An analysis of the polar motion series from VLBI observations

From the reduction of 2893 globally distributed astrometric and geodetic VL BI sessions from August 1979 to the end of 1998, coordinates of 722 radio s ources at J2000.0, coordinates and velocities of 128 stations at J1997.0 an d about 20 years Earth Orientation Parameters were estimated. From the anal ysis of the resultant polar motion series, the following are demonstrated: (i) During the VLBI data span the Markowitz wobble is not exhibited. (ii) T he amplitudes of both annual and Chandler wobble show temporal variations, with the former being more obvious than the latter. (iii) Wavelet analysis shows that all the signals in the polar motion series are characterized by temporal variation in amplitudes. If we take any signal as strictly periodi c, it is impossible to remove it completely from the polar motion series by least-squares fit because the hypothesis of a constant amplitude conflicts with VLBI measurements. (iv) By applying a filter, the secular polar motio n was found to be (2.74 +/- 0.01) mas/a towards (83.9 +/- 0.3)degrees W lon gitude, which is smaller in rate and more westward in direction compared wi th those determined from optical observations or the combination of optical and space geodetic observations.