Ionospheric reflection
The ionosphere — a layer of charged particles 80 to 600 km above the Earth — reflects radio waves below about 30 MHz back to the surface. OTH radar transmits a high-frequency (HF) signal upward at a shallow angle, bounces it off the ionosphere, and illuminates a target area thousands of kilometres away. The echo bounces back the same way.
Why HF, why huge antennas
HF means long wavelengths — 10 to 100 metres. Antennas have to be matched to the wavelength, so OTH radar arrays are kilometres long. Australia's JORN system uses a 3-kilometre transmit array and a separate 3-kilometre receive array. The Soviet Duga 'Russian Woodpecker' was 700 metres tall and 150 metres wide.
What it can and can't see
OTH can detect aircraft, missile launches and large ships at 1,000-3,000 km range. Resolution is poor — kilometre-scale, not metre-scale. It cannot guide weapons; it can only cue other sensors. Stealth shaping doesn't help because the wavelength is too long, but OTH cannot tell an airliner from a fighter at that resolution.
Modern operators
Active OTH systems include Australia's JORN, Russia's Container, the US Relocatable Over-The-Horizon Radar (ROTHR) used for drug interdiction, and several Chinese systems aimed at the Western Pacific. The capability matters strategically: an OTH site can detect a B-21 launch from 3,000 km away even if the B-21 itself is invisible to X-band radar.