||VLBI is a geometric technique: it measures the
time difference between the arrival at two Earth-based antennas
of a radio wavefront emitted by a distant quasar. Using large
numbers of time difference measurements from many quasars observed
with a global network of antennas, VLBI determines the inertial
reference frame defined by the quasars and simultaneously the
precise positions of the antennas. Because the time difference
measurements are precise to a few picoseconds, VLBI determines
the relative positions of the antennas to a few millimeters and
the quasar positions to fractions of a milliarcsecond. Since
the antennas are fixed to the Earth, their locations track the
instantaneous orientation of the Earth in the inertial reference
||The celestial reference frame is defined by VLBI.
The International Astronomical Union has adopted the ~500 extragalactic
radio sources (mostly quasars) used by VLBI as the defining objects
of the celestial reference frame. Extragalactic objects form
a true inertial reference frame because they are at such great
distances that their motions across the sky are undetectable.
Positions of stars in our galaxy are now tied to this reference
frame, and this is the same reference frame used for measuring
Earth orientation. VLBI is unique in its ability to make rapid,
accurate measurements of the orientation of the terrestrial reference
frame with respect to the celestial reference frame.
||VLBI observations are sponsored by over 40 organizations
located in 17 countries. VLBI determines with unequaled accuracy
the terrestrial reference frame (antenna locations on the Earth),
the celestial reference frame (quasar positions on the sky),
and Earth's orientation in space. This map
of VLBI sites is also
available in PDF form.
The International VLBI Service for Geodesy and Astrometry (IVS)
is an international collaboration of organizations which operate
or support Very Long Baseline Interferometry (VLBI) components.
IVS provides a service which supports geodetic and astrometric work
on reference systems, Earth science research, and operational activities.
The IVS groups geodesy and astrometry together because they use the
same observations and the same analysis gives both types of results.
IVS is established under the International Association of Geodesy
An excellent discussion on the history, science, and future of VLBI
can be found in the VLBI
Some of the scientific results derived from VLBI include:
- Motion of the Earth's tectonic plates
- Regional deformation and local uplift or subsidence.
- Definition of the celestial reference frame
- Variations in the Earth's orientation and length of day
- Maintenance of the terrestrial reference frame
- Measurement of gravitational forces of the Sun and Moon on the
Earth and the deep structure of the Earth
- Improvement of atmospheric models
Related VLBI information and data links:
A description of the CDDIS
VLBI data sets is available.