A primary component of the PBO permanent GPS stations are the GPS monuments. The GPS monuments are designed to provide a stable and securely anchored structure, to which the GPS antenna is mounted.
The PBO GPS network consists mainly of two types of monumentation, the deep-drilled, braced monuments (DDBMs) and the short-drilled, braced monuments (SDBMs). In addition to these two primary types are the Alaska thermopile monument, the borehole strainmeter hybrid monument, and the building mount monument. These three types of monumentation are used in special circumstances.
The DDBM is designed to create a highly rigid and immobile structure isolated from surface soil movement and cemented in place at depth. The monument consists of five legs (stainless steel pipes) placed and grouted into drilled holes, and welded together above the surface to create a "tripod" frame. Of the five legs, the center leg is vertical and the four other legs are installed at angles to brace the vertical leg. The holes are drilled to a depth of 40 ft with a truck or track mounted drill rig, and thus vehicle access to the site is required for this type of monument.
The SDBM is similar to the DDBM in appearance, but is used where competent rock is at the surface, and where cost and/or access issues prevent the use of a full size drill rig. The SDBM is composed of 4 legs (solid stainless steel rods), epoxied into rock to a depth of up to 6 ft, and welded together to create a "tripod" frame. The material and tooling needed to construct an SDBM can be transported with a single pickup truck, slung to the site via helicopter, or carried via horseback or even hiked in a short distance. With reasonably good conditions, a crew of two can install a station in two long days. Because the specified antenna height range is slightly lower than that of the DDBM, sky view and multipath obstructions, as well as snow pack, can be more problematic.
In the permafrost region of central and northern Alaska, ground conditions are such that neither the DDBM nor the SDBM will allow for solid anchoring to the sub-surface. There is generally not rock at these locations, and so the thermopile monument is used to achieve a solid bond to the always frozen permafrost layer. The monument consists of a single pressure vessel, filled with high pressure carbon dioxide. The thermodynamic properties of the carbon dioxide allow the monument to stay frozen in place during both the long cold winters, as well as the summer months when the upper layer of the soil thaws.
Where borehole strainmeters have been installed, it is often cost effective and relatively simple to install a GPS antenna mounted to the strainmeter casing, using a hybrid monument. The hybrid monument structure bolts directly to the 6" strainmeter casing pipe, which is then reduced down to a 1.25" nipple, which protrudes from the strainmeter equipment hut and allows attachment of the GPS antenna.
In rare cases, there may be times when none of the above monuments are suitable, generally due to an abundance of buildings and other structures in the area. In such cases, a GPS antenna can be mounted directly to the roof of a building with the building mount monument. This monument is secured to a corner or other stable portion of a structure with epoxy and short stainless steel legs. Ideal building candidates are well established, reinforced masonry structures, 2-5 stories in height.