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Zusammenfassung:
We present a method to measure geocenter motion in the terrestrial reference frame (TRF) defined by a global GPS orbit and clock product. Access to the geocenter is provided through precise orbit determination (POD) of Low Earth Orbiters (LEO) using onboard GPS tracking data. In our method, the geocenter location parameters are explicitly expressed in the observation equations for the LEO tracking data, while the GPS orbital position and clock parameters are held fixed to the product produced by the IGS Analysis Center at JPL. The geocenter location parameters are estimated on a daily basis and the resulting time series describes the motion of the Earth’s instantaneous center of mass sensed by the LEO satellites orbital motion in IGb14 frame defined by the GPS orbit and clock product. The key to the success of this method is the precise modeling of the perturbing forces on the LEOs. When good quality accelerometer measurements are available, such as for GRACE and GRACE-Follow-On missions, the estimated geocenter motion is robust. When the accelerometer measurements are not available, results from multiple LEOs in different orbital planes can be combined to average down the non-common part of systematic errors such as drag and solar radiation pressure force model errors. We present 19 years (2004-2022) of continuous geocenter motion measured using this method. Two types of solutions are compared and features observed in the time series are discussed.
