Fang-Dar Chu

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.

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.

Improvement of the Asia-Pacific TWSTFT network solutions by using DPN results

Two major time and frequency transfer techniques, two-way satellite time and frequency transfer (TWSTFT) and global navigation satellite systems (GNSS: GPS, GALILEO, GLONASS, etc.), are used for the generation of Coordinated Universal Time (UTC)/International Atomic Time (TAI). These time and frequency transfer links comprise a worldwide network and the utilization of the highly redundant time and frequency data is an important topic. Two methods, either TW-only network (i.e., TWSTFT) or singlelink combination of TW and Global Positioning System (GPS), have been developed for combining the redundant data from different techniques. In our previous study, we have proposed a feasible method, utilizing full time-transfer network data, to improve the results of TWSTFT network. The National Institute of Information and Communications Technology (NICT) has recently developed a software-based two-way time-transfer modem using a dual pseudo-random noise (DPN) signal. The first international DPN TWSTFT experiment, using these modems, was performed between NICT (Japan) and Telecommunication Laboratories (TL; Taiwan) and its ability to improve the time transfer precision was demonstrated. In comparison with the conventional NICT-TL TWSTFT link, the DPN time transfer results have higher precision and lower diurnal effects. The estimation also shows that DPN is comparable to GPS precise point positioning (PPP). Because the DPN results show better performance than the conventional TWSTFT results, we would adopt the DPN data for the NICT-TL link and solve the TW+DPN network solutions by using our proposed method. The concept of this application is similar to the so-called multi-technique-network time/ frequency transfer. The encouraging results confirm that the TWSTFT network performance can benefit from DPN data by improving short-term stabilities and reducing diurnal effects. The results of TW+PPP network solutions are also illustrated.

Fully utilization of TWSTT network data by weighted least squares method

The two-way satellite time and frequency transfer (TWSTFT) is a precise time transfer technique and has being used to generate the TAI 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 TWSTFT results.

A digital standard time distribution architecture with its applications

In this paper, an efficient and accurate two-layer standard time distribution architecture is proposed to implement digital time traceability. First, the system uses a clock calibration procedure based on two-way time transfer over a general dialup telephone modem and achieves a couple of milliseconds accuracy. Second, the system uses a delegation time stamping procedure that employs a forward secure proxy signature scheme to provide enhanced validity assurance and extends the RFC 3161 time stamp token to embed the standard time reading. These let users eliminate the blind trust on a commercial time stamping servers clock handling.

The feasibility of applying the neural network technology to the automatic time report system

Utilizing neural network technology, the automatic time report (embryo) system was modified by ChunghWa Telecom. This paper shows how the neural network technology is applied to simulate the operations of supervision and error detection. One may find that the automatic time report system is more complete and suitable for users.

The TWSTFT links circling the world

Two-way satellite time and frequency transfer (TWSTFT), other than the GNSS related techniques, has been one of the most precise time transfer methods. Telecommunication laboratories (TL) has devoted to set up the TWSTFT links with several important Laboratories and achieved the goal of establishing TWSTFT links circling the world. The numerical results of several direct and connected TWSTFT links will be demonstrated in the paper. The results show that a well maintained connected TWSTFT links, even with more than 2 relay stations, can still have good performance.

A study of antenna multipath instabilities in two-way satellite time and frequency transfer

The two-way satellite time and frequency transfer (TWSTFT) has been one of the major tools for atomic clock comparison in the international time community. The performance of TWSTFT nowadays has reached the level of nanosecond. For the further better performance achievement, it is worth studying the instability sources which may contribute to diurnal time residual of TWSTFT. This study conducts an analysis about time errors caused from three antenna multipath cases including reflection, diffraction, and ordinary-extraordinary waves. The results show that the antenna multipath error should not be the essential instability source of diurnal time residual. Since the antenna multi-path error is just a minor instability source, other sources should be considered in future study. We make a recommendation to study temperature dependence, such as that of the multipath interference through cables and that of the transponder of geostationary satellite.

Ionospheric electrodynamics effects on two-way satellite time and frequency transfer

The two-way satellite time and frequency transfer (TWSTFT) has been one of the essential techniques for atomic clock comparison. The performance of TWSTFT nowadays has reached the level of one nanosecond. For achieving further better performance, it is worth studying more for the next-generation TWSTFT. This study attempts to discover the relations between TWSTFT observations and ionospheric electrodynamics effects which can impact radio signals. The interesting new finding implies that the predawn local extrema in TWSTFT time difference patterns maybe result from ionospheric neutral winds.