GPS usage is very prevalent and GPS jamming attacks, the intentional interference with GPS signals to prevent GPS receivers from working correctly, are on the rise. Some people worry about their privacy, others just want to steal things without being followed by the police. We analyzed the behavior of some commercial GPS receivers to see whether a jamming attack could be detected to, e.g., trigger an alarm. We discovered a flaw in the only GPS receiver on the market with a jamming detection mechanism, which permits an attacker to jam the receiver without triggering an alarm.