At the end of this reading, you will no longer look at smartphones the same way. With their motion sensors, they were known to be capable of measuring earthquakes. With their photographic lenses, they transform into microscopes. Some even speak of “smartphonics” to designate all the optical, magnetic and mechanical experiences that can be achieved with these devices.
Now, thanks to a team from Google in California, associated with Harvard (Massachusetts) and Colorado universities, we are learning, in Nature of November 13, that they can also be used to see what is happening above our heads, hundreds of kilometers above sea level, in the ionosphere. This layer of the atmosphere, between 50 and 1,500 kilometers, is ionized, that is to say rich in charged particles, such as electrons, appearing following collisions of solar radiation with molecules in the sky. It helps radio waves travel great distances and is home to the magnificent Northern Lights. Many satellites circulate there.
This region is also subject to disturbances, which, in turn, affect the quality of transmissions. On February 3 and 4, 2022, thirty-eight satellites of the Starlink constellation of the company SpaceX were lost due to a surplus of electrons brought by a solar wind. On November 18, 2023, the explosion at an altitude of 149 kilometers of the Starship launcher from the same company “holed” the ionosphere for an hour. On January 15, 2022, the eruption of the submarine volcano Hunga Tonga (Tonga) briefly stripped an area of the ionosphere of its electrons.
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To monitor these hazards with significant consequences, earthlings have satellites and 9,000 ground stations capturing signals from location satellites (GPS, Galileo, Glonass, etc.). The latter, in fact, need to know the state of the ionosphere between the station and the satellite to enable precise localization. Because the particles can delay the signal and therefore make this position more uncertain. A few hundred nanoseconds less are transformed into tens of meters of error on the ground… These stations constantly estimate the state of the atmosphere and make it possible to compensate for the defects created by its moods.
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But coverage of the globe is not complete. Hence the idea of using smartphones which are equipped with GPS chips to make these measurements everywhere and easily. It is based on two advantages. The first is that modern chips use at least two frequencies, which are not equally affected by disturbances in the ionosphere. The electron concentration is evaluated by comparing the arrival times of these two signals.
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