Kuniyasu Imamura

Two-way satellite time and frequency transfer networks in Pacific Rim region

A two-way satellite time and frequency transfer (TWSTFT) network in the Pacific Rim region is under construction to contribute to the calculation of the international atomic time (TAI). Four major time and frequency institutes in this region have been conducting long-term TWSTFT experiments. In addition to these institutes, several others in the region are planning to join the network. A new type of time transfer modem for TWSTFT is also described.

Two-way time transfer through 2.4 Gb/s optical SDH system

Abstract : An experiment to transfer time and frequency over 2.488 Gbits SDH (Synchronous Digital Hierarchy) systems using 175-km commercial optical fibers has been set up by CRL and NTT. We confirm that the frequency stability of the time comparison data is 10(12)/square root of tau at averaging times above 10 s. This equals that of the Cs frequency standard (H5071A) used in this experiment. The time comparison resolution is of the order of 10(-11) s (square root of time variance). The long-term stability of this system is expected to be better than 1 ns. The time comparison results of this experiment agree well the GPS common-view results.

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.

Upgrading of UTC(CRL)

The problems in the current UTC(CRL) were solved. One problem was a large variation of frequency when a clock was withdrawn from the ensemble, which was caused by an inadequately predicted frequency of each cesium clock in the ensemble. We succeeded in solving this problem by changing the definition of the predicted frequency. Another problem was a variation in the daily-steering frequency of the UTC(CRL). We found that this problem was caused by a large variation in the measured time offset between the clocks. Improving the measured value was not easy, so we adopted a moderate adjustment and thereby succeeded in solving it. These solutions improved the frequency stability of the UTC(CRL).

The first comparative experiment of VLBI and two-way time transfer with better than 1 nsec precision

Time transfer technique comparison between very long baseline interferometry (VLBI) and two way satellite method was firstly performed between Koganei (Tokyo) and Kashima (Ibaragi) over 110 km baseline in November, 1992. Clock variation in two stations are compared by both techniques with a precision of better than 1 nsec. Obtained clock variation by both techniques behaves in the same manner. However, significant scale difference was found in the results. The most likely explanation is that the VLBI clock parameters are coupled with the other adjusted parameters.<<ETX>>

Water vapor estimation using digital terrestrial broadcasting waves: WATER VAPOR ESTIMATION USING DTB WAVES

近年都市部で頻発する局地的大雨(通称ゲリラ豪雨) などの時空間スケールの小さな気象現象は、孤立した 積乱雲の急激な生成・発達により引き起こされる。従 来のレーダー観測ではとらえることが困難だったこの ような現象が、フェーズドアレイ気象レーダー (PAWR)の登場により可視化できるようになってき た。PAWR は半径 60 km の範囲の雨を 30 秒ごとに 三次元観測することができる。さらに、2018 年 3 月 から埼玉大学で観測が開始されたマルチパラメータ・ フェーズドアレイ気象レーダー(MP-PAWR)は、観 測の高速性を保ちつつ偏波の情報を使ってより定量的 な降雨観測を実現している。このようなフェーズドア レイ気象レーダーを用いることで、上空で急発達した 降水粒子が落下に要する 5 ~ 10 分後には地上のどの あたりにどの程度の降水をもたらすか、といった短時 間の予測が可能になってきた。 しかし、防災・減災のための対応が可能となるよう な長いリードタイムを取った予測(20 分~数時間先の 予測)の精度はいまだ十分とは言えない。このような 長い予測の精度向上には数値予報モデルが重要であり、 その予測精度向上にはモデルそのものの改良に加えて、 より多くの観測データを取り込む(データ同化する) ことが必要となってくる。その中でも特に近年期待さ れているのが水蒸気量のデータ同化である。水蒸気は 雨の元となる気体としての水であり、この水蒸気の動 きを早い段階から連続して監視することで、より精度 の高い降雨予測につながると期待されている。本稿で は、NICT が開発し、現在首都圏を中心に実証実験を 実施している地上デジタル放送波(地デジ放送波)を 用いた水蒸気量推定技術を紹介する。

Water vapor estimation using digital terrestrial broadcasting waves

A method of estimating water vapor (propagation delay due to water vapor) using digital terrestrial broadcasting waves is proposed. Our target is to improve the accuracy of numerical weather forecast for severe weather phenomena such as localized heavy rainstorms in urban areas through data assimilation. In this method, we estimate water vapor near a ground surface from the propagation delay of digital terrestrial broadcasting waves. A real-time delay measurement system with a software-defined radio technique is developed and tested. The data obtained using digital terrestrial broadcasting waves show good agreement with those obtained by ground-based meteorological observation. The main features of this observation are, no need for transmitters (receiving only), applicable wherever digital terrestrial broadcasting is available and its high time resolution. This study shows a possibility to estimate water vapor using digital terrestrial broadcasting waves. In the future, we will investigate the impact of these data toward numerical weather forecast through data assimilation. Developing a system that monitors water vapor near the ground surface with time and space resolutions of 30 s and several kilometers would improve the accuracy of the numerical weather forecast of localized severe weather phenomena.

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.

Overview of Japan Standard Time generation

National Institute of Information and Communications Technology (NICT) generates and supplies Japan Standard Time (JST). JST is made from an average atomic time which is calculated by using 18 commercial Cs atomic clocks. In this calculation, there are some original methods in estimating the clock rate and clock weighting. The actual signal of JST is generated by the realization of this average atomic time. In this process, the frequency control method was optimized recently and frequency stability of JST has been largely improved.

Improvement of the Frequency Change in Japan Standard Time

The Japan Standard Time (JST) is calculated from the ensemble of the cesium atomic clocks at NICT Koganei headquarters. JST had a problem that the frequency of the timescale greatly changed by exiting of a clock from the ensemble. We found that the improper presuming method of the clock rate was the cause of this problem. We changed the presuming method so as to reflect the current performance of clocks, and the frequency change at the clock exiting was improved. We also confirmed the relation between the rate of exiting clock and frequency change of the time scale theoretically. It helps us to estimate the influence of the clock exiting.