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A precise frequency dissemination system using optical fiber is studied. The purpose of the system is to transmit a frequency standard with little deterioration to many distant users. It is composed of a phase compensation transmitter, bidirectional optical amplifiers, optical amplified distributor, and receiver. The goal of the system is to achieve stable transmission of hydrogen maser-class signals. For short-term stability, the required optical received power to realize an Allan deviation of 1 × 10–13 (average time of 1 s) is shown. For long-term stability, a new compensation method using a third wavelength transmission is effective to suppress phase fluctuation induced by temperature changes in the fiber. Experimental results show a stability of 8 × 10–17 at 105 s in a fiber link of 160 km in all with one bidirectional amplifier. © 2012 Wiley Periodicals, Inc. Electron Comm Jpn, 95(3): 45–54, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.10372

The remote synchronization system for the onboard crystal oscillator (RESSOX) realizes accurate synchronization between an atomic clock at a ground station and the QZSS onboard crystal oscillator, reduces overall cost and satellite power consumption, as well as onboard weight and volume, and is expected to have a longer lifetime than a system with onboard atomic clocks. Since a QZSS does not yet exist, we have been conducting synchronization experiments using geostationary earth orbit satellites (JCSAT-1B or Intelsat-4) to confirm that RESSOX is an excellent system for timing synchronization. JCSAT-1B, the elevation angle of which is 46.5 degrees at our institute, is little affected by tropospheric delay, whereas Intelsat-4, the elevation angle of which is 7.9 degrees, is significantly affected. The experimental setup and the results of uplink experiments and feedback experiments using mainly Intelsat-4 are presented. The results show that synchronization within 10 ns is realized.

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 × 10-16 (Allan deviation) with the averaging time of 7 × 104 s. The laboratory result is 8.7 × 10-17 (? = 4 × 104 s). The amplifier's long-term stability is promising for stable frequency dissemination in addition to precise frequency comparisons.

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 X 10-13 (t = 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 X 10-13, 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 X 10-13. The results indicate that this method promises to be suitable for most applications, providing an uncertainty of less than 1 X 10-12 at an averaging time of one day.

We have developed a new control method for the QuasiZenith Satellite (QZS) remote synchronization system for an onboard crystal oscillator (RESSOX). The new method utilizes both L1 and L2 positioning signals of the QZS. We also found that precise orbit information and the estimation of delays, such as those caused by the ionosphere and troposphere, are not necessary to realize the RESSOX technology. The estimation of the delays of the L1-, L2- and Ku-band signals caused by the ionosphere and other sources can be calculated separately during feedback control, and the results of Ku-band delay are used to generate the RESSOX control signal. The simulation results showed that synchronization of within 3 ns between the ground site clock and the QZS site clock is achievable. Experimental results on the ground also confirmed that synchronization within 3 ns is feasible.

A remote synchronization system for the on-board crystal oscillator (RESSOX) of the QZSS is proposed. Aimed at offering an alternative architecture for the classic GPS time-keeping system (TKS), the proposed system consists of a synchronization framework where an ultrastable atomic clock, localized at the ground station, is kept synchronized to a second time reference, a voltage-controlled crystal oscillator (VCXO), on-board the QZSS satellite. Based on orbit calculations and delay predictions, feedback and feed-forward control guarantee the correct synchronization of the on-board time reference and the ground-station atomic clock. Compared with the classic GPS TKS, the RESSOX scheme offers several advantages in terms of accuracy, cost, weight, and power consumption. An extensive explanation of this novel design is presented. Differences and advantages of the proposed system are compared with the classic atomic clock scheme.

The quasi-zenith satellite (QZS) system (QZSS) is a Japanese project that was started in 2003 with the aim of positioning services. A simulation and experiment on a new timekeeping method for the QZSS was conducted on the ground. The remote synchronization system for the onboard crystal oscillator (RESSOX) of the QZS functions by maintaining an uplink to receive time information from a ground station (GS) equipped with accurate atomic clocks. The required apparatuses for the RESSOX are described. Some simulation and experimental results obtained on the ground, which indicate the feasibility of synchronization to within 10 ns between the GS and the QZS, are demonstrated and compared.

We present the results from our timing of the millisecond pulsar B1937+21, performed jointly since 1997 on two radio telescopes: the RT-64 in Kalyazin (Russia) at a frequency of 0.6GHz and RT-34 in Kashima (Japan) at a frequency of 2.15 GHz. The rms value of the pulse time of arrival (TOA) residuals for the pulsar at the barycenter of the Solar system is 1.8 μs (the relative variation is ≈10−14 over the observing period). The TOA residuals are shown to be dominated by white phase noise, which allows this pulsar to be used as an independent time scale keeper. The upper limit for the gravitational background energy density Ωg h 2 at frequencies ≈6.5 × 10−9 Hz is estimated to be no higher than 10−6. Based on the long-term timing of the pulsar, we have improved its parameters and accurately determined the dispersion measure and its time variation over the period 1984–2002, which was, on average, −0.00114(3) pc cm−3 yr−1.

Engineering Test Satellite VIII (ETS-VIII) is planning to be launched in 2005 for fundamental studies on satellite positioning technologies and is the one first to have installed a highly precise cesium clock in Japan. To measure the clock offset between the satellite and the TCE earth station, Time Comparison Equipment (TCE) has been developed by NICT. The TCE will be used to measure both code and carrier phases and this system will improve two-way time and frequency transfer. This equipment makes it possible to compare two clocks with high precision. Moreover, TCE and TCE earth stations have abilities to remove the delay caused by terrestrial ionosphere and that between the receivers and transmitters. We estimated measurement precision in code and carrier phases. Furthermore, from the examinations using TCE-EM (Engineering Model), we confirmed that TCE is able to measure the clock phases with a subnanosecond precision by code phase observation and a precision of a few picoseconds by carrier phase observation. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 158(3): 61–71, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20385

It is known that the stability of time transfer with GPS common view in the low latitudes of Asia is worse than in Europe and the United States. The ionosphere fluctuates greatly within ±20° of the equator, which seems to have a large effect on time transfer stability. We compared the results of ionospheric corrections obtained using the Global Ionosphere Map (GIM) with those obtained from actual measurements from the ionosphere. The frequency stabilities of common-view results in the areas close to the equator were worse than those in higher latitudes when using the GIM correction, while the results compensated with the measured ionospheric delay were comparable with those in high latitude areas. We compared the GIM simulated variations with the actual variations in the ionosphere around the equator and found that the accuracies in the lower latitudes were not as good as those in the higher latitudes. This seems to degrade the time transfer stability throughout the lower latitudes of Asia.

Communications Research Laboratory (CRL) has developed a precise measurement system for millisecond pulsar timing using the 34-m antenna at Kashima Space Research Center, one of the smallest antennas for millisecond pulsar observations. Timing observations of the millisecond pulsar PSR 1937+21 have been ongoing since 1997 November at Kashima. From observational data of almost three years, we obtained a timing accuracy of 2.65 µs from daily-averaged residuals, and found the frequency stability to be about 1 ×10 −13 over a period of one year. This result confirms the observational sensitivity of our system, and shows the possibility of constructing a pulsar time scale by using a system with a small antenna.

We have developed a new VLBI system based on the conventional K-4 system, whose data-acquisition mode includes VLBA and VSOP data modes. In developing the new VLBI data-acquisition system, multibit quantization sampling, high-speed (wide-bandwidth) data sampling, and the data acquisition mode were taken into consideration. The new acquisition system has one-bit and two-bit quantization sampling modes for VLBI, and four-bit and eight-bit sampling modes for general-purpose use. The total data rate is up to 256 Mbps, and up to 16 video channels can be selected. Anti-aliasing filtering is carried out in analog (32 MHz), and after sampling, the 16-MHz, 8-MHz, etc. filterings are carried out in the digital filter. The digital filter is used because it results in good phase characteristics. This system is specially designed for the Key Stone Project, and an input interface unit is incorporated in the VSOP.

A highly accurate zero baseline interferometry (ZBI) method for measuring the instrumental delay in VLBI time comparisons has been developed. ZBI is a kind of portable clock method but adds a separately calibratable portable receiver as a reference. Instrumental delay is an important factor in determining the accuracy of time-and-frequency comparisons. Fundamental experiments using the ZBI method show a measuring accuracy of closer than 0.2 ns.

VLBI, SLR, and GPS have been introduced and used to study geodesy and geodynamics in Japan as well as elsewhere in the world. The present article briefly overviews Japanese recent activities concerning the applications of these techniques. VLBI and SLR have been used mainly in international experiments such as the Monitor Earth Rotation and Intercompare Techniques (MERIT) campaign and the Crustal Dynamics Project (CDP). These projects have succeeded in determining the relative plate motions around the Japanese Islands. GPS, on the other hand, has been used mostly to monitor local crustal deformations for earthquake prediction research. The combination of these techniques may lead us to a unified view of crustal deformation and the dynamics of the upper mantle.

In order to optimize GPS (Global Positioning System) time transfer over an intercontinental baseline (France to Japan, about 8800 km), the authors supplement commercial C/A code receivers with dual frequency codeless receivers designed to measure directly the ionospheric delay of the GPS signal. Using data accumulated over a period of two and a half years, it is shown that the long-term precision of time transfer can be improved through the use of these ionospheric measurements. >

A dual-frequency codeless Global Positioning System (GPS) receiver for measuring ionospheric delay, called GTR-2, has been developed. Results of a feasibility study of using GTR-2 for single-frequency very-long-baseline interferometry (VLBI) experiments are presented. GTR-2 uses a cross-correlation property of the P-code signals transmitted from GPS satellites to measure ionospheric total electron content (TEC) with a measurement uncertainty of about 2*10/sup 16/ electrons/m/sup 2/ along the line of site to the satellite. Ionospheric compensation of the signal from radio stars is studied using the TEC data obtained from GTR-2. >

We demonstrate a precision frequency measurement using a phase-stabilized 120-km optical fiber link over a physical distance of 50 km. The transition frequency of the 87Sr optical lattice clock at the University of Tokyo is measured to be 429228004229874.1(2.4) Hz referenced to international atomic time (TAI). The measured frequency agrees with results obtained in Boulder and Paris at a 6*10^-16 fractional level, which matches the current best evaluations of Cs primary frequency standards. The results demonstrate the excellent functions of the intercity optical fibre link, and the great potential of optical lattice clocks for use in the redefinition of the second., 14 pages, 3 figures

A new error adjustment method for remote synchronization of the onboard crystal oscillator for the quasi-zenith satellite system (QZSS) using three different frequency positioning signals (L1/L2/L5) is proposed. The error adjustment method that uses L1/L2 positioning signals was demonstrated in the past. In both methods, the frequency-dependent part and the frequency-independent part were considered separately, and the total time information delay was estimated. By adopting L1/L2/L5, synchronization was improved by approximately 15% compared with that using L1/L2 and approximately 10% compared with that using L1/L5 and a synchronization error of less than 0.77 nanosecond was realized.

The recent development of optical frequency standards has been performed quite rapid and the better uncertainty than that of microwave frequency standards will be realized in very near future. We are evaluating a one-dimensional 87 Sr optical lattice clock developed and located at the University of Tokyo, Tokyo, by using UTC(NMIJ) generated at NMIJ, Tsukuba. The baseline length between those two sites is about 50 km. We constructed a time and frequency transfer link using GPS carrier phase method for this link. We use GIPSY software and a newly developed one for data analysis. Our developed one works in real time using carrier phase data and broadcast navigation data which are obtained from the carrier phase receivers.

A frequency measurement system is developed away from a National Metrological Institute with a frequency link to International Atomic Time (TAI). The absolute frequency of the 1S0 -3P0 clock transition of the 87Sr lattice clock is determined to be 429,228,004,229,875(4) Hz. The frequency measurement system is being further improved by introducing an optical fiber link and by upgrading the GPS link.

A new error adjustment method for remote synchronization of the onboard crystal oscillator for the quasi-zenith satellite system (QZSS) using three different frequency positioning signals (L1/L2/L5) is proposed. The error adjustment method that uses L1/L2 positioning signals was demonstrated in the past. In both methods, the frequency-dependent part and the frequency-independent part were considered separately, and the total time information delay was estimated. By adopting L1/L2/L5, synchronization was improved by approximately 15% compared with that using L1/L2 and approximately 10% compared with that using L1/L5.

The paper discusses a concept and a projected performance of an "optical lattice clock" as a next generation time and frequency standard. Recent experiments on the absolute frequency measurement are presented. The measured Allan standard deviation shows that the fractional frequency instability of the Sr lattice clock has reached 3times10-15 at an averaging time of 1000 s. A preliminary absolute frequency value of the Sr lattice clock is obtained to be 429,228,004,229,876(5) Hz

In the time and frequency transfer and dissemination field, it is important to provide cost effective remote frequency calibration services with an uncertainty of around 10-12 for end users. It is also required to develop ultra precise transfer methods with an order of 10-15 or better uncertainty for the comparison between ultra stable frequency standards which are under developing. This study shows two methods using optical fiber networks to satisfy these demands. First, it is an economical remote calibration method using existing synchronous optical fiber communication networks. The measured frequency stability (the Allan deviation) of the transmission clock is 2times10-13 for an averaging time of one day. The result indicates the method is promising for the simple remote frequency calibration service. Second, it is an ultra precise two-way optical fiber time and frequency transfer method using a newly proposed bi-directional optical amplifier. In this method, wavelength division multiplexing (WDM) signals are transmitted along a single optical fiber. The preliminary measured frequency stability is less than 1015 (tau =104 s) for a 100-km-long fiber with the bi-directional amplifier. It suggests that the method has capability for improving TAI (International Atomic Time) and UTC (Coordinated Universal Time)

Repetition rate of femtosecond frequency comb is controlled by HF frequency standards. We investigate the relation between the stability of HF frequency standards and the optical frequency measurement capability of femtosecond frequency comb.

A multichannel dual mixer time difference (DMTD) system has been developed by the Communication Research Laboratory (CRL) for measurement system to generate UTC(CRL) time scale. This system has a function of measuring time differences between reference signal and 24 DUT signals, simultaneously

The CRL (Communications Research Laboratory) and the JAXA (Japan Aerospace Exploration Agency) are planning to conduct a precise time and frequency transfer between an atomic clock on the ETS-VIII satellite and a ground-reference clock. The CRL is developing special equipment for this precise time transfer link. That conducts the two-way time transfer using the carrier phase measurement method. We expect to obtain a precise measurement exceeding 10 ps between the on-board standards and ground reference clocks. The development of the time comparison equipment, which underwent electrical tests, was finished. We are constructing the ground stations and planning detailed experiments

The time transfer precision of the GPS common-view method with single-channel C/A code receivers has become inadequate for an international time comparison network because of the advances made in atomic clock technology. NICT has been developing carrier-phase time transfer software with geodetic receivers to enable more precise time transfer technology. This paper describes the details of this software and the time transfer precision attainable with this software. Especially, we discuss the influence of tropospheric delay correction models.

We have developed a unique system using an acoustooptic spectrometer for precise timing of millisecond pulsars. This system can handle a signal of 50 MHz bandwidth with a frequency resolution of 200 kHz and a time resolution of 13 /spl mu/s, and it can average 2/sup 24/ (7 h) pulses without any dead time. >

The two way satellite time and frequency transfer (TWSTFT) is one of the most accurate and precise time transfer techniques. But there are possibilities of nanosecond level time transfer errors due to delay variations, mainly caused by temperature variations. This paper reports a measurement technique under development at the Communications Research Laboratory using a multi-channel time transfer modem.

We have been developed a new VLBI system applicable to VLBA and VSOP. This system has been achieved by using a high speed A/D, digital filter and 32 MHz IRM. An XF type correlation processor has been developed by making use of field programmable gate array. >

The Communications Research Laboratory (CRL) has developed an unique system using an acousto-optic spectrometer for precise timing measurements for millisecond pulsars. This system can treat a signal of 50 MHz bandwidth, have a frequency resolution of 200 kHz and a time resolution of 13 /spl mu/s, and average 2/sup 24/ pulses without dead time. >

An L1 band (1575.42 MHz), C/A (coarse/acquisition) code GPS receiver for precise time comparisons has been developed. GPS measurements in common-view have been carried out between the Korea Standards Research Institute (KSRI) and the Communications Research Laboratory (CRL), Japan. The frequency stability of the KSRI master clock has been measured against the master clock of CRL for 50 d. The frequency stability is about 1 part in 10/sup 13/ for the averaging time of 4 d. >

A space geodetic observation network has been established around Tokyo, Japan under a project name of Key Stone Project by Communications Research Laboratory. Three space geodetic methods, i.e. Very Long Baseline Interferometry, Satellite Laser Ranging, and Global Positioning System, are involved in the project. As of September, 1997, VLBI and GPS observation facilities at all four stations are operational, whereas developments of SLR observation facilities are in course of final alignment procedures. Daily VLBI observations began in January 1995 with a single baseline between Koganei and Kashima, and the full network observations with four stations began in September 1996. Observations and data analysis of VLBI measurements are fully automated and the analysis results are produced shortly after all observations of an experiment session finished. GPS observations at four sites began in July 1997 and the automatic data collection and analysis system are under developments.

Results of joint Russian-Japanese timing observations of the millisecond pulsar B1937+21 are presented. RMS of the post-fit residuals of the combined data is 1.8 μs. Secular change of dispersion measure with rate of −0.0012 ± 0.0003 pc cm−3 per year has been observed.