Criminals now use jammers that block GPS reception
The recent appearance of readily available, low-cost GPS jamming devices presents a real and immediate threat to all such tracking and security systems. Criminals now employ jammers that can block both GPS reception and GSM in Europe, and U.S. and other mobile phone systems throughout the world, rendering vulnerable the use of GPS in critical security applications. Other global satellite navigation systems (GNSS) in development will likely share that vulnerability. While not yet deployed for criminal purposes, spoofers that mimic GNSS signals will pose an even greater threat to vehicle security than GPS jammers.
That cheap cigarette-lighter jammers available for purchase over the internet are being bought by people who want to stop employers from tracking their movements within certain jobs. Although it is illegal to possess such a device in the U.S., in other countries the legislation is less strict. In the U.K., for example, whilst it is illegal to operate a GPS jammer, it is legal to own one.
Any kind of GPS jamming poses a great danger to airports and shipping ports in the immediate vicinity. The guy who simply wants to jam his own tracking system has no control over what parts of the critical national infrastructure that jammer takes out.