… Because they [satellite positioning systems] rely on signals from satellites transmitting from an altitude of around 20,000 kilometres (12,400 miles), the signals are very weak, making them vulnerable to accidental or deliberate interference. This can take the form of natural interference, as a result of solar activity, for example; accidental man-made interference due to signal reflection or faulty transmitter equipment; and deliberate jamming of the satellite signal by transmitters that drown it out by broadcasting their own signal on the same frequency.
British police began finding jamming equipment in the possession of criminals about three years ago. This was not surprising, because evidence from satnavs and vehicle-tracking devices had already been used in several successful prosecutions. In July 2010 two men were jailed for a total of 16 years after they admitted being members of a gang that stole 40 lorries and their loads with a total value of £6m ($9.6m). They had used GPS jammers to prevent the vehicles being tracked after the thefts. In Germany, meanwhile, some lorry drivers have used jammers to evade the country’s GPS-based road-tolling system.
“If you do an internet search on GPS jammers, you get over 300,000 hits, with many of these linking to sites offering them for sale,” says Jim Hammond of the intelligent transport systems working group at Britain’s Association of Chief Police Officers (ACPO). “I’d suggest you don’t get that level of hits for products that nobody buys.” ACPO and Britain’s communications regulator, Ofcom, are urging the government to make it easier to prosecute people who use and sell jammers, and to make their possession a criminal act.