DTRF2014: The new realization of the International Terrestrial Reference System (ITRS) of DGFI-TUM
Since many decades, the realization of global and regional terrestrial reference systems is a key research area of DGFI-TUM. In the framework of these activities the institute intensively networks globally, and it takes a leading position as regards the realization of the fundamental Earth-fixed coordinate system, the International Terrestrial Reference System (ITRS).
A reference frame is the realization of a theoretically defined coordinate system (reference system) in the form of concrete points (markers) attached to the solid Earth crust with precisely determined coordinates. Due to physical processes in the Earth system the points on the Earth surface undergo permanent variations in time (e.g., due to plate tectonics or tidal deformation). Hence, typically, a reference frame consists of mean 3D positions of the stations and their linear changes (motions) over a long time period (e.g., 20-35 years of observations).
Relevance and international cooperation
In its role as an "ITRS Combination Centre" within the International Earth Rotation and Reference Systems Service (IERS), DGFI-TUM took the responsibility for the continuous computation of the International Terrestrial Reference Frame (ITRF). The ITRF is the realization of the fundamental Earth-fixed coordinate system with the best possible accuracy and long-term stability. Such a reference frame is an indispensable requirement for various applications in daily life (e.g., for navigation and positioning, for the realization of height systems and precise time systems or for the computation of spacecraft and satellite orbits). Furthermore, it is the backbone for Earth system research by providing the metrological basis and uniform reference for monitoring processes in the context of global change (e.g., ice melting, sea level rise). To ensure up-to-date results with highest accuracy, new computations of the ITRF with the most recent data must be performed every few years.
The International Association of Geodesy (IAG) with its Global Geodetic Observing System (GGOS) has formulated that the terrestrial reference frame should be accurate at a level of 1 mm with respect to the position of the geodetic stations on the Earth's surface and stable with time at a level of 0.1 mm/yr. Taking into account a large range of impact and error sources (e.g., atmospheric influences on the signals of the geodetic observations, non-linear motions of the stations due to geodynamical processes [e.g., tidal deformations of the Earth's body within a few hours, seasonal variations caused by atmospheric, oceanic and/or hydrological loading effects, thermal deformations of telescopes]) this is a big challenge. The current accuracy of about 5 mm for station positions and about 1 mm/yr for velocities does not meet the requirements formulated above (it is worse by a factor of 5-10). The envisaged accuracy of the ITRF is necessary for a reliable and long-term stable estimation of very small signals of global change phenomena such as the mean global sea level rise which is in the order of a few mm per year. This phenomena cannot be measured with sufficient reliability if the reference frame is not accurate and stable enough over decades. Present-day estimations of the mean sea level rise are in a range between 2.9 and 3.4 mm/yr, which differs by almost 20%! French colleagues reported in a recently published and internationally well-recognized review paper: "The major limitation is the realization of the terrestrial reference frame. Ideally, to be useful in long-term sea level studies, vertical land motion should be determined with standard errors that are 1 order of magnitude lower than the contemporary climate signals of 1-3 mm/yr observed on average in sea level records" (Wöppelmann & Marcos, Reviews of Geophysics, doi:10.1002/2015RG000502, 2016). In cooperation with other institutions present research activities of DGFI-TUM focus on dedicated studies and investigations to further increase the accuracy and stability of the terrestrial reference frame.
The present version of the realization of the ITRS computed at DGFI-TUM is called DTRF2014. Since only three institutions worldwide have the capabilities to perform the ambitious computations and to produce solutions of high quality (IGN in Paris, France, and JPL in Pasadena, USA), there are consequently only three different realizations of the ITRS. However, one of them (JPL's solution called JTRF2014) is not directly comparable since it is based on time series of station positions instead of station positions and velocities and, thus, is conceptually different from the ITRS definition. A second comparable realization has been computed by the IGN, called ITRF2014. DTRF2014 and ITRF2014 result from different combination strategies: while the ITRF2014 is based on the combination of solutions, the DTRF2014 is computed by the combination of normal equations. Two solutions are the absolute minimum concerning redundancy (e.g., in order to detect possible inconsistencies or errors). Thus, the activities of DGFI-TUM in this field are mandatory to ensure the high quality of the ITRS realization and are well recognized in the international geodetic community.
The importance of a highly accurate and long-term stable geodetic reference frame as a fundamental requirement for many different applications (as mentioned above) has also been recognized on the highest political and societal level. On February 26, 2015, the UN General Assembly adopted its first geospatial UN resolution to implement a "Global Geodetic Reference Frame for Sustainable Development (GGRF)" under UN mandate (www.unggrf.org). In this framework, DGFI-TUM contributes to current IAG activities concerning the definition of the GGRF together with partners of the Forschungsgruppe Satellitengeodäsie (FGS).
DTRF2014 – Concept and methodology for the computation
The DTRF2014 consists of 1712 globally distributed observing stations, for which the positions and linear motions (i.e., coordinate changes in three dimensions) were computed.
The observations of four different space geodetic techniques were combined:
- Global Navigation Satellite Systems (GNSS)
- Satellite Laser Ranging (SLR)
- Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS)
- Very Long Baseline Interferometry (VLBI)
The DTRF2014 is based on observations from 1980 until the beginning of 2015 (the year in the DTRF specification indicates the last complete year covered by observations of the four space techniques). In 2015, the main effort was the preprocessing of the technique-specific observations performed by many contributing institutions worldwide, and as a result of this step the DTRF2014 input data were provided by the Technique Services (i.e., International GNSS Service [IGS], International Laser Ranging Service [ILRS], International VLBI Service [IVS] and International DORIS Service [IDS]). The ITRS Combination Centre at DGFI-TUM combined these preprocessed data of the four space techniques to derive a unique and consistent reference frame realization. The challenge, thereby, is to optimally connect and integrate the four techniques in order to fully exploit their individual strengths and sensitivities for different parameters (e.g., realization of the origin of the coordinate system in the Earth's center of mass, the orientation of the coordinate axes, the realization of the network scale, the relation to the celestial reference system realized by quasars, and the estimation of Earth orientation parameters). Essential for this combination are stations where several of the different observation techniques are operated in parallel (so-called co-location sites), such as the Geodetic Observatory Wettzell that is jointly operated by the TUM and the Federal Agency for Cartography and Geodesy (BKG). These co-location sites form the core network of the ITRF and are also called "fundamental stations".
DTRF2014 – Analysis of the signals
Each vector in the displayed figures belongs to an observing station (geodetic marker) on the Earth surface. The different colors represent the four mentioned observation techniques. The arrows indicate the direction of the station motion, the length of the vectors provide the corresponding velocity (mm/yr). The horizontal and vertical station motions can be used for the analysis and interpretation of various geophysical processes.
The largest effect visible in the global velocity field is the drift of the tectonic plates. The motion of several stations located on fast moving plates (e.g., islands on the Pacific plate) show velocities of up to 7-8 cm/yr. A zoom on Europe indicates for most parts relatively stable motions of the reference points, but e.g., in the Aegean region the station motions differ from the stable part of Europe indicating the existence of a deformation zone in that area. Deformation zones are regions of the Earth’s crust which do not belong to a “rigid” tectonic plate (i.e., in this case the Eurasian plate) and, thus, the station motions are different. Iceland shows clearly that the station at the east coast moves toward north-east, whereas the other station at the west coast moves toward north-west. This phenomenon is caused by the fact that the boundary of the North American and Eurasian plate goes directly through Iceland, and that both plates move differently.
The Geodetic Observatory Wettzell, which is co-operated by the TUM, is highlighted with a black star in the figure above.
The region of Greenland, Scandinavia and Canada was covered by large ice sheets during the last glacial period. The load of these ice sheets pushed the Earth crust downward over thousands of years. Although a large part of the former ice sheet has melted already, there is still a reaction of the Earth's crust visible in the geodetic observations ("postglacial uplift"). Uplift rates in these regions are up to several centimeters per year, which is clearly visible in the vertical station motions of the DTRF2014.
Large earthquakes often have an impact on the motion of stations (direction and velocity) located in the region of plate boundaries or deformation zones. This is also visible in the DTRF2014. As shown in the left figure above, all reference stations in Chile moved toward north-east before the earthquake in Maule/Chile on February 27, 2010. The earthquake caused a large co-seismic displacement of about 3 m for stations close to the epicenter. After the earthquake, most stations show a movement toward north-west. This behavior is dominated by a relaxation process as a consequence of the co-seismic displacement of the Earth crust.
After such a large earthquake, the reference frame is unusable in the entire region as positions and velocities of the affected stations are not valid any longer. This is one of the reasons why a continuous update and the computation of new realizations of terrestrial reference systems is necessary to ensure the required accuracy.