The radar horizon is a critical area of performance for aircraft detection systems that is defined by the distance at which the radar beam rises enough above the Earth's surface to make detection of a target at low level possible . It is associated with the low elevation region of performance, and its geometry depends on terrain, radar height, and signal processing. This is associated with the notions of radar shadow, the clutter zone, and the clear zone. Airborne objects can exploit the radar shadow zone and clutter zone to avoid radar detection by using a technique called nap-of-the-earth navigation. Definition Without taking into account the refraction through the atmosphere, the radar horizon would be the geometrical distance from the radar to the horizon only taking into account the height of the radar above sea-level, and the radius of the earth (approximately 6.4·103 km): When H is small compared to , this can be approximated by: [The percentage error, which increases roughly in proportion to the height, is less than 1% when H is less than 250 km.] With this calculation, the horizon for a radar at a altitude is . The radar horizon with an antenna height of over the ocean is . However, since the pressure and water vapor content of the atmosphere varies with height, the path used by the radar beam is refracted by the change in density. With a standard atmosphere, electromagnetic waves are generally bent or refracted downward. This reduces the shadow zone, but causes errors in distance and height measuring. In practice, to find , one must be using a value of 8.5·103 km for the effective Earth's radius (4/3 of it), instead of the real one. So the equation becomes: And for the same examples : the radar horizon for the radar at a altitude will be and the one at will be . Furthermore, layers with an inverse trend of temperature or humidity cause atmospheric ducting, which bends the beam downward or even traps radio waves so that they do not spread out vertically. This phenomenon occurs in two circumstances: A thin stable layer of elevated humidity Stable temperature inversion Ducting influence becomes stronger as frequency drops. Below 3 MHz, the whole volume of the air acts as a waveguide to fill in the radar shadow and also reduces radar sensitivity above the duct zone. Ducting fills in the shadow zone,