Just like the limited (3 meter) jammers available retail here to disrupt GPS units, same principle but on a larger scale.
Something tells me though that the Nork's may not have come up withthis themselves...the ChiCom's probably want to test to technology...since as a potential adversary we rely heavily upon the technology in our weapons systems. Especailly since copying Rooskie technology is such a traditional ChiCom tactic.