Global positioning system (GPS) technology has been around long enough — and is so widely used that we have come to take it for granted. If you don’t have a GPS navigation system installed in your car, you probably have something like it on a standalone device from Garmin or TomTom or on your cellular phone. What you may not know is how many other seemingly unrelated systems rely on GPS signals, and how easily they can be thrown into chaos.
The core of the GPS is a constellation of about 30 satellites (the number of working satellites varies) in Earth’s orbit. At any point on Earth, at any given moment, at least six of these will be in the sky, someplace from horizon to horizon. Each satellite carries a very precise atomic clock and a radio that transmits a time signal. GPS receivers also carry an accurate crystal oscillator clock that is used to compare the time signals received from overhead satellites.
The signals travel at the speed of light, and by comparing the time signal from the satellite with that of the onboard clock, the distance from the satellite is determined. Each signal describes a sphere around that satellite, but the point where at least four satellite signal spheres intersect identifies a specific point in space. This calculation is repeated several times each second as you go over the river and through the woods to grandmother’s house.
Motorists depend on GPS signals, but so do cellular telephone systems, power grids, banking systems, aircraft, and marine vessels. Cellular systems use the signals to coordinate handoffs between cell towers and for locating 9-1-1 callers. Electrical power providers use the time signals to control current transfers from one major grid to another. Financial networks — including ATMs — use the signals for timestamps of transactions. Interference with the GPS signal can cause cell phones and pagers to stop working, ATMs to refuse transactions, electrical power to fail, and you to miss grandma’s legendary green bean casserole.
Why would anyone jam a GPS signal? Commercial truckers and delivery services often have tracking devices mounted on their vehicles, and the drivers may want to goof off a bit without the boss knowing about it. High-value shipments may have GPS trackers inside their crates to aid in recovery if the cargo is hijacked. For less than $200 (a lot less if you make it yourself), you can purchase a device that will make the satellite GPS signal disappear. It will also play hell with the GPS signals used by anyone within range of the device.
Yes, they’re illegal, but only if you get caught.
The largest-scale known case of GPS jamming took place in January 2007, when two U.S. Navy vessels in San Diego harbor conducted a training exercise to test procedures in the event of a loss of communications. The Navy didn’t count on operators at air traffic control not being able to see where their airplanes were, the loss of cellular phone service, nonfunctioning ATMs, and the disruption of hospital pagers.
Oops, sorry. My bad.
It’s easy to jam GPS signals because they are so faint to begin with — it’s coming from a satellite around 13,000 miles away. Typical signal strength is around 1.5 dB (television signals at my suburban home range from 25-46 dB). A very small jammer can disrupt the GPS signal for a mile or more. There are people who want to jam GPS signals because they believe the government is secretly tracking their comings and goings, and others who are engaged in smuggling or theft. The trucker who just wants some extra time off might not have a greater criminal enterprise in mind, but he might also be a hijacker. The solo motorist could be a constitutionalist or “sovereign citizen” who prints his own vehicle tags and driver’s license and denies the authority of local laws.
Many states have laws prohibiting the interference of licensed radio signals. The FCC regulates use of the broadcast spectrum on the federal level, and specifically prohibits the use of jammers of any kind. As a local law enforcement officer, you may lack the authority to enforce FCC regulations, but it may be possible for you to seize the jammer or make an arrest under the prevailing laws of your state.
