The most challenging scientific and non-scientific requirements concerning monitoring of displacements with respect to a terrestrial geodetic reference frame do not only demand increasing accuracy and temporal stability of the reference frame, but also high spatial and temporal resolution of this frame as well as low latency of the determination of displacements. Some of these requirements are related to geohazards and in particular early warning. In order to meet these requirements, it is increasingly important to be able to predict reference trajectories for all points on the Earth surface, against which "anomalous motion" can be detected. Such motion is expected prior to volcanic eruptions, land- and rockslides, as well as the failure of infrastructure such as oil platforms, bridges, and reservoir dams.
Current global geodetic reference frames such as the International Terrestrial Reference Frame (ITRF) consist of a set of globally distributed reference points with coordinates given at a reference epoch and associated constant velocity vectors describing the temporal deformation of the secular reference polyhedron through so-called regularized coordinates. The reference points implicitly determine the axes, the Reference Frame Origin (RFO), and the scale of the underlying reference system. In addition to this secular polyhedron, the frame also includes a set of a priori models that describe deviations of the actual motion of the Earth's surface from the secular polyhedron. Since the Earth's secular veolocity field is not well known with high spatial resolution, this approach does not allow us to assign predicted (expected) reference coordinates to any point on the Earth's surface different from the reference points themselves. Thus, the predictive capability of the current reference frame approach is rather limited. However, with the help of precise satellite orbits and clocks, precise point coordinates can be determined in the frame defined by the polyhedron. Precise orbits and clocks are determined on a daily basis in a free solution which is then aligned to the reference frame. The methodology used for this alignment as well as the mathematical model for the reference point motion applied in the space-geodetic analyses determines the degree to which geophysical signals are filtered and potentially aliased into the displacement time series.
The current simple mathematical model of regularized coordinates has three major problem: (1) The actual motion of the reference points is not linear in general. Deviations from linear motion are due to tides, surface loading, and processes in the solid Earth, including pre-, co-, and postseismic displacements. For large earthquakes, the latter can be of regional to global nature. Currently, only tides are taken into account. (2) The methodology used for the alignment of solutions to the GRF as well as the mathematical model for the reference point motion constitute a technique- and solution-dependent filtering of unaccounted geophysical signals, which alias these signals into displacement time series and hampers comparison of observations to model predictions and between techniques. (3) The velocity vectors have errors, which over time can deform the polyhedron considerably, thus requiring frequent updates of the ITRF.
One way to improve the predicability of the reference frame would be the development of a Dynamic Reference Earth Model (DREM, Herring et al., 2007), which predicts the motion of points on the Earth based on a dynamic Earth system model assimilating geodetic and geophysical observations. The model will have to account for all geophysical processes known accurately enough to be modeled with a predefined target accuracy derived from the user requirements. On the one hand, this dynamic system model will require consistent assimilation of geodetic observations of surface displacements, gravity field changes and Earth's rotation perturbations. On the other hand, the model will provide a reference frame against which displacements can be determined that are to a large extent decontaminated for known geophysical processes. Ultimately, the DREM will allow a consistent integration of point and image geodesy on the basis of an Earth system model with high spatial and temporal resolution. The DREM will be particularly useful for applications such as geohazards, monitoring of infrastructure, off-shore activities, and studies of processes in deformation zones not yet modeled by the DREM. The presentation will discuss the steps towards the development of a DREM and address the challenges to be met in terms of integration of point and image observations, data assimilation and necessary extensions of the theory describing deformations of the solid Earth due to surfaces and body forces.
Herring, T.A., Altamimi, Z., Plag, H.-P., Poli, P., & Ray, J., 2007. The future geodetic reference frame, in The Global Geodetic Observing System: Meeting the Requirements of a Global Society on a Changing Planet in 2020 - The Reference Document, edited by H.-P. Plag & M. Pearlman, Global Geodetic Observing System, available at http://geodesy.unr.edu/ggos/ggos2020/.