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15
SOCQUET ET AL.: GPS MEASUREMENTS TO REFINE INDIA/SUNDA RELATIVE MOTION
each campaign, depending on the quality of
measurements, the quality and homogeneity in the
type of antennae used, the mean length of baselines
for the considered campaign (the dispersion increases
with the length of the baselines essentially due to
uncertainties in satellites orbits) and the number of
stations processed simultaneously. Values range
between 1 and 4 mm for the north component and
between 3 and 8 mm for the east component, whereby
more recent data generally show better repeatabilities.
The solution vectors and associated
variance/covariance matrices of the independent daily
solutions have been combined by GLOBK, based on a
Kalman filtering approach, to obtain a multi-session
free-network solution for each campaign with loose
constrains on positions and velocities. Long term
repeatabilities (i.e. the scatter of independent
campaign determinations about their detrended
average) have been performed for this solution and
average values are about1.8 mm for the north
component and 2.9 mm for the east component for
baselines of about 1000 km (Fig S1). Velocities are
not estimated at this stage but rather after the
combination with the GIPSY solution.
1.2. Details on GIPSY processing
The solution computed with the GIPSY-OASIS II
v2.6.1 [Blewitt, et al., 1988] software uses a data set
spanning 1994-2002. Again, GEODYSSEA (1994-
1996-1998), THAICA (1994-1996-2000) and
Sulawesi (1999-2002) campaigns have been reprocessed
independently from the GAMIT solution.
Theses are the data common to the GAMIT/GLOBK
solution, and represent 60% of the sites used in the
final combination. Three years of the permanent
Malaysian (MASS) network (1999-2000-2001),
Sulawesi (1996-1997-1998) and THAICA (2001) data
have been added. Also, up to 30 IGS stations world
wide spread have been included in the processing to
allow an accurate mapping in the ITRF.
The precise point positioning (PPP) strategy has
been chosen, using fixed satellite orbits and clocks
distributed by JPL (Jet Propulsion Laboratory). As in
the GAMIT processing, the ionosphere-free linear
combination of GPS phase and pseudorange data is
used, here with a sample rate of 5 minutes and an
elevation cut-off angle of 15 degrees. Tropospheric
delay parameters (zenith path delay and gradients) are
also estimated. Ocean loading parameters are included
for each GPS location [Scherneck, 1991], and to
account for the different GPS antennae, the NGS
relative antenna phase centre corrections [Mader,
1998] are applied.
Each campaign solution is projected on ITRF-
2000 using up to 25 IGS stations. Then, velocities
are obtained by fitting a linear trend through
station’s position by a mean square minimization.
At that stage, no adjustments of the station positions
are allowed. Outliers (positions differing by more
than 2mm from their estimated value) are removed
from the linear trend estimation. Those can be due
to measurements errors of any kind or co-seismic
induced displacements for example. A detailed
description on the GIPSY solution and discussion of
its uncertainties are given in [Simons, et al.,
submitted].
1.3. Combination
A combined solution was made from the
GAMIT and GIPSY processing. It spans the entire
region from India and Nepal up to the south-eastern
tip of the Indonesian archipelago. This solution was
generated by combining loosely constrained
variance-covariance matrices from the two
independent solutions with the GLOBK Kalman
filter. Common sites (the regional IGS stations we
use and 60% of our campaign sites) enable a strong
unification of the two solutions. The final mapping
in the International Terrestrial Reference Frame
(ITRF) 2000 is done by minimizing (in a mean
square procedure) the departure of a selected list of
IGS stations positions and velocities from their a
priori values in the ITRF2000 [Altamimi, et al.,
2002]. On these stations, the velocity misfits range
between 0 and 2 mm/yr, indicating an accurate
mapping in the reference frame (Table S2). A
comparison of velocities determined independently
(GAMIT/GLOBK vs. GIPSY) at common sites is
also made (Table S3). They differ by no more than
2 mm/yr at most sites, and these differences should
probably be attributed to the usage of different
orbits (IGS vs. JPL) and the slightly different
antenna phase centre corrections (IGS vs. NGS) that
were applied. Velocities in ITRF2000 of most sites
involved in the combined solution with their 1-σ
uncertainties are given in Table S4. The velocities
are plotted with respect to the Sunda block on Fig. 2
in the paper.