Why can ground-based radar not detect an object until it rises above the horizon?

Conventional ground-based radar systems typically operate at microwave and ultrahigh frequencies that propagate in straight lines, similar to light rays. Because these waves travel in a direct path, they are blocked by the curvature of the Earth, which prevents them from detecting objects located beyond the horizon.12

While atmospheric phenomena such as "ducting" can occasionally trap and guide radar energy around the Earth's curvature, this effect is unpredictable and unreliable. To overcome the limitations imposed by the horizon, specialized over-the-horizon radar systems utilize lower frequencies in the high-frequency (HF) radio band. These waves are refracted, or bent, by the ionosphere, allowing them to return to the Earth's surface at distances far beyond the line of sight.32