North Korea has recently improved its ballistic missile launch facility at Musudan-ri (on the northeast coast), and several of the new features are identical to those first seen at the Iranian Semnan launch complex. This is yet another bit of evidence that Iran and North Korea have been sharing ballistic missile technology.
Musudan-ri was completed four years ago and featured an underground fueling system. This made it much easier to prepare one of their multistage ballistic missiles for launch. Prep time could be reduced from 4-5 days to 1-2 days. Previously, the missile would be surrounded by fuel trucks, special equipment, and technical personnel before launch. But now most of that is hidden from view. Thus, with so much of the launch preparation underground, space satellites see less obvious signs of an impending launch. But many aspects of the base are in plain sight and visible to photo satellites.