Just hours before U.S. president Barack Obama delivered his final State of the Union speech in January, two U.S. Navy patrol boats wandered into Iranian waters. The Iranian military intercepted the vessels and captured 10 U.S. sailors, making for an awkward moment as the president took the stage.

The seamen were released the next day, but no military official seemed able to explain why the boats had strayed from their intended path. Defense Secretary Ashton B. Cartersimply said the highly trained crew had “misnavigated.” An investigation is still under way.

Without a clear explanation, the incident prompted speculation that Iran had sent false GPS signals to lure the sailors onto another course. It would not have been easy for the Iranians to hijack the GPS system—military GPS signals are heavily encrypted—but the idea wasn’t inconceivable. In 2011, Iran boasted that it had captured a highly classified drone belonging to the CIA by fooling its GPS to make it land in Iran rather than Afghanistan.

Three years before the drone’s capture, one of us (Humphreys) had developed the only publicly acknowledged GPS spoofer that could perform such a feat. A spoofer transmits false GPS signals, which to a navigation system are indistinguishable from real ones. Meanwhile, the other of us (Psiaki) was hard at work on detectors to catch spoofers in the act.

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