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24.07.18 - Summer series. Students' project (3/9) – The meteorological sensors carried into the upper atmosphere by weather balloons are often lost as they return to earth. As part of their Bachelor’s project, five EPFL students worked on a system to recover this equipment.

Dozens of balloon-borne radiosondes are released into the atmosphere every day. They measure temperature, atmospheric pressure, humidity and wind direction at different altitudes before eventually falling back to earth. In most cases, this high-tech equipment is never recovered. But a group of five EPFL students,* studying either environmental sciences or physics, could change all that. For their Bachelor’s project, they developed a system that can help guide the radiosonde’s trajectory as it falls to earth so that it can be found and reused. Through a single test flight from the roof of a campus building, they proved the feasibility of their concept.

Radiosondes are carried aloft by giant helium-filled weather balloons that expand as they rise through the air – in some cases to an altitude of 30 kilometers – until they eventually pop. A small parachute attached to the onboard equipment automatically opens once the maximum altitude has been reached, preventing the radiosondes from causing damage when they touch down.

The students’ focused their attention on the parachute release. “Our system controls the exposed surface area of the parachute, directing the radiosonde towards air currents that will guide it to an accessible landing spot,” says Hugo Cruz, a student in Environmental Sciences and Engineering. His classmate Lorenzo Donadio adds: “The main thing is to make sure the radiosonde doesn’t end up on the side of a mountain, at the bottom of a lake or in some no man's land.”

Their parachute system is fully automated and run by computer code that the students wrote themselves. It is triggered as soon as the radiosonde starts to fall: a small motor lets out the parachute cords and reels them back in to control the speed of descent. The radiosonde can thus be navigated to an air current that will take it in the desired direction. The system uses up-to-date weather data along with GPS coordinates that are refreshed every 30 seconds relative to a reference point. A tracking device is used to pinpoint where the radiosonde lands.

As light as possible

The students’ invention, while practical and clever, faced some hurdles. For starters, their parachute system had to be able to withstand all sorts of weather, including extremely high winds. It also had to be as light as possible so that the radiosonde could be easily redirected while freefalling. This seriously limited the instruments and materials that they could use. “We also had to master a number of concepts that we had never studied before, especially in computer science and physics,” says Julie Reznicek, an environmental engineering student.

They did a single trial run of their system this past spring, flying their weather balloon to an altitude of around 10,000 meters above Lake Geneva as planned. The equipment was then recovered in a field in Epalinges, just north of Lausanne. While the mechanism used to release and retract the cords worked well, the students noted that the microcontroller lacked the power it needed to record all the data. Some of the students will continue to refine the device this summer, and a second test flight with a larger balloon and more reliable equipment is planned for September. There’s no doubt that these students have set their sights high.

* Julie Reznicek, Hugo Cruz, Lorenzo Donadio,Simon Léo Albers and Guillem Rivas Castellá.

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