> In the event of an imminent nuclear apocalypse, we suspect that many people who have access to private jets will immediately take to the skies and escape city centers.

1. I think the logic behind this particular concept flawed. What's the flight time for an ICBM? 20 minutes if from Russia, and less than that from a submarine? I don't think a billionaire could get to his jet in time, unless he lives on an airstrip like John Travolta. Some might get some early notice if their country planned a first strike (but I doubt it, as loose-lips like that would probably give the enemy notice, too).

2. I think if nuclear war is actually immanent, your best bet of an early warning is an EAS National/Presidential alert (https://en.wikipedia.org/wiki/Emergency_Alert_System), because I'd hope people with access to actual early-warning sensors would cause one to be sent (while they're getting ready for a second-strike attack). But, given the shambolic nature of post-Cold War government, that could be a foolish hope.

The more effective thing is probably something scanning a news feeds for world events that indicate a major crisis progressing up the escalation ladder. Stuff like conflicts involving nuclear powers, threats of nuclear weapon use, reports of unusual activity of emergency command and control aircraft (like going on alert), use of tactical nuclear weapons, etc.