Geodetic data assimilation and estimation of references for climate change investigation – GEODESIE

Our goal is to process all the data available since the early eighties, with a specific data assimilation/combination software, to compute simultaneously and consistently all the geodetic references. We also propose to assimilate the data of the space-geodetic techniques (Doppler Orbitography and Radio-positioning Integrated on Satellite, Global Navigation Satellite Systems – GPS, GLONASS, and Galileo, Lunar/Satellite Laser Ranging, and Very Long Baseline Interferometry) with a common physical modeling and an unified parameter setting. This aims to question the contribution of each technique to the accuracy and stability of the global geodetic frames and to better understand and mitigate all the error sources in the data processing. The quality of the references will be assessed with independent data and geophysical models. We will compute time series of geocentric sea levels from altimeter data and tide gauge records with these references (orbits of satellite altimeters, terrestrial reference frames, etc.).

We assess the level of achievement of the project at 50 %, with important scientific results and technical contributions for different possible leverages of improvement for the geodetic references: assessment of the impact of a possible future new Geodetic global observatory in Tahiti and of a possible future co-location site in space, state-of-the-art processing of all the space-geodetic data and evolution model design, thorough study of modeling links between technique observations, etc. All of this will allow the team to continue the project to its end, on solid foundations.

The GEODESIE project is a project of finalized research, coordinated by the Institut national de l’information géographique et forestière (IGN). It associates also the Centre national d’études spatiales (CNES), Géosciences Environnement Toulouse (GET), the Observatoire de Paris (SYRTE and IMCCE) and the Université de la Rochelle (UMR LIENSs). The project started in March 2017 and lasted 60 months. It will continue, in another framework, for at least 60 months.

We published/submitted five papers in peer-reviewed journals, about innovating topics such as the consideration of spatio-temporal correlations between time series of GNSS station positions and simulations for future new ground instruments or satellites. This project resulted in 19 communications in international and French meetings. The project team was also significantly involved in the processing of the latest version of the global geodetic reference frame, namely ITRF2020.

Many major indicators of climate change are monitored with space observations (sea level rise from satellite altimetry, ice melting from dedicated satellites, etc.). This monitoring is highly dependent on references (positions and velocities of ground observing instruments, orbits of satellites, etc.) that only geodesy can provide. The current accuracy of these references does not permit to fully support the challenges that the constantly evolving Earth system gives rise to, and can consequently limit the accuracy of these indicators. For this reason, in the framework of the Global Geodetic Observing System (GGOS), stringent requirements are fixed to the International Terrestrial Reference Frame (ITRF) for the next decade: an accuracy at the level of 1 mm and a stability at the level of 0.1 mm/yr. This means an improvement of the current quality of ITRF by a factor of 5-10.
Improving the quality of the geodetic references is an issue which requires a thorough reassessment of the methodologies involved. The most relevant and promising method to improve this quality is the direct combination (Combination at Observation Level – COL) of the space-geodetic measurements used to compute the official references of the International Earth Rotation and Reference Systems Service. The GEODESIE project aims at (i) determining highly-accurate global and consistent references (time series of Terrestrial Reference Frames and Celestial Reference Frames, of Earth's Orientation Parameters, and orbits of Earth's observation satellites) with a beyond state-of-the-art space-geodetic data assimilation, and (ii) providing the geophysical and climate research communities with these references, for a better estimation of geocentric sea level rise, ice mass balance and on-going climate changes. Time series of sea levels computed from altimetric data and tide gauge records with these references (orbits of satellite altimeters, Terrestrial Reference Frames and related vertical velocities of stations) will also be provided.
We propose to process all the data available since the advent of space geodesy, with a specific data assimilation software, to compute simultaneously and consistently all the geodetic references and products. We also propose to use all the possible links between the space-geodetic techniques (Doppler Orbitography and Radiopositioning Integrated on Satellite – DORIS, Global Navigation Satellite Systems – GNSS, Lunar/Satellite Laser Ranging – LLR/SLR, and Very Long Baseline Interferometry – VLBI) and to question the contribution of each technique to the accuracy and stability of the Terrestrial Reference Frames. The quality of the references will be assessed with independent data and geophysical models.
The geodetic references will be essential bases for Earth's observation and monitoring to support the challenges of the century. They will be updated operationally after the project. The geocentric time series of sea levels will permit to better apprehend (i) the drivers of the global mean sea level rise and of regional variations of sea level and (ii) the contribution of the global climate change induced by anthropogenic greenhouse gases emissions to these drivers. All the results and computation and quality assessment reports will be available on a Website designed and opened at the beginning of the project.
This project will be an unprecedented opportunity to provide the team involved (and, by extension, the French Groupe de Recherche de Géodésie Spatiale – GRGS) with complete simulation and data assimilation capabilities to:
(i) strengthen its position as an international leader expert on COL, (ii) prepare the future arrival of space missions such as the Geodetic Reference Antenna in SPace – GRASP, and (iii) significantly contribute to the GGOS with accurate references estimated with data assimilation.