Introduction - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 16

radio sources, it was possible to establish an extremely stable and therefore inertial
space reference coordinate system. From long-term and repeated observations at
short intervals, variations in the three baseline vector components could be com-
puted and the polar motion, variations in the speed of the Earth's rotation, plate
movement, and vertical crustal movement could be derived accordingly. Hence, the
VLBI technique is an effective way to study the dynamic state of the Earth. In
combination with the SLR technique and observations of the Earth's tide, dynamic
geodesy emerged and provided strong support for geodynamics. With the matura-
tion of GPS technology, since the 1980s GPS surveying has already become the
chief method of dynamic geodesy.

1.3.2 Trends in the Development of Geodesy

Three hundred years have passed since the establishment of geodesy, and there
have been considerable achievements in studies on the shape of the Earth, the
Earth's gravity field, the measurement of surface point positions, and so on. The
current trends in the development of geodesy are as outlined below. Exhaustive
information about the trends of development is available in the literature (Schwarz
2000; Altamimi et al. 2011; Xu 2010; Xu 2012; Flechtner et al. 2010; Sideris 2009;
Mertikas 2010; Drewes 2009; Pail et al. 2011; Plag and Pearlman 2009).

Modern Geodesy as Represented by Space Geodesy

The rapid development of modern technology, especially the development of laser,
microelectronics, artificial satellites, extragalactic radio source interferometry,
computers, and high-accuracy atomic frequency standard techniques, has given
rise to an important breakthrough in geodesy and resulted in space geodesy with
artificial satellites (signal) and extragalactic radio sources (signal) as the objects of
observation. Such a breakthrough has enabled a relative accuracy of 10 6 to 10 9
of the distance and point position determination on a global arbitrary space scale
and allowed determination of the 3-D position of a surface point with high effi-
ciency within a few minutes or hours. This has radically broken the spatial-
temporal limitation of classical geodesy. With the advent of quantum gravimeters
and superconductor gravimeters, the ground gravity meter has also reached the
microgal level (1 10 8 m/s 2 ), or even much higher accuracy. The satellite

gravity technique involved in space geodesy can obtain gravity field information on
a global scale, including the oceans. The breakthrough in technology has allowed
the discipline to undergo an epoch-making revolutionary transformation and
develop into a new stage of modern geodesy, as represented by space geodesy.
The transformations are primarily:

Gal ¼

μ