6 Transformation to ETRS89

The final combination was calculated in the International Terrestrial Reference System (ITRS), which requires velocities to express coordinates in different epochs. Resulting coordinates from our combination had to be transformed into the European Terrestrial Reference System (ETRS89) in order to minimize coordinate changes in time. The latest ETRS89 realization based on ITRF2005 is maintained via the cumulative EPN solution (Kenyeres, 2009).

In transformation we followed the procedure described in Memo (Boucher and Altamimi, 2008) using two approaches to transform ITRF2005 coordinates and velocities into the ETRF2000:

• a) Two-step transformation: ITRF2005 ⇒ ITRF2000 ⇒ ETRF2000
• b) Direct transformation ITRF2005 ⇒ ETRF2000 using 14-element transformation (three translations, three rotations and scale in basic epoch and their time changes)

Both transformations approaches agrees on submilimetre level and the difference is caused by rounding errors when reproducing coefficients directly from the Memo. The coefficients are more accurately given at http://itrf.ensg.ign.fr/ITRF_solutions/2005/tp_05-00.php. The EPN web-page service (http://epncb.oma.be/_dataproducts/coord_trans), which are using those more accurate coefficients, was finally used in the transformation.