Report | ”Funny Signals” - Kristine Larson uses GPS data for detecting soil moisture and measuring volcanic ash plumes

By the help of GPS signals cars, airplanes and farm tractors find their way. By now the technique has reached our everyday life. But it is also useful for detecting soil moisture or vegetation from space and for measuring volcanic ash plume density to improve flight safety, hopefully. One of the pioneers of such new applications is Kristine Larson. Since 2016 she is Alexander von Humboldt Research Awardee and now works as a visiting scientist at the GFZ in section Space Geodetic Techniques.

Reflected waves

Larson did her PhD in geophysics at the University of California in San Diego. Since 1990 she is working in Boulder and now stays for six month as a visiting scientist at the GFZ. She has used GPS for measuring the motion of tectonic plates and their deformation related to earthquakes. “Continents move just a few centimeters per year. After a while it was too slow for me, I wanted to investigate more rapid processes”, she says with a twinkle in her eye. As a matter of fact the measuring methods were getting better in the late 1990s and not as challenging in a technical way for Larson. But there was another issue bothering the geophysicist: “In the GPS data we gathered for earthquake analysis there were some “funny signals”. We had no idea whether we could successfully eliminate these signals but I wanted to get to the bottom of it!”

It turned out: the funny signals were reflected waves. They did not come directly from the GPS satellites, orbiting the Earth 20,200 km above our heads, to the antenna. Instead, the waves made a small diversion to the ground where they were reflected, finally reaching the antenna from below. “The crazy thing is we can measure these reflected waves”, Larson explains. “If there is snow on the ground the waves change their frequency, soil moisture leads to phase changes, and vegetation water-content affects the amplitude.”

These findings help Larson and her colleagues to describe the properties of surfaces below GPS stations – from the drought-ridden western United States to permafrost soils in Alaska. The data are now being used for climate studies, and satellite sensor validation. Here in Potsdam she will discuss further applications with colleagues from GFZ as well as from the Potsdam based institutes of the Alfred-Wegener-Institute for Polar Research (AWI) and the Potsdam-Institute for Climate Impact Research (PIK).

Investigating volcanic ash plumes

An additional research field is volcanic ash plumes. The fine particles can affect even regions several thousand kilometers away as shown by the eruption of Eyjafjallajökull in Iceland in 2010. For safety reasons thousands of flights in Europe had been cancelled. “Ash plumes are difficult to study using satellites if there are clouds, and radars are not often available near volcanos”, says Larson. “But GPS signals can help us, by looking at the signal distortions from plumes, similar to the way we used GPS to look at reflected signals on the ground.” So GPS could be a new way to monitor erupting volcanoes.

About 1500 active volcanoes are known worldwide, from Italy to Indonesia. Roughly 500 million people live in regions that might be affected by an eruption. Hence, it would be a worthwhile goal to improve monitoring techniques and safety of the people, states Larson.