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interferometric radio observations, and the International Terrestrial
Reference Frame ITRF, based on different space techniques. The
International GPS Service IGS, started in 1994 and evolved to be the
main source for precise GPS orbits as well as for coordinates and

observations from a global set of more than 300 permanently
observing reference stations. At the national level permanent services
for GPS reference data have been established and are still growing,

e.g. CORS in the USA, CACS in Canada and SAPOS in Germany.
_____ Development of basic methods for satellite observations,
and for the computation and analysis of satellite orbits occurred in
1958 to around 1970. This phase is characterized by the optical-

photographic determination of directions with cameras. The main
results were the determination of the leading harmonic coefficients of
the geopotential, and the publication of the first Earth models, for

instance the Standard Earth models of the Smithsonian Astrophysical
Observatory (SAO SE I to SAO SE III), and the Goddard Earth
Models (GEM) of the NASA Goddard Space Flight Center. The only

purely geometrical and worldwide satellite network was established
by observations with BC4 cameras of the satellite PAGEOS.
____ After more than 40 years of satellite geodesy the

development of geodetic space techniques is continuing. We have
significant improvements in accuracy as well as in temporal and
spatial resolution. New fields of application evolve in science and
practice. For the first decade of the new millennium development will

focus on several aspects: − launch of dedicated gravity field probes
like CHAMP, GRACE, and GOCE forthe determination of a high
resolution terrestrial gravity field, − establishment of a next generation

Global Navigation Satellite System GNSS with GPS Block IIR and
Block IIF satellites and new components like the European Galileo, −
refinement in Earth observation, e.g. with high resolution radar
sensors like interferometric SAR on various platforms, − further

establishment of permanent arrays for disaster prevention and
environmental monitoring, and − unification of different geodetic
space techniques in mobile integrated geodeticgeodynamic monitoring

systems.

Task 2. Put up 5 general and 5 special questions to the text.

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