Why do we need VLBI?

Why do we need VLBI?

Very Long Baseline Interferometry is a major space-geodetic technique. VLBI observations help us to measure, describe and understand where we are and how we move — on our planet as well as relatively to the cosmic background. The long time series of geodetic VLBI observations starting in the late 1970s reveal subtle interactions within the Earth system, enable the quantification of troposphere parameters for climate studies and allow to assess geodynamic effects, such as plate tectonics or post-glacial rebound. Hence, VLBI is indispensable for numerous scientific, societal and economic tasks and applications.

Terrestrial reference frames are the basis for land surveying and global change monitoring

In order to meet the requirements of science and society that are meeting increasing challenges on a chancing planet, a highly accurate and stable terrestrial reference frame (TRF) is needed. A high quality and consistent TRF is a prerequisite for reliably monitoring global change, as it facilitates the measurement of minute changes in the system Earth in a long-term and consistent way.  Furthermore, global TRF are the basis for defining regional reference frames used by thousands of land surveyor every day. Considering this important role, and the potential to directly affect crucial eco-political decisions, the quality of the TRF has wide-ranging implications on the future of economical, environmental and societal development on our planet.

Today, the most accurate realization of a TRF is the International Terrestrial Reference Frame (ITRF). The ITRF is a combination product of the different ground- and space-geodetic techniques, combining the specific strengths and unique characteristics of each technique. VLBI significantly contributes to the ITRF combination, in particular for its scale, with the full observation history that covers nearly four decades.

The northermost VLBI station in Ny-Ålesund (Spitsbergen, Norway) at a latitude of nearly 79° north.

Measuring the Earth's Orientation, Rotation and Time

The Earth's rotation becomes visible when capturing the night sky with long exposure time.

Earth Orientation Parameters (EOP) allow us to describe the Earth’s orientation with respect to the celestial background, i.e. the best approximation of an inertial reference system. Hence, knowing the EOP makes it possible to perform coordinate transformations between the terrestrial reference systems, i.e. the system of our station coordinates, and the celestial reference system, where VLBI’s extragalactic observation targets are located. EOP are commonly defined by angles that characterize (a) the wobble of the Earth’s rotation axis w.r.t. the ecliptic referred to as nutation, (b) the location of the rotation axis  w.r.t. the Earth’s body (polar motion) and (c) the Earth’s rotation. The Earth rotation parameter, referred to as dUT1, describes the deviation between the Coordinated Universal Time (UTC) und the Universal time (UT1) and, therefore, implicitly quantifies the length of our days.
VLBI is the only technique that provides the full set of Earth orientation parameters, which are indispensable for positioning and navigation on Earth and in space, and provide valuable information about interactions within the Earth system. In particular, direct measurements of nutation parameters and of the Earth rotation angle (UT1 – UTC) are uniquely provided by VLBI.

Why is dUT1 so important?

dUT1 characterizes the non-uniform rotation of the Earth about its rotation axis. It is extremely important to precisely measure dUT1 because in a time as short as 1 milli-second (0.001 sec) a surface point at the equator moves approximately 0.5 m due to Earth rotation. Hence, any error in the dUT1 parameter directly propagates into an error in the position determined by Global Navigation Satellite Systems (GNSS), such as GPS or ESA’s GALILEO System.
That is why the IVS organizes a dedicated VLBI session (“intensive session")  in order to estimate dUT1 every day throughout the year!

Grasp the bigger context (with the help of celestial reference frames)

Geodetic VLBI plays a unique role in the practical realization and maintenance of the International Celestial Reference Frame (ICRF)

The ICRF is mainly used in the fields of astrophysics and astrometry , where it is utilized for studies about Active Galactic Nuclei (AGN) or stellar proper motions. However, it also plays a crucial role for geodesy where a precise Celestial Reference Frame (CRF) is needed, among others, for the exact determination of the EOP. Further, the ICRF is used for phase referencing in astronomic Very Long Baseline Interferometry (VLBI) where the compactness of the sources is exploited. The high angular resolution of VLBI is vital for spacecraft tracking where the angular distance of a spacecraft to the stable background CRF helps to accurately fly to Mars and beyond. These applications put high demands on the ICRF with the need for highest astrometric accuracy while spatial coverage should be as dense as possible. For example, spacecraft tracking needs high accuracy in the ecliptic plane, since most spacecraft move within this region. The requirements for phase referencing in astronomic VLBI comprises a dense spatial coverage because a calibrator source in near vicinity of the investigated object is desired.

Global Geodetic Reference Frames (GGRF)

The importance of Global Geodetic Reference Frames (GGRF) for any kind of positioning and navigation was recognized at highest levels when in February 2015 the United Nations General Assembly adopted the resolution “A Global Geodetic Reference Frame for Sustainable Development (A/RES/69/266)" – the first resolution recognizing the importance of a globally-coordinated approach to geodesy. And VLBI certainly plays an important and unique role here.
Please find more information on the United Nations Global Geospatial Information Management (UN-GGIM) Working Group and on the Global Geodetic Reference Frame (GGRF) at https://www.unggrf.org/.

Global Geodetic Observing System (GGOS)

The Global Geodetic Observing System (GGOS) has been established as a flagship component by the International Association of Geodesy (IAG) in 2003. GGOS works with the IAG components to provide the geodetic infrastructure necessary for monitoring the Earth system and for global change research.

VLBI, coordinated by the International VLBI Service for Geodesy and Astrometry (IVS), is an intrinsic part of GGOS and contributes to many IAG products. Learn more about GGOS at http://www.ggos.org/.