Geodetic Very Long Baseline Interferometry (GeoVLBI) is a space-based technique that allows for accurate measurements of the Earth and its orientation, in inertial space. The technique is based on observations of compact radio sources with radio telescopes distributed around the globe (see Fig. 1). The observed radio sources are called quasars and located at billions of light years from the Earth. They define an inertial frame in which the Earth’s orientation is measured. Telescopes simultaneously record the radio signals from quasars. At each radio telescope station the signal is time-stamped with very precise atomic clocks. Afterward, all data are collected and processed at the correlation centers, where differences in arrival times of signals at the various telescopes (time delays) are determined. From the time delay observation it is possible to estimate and study various geodetic parameters describing Earth rotation and nutation, plate tectonics, troposphere and others.
Geodetic VLBI contributes to:
- Terrestrial Reference Frame (TRF)
- Celestial Reference Frame (CRF)
- Earth Orientation Parameters (EOP) – UT1, polar motion, nutation, precession
- Troposphere parameters
- Geophysical models (ocean tides, Earth solid tides, Earth crust deformation and and plate motion, post- glacial rebound, etc.)
The GeoVLBI observations and data analysis are coordinated by the International VLBI Surveys for Geodesy and Astrometry (IVS)
GeoVLBI observations in Finland
Since 2004 Finnish Geospatial Research Institute has performed geodetic VLBI observations using 13.7 m antenna located at the Metsähovi Radio Observatory of Aalto University. This antenna has been used for 4-8 dedicated campaigns per year for GeoVLBI, with the remaining of the time the telescope devoted to astronomical observations. The GeoVLBI observations are performed with S/X bands cryogenic receiver.
Figure 2. Metsähovi radio telescope which is used also for geodetic VLBI.
In the coming years (2016-2019) FGI will build a new VLBI2010 Global Observing System (VGOS) telescope at the Metsähovi Geodetic Fundamental Station. The new telescope will be a part of the international VGOS network. The main goals of VGOS are continuous observations to monitor Earth orientation parameters and station positions with the goal of 1 mm accuracy in position and 0.1 mm/yr in velocity. VGOS concept is developed to meet the highly demanding accuracy requirements. The new VGOS telescope will perform broadband observations (2-14 GHz). The radio telescope dish with a diameter of 13.2 m is able to rotate with high speed and perform more extensive observations during a 24-hours session. This observation strategy leads to the high accuracy of the next-generation GeoVLBI technique. More details on the VGOS project in Finland.
Figure 3. An example of a VGOS telescope (The RAEGE Station in the Island of Santa Maria, Azores).