Radar horizon

The radar horizon is a critical area of performance for aircraft detection systems, defined by the distance at which the radar beam rises enough above the Earth's surface to make detection of a target at the lowest level possible.

It is associated with the low elevation region of performance, and its geometry depends on terrain, radar height, and signal processing.

[1] Without taking into account the refraction through the atmosphere, the radar horizon would be the geometrical distance

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.

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.

The Clutter Zone is where radar energy is in the lowest several thousand feet of air.

wide will illuminate millions of square feet of surface by the time the radar pulse reaches 10 miles (16 km).

The antenna for radar with no signal processing clutter-reduction improvement is not normally aimed near the ground to avoid overwhelming computers and users.

[3] Pulse-Doppler radar can reduce clutter by over 60 dB, which can allow objects smaller than 1-square-foot (0.093 m2) to be detected without overloading computers and users.

The Clear Region is the zone that begins several kilometers beyond the radar horizon at low elevation angles.

There is no clear region in areas with weather and heavy biological activity (rain, snow, hail, high winds, and migration).

A number of radar systems have been developed that allow detection of targets in the shadow zone.

Three systems are generally used; the most common uses the ionosphere as a reflector and beams the signal skyward and then listens for the tiny signals that are returned from the sky, others use a bistatic arrangement with distant antennas looking for objects that pass between them, and a small number of systems use "creeping waves" that travel into the shadow zone.