What did 38 North Say?
The institute, based in Washington, posted the satellite images on its website.
South Korea's planned development for the system, which has the capability to destroy incoming ballistic missiles at an altitude of about 12.4 miles, comes at a time when North Korea may be on an "aggressive schedule" to deploy its first operational ballistic missile submarine.
Images from November 5 showed parts being moved into and out of areas near constructions halls, indicating a shipbuilding program is in progress at the Sinpo base on North Korea's east coast.
"Throughout 2017 there has been continued movement of parts and components into and out of the two parts yards adjacent to the constructions halls in the center of the shipyard", 38 North researcher Joseph S. Bermudez Jr. wrote, adding that he has also detected crane movement. Images of the Sinpo South Shipyard taken on November 5 showed what appeared to be a submarine's pressure hull in the yard.
Submarines and the ability to launch ballistic missiles at sea offer the North the ability to slip behind enemy defenses in combat. Ejection tests are expected in the near future for submarine-launched ballistic missiles Pukguksong-1, a possible Pukguksong-3, and other missiles, Newsweek reported.
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Satellites captured images of what experts believe is a launch canister at the missile test stand, likely indicating that Pyongyang has been testing an ejection mechanism for launching missiles from a submarine.
The images corroborated the fact that the USA military intelligence was suspecting since the end of October - that a new diesel-electric submarine under construction in North Korea's shipyard.
North Korea has openly been working on developing a nuclear tipped missile that would be capable of reaching the United States and in September reported a successful test of a thermonuclear hydrogen bomb that could be attached to an intercontinental ballistic missile capable of reaching the USA mainland.
But the past two months have been relatively quiet with Kim failing to fire a ballistic weapon since September 15. Experts cited by CNN said in August that if North Korea had fired an intercontinental ballistic missile on a standard trajectory, it could have hit major US cities such as Los Angeles, Chicago or Denver.
Kim Jong-un has carried out dozens of missile tests this year as well as its largest nuke test.