Kennelly–Heaviside layer
This layer became a popular means of communication in the late 1920's when AM signals (Amplitude Modulated) were transmitted over long distances. AM signals can bounce off the Kennelly-Heaviside. They can go further than signals which cannot bounce off the layer. This means that the sender and receiver of the signal could be over the horizon from each other, and the signal can bounce off the layer in the sky in between them.