Featherstone et al: GPS-geodetic deformation monitoring 
deformation is monitored using coordinate time-series 
formed from episodic measurements, such seasonal 
effects could result in an under-sampled or aliased signal, 
resulting in biased velocity estimates. Hence in the 
presence of seasonal variations, simple linear regression 
techniques to estimate velocities (as above) may be 
inappropriate. 
Another important consideration when computing 
station velocities is the correlation of the position error 
estimates between measurement epochs ( e.g . Williams 
2003). From an analysis of continuous GPS data, the 
resulting coordinate estimate each day, Mao et al. (1999) 
suggests that the velocity error may be underestimated 
by factors of 5-11 if such correlations are ignored. In the 
episodic approach to be used for the SWSZ network this 
is perhaps less critical, but nevertheless will be 
considered. When computing station velocities, the 
stability of the geodetic monument should also be 
assessed, namely that it remains firmly anchored in the 
ground and represents movement of the Earth's crust, 
not simply a local effect. This was addressed in the new 
SWSZ network as best as possible by only establishing 
sites on firm bedrock. 
Forming coordinate time-series (and subsequently 
station velocity estimates) using GPS has the added 
complication that dynamic reference frames such as the 
ITRF are regularly updated, typically every 3-4 years. 
This becomes an issue for long-term episodic GPS 
deformation monitoring, since the precise satellite orbits 
attainable from the IGS are provided in the most recent 
realisation of the ITRF, which may be different from that 
used in the data processing of a previous GPS survey. In 
the data-processing approach adopted here, the 
estimated station coordinates are essentially in the same 
reference frame as the satellite orbit (as inferred via the 
control stations used). Since velocities can only be 
computed from coordinate estimates that are expressed 
in a common reference frame, it is usually necessary to 
re-process the data from a previous survey when the 
latest realisation of the ITRF becomes available, or 
transform the coordinates from the previous realisation 
of the ITRF to the most recent realisation (Boucher & 
Altamimi 1996). Therefore, these two different 
approaches will be experimented with after subsequent 
epochs are measured across the SWSZ. 
The consortium intends to conduct a re-occupation of 
the 48-point network as soon as 2004. Depending on the 
number of different GPS receivers and antennas 
available, several reoccupations will be used to estimate 
inter-instrumental biases so as to better define the 
accuracy of the computed coordinates. However, where 
possible, the same GPS receivers and antennas will be 
used at the same stations as used for the epoch-one 
survey so that common systematic errors will cancel. 
Once reoccupations have been undertaken in 2004, and 
probably again in 2006, the GPS data will be reprocessed 
(using more sophisticated algorithms and techniques that 
may be available at that time, as well as implementing a 
consistent ITRF realisation) to give the first estimates of 
both absolute and relative station velocities in the SWSZ. 
These data can be analysed in a variety of ways, from 
simple vector plots through to stress and strain inversion 
(e.g. Wu et ah, 2001), in order to extract information 
relevant to Geoscience Australia's earthquake hazard 
research, as well as other programmes being undertaken 
by the consortium members. 
Acknowledgements: We would like to thank Geoscience Australia, New 
Zealand Institute of Geological and Nuclear Sciences, Western Australian 
Department of Land Information, Curtin University of Technology and 
the University of Western Australia for providing funding for this project. 
We would also like to thank the reviewers (Paul Tregoning and one 
anonymous) and the editor (Phil Withers) for their constructive critiques 
of this manuscript. 
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