How to integrate GNSS data into a smart deformation monitoring strategy: Your questions answered

NavStar’s Robbie Praeg and GroundProbe’s Antonio Rocha recently presented a live webinar, How to Integrate GNSS Data into a Smart Deformation Monitoring Strategy. The duo explored using NavStar’s GNSS technology and data visualization platform, GeoExplorer to monitor displacement alongside other in-situ sensors. 

This webinar covered: 

• An overview of GNSS monitoring devices 
• The benefits and considerations of using GNSS in remote and harsh environments 
• Real-world examples of monitoring applications using GNSS data 
• How to best view GNSS data alongside data sets from other sensors 
   

We received several questions during the webinar that we’ve answered below: 

Question: What’s the maximum spacing between the base station and the rovers? 

Robbie: This is highly dependent on the specific site topography, potential obstacles, and possible interference from other devices using radio. For reference, using 900 MHz radio, in a number of instances we have reached Base Station – Rover communication spaced over 30 km apart.  A Repeater can be used in instances where maximum spacing may be exceeded, to bounce data further along a radio network or to rebroadcast corrections from the base. 
 
 

Question: At what depth should the anchor that supports the GMS be located?  

Antonio: It depends on your site conditions. It could be as simple as a vertical pole driven into the ground or an installation onto a small cement base. What we want to avoid is for your GNSS device to capture the movement of a fragile installation. If the installation is stable, your readings can be trusted. 
 

Question: How do the batteries perform in temperatures colder than -30 °C? 

Robbie: NavStar’s origins are in Kelowna, on the west coast of Canada, where it can get cold. There are many installations in very cold climates, some being in Northern Canada, for example. With any battery, you’re going to get some variability relative to the conditions externally, and measurement interval chosen. The GMS units have been installed in many cold climates and are capable of reaching a lifespan of up to 3 years, even in these Northern parts of Canada where you’re getting – 30°C below and beyond. Some of these areas where you’re lucky with -30°C in the winter. This unit is designed to be installed in harsh climates. 
 

Question: Does the amount and the scale of noise make this system unsuitable for monitoring with a view to identify critical situations?  

Antonio: If you remember the discussion about monitoring strategy, GNSS is closer to the base—so less or smaller geotechnical risk. We’re talking about a system that will produce data or information every 30 minutes with a precision of 10 to 15 mm.

If we think of slope stability radars for a safety-critical solution, along with thinking of alarms and immediate decisions to the operations, in terms of an equation, then GNSS would be more on the background monitoring and the trend monitoring. With GNSS, you could know those movements, know the trends, from the beginning, and as the risk increases, you would look for another monitoring solution that is faster and more precise and has bigger coverage. 
 

Question: Do you have experience or recommendations for establishing “moveable” GNSS monuments? For example, if you want a GNSS survey-monument established in a location where construction may require it to be temporarily relocated to another monitoring site of interest, is there a configuration that would allow for relatively low-effort relocation (i.e. without digging up buried posts)? 

Robbie: A key requirement for a GMS unit is that it is stable—that’s the main piece. After that, the user would have the ability to transport the unit to an alternative location, input the new coordinates into something like the GeoExplorer platform, so that it can collect data again. There are options for moving. 
 

Question: What are the sampling/reporting rates and corresponding accuracies that you can accomplish, across various aggregations?

 
Robbie: The default sampling rate is readings every 60 minutes. As for the data itself, we’ll see averaging over 24 hours to get the best results. Each time a new measurement is received, that reading would be added to that data set to create the average. It’s a rolling average value being recomputed each time a new measurement added within the platform.  

 
Question: Can GNSS be used in tunneling projects? 

Robbie: An important requirement for these sensors is the rovers require close to a 360-view of the sky to be able to communicate with that satellite.  While there is opportunity for GNSS rover installations to complement monitoring solutions in tunnel projects, directly subsurface deployments would not be favorable in most circumstances.  
 
This transcript has been edited for clarity. Watch the webinar on-demand to learn more about how GNSS can be used with GeoExplorer 
 
 

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