Over the past two weeks, North Korea has used jamming equipment to interfere with global positioning system (GPS) signals near South Korea’s two largest airports outside its capital city Seoul and across the center of the Korean peninsula. The jamming caused no accidents or loss of life, but it demonstrates that North Korea is getting more and more brazen in its efforts to mess with South Korea's high-tech infrastructure.

In 2010, South Korean Defense Minister Kim Tae-young said that North Korea had imported truck-based jamming systems from Russia that could jam GPS signals out to a 100 kilometer radius. Since then, there have been at least three GPS-jamming incidents along the border—which is just 60km north of Seoul. GPS jamming could conceivably be used by itself or in combination with other electronic and network-based attacks to disrupt South Korea’s highly digital society, and perhaps cause aircraft or ships to stray into North Korean territory. And as Ars reported in February, public safety experts say that even jamming on a small scale can create a security threat, especially in coastal waters like those west of Seoul.

In the latest incident, according to a report from the Korea Herald, a total of 553 aircraft flying in and out of South Korea’s Inchon and Gimpo airports reported failures of GPS systems, as did hundreds of ships and fishing boats in the West Sea. The jamming signals, which were first detected on April 28 and apparently ended on May 6, were traced to the North Korean border city of Kaeson. The jamming—the most large-scale effort to block GPS signals by North Korea thus far—is seen by South Korean defense officials as a demonstration of the North’s wider efforts to build up electronic and cyber warfare capabilities.

It’s also a much cheaper way to attempt to intimidate the South than the recently failed attempt to put a satellite into orbit. Lee Sang-wook, chief of South Korea’s Electronics and Telecommunications Research Institute’s satellite navigation research team, told the Herald, “If it takes the right approach, North Korea can send out jamming signals over a wide bandwidth, affecting the greater number of facilities without consuming much energy, say, for costs as low as 100,000 won (about $88 US).”

In addition to its electronic warfare efforts, the North Korean military is also reportedly building up its hacking prowess. South Korean and US officials blamed North Korean military intelligence for a rash of cyber-attacks on financial institutions last May, as well as distributed denial of service attacks against South Korean government and US military websites last March. In his book Cyber War, published in April, Richard Clarke, the former White House counter-terrorism czar and special advisor on cyber security, wrote that the North Korean People’s Army has units with hundreds of hackers—many of them working out of China. Some are employed in psychological operations to spread propaganda and infiltrate social networks, while a larger percentage are dedicated to attempts to take down South Korea’s IT and communications infrastructure.