Introduction - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 15

the Molodensky theorem, which enabled rigorous calculation of the distance
between surface points and the reference ellipsoid (i.e., geodetic height) directly
from terrestrial gravimetric results without any reduction. It is significant that the
theorem allows direct and rigorous calculation of geodetic heights of the surface
points by avoiding the geoid, which cannot be calculated rigorously in theory.
Using geodetic heights, the observed values on the Earth's surface can be accu-
rately reduced to the ellipsoid, which enables processing of astro-geodetic survey-
ing data to avoid errors caused by inaccurate reduction. With the Molodensky
theorem, the method of astro-gravimetric leveling and the normal height system
emerged and were widely adopted by many countries.

Development of Satellite Geodesy

Up to the middle of the twentieth century, geometric geodesy and physical geodesy
were well developed. However, measurements of the shape of the Earth and the
Earth's gravity field were not satisfactory because astro-geodetic surveying can
only be conducted on land, not across the oceans. There was only a limited amount
of data from gravity surveys in areas like oceans, high mountains, and deserts. It
was not until the launch of the first artificial satellite in 1957 that satellite geodesy
began to emerge and geodesy developed into a brand new technique.
Not long after the emergence of artificial satellites, the satellite method was used
to measure the flattening of the Earth ellipsoid precisely. More than a decade later,
the measuring precision of the semimajor axis of the Earth ellipsoid reached

5m,
the coefficient of the spherical harmonics expansion of the Earth's gravity field was
reliably deduced to the order of 36, and the global geodetic coordinate system had
been established through satellite tracking stations. Modern GPS positioning tech-
niques enable high-accuracy measurement of the geocentric coordinates of any
surface point in the world geodetic coordinate system based on the precisely
measured elements of satellite orbit. The use of the satellite radar altimetry tech-
nique to measure geoid undulation of the seawater has also yielded good results.
The technique of satellite gravimetry is developing and also has great potential.

Development of Dynamic Geodesy

The Earth's crust is not still; it moves slightly and slowly due to lunisolar attraction,
tectogenesis, and so on. Without precise surveying methods, such movement cannot
be measured accurately. The VLBI technique was born in 1967. At the two ends of
the baseline, which was thousands of kilometers long, radio-receiving antennae
were established to receive synchronous signals from extragalactic quasar radio
sources. By interferometry, the three baseline vector components of this baseline in
the inertial coordinate system could be computed with centimeter-level accuracy.
Quasar radio sources are extremely far away from the Earth and so have almost no
angular motion relative to the Earth. Thus, from the given positions of some quasar