By Liz Kalaugher at the European Geosciences Union General Assembly in Vienna
As well as helping millions of smartphone owners and car drivers find their way around unfamiliar territory, GPS data have become vital to environmental and Earth scientists. That includes those, like Kristine Larson of the University of Colorado, Boulder, US, who monitor small ground movements in areas prone to earthquakes. (Larson, incidentally, was also the first to show that GPS can measure seismic waves during large earthquakes.)
Around 8 years ago, Larson realised that there was more to GPS data than reached the eye, or rather than reached the antenna directly. The GPS antennas installed in field sites around the world could, as well as detecting beams straight from the satellite, also gain data from signals that bounce off the ground and reach the detector fractionally later. These reflected signals reveal information about the Earth’s surface, such as snow height, vegetation water content, moisture in the top 5 cm of soil, or sea level.
The system works like an interferometer, Larson explained at a press conference at the European Geosciences Union (EGU) General Assembly in Vienna. Since the reflected signals take longer to reach the antenna, they’re out of phase with the beams that get there directly, which sets up an interference pattern. And because the satellites sending out the signals are moving, the interference pattern changes continually. Unsurprisingly enough, the technique is called GPS Interferometric Reflectometry. In a nutshell, the amplitude, phase and frequency of the reflected signals provide plenty of information about the ground that’s bouncing them back. Sea-level rise or the addition of snow changes the height at which the reflection takes place, altering the distance the beam travels. If there’s wet vegetation on the ground, it attenuates the signal, whilst beams will penetrate moist soil further.
The resulting measurements, which can cover roughly 1000 square metres of ground around the antenna, are valuable for climate scientists, weather forecasters, water managers, and engineers that validate satellites (SMOS). Since most GPS networks were installed with taxpayers’ money, they provide data without charge; Larson says that “essentially you have a measurement network for free”.
Larson’s pilot in the western US is using data mainly from the EarthScope Plate Boundary Observatory – you can see daily results for around 500 sites at http://xenon.colorado.edu/portal. The technique is applicable to any GPS network; Larson now plans to use it for monitoring worldwide.
- The EGU has awarded Larson this year’s EGU Huygens medal in recognition of her work.