GPS-based Sub-Hourly Polar Motion Estimates: Strategies and Applications.

摘要

Since their advent in the late 1970's, satellite geodetic techniques have revolutionized the monitoring of the Earth's orientation in inertial space and significantly contributed to advances in geodynamics. Long- and mid-term features manifesting in the Earth Rotation Parameters power spectra are, in general, accurately determined and explained. In contrast, sub-daily fluctuations affecting polar motion and length-of-day are not known as precisely. The lack of highly-resolved time series is currently a strong limiting factor for gaining more insight into rapid polar motion - the parameter of interest here - and its geophysical implications. This thesis was aimed at customizing an estimation strategy targeting the recovery of polar motion at very high temporal resolution based on GPS ground observations only. To this end, the trade-space existing between the precise determination of the GPS satellite orbits and clocks themselves and the retrieval of the Earth's pole coordinates at sufficient accuracy for geodetic purposes was extensively investigated. The strategy design and underlying rationale are described. Candidate strategies are presented and results obtained from the reanalysis of one year of data are shown and analyzed. The challenges associated with the determination of ultra-rapid polar motion are discussed. In particular, mathematical singularities stemming from various sources are emphasized and handling techniques proposed. Difficulties inherent to the quality assessment of the estimates are stressed and the methodology employed for conducting relevant performance analyses is detailed. The validity of the solutions generated is demonstrated through the recovery of geophysical signals such as the major semidiurnal and diurnal ocean tides and large-scale oceanic and atmospheric circulations. Among other geophysical applications, preliminary results for the detection of so-called megaquakes are shown based on the 2011 M9.0 Honshu earthquake event. Recommendations and guidelines for the determination of polar motion at high frequencies are formulated based on the numerous test cases studied.