The International VLBI Service for Geodesy and Astrometry (IVS) Further advancing the technique for the best possible VLBI products

Accurate information about the orientation of the Earth in space is an essential prerequisite for every kind of navigation on Earth and in space. Very Long Baseline Interferometry (VLBI) is the only space geodetic technique capable of measuring the full set of Earth orientation parameters, i.e. the position of the rotation axis with respect to the sky, with respect to the Earth’s surface, and the rotation angle around the axis. In VLBI, globally distributed radio telescopes observe the signals from extragalactic radio sources (mostly quasars) billions of light years away. The signals are then time-tagged using hydrogen masers at the stations and sent to a correlator, where the difference in arrival time (delay) is determined by cross correlation. With a sufficiently large number of delays, Earth orientation parameters are determined in the analysis step as the result of an adjustment based on the leastsquares method. Of particular importance is the Earth’s rotation angle around its axis, because satellite techniques cannot derive this parameter. Actually, VLBI is able to determine that angle (with respect to atomic time) with an accuracy of several microseconds, corresponding to millimetrelevel at the Earth’s surface.

Why is VLBI important?

VLBI is the technique for realizing the celestial reference system at radio wavelengths with positions of thousands of quasars in the sky. In the best cases, we reach accuracies better than 30 microarcseconds, which corresponds to a tennis ball on the Moon as observed from Earth. Unfortunately (for geodesists) – or interestingly (for astrophysicists) – the position and the structure of the quasars depends on frequency, i.e. it makes a small but significant difference whether we use bandwidth in the S, X, K or Ka band for the observations. In general, however, it is getting more and more difficult to find undisturbed frequencies for the observations because of the increasing amount of artificial signals emitted from ground and space in the microwave domain. Moreover, VLBI plays an important role in the realization of the terrestrial reference system by coordinates and velocities of thousands of stations on the Earth’s surface. VLBI observations are particularly valuable for the determination of the size (scale) of the station network, which is of the utmost importance for measuring tiny effects, such as sea level rise.

The International VLBI Service for Geodesy and Astrometry (IVS) is coordinating all the tasks within the VLBI chain to derive the best possible VLBI products and to further advance the technique. These steps include the scheduling and performing of the observations with the network stations, data transfer and correlation, as well as analysis and distribution of the results.

The IVS is an international collaboration of organizations founded in 1999 as a service of the International Association of Geodesy (IAG) and of the International Astronomical Union (IAU).

By Johannes Böhm, TU Wien, Austria