The International global Terrestrial Reference System (ITRS) is the basis for describing the figure of the Earth and for referencing processes at the Earth's surface, in the Earth's immediate space environment and in the Earth's interior. The ITRS is realized by positions and velocities of space-geodetic observing stations, which are derived from a combination of the space geodetic techniques Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Global Satellite Navigation Systems, e.g. GPS, and the Doppler measurement system DORIS (see Figure). The techniques provide individual strengths with respect to the determination of geodetic parameters - in particular the parameters of the geodetic datum. The different potentials of the techniques can be exploited in an optimal way in the combination.
The computation of the reference frame is performed as shown in the figure. In a first step the time series of normal equation systems of the individual techniques are analyzed and combined to one normal equation system per technique. In a second step the normal equation systems of the different techniques are combined.
- Flowchart of the ITRS realization at DGFI
The International Terrestrial Reference System (ITRS) is a conventional global reference system. Its realization is the International Terrestrial Reference Frame (ITRF). The International Earth Rotation and Reference Systems Service (IERS -> www.iers.org) is responsible for the computation of the ITRF, which is performed every 3-5 years. The DGFI as one of the ITRS Combination Centres is in charge with the computation of the ITRF. The latest realization is the DTRF2008.
The DTRF2008 (Seitz et al., 2012a) is a reference frame of high precision and long-term stability. The major goal of the current research activities of the ITRS Combination Center at DGFI is to further improve its accuracy, stability, consistency and actuality.
Simultaneous determination of TRF, CRF und EOP
Besides the listed research topics the common realization of the ITRS and the International Celestial Reference System (ICRS) is a further central task. Consistently to both reference frames time series of Earth Orientation Parameters (EOP) - which allow for a transformation between the frames - are determined. Like the ITRS is realized by positions and velocities of stations, the ICRS is realized by positions of radio sources (QUASARs or radio galaxies), which are observed by VLBI.
A simultaneous determination of the reference frames and the linking EOP guarantees for the consistency of the products. So far, ITRS and ICRS are realized independently at different computation centres. For the first time, DGFI realizes the ITRS and the ICRS together with the Earth Orientation Parameters - which allow for a transformation between both systems - consistently in one adjustment.
In our computation, we considered data of Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR) and Global Navigation Satellite Systems (GNSS). The parameter considered in the realization are station coordinates, quasar coordinates and the EOP, i.e. the coordinates of the terrestrial and the celestial pole (polar motion and nutation), UT1-UTC and their derivation in time. Altogether about 45,000 parameters are solved. In order to realize the orientation of the frames only non-deforming minimum conditions are applied.
Seitz M., Angermann D., Bloßfeld M., Drewes H., Gerstl M.
: The 2008 DGFI Realization of the ITRS: DTRF2008.
Journal of Geodesy, Volume 86, Issue 12, pp 1097-1123 , DOI:10.1007/s00190-012-0567-2
Seitz M., Heinkelmann R., Steigenberger P., Artz T.: Common Realization of Terrestrial and Celestial Reference Frame. In: Alef W., Bernhard S. and Nothnagel A. (Eds.) Proceedings of the 20th Meeting of the European VLBI Group for Geodesy and Astronomy, Schriftenreihe des Instituts fÃ¼r GeodÃ¤sie und Geoinformation der UniversitÃ¤t Bonn, 22, 2011
Angermann D., Müller H.: On the strength of SLR observations to realize the scale and origin of the terrestrial reference system. In: Sideris, M.G. (Ed.): Observing our Changing Earth, IAG Symposia, Vol. 133, 21-27, 2009
Angermann D., Drewes H., Gerstl M., Krügel M., Meisel B.: DGFI combination methodology for ITRF2005 computation. In: Drewes, H. (Ed.): Geodetic Reference Frames, IAG Symposia, Vol. 134, 11-16, 2009
Seitz M.: Kombination geodÃ¤tischer Raumbeobachtungsverfahren zur Realisierung eines terrestrischen Referenzsystems. DGK Reihe C 630, 2009
Angermann D., Drewes H., Krügel M., Meisel B.: Advances in terrestrial reference frame computations. IAG Symposia, Vol. 130, 595-602, Springer, 2007