B. SYSTEM CLASSIFICATION 



Navigation and ship -positioning systems are generally classified as 

 hyperbolic, ranging, or azimuthal . This classification refers to the radiation 

 patterns, which are illustrated in Figure III -2. 



I ■ HYPERBOLIC POSITION -DETERMINING SYSTEMS 



All hyperbolic position -determining systems define a hyperbolic line- 

 of -position by measuring the difference in transmission time bety/een signals 

 transmitted simultaneously, or with a fixed delay, from two fixed stations; these 

 stations represent the foci of the hyperbolic line of position. To generate a fix, 

 two lines of position (LOP) must be generated, and a third station, which serves 

 as a master to each of two slave stations, is required. The time -difference 

 error makes the actual position line indeterminate between two adjacent position 

 lines . This distance is called the lane width and corresponds to one -half wave 

 length of the transmitted frequency. Either pulsed or continuou's wave trans- 

 mission is used. Some systems use a combination of the two. 



The actual error of the position fix with such a system is proportional 

 to the largest dimension of a diamond formed by the intersection of adjacent 

 paired lines of position . The positional error increases as the distance from 

 the stations is increased . Loran-A, Loran-C, Decca Navigator, andOmegaare 

 examples of hyperbolic navigation systems . 



2. RANGING SYSTEMS 



Distance -measuring (ranging) systems generate a circular pattern. 

 The master interrogates the shore stations; these stations respond, and the 

 round -trip time is measured at the master station. Either pulsed or continuous 

 wave may be used. It is clear from Figure III -2b that this arrangement yields 

 higher accuracy over a larger area than hyperbolic systems do. However, the 

 systems required to generate circular patterns are usually more complex, and 

 several frequencies are often required to generate and transfer the required in- 

 formation. Ranging systems such as Shoran and Raydist have found their great- 

 est use in hydrographic work where the ranges are 200 nautical miles or less . 

 Lower -frequency systems such as EPI have been found useful to ranges of 400 

 nautical miles . 



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