Chia-Shu Liao

Sagnac Effect and Diurnal Correction on Two-Way Satellite Time Transfer

Precision of two-way satellite time and frequency transfer (TWSTFT) highly depends on the residual nonreciprocity delays; one of them is caused by the Sagnac effect. In this paper, we employ the satellite ephemeris data to compute the precise Sagnac effect and discuss the possible additive effect related to the Sagnac effect. We also describe a method to calculate the diurnal correction based on the time derivative of the Sagnac effect. Through the analysis of the link between the National Institute of Information and Communications Technology in Japan and the Telecommunication Laboratories in Taiwan, we demonstrate that the correction was useful to improve the TWSTFT data. The time deviation at an 8-h averaging time is reduced from 346 to 156 ps after the correction.

Full Utilization of TWSTT Network Data for the Short-Term Stability and Uncertainty Improvement

The two-way satellite time and frequency transfer (TWSTFT) is a precise time transfer technique and has being used to generate the international atomic time since 1999. Nowadays, TWSTFT links have formed a worldwide network and the large amount of TWSTFT data become highly redundant. To fully utilize the data of TWSTFT network and improve the TWSTFT results, a feasible method of processing data is proposed in this paper. According to the stability of each link, we assign a weighting value to each related equation and then solve the matrix equation with the weighted least squares method. We perform data analysis with practical time transfer data, and the results show that it is an effective method to improve the short-term stabilities and uncertainties of TWSTFT results.

Remote frequency control via IEEE 1588

A new scheme of remote frequency control based on the IEEE 1588 standards, a precision time synchronization protocol (PTP) is proposed in this paper. A remote oven-controlled crystal oscillator (OCXO) is steered by integrating the PTP system, fuzzy controller and D/A converter, such that its frequency can follow a primary cesium atomic clock used as the master clock of the PTP system. Experimental results show that for averaging times of one day, the frequency stability of the OCXO can be improved from a few parts in 109 to 1012.

Formation of a paper neural-fuzzy time scale in the eastern Asia

This work presents a novel scheme to simulate a paper time scale by utilizing ensemble clock data of time laboratories in the eastern Asia-Pacific region. Based on integration of the above data with a neural-fuzzy prediction approach, the time difference with a local coordinated universal time is analyzed by a reference time scale devised in this work. Validity of the proposed scheme is investigated by undertaking a 250-day long neural-fuzzy predictor that is based on the calculated clock ensembles from Asian time laboratories. In the Asia-Pacific rim region, TWSTFT links from four laboratories, including the National Institute of Information and Communications Technology of Japan (NICT), the National Time Service Center of China (NTSC), the Korea Research Institute of Standards and Science of Korea (KRISS), and the Telecommunication Laboratories of Taiwan (TL), have formed a time transfer network based on use of multi-channel TWSTFT time transfer modems. Thus, a paper neural-fuzzy reference time scale by using the ensemble clock data among NICT, NTSC, KRISS, and TL in the Asia-Pacific region is simulated. Moreover, the performance of the proposed time scale is evaluated.

Frequency Calibration Based on the Adaptive Neural–Fuzzy Inference System

A new scheme of frequency calibration based on adaptive neural-fuzzy inference system (ANFIS) is proposed in this paper. In normal mode, an oven- controlled crystal oscillator (OCXO) is steered by integrating time interval counter (TIC), fuzzy controller, D/A converter, etc., such that its frequency can follow a primary cesium atomic clock. In addition, the ANFIS is applied when the system enters holdover mode. Experimental results show that the frequency stability of the OCXO can be improved from a few parts in 109 to 1013 for averaging times of one day, as well as the performance could be maintained within a few parts in 1010 over one day in the holdover mode.

Design of GPS Remote Calibration System

A new design of GPS remote time and frequency calibration system is presented. Its hardware was composed with a time interval counter card, a GPS receiving modular and a controller. The counter card was manufactured with modeled number GT200 by Guide Technology, Inc. The Trimble ThunderBolt GPS disciplined clock was used as the GPS receiving modular. A personal computer with Redhat Linux operation system was utilized for the controller. The software is written by C language which controlled the GPS modular, the counter card, and data collecting. The characteristic of the counter card and GPS receiving modular were performed. A series of test were executed to valuate this system. The short baseline common clock test with hydrogen maser was studied for the system noise level. The direct phase comparison in laboratory, GPS short baseline test in laboratory, and GPS field test within 10 km between hydrogen maser and cesium clock were also experimented two methods were applied to evaluate the system performance and there were GPS all-in-view and phase difference comparison. The later one could illustrate the short-term frequency stability. The former method could demonstrate the longer term stability and time characteristic of the clock at remote site. The accuracy and stability of the experimental result of the short baseline common clock with hydrogen maser reached to a few parts of 10-13 and the TDEV values were lower than the ITU-T G.811 requirements. The designed system with the capability could be applied for the remote time/frequency calibration and for the monitoring the performance of the primary reference clocks (PRC) of the digital telecommunication network, too.

Fast computation of time deviation and modified Allan deviation for telecommunications clock stability characterization

Time and frequency characterization of precision clocks and oscillators is an important task in the maintenance of time and frequency standards. Recently, time domain signal characterization of clocks is generally preferred and several performance measures of clocks and oscillators are defined in ITU-T Recommendation G.810. Among these measures, maximum time interval error (MTIE), time deviation (TDEV) and modified Allan deviation (MDEV) are frequently used, especially in telecommunications measurement. However, the direct computation of MTIE, TDEV and MDEV, defined in ITU-T Recommendation G.810, tends to be unmanageable when the number of samples becomes large. We propose a fast computation approach for TDEV and MDEV. The approach is based on the recursive algorithm and the computation exactly conforms to the ITU-T G.810 definition. As compared with the direct computation approach, the time complexity of the proposed approach is reduced by a factor of N, the number of samples. It reveals that the real-time measurement or monitoring will be feasible by employing the proposed computation approach.

A study of ionospheric delay corrections for next-generation two-way satellite time and frequency transfer

Time scale difference has been compared at nanosecond level so far by using two-way satellite time and frequency transfer (TWSTFT), benefiting from many signal delays are pretty canceled out due to the reciprocity of propagation path. For next-generation TWSTFT which compares time scale difference at sub-nanosecond level, further studies for signal delays are necessary. This work studies the impact of the ionospheric delay corrections on next-generation TWSTFT. The results demonstrate that the ionospheric corrections vary with both latitude and solar activity. At low latitudes and during moderate or high solar activity, they can reach 200 picoseconds, and thus can affect the time precision of sub-nanosecond next-generation TWSTFT.

Formation of a Real-Time Time Scale With Asia-Pacific TWSTFT Network Data

This paper presents a novel scheme to generate a real-time paper time scale by utilizing ensemble clock data and two-way satellite time and frequency transfer (TWSTFT) data of time laboratories in the Asia-Pacific region. Based on integration of the aforementioned data with a specific weighting method and a fuzzy prediction approach, the reference paper time scale devised in this paper can be generated in real time. The validity of the proposed scheme is investigated by undertaking about one years worth of experiments, which are based on the calculated clock ensembles from TWSTFT links among Asian time laboratories. Moreover, the performance of the proposed scheme is evaluated with respect to the time scales of Coordinated Universal Time (UTC) and local realizations of UTC. The simulated real-time reference time scale based on the proposed scheme is within 10 ns with UTC, and time deviation is less than 0.6 ns for a time interval exceeding 60 days.

Application of ANFIS for Frequency Syntonization Using GPS Carrier-Phase Measurements

A new scheme of frequency syntonization based on the GPS carrier-phase measurements using an adaptive neural-fuzzy inference system (ANFIS) is proposed in this paper. A GPS carrier-phase disciplined oscillator (GPSCDO), which uses pure carrier-phase measurements to steer an oven-controlled crystal oscillator (OCXO) has been designed. High short-term frequency stability of the GPSCDO is obtained. However, like traditional GPSDO, the long-term performance of the GPSCDO is limited due to the atmospheric effect. Therefore, an ANFIS is employed in an attempt to improve the long-term frequency stability of the GPSCDO. By applying the off-line data, which are predicted by the ANFIS, the remote GPSCDO can correct its estimations of the frequency offsets, which is used as input of a fuzzy controller. Experimental results show that for averaging times of one day, the frequency stability of the GPSCDO can be improved from a few parts in 1012 to 1014.