Researchers have discovered that nuclear tests leave a mark on the part of the upper atmosphere known as the ionosphere. After a nuclear explosion, there is a change in ionospheric electron density, as a bubble of disturbed particles spread out from the test site and across the planet. As the GPS signal passes through the edge of the bubble, the change in electron density disturbs the signal in a noticeable way.
Even though the GPS system was designed for location purposes, the technology has always been especially sensitive to atmospheric disturbances, said Dorota Grejner-Brzezinska, a professor of geodetic science at Ohio State University. The GPS system 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.
According to Ralph von Frese, professor of earth sciences at Ohio State University and senior author on the study, “Its 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.”
Jihye Park, a doctoral student in geodetic science at the university, says that international authorities already possess several methods for detecting illegal nuclear tests. But the GPS system data is a good complement to these other ways of detecting nuclear tests.
For example, seismic detectors pick up shock waves on land, and acoustic sensors monitor for shock waves through water and the air for tests that happen above ground. Chemical sensors detect airborne radioactive gas and dust as definitive evidence of a nuclear explosion. However, these particles may be lacking if the explosion is contained deeply below ground.
Park wrote computer algorithms that search GPS signals for patterns indicating a sudden fluctuation in atmospheric electron density in specific locations, which is what happens when a shockwave pushes a bubble of air through the atmosphere.