Geodetic Data Collection Techniques - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 46

1989) and Etalon-2 (in May 1989) launched by Russia (the former Soviet Union),
and others are dedicated laser ranging satellites for the applied research of
geodynamics and geodesy. Satellites are especially suitable for geodetic research
and observation because of their high and stable orbits, small area-to-mass ratio,
symmetrical spherical shape, long accumulation time for observational data, etc.
Figure 2.12 shows the shape of such satellites. They are spherical with a diameter of
60 cm, installed with 426 laser reflectors on their surfaces. They are still used for the
common geodetic observation of SLR nowadays and have, among others, provided
much data for establishment of the terrestrial reference frame and determination of
the Earth rotation parameters. SLR has become one of the main techniques for
satellite orbit determination because of its high precision distance measurement
ability. Many reconnaissance satellites, meteorological satellites, Earth resources
satellites, and oceanic satellites have all been equipped with laser reflectors so as to
carry out more precise measurement and control of satellites by means of the SLR
technique.

2.3.3 Very Long Baseline Interferometry

Very long baseline interferometry (VLBI) came into development in the late 1960s
and is a radio interferometric observation technique that can combine two radio
telescopes thousands of kilometers apart into a radio interferometry system with
very high resolution. Since the line between the two telescopes is known as the
baseline, VLBI is called very long baseline interferometry. Its resolution has now
been improved to (the magnitude of) 0.1 mas with extension of the baseline.
Because of the super-high resolution of VLBI, it has been widely applied in
many fields like astronomy, geophysics, geodesy, and space technology for appli-
cations such as radio astronomy, accurate determination of the Earth's rotation
parameters, crustal deformation detection, exploration of deep space and the
ionosphere, etc.

Principles of Geodetic VLBI

Celestial bodies observed by VLBI are extragalactic radio sources, which are
usually in deep space 100 million light years from Earth. When the electromagnetic
wave radiated from the celestial bodies reaches the Earth's surface, its propagation
distance is much further than that of the baseline in VLBI, so at this moment the
movement of the wave front can be assumed to be parallel propagation and the
wave is called a plane wave. On account of the different distances between the two
antennae and a certain radio source, we get a distance difference L. Therefore, the
time span of the radio signal from the same wave front to either antenna will be
different, resulting as a time delay ˄ g . According to the geometric relationship

shown in Fig. 2.13 we get: