Yi-Jiun Huang

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.

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.

Design and Evaluation of an Open-Loop Receiver for TWSTFT Applications

The two-way satellite time and frequency transfer (TWSTFT) technique provides uncertainty at the nanosecond level for time differences measured over an intercontinental baseline. The time-difference results occasionally reveal daily variations of a few nanoseconds due to multiple effects. These daily variations are called diurnals, and they increase the uncertainty of the time-difference measurements. Since a time difference is obtained by combining two time-of-arrival (TOA) measurements of two receivers, the receivers may contribute the diurnals if they are not reciprocal. To improve reciprocities, a TWSTFT receiver based on the open-loop (OL) method was proposed and implemented on a software-defined radio receiver (SDR). For evaluation, the OL-based SDR was compared with a conventional receiver in a short-baseline TWSTFT experiment. The experimental results indicate that the diurnals disappeared with the OL-based SDRs while a diurnal of ±0.2 ns could be observed with conventional receivers. These results imply that the diurnals partially depend on TOA measurement methods used in the TWSTFT receiver.

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.

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.

Long-term inconsistency between TWSTFT and GPS time transfer results in PTB-TL and NICT-TL time links

In this paper, we compared the TWSTFT and GPSPPP data for the PTB-TL, PTB-NICT, and NICT-TL time transfer links. We investigated the double difference between the time transfer data of TWSTFT and GPSPPP from 2005 to 2010. The results show two time transfer techniques had a conspicuously inconsistency and the patterns of double difference results were similar in PTB-TL and PTB-NICT links. The double difference results of all direct links exhibited the annual varying patterns, whose peak-to-peak phase difference in 1-day moving average are 4.5 ns both for the PTB-TL and PTB-NICT links, and 2 ns for NICT-TL link.

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.

Uncertainty evaluation of 2013 TL METODE link calibration tour

We targeting analyzed the UTC link delay of GPS station PTBB of PTB to TWTF of TL and expressed it as 3 groups: the uncompensated GPS common clock difference measurements, cable delay measurement, and the total delay variations of fixed GPS stations and travelling calibrator used in this METODE calibration tour. The total delay variation of fixed GPS stations PTBB and TWTF was evaluated by monitoring their long-term CCD of the same type receiver more than 350 days. For the BIPM travelling calibrator BP1C and BP0U, we used a moving cesium clock method to evaluate their instability of total delay in different antenna position. Our study was helpful for clarifying the uncertainty composition of the current PTB-TL link and could reduce it.

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.