Accuracy Assessment Of Polar Motion And Universal Time (UT1) Observed By VLBI At Sub-Diurnal Periods
Öcal , Mehmet Fikret
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Earth rotation parameters (ERP): polar motion coordinates and Universal Time (UT1) have a significant impact basically on the terrestrial and celestial reference system relations, orbit determination, and deep-space navigation. Very Long Baseline Interferometry (VLBI) is the only space-based geodetic technique that can monitor both Earth rotation parameters and nutation offsets simultaneously. International VLBI Service for Geodesy and Astrometry (IVS) organizes VLBI observations for 1-2 hours or 24 hours called as Intensive or Standard sessions, respectively. Intensive sessions are carried out for UT1 determination only by 2-3 VLBI stations on a daily basis. However, Standard sessions, that are suitable for monitoring Earth rotation parameters, are performed by 5-8 VLBI stations twice a week. IVS suggests a parametrization for UT1 determination from the analysis of Intensive sessions that contain 15-55 observations. In fact, the estimation of the tropospheric delay in short time intervals during the Intensive session analysis is not possible due to few observations. In order to increase the accuracy of UT1 determination from the analysis of the Intensive sessions, troposphere signal delays and troposphere gradients derived from the analysis of the Global Navigation Satellite Systems (GNSS) observations are involved in the analysis of the VLBI Intensive sessions between 2008 and 2018 in this study. Statistical comparisons show that length-of-day (LOD) values obtained from the proposed analysis strategies of this thesis (NewUT1) are 2-3mus/day more accurate than those of IVS standard analysis. NewUT1 series are daily updated at Hacettepe University servers for the use of researchers globally. On the other hand, the major reason for ERP variations at semi-diurnal and diurnal periods is ocean tides. International Earth Rotation and Reference System Service (IERS) recommended a model for predicting these variations derived from geodynamical models nearly twenty-five years ago. However, the International Association for Geodesy (IAG) propounded the necessity for the development of a new model based on the state-of-the-art space/satellite geodetic techniques. For this reason, the other research objective of this thesis is modeling the sub-daily ERP variations due to ocean tides by analyzing the VLBI Standard sessions between 2000 and 2018. For this purpose, time series of ERP are decomposed into sine and cosine functions at tidal periods using least squares (LS), singular value decomposition (SVD), and complex demodulation (CD) solution methods. Estimated tidal amplitudes from different solution methods are compared with those of IERS model and also between each other using the metrics of phasors, Fourier transform, and time series. Amplitudes estimated from least squares method have the best agreement with those of IERS recommended model. Furthermore, tidal amplitude estimation software with a graphical user interface called TIDEST is developed in the MATLAB environment in order to provide easy-use for the estimation of tidal amplitudes from ERP time series within the study.