In radar technology, the range is limited by the wave propagation of the radar frequency used and the curved surface of the earth. Frequencies above a few megahertz are transmitted in line-of-sight, while lower frequencies in the shortwave range are transmitted as space waves.
Space waves are reflected off atmospheric conducting layers of the ionosphere and can thus bypass the limitation imposed by the curvature of the earth or by topological elevations. Over-the-horizon(OTH) radars take advantage of these physical conditions. They enable reconnaissance far beyond the horizon. Because OTH radars transmit at relatively long wavelengths, the antenna arrays were also correspondingly large. Therefore, over-the-horizon radars were only used in the 1960s and 1970s and were replaced by satellite-based reconnaissance systems.