Analysis of 25 Years of Polar Motion Derived from the DORIS Space Geodetic Technique Using FFT and SSA Methods

Polar motion (PM) has a close relation to the Earth’s structure and composition, seasonal changes of the atmosphere and oceans, storage of waters, etc. As one of the four major space geodetic techniques, doppler orbitography and radiopositioning integrated by satellite (DORIS) is a mature technique that can monitor PM through precise ground station positioning. There are few articles that have analyzed the PM series derived by the DORIS solution in detail. The aim of this research was to assess the PM time-series based on the DORIS solution, to better capture the time-series. In this paper, Fourier fast transform (FFT) and singular spectrum analysis (SSA) were applied to analyze the 25 years of PM time-series solved by DORIS observation from January 1993 to January 2018, then accurately separate the trend terms and periodic signals, and finally precisely reconstruct the main components. To evaluate the PM time-series derived from DORIS, they were compared with those obtained from EOP 14 C04 (IAU2000). The results showed that the RMSs of the differences in PM between them were 1.594 mas and 1.465 mas in the X and Y directions, respectively. Spectrum analysis using FFT showed that the period of annual wobble was 0.998 years and that of the Chandler wobble was 1.181 years. During the SSA process, after singular value decomposition (SVD), the time-series was reconstructed using the eigenvalues and corresponding eigenvectors, and the results indicated that the trend term, annual wobble, and Chandler wobble components were accurately decomposed and reconstructed, and the component reconstruction results had a precision of 3.858 and 2.387 mas in the X and Y directions, respectively. In addition, the tests also gave reasonable explanations of the phenomena of peaks of differences between the PM parameters derived from DORIS and EOP 14 C04, trend terms, the Chandler wobble, and other signals detected by the SSA and FFT. This research will help the assessment and explanation of PM time-series and will offer a good method for the prediction of pole shifts.

The status of Chandler wobble

<p>The Chandler wobble (CW) and Annual wobble (AW) are the main components of the Earth’s Polar motion, which play an important role in our understanding of their excitations. The Fourier Basis Pursuit Band-Pass Filtering (FBPBPF) method, which can effectively suppress the edge effect, are applied to extract the CW and AW in Earth's polar motion during 1900-2016. Through analyze the variation of CW extracted by the FBPBPF method, we find that the amplitude of the CW has been diminishing since 1995. However, the amplitude of the CW had stopped decline in the last year, and start to increase at now.</p>

Analysis of Long Time Series of Polar Motion

Two long time series of polar motion were analysed with respect to a linear drift, decadal variations, Chandler wobble and annual wobble: the C01 series published by the International Earth Rotation Service (IERS) and the pole series which J. Vondrák, obtained by re-analysis of the classical astronomical observations using the HIPPARCOS reference frame (1899.7–1992.0). By a least-squares fit the linear drift of the pole, usually called ‘secular polar motion,’ was determined to 3.31 milliarcseconds/year (mas/yr) toward 76.1° West longitude. For this fit the a priori correlations within each pair of pole coordinates were taken into account, and the weighting function was calculated by estimation of empirical variance components. The decadal variations of the pole path were determined by Fourier analysis. Using a sliding window analysis, the variability of the periods, the amplitudes and the phases of the Chandler wobble and annual wobble was investigated. The variances of the results and the number of iterations needed to get a convergence in the nonlinear approach show that the new time series by Vondrák is more homogeneous and consistent than the IERS C01 series.