That should worry authorities. It’s not hard to convert a cellphone into a remote trigger for an improvised explosive device. But it is hard—and illegal in most cases—to jam cell signals without inflicting extensive collateral damage that could be even worse than the harm a small IED might cause.
“Jamming technology generally does not discriminate between desirable and undesirable communications” is how the Federal Communications Commission put it in a fact sheet (PDF). “Use of cell phone jammers poses an unacceptable risk to public safety.”
Cellphones are basically just small, sophisticated radios. They convert a radio signal into an electrical current and then process that current into sound. Incorporated into a bomb, a cellphone’s electrical current is enough to jolt a small detonator charge, which in turn can set off the main explosive.
Modifying a cellphone into a trigger is so easy that some DIYers rig up old phones to set off fireworks. All it takes is a phone, five bucks worth of parts, and a few minutes of tinkering—plus, for first-timers, any one of scores of easy-to-follow internet tutorials.
Working the trigger is equally simple. Just get a safe distance away from your bomb and dial the number of the phone attached to the detonator. Boom.
In theory, it’s a fairly straightforward process to jam a cell signal and thus prevent a bomb from exploding. Cellular jammers—you can illegally buy them online for a few hundred dollars—work by flooding radio channels with jibberish signals, essentially crowding out the particular signal a cellphone is looking for.
But cellular jammers—any radio jammers, really—are fairly indiscriminate. Try to jam one phone or a few phones and, in practice, you’ll end up wiping out communications across a wide area, potentially causing greater insecurity than you’re preventing by blocking a bomb detonation.
To be sure, cellphone jamming in a small or fixed area can be useful. Many prisons jam incoming signals. The U.S. Secret Service reportedly possesses jammers that accompany presidential motorcades.
The U.S. military equips many of its armored vehicles with radio jammers, creating an electronic bubble in which many remotely triggered bombs won’t detonate. More powerful airborne military jammers can sweep away signals underneath the emitting aircraft.
But jamming cellular signals across a wider area can be highly problematic. For starters, the most modern cellular services take advantage of what’s called “frequency-hopping.” That is, they can rapidly move signals across different frequencies more or less to avoid overcrowding. This complicates jamming. The more a signal hops, the more frequencies you’d have to jam.
Consider this scenario. Police get a credible tip that a terrorist is planning to detonate an IED by cellphone… somewhere. The cops set up jammers across a range of frequencies all over the city, perhaps for hours. Yes, authorities might stop the bomb from exploding. But they’ve also jammed 9-1-1 and many of their own communications.
Medical emergencies and crimes in progress could go unanswered. Thousands of crises of a personal nature might go unresolved. Business would suffer hugely. Unable to make plans and connections, untold throngs of commuters could find themselves lost or stranded.
The military certainly appreciates this problem. Ground convoys must turn off their jammers when they enter bases, lest they shut down the whole base’s communications. Airborne jammers require careful coordination with other forces, as their powerful blocking signals could wipe out friendly troops’ own radios and phones and potentially scuttle delicate battle plans.
The risk of collateral damage is one reason why the federal government bans most cellular jamming. It’s illegal to sell a cell-jammer in the United States, so most buyers order theirs online from overseas retailers—and usually get away with it.
The jamming-ban isn’t actually new. The Communications Act of 1934 states that “no person shall willfully or maliciously interfere with or cause interference to any radio communications of any station licensed or authorized by or under [the Communications] Act or operated by the United States government” (PDF).
“Jammers cannot be marketed or operated in the United States except in the very limited context of authorized, official use by the federal government,” the FCC explained in its fact sheet. In May, the FCC fined a Florida man $48,000 for operating a cell-jammer during his commute to work, apparently in order to prevent other commuters from driving while on their phones and distracted.
Legally speaking, the Department of Homeland Security or some other federal agency could authorize cell-jamming in support of local law enforcement. Some police departments have been caught jamming signals without federal approval. Devices called “stingrays” act as decoy cellular towers. Deployed by police eavesdroppers, stingrays can intercept cellular calls for purposes of gathering evidence.
But if a stingray is sucking up cell signals, that means calls—including, for example, 9-1-1 calls—aren’t reaching actual cellular networks. On that basis and others, Georgetown University professor Laura Moy filed a complaint to the FCC over the Baltimore Police Department’s use of stingrays.
“In a clear violation of law, BPD has no license whatsoever to operate its [cellular-site] simulator equipment on frequency bands that are exclusively licensed to cellular phone carriers in Baltimore,” Moy wrote. “BPD further violates the Communications Act by willfully interfering with the cellular network through its use of [cell-site] simulator equipment.”
To be clear, local authorities can skirt the 1934 law by simply asking telecoms to briefly shut down cellular service in a particular area, as city officials in San Francisco did in 2011 as they tried to disrupt a planned protest of a police shooting.
Voluntary cellular shutdowns aren’t necessarily illegal, but like jamming, they do run the risk of interfering with emergency responders, business, and travel. So while cellphones have made it easier for terrorists to set off homemade bombs, cellphone-jamming probably isn’t the best tactic for stopping them.