EPOS-GNSS, Double-Difference Processing

 

Context

EPOS
EPOS (European Plate Observing System) is a multidisciplinary research infrastructure that produces data and services for the Earth Science research community.

EPOS-GNSS
The EPOS-GNSS or ‘GNSS Data and Products’ group is an EPOS component that brings together 12 partners across Europe and aims to archive and disseminate GNSS (Global Navigation Satellite System) data from permanent stations throughout Europe (using the RINEX international exchange format), software tools, as well as derived products (daily solutions, position time series, velocity fields, and deformation fields).

EPOS in Operational Phase
EPOS began with an Implementation Phase (EPOS-IP, 2015–2018). At that time, a prototype solution was produced by OSUG/ISTerre. After a Pilot Operational Phase, EPOS entered its Operational Phase in 2023.

GNSS Products over Europe

Description of the solution
The OSUG/ISTerre processing centre first generates position time series using the MIT GAMIT-GLOBK tool. The solutions are calculated using the double-difference approach. A complete reprocessing is carried out regularly to integrate stations newly distributed via EPOS, and daily automatic processing is performed to keep the time series up to date.

Next, the time series are analysed to estimate offsets (position changes due to a change in the instruments used or a high-magnitude earthquake close to the site). These offsets are available on the EPOS-GNSS product portal, in Data Controls / Offsets.

Finally, the time series are analysed to estimate secular GNSS velocities.

The latest solution generated in 2024 comprises 1,510 stations across Europe and includes data covering a 24-year period (2000-2023 included). The database consists of all stations with data distributed via the EPOS-GNSS data portal: the stations are located in 39 European countries.

Returning to the estimation of velocities, after technical validation, the MIDAS software emerged as an effective, fast and most appropriate method. This software is an automatic estimator of time series trends (Blewitt et al., 2016). Using a statistical method based on a set of pairs of positions separated by one year, it provides GNSS velocities that are highly insensitive to outliers, jumps and seasonal signals.

Horizontal and Vertical Velocity Maps

Horizontal velocities relative to the Eurasian plate. This velocity field is composed of 1510 stations that meet the quality criteria defined in the text below.

Vertical Velocity Map (Focus on the Alps and the Apennines)

Three categories of robustness for MIDAS velocities have been defined. Robustness is quantified through two MIDAS control points:

  • The maximum number of outliers that can be tolerated.
  • The number of offsets (changes in position from one day to the next, mainly due to changes in instruments) wrt to duration, in order to maintain a reliable estimate of the velocity.

( For more information on MIDAS and the two MIDAS control points, see Blewitt et al., 2016).

We exclude from our analysis the stations for which the MIDAS control points are not satisfied. In addition, we exclude stations for which the error estimated by MIDAS exceeds the following values:

  • Horizontal error > 0.5 mm/yr
  • Vertical error > 1.5 mm/yr

This filtering mainly concerns sites for which the time series is too short to accurately constrain the velocity estimate.
As a result, the map above includes velocities for 1510 stations, with 504 stations excluded.

Obtain the solutions (i.e., time series and velocity field)
The solutions (position time series and velocity field) are available via the products portal EPOS-GNSS.
Time series are also available on the FormaTerre ‘Geodesy Plotter’ portal (UGA_Epos-Fr_GG solution).

European Partners
UBI (Portugal), ROB (Belgium), OCA (France), CNRS (France), INGV (Italy), SGO (Hungary), BKG (Germany), LM (Sweden), WUT (Poland), LRU (France), NOA (Greece), OGS (Italy).

ISTerre-OSUG team
Anne SOCQUET, Observatory Physicist
Andrea WALPERSDORF, Observatory Physicist
Gaël JANEX, Research Engineer

Contact
Anne SOCQUET

Funding
European Research Infrastructure EPOS

Links
EPOS
EPOS-France
RENAG