How do over-the-horizon radars provide earlier warning of ballistic missile launches?

Over-the-horizon (OTH) radars provide earlier warning of ballistic missile launches by overcoming the line-of-sight limitations of conventional microwave radars, which are restricted by the curvature of the Earth.12

While conventional systems can only detect objects once they appear above the horizon, OTH radars operate in the high-frequency (HF) radio band (3 to 30 MHz). These radio waves are refracted, or bent, by the ionosphere, allowing the signals to return to the Earth's surface at distances far beyond the horizon—typically between 500 and 2,000 nautical miles.132

These systems detect ballistic missiles by identifying disturbances in the received signal caused by the missile's exhaust plume as it traverses the ionosphere. There are two primary configurations for this detection:13
* Forward scatter: The signal is transmitted from one location and received several thousand miles away on the opposite side of the launch point.3
* Back-scatter: The signal is received at the same location as the transmitter, similar to conventional radar operations.3

By monitoring these ionospheric fluctuations, OTH radars provide critical, timely alerts that are not possible with standard surface-based systems.13