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The beacon signals from a geostationary satellite were observed using three-element very long baseline interferometry (VLBI), and the phase fluctuations along the baselines between three sites were obtained. The atmospheric phase fluctuations at each observation site were derived from the baseline phase fluctuation data. The fluctuations were classified into three time-interval regions based on the dependence of the Allan standard deviation of the fluctuations on the time interval. In the region where the interval was less than a few seconds and in the one where it was greater than one hundred seconds, the curve of the Allan standard deviation was steep and showed the property of white phase noise. In the region between these two regions, the dependence of the Allan standard deviation on the time interval was weak. The magnitude of the Allan standard deviations for the three observation sites showed time variations in the region where the time interval was longer than a few seconds. Comparison with the weather conditions suggested that the time variations of the Allan standard deviation reflected atmospheric instability above the sites. Index Terms--Allan standard deviation, atmospheric stability model, beacon wave, geostationary satellite, observation site atmospheric phase fluctuation, very long baseline interferometer (VLBI).

This paper reports the statistical characteristics of phase fluctuations obtained by a very long baseline interferometry, which received beacon waves from a geostationary satellite. Observations were made in different weather conditions by the 6-m Kagoshima and the 10-m Mizusawa radio telescopes, which were 1284 km apart from one another. Atmospheric phase fluctuations ranging from 0.2 to about 1000 s were detected. To study a variety of statistical characteristics of these phase fluctuations, the Allan standard deviation, [[sigma].sub.y] ([tau]), the temporal structure function, [D.sub.[phi]] ([tau]), and the square root of power spectrum, [D.sub.[phi]] (f), were calculated from the observation results. These qualities were found to depend on the time interval [tau] or the frequency f (f : 1/2[tau]) as follows: [[sigma].sub.y] ([tau]) [infinity] [[tau].sup.0.6] [[tau].sup.0.3], [D.sub.[phi]] ([tau]) [infinity] [[tau].sup.0.9], [[tau].sup.1.3] for 0.2 s < [tau] < 1.5 s and 1.5 s < [tau] < 400 s. [G.sub.[phi]](f) [infinity] [f.sup.-0.8] for 2.5 Hz > f > 0.33 Hz and [G.sub.[phi]](f) [infinity] [f.sup.-1.3] for 0.33 Hz > f > 0.00125 Hz. The curves of [[delta].sub.y] ([tau]) and [D.sub.[phi]]([tau]) exhibited shifts whose magnitudes followed the weather order clear, cloudy, and rainy. For [tau] > 1.5 s (or f < 0.33 Hz), the experimental results were consistent with the theoretical values predicted by Kolmogorov's turbulence theory and a frozen-screen model, but for [tau] < 1.5 s (or f > 0.33 Hz), the model has to be modified to explain the experimental results. Index Terms--Allan standard deviation, atmospheric phase fluctuation, beacon wave from geostationary satellite, temporal structure function, very long baseline interferometer (VLBI).

Precise frequency comparisons are becoming more urgent given the recent rapid progress in next-generation frequency standards. This paper describes a new type of bidirectional optical amplifier that overcomes the fiber loss limits that have prevented accurate frequency comparisons between widely separated places; such comparison is realized by bidirectionally transmitting wavelength-division-multiplexed (WDM) signals along a single fiber. The proposed optical amplifier has an optical isolator in each two-way channel divided by wavelength filters to suppress the optical reflection that causes amplification instability. The additional insertion optical loss due to this method is about 1.5 dB. The optical gain greater than 30 dB is obtained for both signals with good optical isolation of 65 dB. A radio-frequency reference signal can directly be sent by simple intensity modulation and direct detection (IM-DD) devices in the 1550-nm region widely used in telecommunication networks. Phase comparisons of the received signals and the frequency standards at each terminal are used for frequency comparisons. The amplifier is tested in the field using two hydrogen masers. A 120-km fiber with loss of 52.5 dB is used to connect the National Metrology Institute of Japan (NMIJ) to the University of Tokyo. Because of this loss, an amplifier is needed to realize sufficient receiving power. The frequency stability of the system with a 10-MHz direct transmission is evaluated by returning the optical signal from a halfway point [55 km from the NMIJ] where the amplifier is installed. The result is 2.6 x 10-16 (Allan deviation) with the averaging time of 7 x 104 s. The laboratory result is 8.7 x 10-17 (τ = 4 x 104 s). The amplifier's long-term stability is promising for stable frequency dissemination in addition to precise frequency comparisons. [ABSTRACT FROM AUTHOR]

This paper describes a simple and cost-effective method of frequency dissemination. In current digital communication networks, node clocks are hierarchically synchronized to the atomic master clock through fiber links. This synchronized network is used as an intermediate link for remote calibration services like the global positioning system common-view method. A prototype reference signal generator has been developed for recovering the communication clock signal and synthesizing a 10-MHz signal from it. The generator output frequency at the client site can be traced to coordinated universal time (UTC) National Metrology Institute of Japan (NMIJ) with some uncertainty, depending on the stability of the node clocks and the distance from the master clock. The stability performance of the generated reference signal has been tested at Okinawa-the farthest prefecture from Tokyo, where the master clock is located (baseline distance of 1500 km). The primary rate (1.544 MHz) for telecommunication services was chosen for the 10-MHz signal generation in the experiment. A sinusoidal phase fluctuation within a one-day period is dominantly observed. This fluctuation is mainly caused by fiber expansion and contraction due to normal daily temperature changes. It degrades the stability (Allan deviation) to the level of 5 × 10-13 τ = 40 000 s, which is almost half a day). However, the major part of the phase fluctuation can be canceled by averaging a full day's data. In this case, the Allan deviation becomes 1 ×-13 which is obtained at Okinawa over ten consecutive days of measurement. The worst average frequency offset relative to UTC (NMIJ) (one-day averaging) is -6.3 × 10-13. The results indicate that this method promises to be suitable for most applications, providing an uncertainty of less than 1 × 10-12 at an averaging time of one day. [ABSTRACT FROM AUTHOR]

The article discusses an error adjustment process for the remote synchronization of the Japanese quasi-zenith satellite system's (QZSS') onboard crystal oscillator. For the synchronization process, three different frequency positioning signals (L1/L2/L5) are used. The QZSS, which is a three-satellite navigation/positioning system, was developed to enhance the positioning performance of the global positioning system (GPS) that is currently used in urban areas. The National Institute of Advanced Industrial Science and Technology, Japan, has planned a new time-keeping method of the QZSS, which is called the remote synchronization system for an onboard crystal oscillator.

We performed spectroscopic observations of the 698-nm clock transition in 87Sr confined in an optical lattice using a laser linewidth transfer technique. A narrow-linewidth laser interrogating the clock transition was prepared by transferring the linewidth of a master laser (1064 nm) to that of a slave laser (698 nm) with a high-speed controllable fiber-based frequency comb. The Fourier-limited spectrum was then observed for an 80-ms interrogating pulse. We determined that the absolute frequency of the 5s21S0–5s5p 3P0 clock transition in 87Sr is 429 228 004 229 872.0 (1.6) Hz referenced to the SI second. [ABSTRACT FROM AUTHOR]

We demonstrate an improved absolute frequency measurement of the 1S0–3P0 clock transition at 578 nm in 171Yb atoms in a one-dimensional optical lattice. The clock laser linewidth is reduced to ≈2 Hz by phase-locking the laser to an ultrastable neodymium-doped yttrium aluminum garnet (Nd:YAG) laser at 1064 nm through an optical frequency comb with an intracavity electrooptic modulator to achieve a high servo bandwidth. The absolute frequency is determined as 518 295 836 590 863.1(2.0) Hz relative to the SI second, and will be reported to the International Committee for Weights and Measures. [ABSTRACT FROM AUTHOR]

We report an uncertainty evaluation of an 171 Yb optical lattice clock with a total fractional uncertainty of 3.6×10 -16 , which is mainly limited by the lattice-induced light shift and the blackbody radiation shift. Our evaluation of the lattice-induced light shift, the density shift, and the second-order Zeeman shift is based on an interleaved measurement where we measure the frequency shift using the alternating stabilization of a clock laser to the 6s 2 1 S 0 -6s6p 3 P 0 clock transition with two different experimental parameters. In the present evaluation, the uncertainties of two sensitivity coefficients for the lattice-induced hyperpolarizability shift d incorporated in a widely used light shift model by RIKEN and the second-order Zeeman shift a Z are improved compared with the uncertainties of previous coefficients. The hyperpolarizability coefficient d is determined by investigating the trap potential depth and the light shifts at the lattice frequencies near the two-photon transitions 6s6p 3 P 0 -6s8p 3 P 0 , 6s8p 3 P 2 , and 6s5f 3 F 2 . The obtained values are d=-1.1(4) μ Hz and a Z =-6.6(3) Hz/mT 2 . These improved coefficients should reduce the total systematic uncertainties of Yb lattice clocks at other institutes.