OCEAN ELECTRONIC NAVIGATIONAL AIDS 45 



MAXIMUM AND MINIMUM RANQE^ 



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SCHEMATIC SKETCH ILLUSTRATING MINIMUM RANGE. ANY 

 OBJECT CLOSER THAN THE MINIMUM RANGE WOULD NOT BE 

 PICKED UP. 



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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 desired for operating in confined areas, has a lower limit 

 dependent largely on the resolution. 



The size of the PPI scope will to some extent govern the range scales that 

 should be used. The size of the scope, however, is essentially a matter of 

 individual preference. As the Radar provides an excellent means of pre- 

 cisely measuring range and bearing it is believed that range and bearing 

 knobs should be provided with precise means of determining these factors, 

 particularly on the better type Radars. Even less expensive Radars should 

 retain the means for precise bearing measurement, as this factor is pre- 

 dominant in determining whether or not two ships are on a collision course. 

 The methods of determining range generally used are by means of a movable 

 range ring geared to a dial or counter and by using fixed range markers 

 (circles) to which the range of an object may be referred. Means should be 

 provided to eliminate the range circles when not in use. 



In measuring bearing, there are two methods available, the first of which 

 is by considering the top of the scope as being in line with the bow of the ship 

 and measuring the bearing relative to the bow of the ship. This method has 

 distinct disadvantages: As the ship turns in one direction, the pips on the 



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