Citation: Guo, Y.; Gao, S.; Bai, Y.; Pan,
Z.; Liu, Y.; Lu, X.; Zhang, S. A New
Space-to-Ground Microwave-Based
Two-Way Time Synchronization
Method for Next-Generation Space
Atomic Clocks. Remote Sens. 2022, 14,
528. https://doi.org/10.3390/rs
14030528
Academic Editors: Kamil Krasuski
and Damian Wierzbicki
Received: 8 December 2021
Accepted: 19 January 2022
Published: 22 January 2022
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Article
A New Space-to-Ground Microwave-Based Two-Way Time
Synchronization Method for Next-Generation Space
Atomic Clocks
Yanming Guo
1,2
, Shuaihe Gao
1,
*, Yan Bai
1
, Zhibing Pan
1
, Yinhua Liu
1
, Xiaochun Lu
1,2
and Shougang Zhang
1,2
1
National Time Service Center, Chinese Academy of Sciences, Xi’an 710600, China;
guoyanming@ntsc.ac.cn (Y.G.); by@ntsc.ac.cn (Y.B.); panzhibing@ntsc.ac.cn (Z.P.); liuyinhua@ntsc.ac.cn (Y.L.);
luxc@ntsc.ac.cn (X.L.); zhangshougang@ntsc.ac.cn (S.Z.)
2
University of Chinese Academy of Sciences, Beijing 100039, China
* Correspondence: gaoshuaihe@ntsc.ac.cn
Abstract:
The accuracy of time synchronization can be significantly increased by enhancing the
performance of atomic clocks. Future-generation time-frequency loads will be equipped with the latest
ultrahigh-precision atomic clocks (with a day stability better than 10
−17
) and will leverage advantages
of the space environment such as microgravity and low interference to operate a new generation
of high-performance time-frequency payloads on low-orbit spacecraft. Moreover, using the high-
precision time-frequency system of ground stations, low-time-delay high-performance time-frequency
transmission networks, which have the potential to achieve ultrahigh-precision time synchronization,
will be constructed. By considering full link error terms above the picosecond level, this paper
proposes a new space-to-ground microwave two-way time synchronization method for scenarios
involving low-orbit spacecraft and ground stations. Using the theoretical principles and practical
application scenarios related to this method, a theoretical and simulation verification platform was
developed to research the impact of the attitude, phase center calibration, and orbit determination
errors on the single-frequency two-way time synchronization method. The effectiveness of this new
method was verified. The results showed that when the attitude error is less than 72 arc seconds
(0.02
◦
), the phase center calibration error is less than 1 mm, and the precision orbit determination
(POD) error is less than 10 cm (three-axis). After disregarding nonlink error terms such as equipment
noise, this method can attain a space-to-ground time synchronization accuracy of better than 1.5 ps,
and the time deviation (TDEV) of the transfer link is better than 0.7 ps @ 100 s, which results in
ultrahigh-precision space-to-ground time synchronization.
Keywords:
low-orbit spacecraft; new generation of high-performance time-frequency payloads;
microwave; two-way time synchronization
1. Introduction
At present, optical fiber link time comparison technology [
1
], which can achieve the
highest picosecond time synchronization accuracy, is the most accurate of the commonly
used long-distance time transfer methods. However, broad applications of this technology
have been problematic due to limitations of the atmospheric environment. Moreover,
the microwave satellite-to-ground/intersatellite two-way time comparison system, which
is widely used in navigation systems, is limited by the performance of spaceborne atomic
clocks and the error processing method of the comparison link. This system can only
achieve sub-nanosecond time synchronization accuracy [
2
], which limits the application of
high-precision time-frequency references in space science.
The time-frequency performance of space atomic clocks will be better [
3
,
4
] than that
of terrestrial atomic clocks in a near-Earth microgravity environment, thus enabling a time
Remote Sens. 2022, 14, 528. https://doi.org/10.3390/rs14030528 https://www.mdpi.com/journal/remotesensing
