Study On The Response Of GNSS Vertical Timing To Thermal Expansion Effects And Environmental Loading

The Global Navigation Satellite System(GNSS)is a commonly employed technique in space geodesy that provides a large amount of data for researches about geodesy and geodynamics.The GNSS coordinate time series not only contains tectonic deformation but also non-tectonic deformation,which is primarily exhibited in the vertical time series.These nontectonic deformations are mainly caused by the environmental loading and thermal expansion effects,which can lead to significant errors in estimating velocity.In addition,the noise in GNSS time series is also unfavorable for estimating station motion parameters.Therefore,investigating the response of GNSS vertical coordinate time series to thermal expansion and environmental loading and removing noise from the time series can lead to an accurate estimation of station motion.In this study,the vertical coordinate time series of 223 GNSS stations,provided by the Crustal Movement Observation Network of China(CMONOC),are analyzed to investigate the response of GNSS vertical coordinate time series to thermal expansion and environmental loading using the processing and analysis methods.In addition,the denoising methods are also introduced to improve time series analysis accuracy.The main research contents and conclusions are:(1)This paper first calculated the displacement time series of the thermal expansion effect and environmental loading on 223 GNSS stations,which are based on the temperature data and the environmental loading grids provided by the European Medium-term Weather Prediction Center and the German Geoscience Research Center,respectively.Then the magnitude and distribution characteristics of the thermal expansion effect and environmental loading are analyzed.The results suggest that the average annual amplitudes of the thermal expansion effect,environmental load and the summation of their contributions for 223 GNSS stations are1.125 mm,4.572 mm and 5.172 mm,respectively.The annual amplitude of the thermal expansion effect gradually increases with the increase of latitude of the GNSS sites,and the magnitudes of the environmental loading displacement in the southwest of mainland China are larger than that of other areas.The regional characteristics of the distribution of total impact are similar to those of the environmental loading,while the annual amplitudes of the total impact for most stations are larger than that of the environmental loading due to the thermal expansion effects.(2)We investigate the influences of correcting the thermal expansion effect and environmental load on the optimal noise model of GNSS coordinate time series,using the Bayesian information criterion.After the thermal expansion effect and environmental loading were corrected,the optimal noise models changed for 53 and 54 GNSS stations distributed in the central and southwestern mainland China.The optimal noise models are white noise plus flicker noise for 197 stations after correction of both effects.Considering the optimal noise model and the both two effects,the average differences of the velocity and the velocity uncertainty are 0.547mm/a and 0.759mm/a,respectively.(3)The relationships between the displacement caused by environmental loading,thermal expansion effects and the common-mode error that also includes seasonal signals are analyzed.We found that the correlation between the thermal expansion effect and the common-mode error is weak,while the correlation between the environmental loading and the common-mode error is strong.In addition,this study proposes a denoising method which combines the Complementary Ensemble Empirical Mode Decomposition(CEEMD)and the seasonal signal in GNSS time series.The method,named CEEMD＿T,takes the average period of the intrinsic mode function component obtained from CEEMD as the criterion for screening the noise component.The proposed denoising method not only work well on decreased the flicker noise value,velocity uncertainty and RMS,but also completely preserve the seasonal signal.Based on the optimal noise model and CEEMD＿T,the influences of the thermal expansion effect and environmental loading on seasonal signals in GNSS coordinate time series are analyzed.after correcting the thermal expansion effect and environmental loading for 223 GNSS station from CMONOC,the average correction rates of the annual amplitude are 10.3% and 41.0% for 59.2%and 70.4% of total stations,respectively.After correcting both the thermal expansion effect and environmental loading,the annual amplitudes of GNSS time series decrease decreases by about50%,and the RMS of 133 stations(accounting for 59.6%)are reduced.The average RMS the improved stations drop from 13.982 mm to 12.377 mmwith 13.7% of average rate.