With total stations, asset monitoring is much more common and better known than monitoring with spatial sensors for several reasons. Total stations are an amazing choice in most of the situations provided that their ability to monitor up to hundreds of points from a single setup in real-time and high accuracy. But GNSS monitoring is useful for several challenging monitoring projects which require the special capabilities of GNSS receivers, in time by themselves and sometimes in combination with total stations. The GNSS capabilities can be divided into two categories which are long term and real-time.
Often with total stations, Real-time monitoring is performed when a large number of points are required for measuring but GNSS is much better in specific situations. Like for instance, in cases where just a few points are required to be monitored like on a dam, GNSS receivers can be the better option as they work in all weather conditions and they do not need long lines of sight to ground-based control and they continuously keep updating 3D position without the need of reference to other earthbound points. As a real-life example, let’s take the example of designed pipeline monitoring systems. This system passes through a deep valley and river with structures supporting which are set unavoidably in permafrost. Just a few points are required to be monitored, in near real-time for detecting any long-term instability or rapid thawing. Reliable long sightlines were not available as most of the surrounding area was also permafrost. When 3D positing capacity in extreme weather is lost, it can be a real problem as snow loads and heavy rains are a precipitating form for land movement. Another specialty of GNSS is Real-time monitoring at high speed like for example tracking the sway of a bridge at mid-span.
Long-term monitoring is also well suited for GNSS. Monitoring with receivers may be at least as common as total station monitoring for assets like dams, bridges, landslide areas, and open-pit mines in cases where rapid catastrophic failure is not expected, but knowledge of relatively small movement over a month can be valuable. A great example is the offshore drilling platforms which rarely fail quickly but monitoring the movement which is compared to the seabed can be incredibly useful for safety and operation. Without stable back sights available off-platform, total stations can fail and may not have much use. As real-time feedback on position is not so critical on projects like this, usually adequate information can be offered with very few receivers acting as rovers without the support of an RTK network.
Another interesting use of GNSS receivers for monitoring networks is when co-located with total stations as a positional check. Like for instance, in case of a landslide monitoring situation where a total station may need to be placed in a location that is itself unstable, a prism-mounted receiver can be placed nearby and included in the monitoring routine of the total station. This offers an excellent check on the absolute position of the occupied point and the overall network. This is because they are very accurate clocks when added to being good spatial sensors, sometimes GNSS receivers are used for providing timing for sensors like accelerometers.
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