Chapter 6— PRINCIPLES OF SONAR 





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A STEP-BY-STEP GENERATION OF SPIRAL SCAN 



B SYNCHRONIZING THE SWEEP WITH THE 

 SCANNING SWITCH 



71.121 

 Figure 6-7. — Generation of spiral scan. 



scope at a linear rate. On reaching the scope's 

 outer edge, full deflection is achieved and the 

 sweep is then cut off. 



Rotation of the scanner is at such a speed 

 that you don't see a spiral sweep, but what 

 appears to be an expanding circular sweep. 

 An echo is seen on the scope as a brightening 

 of the sweep, at a distance from the scope 

 center, and in a direction corresponding to the 

 target's range and bearing. 



During the interval between the end of one 

 sweep and the beginning of the next, the cursor 

 is electronically produced on the scope. Because 

 of the long persistency of the CRT, the target 

 echo remains visible for a short time, allowing 

 the operator to determine accurately the target's 

 range and bearing. Bisecting the echo with the 

 cursor gives the bearing of the target. By 

 adjusting the length of the cursor so that its 



tip touches the echo, the target's range is deter- 

 mined. 



Also located on the sonar control-indicator 

 are various switches and controls. Their purpose 

 is to give a better target presentation. These 

 switches and controls, together with complete 

 operating procedures, are explained in the manu- 

 facturer's technical manual supplied with each 

 sonar system. 



Rotating Directional Transmission 



Conventional scanning sonars formerly were 

 of the type just discussed; that is, only omni- 

 directional transmission was possible. Modern 

 sonars have an operating feature called rotating 

 directional transmission (RDT), which operates 

 on a principle similar to the searchlight type 

 of transmission. The RDT provides greater 

 transmitted power thaii the conventional scanning 



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