Site Stability
The stability of a GNSS site is mainly dependent on the quality of the foundation, antenna monument and antenna mount.
Monument Foundation¶
Ideally, IGS CORS antenna monuments are structurally fixed to bedrock using drilled-braced tripod structures or tapered pillar type monuments. This is especially important for stations contributing to the realisation of the ITRF.
Roof or structure mounted antennas (e.g., attached to a wall) should be avoided unless environmental or economic circumstances don’t allow a different approach. Those sites may be fixed to bedrock, a concrete foundation in stable soil, and on load bearing components of concrete, brick, or masonry structures, preferably near the intersection of two walls. Due to thermal expansion and wind loading, structures less than ten metres in height are preferred.
Structures with high thermal expansion and buildings undergoing post-construction settlement should be avoided. To reduce the effect of multipath from the building, avoid structures with metal cladding or roofs.
Antenna Monuments¶
The monument of a GNSS site should be designed to provide a stable and securely anchored structure to which the antenna is mounted. For all IGS CORS, the required characteristics of the monument include:
- Short-, medium-, and long-term stability,
- Long-term survivability,
- Minimal multipath,
- Sufficient height to minimise obstructions,
- Simple design for ease of manufacture, installation and maintenance,
- Low maintenance,
- Corrosion, erosion, and subsidence resistant,
- Capability of bearing the mass of the antenna,
- In reasonable distance to the receiver,
- Tamper-proof design.
If possible, it is recommended to attach the monument to solid bedrock to guarantee an extremely stable foundation. The depth of the monument anchor should be chosen in a way that is unaffected by frost action. The monument should be higher than snowfall levels.
The width of the top of the monument should be less than the antenna diameter to minimise multipath from the monument edge and upper surface. It is recommended to avoid the amount of metal in close proximity to the antenna. In general, materials with low coefficients of thermal expansion should be used when high temperature variations are expected. If thermal expansion is a concern, insulation should be implemented. Aluminium monuments should be avoided due to the increased thermal expansion of aluminium. Roof monuments should place the antenna a minimum of 50 centimetres above the roof. Avoid structures with a metal roof where possible. If a structure with a metal roof or other reflective surface is used, avoid antenna heights that are multiples of GNSS carrier phase wavelengths (19 or 24 cm).
Use stainless steel bolts and fittings. Use through-bolts that penetrate the entire wall thickness for solid concrete walls unless the through bolt fitting will be exposed in an interior workspace such as an office or hallway. The use of stainless-steel backing plates will help distribute the force of through bolts. Avoid through bolts on cavity walls, as this may pull the skins together and weaken the foundation structure.
Antenna Mounts¶
The antenna mount (or adapter) connects the GNSS antenna to the monument. Once installed the antenna mount should lock the antenna in place so that it cannot be moved or rotated. When an antenna is removed and replaced, the mount should return the antenna reference point to within 0.5 mm and 1° of its original location and orientation. The antenna mount must maintain a level antenna, oriented to within 5° of true North for antenna calibrations to be effective.