US satellites are constantly surveying North Korea looking at known missile launch sites and watching for any signs of launches from areas they may not know about.
Launches are detected almost instantly because the missiles pick up the infrared heat signature of the launch. Then other classified intelligence systems begin to quickly calculate the trajectory of the missile based on the telemetry or electronic signals it emanates. That allows US commanders to quickly calculate the path of the missile and the estimated target point.
The trajectory and estimated target point can be calculated within minutes of launch.
There is a full interagency communication process during a North Korean missile launch which would involve command centers from the Pentagon, the intelligence community and major US military commands overseas. Each quickly would weigh in with their calculation of the specific threat the missile may pose.
The North American Aerospace Defense Command headquartered in Colorado makes the determination whether the missile poses a threat to the continental US.
The Pacific Command, headquartered in Hawaii, determines if the missile poses a threat to Japan, South Korea, Guam or Hawaii.
The Strategic Command determines if the missile poses a threat to US satellites in space. Ballistic missiles such as those North Korea has fired go high into space, where satellites are operating.
If any of these commands detect a threat, it then moves to a quick decision about shooting down the missile with any number of missile defense systems that may be in the target area. The President would be told and consulted, but if the threat is imminent, the US military has the right to self-defense -- and could make the decision.
With its latest threat, North Korea is suggesting it could launch four missiles which could potentially overwhelm the THAAD missile defense system and strike near Guam. But US officials say they may be misreading US capabilities -- the US has other assets in the region, mainly Navy ships that could shoot down the missiles as well as THAAD.
Of course, if the US determines that North Korea has launched a missile targeting the US, the President would have to decide whether to retaliate.