WG 1.2.1: Assessing impacts of loading on Reference Frame realizations

Chair: Anthony Mémin (Université Nice, France)
Vice-chair: Anna Klos (Military University of Technology, Poland)

Terms of Reference

Non-tidal loading (NTL) deforms the Earth's surface adding variability to the coordinates of geodetic sites. The effects of NTL are already observed in geodetic time series from VLBI, SLR, DORIS and GNSS techniques. They occur in a wide range of period, from sub-daily to centennial time scale. They also have an impact on crustal velocity estimates and as a consequence on the realization of the terrestrial reference frame. It has been shown that unconsidered NTL effects can bias estimates of geodetic vertical velocity by 0.5 mm/yr over the continent to more than 1 mm/yr in the southern tropical regions between 1993 and 2014 (Santamaría-Gómez and Mémin 2015). It is five to more than ten times larger than the requirement of the Global Geodetic Observing System on interannual to secular time scales and about one-third of the current rate of sea level rise. Geodetic techniques require accurate global circulation models to allow precise estimation of the Earth's surface displacements to reduce the variability of position time series, in addition to the corresponding time-variable gravity field affecting the orbits of artificial satellites. Correcting for NTL at the observation level reduced for example the variability of GNSS time series by up to 7 mm (Männel et al. 2019).  According to the 2010 IERS conventions, there are currently no recommended surface-mass change models (atmosphere, ocean circulation, ocean response to atmospheric changes, hydrology, past- and present-day ice-mass, sea level) nor Earth models (1D vs 3D, elastic, visco-elastic, rheology, coastline definition) to account for NTL deformation in geodetic position time series. Hence, a better understanding of NTL contribution to geodetic time series is required. Also, several studies have already shown that a posteriori corrections slightly decrease the variance factor of a Terrestrial Reference Frame (TRF) multi-technique combination but the improvement at some sites was also counterbalanced by degradation at others. The accuracy and precision of current space geodetic techniques are such that several scientific studies have already considered atmospheric loading corrections at the observation level. However, there still exist open questions regarding the application of loading corrections for the generation of operational geodetic products, either a priori or a posteriori.

References:

Santamaría-Gómez A, Mémin A (2015) Geodetic secular velocity errors due to interannual surface loading deformation. Geophys J Int Exp Lett 202 (2):763–767, doi:10.1093/gji/ggv190.

Männel B, Dobslaw H, Dill R, Glaser S, Balidakis K, Thomas M, Schuh H (2019) Correcting surface loading at the observation level: impact on global GNSS and VLBI station networks, J Geod, doi.org/10.1007/s00190-019-01298-y.

Objective

The principal objectives of the scientific work are to assess the effects of load and Earth models and their applications for TRF utilization and to assemble specific recommendations for users and future IERS conventions.

Program of Activities

 ·         Create and maintain an updated list of loading studies: models and observations.

·         Compare and assess differences between existing load models.

·         Assess ice and sea level change loading deformation.

·         Assess the propagation of loading model errors and differences in using several Earth models into the site coordinates, TRF parameters and the ITRF.

·         Determine whether load models should be applied a priori or a posteriori.

·         Organize meetings during international conferences (EGU, AGU…).

·         Suggest recommendations for IERS conventions.

List of members (not yet finalized)

Jean-Paul Boy (France)

Kristel Chanard (France)

Benjamin Maennel (Germany)

Anthony Mémin (France), Chair

Laurent Métivier (France)

Manuela Seitz (Germany)

Giorgio Spada (Italy)

Daniela Thaller (Germany)

Wouter van der Wal (The Netherlands)

Corresponding members

Christopher Kotsakis (Greece)