Shin'ichi Hama

Japan-US time comparison experiment for realizing better than 1-ns accuracy by using a radio interferometric technique

A zero baseline interferometry (ZBI) experiment was conducted to determine the differential instrumental delay between two stations (Kashima, Japan, and Richmond, USA). It enabled the authors to find an absolute time comparison with an accuracy of better than 1 ns. ZBI experiments in Japan were conducted in winter and summer to find the delay dependency on temperature. >

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.

Accuracy of two-way satellite time and frequency transfer via non-geostationary satellites

In a manner different from the two-way satellite time and frequency transfer (TWSTFT) via geostationary satellites which has been employed for subnanosecond time transfer between widely separated earth stations, TWSTFT via non-geostationary satellites has a problem attaining subnanosecond accuracy because of satellite motion. The impact of satellite motion on the accuracy of TWSTFT via non-geostationary satellites is wholly attributed to the non-reciprocal geometric path. We have analysed the non-reciprocal geometric path caused by satellite motion in order to evaluate the accuracy of TWSTFT via non-geostationary satellites. Our analysis shows that the non-reciprocal geometric path introduces significant error to the accuracy of subnanosecond time transfer. Using a numerical model, we have demonstrated that ordinary orbit estimation can correct the non-reciprocal geometric path in TWSTFT via non-geostationary satellites to subnanosecond accuracy. Residuals of the non-reciprocal geometric path correction with respect to the orbit estimation accuracy and baseline length are presented. As an example of a non-geostationary satellite orbit, we employed an orbit of a quasi-zenith satellite orbiting the Earth in a period of one day, in synchronization with the Earths rotation and with an orbital inclination of 45°.

A new VLBI data acquisition and processing system, K-4

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.<<ETX>>

Pulsar VLBI experiment with the Kashima (Japan)–Kalyazin (Russia) baseline

Abstract The position of PSR0329+54 on the International Celestial Reference Frame was measured at epochs March 1995, May 1996, and May 1998. Our observations detected the proper motion of PSR0329+54. The position and proper motion agreed well with the position determined by Bartel et al. (1985) . From combined analysis with our data and that of Bartel, the proper motion of PSR0329+54 was determined: μ α =+17.4±0.3 mas yr −1 , μ δ =−11.0±0.3 mas yr −1 . These results are consistent with the value by Harrison et al. (1993) measured with the MERLIN interferometer. We also determined the coordinates of PSR0329+54 very accurately within the ICRF: α =03 h 32 m 59 s .3761±0 s .0002, δ =54°34′43′′.5119±0′′.0015 at 1995.

Development of VLBI time transfer system toward 0.1 nsec accuracy using transportable ground terminal for calibration

To improve the accuracy of time transfer in very long baseline interferometry (VLBI) technique, it is planned to perform a short baseline interferometry (SBI) experiment instead of zero baseline interferometry (ZBI) which was demonstrated in Japan-US baseline in 1986. A transportable VLBI receiving system which consists of 1.2 m antenna and delay calibration system is under development. The source flux available in SBI is weaker than ZBI because extended radio sources are resolved with distance. However, an improved VLBI data recorder with recording speeds up to 256 Mbps provides us higher sensitivity to observe weak sources in spite of small aperture antenna. Using the above system, it is easier to apply any VLBI stations for time transfer. It is expected that the new system will have 0.1 nsec level accuracy in time transfer. To confirm time transfer accuracy, a closure test will be made using more than three stations.<<ETX>>

Dissemination of UTC(NICT) by means of QZSS

The Japanese Quasi-Zenith Satellite System (QZSS) offers the possibility to transmit information with unprecedented bit rates via the L-band experimental (LEX) signal. This feature can be used to disseminate Japan Standard Time, i.e. UTC(NICT) to any user who is capable to receive the new QZSS signal. Various modes for the transmission of the timing information as well as a dedicated ionosphere correction model allow the users to instantaneously realize UTC(NICT) across Japan with an uncertainty of a few nanoseconds.

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.

Development of QZSS-mobile station

As we designed and constructed the QZSS mobile station, we transported it to Okinawa Electromagnetic Technology Center and Kashima Space Technology Center. At each place we carried out time comparison experiments. This paper describes some results of the time comparison experiments.