 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.Without taking into account the refraction through the atmosphere, the radar horizon would be the geometrical distance formula_1 from the radar to the horizon only taking into account the height H of the radar above sea-level, and the radius of the earth formula_2 (approximately 6.4·103 km)