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 In
cooperation with many international partners, NASA's Solid
Earth Science Program plays a key role in establishing, maintaining,
and operating global geodetic networks. Currently the networks include
SLR, VLBI, and the GPS ground system. Relevant data products and
valuable services are provided to the worldwide research community
through the International Laser Ranging Service, the International
VLBI Service, and the International GPS Service, respectively. Precise
3-D crustal motions are determined by all three networks, with dense
GPS arrays particularly useful for regional tectonic and earthquake
cycle studies.
Beyond
their scientific value, these data, together with precise determination
of the 3-D geocenter motion by SLR and GPS, constitute the geodetic
elements that define the International Terrestrial Reference Frame
(ITRF), which is the basis for all geodetic measurements described
in this report. The ITRF is geometrically connected to the Celestial
Reference Frame via Earth Orientation Parameter (EOP) time series,
which are determined primarily by the VLBI technique and contain
a wealth of geophysical and climatic information. The ITRF and EOP,
and hence the networks, should continue to be maintained and improved
and their data routinely acquired at the best possible accuracy
and temporal resolution.
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