[IGSMAIL-4321]: Modeled GPS Time Series and Coordinates Tool
Yehuda Bock
ybock at igpp.ucsd.edu
Mon Mar 24 23:20:49 PST 2003
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IGS Electronic Mail 24 Mar 23:17:45 PST 2003 Message Number 4321
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Author: Y. Bock, P. Jamason, L. Prawirodirdjo, P. Fang
SOPAC analyzes data from about 700 GPS sites divided into
several global and regional sub-networks, using the GAMIT/GLOBK
software. We completed a re-analysis of its GPS data holdings
beginning with the GIG campaign in January 1990 and ending in 2001
when the re-analysis caught up with the operational analysis. The
terrestrial reference frame is realized by the adoption of a core set
of epoch coordinates and velocities of global tracking stations, as
specified by the IGS in ITRF. ITRF2000 is of course its latest
incarnation. For each day, 7 free parameters (3 translations, 3
rotations and scale) are estimated to minimize the deviations (by
weighted least squares) between the unconstrained GAMIT/GLOBK
solution coordinates and the core set of globally distributed station
coordinates, resulting in a set of coordinates for all sites. The
on-line position time series are updated once a week, and a few time
series span more than 12 years.
The current realization of the terrestrial reference frame is
limited by the assumption that it is sufficient to describe the
changing coordinates of the core sites by a single secular motion,
i.e., a horizontal velocity related to plate tectonic motion. The
reality is that the time series of (global and regional) coordinate
positions exhibit a complex spatial and temporal behavior, including
some combination of unmodeled seasonal and tidal effects, non-secular
tectonic motions, vertical motions, as well as apparent motions due
to atmospheric effects, local motions, instrumental effects, site'ing
deficiencies, and inaccurate metadata. Building on the work of
Nikolaidis [2002 - http://sopac.ucsd.edu/other/research.html], we
model the daily position time series for offsets (coseismic or
otherwise), linear velocity, and annual and semi-annual seasonal
terms (for stations that have at least two years and one year of
data, respectively). For stations that experience postseismic
relaxation, an exponentially decaying term is also estimated. In some
cases where a station experiences coseismic and postseismic
deformation, changes in the linear velocity are also estimated. All
parameters are estimated with full white noise + flicker noise
covariances based on a noise analysis (maximum likelihood method) of
the GPS position time series. Their uncertainties may thus be
considered realistic.
The SOPAC coordinate time series (global and regionally filtered
for select networks) can be accessed at
ftp://garner.ucsd.edu/pub/timeseries. A listing of velocities in xyz
(ITRF2000) and neu (WGS84) is available at:
http://sopac.ucsd.edu/cgi-bin/refinedModelVelocities.cgi. The model
parameters are available by site at
http://sopac.ucsd.edu/cgi-bin/refinedModelTerms.cgi. Modeled time
series plots are also available at
http://sopac-devel.ucsd.edu/cgi-bin/refinedTimeseriesListing.cgi and
are currently shown in terms of neu components before and after the
model is applied.
We believe that this approach has several distinct advantages:
1) More accurate representation of the terrestrial reference frame
and epoch-specific coordinates.
2) Easier detection of anomalous site positions due to real and
apparent motions.
3) Better indication of the accuracy and precision of site
coordinates and their derived parameters.
Any feedback is appreciated.
Acknowledgments: Rosanne Nikolaidis, Simon Williams, MIT GAMIT/GLOBK
group, colleagues at CSRC, IGS, NOAA/FSL, SCIGN, UNAVCO.
--
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Yehuda Bock e-mail: ybock at ucsd.edu
Research Geodesist Phone: (858) 534-5292
& Senior Lecturer Fax: (858) 534-9873
WWW: http://sopac.ucsd.edu/; http://csrc.ucsd.edu/
Director, Scripps Orbit and Permanent Array Center (SOPAC)
Director, California Spatial Reference Center (CSRC)
Address:
University of California San Diego
Institute of Geophysics and Planetary Physics
9500 Gilman Drive DEPT 0225
La Jolla, CA 92093-0225
FedEx Address:
Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics
Scripps Institution of Oceanography
8785 Biological Grade, Room 4208
La Jolla, CA 92037
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