Instrumentation & Measurement Magazine 25-6 - 19

Evaluation of Asia-Europe TWSTFT
Links Using the Express-80
Satellite
Weixiong Wang, Shaowu Dong, Wenjun Wu, Xiang Wang, Zhe Gao, and Dong Guo
T
wo-way satellite time and frequency transfer (TWSTFT)
is extensively used by the Bureau International des
Poids et Mesures (BIPM) in the calculation of coordinated
universal time (UTC). The Asia-Europe TWSTFT links
using the Express-80 satellite were reestablished in June 2021.
This paper uses monthly measurements to evaluate the time
comparison performance of the Asia-Europe TWSTFT network.
We select TWSTFT data for analysis on the long and short baselines
over the same period, respectively. Since the TWSTFT link
has not been calibrated, we take the GPS precise point positioning
(PPP) link, independent of TWSTFT, as a reference and align
the TWSTFT results with it. Then, the results from the TWSTFT
and GPS PPP solutions are compared on the different baselines.
Finally, the modified Allan deviation (MDEV) and the double
clock difference (DCD) between the TWSTFT link and the GPS
PPP link are used to evaluate the performance of the TWSTFT
results. The results show that the TWSTFT results are consistent
with the results of GPS PPP time transfer, and the frequency
stability of TWSTFT links on long and short baselines can reach
3E-15 or better with an averaging time of 1 d. The standard deviation
of the DCD results is less than 0.4 ns on different baselines,
which ensures the accuracy of the TWSTFT link. The Asia-Europe
TWSTFT link can be used for UTC time transfer after calibration.
Research Status of TWSTFT
Two-way satellite time and frequency transfer (TWSTFT) is a
primary time comparison technique over large distance that
has been widely used in global timekeeping laboratories for
the generation of International Atomic Time (TAI) and Coordinated
Universal Time (UTC) by the Bureau International des
Poids et Mesures (BIPM) since 1999 [1]. TWSTFT also serves as
a major time transfer method for ground-based reference clock
systems, including GPS, GLONASS, Galileo and BeiDou [2].
Currently, the time uncertainty of TWSTFT is less than 1 ns,
as well as the frequency uncertainty is near 1E-15 at an averaging
time of one day [3]. However, TWSTFT is disturbed by
Fig. 1. Asia-Europe TWSTFT network.
September 2022
IEEE Instrumentation & Measurement Magazine
1094-6969/22/$25.00©2022IEEE
19

Instrumentation & Measurement Magazine 25-6

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