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SIMONS ET AL.: GPS MEASUREMENTS IN S.E. ASIA TO REFINE SUNDALAND MOTION X - 3
work was re-measured by ENS in 2000. These 3 additional networks
increase the available GPS points in this region by 15.
In Malaysia, (Region 2 in Fig. 1 the included Malaysia Active
GPS System (MASS) consists presently of 17 permanent GPS
tracking stations situated at strategic locations, some at tide gauge
stations. The MASS network was setup by the Department of Survey
and Mapping, Malaysia (DSMM) at the end of 1998. The
MASS network has a good spacing on both the Peninsula and East
Malaysia. While the largest part of Malaysia is located on the stable
part of Sundaland, the network is very suited to investigate any intrablock
deformations in Malaysia, especially near/in the Sundaland
boundaries in the eastern part of East-Malaysia (Sabah province).
Three full years of permanent GPS data (1999-2001) were included
in this paper, as well as 2-3 weeks of MASS data in 1998, 2002 and
2003. In this region, the addition of the MASS network increases
the number of available geodetic points from 5 to 22.
The island of Sulawesi, Indonesia, together with the surrounding
region (Region 3 in Fig. 1, is more complex from a geodynamic
viewpoint [Walpersdorf et al., 1998; Simons et al., 2000;
Vigny et al., 2002]. Therefore the existing GEODYSSEA network
was most densified here and re-measured yearly since 1996. The
observed network also includes 2 fault transects (Palu-Koro and the
Gorontalo fault). Since 1999 permanent stations have also been installed
in Sulawesi, mainly to study the behavior of the Palu-Koro
fault (at present 6 stations), in both east-west and north-south direction.
Although all Sulawesi GPS network data was processed along
with the other data on S.E. Asia, a detailed treatment of the results
for Sulawesi is beyond the scope of this paper. Hence, mainly the
motion of the western part of Sulawesi with respect to Sundaland
will be discussed later on. The Sulawesi GPS network was densified
from the original 8 GEODYSSEA sites in 1994, up to more than 30
in 2003, plus an additional 25 fault transect points.
Since 1997, 3 new sites (NTUS, BAKO and PIMO) from the
IGS became operational in S.E. Asia. These sites have also been
included in the network described here. The BAKO site was also
observed as part of the GEODYSSEA network, and the velocity
estimates of these IGS sites were not constrained to those of ITRF
solution of 2000. The time series included for these sites in the
ITRF-2000 solution are relatively short, and hence the velocity estimates
given are not sufficiently accurate for our study. Finally, data
was included from 1 permanent site in Sumatra, Indonesia.
In total, data from about 100 (125 including Sulawesi fault transect
points) GPS points in South and South-East Asia was uniformly
(re)processed, along with an additional data set of about 30 IGS stations.
A summary of the available GPS network sites is given in
Table 1.
The campaign-style GPS data from the Asia Pacific Regional
Geodetic Project (APRGP), which is also available for S.E. Asia
since 1997, was not included in this paper. Near almost all APRGP
sites located on Sundaland, the GPS database used in this paper contains
permanent GPS data. In addition tests conducted with this part
of the APRGP data indicate that the quality of the data (contributed
separately by each Asian APRGP participant) is not always consistent
and sometimes noisier. This can also be noticed in Socquet et al.
[2003, subm.] where 1997-2000 APRGP data on Indochina was included
to a GPS network similar to Table 1, and relatively higher
velocity residuals with respect to Sundaland at these APRGP sites
can be observed.
Because one of the goals of this paper is to present the velocity
estimates in ITRF-2000, a number of permanent IGS GPS stations
needed to be included. Previous experience with mapping local networks
in S.E. Asia into ITRF, has shown that if only regional IGS
stations are selected the mapping may not always be optimal, and
position, and hence velocity errors may occur. The main reason is
that even today, the available IGS network with valid ITRF-2000 solutions
in S.E. Asia is still limited. A number of new IGS sites in the
region have become available since 1996, but their time series are
relatively short, and the data quality sometimes lower than average
(e.g. IISC, GUAM, LHAS, NTUS and BAKO). Moreover, some
stations have been affected by nearby earthquakes (e.g. COCO),
and hence they don’t always fit the linear velocity trend given by
ITRF, nor are official position jumps made available. Some stations
had such short time series at the time they were included in an
ITRF solution, that their listed coordinates and velocity estimates
were much less accurate (e.g. KARR in ITRF-97 and PIMO in
ITRF-2000). Because ITRF gives a velocity estimate together with
position at a certain reference epoch (01-01-97 in the last two ITRF
solutions), any errors in the predicted position become larger as the
analyzed GPS data lies further away from the actual time series
used in ITRF-2000. This might not always become clearly visible
in the mapping process, because with few regional IGS sites available,
part of the position misfits can be absorbed by the Helmert
transformations, which are commonly used in the transformation of
network solutions.
If beside regional IGS stations, also a number of well determined
global IGS sites are included in the analysis, the above mapping
problems can be avoided. Furthermore, some of these included
IGS stations can be excluded from the mapping process,
and used to check/validate the quality of the ITRF transformation
and/or provide more accurate ITRF velocities for these sites in the
analyzed time period. Therefore, IGS data from 14 regional and
17 global stations (Fig. 2) were retrieved from the IGS databases
(http://igscb.jpl.nasa.gov, [Beutler et al., 1994]). The global IGS
sites were selected upon their availability from 1994 until the end
of 2003, and because the majority of them are ’long-running’ permanent
IGS stations. This ensures that these sites have very well
determined coordinates and velocity estimates, with the most recent
ones provided by ITRF-2000 (http://lareg.ensg.ign.fr).
3. GPS Data Analysis
In this section the results of the GPS data analysis with the
GIPSY-OASIS II v2.6.1 [Blewitt et al., 1988] software are discussed.
The software includes a processing method that allows permanent
GPS networks (up to 100 stations) to be processed in relatively short
time periods, including ambiguity fixing for the whole network.
For the complete data set from 1994 to 2003, daily fiducialfree
network solutions were computed with the GIPSY precise
point positioning (PPP) strategy. With this strategy, each station
can be solved for individually, using precise (JPL) satellite orbits,
satellite clocks and corresponding earth rotation parameters
(ftp://sideshow.jpl.nasa.gov). Because the orbits and clocks are held
fixed in this technique, all correlations among the different stations
are not explicitly computed. Therefore a network can be processed
in a much more computationally efficient way, as opposed to the
traditional technique where the complete network is processed simultaneously.
The individual PPP station coordinate solutions were computed
using the ionosphere-free linear combination of the zero-differenced
GPS phase and pseudorange data at 5 minute intervals with an elevation
cut-off angle of 15 degrees. To account for tropospheric effects,
the zenith path delay (using Niell’s model [Niel, 1991]) and gradients
are estimated at each time interval. If required, the default
apriori value of the wet tropospheric delay (10 cm) and the apriori
station coordinates are iterated upon. Data from GPS satellites
that were undergoing maintenance during part of the processed day
(ftp://tycho.usno.navy.mil/pub/gps/) were removed. Ocean loading
parameters for all site locations were obtained from H.-G. Scherneck
and M.S. Bos (http://www.oso.chalmers.se/˜loading/[Scherneck,
1991]) and ocean loading modeling was applied to each site in
the network. Finally, because different antennae were used in
the network, the NGS relative antenna phase center corrections
(http://www.ngs.noaa.gov/ANTCAL [Mader, 1998]) were applied.
In previous analyses, the IGS antenna phase center corrections
[Rothacher and Mader, 1996] were used. Unfortunately, the GPS
network in this analysis contains GPS antennae that were not included
in the original official table provided by IGS (igs01.pcv).
As these new GPS antennae became available, IGS included the
NGS antenna phase center corrections for most of these antennae.
However, differences can be observed between the IGS and NGS
tables for some (older) antenna types also used in this network.