When North Korea conducted its second known nuclear bomb test on May 25, 2009, the country's leaders took extreme care to conceal the details of the event. They detonated the bomb a kilometer or so beneath the earth, so no radiation could escape; radiation clues could have enabled member countries of the Comprehensive Nuclear Test Ban Treaty Organization to more accurately determine the type and size of the bomb tested.
What the rest of the world knows about the bomb was learned from seismic waves. Tremors registering at 4.52 on the Richter scale confirmed that the test took place just before 1 a.m. Coordinated Universal Time near the village of P'unggye and suggested that the yield was on the order of a few kilotons.
At the time, however, the May 25 bomb also sent a different signature, this one into the atmosphere. It did not release radioactive gas or dust, as would be the case for a bomb detonated on the Earth's surface. Rather, it released a shockwave -- a bubble of disturbed air that spread out from the test site across the planet and high into the ionosphere.
It was, in fact, 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.
We hope that the discovery, which grew out of our efforts to improve the planet's global positioning system (GPS), will be useful to the test-ban organization as a supplementary means of detecting clandestine nuclear tests. It may even give the United States more reason to ratify the Comprehensive Test Ban Treaty.
GPS might at first seem like an unlikely source for nuclear test detection. But if the technology was designed for location purposes -- and it has found many such purposes in the pursuit of science -- GPS signals have always been especially sensitive to atmospheric disturbances. GPS radio waves must pass from transmitters on satellites high above the planet down to ground-based receivers. Air molecules in-between -- more specifically, electrons and other charged particles in the ionosphere -- interfere with the signal, generating position error.
Part of our research concerns how to compensate for that interference. Whether our team is working on antenna calibration in the harsh environment of Antarctica, ionospheric modeling in the Global Navigation Satellite Systems network, or tropospheric modeling for precise and rapid GPS positioning in mountainous areas, we are working to boost GPS precision.
Over the years, we have learned that atmospheric conditions, such as the density of charged particles in the ionosphere, alter GPS signals in predictable ways. Time is the heart that drives GPS, and the presence or absence of particles in the atmosphere may speed up GPS radio signals or slow them down. By knowing how different conditions affect the signal speed, we can construct computational algorithms to compensate and maximize precision so GPS data will be useful for science.
When the bomb test occurred in 2009, we happened to be examining this problem of ionospheric disturbance. We knew that previous research had proven that underground nuclear explosions disturb the ionosphere. We wondered: Could the North Korean explosion have disturbed this layer of atmosphere to the level detectable by GPS signals? And could a sufficient number of GPS stations near North Korea be found that had continuously collected the data needed to examine the event? If so, we could take one of GPS's inherent vulnerabilities and turn it into something useful.
We quickly gathered data from 11 GPS receivers -- six belonging to the South Korean GPS network and five belonging to the International GNSS Service (IGS) and scattered around Eastern Asia. The data indicated a sudden spike in atmospheric electron density just after the underground test.
We calculated that within 25 minutes, the leading edge of the shockwave had traveled 225 miles to the nearest GPS station in our study, which was located in Inje County, Gangwon Province, South Korea. By the end of the first hour, the disturbance had reached all 11 stations. The shockwave propagated through the air at 9 miles per minute, or 540 miles per hour. Based on the timing of the shockwave at the different receiver locations, we were able to trace the origin of the explosion back to P'unggye, in agreement with the seismic data for the event.
So we believe that GPS could work as a complement to other detection methods used by the test-ban organization and could help confirm that a nuclear test has taken place -- especially when the test is underground, making its emissions into the air very subtle and otherwise nearly impossible to detect.
When the Nuclear Test Ban Treaty was formalized in 1996, the United States was among nine nations that did not ratify it. In part, US officials objected that technologies of the time were not reliable enough to ensure accurate detection of secret nuclear tests. Subsequent advances in computer science, seismology, and chemical sensors have begun to quell those objections. Perhaps the addition of GPS to the list will help to suppress them further.
