The FCC slammed a Chinese GPS jammer vendor Thursday with a $34.9 million fine, the largest in history, following reports earlier this month to a high level GPS advisory group that starkly illustrates the jammer problem: Every three or four trucks tracked at the Portland International Airport in Oregon emitted signals in the GPS band, a sign of jamming.

The FCC said C.T.S. Technology Ltd has sold jammers to U.S. consumers for the past two years, including some high powered 100 watt models that the company said can blank out GPS signals within a range of 1,000 meters, or more than a half mile. The strength of a GPS signal at a receiver is measured in milliwatts.

“Signal jammers present a direct danger to public safety, potentially blocking the communications of first responders,” said Travis LeBlanc, acting chief of the FCC Enforcement Bureau. “Operating a jammer is also illegal, and consumers who do so face significant civil and criminal penalties,” which includes fines up to $100,000.

In August 2013, the FCC fined a trucker caught operating a GPS jammer near the Newark airport $32,000.

Logan Scott, a GPS consultant, told a meeting of the National Space-Based Positioning, Navigation, and Timing Advisory Board in Washington on June 3 that some truckers use GPS jammers to disable GPS tracking systems installed by fleet managers. He added that truckers hauling hazardous waste may use GPS jammers in order to avoid taxes and fees.

On March 5, Rohde &Schwarz USA, a manufacturer of mobile radio test and measurement equipment, set up a monitoring van on a highway south of the Portland airport and determined that “about every third or fourth truck was radiating at or near the GPS L1 [frequency of 1575.42 MHz].” This included a tandem trash hauler truck, whose driver continued to operate the jammer even after he was caught, Scott said.

Kama Simonds, a spokeswoman for the Portland airport, said the airport was unaware of the March GPS jammer monitoring experiment and subsequent report.

The FCC, in its enforcement action against C.T.S, said the company offered jammers designed to knock out vehicle tracking systems and has more than tripled the number of jammers on offer for sale in the U.S. from 78 in 2013 to 285 today. Despite the record fines against C.T.S., a Google search for GPS jammers results in thousands of hits for websites offering the devices for sale – with some priced as low as $25.