| The Earth's geopotential field plays an important role in geophysics and related fields.According to the general relativity theory(GRT),a precise clock runs at different rates or frequencies at different geopotentials.Therefore,one may determine the geopotential difference between two arbitrary stations by comparing the clocks'ticks or their frequencies.This method is called“relativistic geodesy”or“chronometric geodesy”.The key of the geopotential determination via chronometric geodesy is the high-precise time and frequency besides their comparisons.Recent years,the accuracy of the atomic clocks has been continuously improved,therefore,the determination of the geopotential difference as well as orthometric height(OH)through relativistic method gradually appears.This method can expand the way of geopotential determination,and with the rapid development of time and frequency science,it is prospective to achieve the cross-continent and cross-ocean transmission measurement of OH and consequently the International Height Reference System unification.This paper takes the problem of geopotential determination as the research object,introduces the concept of relativistic geodesy,and deduces the principle of the time-compared-determined geopotentials.In this paper,the Common View method via global navigation satellite system(GNSS CV)is adopted for comparing clocks'signals between different stations,and the implementation plan of clock transportation experiments via GNSS CV are discussed thereby.Here we conduct data processing for the clock transportation experiments implemented by the time-frequency research team of Wuhan university,and achieve the time-compared-determined geopotential difference as well as OHs,which verified the effectiveness of this method.Finally,we make comparisons between the experimental results and the results determined by Earth Gravitational Model 2008(EGM2008),and give the accuracy evaluation of our experiments.Due to the fact that the hydrogen atomic clocks in the study are at 10-15/day level,and the corresponding results of geopotentials determination are limited at tens of meters to hundreds of meters,this is consistent with the results of the transport clock experiments,which verifies the feasibility and effectiveness of the theory in this study.The main work and conclusions of this paper are as follows:(1)Introduce the concept of relativistic geodesy.The research dynamics of relativistic geodesy,time and frequency reference as well as high-precise time comparison technology are discussed,and the principle of geopotential difference determination via time-compared method is derived.(2)The high-precise time comparison is the core of relativistic geodesy.We discuss the principle and error sources of the GNSS CV method,and analyzes the accuracy of error sources and their influence on the determination of geopotentials.We discuss some basic concepts and the method of data quality evaluation in the field of time and frequency domain.(3)Design and carry out the clock transportation experiment for twice,detailing the experimental ideas,experimental facilities,and the specific implementation process of the experiments.For the experimental results,we finish the data processing from observations to original clock offsets via self-written program,and finished the data preprocessing from original clock offsets to preprocessed data set.(4)The Ensemble empirical mode decomposition(EEMD)method is introduced into the analysis of clock offset signals,and its effectiveness in extracting linear trend signals is verified via simulation experiments.We perform EEMD decomposition on the preprocessed data set and the geopotential relevant signals are then extracted.According to the principles of this paper,the determination of the geopotential difference as well as OHs are completed,and the results verify the validity of the time-compared method.Finally,we make comparisons between the experimental results and the results determined by EGM2008,and give the accuracy evaluation of our experiments. |
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