The individual point positions were merged into daily full-network
solutions. Due to the nature of PPP processing, the different station
positions are modelled as uncorrelated. Nevertheless the phase
ambiguities can still be fixed to improve the position in the east–west
direction. The daily ambiguity-fixed solutions were combined into 7-
day campaign averaged solutions using 7-parameter Helmert transformations.
In order to condense the results and to facilitate the detection
and downweighting of outliers, the overall repeatability weighted root
mean square (WRMS) statistics of the 7-day combination solutionwere
used to scale the formal errors in their variance–covariance matrices.
Next, the absolute median algorithm(Rousseeuw, 1998)was applied to
detect and remove outliers prior to the computation of final coordinate
repeatabilities. These values are direct indicators for the internal
precision of each station. In general, the GPS points CHON and BANH
perform very well, with daily coordinate repeatabilities (1994–2009)
typically ranging from 1 to 5 mm for the horizontal components and
from 3 to 15mm for the vertical component.