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RADAR HORIZON / LINE OF SIGHT

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There are limits to the reach of radar signals. At the frequencies normally used for radar, radio waves usually travel in a straight line. | RADAR HORIZON LINE OF SIGHT There are limits to the reach of radar signals. At the frequencies normally used for radar radio waves usually travel in a straight line. The waves may be obstructed by weather or shadowing and interference may come from other aircraft or from reflections from ground objects Figure 1 . As also shown in Figure 1 an aircraft may not be detected because it is below the radar line which is tangent to the earths surface. Some rules of thumb are Range to horizon rnm 123 firaa w hmfi RADAR CLUTTER AND SHADOWING Figure 1. Radar Horizon and Shadowing Range beyond horizon over earth curvature rnm i-24 hrodor with h inft In obtaining the radar horizon equations it is common practice to assume a value for the Earth s radius that is 4 3 times the actual radius. This is done to account for the effect of the atmosphere on radar propagation. For a true line of sight such as used for optical search and rescue the constant in the equations changes from 1.23 to 1.06. A nomograph for determining maximum target range is depicted in Figure 2. Although an aircraft is shown to the left it could just as well be a ship with radars on a mast of height h . Any target of height or altitude H is depicted on the right side. See also Section 5-1 on ducting and refraction which may increase range beyond these distances. 200------ 200 5000------ 4000------ 3000------ 2000------ 1000------ 500------ 200------- 100------- .5 0------- 150------150 100------100 50------50 0 0 ----- 5000 ----- 4000 ----- 3000 ----- 2000 ------1000 -----500 ------200 ------100 0 P R H FEET nauticalmiles feet Figure 2. Earth Curvature Nomograph 2-9.1 This data was expanded in Figure 3 to consider the maximum range one aircraft can detect another aircraft using RNM 1 -23 hradar target with h in feet It can be used for surface targets if Htarget 0. It should be noted that most aircraft radars are limited in power output and would not detect small or surface objects at the listed ranges. Figure 3.