What does near vertical incidence sky-wave (NVIS) propagation enable?

Near Vertical Incidence Sky-wave (NVIS) is a propagation technique where radio waves are transmitted almost straight up into the ionosphere. Because they hit the ionosphere at a steep angle, they reflect almost straight back down, covering a local or regional area, typically between 50 to 650 kilometers. This creates an 'umbrella' of coverage that eliminates the skip zone and drops signals directly down into valleys or behind mountains, making it ideal for medium-range communications in difficult terrain. It requires operating below the critical frequency; otherwise, the signals would punch right through the ionosphere into space.