Space methods - Physical Geodesy - page 269

reference surface for a number of geodynamic processes (subject to conti-
nents, oceans, ice masses, atmosphere, etc.) and their interaction. The mass
inhomogeneities are a necessary prerequisite to understand convection mo-
tions in the earth's mantle which are responsible for plate tectonics. Some
large and many small lithospheric plates with a thickness of some 100-200 km
move with a relative velocity of some centimeters per year. At the edges of
the plates, seismic zones and volcanoes are situated.
Many time-dependent earth-related processes can be regarded as changes
of the mass distribution and, thus, influence the gravity field, e.g., ocean cir-
culation, ice mass variations, sea level change, tides, volcanism, post-glacial
rebound. These variations may be categorized according to their periodicity.
Some of these effects are extremely long-periodic or secular, e.g., plate tec-
tonics with about 100 million years. In contrast, changes of the ice masses
may amount to some 10 years only; even immediate events like earthquakes
may occur.
These variations are referred to a global physical reference surface, the
geoid. Therefore, the more accurately we know the geoid, the better we
accurately understand the previously mentioned effects. Referring to various
disciplines, the earth's gravity field is important for, e.g., geodesy, geophysics,
oceanography, and climatology.

Geodesy
As mentioned in Sect. 5.3, GPS has revolutionized geodesy in many respects.
Despite the tremendous importance of GPS, in Sect. 5.4 it was shown that
the user of GPS gets only geometric quantities: WGS 84 coordinates, i.e.,
geocentric rectangular coordinates X, Y, Z or, computed from them, ellip-
soidal coordinates ϕ, λ, h (see Sect. 5.6.1). Therefore, the height obtained by
GPS, i.e., the ellipsoidal height h , is purely geometric. To transform these
heights into orthometric heights H by H = h−N , the geoidal undulation N
is required. Using satellites to determine the earth's gravity field, a globally
uniform height system will result.
Additionally, an accurate knowledge of the earth's gravity field improves
the orbit determination of satellites.

Oceanography
The sea surface topography (SST), i.e., the difference between the geoid and
the mean sea surface, can be determined when combining satellite altimetry
data and the earth's gravity field data. From Fig. 7.7 we obtain the relation

h = N + SST + ∆ H + a, (7-47)