From ABC News:

The North is believed to be nearing completion of an electromagnetic pulse bomb that, if exploded 25 miles above ground would cause irreversible damage to electrical and electronic devices such as mobile phones, computers, radio and radar, experts say.

The current attempts to interfere with GPS transmissions are coming from atop a modified truck-mounted Russian device. Pyongyang reportedly imported the GPS jamming system from Russia in early 2000 and has since developed two kinds of a modified version.