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International VLBI Service for Geodesy and Astrometry (IVS)


The function of the International VLBI Service for Geodesy and Astrometry is to organize and run appropriate VLBI observing programs to establish and maintain terrestrial and celestial reference frames and to monitor closely the Earth’s rotation. For this purpose, the service is structured around different entities – VLBI observing stations (radiotelescopes), correlators, data centers, analysis centers,… – which cover the entire VLBI chain, from the data acquisition to the dissemination of the end VLBI products.


Standard IVS observing sessions are carried out with networks of about 10 radiotelescopes and have a duration of 24 hours. There are 150 to 200 such sessions every year. In addition, shorter “intensive” sessions, only one-hour long and using two or three radiotelescopes, are conducted daily. VLBI data comprise interferometric delays which are the basis to estimate geodetic and astrometric parameters (radiotelescope positions, Earth’s orientation, source positions) along with complex visibilities (amplitudes and phases) which are used to image the sources. All IVS data are publicly available immediately after correlation.

Involvement of OASU

As an analysis center, the LAB contributes to both the data analysis and the development of VLBI observing programs. These are guided by the IVS objectives, especially as regards the celestial reference frame, i.e. the ICRF (International Celestial Reference Frame). In details, the activities are the following :

  • Regular IVS data analysis to monitor the Earth’s rotation and maintain reference frames. The multi-technique software package GINS from CNES is used to this end. Developing the VLBI part of GINS also falls within the scope of this activity.
  • Regular imaging of the ICRF sources and assessment of their astrometric quality. All VLBI maps and the derived compactness and structure correction maps that qualify such quality are disseminated through the Bordeaux VLBI Image Database (BVID).
  • Implementation of VLBI observing programs to strengthen the ICRF, for example by searching for new sources to densify the frame or to improve its alignment with the Gaia optical frame. Such programs are carried out in particular with the European VLBI Network (EVN)  and the Very Long Baseline Array (VLBA).

Also of significance is the development and maintenance of the dynamic web site IVS-Live which allows one to monitor IVS sessions in real time.


The VLBI observing technique by its very nature (interferometry using intercontinental baselines) requires international cooperation. In all, the IVS brings together about forty institutions (including OASU and Paris Observatory) in 20 countries, which collaborate to carry out the desired observing programs and disseminate data and products.