequency accuracy of the optical clock supplied from the frequency distribution device is extremely high. Therefore, the time reference device 53 shown in Fig. 5 and 6 can prevent time errors with the same accuracy as the optical clock 51 by maintaining the time using the frequency of the optical signal supplied from the frequency distribution device 52, even during periods when time information from the GNSS is interrupted.
[0016] However, optical clocks generally have difficulty in continuous operation for long periods of time. Furthermore, when disturbances such as vibrations occur, the optical clock and the optical network may temporarily lose synchronization. Therefore, in an environment where a highly accurate clock frequency must be continuously supplied to a time reference device for a long period of time, as in the example shown in Figure 2, it is practically difficult to use optical clocks.
[0017] The present invention has been made in consideration of the above situation, and aims to provide a time reference system and a time synchronization control method that can reduce the time error in the reference time provided to users in a situation where high-priority time information such as GNSS is interrupted.
[0018] The time synchronization control method for a time reference system of the present invention uses a first reference frequency generation unit capable of internally generating a first frequency signal with a stable frequency, and a second reference frequency generation unit capable of outputting a second frequency signal with higher accuracy than the first frequency signal, monitors the operating state of the second reference frequency generation unit, and when the second frequency signal is not present or is unstable, preferentially selects the first frequency signal over the second frequency signal, and when the second frequency signal is present and stable, preferentially selects the second frequency signal over the first frequency signal, and uses the selected first frequency signal or second frequency signal for accurate time synchronization.
[0019] According to the time reference system and the time synchronization control method of the present invention, it is possible to reduce the time error of the reference time provided to a user in a situation where high-priority time information such as GNSS is lost. In other words, since the first frequency signal and the second frequency signal are selected in consideration of priority depending on the situation, the first frequency signal and the second frequency signal do not need to be continuously supplied for a long period of time. Therefore, it is possible to improve time accuracy by using, for example, an optical clock that is difficult to operate continuously for a long period of time, and it is less likely that periods when the optical clock's frequency is not synchronized or sudden short-term desynchronization will have a significant impact on time accuracy.
[0020] FIG. 1 is a block diagram showing a typical time reference device and the main components connected to it. FIG. 2 is a time chart showing the relationship between the time when a time signal from a GNSS satellite is lost and the clock supply period. FIG. 3 is a block diagram showing an example configuration of a communication system assumed when the time reference device uses an optical clock. FIG. 4 is a graph showing the typical time accuracy of four types of clocks. FIG. 5 is a block diagram showing Example 1 of a configuration of a communication system connecting an optical clock and a time reference device. FIG. 6 is a block diagram showing Example 2 of a configuration of a communication system connecting an optical clock and a time reference device. FIG. 7 is a time chart showing an example operation of a time reference system in an embodiment of the present invention. FIG. 8 is a block diagram showing the internal configuration and peripheral components of a time reference system in an embodiment of the present invention. FIG. 9 is a flowchart showing a characteristic example operation of a time reference system in an embodiment of the present invention. FIG. 10 is a block diagram showing a modified example of a time reference system in an embodiment of the present invention.
[0021] An embodiment of the present invention will be described below with reference to the accompanying drawings. <Outline of time synchronization control method> An example of the operation of a time reference device 10 in an embodiment of the present invention is shown in Fig. 7. Also, an example of the configuration of a time reference system 100 including this time reference device 10 is shown in Fig. 8.
[0022] The time reference device 10 of this embodiment is in an environment where it can use an ultra-high precision frequency signal (second frequency signal) supplied from an optical clock or the like, similar to the time reference device 44 shown in Fig. 3. The time reference device 10 of this embodiment is also configured to simultaneously use a frequency signal (first frequency signal) that is less precise than the optical clock but can be used continuously. The first frequency signal can be generated using, for example, a cesium atomic clock.
[0023] In the example shown in Figure 7, a situation is assumed in which, at the location where the time reference device 10 is located, time information from the GNSS satellite is lost at time tx, and the second frequency signal is stably output at time t0.
[0024] The second frequency signal supplied from the optical clock or the like is highly accurate, but it takes some time for the second frequency signal to reach a stable synchronized state after the optical clock or the like starts operating. On the other hand, the first frequency signal can be used continuously at all times.
[0025] Therefore, the time reference device 10 of this embodiment preferentially selects the first frequency signal at time tx when time information from the GNSS satellite is lost, and uses this first frequency signal for time synchronization. In addition, the time reference device 10 starts monitoring from time tx whether the second frequency signal is being output stably.
[0026] Then, when the time reference device 10 detects that the second frequency signal is being stably output at time t0, it ends the free-running period T1 shown in Fig. 7, preferentially selects the second frequency signal, and starts time synchronization to synchronize with the second frequency signal. From time tx, when time information from the GNSS satellite is lost, until time t1, 14 days later (the period T1 + T2), it is necessary to maintain the time error within a specified range (for example, 100 ns).
[0027] As shown in Figure 7, during the free-running period T1, a relatively low-accuracy first frequency signal is used, resulting in a relatively large time error per unit time in the reference time output by the time reference device 10. However, because the free-running period T1 ends before a long time has elapsed since time tx, the time error at time t0 is maintained within the specified range (e.g., 100 ns). Furthermore, during the period (T2) from time t0 to t1, time synchronization is performed using an ultra-high-accuracy second frequency signal obtained by an optical clock or the like. As a result, almost no time error occurs during the period from t0 to t1, and even at time t1, 14 days after time tx, the time error is maintained within the allowable error range (e.g., 100 ns).
[0028] <Configuration example of a time reference system> The time reference system 100 shown in Fig. 8 includes a time reference device 10 and an optical clock network 20. The time reference signal SG1 input to the time reference device 10 can be obtained, for example, by receiving radio waves from a GNSS satellite. The signal actually obtained from the GNSS satellite is a digital signal including latitude, longitude, time, etc., and the time reference device 10 inputs the signal SG1 as an electrical reference time signal based on the received digital signal.
[0029] The optical clock network 20 shown in Fig. 8 can be configured using, for example, the optical clock 41, synchronization network 42, and frequency distribution system 43 shown in Fig. 3. The frequency distribution system 43 is equipped with functions such as frequency synchronization, distribution, and photoelectric conversion. Therefore, the optical clock network 20 can supply the time reference device 10 with information on a reference frequency having the same accuracy as that of the optical clock 41 located in a remote location away from the time reference device 10 as the signal SG3.
[0030] Within the optical clock network 20, reference frequency information is transmitted over the network as an optical signal, but the signal SG3 that the optical clock network 20 supplies to the time reference device 10 is an electrical signal that conforms to the interface specifications of the time reference device 10. Of course, if the time reference device 10 is compatible with optical signal input, the reference frequency may be supplied in the form of an optical signal as the signal SG3.
[0031] The time reference device 10 shown in FIG. 8 has, as its main functions, a time recovery block 11, a local frequency clock unit 12, a local time scale unit 13, a frequency interface (Fre. I / F) 14, a time interface (Time I / F) 15, a phase interface 16, and a frequency interface 17.
[0032] The time recovery block 11 can input a reference time signal SG1 from a GNSS or the like. The time recovery block 11 can also transmit a special signal SG2, which will be described later, to the optical clock network 20. The time recovery block 11 can also supply signals SG11, SG12, and SG13 to the local time scale unit 13. The signals SG12 and SG13 each contain time and phase information. The time recovery block 11 can also supply a signal SG14 to the local frequency clock unit 12. The signal SG14 contains frequency information obtained from a GNSS or the like.
[0033] The local frequency clock unit 12 has a function of internally generating a unique local frequency clock signal. The accuracy of this local frequency clock is, for example, 10 －１２ In this case, for example, in a free-running period of 1000 [s], －１２ ×1000 [s] = 10 －9 Since [s] = 1 [ns], this is sufficient frequency accuracy for a free-running period of about 1000 [s], in accordance with the standard of limiting the time error to 100 [ns] over two weeks. Furthermore, the local frequency clock unit 12 outputs a frequency appropriately selected from multiple types of frequencies according to the predetermined priority and the situation. Priorities 1 to 3 for the multiple types of frequencies are determined in advance as follows: Priority 1: Frequency of signal SG14 (GNSS frequency) Priority 2: Frequency of signal SG3 (frequency with optical clock accuracy: second frequency signal) Priority 3: Local frequency clock (internally generated frequency: first frequency signal)
[0034] The local time scale unit 13 can output information about the GNSS reference time input as signal SG12 to the time interface 15. Furthermore, in a situation where the GNSS reference time input as signal SG12 is interrupted, the local time scale unit 13 adjusts the local time so that it is synchronized with signal SG15 of the frequency selected by the local frequency clock unit 12, and outputs information about the local time to the time interface 15. In other words, the local time scale unit 13 performs holdover.
[0035] <Operation of the Time Reference System> An example of a characteristic operation of the time reference system 100 shown in Fig. 8 is shown in Fig. 9. The operation of Fig. 9 will be described below.
[0036] The time recovery block 11 in the time reference device 10 constantly monitors the state of the signal SG1 in step S11 to check whether the signal from the GNSS is being detected and whether it is interrupted. If the time recovery block 11 is detecting a signal from the GNSS (Yes), the process repeats step S11. Then, if the time recovery block 11 detects that it is no longer detecting a signal from the GNSS (No), the process proceeds to step S12.
[0037] In step S12, the time recovery block 11 uses the signal SG2 to instruct the optical clock network 20 to start transmitting an optical frequency signal. The signal SG2 is, for example, an electrical signal, and is used as a notification to the synchronization monitoring unit 201 that controls the optical clock network 20. Thereafter, the process returns to step S11.
[0038] When the synchronization monitoring unit 201 of the optical clock network 20 detects the instruction sent by the time recovery block 11 in step S12 via signal SG2, it starts the operation of the optical clock 41 and starts frequency synchronization processing in step S13. In step S14, the synchronization monitoring unit 201 determines whether synchronization is complete. If synchronization is not complete (No), the process repeats step S14. Then, when frequency synchronization is complete, the process proceeds from step S14 to the next step S15, where the optical clock network 20 starts sending out signal SG3, which is a high-precision frequency signal.
[0039] After starting to send the signal SG3, the synchronization monitoring unit 201 of the optical clock network 20 monitors in step S16 whether or not there is a loss of frequency synchronization within the optical clock network 20. For example, if an abnormality occurs in the operation of the optical clock 41, or if a loss of synchronization occurs in an optical clock or optical frequency comb within the optical clock network 20 due to an external disturbance such as vibration, the synchronization monitoring unit 201 of the optical clock network 20 detects this in step S16 and proceeds to the next step S17. In step S17, the synchronization monitoring unit 201 of the optical clock network 20 temporarily suspends the sending of the signal SG3, returns to step S13, and restarts frequency synchronization within the optical clock network 20.
[0040] The local frequency clock unit 12 outputs one frequency selected from three types of frequencies as signals SG15 and SG16 in step S18 according to the predetermined priorities and circumstances. Priority 1: Frequency of signal SG14 (GNSS frequency) Priority 2: Frequency of signal SG3 (frequency with optical clock accuracy: second frequency signal) Priority 3: Local frequency clock (internally generated frequency: first frequency signal)
[0041] That is, when the time reference device 10 can receive signals from the GNSS, signal SG14 has the highest priority, so the local frequency clock unit 12 selects the frequency of signal SG14 in step S18 and outputs it as signal SG15.
[0042] Furthermore, if the signal from the GNSS reaching the time reference device 10 is interrupted, the local frequency clock unit 12 selects the frequency of the signal SG3, which has the second priority, in step S18 and outputs it as the signal SG15. However, for example, immediately after the optical clock network 20 is started up or in a situation where synchronization is lost, the optical clock network 20 does not send out the signal SG3, so the local frequency clock unit 12 selects the local frequency clock, which has the third priority, in step S18 and outputs it as the signal SG15.
[0043] When the signal from the GNSS is interrupted, the local time scale unit 13 controls the local time so that it maintains (holds over) the time using the frequency of the signal SG15 output by the local frequency clock unit 12, and sends the local time information to the time interface 15 (steps S19, S20).
[0044] 9, signals SG14 and SG3 do not appear for a while from time tx when the signal from GNSS reaching the time reference device 10 is lost until synchronization of the optical clock network 20 is established, so the local frequency clock unit 12 selects the local frequency clock. Also, the local time scale unit 13 generates a local time held over by the local frequency clock. In other words, during the free-running period T1 from time tx to time t0 shown in FIG. 7, the accuracy of the local frequency clock affects changes in the time error.
[0045] Furthermore, once synchronization of the optical clock network 20 is established, signal SG3 is output from the optical clock network 20, and the local frequency clock unit 12 selects signal SG3, which has the second highest priority, and outputs it as signal SG15. Furthermore, the local time scale unit 13 generates local time synchronized with the frequency of the highly accurate signal SG3. In other words, during the period T2 from time t0 to time t1 shown in FIG. 7, the frequency of signal SG3, which has the same accuracy as the optical clock 41, affects changes in the time error.
[0046] Since the free-running period T1 shown in Figure 7 can be shortened, it is easy to maintain the time error at time t0 within the tolerance (100 [ns]) even if the accuracy of the local frequency clock is relatively low. Furthermore, since synchronization with an ultra-high precision frequency can be achieved by using an optical clock or the like after time t0, it is also easy to maintain the time error within the tolerance (100 [ns]) during the period from time tx when the GNSS signal is interrupted to time t1. Furthermore, although it is generally difficult for optical clocks to operate continuously for long periods of time, when the control of Figures 7 and 9 is performed, the frequency of the local clock is used again to perform time holdover during the period when the frequency of the optical clock is interrupted, so the optical clock can be used practically.
[0047] <Modification of the Time Reference System> The configuration of a modified time reference system 100A is shown in Fig. 10. This configuration is useful when the accuracy of the local frequency clock inside the time reference device is low, e.g. －6 In this case, for example, in a free-running period of 1000 [s], －6 ×1000 [s] = 10 －3 Since [s] = 1 [ms], the standard requires a time error of 100 [ns] over two weeks, but the time error will exceed the reference value by less than 1000 [s]. Therefore, the time reference system 100A shown in Figure 10 includes a time reference device 10A, an optical clock network 20, and a continuously operable high-precision frequency reference device 25, such as a cesium atomic clock. The high-precision frequency reference device 25 functions as a third reference frequency generator capable of outputting a third frequency signal with a stable frequency. The optical clock network 20 functions as a second reference frequency generator capable of outputting a second frequency signal with higher precision than the third frequency signal.
[0048] As can be seen from the example shown in Fig. 4, the frequency accuracy of the signal SG4 output by the high-precision frequency reference device 25 in Fig. 10 is lower than the frequency accuracy of an optical clock. However, the high-precision frequency reference device 25 is capable of continuous operation for a long period of time. Furthermore, when the frequency of the high-precision frequency reference device 25 is used for only a relatively short period, such as the free-running period T1 shown in Fig. 7, the increase in time error can be kept relatively small.
[0049] The time reference device 10A in FIG. 10 is equipped with a frequency interface (Opt.Fre.I / F) 14A for inputting the frequency of the signal SG3, and a frequency interface (Cs Fre.I / F) 18 for inputting the frequency of the signal SG4.
[0050] The local frequency clock unit 12A inside the time reference device 10A can appropriately select one of three types of signals SG14, SG3, and SG4 as a high-precision frequency signal according to predetermined priorities and the situation. Priorities 1 to 3 are defined as follows: Priority 1: Frequency of signal SG14 (GNSS frequency) Priority 2: Frequency of signal SG3 (frequency with optical clock precision: second frequency signal) Priority 3: Frequency of signal SG4 (frequency with precision of the high-precision frequency reference device 25: third frequency signal)
[0051] That is, in a situation where the time reference device 10A can receive signals from the GNSS, signal SG14 has the highest priority, so the local frequency clock unit 12A selects the frequency of signal SG14 in step S18 and outputs it as signal SG15.
[0052] Furthermore, if the signal from the GNSS reaching the time reference device 10A is interrupted, the local frequency clock unit 12A selects the frequency of the signal SG3, which has second priority, in step S18 and outputs it as the signal SG15. However, for example, immediately after the optical clock network 20 is started up or in a situation where synchronization is lost, the optical clock network 20 does not send out the signal SG3, so the local frequency clock unit 12A selects the signal SG4, which has third priority, in step S18 and outputs it as the signal SG15.
[0053] When the signal from the GNSS is interrupted, the local time scale unit 13 controls the local time to maintain (hold over) the time using the frequency of the signal SG15 output by the local frequency clock unit 12A, and sends the local time information to the time interface 15 (steps S19, S20).
[0054] 9, signals SG14 and SG3 do not appear for a while from time tx, when the signal from GNSS reaching the time reference device 10A is lost, until synchronization of the optical clock network 20 is established, so the local frequency clock unit 12A selects the frequency of signal SG4 and outputs it as signal SG15. Furthermore, the local time scale unit 13 generates a local time held over by the local frequency clock. In other words, during the free-running period T1 from time tx to time t0 shown in FIG. 7, the frequency accuracy of signal SG4, i.e., the accuracy of the high-precision frequency reference device 25, affects changes in the time error.
[0055] Furthermore, once synchronization of the optical clock network 20 is established, signal SG3 is output from the optical clock network 20, and the local frequency clock unit 12A selects signal SG3, which has the second highest priority, and outputs it as signal SG15. Furthermore, the local time scale unit 13 generates local time synchronized with the frequency of the highly accurate signal SG3. In other words, during the period T2 from time t0 to time t1 shown in FIG. 7, the frequency of signal SG3, which has the same accuracy as the optical clock 41, affects changes in the time error.
[0056] Since the free-running period T1 shown in FIG. 7 can be shortened, for example, when the frequency of the external signal SG4 is within the frequency accuracy of 100% of the high-precision frequency reference device 25, －１２ If the time error at time t0 is within the allowable value (100 ns), it is easy to maintain the time error at time t0 within the allowable value (100 ns).
[0057] Furthermore, after time t0, synchronization with an ultra-high precision frequency can be achieved by using an optical clock or the like, and it is therefore easy to maintain the time error within the tolerance (100 ns) for the period from time tx when the GNSS signal is interrupted to time t1. Furthermore, although it is generally difficult for an optical clock to operate continuously for a long period of time, when the control of Figures 7 and 9 is performed, it is sufficient to operate the optical clock only for the relatively short period from time tx when the GNSS signal is interrupted to time t1, and time holdover is performed again using the frequency of the high-precision frequency standard device 25 during the period when the frequency of the optical clock is interrupted, so the optical clock can be used in a practical manner.
[0058] <Possible variations other than those described above> In the configurations shown in Figures 8 and 10, it is assumed that a reference time signal from GNSS is input to the time reference device 10 as signal SG1, but it is also possible to replace it with a signal other than GNSS.
[0059] For the local frequency clock built into the local frequency clock unit 12 shown in Figure 8, it is expected that a device with appropriate frequency accuracy will be used as appropriate depending on the allowable error of the reference time and the control timing required by the time reference system 100.
[0060] Instead of the optical clock network 20 shown in Figures 8 and 10, another facility capable of ensuring the same level of frequency accuracy may be connected. Also, instead of the high-precision frequency standard device 25 shown in Figure 10, another facility capable of ensuring the same level of frequency accuracy may be connected.
[0061] <Features of the time reference device and its time synchronization control method> Characteristic features of the time reference device and its time synchronization control method of the present invention are listed in the following [1] to [8]. [1] A time synchronization control method for a time reference system, comprising: utilizing a first reference frequency generation unit capable of internally generating a first frequency signal (local frequency clock) with a stable frequency; and a second reference frequency generation unit (optical clock network 20) capable of outputting a second frequency signal (signal SG3) with higher accuracy than the first frequency signal; monitoring the operating state of the second reference frequency generation unit (steps S14 and S16); in a state where the second frequency signal is absent or unstable, preferentially selecting the first frequency signal over the second frequency signal (step S18); in a state where the second frequency signal is stably present, preferentially selecting the second frequency signal over the first frequency signal (step S18); and utilizing the selected first frequency signal or the second frequency signal for accurate time synchronization (step S19).
[0062] According to the time synchronization control method for the time reference system described in [1] above, even if the second reference frequency generation unit uses an optical clock, the optical clock only needs to be activated when the GNSS signal is interrupted (time tx), eliminating the need to operate the optical clock continuously for a long period of time, allowing for practical system operation. Furthermore, during the free-running period (T1) from when the optical clock is activated until its frequency synchronization stabilizes, local time synchronization can be maintained using the first frequency signal. Furthermore, since the free-running period (T1) is relatively short, it is easy to maintain the time error that occurs during this period within a tolerance (e.g., 100 ns). Furthermore, during the period (T2) after the end of the free-running period (T1), the frequency of the ultra-high-precision optical clock can be used, allowing the time error to be maintained within a tolerance (e.g., 100 ns) for a period of, for example, 14 days or more.
[0063] [2] Further utilizing a third reference frequency generating unit (high precision frequency reference device 25) capable of outputting a third frequency signal with higher precision than the first frequency signal, monitoring the operating state of the second reference frequency generating unit (optical clock network 20), and in a state where the second frequency signal is not present or is unstable, preferentially selecting the third frequency signal over the second frequency signal, and in a state where the second frequency signal is present and stable, preferentially selecting the second frequency signal over the third frequency signal, and utilizing the selected third frequency signal or the second frequency signal for accurate time synchronization.
[0064] The time synchronization control method for a time reference system according to the above item [1]. According to the time synchronization control method for a time reference system according to the above item [2], even when a sufficiently accurate internal clock is not available, it is easy to maintain the time error that occurs during holdover within a tolerable value.
[0065] [3] A time synchronization control method for a time reference system described in [2] above, which monitors the state of a time reference signal (signal SG1) having a higher priority than the second frequency signal, and when the time reference signal appears normally, performs time synchronization based on the time reference signal (steps S18, S19), and when the time reference signal is not detected, uses the selected first frequency signal or second frequency signal for accurate time holdover (step S19).
[0066] According to the time synchronization control method for the time reference system described in [3] above, the first frequency signal and the second frequency signal are appropriately used when the time reference signal is not detected, so that it is possible to reduce the time error that occurs, for example, during the period (time tx to t1) when the time reference signal obtained from the GNSS is interrupted and a holdover of the reference time is performed.
[0067] [4] A time synchronization control method for a time reference system according to the above [3], wherein when the time reference signal is not detected, synchronization processing of the second reference frequency generation unit is started (steps S11, S12), when completion of synchronization of the second reference frequency generation unit is detected, output of the second frequency signal is started (steps S14, S15), when loss of synchronization of the second reference frequency generation unit is detected, output of the second frequency signal is stopped (steps S16, S17), and while the second frequency signal is not synchronized, the first frequency signal or the third frequency signal is preferentially used for time holdover.
[0068] According to the time synchronization control method for a time reference system described in [4] above, when the time reference signal is not detected, the synchronization process of the second reference frequency generation unit is started, so that even in a situation where a signal such as a GNSS is interrupted at an unexpected timing (time tx), it is not necessary to keep an optical clock or the like running at all times. Furthermore, when synchronization of the second frequency signal is not established or is lost, output of the second frequency signal is stopped, so that an increase in time error caused by the second frequency signal can be suppressed.
[0069] [5] A time reference system comprising: a first reference frequency generation unit (local frequency clock unit 12) capable of internally generating a first frequency signal (local frequency clock or signal SG4) with a stable frequency; a second reference frequency generation unit (optical clock network 20) capable of outputting a second frequency signal (signal SG3) with higher accuracy than the first frequency signal; a synchronization monitoring unit (synchronization monitoring unit 201: steps S14 and S16) that monitors the operating state of the second reference frequency generation unit; a signal selection unit (local frequency clock unit 12) that, when the second frequency signal is not present or is unstable, preferentially selects the first frequency signal over the second frequency signal, and, when the second frequency signal is present and stable, preferentially selects the second frequency signal over the first frequency signal (step S18); and a time synchronization unit (local time scale unit 13) that uses the first frequency signal or the second frequency signal selected by the signal selection unit for accurate time synchronization.
[0070] According to the time reference system having the configuration described in [5] above, even if the second reference frequency generation unit uses an optical clock, the optical clock only needs to be activated when the GNSS signal is interrupted (time tx), eliminating the need to operate the optical clock continuously for a long period of time, allowing for practical system operation. Furthermore, during the free-running period (T1) from when the optical clock is activated until its frequency synchronization stabilizes, local time synchronization can be maintained using the first frequency signal. Furthermore, since the free-running period (T1) is relatively short, it is easy to maintain the time error that occurs during this period within a tolerance (e.g., 100 ns). Furthermore, during the period (T2) after the end of the free-running period (T1), the frequency of the ultra-high-precision optical clock can be used, allowing the time error to be maintained within a tolerance (e.g., 100 ns) for a period of, for example, 14 days or more.
[0071] [6] The time reference system according to claim 5, further comprising a third reference frequency generation unit (high precision frequency reference device 25) capable of outputting a third frequency signal having higher precision than the first frequency signal, wherein the signal selection unit (local frequency clock unit 12) preferentially selects the third frequency signal over the second frequency signal when the second frequency signal is not present or is unstable, and preferentially selects the second frequency signal over the third frequency signal when the second frequency signal is present and stable, and wherein the time synchronization unit (local time scale unit 13) uses the third frequency signal or the second frequency signal selected by the signal selection unit for accurate time synchronization.
[0072] According to the time reference system having the configuration described in [6] above, even if a sufficiently accurate internal clock is not available, it is easy to maintain the time error that occurs during holdover within an allowable value.
[0073] [7] The time reference system described in [6] above, further comprising a time reference monitoring unit (step S11) that monitors the state of a time reference signal (signal SG1) that has a higher priority than the second frequency signal, wherein the time synchronization unit performs time synchronization based on the time reference signal when the time reference signal appears normally, and when the time reference signal is not detected, performs time holdover based on the first frequency signal or the second frequency signal selected by the signal selection unit (steps S18, S19).
[0074] According to the time reference system having the configuration of [7] above, the first frequency signal and the second frequency signal are appropriately used when the time reference signal is not detected, so that it is possible to reduce the time error that occurs, for example, during the period (time tx to t1) when the time reference signal obtained from the GNSS is interrupted and a holdover of the reference time is performed.
[0075] [8] The time reference system according to [7] above, wherein the second reference frequency generation unit starts synchronization processing of the second frequency signal when the time reference signal is not detected (steps S11, S12, S13), the second reference frequency generation unit starts outputting the second frequency signal when it detects completion of synchronization of the second frequency signal (steps S14, S15), the second reference frequency generation unit stops outputting the second frequency signal when it detects loss of synchronization of the second frequency signal (steps S16, S17), and while the second frequency signal is not synchronized, the first frequency signal or the third frequency signal is preferentially used for time holdover.
[0076] According to the time reference system having the configuration of [8] above, when the time reference signal is not detected, the synchronization process of the second reference frequency generation unit is started, so that even in a situation where a signal such as a GNSS is interrupted at an unexpected timing (time tx), it is not necessary to keep an optical clock or the like running at all times. Furthermore, in a situation where synchronization of the second frequency signal is not established or is lost, output of the second frequency signal is stopped, so that an increase in time error caused by the second frequency signal can be suppressed.
[0077] 10, 10A Time reference device 11 Time recovery block 12, 12A Local frequency clock unit 13 Local time scale unit 14, 14A, 17 Frequency interface 15 Time interface 16 Phase interface 20 Optical clock network 201 Synchronization monitor unit 25 High precision frequency reference device 31 High precision time reference device 32 GNSS 33 High precision frequency reference device 34 High precision time synchronization device 41 Optical clock 42 Synchronization network 43 Frequency distribution system 44 Time reference device 45 GNSS 51 Optical clock 52 Frequency distribution device 53 Time reference device 54 Optical network 100, 100A Time reference system SG1, SG2, SG3, SG4 Signal SG11, SG12, SG13, SG14, SG15, SG16 Signal T1 Self-running period
Claims
1. By utilizing a first reference frequency generation unit capable of internally generating a first frequency signal with a stable frequency, and a second reference frequency generation unit capable of outputting a second frequency signal that is more accurate than the first frequency signal and generated by a light clock, The operation status of the second frequency signal generated by the second reference frequency generation unit is monitored to determine whether or not it is out of sync. When the second frequency signal is absent or not stably synchronized, the first frequency signal is selected preferentially over the second frequency signal. When the second frequency signal exists and is stably synchronized, the second frequency signal is selected preferentially over the first frequency signal. The selected first frequency signal or the second frequency signal is used for precise time synchronization. A method for controlling time synchronization in a time-based system.
2. A third reference frequency generation unit capable of outputting a third frequency signal with higher accuracy than the first frequency signal is further utilized. The operating state of the second reference frequency generation unit is monitored, When the second frequency signal is absent or not stably synchronized, the third frequency signal is selected preferentially over the second frequency signal. When the second frequency signal exists and is stably synchronized, the second frequency signal is selected preferentially over the third frequency signal. The selected third frequency signal or the second frequency signal is used for precise time synchronization. A time synchronization control method for a time reference system according to claim 1.
3. The state of the time reference signal, which has a higher priority than the second frequency signal, is monitored. When the aforementioned time reference signal is present, time synchronization is performed based on the aforementioned time reference signal. If the aforementioned time reference signal is not detected, the selected first frequency signal, the third frequency signal, or the second frequency signal is used for accurate time holdover. A time synchronization control method for a time reference system according to claim 2.
4. When the aforementioned time reference signal is not detected, the synchronization process of the second reference frequency generation unit is started. When the synchronization of the second reference frequency generation unit is detected to be complete, the output of the second frequency signal is started. When the second reference frequency generation unit detects a loss of synchronization, the output of the second frequency signal is stopped. While the second frequency signal is not synchronized, the first frequency signal or the third frequency signal is preferentially used for time holdover. A time synchronization control method for a time reference system according to claim 3.
5. A first reference frequency generation unit capable of internally generating a first frequency signal with a stable frequency, A second reference frequency generation unit that is more accurate than the first frequency signal and capable of outputting a second frequency signal generated by a light clock, A synchronization monitoring unit monitors the operational status of the second frequency signal generated by the second reference frequency generation unit, including whether or not it is out of sync. A signal selection unit that, when the second frequency signal is absent or not stably synchronized, prioritizes selecting the first frequency signal over the second frequency signal, and when the second frequency signal is present and stably synchronized, prioritizes selecting the second frequency signal over the first frequency signal. A time synchronization unit that uses the first frequency signal or the second frequency signal selected by the signal selection unit for accurate time synchronization, A time reference system equipped with the following features.
6. The system further includes a third reference frequency generation unit capable of outputting a third frequency signal with higher accuracy than the first frequency signal, The signal selection unit, when the second frequency signal is absent or not stably synchronized, prioritizes selecting the third frequency signal over the second frequency signal, and when the second frequency signal is present and stably synchronized, prioritizes selecting the second frequency signal over the third frequency signal. The time synchronization unit utilizes the third frequency signal or the second frequency signal selected by the signal selection unit for accurate time synchronization. The time reference system according to claim 5.
7. The system includes a time reference monitoring unit that monitors the state of a time reference signal with a higher priority than the second frequency signal, The time synchronization unit performs time synchronization based on the time reference signal when the time reference signal is present, and performs time holdover based on the first frequency signal, the third frequency signal, or the second frequency signal selected by the signal selection unit when the time reference signal is not detected. The time reference system according to claim 6.
8. The second reference frequency generation unit starts the synchronization process for the second frequency signal when the time reference signal is not detected. The second reference frequency generation unit starts outputting the second frequency signal when it detects that the synchronization of the second frequency signal is complete. The second reference frequency generation unit stops outputting the second frequency signal when it detects a desynchronization of the second frequency signal. While the second frequency signal is not synchronized, the first frequency signal or the third frequency signal is preferentially used for time holdover. The time reference system according to claim 7.