At this week's Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) meeting, American researchers will unveil a new tool for detecting illegal nuclear explosions: the Earth’s global positioning system (GPS).
While other methods, such as seismic detectors, acoustic sensors and chemical sensors can in many circumstances do the same job, they often fail to detect explosions deep underground.
"GPS is a complement to these other methods, and can help confirm that a nuclear test has taken place – especially when the test was underground, so that its effect in the air is very subtle, and otherwise nearly impossible to detect," says Jihye Park, a doctoral student at Ohio State University.
The technique relies on the fact that even underground nuclear tests leave their mark on the upper atmosphere - and this can be picked up by GPS.
"GPS signals must pass from transmitters on satellites high above the planet down to ground-based receivers,” says professor Dorota Grejner-Brzezinska. "Air molecules – more specifically, the electrons and other charged particles in the ionosphere – interfere with the signal, generating position error."
Park wrote computer algorithms to search GPS signals for patterns indicating a sudden fluctuation in atmospheric electron density in specific locations.
And when the researchers examined GPS data recorded the same day as a North Korean nuclear test in 2009, they found that GPS stations in nearby countries registered a change in ionospheric electron density, within minutes, as a bubble of disturbed particles spread out from the test site and across the planet.
"It's as if the shockwave from the underground explosion caused the earth to 'punch up' into the atmosphere, creating another shockwave that pushed the air away from ground zero," said Ralph von Frese, professor of earth sciences at Ohio State University.
Based on the timing of the shockwave, the researchers traced the origin of the explosion back to P’unggye, in Hamyong Province, North Korea. This finding agrees with seismic data from the event, which was collected by the CTBTO and the US Geological Survey.