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SIMONS ET AL.: GPS MEASUREMENTS IN S.E. ASIA TO REFINE SUNDALAND MOTION X - 5
the multi-day averaged (local) solutions, and hence the daily coordinate
repeatabilities shown in Table 2, give a realistic indication of
the internal accuracy of the network processed. In a final stage, the
formal errors, which typically are underestimated in the GIPSY PPP
strategy, are scaled using the overall repeatability of each averaged
solution. This results in realistic errors, which in turn, will result
in more realistic formal errors in the site velocity estimation, and
automatically weighs the contribution of each campaign/weekly averaged
solution. Therefore it is not required to ’artificially’ scale up
the formal errors as sometimes is done.
The daily station coordinate repeatabilities, together with the
number of final and removed station coordinate solutions, are given
in Table 2 as the Root-Mean-Square (RMS) of the daily station coordinate
residuals for each campaign and/or year of the permanent
data processed. The daily repeatabilities are in general better than
respectively 2 and 4 mm for the horizontal positions and around
9 mm for the height. A clear improvement can be seen since 1997,
mainly due to an improvement in the GPS orbits and the amount
of GPS satellites available. The measurements of 1996 exhibit the
highest residuals, and are probably caused by a poorer quality of the
GPS orbits in the observation time window of this year.
An overview of the daily coordinate repeatabilities for the IGS
stations can also be seen in Table 2. These show similar values as
for the S.E. Asia network, and also here the 1996 positions show
higher daily offsets. The east component is sometimes a bit larger,
because the ambiguity resolving works less well for the global IGS
sites included (baselines > 2000 km to the nearest site). It is interesting
to notice that (relatively) more outliers were removed between
1999 and 2001, mainly for stations located in the equatorial region.
All these sites used Rogue SNR-8000 receivers, which had problems
with tracking GPS satellites during periods of increased solar
activity (higher ionospheric delays). All these receivers have been
replaced in the past 4 years with upgraded or new GPS receivers.
3.2. Mapping solutions in ITRF-2000
Each (weekly/campaign) multi-day averaged solution was projected
onto ITRF-2000 coordinate sets, each containing the positions
of the 31 IGS stations at the middle epoch of each
analyzed week/campaign. The ITRF-2000 GPS SINEX file
(http://lareg.ensg.ign.fr) is propagated to each of these epochs, and
contains the full covariance matrix for the IGS stations subset.
In the mapping process, any multi-day averaged IGS coordinate
solutions, identified as (horizontal and/or vertical) clear outliers with
respect to the propagated ITRF-2000 solution were downweighted.
The coordinate residuals of the IGS sites for all epoch transformations
are shown in Table 3.
The coordinate residuals of all IGS stations are small and consistent,
with RMS values of 2 to 3 mm for the horizontal and about
8 mm for the vertical position. These numbers correspond with
the repeatabilities for the IGS sites in the above section, and hence
confirms (only stable and linearly moving stations were used) the internal
accuracies. From the total of 31 included stations, 13-23 were
used in the mapping of all weekly/campaign solutions from 1994
to 2003. The ITRF positions of BAKO, NTUS, PIMO (all in S.E.
Asia), MKEA, KARR, SHAO and KUNM (mapping quality check)
were estimated along with the S.E. Asia network. Since DARW and
XIAN are relatively new in IGS and ITRF-2000, these stations were
also not considered in the mapping. Finally, a number of sites, in
chronological order COCO, KOKB, TSKB, GUAM, GOLD, FAIR
and KOSG were automatically rejected from the mapping between
1999 and 2003. This was due to jumps in the time series, which
all could be traced back to seismic events, antenna changes and/or
changes in the site marker/antenna setup. This GPS analysis allowed
the detection of horizontal jumps larger than 2 mm and about
10 mm in the height in the time series of the permanent sites.
The results of Table 3 together with the daily coordinate repeatabilities
of the S.E. Asia and IGS sites (Table 2), indicate that the
global accuracy with respect to ITRF-2000 of each multi-day averaged
solution is in the range of 2-4 and 4-6 mm for the horizontal
position components, and around 9-15 mm for the height.
3.3. Velocity estimates in ITRF-2000
In this paper only steady state site velocities are given, and these
can be easily estimated by computing a (linear) fit through all ITRF-
2000 mapped weekly site coordinates, and checking the residuals at
each point in the time series. Also the velocity differences between
this solution and the ITRF-2000 will be verified, for both sites that
were and were not used in the mapping process.
A linear velocity model was fitted through all the available ITRF-
2000 mapped coordinates for each individual site making use the
GIPSY software utilities. In this process, combined coordinate solutions
in ITRF-2000 identified as clear outliers with respect to the
linear velocity trend are downweighted. Jumps in site position can
also be estimated, but this is primarily useful for time series of continuously
operated stations, unless a campaign site was measured
many times, both before and after a jump in position.
The coordinate residuals for all the S.E. Asia and IGS sites w.r.t.
to their estimated velocity trend are given in Table 4 for each of the
Table 5. Residuals IGS stations w.r.t. linear trend fitting. Conform
Table 4 the RMS of the residuals from the linear trend
fitting are shown, but this time for individual IGS stations that
were not used in the mapping into the ITRF-2000
IGS Weekly RMS Residuals (mm)
Station Solutions North East Up
BAKO / Indonesia 189 (- 5) 3.2 3.4 9.7
NTUS / Singapore 145 (- 7) 2.9 3.6 9.2
PIMO / Phillipines 122 (-16) 3.9 4.0 12.4
KUNM / China 157 (- 1) 2.5 3.6 8.5
XIAN / China 79 (- 0) 2.5 1.9 6.8
DARW / Australia 156 (- 8) 2.2 3.3 8.6
KARR / Australia 200 (- 1) 1.5 2.6 8.1
MKEA / Hawaii 184 (- 5) 1.7 3.4 8.4
latitude (cm)
longitude (cm)
vertical (cm)
15
10
5
0
-5
-10
-15
-20
trend = -8.40 _+ 0.14 mm/yr
1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004
15
10
5
0
-5
-10
-15
-20
trend = 31.90 _+ 0.19 mm/yr
1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004
15
10
5
0
-5
-10
site KUAL
-15
-20
trend = 1.20 _+ 0.67 mm/yr
1994 1995 1996 1997 1998 1999
year
2000 2001 2002 2003 2004
Figure 3. MASS site KUAL: Coordinate time series in ITRF-
2000. This site in Malaysia was originally a GEODYSSEA site,
and became a permanent station at the end of 1998. The figure
shows a marked outlier in 1997, and a vertical only outlier
in 2001. The errors on the velocity component are 1-values,
which do not require any further scaling.