VLBI is an advanced space geodetic technique that can measure a distance
of thousands of kilometers between its antennas with an accuracy of few
millimeters, by receiving radio signals from deep space as far as several
billion light years away.
1) Giant parabola antennas receive faint radio signals emitted from far-distant
celestial objects*. Due to the position difference between the antennas, there is a slight difference of about 0 to 0.02 seconds between the antennas in the receiving time of a particular signal from the same object. This delay time is measured to a precision of one 10-billionth of a second with a precise atomic clock installed at each antenna (see right picture).
2) By multiplying the delay time and the velocity of radio waves (about 300,000
km/s), we can obtain the distance between the antennas viewed from the
direction of the radio waves (see above figure).
3) If we repeat such a measurement to three or more celestial objects at different directions in space, we can obtain the three-dimensional relative positions between the antennas. Normally one observation, lasting for 24 hours, yields 1,200 GB of data (worth of 2,000 CD-ROMs) per station from a total of about 500 celestial objects.
4) VLBI antennas around the world performing an observation together, it
is possible to know the three dimensional position of each antenna as well
as the rotation of the Earth.
VLBI uses a hydrogen maser atomic clock, the most accurate clock on Earth,
which loses only one second in 100 million years.
This high precision is the secret of the reliability of VLBI observations.
These objects are called the "quasi-stellar radio sources", or
"quasars" for short. Quasars are considered to be the very bright
centers of some distant galaxies, probably due to the presence of a super
massive black hole at the center. About 50 quasars such as the 0552+398(Charioteer)
and 4C39.25(Leo Minor) can be used for geodetic VLBI observations.