Since its signals comes from satellites at very high altitude, GPS relies on very weak signals that are extremely vulnerable not only to spoofing attacks but also to jamming – the deliberate or accidental transmission of radio signals that interfere with regular communications. This weakness has already been exposed in a few incidents in the past. In late 2009, GPS receivers at Newark Airport were going down intermittently every day and no one understood why. It took two months to track down the source of the interference: a driver had installed in his truck an illegal GPS jammer – which can easily be bought online for $50– so that his employer couldn’t track his every move. In 2001, a boat’s television antenna preamplifier took out GPS over the entire harbor town of Moss Landing, just south of San Francisco, for weeks. - WarLord