What is Very Long Baseline Interferometry (VLBI)?

Introduction

VLBI conception
- NASA VLBI group

The basic observable in geodetic Very Long Baseline Interferometry (VLBI) is the time difference of arrival times of a radio wave front emitted from a distant quasar, received at two radio telescopes up to several thousand kilometres apart. The precise time difference measurements called the delay are used to determine the relative positions of the antennas to within a few millimetres. This image is a VLBI concept for the NASA VLBI group. Select for a larger gif_40k image.

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The History of VLBI

The VLBI concept was introduced by Russian radioastronomers in 1965 and technically implemented in the USA in 1966. Since 1983, regular geodetic and astrometrical VLBI sessions have been performed internationally under coordination of the Goddard Space Flight Centre (GSFC, Maryland, USA).

Australia participated in the first intercontinental VLBI experiments in the late 1960s jointly with American, Canadian and Russian scientists. Currently, three Australian radiotelescopes - Hobart, Parkes and Tidbinbilla - participate in geodetic VLBI experiments. Read more details about Geodetic VLBI in Australia.

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Networks

VLBI observatory network
- NASA VLBI group

About 50 VLBI antennas are distributed around the world and which are combined in networks for different special programs. There are three permanent networks that operate for geodetic research in local regions - the European network, the Asia-Pacific network and the North American network. Image is of the VLBI observatory network for NASA VLBI group. Select for a larger gif_25k image.

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The Role of Geodetic VLBI

Geodetic VLBI observations provided the first direct confirmation of tectonic plate motion at the end of the 1980s. Now VLBI observations measure the motions of stations with an accuracy better than 1mm/year together with other space geodetic techniques such as Global Positioning System (GPS) and Satellite Laser Ranging (SLR).

VLBI observations to stable distant quasars are an important component for the establishment of a reference system of coordinates in the sky - defining the the Celestial Reference Frame. The radiosystem (positions derived from radio observations to quasars) has replaced the traditional optical reference system based on star positions. The optical system which was used for the last 200 years had an average accuracy (of star positions) to about 10 milliarcseconds (equivalent to 0".01). The current average accuracy of quasar positions observed by radiosystems is about 0.1-0.2 milliarcseconds (50-100 times better); therefore the International Astronomical Union has recommended using radiosystems for the definition of the International Celestial Reference Frame.