Radar Horizon & Ducting Calculator
Radio wave propagation: Calculate geometric & 4/3 refracted radar horizons, sea-skimmer detection ranges, and atmospheric ducting trapping frequencies.
Antenna & Target Altitudes
Line-of-Sight & Ducting Horizons
Recommended Tools & Equipment
Tested hardware and components for high reliability
Frequently Asked Questions
Why does atmospheric refraction bend radar waves around the Earth?
Atmospheric pressure, temperature, and water vapor decrease with altitude, causing the refractive index (n) of air to decrease upward (dN/dz ā -39 N-units/km). Because electromagnetic waves travel faster through lower-density upper air than dense lower air, wavefronts bend slightly downward toward the surface. In standard atmosphere, this downward curvature is modeled mathematically by replacing Earth's actual radius with a 4/3 effective Earth radius (k = 1.333).
Why do sea-skimming anti-ship missiles present such an extreme defensive challenge?
Sea-skimming cruise missiles (like Exocet, Harpoon, or Kalibr) fly just 3 to 5 meters above the waves. For a shipboard mast-mounted radar at 25 meters, the geometric radar horizon against a 5-meter target is only ~16 nautical miles (~30 km). A supersonic sea-skimmer traveling at Mach 2.5 covers this distance in less than 35 seconds, leaving seconds for automated electronic countermeasures and CIWS interception.
What is an evaporation duct and how does it enable Over-the-Horizon radar detection?
An evaporation duct is a permanent shallow atmospheric layer (typically 5 to 25 meters thick) immediately above the ocean surface caused by intense humidity gradients. When the modified refractivity gradient dM/dz becomes negative, the air acts like a giant leaky dielectric coaxial waveguide. Radar waves above the cutoff frequency become trapped between the sea surface and the duct ceiling, propagating hundreds of miles beyond the visual horizon.