> “thousands of flights a day could be impacted with delays and cancellations”
I don't think this was the work of a rogue environmentalist hacker, but what gear would a lone terrorist need to do this?
Not much, unfortunately. That's why there are redundant navigation sensors and kalman filters in modern electronic flight navigation systems. Aircraft flew and landed way before GPS existed. We went to the moon with less technology, in fact.
The problem with those backups is that GPS is so easy to use and reliable that the skills to fly without them might atrophy.