VLBI

 

The Very Long Baseline Interferometry technique (VLBI) has been employed in geodesy for nearly 40 years. Covering intercontinental baselines with highest accuracy, monitoring Earth rotation at the state of the art and providing the quasar positions as the best approach to an inertial reference frame, VLBI significantly contributed to the tremendous progress made in geodesy over the last decades. VLBI was a primary tool for understanding the global phenomena changing the “Solid Earth”. Today VLBI continuously monitors Earth rotation and its variations and also crustal movements in order to maintain global reference frames, coordinated within the International VLBI Service for Geodesy and Astrometry (IVS) – a Service of the IAG and IAU. Science and applications set the requirements for the realization and maintenance of global reference frames at VLBI’s technical limitations. VLBI, as the unique technique for providing a celestial reference frame and for deriving the full set of Earth rotation parameters, plays the fundamental role of generating the basis for many applications and research in the geosciences. VLBI measures the time differences in the arrival of microwave signals from extragalactic radio sources received at two or more radio observatories. Analyses of such VLBI experiments now permit measurements of relative positions of points on the Earth’s surface and of angles between celestial objects at levels of better than one cm and one nanoradian, respectively. The accuracy refinements are expected to continue until insurmountable problems are reached, probably in the sub-milimeter accuracy regime. Geodetic measurements on the Earth surface have been enormously expanded and densified by satellite tracking methods developed during the past decade, but VLBI remains the prime technique for astrometry employing natural radio sources. It is unique in its availability to measure the Earth’s orientation in an inertial frame of reference. Detection and processing of the extremely weak signals from radio sources requires long integration times, sophisticated data acquisition systems, and customized computers. Analyses of VLBI observables involve an uncommonly broad cross section of the disciplines of physical science, ranging from consideration of the effects of the Earth’s internal structure on its dynamics, to the tectonic plate motions and various terrestrial tidal effects, to quantification of turbulence in the atmosphere, to special-relativistic description of the radio signals travelling from the distant sources, to general relativistic retardation. With the exception of gravimetric and oceanographic experiments, VLBI is perhaps the most demanding technique for many aspects of global Earth models.

References

Schuh H, Behrend D (2012) VLBI: A fascinating technique for geodesy and astrometry. J Geodyn 61, 68-80, doi: 10.1016/j.jog.2012.07.007

Sovers, O.J., Fanselow J.L. ve Jacobs, C.S., (1998) Astrometry and geodesy with radio interferometry: experiments, models, results. Reviews of Modern Physics, 70, No. 4, doi:10.1103/RevModPhys.70.1393

[URL 1] http://ivscc.gsfc.nasa.gov/

 

Some IVS Analysis Centers