Although cell phone jammers are sold all over the Internet, these jammers are illegal in most countries. When it comes to using a GPS jammer, or blocker, in your own vehicle, to hide your location from anyone who tries to track you, that's more of a gray area. True, there was a famous court case this year that said police needed a warrant to track your car, but that law had holes in it big enough to drive a truck through. According to a police blog:
"If law enforcement officials can articulate a serious threat ... to the general public, and that the tracking of a vehicle is essential to guard against that threat, then it is not necessary to have a search warrant for the installation of a tracking device. Only reasonable suspicion, and not probable cause, is required under this exception."
Do my wife and I carry GPS jammers in our various vehicles? Should you and your mate carry jammers in your vehicles? (These are rhetorical questions.)