Citation: Hoang, A.T.; Shen, Z.; Wu, K.; Ning, A.; Shen, W. Test of Determining Geopotential Difference between Two Sites at Wuhan Based on Optical Clocks’ Frequency Comparisons. Remote Sens. 2022, 14, 4850. https://doi.org/10.3390/ rs14194850 Academic Editor: Alexander Braun Received: 24 August 2022 Accepted: 24 September 2022 Published: 28 September 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). remote sensing Article Test of Determining Geopotential Difference between Two Sites at Wuhan Based on Optical Clocks’ Frequency Comparisons Anh The Hoang 1,2 , Ziyu Shen 3, *, Kuangchao Wu 1 , An Ning 1 and Wenbin Shen 1,4 1 Time and Frequency Geodesy Center, Department of Geophysics, School of Geodesy and Geomatics, Wuhan University, Wuhan 430079, China 2 School of Agriculture and Natural Resource, Vinh University, Vinh City 460000, Vietnam 3 School of Resource, Environmental Science and Engineering, Hubei University of Science and Technology, Xianning 437100, China 4 State Key Laboratory of Information Engineering in Surveying, Mapping and Remote Sensing, Wuhan University, Wuhan 430079, China * Correspondence: zyshen@hbust.edu.cn Abstract: Applications of optical clocks in physical geodesy for determining geopotential are of increasing interest to scientists as the accuracy of optical clocks improves and the clock size becomes more and more compact. In this study, we propose a data processing method using the ensemble empirical mode decomposition technique to determine the geopotential difference between two sites in Wuhan based on the frequency comparison of two optical clocks. We use the frequency comparison record data of two Ca + optical clocks based on the optical fiber frequency transfer method, provided by the Innovation Academy for Precision Measurement Science and Technology, Chinese Academy of Sciences (Wuhan, China). By optical clock comparisons we obtained a geopotential difference of 42.50 ± 1.03 m 2 ·s -2 (equivalent to height difference of 4.33 ± 0.11 m) between the two sites, which is excellent compared to the geopotential difference of 42.56 ± 0.29 m 2 ·s -2 (equivalent to height difference of 4.34 ± 0.03 m) measured by a spirit leveling. The results show that the optical fiber frequency transfer method is promising in determining the geopotential and potential for unifying the world height system. Keywords: OFFT; optical fiber; frequency transfer; geopotential; orthometric height 1. Introduction Based on Einstein’s general relativity theory (GRT), clocks in positions with higher potentials run faster. Thus, when comparing the clock’s frequencies at two points, we can determine the geopotential difference between those two points. Such applications of atomic clocks in geodesy are extensively investigated by scientists [1–5]. Studies show that to determine the geopotential difference with an accuracy of 0.1 m 2 ·s -2 (equivalent to the orthometric height difference of 1 cm accuracy), we should determine the time shift or frequency shift at the accuracy level of about 1 × 10 -18 . With the development of clock technology, the accuracy of atomic clocks made signifi- cant improvements. Over the past decade, optical atomic clocks with frequency uncertain- ties of 10 -18 , or even higher levels were consecutively generated [6–10], with the ability to sence a 7 mm variation in height [10]. In another aspect, the atomic clocks become more compact to be easily transported. There are two ways to compare the frequencies between two remote clocks. The first one is transmitting frequency signals between two points via the GNSS satellite [11–17], which many scientists studied; for instance, the recently proposed tri-frequency combina- tion approach (TFCA) [14]. The basic idea of the TFCA is to use three microwave links between a satellite and a ground station to determine the gravitational frequency shift and then the geopotential difference between them. Suppose the ground station emits a Remote Sens. 2022, 14, 4850. https://doi.org/10.3390/rs14194850 https://www.mdpi.com/journal/remotesensing