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 Proactive disclosure Print version   | |  New GPS monument design for permanent GPS installations in the Western Canada Deformation Array
M. Schmidt, H. Dragert, W. Hill, N. Courtier
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This paper was presented at the IGS Network Workshop 2000, 12 - 14 July, 2000, Soria Moria, Oslo, Norway
Up until 1998, the monumentation for GPS antenna installations in the Western Canada Deformation Array (WCDA) consisted of steel reinforced concrete piers anchored in bedrock. While these piers provide level, stable monumentation that permits the antenna to be oriented to north, there is little if any security against theft of the choke-ring antenna. Furthermore, these conventional concrete piers may be more vulnerable to near-field EM effects due to the "EM cavity" formed by the space between the top of the pillar and the bottom of the antenna groundplane. A new pier design incorporating a stainless steel pedestal anchored in bedrock provides a level surface for the antenna with an antenna groundplane location several wavelengths above the ground, the ability to orient the antenna as required and a more secure method for attaching the antenna thus minimizing the chance of theft. The use of external PVC shielding combined with sand in the interior of the stainless steel pipe minimizes the effect of diurnal temperature variations. The new antenna domes developed by SCIGN have also been adopted as part of the standard WCDA installation in order to provide snow protection and minimize variations in delay from one site to the next. The installation of this new pier is considerably easier compared to the traditional concrete pier especially at remote sites. The first of these new monuments was installed at the station CHWK in the fall of 1998. Recently, similar stainless steel piers have been installed at new sites on the remote west coast of Vancouver Island. The specification and installation of the new monument are discussed and compared to concrete piers. 
The Western Canada Deformation Array (WCDA) is an array of permanent GPS reference stations located in southwestern British Columbia. Established by the Geological Survey of Canada (GSC) starting in 1991, the array is intended to monitor crustal motion on a regional scale within the tectonically active Cascadia Subduction Zone. The first two stations established, DRAO and ALBH, are co-located with mobile VLBI sites. The GSC operates additional stations outside the Cascadia Subduction Zone: WHIT, located in the St. Elias region; FLIN and DUBO located in central Canada for post glacial rebound studies. The GPS data from all sites is forwarded to the International GPS Service (IGS) and are also used by Geomatics Canada to maintain the Canadian National Spatial Reference Frame. The WCDA forms the northern extent of the Pacific Northwest GPS Array (PANGA). Four new WCDA sites are being established in 2000 (see blue squares on map - above) - these new sites as well as station CHWK use the new stainless steel antenna pedestal.
At present all WCDA sites are equipped with AOA Dorne_Margolin choke_ring antennas mounted on forced_centre monuments. The stability of the GPS antenna is paramount to ensure that a tectonic signal and not a monument displacement signal is derived. Up until 1998 all WCDA monumentation utilized concrete piers. With the exception of station FLIN, the monument construction utilized steel reinforcement rods within the concrete with additional steel reinforcement in the bedrock to depths of up to 3 metres. These concrete piers are topped with a levelled, brass force_centering plate to permit unambiguous placement of the GPS antenna. Initially 0.5 or 1.0 mm circular shims were used to permit orientation of the antenna while still maintaining a tight threading of the antenna to the base plate. It was found that improvements were required in order to guarantee correct orientation as well as to facilitate access to the RF connector. A 10 cm high aluminum antenna base with a stainless steel insert, designed by the Geodetic Survey Division, NRCan, was therefore introduced. This base permits orientation of the antenna as well as easier access to the RF connector.
Station FLIN was established as part of a joint NRCan / NASA post-glacial rebound study. Given that the vertical component is especially critical and given the annual temperature variation expected (-40 to +40 degrees C.) this pier was constructed using special materials. A 4.5 metre Invar rod is grouted 3m in the bedrock. The top 1.5m of the rod is housed in a PVC sleeve inside of a special super-plasticized, high fly ash concrete mix with non-reactive aggregate and polypropylene fibres developed at NRCan's CANMET laboratories. The concrete is anchored to a depth of 60 cm in the bedrock. A UNAVCO mount is used to attach and orient the antenna.
The expansion of the WCDA to sites which are less secure and / or which utilize existing structures forced a re-examination of the use of concrete piers for the robust mounting of GPS antennas. The criteria for the new design included:
The final design permits installation of the pedestal on top of any concrete surface or competent bedrock. Fig A shows the newly installed pedestal at station NTKA on the remote west coast of Vancouver Island. Fig B provides a schematic of the new pedestals. The top of the monument is levelled during the construction phase. Antenna security is provided by bolting the choke ring antenna to a security ring, which once installed, cannot be removed without the use of specialized tools. The antenna element is protected by replacing the antenna radome bolts with bolts which have special security heads, again requiring specialized tools to remove. The use of a solid stainless steel pier, properly installed using special plate bonding epoxy grout insures long term monument stability. The effect of solar radiation is minimized by using a reflective white PVC pipe over the stainless steel column; an air space separates the two. In addition the interior of the column is filled with fine grained, dry sand which is intended to act as a thermal moderator.
[Click on an image thumbnail to view a larger image, notice]
The security system minimizes the chance of the choke ring antenna assembly being removed from the top of the pedestal. Except for the insert all components are made from T-316L stainless steel. The insert, which threads into the base of the antenna, is made from anodized aluminum. The choke ring assembly is drilled in a predetermined pattern to permit security bolts to fasten the choke ring to the security ring. Installation of security mechanism Once the PVC solar shield is in place the security mechanism and antenna are put in place:
Security bolts
The 2" security bolts (E)
are flush with the inside base of the choke ring assembly - specialized
tools are required to install / remove these bolts; At some sites
the antenna radome threaded bolts (*)
have been replaced by security bolts - pictured in this image is the
regular bolt along with a security bolt.
Aluminum insert
The aluminum insert (see fig. 1 & 2 ) has a standard 5/8" 11NC thread to permit direct attachment to the choke ring assembly. Aluminum was chosen over stainless steel to:
The 5/18" groove ensures that the cylinder cannot be pulled up as the hex bolts aline with the groove. Notes:
The base plate is constructed of 5/8" (1.6 cm) thick stainless steel. The size of the base plate can be varied to suit the installation. The standard size used to date has been 24" X 24" (61 cm X 61 cm), although a larger base plate was used at CHWK to accommodate the in situ steel reinforcement bars in the concrete reservoir on top of which the pier was placed. Four triangular braces are placed at 90 degrees to each other facing the side of the base plate so as not to interfere with the corner rods and bolts. The reflective PVC pipe is slit in order to accommodate the four braces.
The grout used to seal the area between the base plate and the rock (or concrete) surface is a special epoxy based, non-shrinking grout, Sikadur 42 Grout Pak Multi-Flo. It is a three component grout specifically designed for seating base plates for heavy objects. The three, pre-proportioned components (resin, hardener, aggregate) can be mixed in various ratios to optimize the flow of the grout given different temperature regimes (recommended minimum temperature is +5 °C). Once set, the grout is corrosion, impact, stress and chemical resistant. It has a high compressive strength and is resistant to effects from vibrations. Surface preparation of both the underlying surface (rock, concrete, etc.) and the contact surface of the base plate is important. Both must be clean and free of any agents likely to affect the bonding of the grout. Under normal temperature conditions the forms can be removed within 24 hours.
Three different types of domes are or have been used in the WCDA. The domes provide protection from snow accumulation, wildlife as well as vandalism. The use of metallic RF skirts was introduced in an effort to minimize the near field multi-path for those sites where the antenna is installed on a concrete pier.
The 'EMRA' dome has been in use since the start of the WCDA. It consists of an acrylic dome, attached to an aluminum flange plate which in turn is attached to the bottom of the choke ring assembly. The flange plate extends the effective ground plane of the antenna by 2.5cm. The 'RF skirt (aluminum in this figure) covers the cavity between the bottom of the choke ring / flange plate and the top of the concrete pier and brass base plate.
The SCIGN Short dome (SCIS) is being phased into the WCDA and will replace all the EMRA domes. The use of the SCIS dome obviates the need for the flange plates. The 'RF skirt is being upgraded to stainless steel mesh from the aluminum screens. It is expected the stainless steel screens will have a longer life than the aluminum which appear to weather rapidly. The stainless steel mesh screens are fastened in panels using stainless steel gear clamps; one panel facilitates access to the RF cable for servicing.
The UNAVCO acrylic dome (UNAV) was used at one station only (CHWK). It has been replaced by the SCIS dome. Note: The RF skirts are not required with the new stainless steel GPS piers.
Steps in relative height Effects of changes to antenna environment The effects of changes to the antenna environment such as adding an antenna base, adding or changing domes and adding RF skirts must be accounted for when deriving rates of motion. The 'raw' relative vertical changes (red line - top) and the relative vertical change once the steps have been removed (green line - bottom) between the reference station DRAO and station WILL are plotted in the above figure. There are five clear steps visible in the uncorrected graph. Before examining each step it is important to understand that the magnitude of these steps are a function of the analysis technique used and are not an absolute calibration. For this time series the CGPS22 analysis package was used. The first step (1) is coincident with the switch from P-code tracking to Y-code tracking (+11.2mm +/- 1.4mm, 1 sigma). Steps 2 - 5 are a direct effect of changes to the local environment at either DRAO or WILL. The addition of the EMRA dome and flange plate at WILL yielded an apparent relative vertical change of 20.6mm +/- 0.9mm (step 2). The 3rd step noted (+8.1mm +/- 0.8mm) was due to the addition of the 10cm antenna base (see fig 5) at DRAO. Step 4 (-11.7mm +/- 0.8mm) was due to two simultaneous changes made at WILL, the addition of a 10 cm base and an RF skirt. The last major step, No. 5, of -7.5mm +/- 0.7mm is due to the addition of an RF skirt at DRAO. Close scrutiny yields a 6th step at about Day 1500. This is due to a reference frame change and is not considered to be statistically significant. However what is significant is the corrected time series v.s. the uncorrected. Any time series regression analyses must allow for step functions or resulting estimates of long term linear trends will be biased. Clearly the slope of the two graphs are significantly different. Change of dome
A UNAVCO dome was installed at station CHWK in 1998. A slight crack developed in the dome due to thermal contraction and was replaced by a SCIGN short dome (SCIS dome). The graph in fig. 30 shows the effect of this changeover. A step in the vertical of -8.1mm +/- 1.1 mm was introduced as a direct result of a change in dome. The analysis was carried out using Bernese 4.2 and as with the results shown in fig. 30 it must be emphasized that this is not an absolute calibration. The analysis strategy, including tropospheric modelling and elevation cutoffs, can affect the magnitude of the step.
The Western Canada Deformation Array (WCDA) is a network of automated continuous Global Positioning System (GPS) stations located in southwestern British Columbia. Started by the Geological Survey of Canada in 1991, the network has gradually expanded to the current 8 sites which now also serve as the northern portion of the Pacific Northwest Geodetic Array (PANGA). The initial objectives of this regional array were to 1) provide high-quality GPS data for global geodynamic studies; 2) provide a precise, common reference frame for all deformation surveys carried out in this active seismic region; and 3) serve as a strainmeter to map regional strain and monitor possible transient strain signals. As such, the WCDA was a new tool being adopted in a program of crustal deformation studies that had utilized tide gauge, levelling, precise gravity, and trilateration surveys. The utility of this new tool has proven itself several times over: The infra-structure is in place and we currently collect, verify, distribute, and archive the continuous GPS data in near-real time with minimal manual intervention and therefore minimal manpower; continuous regional reference stations allow the integration of strain results from repeated campaigns, and horizontal strain tensors can be derived in a common regional framework; relative crustal motions have been resolved with a precision of better than 1 mm/yr within a time span of two to three years and these observed motions are helping to constrain models of the Cascadia Subduction Zone; seasonal and other transient signals have been observed emphasizing the usefulness of continuous GPS coverage; sudden shifts in positions have been found to be caused by near-field effects on the antenna phase-centre location and pointed out the critical importance of antenna set-up. Although already invaluable to current studies of regional crustal dynamics, the sparseness and limited extent of this array present problems. The small number of continuous sites makes each site critical for the robust estimate of regional strain and great care must be exercised, beginning with the installation of stable monuments through to the elimination of non-tectonic effects in the analysis of data. This sparseness also prevents the resolution of variations in the strain field related to active structures of modest (~100 km) spatial scales and it prevents the clear identification of aseismic signals. With its current geographical coverage, the WCDA network limits deformation monitoring to the north Cascadia margin and leaves unmeasured the crustal motions associated with the extensive seismic activity along the Queen Charlotte Fault and regions further north.
The authors wish to acknowledge the collaborative role of the Geodetic
Survey Division, Geomatics Canada. Ongoing funding for the operation of
the WCDA is provided by Natural Resources Canada; collaborative resources
are provided by the University of Victoria through the GEOIDE Project,
specifically infrastructure costs for the establishment of four new sites
on Vancouver Island. The efforts of Yuan Lu (software development), Brian
Schofield (daily data management) and Alex Smith (UVic - GEOIDE) are gratefully
acknowledged as is the GPS data analysis carried out by Joe Henton and Tom James.
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