It was 6:30 a.m. on a Saturday in 2003 when Kristine Larson realized what she was seeing.
Larson, a professor in aerospace engineering sciences, had been using data from global positioning system (GPS) receivers to record the slow movements of tectonic plates. But that morning she was looking at data recorded soon after the magnitude 7.8 Denali Fault earthquake.
She recalls looking at the GPS data and realizing she was seeing seismic waves that had traveled more than 500 miles from the rupture. Most people thought that GPS would never be accurate enough to see these ground motions, which are traditionally measured by seismometers.
"No one had ever seen that before," says Larson. "Ever. I remember being so excited to share the news, but I couldn't call anyone because it was too early. So I waited until 9 o'clock to call a colleague, because my mother had always told me it was not polite to call people before 9 in the morning."
This is what happens when a geophysicist, who is also an engineer, starts thinking "outside the box." Redeﬁned uses of high-precision GPS are lead ing geoscientists to a new understanding of not just plate tectonics and seismology, but soil moisture, snow depth, vegetation water content, atmospheric water vapor, sea level, ionospheric electron content and volcanic eruptions.
Since that early morning revelation a decade ago, Larson's unconventional uses of GPS have proliferated. Not only was she the ﬁrst person to use a GPS receiver to measure seismic waves, she was the ﬁrst to show that it could be used to measure how rapidly Greenland's ice sheets were moving.
Larson - who came to CU-Boulder in 1990 with a bachelor's degree in engineering sciences and a PhD in geophysics - has been pushing the boundaries of what people thought could be done with GPS for scientiﬁc and engineering applications for more than 20 years.
"For the general public, GPS is just a way to get from point A to point B," says Larson. "For Earth scientists, it's mostly used to measure how the ground moves. But you can extract an amazing amount of information from GPS signals. You just need to think about it."
Her pioneering GPS analysis techniques have been used to study earth quakes in San Simeon, Calif., and Hokkaido, Japan, as well as measuring tectonic motions in California, Alaska, Mexico, Hawaii and the Himalayas (including a GPS-based remeasurement of Mt. Everest that determined it is seven feet taller than previously thought). More recently, Larson and her colleagues have been using data from an existing GPS network located in the western United States to measure snow depth, vegetation water content and soil moisture. The statistics Larson compiles are derived from reﬂected GPS signals. These data can be used to predict drought or the amount of water from snowmelt.
Since GPS signals can also reﬂect off water, Larson also showed that if the GPS receiver is close to the shore, it can serve as a tide gauge. The information gleaned from these sea level data can alert ﬁshermen and boaters when it's safe to leave or enter a harbor. And tide records provide another way for scientists to monitor global ocean levels.
Larson's most recent innovation is to use GPS to detect ash in volcanic plumes. If ash is drawn into an airplane engine, it can cause the plane to stall and crash. By looking at the GPS data from a volcano near the Anchorage airport in Alaska, she was able to determine not only that there was a volcanic eruption, but how fast it erupted. The implications for this new understanding are huge for air travel, as evidenced by the 2010 eruption of Eyjafjallajökull in Iceland, which caused enormous disruption to air travel between the United States and Europe for almost a week. Larson has written a proposal to the National Science Foundation to make GPS plume monitoring an operational system for airports.
Larson's passion for all things GPS has led her to develop an outreach website, called GPS Spotlight, for middle and high school students.
"I think GPS is so cool and I want everybody to know how cool it is," she says. "And I want them to know how useful it is for science too."