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measurement system can be formed by making a baseline between the space and
ground stations so as to determine directly the geocentric coordinates of the ground-
based station. Because all the VLBI antennae around the world take part in the
space VLBI observation, a complete terrestrial reference system can be established
independently using space VLBI technology itself. A VLBI station can not only be
an observed object for various artificial satellite tracking stations on the ground, it
can also be a space station in the orbit of artificial satellites to observe extragalactic
radio sources directly. Thus, the direct connection and unification of the artificial
satellite dynamic reference system and the radio source reference system can be
realized. In addition, by means of space VLBI, an agreement can be made in VLBI
about conversion between the Conventional Terrestrial Reference System (CTRS)
and the Conventional Celestial Reference System (CCRS) to obtain a unified
celestial and terrestrial reference system (i.e., a unified rotation and scale system
with a commonly defined origin). Such unification of coordinate systems is of great
significance for research in geodesy and other related fields.
2.3.4 Satellite Altimetry
In the 1980s, satellite altimetry (SA) appeared along with the application and
development of computer technology, space technology, satellite telemetry, and
remote sensing technology. SA employs microwave radar altimeters installed in
satellites, radiometers, synthetic aperture radar, and other equipment to measure in
real time the distance from a satellite to the ocean's surface, the effective wave
height, and the backscattering coefficients, and to carry out research in geodesy,
geophysics, and oceanography through data processing and analysis.
SA data can determine the marine geoid and solve the gravity anomaly of the
ocean to compensate for the data gap in gravity measurement of marine areas.
Therefore, SA plays an important role in establishing an Earth gravity field model
with high accuracy and high resolution. The US Federal Geodetic Control Sub-
committee (FGCS) noted that what the ocean altimetry satellite Seasat does within
3 months would take 200 years and cost 2 billion US dollars if done by marine
gravimetry. Besides, SA data can also be used in oceanographic studies such as the
measurement of the width, boundary, and velocity of ocean currents; tidal fluctu-
ations; sea surface topography; and mean sea level changes.
The Basics
In SA, a microwave radar altimeter mounted on a satellite (the carrier) transmits
microwave signals to the ocean's surface. This radar impulse reaches the ocean's
surface and then returns to the radar altimeter by reflection. According to echo
theory, we can obtain three observed quantities after the return of the radar pulse,
including:
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