North Koreans have GPS jammers
North Korea is apparently actively using in-vehicle systems to jam GPS signals that overwhelm signals from tracking satellites. South Korea couldn't find the jammers because the Army ran them for about ten minutes at a time and then moved them.
Why is this important? Presumably a southern invasion would require some kind of GPS signal for logistical coordination. Some secret systems that we don't know about do not support satellite signals available to military and civilian equipment. By drowning out GPS signals sent from distant satellites, their frequencies explode from closer range.
GPS signal "very weak". A satellite transmits no more energy than a car headlight, and it transmits it in space 20,000 kilometers away. If you yell at it with a moderately powered jammer nearby, you'll drown out the GPS signal and the receiver will lose service.
The North Koreans are believed to have old Soviet jammer gps purchased about 10 years ago and their own homemade equipment. These wheeled jammers are located on the top of the mountain in Kaesong province near the border for maximum coverage.
South Korean military reports suggest that the Kim regime may have acquired high-powered jammers that can scramble GPS receivers up to 250 miles away.
Inexpensive internet sales jammers, favored by truckers looking to avoid boss scrutiny, often only cover a 30-foot radius.