“The ITRF serves as a framework and a metrological basis in Earth Science applications: Geophysics, Oceanography, Altimetry, Earth Rotation, Mean Sea level, Positioning, Navigation and geo-referencing.”

For many Earth Science applications as well as for satellite navigation, geodesy—the science of measuring and monitoring the size and shape of the Earth and the location of points on its surface—has the potential to provide a global Terrestrial Reference Frame (TRF) to which valuable observations of parameters that govern the Earth system are related: Earth rotation and its gravity field, tectonic motion, crustal deformation, mean sea level, geocenter motion, etc.

The paper describes the implementation of the 2000 version of the International Terrestrial Reference Frame (ITRF), called ITRF2000. The ITRF is used as the metrological standard against which measurements and parameters of the Earth System are compared. It is based on a combination of individual TRF solutions derived from observations of space geodesy techniques (VLBI, SLR, GPS, DORIS) and determined by the corresponding international services: IVS, ILRS, IGS and IDS.

The ITRF is under the responsibility of the International Earth Rotation and Reference Systems Service (IERS). These international services are part of the International Association of Geodesy (IAG), leading a flag program called Global Geodetic Observing System (GGOS) which is recognized as one of the Global Earth Observing System of Systems (GEOSS) of the Group on Earth Observations (GEO).

A new and improved version of the ITRF, namely the ITRF2005, is now published, providing to the users positions and velocities of all observatories operated by space geodesy techniques. In addition it provides a consistent series of Earth Orientation Parameters spanning 26 years, as well as a velocity field allowing to infer motion and deformation of 15 tectonic plates.

  Does it describe a new discovery or a new methodology that’s useful to others?

The paper describes a rigorous combination methodology, allowing it to gather the strengths of all space geodesy techniques for the realization of a consistent, homogeneous, long-term stable ITRF. The ITRF serves as a framework and a metrological basis in Earth Science applications: geophysics, oceanography, altimetry, Earth rotation, mean sea level, positioning, navigation, and geo-referencing.

  Could you summarize the significance of your paper in layman’s terms?

In order to know the exact location of any point, anytime, anywhere over the Earth’s surface, a TRF, co-moving with the deformable Earth in its diurnal motion, needs to be available and with respect to which point position is referred.

Point positions are not observable quantities, but rather estimated from measurements operated by space geodesy techniques. There are many possible TRFs, depending on the used technique, physical modelling, and dealt observations.

The paper describes the implementation of the 2000 version of the ITRF based on a multi-technique combination. The ITRF is used as a standard frame for geo-referencing applications over the Earth’s surface, and so, using GPS or Galileo in the very near future, allows precise positioning in the ITRF.

In addition to geo-referencing applications, continuous observations collected at space geodesy observatories anchored at the Earth’s crust, allow one to infer the Earth’s Rotation and the variation of its gravity fields, tectonic motion, and displacement due to earthquakes, post-glacial rebound, local subsidence, and other types of crustal deformation.

The parameters describing these phenomena are neither observable nor absolute quantities and therefore should be related to a long-term stable TRF, which is the ITRF.

  How did you become involved in this research?

As many other worldwide institutions, the French Institut Géographique National (IGN) contributes to the IERS activities by hosting the ITRF Center. Since the early 1980s, along with many other colleagues, I contributed and still contribute to the ITRF research activities with the aim of continuously improving the accuracy of the ITRF definition and determination by improving the physical and stochastical multi-technique combination models.

  Are there any social or political implications for your research?

The prerequisite for precise quantification of the parameters describing these geophysical phenomena of the changing planet, with direct implications to water cycles, ice sheets, sea level rise, climate change, global warming, and geohazards, is the availability of numerous, globally distributed, and continuously observing space geodesy instruments.

Unfortunately, severe budget cuts worldwide limit our geodetic networks and prevent the renovation of degrading instrumentations and, in particular, VLBI and SLR systems that are indispensable, along with GNSS and DORIS, for the long-term maintenance and improvement of the ITRF.

Dr. Zuheir Altamimi
Head, IERS ITRF Center
Institut Géographique National
ENSG/LAREG
Champs-sur-Marne, FRANCE

Related Links:
•	http://itrf.ensg.ign.fr
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