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The radar Horizon is a critical area of performance for aircraft detection systems that is defined by the distance at which the radar beam raises enough above the Earth's surface to make detection of a target at low level impossible. It is associated with the low elevation region of performance and its geometry depends upon 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 H of the radar, and the radius of the earth (6.4·103 km). : With this calculation, the horizon for a radar at altitude is . The radar horizon with an antenna height of over the ocean is . However, since the pressure and content in water vapor 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 fault in the 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 altitude will be and the one at will be . Furthermore, layers with inverse trend of temperature or humidity cause atmospheric ducting which bend downward the beam, or even traps radio waves so that they do no spread out vertically. This phenomenon occurs in two circumstances: * A thin stable layer of elevated humidity * Stable temperature inversion Ducting influences becomes stronger as frequency drops. The whole volume of the air acts as a waveguide below 3 MHz to fill in the radar shadow and also reduces radar sensitivity above the duct zone. Ducting fills in the shadow zone, extends the distance of the clutter zone, and can create reflections for low PRF radar that are beyond the instrumented range. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「radar horizon」の詳細全文を読む スポンサード リンク
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