International Terrestrial Reference Frame

The International Terrestrial Reference Frame ( ITRF ) is the currently valid, centimeter-accurate implementation of the terrestrial reference system ITRS . This global coordinate frame for the earth is the result of numerous international measurement campaigns and an ongoing cooperation between institutions from geodesy , space travel and astronomy .

The ITRF consists of a directory of the three-dimensional Cartesian coordinates and velocities of around 400 high-precision survey points distributed around the world , the positions and plate-tectonic movements of which determine the ITRS numerically. The ITRF points are also precisely integrated into the respective (regional) land survey .

In contrast to geophysical and dynamic earth models , the forces causing continental drift are not taken into account in the ITRF , which is why it is one of the models of plate kinematics .

The European analogue of the ITRF is the European Terrestrial Reference Frame ( ETRF ).

Reference system of the earth's body

In order to be able to determine the position of points on the earth's surface , geodesy and astronomy require a precisely defined coordinate system .

Since neither the origin nor the axes of such a reference frame can be realized and made accessible in nature , it is defined indirectly - through a larger number of fixed points . Their concrete coordinate values turn a theoretically modeled reference system (IT RS ) into a practically usable frame of reference (IT RF ) that fixes the axes of the system.

Its origin is the geocenter , the center of gravity of all masses on earth. It represents the center of gravity for all artificial earth satellites , which is why one can determine its position in relation to the earth's surface from observed satellite orbits.
The Z-axis is the mean rotation axis of the earth (reference pole of the IERS ), the X-axis is perpendicular to the Z-axis and pierces the prime meridian. The Y-axis is in turn perpendicular to the X- and Z-axes and thus completes the right-hand system . At the same time, the X and Y axes span the equatorial plane.

Measurements for ITRF

The determination of the coordinates of the satellite stations and their long-term changes (e.g. due to plate tectonics) is carried out using various methods of satellite geodesy and radio telescopes . In detail these are:

Microwave measurements to the high navigation satellites of the Global Positioning System (GPS),

supplemented by other GNSS methods such as GLONASS and from 2009 also Galileo

the DORIS Doppler radio system

Distance measurements ( Satellite Laser Ranging SLR) to medium- height geodetic satellites

Laser Ranging to the Moon ( Lunar Laser Ranging LLR)

VLBI (radio interference measurement to quasars )

and their connection with the astrometric system of the currently most accurate star catalog FK6 .

Viewed from a different perspective, one could also say:

the coordinate frame ITRF is the directory of coordinates while
the ITRS reference system describes the geometry and dynamics of the earth's body. It contains the theory and the associated constants .

ITRF97 and ITRF2005

The earth is not a completely rigid body, but rather deformable. For example, it gives in to the tidal forces that the moon exerts on it by about half a meter. In addition, plate tectonics leads to constant shifting of the continents by one to ten centimeters per year.

These shifts in the earth's crust could only be detected indirectly until about 10 years ago. Now the accuracy of the geoscientific measurements has increased so much that they can be determined within a year.
It is therefore necessary to continuously improve the terrestrial reference system through measurements and further development of the models. Since the 1990s , the most precise measurement methods have been combined in some cases into “annual solutions” - in international cooperation between institutions of the International Union for Geodesy and Geophysics (IUGG) and the IERS. The largest regular contribution from Europe to IERS and ITRF are the VLBI and GPS measurements of the German Federal Agency for Cartography and Geodesy (BKG).

The rotation data of these annual solutions have meanwhile (compared to the system of quasars) reached centimeter accuracy , over a few years even 3 mm or 0.0001 ". The continental drift is taken into account by special methods of higher mathematics . The earth models calculated in this way receive a year:

ITRF97 is a particularly precise solution due to various projects and has been used for several years to compare different models.
ITRF2005 is currently the most accurate model and is the reference system for earth deformations.

literature

Z. Altamimi, P. Sillard, C. Boucher (2002): ITRF2000: A new release of the International Terrestrial Reference Frame for earth science applications , Journal of Geophysical Research, Vol. 107, B10, doi : 10.1029 / 2001JB000561

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