Subject: [IGSMAIL-4321]: Modeled GPS Time Series and Coordinates Tool
From: Yehuda Bock <[email-masked]>
To: UNAVCO Mail <[email-masked]>, IGS Mail <[email-masked]>, SCIGN Mail <[email-masked]>, CSRC General Mail <[email-masked]>
Cc: Rosanne Nikolaidis <[email-masked]>, Simon Williams <[email-masked]>
Date: Mon, 24 Mar 2003 23:20:49 -0800

******************************************************************************
IGS Electronic Mail      24 Mar 23:17:45 PST 2003      Message Number 4321
******************************************************************************

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.

-- 
***********************************************
Yehuda Bock		e-mail: [email-masked]
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
***********************************************