Tadahiro Gotoh

Ultra-stable frequency dissemination via optical fiber at NICT

We have developed a radio-frequency (RF) dissemination system using optical fibers. The phase noise induced during the transmission is actively cancelled by the compensation system with a voltage-controlled crystal oscillator. A first proving test was conducted on an urban telecom fiber link with a length of 10 km, and a frequency stability of 1 X 10-17 was achieved at an averaging time of one day. As an application of ultrastable frequency dissemination, a 1-GHz signal based on a cryogenic sapphire oscillator was transferred through a 25-km fiber and used as a microwave reference for an optical frequency comb. A fractional frequency stability of an ultranarrow clock laser for a Ca+ ion optical frequency standard was measured by the comb as 9 X 10-15 at 1 s, which included both laser stability and transferred reference stability.

Carrier-phase two-way satellite frequency transfer over a very long baseline

In this paper we report that carrier-phase two-way satellite time and frequency transfer (TWSTFT) was successfully demonstrated over a very long baseline of 9000 km, established between the National Institute of Information and Communications Technology (NICT) and the Physikalisch-Technische Bundesanstalt (PTB). We verified that the carrier-phase TWSTFT (TWCP) result agreed with those obtained by conventional TWSTFT and GPS carrier-phase (GPSCP) techniques. Moreover, a much improved short-term instability for frequency transfer of 2 × 10−13 at 1 s was achieved, which is at the same level as previously confirmed over a shorter baseline within Japan. The precision achieved was so high that the effects of ionospheric delay became significant; they are ignored in conventional TWSTFT even over a long link. We compensated for these effects using ionospheric delays computed from regional vertical total electron content maps. The agreement between the TWCP and GPSCP results was improved because of this compensation.

Development of a GPU-Based Two-Way Time Transfer Modem

We have developed a new two-way time transfer modem to improve the time transfer precision of remote clock comparison. As a timing signal, we apply a binary offset carrier, which is similar to those signals used for the next-generation Global Navigation Satellite Systems. We took advantage of versatile A/D and D/A converters, and most of the digital signal processing stages were realized by software, running on an off-the-shelf PC. This enabled us to realize the complete system with cheaper equipment, leading to an affordable low-cost modem. For the real-time digital signal processing stages implemented in software, we relied on a graphics processing unit (GPU) developed for computer game enthusiast. The developed modem can receive four channels at the same time with a single GPU card. We performed two-way satellite time transfer experiments using these modems between Japan and Taiwan. The obtained results are consistent within 200 ps with respect to the results of GPS carrier phase time transfer. As a consequence, we improved the time transfer precision by nearly one order of magnitude as compared to a conventional two-way modem without increasing the connection fees caused by commercial communication satellites.

First international two-way satellite time and frequency transfer experiment employing dual pseudo-random noise codes

Two-way satellite time and frequency transfer (TWSTFT) is one of the main techniques used to compare atomic time scales over long distances. To both improve the precision of TWSTFT and decrease the satellite link fee, a new software-defined modem with dual pseudo-random noise (DPN) codes has been developed. In this paper, we demonstrate the first international DPN-based TWSTFT experiment over a period of 6 months. The results of DPN exhibit excellent performance, which is competitive with the Global Positioning System (GPS) precise point positioning (PPP) technique in the short-term and consistent with the conventional TWSTFT in the long-term. Time deviations of less than 75 ps are achieved for averaging times from 1 s to 1 d. Moreover, the DPN data has less diurnal variation than that of the conventional TWSTFT. Because the DPN-based system has advantages of higher precision and lower bandwidth cost, it is one of the most promising methods to improve international time-transfer links.

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.

Comparison Study of GPS Carrier Phase and Two-Way Satellite Time and Frequency Transfer

A dual frequency GPS and a geostationary communication satellite based time and frequency transfer network have been established for the Asia-Pacific region. Some stations link to Europe with both methods. Consequently, we can compare the performance of GPS carrier phase time or frequency transfer to a two-way satellite time and frequency transfer, and vice versa. The difference between the two methods shows good consistency in the regional network. On the other hand, for intercontinental links there is an obvious variation with the diurnal and secular changes.

Development of a GPU based two-way time transfer modem

We have developed a new two-way time transfer modem, which adopts a dual pseudo-random noise (DPN) code. The usage of DPN signal dramatically improves time transfer precision without increasing connection fees of commercial communication satellites. In addition, the new modem is based on software-defined radio technology. As the only drawback of such a software implementation would be the processing speed, we overcome this problem by using a graphics processing unit (GPU) as a parallel platform. The usage of GPU enables real-time signal processing with the new modem.

First comparison of primary frequency standards between Europe and Asia

In December 2006, PTB and NICT simultaneously operated their caesium atomic fountain primary frequency standards, PTB-CSF1 and NICT-CsF1 for 15 days starting from MJD 54079. This was the first direct comparison of primary frequency standards between Europe and Asia. The fountains were compared to local H-masers and the frequency difference between the H-masers was determined directly by TWSTFT with frequency transfer uncertainty below 10-15. PTB-CSF1 and NICT-CsF1 showed good agreement within the stated uncertainties.

Concept for an accurate calibration of long baseline two-way satellite time and frequency transfer (TWSTFT) links via two separated transponders on one telecommunication satellite

Two-way satellite time and frequency transfer (TWSTFT) is the technique which allows time scale comparisons at the nanosecond level. Until now, this uncertainty level is restricted to those institutes which both can exchange signals through the same single transponder of one telecommunication satellite. Only in this case the calibration scheme of circulating a portable TWSTFT reference station is applicable. This condition is not fulfilled in the operational intercontinental links, e.g. between Asia and Europe. Calibration of the TWSTFT link is in such cases achieved by adjusting the TWSTFT results to those obtained with a calibrated GPS link. The uncertainty is then limited by the uncertainty of the GPS link, which is commonly estimated to be at the level uB = 5 ns. We propose an experimental configuration which allows determination of all important delay differences in a long baseline link including the differential delays between the operational ground stations as well as the differential transponder delay between the two separated transponders used on the satellite. The latter has never been determined on a flying satellite. We propose to exchange two portable TWSTFT stations and two GPS receivers between the two ground stations to be calibrated. We estimate the overall uncertainty for the calibration to be at a level of 2 ns. We also discuss briefly the ability of calibrating a TWSTFT link as a whole by means of GPS on the same level of uncertatinty.

GPS carrier phase and precise point positioning time scale comparisons using different software packages

Precise point positioning (PPP) software combines dual frequency code and phase GPS observation data with precise information about the satellite clocks and orbits provided by the International GNSS Service (IGS). It is also a very interesting method for time metrology. Error sources like tropospherical path delay and site displacements have to be modeled or estimated, while the error induced by the first order ionosphere terms can be canceled due to the usage of both GPS frequencies. In case of long baselines the atmospherical effects dominate. Here the physical correction models become important. In this work we compare several precise point positioning (PPP) software packages and study not only the clock solution but also other estimated parameters like positions and zenith tropospheric zenith path delays in order to find the software that is most suitable for time transfer. Especially the repeatability of the position could be a good indicator for quality. We use Dicom GTR50 time and frequency transfer receivers connected to local time scale realizations on baselines with different lengths.