WingtraOne and Septentrio GNSS Receivers Team Up to Provide 3D Positioning Accuracy for Avalanche Prevention

Avalanches can be extremely dangerous especially for off-piste skiers as well as for small towns situated below the slopes. As ski season comes upon us, it’s a good idea to check the avalanche conditions of the ski slopes you plan on frequenting. There are over 1000 avalanches occurring every year in the Swiss Alps alone. Local communities put up steel fence barriers along the slopes to prevent avalanches from encroaching near their town. To build such snow barriers, steep rock faces and cliffs need to be surveyed with utmost precision.

Swiss-based survey and mapping company Darnuzer Ingenieure AG has the expertise in surveying snowy peaks such as those above the famous resort town of Davos, located in the Swiss Alps near the Austrian border. Above the city, towers the magnificent Schiahorn mountain, its slopes lined with pistes and chairlifts. To protect the town and the skiers, Davos community plans to build avalanche barriers along the steep slopes. The ideal location is along one square kilometer on the Parsenn slope, right above a ski piste. To plan the works along this uneven rock face, a detailed 3D reconstruction of the area is needed. Their WingtraOne, fixed-winged drone features a top-quality camera, combined with a Septentrio high-performance GGNSS receiver,* according to company materials. It can fly at 150 m altitude making orthophotos without the need of GCPs (Ground Control Points) or a real-time base station link. GCPs are markings on the ground with a known location, used as a positioning reference when making orthophotos. Placing GPSs on the ground is time consuming work. Darnuzer’s surveyors can get their job done over 4 times faster with the Wingtra drone where no GCPs are required. After a couple of hours of flight, the survey job is done.

Figure 1: Steel fences hold the weight of snow and prevent avalanches during heavy snowfall. Before such snow fences can be constructed, the hill needs to be surveyed with high accuracy.

“Even in the valleys where satellites visibility is low, we always get the positioning we need due to the receiver having access to several navigational satellite systems. The positioning accuracy we are looking for is generally 2-5 cm,” says Dr. Bruno Wirth, senior surveyor at Darnuzer Ingenieure AG. “The Wingtra system has worked out-of-the-box and we have never had any issues with interference thanks to Septentrio’s GNSS* technology.”

The wide-open space between the peaks provides few barriers for radio waves, which might be broadcast by near-by communication towers. Such radio waves also known as Radio Frequency Interference (RFI) can overpower GNSS satellite signals and interfere with GPS receiver operation, reducing accuracy or even causing loss of positioning. Robust interference mitigation technology is key to ensuring continuous and trustworthy positioning of the GNSS receiver.

Maria Simsky, Technical Writer, Septentrio answered some questions for GISCafe Voice about Septentrio’s high performance GNSS receiver and post-processing engine GeoTagZ.

Figure 2: The WingtraOne VTOL drone uses a high-precision GPS/GNSS receiver to accurately
geotag its images.

1. What is the accuracy of a drone compared to the GCP coverage?
The mean accuracy error has improved by a factor of 3 since transitioning from GCPs to high-precision GNSS on the drone.

2. What is an intuitive Wingtra workflow?

The WingtraOne VTOL drone Vertically Takes-Off and Lands on a rocky slope and flies automatically, meaning no drone pilot training required and no risk of damaging the payload with “belly” landings. A single surveyor takes the drone to the rocky Parsenn slope during the summer season, capturing ground images without snow needed for the 3D model. As the drone flies, it takes high-resolution images which are immediately geotagged and time stamped. Darnuzer’s surveyors then post process the images for RTK (Real Time Kinematic) centimeter accuracy using the intuitive Wingtra workflow, powered by the Septentrio post-processing engine GeoTagZ. The orthophotos are then processed by Pix4D software and a detailed 3D Digital Terrain Model (DTM) of the Parsenn slope is created.

First images are taken with the drone without GCPs or base-station link. Then GNSS post-processing it done with GeoTagZ to achieve RTK cm-level accuracy. Then Pix4D software is used to stitch the orthophotos together and to create the 3D model of the area.

Figure 3: Post-processing software like Septentrio GeoTagZ georeferences images with cm-level accuracy to enable the creation of a 3D models such as the Parsenn slope shown on this image.

3. Explain the positional accuracy in the mountains.
In the low satellite visibility environment of the mountains highest-accuracy can still be achieved. Multi-constellation multi-frequency GNSS technology delivers robust positioning due to its ability to  communicate with any of the navigation satellites visible in the sky.

4. How does the interference mitigation technology work?
AIM+ technology detects and neutralizes interference resulting in faster set-up, reduced downtime and secure operation. AIM+ protects against simple narrow-band interference as well as more complex wide-band interference, including jamming and spoofing. Your receiver’s web interface allows you to analyze interference with the spectral plot, to be able to determine the type of interference and its possible source.

Narrow-band interference can be caused by electronic devices and effects only a small portion of the GNSS frequency spectrum. To mitigate the effects of narrow-band interference, 3 notch filters can be configured either in auto or manual mode. These notch filters effectively remove a narrow part of the RF spectrum around the interfering signal. The L2 band, being open for use by radio amateurs, is particularly vulnerable to this type of interference. The effects of wideband interference, both intentional and unintentional, can be mitigated by enabling the WBI mitigation system. The WBI system also reduces, more effectively than traditionally used pulse-blanking methods, the effects of pulsed interferers.

Figure 4: One of the most famous landslides of 2019 was the one that halted the cyclists during stage 19 of Tour De France.
Image courtesy Twitter, @joscelinryan.

5. Explain why you are doing tree and rock removal. Is this used for stone avalanches as well?
Tree and rock removal was done to smooth out the 3D model. Darnuzer needed to provide ground heights since avalanche barriers were going to be built by putting concrete sockets into the ground.

6. How is ground movement monitored?
The drone flies over the area taking geotagged images and post-processing for centimeter accuracy (similar process as with avalanche barriers). Orthophotos are then compared with those of the previous year. Darnuzer manually measures the distance between multiple landmarks such as rock to calculate displacement.

Snow avalanches are not the only type of avalanches posing danger in the steep Alpine mountains. Stone avalanches and landslides are just as hazardous. The Val Bruna is a steep, rocky slope rising above the ski resort village of Müstair. Heavy rainfall erodes the soil, creating a potential landslide threat. Every year Darnuzer’s WingtraOne drone flies over the Val Bruna slope surveying soil movement. Precise measurement of ground movement is key in helping experts asses the risk of landslides in the coming year.

*GNSS refers to Global Navigation Satellite System such as the American GPS, Russian GLONASS, European Galileo, Chinese BeiDou. These satellite constellations broadcast positioning and timing information which is picked up by the a GNSS receivers on Earth and used to calculate its global position.

Tags: data, geospatial, GIS, GNSS, GPS, imagery, intelligence, laser scanner, LiDAR, location, mapping, maps, mobile, mobile mapping, remote sensing, satellite imagery

Categories: analytics, avalanches, climate change, cloud, data, field GIS, geospatial, GIS, government, GPS, laser radar, lidar, location based sensor fusion, location based services, location intelligence, mapping, mobile, photogrammetry, public safety, remote sensing, satellite based tracking, satellite imagery, sensors, Septentrio, spatial data, survey, UAS, UAV, UAVs

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