January 21, 2016.
By Zuheir Altamimi,
IGN France, IAG Vice-President
The new version of the International Terrestrial Reference Frame (ITRF) was released in January 21, 2016, and is available at the dedicated website: http://itrf.ign.fr/ITRF_solutions/2014/
As recognized by the United Nations General Assembly resolution on the Global Geodetic Reference Frame for Sustainable Development (http://www.unggrf.org/
), the ITRF (http://itrf.ign.fr/
) is fundamental to various positioning (location-based), satellite orbit determination and Earth science applications. It is adopted and used by the international geodetic community, at the global and regional levels, as the standard and most accurate terrestrial reference frame available today. Its parent system (i.e. mathematical and physical formulation), namely the International Terrestrial Reference System (ITRS), is formally adopted and recommended by the International Union of Geodesy and Geophysics (IUGG) for all Earth science applications.
The ITRF, developed and maintained by the International Earth Rotation and Reference Systems Service (IERS - http://www.iers.org
), a Service of the International Association of Geodesy (IAG), since the past 30 years. Its construction is based on a rigorous combination of geodetic products (temporal station coordinates and Earth Orientation Parameters EOPs), provided by the main four space geodetic techniques, through their co-located measuring instruments at a certain number of core sites, called co-location sites. The space geodetic techniques that provide measurements for the computation of the ITRF include the Global Navigation Satellite Systems (GNSS), Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI), and Doppler Orbitography Radiopositioning Integrated by Satellite (DORIS). These space geodetic techniques are organized as scientific services under the umbrella of the IAG: the International GNSS Service, formerly the International GPS Service (IGS http://igs.org/
), the International VLBI Service (IVS http://ivscc.gsfc.nasa.gov
), the International Laser Ranging Service (ILRS http://ilrs.gsfc.nasa.gov
) and the International DORIS Service (IDS http://ids.cls.fr
The ITRF2014 solution includes 1499 stations, located at 975 sites, about 10% of which are co-located with 2, 3, or 4 distinct space geodetic instruments as illustrated by Figure 1. The ITRF2014 input data comprises (1) the IAG technique service solutions provided in the form of daily or weekly time series of station positions and daily EOPs (Polar Motion from satellite techniques and VLBI, and Universal Time and Length of Day from VLBI only), as detailed in Table 1, and (2) 239 local tie vectors provided by the owners of the co-located instruments, in 139 SINEX files with full variance-covariance information.
The Earth is deforming, subject to multiple geophysical solicitations, such as tectonic plate motion, Earthquakes and loading effects generated by the atmosphere, ocean circulation and terrestrial hydrology. While tectonic plate motion will be an additional product of ITRF2014 that is coming soon, Post-seismic deformation and loading effects have to be considered during the ITRF2014 processing in order to improve the accuracy and precision of the frame.
Table 1. Summary of ITRF2014 space geodesy input data
|Service/Technique||Number of Solutions||Time span|
|IGS/GNSS/GPS||7714 daily||1994.0 2015.1||IVS/VLBI||5328 daily||1980.0 2015.0|
|IDS/DORIS||1140 weekly||1993.0 2015.0|
Two innovations were then introduced in the ITRF2014 processing, namely:
- To account for the loading seasonal effects, annual and semi-annual terms were estimated, using a sinusoidal function, for stations with sufficient time-spans of the 4 techniques during the stacking processes of the corresponding time series;
- Post-Seismic Deformation (PSD) models, using logarithmic or/and exponential functions, were determined by fitting GNSS/GPS data at major GNSS/GPS Earthquake sites. The PSD models were then applied to the 3 other techniques at Earthquake Co-location sites.
The main benefit of estimating seasonal signals and post-seismic deformations is to be able to infer a robust and accurate velocity field of the ITRF2014 sites, as illustrated by Figure 2.