IERS Terrestrial Reference System

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The IERS Terrestrial Reference System or (simplified) International Terrestrial Reference System , abbreviated ITRS , is the internationally agreed, earth-fixed, worldwide reference system of terrestrial Cartesian coordinates that form the IERS Terrestrial Reference Frame (ITRF) .

It is the result of an ongoing cooperation between institutions from geodesy , space travel and astronomy and an important part of the International Earth Rotation and Reference Systems ( IERS ) service.

Definition of the reference system

Its origin is the geocenter (center of mass of the earth), whose position to the earth's surface results from the orbits of artificial earth satellites . The Z-axis is the mean rotation axis of the earth (reference pole of the IERS ), the X-axis falls in the 0 ° meridian plane ( Greenwich ), which is determined by coordinates of reference stations (see below, ITRF). The Y-axis completes the tripod of the legal system.

Since the origin and the axes of a coordinate system in nature cannot generally be materially realized and made accessible, they are defined by a more or less large number of fixed or fixed points . The system is then implemented indirectly through the coordinates of these fixed points, and the reference system becomes a reference frame made up of concrete numerical values. So z. B. the (resting) inertial system of astronomy ( International Celestial Reference System , ICRS ) realized by several hundred quasars and their connection with the star coordinate system of the FK5 .
The "system earth " rotates within this inertial system and is defined - like a large, precise top - by its top axis and its rotation . For this purpose, however, "points on the top" itself - that is, on the earth's body - must be defined:

Realization through measurements and ITRF

This specific form of the ITRS is called ITRF (F for frame , "coordinate frame"). ITRF currently consists of around 400 high-precision surveying points (partly at observatories and fundamental stations ) distributed around the world , whose positions and plate tectonic movements are numerically determined by the ITRS. The ITRF points are also precisely integrated into the respective (regional) land survey .

The coordinates and their long-term changes are determined

Viewed from a different perspective, one could also say:

Annual solutions of the ITRF

The earth is not a rigid body, but gives in to external and internal forces to a certain extent. The earth already "breathes" about half a meter due to the tides caused by the moon . Influences of plate tectonics lead to permanent shifts of the continents by 1–10  cm per year, while the accuracy of geoscientific measurements is increasing continuously.

It is therefore necessary to improve the reference system through ongoing measurements and through 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 IUGG institutions and the IERS earth rotation service. The largest regular contribution from Europe to IERS / ITRF / ICRF are the VLBI and GPS measurements of the BKG .

The rotation data of these annual solutions have meanwhile (compared with the system of quasars) a precision of <1 cm, over a few years even 3 mm or 0.0001 ″. The continental drift is taken into account by special mathematical conditions (e.g. no net translation / rotation ). These model solutions are given a year; a solution that was particularly precise through various projects was ITRF97, which was used to compare different models. The ITRF2000 is currently used for such purposes (and in the future probably 2005).

ITRF 2005

There are some improvements and innovations compared to the ITRF2000. On the one hand, it contains data up to the end of 2005, so that newer stations could also be taken into account, or old stations received significantly improved coordinates. In addition, for the first time, non-linear station movements were taken into account by estimating a new coordinate for these stations, for example after earthquakes.

See also

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