Most of the global plate velocity maps developed by American workers using SLR and VLBI
have been published as part of the NASA Crustal Dynamics Project volumes. In general, there is good
agreement between VLBI and SLR derived station velocities far from plate boundaries, and the
predictions of NUVEL-1 (Northwestern University VELocity model 1) [ DeMets et al., 1990].
NUVEL-1 combines spreading rates, transform fault azimuths, and earthquake slip vectors to
predict relative plate motion. This relative motion is described by a rotation vector, known
as an Euler vector. The magnitude of the Euler vector is proportional to
the angular velocity and its direction is called the Euler pole. Euler vector predictions can
be converted into station velocities and compared with geodetic measurements. SLR and VLBI observations across
plate boundaries are 6.3
0.9% slower than the geologic rates used to develop NUVEL-1 (averaged
over the last 3 million years) [ Robbins et al., 1993].
Subsequently, DeMets et al. [1994] revised NUVEL-1, using what
is believed to be a more accurate magnetic time scale. This new model, NUVEL-1A, is 4.4%
slower than NUVEL-1, bringing short term geodetic measurements into even closer agreement with
geologic estimates. For further discussion of global plate models, see DeMets [this volume].
Robbins et al.[1993] summarize both VLBI and SLR measurements by American workers (in many cases with European colleagues participating in data collection). Whereas Robbins et al. [1993] concentrated on calculating station velocities, Robaudo and Harrison [1993] used these same datato determine Euler vectors for the Australia, Pacific, Eurasia, and North American plates. Discrepancies between NUVEL-1 and geodetic Euler vectors were generally within errors in the geodetic estimates. Unfortunately, the SLR and VLBI data cover very few tectonic plates. For example, there are no SLR observations on the African continent south of the Sahara, and there are VLBI observations at just one site in Africa. In addition to providing evidence of global plate tectonics, VLBI and SLR measurements are extremely important for defining and maintaining the terrestrial reference frame.
GPS has begun to make a contribution to global tectonic studies as well. The global GPS
tracking network, known as the IGS (International GPS Service)
network, is coordinated and operated jointly by international government agencies.
The IGS network has been developed primarily to support regional
measurement campaigns and provide precise GPS
ephemerides [ Beutler and Brockmann, 1993]. It is far more extensive
than the SLR and VLBI networks, although there are still
many more receivers in Europe and North America than in the southern hemisphere. Three
American groups are routinely analyzing data from this network: the Jet Propulsion Laboratory (JPL),
the National Oceanic and Atmospheric Administration (NOAA), and the
Scripps Institution of Oceanography. Work is in progress by these groups to
calculate secular velocities for the IGS network, which will eventually be
interpreted in terms of global tectonics [ Heflin et al., 1993; Calais et al., 1993].
Larson and Freymueller [1995] used data collected by the global GPS tracking network to
estimate relative Euler poles for the Antarctic, Australian, and Pacific plates. Euler vector
determinations for the Australia-Antarctic plates agree with NUVEL-1A to better than
a standard deviation using
global GPS data spanning 
three years. On the other hand, the Pacific plate velocities estimated in
the same study found significant differences at the site in French Polynesia. This GPS estimate
agrees with the SLR observations reported by Robbins et al. [1993], but disagrees significantly
with NUVEL-1A. In addition to these permanent GPS installations, there have been numerous
international GPS experiments conducted in support of geophysical studies. We summarize the
efforts of three groups that have been published in the last four years.