Hydrogen Atomic Clock Research Articles

Steering the Frequency of Hydrogen Master Based on Pulsar Observations

Hydrogen masters have high short-term stability, using them as master clocks can generate high-precision local standard time in a short period of time. However, the hydrogen maser has a frequency drift phenomenon, which leads to its poor long-term stability, thus affecting the accuracy of the local time. The rotation of the millisecond pulsar is highly stable. With the help of its high long-term stability, it can regularly control the frequency of the hydrogen atomic clock, thus control the real-time signal. The stability of four millisecond pulsars in the second data set released by the International Pulsar Timing Array (IPTA) is analyzed. At the same time, the frequency stability of a hydrogen maser of the National Time Service Center of the Chinese Academy of Sciences (NTSC) is analyzed by Hadamard variance. Finally, a strategy for steering the frequency of hydrogen master using pulsars is given.

SISRE of BDS-3 MEO: Evolution as well as Comparison between D1 and B-CNAV (B-CNAV1, B-CNAV2) Navigation Messages

The signal-in-space range error (SISRE) has a direct impact on the performance of global navigation satellite systems (GNSSs). It is an important indicator of navigation satellite space server performance. The new B-CNAV navigation messages (B-CNAV1 and B-CNAV2) are broadcast on the satellites of the Beidou Global Navigation Satellite System (BDS-3), and they are different from D1 navigation messages in satellite orbit parameters. The orbit accuracy of B-CNAV navigation messages lacks analyses and comparisons with D1. The accuracy and stability of the new hydrogen and rubidium clocks on BDS-3 satellites need annual analyses of long time series, which will affect the service quality of this system. Based on precise ephemeris products from the Center for Orbit Determination in Europe (COD), the orbit error, clock error, and SISRE of 24 medium Earth orbit (MEO) satellite D1 and B-CNAV navigation messages of BDS-3 were computed, analyzed, and compared. Their annual evolution processes for the entire year of 2022 were studied. Thanks to the use of inter-satellite links (ISLs) adopted by BDS-3 MEO satellites, the ages of the ephemeris are accurate and the percent of ages of data, ephemerides (AODEs), and ages of data and clocks (AODCs) shorter than 12 h were 99.95% and 99.96%, respectively. In addition, the broadcast orbit performance was also improved by ISLs. The root mean square (RMS) values of the BDS-3 MEO broadcast ephemeris orbit error were 0.067 m, 0.273 m, and 0.297 m in the radial, cross, and along directions, respectively. Moreover, the 3D RMS value was 0.450 m. Thanks to the use of new orbit parameters in the B-CNAV navigation messages of BDS-3 MEO, its satellite orbit accuracy was obviously better than that of D1 in the radial direction. Its improved accuracy can reach up to about 1.2 cm, and the percentage of its accuracy improvement was about 19.06%. With respect to clock errors, the timescale differences between the two clock products were eliminated to assess the accuracy of broadcasting ephemeris clock errors. A standard deviation value of 0.256 m shows good performances as a result of the use of the two new types of atomic clocks, although the RMS value was 0.541 m due to a nonzero mean bias. Overall, the accuracy of atomic clocks was good. For the new hydrogen and rubidium atomic clocks, their RMS and standard deviation were 0.563 m and 0.231 m and 0.519 m and 0.281 m, respectively. The stability of the former was better than that of the latter. However, due to the nonzero mean bias the latter was better than the former in accuracy. The RMS value of the SISRE of BDS-3 MEO’s broadcast ephemeris was 0.556 m, and the value was 0.920 m when it had a 95% confidence level. In contrast, after deducting the influence of the clock error, the value of SISRE_ORB was 0.092 m. Since the satellite clock error was substantially larger than the orbit radial error, the SISRE was mainly affected by the clock error, and their annual evolutions were consistent. Because of the improvement to the B-CNAV’s navigation message with respect to orbit radial accuracy, SISRE_ORB has improved in accuracy. Compared to D1, it had a significant effect on improving the accuracy of SISRE_ORB, and the percentage of the accuracy improvement was 8.40%.

Measuring the gravity potential between two remote sites with CVSTT technique using two hydrogen clocks

ABSTRACT According to General Relativity Theory (GRT), by comparing the frequencies between two precise clocks at two different stations, the gravity potential (geopotential) difference between the two stations can be determined due to the gravity frequency shift effect. Here, we conduct a clock-transportation experiment for measuring geopotential differences based on frequency comparisons via satellite links between two remote hydrogen atomic clocks. Based on the net frequency shift between the two clocks in two different periods, the geopotential difference between stations of the Beijing 203 Institute Laboratory (BIL) and Luojiashan Time-frequency Station (LTS) is determined. Comparisons show that the experimental result deviated from the reference of Earth gravity model EGM2008 result by (38.545.9) m in Orthometric Height (OH). The results are consistent with the frequency stabilities of the hydrogen clocks (at the level of ) used in the experiment. With the rapid development of time and frequency science and technology, the approach discussed in this study for measuring the geopotential is prospective and thus, could have broad applications.

Test and Analysis of Timekeeping Performance of Atomic Clock.

At present, there are few articles about the timekeeping performance of domestic atomic clocks in their moving state. In this paper, the frequency stability changes of hydrogen atomic and cesium atomic clocks in stationary and moving states are compared and analyzed; the frequency stability of the atomic clock at the beginning of its transition from moving state to stationary state is tested and analyzed; the influence of three main noises of atomic clocks on frequency stability is analyzed; and finally, the difference in the predictability of atomic clocks in moving and stationary states is analyzed. The results show that: (1) in the moving state, the frequency stability of a hydrogen clock decreases by 1-2 orders of magnitude, and the frequency stability of a cesium clock decreases by 0.5 orders of magnitude; (2) in the recovery stage, the frequency stability of hydrogen and cesium clocks is between that in static and moving stages, but the frequency stability fluctuates greatly in this stage; (3) in the moving state, the three main noises of the atomic clock all increase, of which the increase in the white noise of phase modulation is the largest, indicating that it is the most sensitive to vibration and has the greatest impact on the frequency stability of the atomic clock during the moving period; (4) in the mobile state, the RMS of the prediction data of the hydrogen clock and cesium clock greatly increases compared with that in the static state.

Simulation analysis of hydrogen atomic clock double state-selection beam optical system

Hydrogen maser uses the transition frequency of hydrogen atom at hyperfine energy level of ground state to realize precise timing. It has excellent frequency stability, especially in medium- and short-term, and low frequency drift. It has been used as high-precision frequency standard in engineering fields such as time keeping, navigation, and very long baseline interferometry. Clock transition of hydrogen maser is the transition between states of <inline-formula><tex-math id="Z-20221227162344">\begin{document}$|F = 1, m_{\rm F} = 0\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221363_Z-20221227162344.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221363_Z-20221227162344.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="Z-20221227162352">\begin{document}$|F = 0, m_{\rm F} = 0\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221363_Z-20221227162352.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221363_Z-20221227162352.png"/></alternatives></inline-formula>. State selection is realized by state selection magnet, through which high energy atoms are converged and low energy atoms are dispersed. In conventional magnet state-selecting system, both atoms of <inline-formula><tex-math id="Z-20221227162429">\begin{document}$|F = 1, m_{\rm F} = 0\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221363_Z-20221227162429.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221363_Z-20221227162429.png"/></alternatives></inline-formula> states, which are required for the maser transition, and useless atoms of <inline-formula><tex-math id="Z-20221227162419">\begin{document}$|F = 1, m_{\rm F} = 1\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221363_Z-20221227162419.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221363_Z-20221227162419.png"/></alternatives></inline-formula> states are focused into storage bulb, which places restrictions on the medium- and long-term frequency stability performance of hydrogen maser. In order to further improve the quality of atomic transition spectral lines and the performance of hydrogen maser, double state-selection beam optical system which is based on the Majorana transition mode is constructed through calculations and simulations. In this work, we use Majorana method to invert atomic states. The magnetic field required for Majorana transition is established by using two coils with reverse current. The two coils are separated by 71 mm, and the coil axes are aligned with the direction of atomic beam. The other two pairs of transverse Helmholtz coils are separated by 22 mm in the center of the state reversal to adjust the zero point of magnetic field, which should coincide with the atomic beam to ensure a complete reversal of atomic polarity. The state reversal region is surrounded by four magnetic shields to reduce the influence of stray magnetic fields. Relationship between selected-state magnetic field gradient and distance of magnetic poles is analyzed by simulation, and trajectories of the atoms with high and low energy under different selected-state magnetic fields are calculated. The utilization and purity of high energy state atoms entering into bulb atoms are obtained. The purity of the selected <inline-formula><tex-math id="Z-20221227162405">\begin{document}$|F = 1, m_{\rm F} = 0\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221363_Z-20221227162405.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221363_Z-20221227162405.png"/></alternatives></inline-formula> state atoms reaches 99% and the utilization rate is 58%. This is ideal for engineering applications. It effectively enhances the proportion of <inline-formula><tex-math id="Z-20221227162411">\begin{document}$|F = 1, m_{\rm F} = 0\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221363_Z-20221227162411.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20221363_Z-20221227162411.png"/></alternatives></inline-formula> state atoms entering into the atomic storage bulb and ensures the utilization of atoms. We verify the state-selection beam optical system experimentally. By turning on double state-selection system the maser signal can be enhanced. By adjusting the coil current of the double state-selection system, the maser signal varies with coil current, which verifies the effectiveness of double state-selection system.

On-orbit evaluation of influence of magnetic-induced frequency shift on space borne atomic clock

The output frequency of an on-board atomic clock changes with a change in the external magnetic field as it is a magnetic sensitive device. In this paper, the phase comparison method is employed for the main and standby atomic clocks to evaluate the frequency stability of 10 Medium Earth Orbit (MEO for short) navigation satellite atomic clocks equipped with hydrogen atomic clocks. The average short-term stability is higher than 1×10−12 τ −1/2, and the daily stability is higher than 7×10−15. Based on this stability level, this paper presents a preliminary evaluation of the magnetic frequency shift in the atomic clock in the navigation satellite. The magnetorquer can cause a shift in the magnetic frequency of the atomic clock to an order of 1×10−13. In addition, the continuous operation of the magnetic torque converter for 1 h will cause the prediction error of the clock difference to be 0.4 ns. In this paper, the working state of the on-orbit magnetorquer is equivalent to the magnetic frequency shift noise, and its influence on the stability of the on-board atomic clock is simulated and analyzed. The simulation results show that the magnetic frequency shift noise worsens the long-term stability of the on-board atomic clock, especially for those with higher stability. Furthermore, the possible ways of minimizing the effect of the magnetic frequency shift on the atomic clock are discussed.

Frequency-temperature effect of hydrogen maser: Theoretical analysis and temperature control optimization.

The internal temperature environment of a hydrogen maser (H maser) is one of the main factors, which limit the frequency stability of hydrogen atomic clocks (HACs). In the present study, the thermodynamic interactions between the atomic transition frequency and the cavity-bulb assembly affecting the H maser were investigated, and the cavity-pulling effect and the bulb wall frequency shift effect induced by the change in temperature were quantitatively analyzed and calculated. Moreover, the effect of the temperature gradient on the temperature sensitivity of the frequency stability (i.e., the frequency-temperature effect) was qualitatively analyzed. The precision temperature control system was optimized based on the HAC temperature stability requirement through the simulation of the temperature field for different heating pattern methods. The optimization effect was verified experimentally, and the results show that after optimizing the design, the temperature stability is improved from ±0.005 K to ±0.001 K, and the frequency deviation is decreased from 3 × 10-15 to 1 × 10-15. The research results may provide theoretical and practical references for improving the frequency stability and accuracy of HACs.

The NTSC VLBI System and its application in UT1 measurement

In order to measure the Universal Time (UT1) in real time, National Time Service Center (NTSC) has built a VGOS-like (VLBI Global Observing System) broadband VLBI network, which includes three 13-m radio telescopes located in Jilin, Sanya and Kashi, and a data analysis center in Xi’an. Each station is equipped with a highly stable hydrogen atomic clock and a self-developed VLBI backend, and is colocated with two GPS receivers. This VGOS-like VLBI network may play an important role in improving the Chinese broadband VLBI technology and making valuable contributions to domestic VLBI measurements of UT1. In this paper, we introduce the specifications of this VLBI network, and present the UT1 measurements at C-band conducted in 2018 using the Jilin-Kashi baseline of this network. The comparisons between our UT1 estimates and those provided by IERS suggest that the NTSC VLBI network is capable to determine UT1 accurate at the level of 58.8 μs.

A preliminary experiment of determining the geopotential difference using two hydrogen atomic clocks and TWSTFT technique

General relativity theory (GRT) concludes that a precise clock ticks at different running rates if it is under the influence of different geopotentials. Therefore, by comparing the running rates of clocks at arbitrary two stations, the geopotential difference between them can be determined. In this study, with the help of two hydrogen atomic clocks (noted as H-masers), using the two-way satellite time and frequency transfer (TWSTFT) technique, we carried out experiments of the geopotential difference determination at the China Aerospace Science & Industry Corporation (CASIC), Beijing. Here the ensemble empirical mode decomposition (EEMD) method is adopted to remove periodic signals included in the original observations. Finally, the clock-comparison-determined geopotential difference in the experiments is determined. Results show that the difference between the geopotential difference determined by GRT and that determined by measuring tape is about 1316.1 ± 931.0 m2s-2, which is equivalent to 134.3 ± 95.0 m in height, and in consistence with the stability of the H-masers applied in the experiments (at the level of 10−15/day). With the rapid improvement of atomic clocks' accuracy, the geopotential determination by accurate clocks is prospective, and it is promising to realize the unification of the world vertical height system (WVHS).

Hydrogen atomic clock difference prediction based on the LSSVM

The atomic clock prediction is the key step for constructing atomic clock time scales and implementing atomic clocks steering, and its prediction level directly affects the accuracy and stability of atomic clock time scales. Aiming at the non-linear and non-stationary characteristics of the hydrogen atom clock, the least squares support vector machine (LSSVM) hydrogen clock prediction algorithm is proposed, which is verified by the actual hydrogen atomic clock data of the timekeeping laboratory in National Institute of Metrology, China. The results show that compared with the linear prediction algorithm and the support vector machine (SVM) prediction algorithm, the least squares SVM prediction algorithm improves the accuracy of the clock prediction, and its root mean square error is, respectively, reduced by 50% and 29%.

Early analysis of precise orbit and clock offset determination for the satellites of the global BeiDou-3 system

Eight new-generation BeiDou satellites (BeiDou-3) have been launched into Medium Earth Orbit (MEO), allowing for global coverage since March 2018, and they are equipped with new hydrogen atomic clocks and updated rubidium clocks. Firstly, we analyzed the signals for the carrier-to-noise-density ratio (C/N0) and pseudorange multipath (MP) by using international GNSS (Global Navigation Satellite System) Monitoring and Assessment System (iGMAS) station data, and found that B1C has a lower C/N0, and B2a has the same level of C/N0 as the B1I and B3I signals. For pseudorange multipath, compared with the BeiDou-2 satellites, the obvious systematic variation of MP scatters related to the elevation angle is greatly improved for the BeiDou-3 and BeiDou-3e satellites signals. For the signals of the BeiDou-3 satellites, the order of the Root Mean Square (RMS) values of multipath and noise is B3I < B1I < B2a < B1C. Then, the comparison of the precise orbit determination and clock offset determination for the BeiDou-2, BeiDou-3, and BeiDou-3 experimental (BeiDou-3e) satellites was done by using 10 stations from iGMAS. The 3D precision of the 24 h orbit overlap is 24.55, 25.61, and 23.35 cm for the BeiDou-3, BeiDou-3e, and BeiDou-2 satellites, respectively. BeiDou-3 satellite has a comparable precision to that of the BeiDou-2 satellite. For the precision of clock offset estimation, the Standard Deviation (STD) of the BeiDou-3 MEO satellite is 0.350 ns, which is an improvement of 0.042 ns over that of the BeiDou-2 MEO satellite. The stabilities of the BeiDou-3 and BeiDou-3e onboard clocks are better than those of BeiDou-2 by factors of 2.84 and 1.61 at an averaging time of 1000 and 10,000 s, respectively.

Characteristics and performance evaluation of Galileo on-orbit satellites atomic clocks during 2014–2017

The on-orbit characteristics and service performance of Galileo navigation system (Galileo) has been drawn attentions since Galileo is the only one global navigation system which is normally equipped with large-scale, high precision and performance hydrogen atomic clocks. Based on a period of 1000 days’ Galileo satellite clock offset products, this article analyzed the performance changes of Galileo satellite clocks when the working clock is switched to the backup clock. The onboard clock phases, frequencies, frequency drifts, the polynomial fitting residuals and the stabilities have been obtained. Results show that the frequency drifts of Galileo satellite clocks can reach up to a magnitude of 10−19 and the fitting precisions are superior to 0.3 ns, while the stabilities in the time length of ten thousand seconds can achieve 10−14. These mainly can be due to the onboard high performance and precision hydrogen atomic clocks.

Clock Differences Prediction Algorithm Based on EMD‐SVM

A new prediction algorithm based on Empirical model decomposition (EMD) and Support vector machine (SVM) is put forward in this paper, and this algorithm solves the problem of the hydrogen atomic clock differences prediction, which is affected by the non-linearity and non-stability. The clock differences were decomposed into Intrinsic mode functions (IMF) and the residual series. The suitable kernel function and parameters were chosen to build the different SVM for predicting each IMF and the residual series. Each prediction result was summed to obtain the clock differences prediction. Results show that the EMD-SVM algorithm is effective compared with the linear regression and single SVM . The relative prediction error is reduced from 0.4327% to 0.2371%, and the dispersion is less than other methods.

Simulation of Nikola Tesla Atmospheric (Maser) Discharges

In this paper, an attempt is made in the laboratory to simulate Nikola Tesla atmospheric maser discharges. The ground lightning bolt, commonly observed, takes the form of filamentary lightning channels called the leader and the return strokes. It is taken that Nikola Tesla atmospheric discharges are obtained when a powerful high-frequency source triggers the atmospheric discharge and enables the energy stored in the cumulonimbus clouds to be released in the form of maser radiations, which are accompanied by a diffuse glow-like discharge covering a large area, as in the case of the aurora borealis. The main point in the current investigation is the generation of the powerful hydrogen atom (maser) line at 1.420 GHz recorded under the condition of high humidity present in air. This suggests that the driving frequency of high power at a frequency close to 1 GHz is capable of electrolysis of the water vapor present in air, dissociation of the hydrogen molecule into atoms, excitation of atomic hydrogen, and consequent emission from the hydrogen atoms in the manner found in the hydrogen atomic clock.

Phase group synchronization between any signals and its physical characteristics

The phase group synchronization between any signals is further revealed, which is based on proposing the new concepts of the greatest common factor frequency, the least common multiple period, quantized phase shift resolution, equivalent phase comparison frequency and so on. Then the problem of phase comparison and processing between different frequency signals is solved and shown in detail. Using the basic principle and the variation law of group phase difference, the frequency stability better than 10−14/s can be easily obtained in the time & frequency measurement and control domain, and experimental results also show the phase relations between atomic energy level transition signal and the locked crystal oscillator signal in an active hydrogen atomic clock are strict phase group synchronization, and locked precision with 10−13/s can be reached based on phase group synchronization. The phase group synchronization can provide technical support to frequency linking among radio frequency, microwave and light frequency.

Testing Local Position Invariance with Four Cesium-Fountain Primary Frequency Standards and Four NIST Hydrogen Masers

We report the most sensitive tests to date of the assumption of local position invariance (LPI) underlying general relativity, based on a 7 yr comparison of cesium and hydrogen atomic clocks (frequency standards). The latest results place an upper limit that is over 20 times smaller than the previous most sensitive tests; this is consistent with the null shift predicted by LPI. The result is based on precise comparisons of frequencies of four hydrogen masers maintained by NIST, with four independent Cs fountain clocks--one at NIST and three in Europe--as the Sun's gravitational potential at Earth's surface varies due to Earth's orbital eccentricity.