Introduction - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 6

system and accurate positions of surface points (e.g., the launch point and tracking
station) in this system; the other is a precise global gravity field model and accurate
gravity field parameters (gravity acceleration, the deflection of the vertical, etc.) of
the surface points.
A geodetic coordinate system is used to describe a spacecraft's movement
relative to the Earth, which is realized by a certain number of datum points with
known precise geocentric coordinates distributed on the Earth's surface. Its estab-
lishment includes determination of the orientation of its coordinate axes and a
normal Earth ellipsoid defined by four fundamental parameters (a, J 2 , ω, and
GM) (see Sect. 4.2.1 ) . The space tracking, telemetering, and command (TT&C)
network, composed of a TT&C station (including a TT&C ship) in the space
project, is adopted to determine the moving state (orbit and attitude) and working
state of the spacecraft. It controls and adjusts the moving state of the spacecraft,
builds their normal state, and manages aircraft under their moving state over a long
term. The precise position of the TT&C station in the geodetic coordinate system is
accurately determined by geodetic methods while the position of the spacecraft is
solved from the given station coordinates of the TT&C through measuring the
radial distance, range rate, azimuthal angle, and the like between the TT&C station
and the spacecraft during operation.
The gravity field model provides prior constraint on the gravity field for analysis,
description, and design of all mechanical behavior of moving objects on the Earth's
surface and in outer space. Precise satellite orbit determination relies on the level of
accuracy of the known expansion coefficients of the disturbing gravity potential in
its dynamic equation of orbital motion. The lower order Earth gravity field model
can ensure the accuracy of the orbit determination of a low Earth orbit (LEO)-
satellite at the decimeter level. Space microgravity, a marginal discipline, emerged
with advancement of interplanetary exploration technology. It is concerned primar-
ily with the microgravity effect of tested objects on the spacecraft and is based
chiefly on the high precision Earth gravity field model.
1.2.3 Applications of Geodesy in Geoscience Research
The components, movement, and development of the Earth system are observed
and revealed by different branches of geoscience from different aspects using
different methods. Geodesy places special emphasis on the study of the Earth's
geometric (spatial) characteristics and fundamental physical characteristics (the
gravity field) and describes their changes. Plate tectonics was developed in the
late 1950s and early 1960s and has led the revolutionary progress of geosciences,
which is significant for establishing the scientific view of “mobilism” in geoscience.
The progress of modern geodesy and the introduction of space geodesy are essential
in fostering geoscience development because geodesy enables extensive acquisition
of information about the Earth's movement and allows the fundamental status of
geodesy to be strengthened more profoundly. Modern geodetic techniques have