Fumimaru Nakagawa

Development of multichannel dual-mixer time difference system to generate UTC (NICT)

A multichannel dual-mixer time difference (DMTD) system has been developed by the National Institute of Information and Communications Technology (NICT) for a measurement system to generate the UTC(NICT) time scale based on Coordinated Universal Time (UTC). This system measures time differences between a reference signal and 24 device under test (DUT) signals, simultaneously. We have confirmed that this system has enough accuracy to measure hydrogen maser and cesium clocks at an averaging time of 1 s.

Carrier-phase-based two-way satellite time and frequency transfer

We performed measurements of carrier-phase-based two-way satellite time and frequency transfer (TWST-FT) with an A/D sampler and conventional TWSTFT system. We found that an instability resulting from a local signal at the satellite transponder was negligible. The short-term stability of 4 × 10-13 at 1 s was achieved in a short-baseline measurement. The results showed good agreement with the GPS carrier phase.

The New Generation System of Japan Standard Time at NICT

NICT has completed a new generation system for the realization of Japan standard time. There are various renewals in this system. One of the big changes is the introduction of hydrogen masers as signal sources for UTC(NICT) instead of Cs atomic clocks. This greatly improves the short-term stability of UTC (NICT). Another big change is the introduction of a newly developed 24ch dual-mixer-time-difference system (DMTD) as the main tool for measurements. The reliability of the system is also improved by enhanced redundancy and monitoring systems. The new JST system is in regular operation since February 2006.

Carrier-phase TWSTFT experiments using the ETS-VIII satellite

The National Institute of Information and Communications Technology (NICT) has developed and tested carrier-phase two-way satellite time and frequency transfer (TWSTFT) as a next-generation technique applicable to greater distances and resulting in higher precision. A method different from that used for code-based TWSTFT is required for carrier-phase TWSTFT. We propose a new method based on variations of carrier phases for the propagation of signals.Using the ETS-VIII satellite launched by the Japan Aerospace Exploration Agency (JAXA) and the Time Comparison Equipment (TCE) system developed by NICT, carrier-phase TWSTFT experiments were carried out between two ground-based hydrogen masers separated by a baseline of 110 km. Our tests showed that the frequency difference between two hydrogen masers can be measured for averaging times larger than 1000 s and the precision of carrier-phase TWSTFT is about 100 times better than that of code-based TWSTFT.

SI-traceable measurement of an optical frequency at the low 10^−16 level without a local primary standard

SI-traceable measurements of optical frequencies using International Atomic Time (TAI) do not require a local primary frequency reference, but suffer from an uncertainty in tracing to the SI second. For the measurement of the 87Sr lattice clock transition, we have reduced this uncertainty to the low 10-16 level by averaging three sets of ten-day intermittent measurements, in which we operated the lattice clock for 104 s on each day. Moreover, a combined oscillator of two hydrogen masers was employed as a local flywheel oscillator (LFO) in order to mitigate the impact of sporadic excursion of LFO frequency. The resultant absolute frequency with fractional uncertainty of 4.3 × 10-16 agrees with other measurements based on local state-of-the-art cesium fountains.

Time Scales Steered by Optical Clocks

Although optical frequency standards made rapid progress these days, microwave standards are still employed as source oscillators of time scales because an oscillator free from phase jumps is a prerequisite. Currently, for high-end users such as national metrology laboratories, hydrogen masers (H-masers) have adequate balance of the stability and reliability. Thus, optical clocks may play the role of the standards to which the time scales refer to in order to adjust their scale intervals. The benefit of using optical frequency standards, in this case, would be the capability to evaluate the scale interval of the H-maser more quickly. To investigate such possibilities of “optical” steering, we evaluated the behavior of an H-maser over a few months with reference to a 87Sr lattice clock. The evaluations clearly demonstrated a stable linear drift of the H-maser frequency, indicating the capability of compensating the drift for a stable time scale. This prospect was also supported by a numerical simulation based on the record of H-maser-UTC(NICT)-UTC link.

Error correction of precise time transfer experiment between ground and ETS-VIII

The Engineering Test Satellite-VIII (ETS-VIII) is a Japanese geostationary satellite. Its missions include basic satellite positioning experiments using onboard atomic clocks. The National Institute of Information and Communications Technology (NICT) developed special equipment for this time transfer link. This link makes precise time transfer between the onboard atomic clock and a ground reference clock using two way time transfer method and carrier phase measurement for the first time in the world. We have corrected ionosphere error by two received downlink measurement data and compared to obtain a precision as better than 3ps between both clocks.

First experiment of precise time transfer using ETS-VIII satellite

The Engineering Test Satellite-VIII (ETS-VIII) is a Japanese geostationary satellite. Its missions include basic satellite positioning experiments using on-board atomic clocks. The NICT (National Institute of Information and Communications Technology) developed special equipment for this time transfer link. This link makes precise time transfer between the on-board atomic clock and a ground reference clock using two-way time transfer method and carrier phase measurement for the first time in the world. We expect to obtain an exceedingly precision as few ps between both clocks. We also estimated the range between the satellite and the ground station using the time transfer data.

Months-long real-time generation of a time scale based on an optical clock

Time scales consistently provide precise time stamps and time intervals by combining atomic frequency standards with a reliable local oscillator. Optical frequency standards, however, have not been applied to the generation of time scales, although they provide superb accuracy and stability these days. Here, by steering an oscillator frequency based on the intermittent operation of a 87Sr optical lattice clock, we realized an “optically steered” time scale TA(Sr) that was continuously generated for half a year. The resultant time scale was as stable as International Atomic Time (TAI) with its accuracy at the 10−16 level. We also compared the time scale with TT(BIPM16). TT(BIPM) is computed in deferred time each January based on a weighted average of the evaluations of the frequency of TAI using primary and secondary frequency standards. The variation of the time difference TA(Sr) – TT(BIPM16) was 0.79 ns after 5 months, suggesting the compatibility of using optical clocks for time scale generation. The steady signal also demonstrated the capability to evaluate one-month mean scale intervals of TAI over all six months with comparable uncertainties to those of primary frequency standards (PFSs).

Relativistic effect correction for Clock Transport

NICT carried out the first calibration of the GPS link between Koganei and Kobe by using GPS, TWSTFT and Clock Transport (CT). We presented the result of the calibration in EFTF 2015. The differential correction of GPS link by GPS, TWSTFT and CT were 102.5, 102.1 and 104.9 ns respectively. The CT result showed a discrepancy of 2 ns. By applying the relativistic effect correction for CT, we confirmed a good agreement of the results obtained by their three techniques.