Geodetic Very Long Baseline Interferometry (VLBI)
A brief Note compiled by: Prof. Madhav N. Kulkarni, IIT Bombay
Since the inception of the Very Long Baseline Interferometry (VLBI) system in
1967, this radio interferometry technique has revolutionised the fields of geodesy,
geodynamics and astrometry. With the unprecedented precision and resolution made
possible by VLBI various tasks such as geodetic positioning, reference frames
interconnections, Earth rotation and polar motion monitoring, estimating the crustal
movements, and mapping the extra-galactic radio sources, have attained accuracies of the
levels never reached before. Due to its tremendous potential “to provide important new
geodetic, geophysical, and astrometric information” geodetic VLBI “moved from an era
of development and demonstration to routine operational activities into a period of
refinement, rapid extension, and widespread recognition”. Thus, VLBI is the most precise
Geodetic technique available today, with numerous applications in different fields.
Basic Principal of VLBI :
The VLBI system consists of an array of two or more dish antennas that observe
the radio signals received from the same extragalactic radio source like quasar
simultaneously. By observing the difference in times of arrival of the signal at the two
locations to a very high degree of accuracy (about 10-14 to 10-16 sec), and knowing the
angular co-ordinates of the radio source, the baseline vector between the two antennas
can be estimated (see Fig.). As no direct connection is required between the two
antennas, very long baselines of the order of thousands of km can be observed.
Accuracies of the order of few mm in measurements of lengths of such long baselines are
routinely achieved, thus making the geodetic VLBI the most precise geodetic technique
available today.
Historical Background :
The distant radio sources like quasars, situated in far-away galaxies at millions of
light years from the Earth, emitting radio waves continuously, were discovered in the
early part of this country. However, it was only in 1967 that practical implementation of
the principle of radio interferometry was established. The early interferometric systems
had inter-connected antennas, thus limiting the separation of the antennas to a maximum
of few hundred metres, and the resolution of few milliarcseconds (mas). With the need to
measure longer baselines, it became necessary to remove this inter-connection, thus
giving birth to the VLBI technique. Within the twenty five years of its operation, the
VLBI technique has become one of the most important systems in the geodetic,
geodynamic, geophysical and astrometric fields.
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Instrumentation :
The VLBI instrumentation can be divided into several sub-systems :
i)
ii)
iii)
iv)
v)
vi)
VLBI Dish Antenna
Receiver
Frequency standard
Recorder
Calibration Unit
Correlator
The dish antenna varies in size from the large permanently installed antennas,
upto 300 meter in diameter, to the small 5 metre antennas of the mobile VLBI systems.
The antenna collects the weak radio signals from the quasar being observed, and
continuously tracks the radio source by rotating in space. The front end receiver receives
this signal and amplifies it. The frequency standard (atomic clock) used is of rubidium
and cesium beam clock type or of hydrogen masers, with stabilities of the order of 1 part
in 1014 or better. The signals received are recorded on magnetic tapes and later correlated
in the correlator unit for post processing. The system is calibrated for environmental and
hardware effects, in order to correct for these effects.
Observables :
The basic observables in geodetic VLBI are time delay (Difference in the times of
arrival of the signal at the two antennas) and time delay rate (time derivative of delay).
The time delay observable can be of two types - phase delay, based upon phase of the
radio signal wave, or group delay. These VLBI observables contain information related to
the length and orientation of the baseline (or relative positions of the antennas), the
parameters defining the Earth rotation and polar motion, and the atmospheric and other
instrumental effects. Thus, it is possible to estimate baselines or the Earth rotation
parameters (ERPs) from these VLBI observations, to a very high degree of accuracy.
Geodetic Applications :
Due to the very high precision offered by VLBI, the technique has found several
important applications in the fields of geodesy, geodynamics geophysics, and astrometry
and astrophysics. Measurement of inter-continental baselines to cm-level precision,
establishment of a world-wide control network of geodetic stations, monitoring the plate
tectonic motion, Earthquake prediction, determination of the ERPs to monitor Earth
rotation and polar motion, establishment of an inertial reference, calibration of other
relative new geodetic systems like the Global Positioning System (GPS), control of scale
and orientation of geodetic control networks, etc., are some of the important geodetic
applications of this system. The VLBI technique also has some very challenging
applications in other fields like astrometry, including the study of the Universe and the
quasars, pulsars, black bodies etc. VLBI defines and maintains the extra-galactic celestial
frame. It holds together the International Celestial and Terrestrial Reference Systems. It
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provides the long-term monitoring of universal time, precession and nutation. The VLBI
antennas used for IERS have diameters ranging from 9 m to 70 m. They are operated in
several tens of sites. The precision of observations of the interferometric time delay is 10
picoseconds. VLBI defines and maintains the extragalactic celestial frame. It holds
together the International Celestial and Terrestrial Reference Systems. It provides the
long-term monitoring of Universal Time, precession and nutation. The VLBI antennas
used for IERS have diameters ranging from 9m to 70m. They are operated in several tens
of sites. The precision of observations of the interferometric time delay is 10
picoseconds.
Future Scenario :
With the rapid advances in the instrumentation, electronics, data processing and
adjustment techniques, the field of geodetic VLBI is progressing at a fast pace. With
several geodetic VLBI stations continuously carrying out interferometric observations,
and new stations being established, including the proposed geodetic VLBI station in
India, and with the array of mobile VLBI instruments augmenting this global VLBI
network, this technique is poised to become the most promising high precision geodetic
technique in the near future. With the precision and with VLBI limited mainly by the
physical limitations on the length of the baseline due to the size of the Earth, it is
proposed to launch dedicated satellites with VLBI antennas in space. This new technique
of Space VLBI offers exciting possibilities in geodetic and astrometric applications [See
Kulkarni, 1992].
Space Very Long Baseline Interferometry (Space VLBI):
Space VLBI is an extension of VLBI into the space, where one or more of the
antennas will be mounted on the satellite. This extended baseline improves the resolution
of the system, thus enabling the scientists to study stars and galaxies farther than before.
Scientists hope to study the galaxies over 18 billion light years away with this system,
and as the origin of the Universe is estimated to be about this, it is likely that signal
emitted 18 billion years ago, or at the time of the ‘Big Bang’ can be studied, thus
enabling us to virtually ‘see’ the Origin of Universe. The geodetic applications of this
system include orbit determination for the satellites, monitoring Earth rotation, defining
Reference Frames, estimating Earth’s gravity field, etc.
The first Space VLBI satellite-VLBI Space Observatory Programme (VSOP) of
Japan was launched in 1997, and the second satellite-RADIOASTRON of Russia is in
final stages. Second generation SVLBI missions under planning by NASA, ESA, Japan,
Russia, etc. Space VLBI Study Groups all over the world are involved in intense research
activities for analysis of data from these satellites.
The Geodetic Space VLBI Study Groups are now in the process of software
development and Data analysis for geodetic applications. At the Ohio State University,
Columbus, USA, Dr. Jozsef Adam carried out theoretical research in 1990, in a NASA-
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funded research project under the guidance of Prof. Ivan I. Mueller, followed by software
development and computer simulation by Dr. Madhav Kulkarni in 1990-92, which
established the utility of the Space VLBI as a Geodetic tool. Later, Adam and Kulkarni
investigated the orbit determination capability of Space VLBI data in 1993 at the Satellite
Geodetic Observatory (SGO), Hungary. SGO has now formed an international project
named Geodesy Demonstration Experiment (GEDEX) for this purpose. A total of 7
scientists’ team, headed by Dr. Kulkarni, initiated the software development for GEDEX
at SGO in 1997, and the work is still continuing. The success of GEDEX would lead to
the future applications of Space VLBI in other Geodetic fields as well. The GEDEX is
likely to continue till 2010, and may be extended beyond that, depending on the results
obtained. In India, even though no Space VLBI research group exists, some scientists,
from National Centre for Radio Astronomy (NCRA), TIFR, Pune, and Physical Research
Labs.(PRL), Ahmedabad, etc. are involved in the international research projects in this
field.
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Principle of Geodetic VLBI
The 305 metre VLBI Antenna in USA
The 20 metre VLBI Antenna at Wettzell, Germany
The 6 metre VLBI Antenna at Wettzell, Germany
The 34 metre VLBI Antenna at Wettzell, Germany
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