Journal of the Royal Society of Western Australia, 87(1), March 2004 
California (e.g. Bock el al. 1997) to obtain continuous 
coordinate time-series for the points occupied, from 
which velocities can be computed and surface 
deformation inferred. However, this would be an 
extremely costly undertaking across the reasonably 
remote SWSZ, with each station costing up to 
approximately AS70000 to install. A more economically 
efficient compromise is to establish a network of 
permanent ground monuments installed rigidly on 
bedrock, coupled with episodic repeat GPS surveys 
collected over several days per station, as was used by, 
for example, Clarke el al. (1998) and Pan el al. (2001). This 
episodic monitoring approach has been adopted in the 
SWSZ. 
Unlike conventional terrestrial surveying methods, 
modern geodetic-GPS also offers the opportunity to 
directly measure both the absolute (i.e. in a global 
reference frame) and relative (i.e. between points in the 
local region of interest) deformation of the ground 
monuments. However, the small amount of deformation 
expected in the SWSZ still poses a technical challenge to 
continuous, and more so the episodic, GPS deformation 
monitoring. Based on the inconclusive results of the 
previous geodetic surveys over an approximate 30-year 
time span, the most important consideration is to 
determine if the new GPS-geodetic time-series will give 
an accurate representation of actual surface deformation, 
not simply an artefact of errors associated with the 
measurement and data processing techniques. As such, it 
is necessary to eliminate systematic errors from the 
episodic GPS-geodetic monitoring network as practically 
and economically possible. 
Early in 2002, a 48-point network of permanent 
ground monuments was established by the consortium 
(Fig 1). For each of the 48 sites. Geoscience Australia 
selected potential granite rock sites from digital 
geological maps. DLI then used these to select the final 
site location and installed near-ground-level forced- 
centring pillars in bedrock. These ground monuments 
(Fig 2) were set using three epoxy-resined bolts set in 
drill-holes in the bedrock with nuts to level the stainless 
steel baseplate with respect to the local vertical. Once 
level, the screws and baseplate were set in more epoxy 
resin and fast-setting concrete, respectively. A standard 
5/8-inch (Whitworth) thread had previously been set in 
each baseplate for the GPS antennas, or other geodetic 
instruments, to be re-centred exactly during each 
episodic occupation. Reference marks were also set at 
~120-degree intervals and at ~3 m surrounding each 
mark, so that if disturbed or destroyed, the primary mark 
can be relocated to millimetre-precision. 
Between 30 April and 22 May 2002, approximately 20 
sites were simultaneously surveyed near-continuously 
(except for equipment failures at a few sites) for between 
5 and 7 days. Each site used forced-centred GPS 
antennas, oriented as closely as possible to north using 
shims [washers of different diameter], on the permanent 
ground marks (Fig 3). Dual-frequency code and carrier- 
phase data were logged at a 30-second interval from all 
GPS satellites above a 5-degree elevation angle. The 
geodetic GPS receivers used were Ashtech Z Surveyor 
and Z12, Trimble 5700 and Leica CRS1000. The GPS 
antennas used were Ashtech [Dorne-Margolin-type] 
choke-ring, Trimble Zephyr (Fig 2), and Leica [Dome- 
Figure 2. A newly installed ground monument (left); the ground 
mark to the right is part of the existing Western Australian 
geodetic network maintained by DLI. 
Margolin-type] choke-ring (Fig 3). The Dorne-Margolin- 
type antennas reduce multipath (reflected GPS signals) 
by the geometry and depth of the choke-rings, and the 
Zephyr antennas use advanced signal processing to 
reduce multipath (see Dawson 2002). Cost considerations 
and equipment availability among the consortium 
members precluded the preferable use of the same 
models of GPS receivers and antennas. 
Five Ashtech Micro Z GPS receivers and antennas 
were operated continuously for the entire campaign (at 
newly established ground marks SZ07, SZ15, SZ20, SZ33 
and SZ48 (Fig 1)). This provided a "backbone" of stations 
in order to precisely link the surveys. The remaining sites 
were occupied in three near-one-week phases (i.e. 
approximately 20 simultaneous occupations including 
the five continuous sites) from north to south with three 
survey teams moving and maintaining around five 
stations each. The mobile survey teams would 
periodically check the receivers, which in some cases 
proved essential because of power failures. At least 5 
days of near-continuous dual-frequency GPS data were 
obtained from all but two of the sites. 
Figure 3. The GPS receiver was powered by a combination of 
solar panels and car batteries and left unattended (excepting 
checks) during each site occupation. 
4 
