Analyses Of Celestial Pole Offsets With VLBI,LLR And Optical Observations

The Earth is not a perfect rigid globe,the existence of its eccentricity,the fluxion and the interactions of its inner parts give rise to the minor motions except for the diurnal self-rotation,including precession,nutation and polar motion.These motions are described with a set of Earth Orientation Parameters?EOP?.Techniques based on radio and laser ranging have been developing since the late 1960s,and thus allow us to measure the changes in the rotation rate and in the orientation of the rotation axis of the Earth.The precession-nutation model currently adopted by International Astronomical Union?IAU?is developed in 2003 and it has been 17 years.In this period of time,theories and observation techniques have both been improved,thus the examination and improvement of the model has become essential.Very Long Baseline Interferometry?VLBI?plays a dominant role in the determination of Earth rotation.However,VLBI has its limitations such as the relatively short history for determining the long-periodic parts,and thus requires external check and complementation from other techniques.This work aims to explore the possibilities of determining the long-period part of the precession-nutation of the Earth with techniques other than VLBI.Lunar laser ranging?LLR?is chosen for its relatively high accuracy and long period.Results of pre-vious studies could be updated using the latest data with generally higher quality,which would also add ten years to the total time span.Historical optical data are also analyzed for their rather long time-coverage to determine whether it is possible to improve the current Earth precession-nutation model.Celestial pole offsets?CPO?series were obtained from VLBI,LLR,and optical observations and were analyzed separately by weighted least-square fits of three em-pirical models,including a quadratic model,a linear term plus an 18.6-year nutation term,and a linear term plus two nutation terms with 18.6-year and 9.3-year periods.We also present discussions on the uncertainties and possible improvements of the LLR-determined CPO series.Joint analyses of VLBI and LLR data is also presented for further discussion.Finally,the state-of-art and perspectives on the nutation determi-nation with VLBI and Global Navigation Satellite System?GNSS?observations are presented as a future plan.We improved the determination of the nutation terms with both VLBI and LLR data.The VLBI data present a most reliable feature of the CPO series with the highest accuracy,and they are most important for determining the precession-nutation of the Earth.The fitting results show an underestimation of the precession rate in d X by about0.3 mas·cy^-1,and an underestimation of the amplitude of the 18.6-yr nutation by about35?as.The standard errors of CPO obtained from the LLR technique have reached a level of several tens of microarcseconds after 2007,but they are probably underesti-mated because the models used in the calculation procedure are not perfect.The results show no evident correction to the precession rate,but a deviation of about-0.3 mas for all the coefficients of the 18.6-yr nutation.Nevertheless,the poor time resolution of LLR CPO series is also a disadvantage.However,this still indicates that LLR has the potential to determine celestial pole offsets with a comparably high accuracy with VLBI in the future and to serve as an independent check for the VLBI results.The current situation of LLR observations is also analyzed to provide suggestions for future improvements.The typical standard error of CPO series from historic optical observa-tions is about two hundred times larger than that of the VLBI series and can therefore hardly contribute to the contemporary precession-nutation theory.