The U.S., South Korea and Japan participated in joint missile-tracking exercises this week.
Those trilateral drills are the first since North Korea's missile test late last month. The drills also follow a new report from North Korea watchdog group 38 North. The report alleged satellite photos showed the North was constructing a submarine capable of launching ballistic missiles.
Submarine launch ballistic missiles, or SLBMs, present a particular challenge because they're harder to detect before launch. Military personnel are using computer simulations to practice tracking North Korean submarine-launched missiles.
