. Electronic navigational aids; loran, radiobeacon, and radarbeacon systems and loran, radio-direction-finder, and radar ship equipment . BCV3NO nva. (»mr / m^ 3xom» at jegfv SCHEMATIC SKETCH ILLUSTRATING MAXIMUM RANGE. IS PICKED UP ON SCOPE BUT MOUNTAIN IS NOT. FIGURE 4-4.—Maximum and minimum range. able speed of antenna rotation is tied in with the cruising speed of the vessel and the retentivity of the PPI tube. One might reason that by increasing the frequency and repetition rate, more desirable coverage might be obtained with a small antenna. This, however, is not true. We cannot pulse fa
. Electronic navigational aids; loran, radiobeacon, and radarbeacon systems and loran, radio-direction-finder, and radar ship equipment . BCV3NO nva. (»mr / m^ 3xom» at jegfv SCHEMATIC SKETCH ILLUSTRATING MAXIMUM RANGE. IS PICKED UP ON SCOPE BUT MOUNTAIN IS NOT. FIGURE 4-4.—Maximum and minimum range. able speed of antenna rotation is tied in with the cruising speed of the vessel and the retentivity of the PPI tube. One might reason that by increasing the frequency and repetition rate, more desirable coverage might be obtained with a small antenna. This, however, is not true. We cannot pulse faster than the time required for the return of echoes. With too low a pulse repetition rate, targets will be passed over as the sur- rounding area is scanned, while too slow a scanning rate will not keep a con- tinuous picture on the PPI scope because of fading of the illumination after the beam has swept by the target. Therefore, with a slow antenna rotation a low pulse repetition rate should be used and conversely, with a fast antenna rotation, a high pulse repetition rate would be needed. A compromise is consequently necessary. A speed of antenna rotation from 6 to 15 times per minute and a pulse repetition rate greater than 800 pulses per second pro- duces the best results. INDICATOR DISPLAYS.—On the indicator is presented all the information col- lected by the Radar. Although there are many ways of presenting this infor- mation the PPI is considered the most advantageous and desirable from a mariner's point of view. While the distance from the center of the scope to the outer edge represents the range from the Radar to the Radar horizon, this range may be set to suit the individual needs of the user depending upon the areas in which he will operate and possible use to the scale of the naviga- tional charts. Generally a number of different range scales are available to be selected at will. The choice of the lowest range scale, although dependent on magnification desire
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