Tài liệu RADAR HORIZON / LINE OF SIGHT pdf

GROUND CLUTTER WEATHER CLUTTER INTERFERENCE RADAR HORIZON SHADOWING h FEET R NAUTICAL MILES H FEET ANTENNA HEIGHT POINT "H" H = 0.672(R-1.22 h) 2 5000 4000 3000 2000 10,000 1000 500 200 100 50 0 250 200 150 100 50 0 10,000 5000 4000 3000 2000 1000 0 500 200 100 50 25 25 250 200 150 100 50 0 h H SHADOW R R NM ' 1.23 h radar with h in ft R NM ' 1.23 h radar % h target with h in ft 2-9.1 Figure 1. Radar Horizon and Shadowing Figure 2. Earth Curvature Nomograph 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): Range (beyond horizon / over earth curvature): 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. 5 10 15 20 25 30 35 150 450 400 350 300 250 200 10 k ft 20 k ft 30 k ft 40 k ft TARGET ALTITUDE (k feet) RADAR AIRCRAFT ALTITUDE 0 10 20 30 40 50 60 70 80 90 100 12 11 10 9 8 7 6 5 4 ANTENNA HEIGHT (feet) R NM ' 1.23 h radar % h target R Visual (NM) ' 1.05 Acft Alt in ft R ESM (NM) ' 1.5 Acft Alt in ft R max (NM) ' 1.23 h r 2-9.2 Figure 3. Aircraft Radar vs Aircraft Target Maximum Range Figure 4. Ships Radar Horizon with Target on the Surface This data was expanded in Figure 3 to consider the maximum range one aircraft can detect another aircraft using: (with h in feet) It can be used for surface targets if H = 0. It should be target noted that most aircraft radars are limited in power output, and would not detect small or surface objects at the listed ranges. Other general rules of thumb for surface "targets/radars" are: For Visual SAR: For ESM: Figure 4 depicts the maximum range that a ship height antenna can detect a zero height object (i.e. rowboat etc). In this case "H" = 0, and the general equation becomes: Where h is the height of r the radar in feet. . Curvature Nomograph RADAR HORIZON / LINE OF SIGHT There are limits to the reach of radar signals. At the frequencies normally used for radar, radio waves usually. it is below the radar line which is tangent to the earths surface. Some rules of thumb are: Range (to horizon) : Range (beyond horizon / over earth curvature):