The Japanese press were the first to report Tuesday that North Korea was preparing to launch a ballistic missile.
Kyodo News and the Sankei Shimbun both reported that the Japanese government detected signals from North Korea suggesting it was poised for another missile launch and warned it could take place "within a few days."
But South Korea and the U.S. had also picked up the signals from "telemetry" or remote signals on Monday suggesting an impending launch, according to the Asahi Shimbun.
Telemetry is an automated communications process where measurements and other data are gathered at remote points and transmitted to receiving equipment for monitoring. It is used to monitor fires, theft, electricity and gas usage as well as pollution.
Bluetooth is also a form of telemetry.
This proved a giveaway because ballistic missiles send telemetric signals to control centers, and North Korea apparently tests the signals before firing missiles.
The U.S. and Soviet Union invested huge amounts of money during the Cold War to monitor telemetry signals from enemy missiles.
A Haeseong-2 missile is being fired from an Aegis destroyer in the East Sea on Wednesday. /Courtesy of the Navy