160 



THE CALCULATION OF RADAR DETECTION PROBABILITY 



situation is often true in matters of this nature and is often not generally- 

 recognized until after elaborate studies have been made, if at all. 



A question which often comes up in connection with discussions of 

 beamwidth and scanning is, why scan at all? Why not simply use a wider 

 beam and a fixed antenna? This thought has a good deal of merit to it. The 

 loss in gain due to the use of a wide beam can be oflFset by the integration of 

 a much larger number of pulses, and the actual detection ranges might very 

 well be comparable. A narrow beam, though, has other advantages which 

 make its use desirable. One of these is that upon detection, the location of 

 the target is known at once so that tracking can commence immediately. 

 Further, the resolution which can only be provided by a narrow beam is 

 often a basic tactical requirement of the system (see Paragraph 2-13). In 

 addition, a narrow beam is often required to give sufficient accuracy during 

 track or to provide a means for narrowing the scan area and "search- 

 lighting" a suspected target. 



Types of Scans. Fig. 3-11 shows some scan patterns which have been 

 used with pencil beam systems. The most common type of scan is a simple 



Multi ■ Bar Raster Scan 

 (A) 



Two - Bar Scan with 



Conical Lobing 



(Palmer Scan) 



(B) 



Fig. 3-11 Some Possible Scan Patterns with a Pencil Beam System 



constant-velocity raster scan with a fly-back at the bottom of the pattern. 

 With a large area to be covered, up to seven or eight bars might be required. 

 Very often the basic scan is modified by a lobing motion. Conical or circular 

 lobing may be used during track to generate angular error signals. During 

 search the lobing may be left on, either because there is no convenient way 

 in which to stop and start the lobing motor or because the larger equivalent 

 beamwidth can be utilized to cut down on the number of scan bars. When 

 this is done, the circular lobing motion combined with the constant-velocity 

 azimuth motion produces a cycloidal scan of the beam centers (Fig. 3-llb). 

 This type of scanning motion is often referred to as a Palmer scan because 

 of its resemblance to a pennmanship exercise. The cardioid and spiral scans 

 shown in Fig. 3-11 represent attempts to minimize the fly-back or dead 

 time. They are not generally regarded as normal designs, but may be 

 required for some applications. 



