​Geodetic very long baseline interferometry

Very accurate geodetic measaruments of, e.g., distances on Earth, can be made with radio astronomical techniques. In astronomy, high spatial resolution, i.e., images with fine details, requires large telescopes. The highest resolution in radio astronomy is achieved by using several radio telescopes, often in different countires or even on different continents, which simultaneously observe the same celestial objects. The data from all telescopes are sent to a central processor where images of the observed sourced can be produced. The technique is called Very Long Baseline Interferometry (VLBI). The same technique can also be used to study the Earth: distances between continents can be measured very accurately and Earth's rotation rate can be determined.

Onsala Space Observatory is active in VLBI research. Astronomical VLBI observations are made with the 20 m and 25 m telescopes, and geodetic VLBI observations with the 20 m telescope and with the Onsala twin telescope (OTT, see below). The observatory is a member of the European VLBI Network (EVN), the Global mm-VLBI Array, and the International VLBI Service for Geodesy & Astrometry (IVS). ​

The VLBI back-end consists of digital base band converters (DBBC). Data is recorded on the Mark 5 system or transported directly to the correlator through the internet (e-VLBI). We have 2 parallell VLBI systems and can observe with the 20 m and 25 m telescope in VLBI-mode simultaneously (but only the 20 m telescope is currently used for geodesy VLBI).

The Onsala 25 m telescope participated already in 1968 in the first transatlantic VLBI experiment. In January 2004, the first e-VLBI experiment took place between Onsala Space Observatory and radio telescopes in Westerbork, Holland, and Cambridge, England.

One example of a result from geodetic VLBI observations comes from measurements of the distance between radiotelescopes in Onsala and in Westford (on the east cost of the USA) during more than 25 years. The results show that the Eurasian and North American tectonic plates move away from each other with 16.5 mm/year, in good agreement with geological models.

One of the OTT antennas

twin telescope (OTT) for geodetic VLBI, consisting of two large parabolic antennae, was inaugurated in May 2017. Each antenna is 13.2 metres in diameter. With a twin telescope we can observe continuously. The telescopes take turns: while one observes, the other moves to the next target. The measurements become more accurate.

Read our information leaflet on the Onsala Twin Telescope:

Read our News item about the OTT inauguration.



One of the antennas of the Onsala twin telescope (OTT).
Credit: Magnus Thomasson

Links, schedules:

International VLBI Service for Geodesy & Astrometry

Geo-VLBI schedules
(The Onsala 20 m telescope is here called On, and the Onsala Twin Telescopes are called Oe and Ow.)

Onsala astro-VLBI schedule​

The 20 m telescope

Contact person:

Rüdiger Haas
Email:, Phone: +46 31-772 5530


Page manager Published: Mon 13 Dec 2021.