A, DECCA TWO-RANGE SURVEY SYSTEM 



1. GENERAL 



The Decca Two -Range Survey system is a more recent development 

 of the standard Decca Navigator system. It is used primarily for hydrographic 

 surveying purposes where accuracy is important and a shorter range capability 

 is acceptable. In this system the master transmitter is shipborne, as is the 

 receiver, and the two shore-based slave stations are light in weight and mobile. 

 Operational principles are the same as in standard Decca, but the geometry of 

 the system leads to circular instead of hyperbolic patterns . Under favorable 

 conditions. Two -Range Decca produces position fixes in error by only 25-60 feet. 



2. DESCRIPTION 



In 1950 there arose an operational requirement for radio navigation 

 to aid in a marine survey in Antarctica, where only two suitable sites for shore 

 stations were available. The Two-Range Decca system which was developed in 

 response to this need has evolved into a useful survey tool which lends itself 

 well to small, mobile equipment. 



Figure V-1 illustrates the typical two-range layout. A master trans- 

 mitter on the ship radiates a cw signal of frequency 12 f. The red slave station 

 ashore receives this transmission and radiates a signal of frequency 8 f, in such 

 a manner that the slave and master signals are phase -locked at the common 

 multiplied -up frequency of 24 f. The lines of constant phase difference are now 

 circles with a center at the red slave and a lane width of about 420 meters . 

 Similarly, the master and green slave produce an intersecting circular net with 

 a lane width of about 280 meters . Fixes are obtained throu^ the intersection 

 of a green and a red position line, as in standard Decca. 



A notable characteristic of the system is that because the lane width 

 is constant with increasing range, position accuracies of 1-2 meters are 

 achievable. Furthermore, the angle of intersection of red and green position 

 lines does not deteriorate with range . Thus accurate fixes are obtainable at 

 ranges well in excess of 150 miles. A compensating disadvantage is the need 

 for a transmitting station and receiver aboard each ship which desires to use 

 the Two-Range Decca. 



A more serious limitation is the high degree of pattern ambiguity. 

 Before a fix can be obtained, ship position must be known to better than ±210 

 meters and ±140 meters from the red and green slaves, respectively. 



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