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satellite orbit
a
h
H
mean sea surface
SST
instantaneous
sea surface
geoid
N
ellipsoid
Fig. 7.7. Satellite altimetry
where h is the ellipsoidal height of the altimeter satellite (based on orbit
computations), N is the geoidal height, SST is the sea surface topography
to be derived, ∆ H is caused by the instantaneous tidal effect, and a is
the altimeter measurement. Note that (7-47) is a simplified representation,
since, e.g., usually SST is split into a dynamic and a constant part. Refer to
Seeber (2003: Sect. 9.3.1) for more details.
Knowing the sea surface topography, ocean currents and circulations may
be explained, which is highly interesting for our understanding of the global
energy transport. Ocean currents together with their time variations are an
important indicator for climatic changes.
This method suffers from different accuracy influences in the results:
when referring the mean sea surface to the ellipsoid, centimeter accuracy
could be achieved. Involving the gravity model and referring the sea surface
topography to the geoid as in Fig. 7.7, an improved geoid is required for a
consistent accuracy level.
Geophysics
As mentioned earlier, the earth's gravity field reflects the mass inhomo-
geneities in the interior of the earth. Knowing gravity values on the earth's
surface and, in addition, complementary data (e.g., magnetic and seismic
data), improved models for the structure and processes in the earth's inte-
rior may be obtained. These processes may cause the movement of tectonic
plates which are responsible for earthquakes. Thus, we see that the gravity
field is the fundamental link in a chain of interactive processes. Using more
descriptive terms, an improved knowledge on the gravity field may yield
more accurate methods to predict earthquakes. This justifies any effort on
the determination of the earth's gravity field.
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