extends the distance of the clutter zone, and can create reflections for low PRF radar that are beyond the instrumented range. Limiting factors Shadow Zone Objects beyond Dh will be visible only if the height satisfies the following requirement: where is the target height and is the target range. Objects below this height are in the radar shadow. Clutter Zone The Clutter Zone is where radar energy is in the lowest several thousand feet of air. This extends to a distance of about 120% of the radar horizon. There are a large number of reflectors on the ground at these elevation angles. Prevailing winds of about 15 mile/hour cause these reflectors to move, and this wind stirs up smaller objects into the air. This interference is called clutter. The clutter zone includes the littoral zone and terrain when operating on or near land. A beam wide will illuminate millions of square feet of surface by the time the radar pulse reaches . Targets are generally much smaller, so will be masked by clutter. Clutter reflections can create unwanted false targets. The antenna for radar with no signal processing clutter-reduction improvement is not normally aimed near the ground to avoid overwhelming computers and users. Moving Target Indication (MTI) can reduce clutter by about 35 dB. This allows objects as small as to be detected. Prevailing wind and weather can degrade MTI performance, and MTI introduces blind velocities. Pulse-Doppler radar can reduce clutter by over 60 dB, which can allow objects smaller than to be detected without overloading computers and users. Systems using pulse-Doppler signal processing with speed rejection set above the wind speed have no clutter zone. This means that the clear region extends all the way to the ground. Clear Region The Clear Region is the zone that begins several kilometers beyond the radar horizon at low elevation angles. The clear region is also the zone above low elevation angles with clear skies. There is no clear region in areas with weather and heavy biological activity (rain, snow, hail, high winds, and migration). Over-the-horizon A number of radar systems have been developed that allow detection of targets in the shadow zone. These systems are collectively known as over-the-horizon radars. 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