57 CHC i83 User Manual
Large Reflective Surfaces Nearby
Your receiver trusts that the signals that it receives have traveled directly from the satellite to your antenna. Large
nearby surfaces present opportunities for the receiver to have signals arrive having taken multiple paths (multipath) or
entirely the wrong path.
Not only do these tanks block the view to the South, but they also have metal-reflective surfaces that provide a multiple
length signal path for every signal from every satellite to the observation area:
(this image is looking South)
Flat metal surfaces are bad. Corrugated metal surfaces (like corrugated roofing) are even worse. Some mirrored glass
windowing used on building exteriors is reflective at microwave frequencies. Box truck bodies, metal buildings, metal
roofs and open water are all potential sources of multipath.
Deep Canyons
Locations at the bottom of deep canyons, especially East-West trending canyons will present full, 100% obstruction
below the ridge line to the South. Most of the GPS satellites are to the South. This is BAD.
RBUT (below) is a NGS CORS site and is the closest CORS site to the iGage office in Salt Lake City Utah. This site is
hindered by a solid mountain 30° mask to the South. This could be a challenging location for GPS observations and is not
a great location for a CORS site.
Moving further North would gain elevation, effectively lowering the southern mask.
Power Poles
< 500 KV DC Transmission Lines and Tower
This class-1 elevation bench mark with measured gravity is unfortunately in a location that is no longer suitable for GPS
observations. It was set prior to the construction of the power line. This is a BAD location.
You should avoid locations that are under high voltage transmission lines and have large steel towers directly to the
south.
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