One of the fundamental goals of space geodesy is to quantify changes in space and time of the Earth’s surface, and consequently to determine point positions as accurately as possible. Meanwhile, point positions are neither observable nor absolute quantities, and so have to be determined with respect to some reference. We refer to ‘‘terrestrial reference system’’ (TRS) as the mathematical object satisfying an ideal definition and in which point positions will be expressed. However, the access to point positions requires some observational means allowing their link to the mathematical object. We therefore call a ‘‘terrestrial reference frame’’ (TRF), a physical materialization of the TRS, making use of observations derived from space geodetic techniques, and to which the users have access through station coordinates as a function of time. The distinction between ‘‘system’’ and ‘‘frame’’ is then subtle since the former is rather invariable and inaccessible while the latter is accessible and perfectible. The main physical and mathematical properties (or defining parameters) of a TRS (at the theoretical level) or of a TRF (at the realization level) are the origin, the scale, the orientation and their time evolution. How is the Earth deforming due to, e.g., plate tectonics, co-seismic and post-seismic deformations, global geophysical fluid dynamics, or current ice melting? How to accurately determine point positions at the Earth surface that is constantly deforming? What is the rate of sea level rise, its spatiotemporal variability, and its impact on climate change? Global geodesy is one of the key Earth sciences that not only measures changes of the Earth system in space and time but also is the only science that provides the indispensable standard against which the changes and their variability are quantified and properly referenced. In order to answer these scientific questions, fundamental to understanding the Earth dynamics, it is critically important to ensure the continuous availability and updates of an accurate, long-term stable and truly global TRF, as a unique standard reference, to ensure inter-operability and consistency of geodetic products and to adequately exploit various measurements collected by ground-based sensors, or via artificial satellites.
The International Terrestrial Reference System (ITRS) and its realization, the International Terrestrial Reference Frame (ITRF) are recommended by a number of international scientific organizations, such as the International Association of Geodesy (IAG) and the International Union of Geodesy and Geophysics (IUGG) for Earth science and operational geodesy applications.
The lectures will recall the basic concepts of a TRS and a Crust-based TRF, including the rank deficiency problem related to the estimability of station coordinates, and so will focus more on the TRF and its possible approaches of representation. It is indeed natural to question how the TRF should be implemented in order to best describe the shape of a constantly deforming Earth’s surface. The types of deformations in question include linear and nonlinear displacements of seasonal and non-seasonal nature, as well as Post-Seismic Deformation (PSD) caused by major earthquakes.
The lectures will recall the principles of the main four space geodetic techniques, their systematic errors and their contribution to the TRF defining parameters and to the ITRF. The four space geodetic techniques are: Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), Global Navigation Satellite Systems (GNSS), Satellite Laser Ranging (SLR), and Very Long Baseline Interferometry (VLBI). Will also be part of the lectures: methods for combination of TRF solutions from a single or several techniques, as well as stacking TRF time series to produce station positions at a reference epoch, velocities and periodic signals.
A significant part of the lectures will detail the ITRF genesis, its development over the years, current achievement in terms of geodetic and geophysical results, but also the strengths and weaknesses of the geodetic infrastructures of the four geodetic techniques contributing to its construction.
The lectures will terminate by presenting and discussing the main results of the latest ITRF release, namely the ITRF2020. The latter is provided in the form of an augmented reference frame so that in addition to station positions and velocities, parametric functions for both PSD and seasonal signals (expressed in the Center of Mass frame as sensed by SLR data) are integral parts of the frame.
Future plans for ITRF model updates to meet most demanding user needs conclude the lectures.