It may well also be that, as a result of several previous jamming events by the North, South Koreans have taken additional precautions. These could include upgrading GPS receivers, improving backup clocks, and increasing the use of their terrestrial Loran-C system alongside GPS. All would help ensure timing and navigation services remain uninterrupted during jamming incidents.
Loran is particularly useful during GPS jamming events that use high power transmitters or that last for extended periods. South Korea has an active program to upgrade its Loran-C system to the eLoran standard to make it more precise and even more difficult to disrupt.
North Korea has never attempted to disrupt the Loran systems operated by Russia, China, South Korea and Japan.