D. DECCA 



1. GENERAL 



Standard Decca is a ship -positioning and navigation technique devel- 

 oped in Britain and introduced commercially in 1946 by the Decca Navigator Co . , 

 Ltd. It is now in extensive use throughout the British Commonwealth and in 

 Greenland, Sweden, the Persian Gulf, and the eastern coast of Canada. (See 

 Figures IV -4 and IV -5.) 



Unmodulated continuous -wave transmissions occupying narrow fre- 

 quency bands in the range 70-130 kcps are employed to generate hyperbolic lines 

 of constant phase difference. Phase comparison circuits on a shipborne receiver 

 utilize two or more of these lines to develop a position fix. Depending on condi - 

 tions of season and time of day, useful maximum ranges vary between about 200 

 and 500 miles, while 95% rms radial errors are roughly 1/4 to 4 miles . 



2. DESCRIPTION 



Decca chains generally consist of a master station around which are 

 disposed three slave stations at distances of 60-100 nautical miles . Thus , three 

 intersecting hyperbolic patterns are generated, from which the user selects the 

 two lines intersecting at die best angle for accurate cross -fix at his location. 



Conceptually, the master station and a phase -locked slave station 

 transmit unmodulated cw waves of the identical frequency. The shipborne 

 receiving equipment then phase -compares the two signals to obtain a position 

 line. In practice, two waves of equal frequency cannot be phase -compared, as 

 they would combine at the receiver into a single wave. The desired effect is 

 achieved by operating the two transmitters at two harmonics of a fundamental 

 frequency f and then comparing phases at the receiver at a common multiplied - 

 up frequency. (See Table IV -4.) The phase differences are displayed on three 

 pointer -type phase meters, known as Decometers (one for each master /slave 

 combination) . On board ship the Decometer readings are usually plotted manu- 

 ally on a chart overprinted with correspondingly numbered Decca -grid lines . 

 Airborne equipment generally provides a continuous automatic plot of the position 

 fix. 



The space between the two adjacent hyperbolic position lines having 

 the same phase difference is known as a lane. In practice, interpolation of 0.01 

 lane can be achieved, corresponding to a position change of several meters along 

 the base line where the lane is narrowest. Typical values of base-line lane width 

 are given in Table IV -5 . 



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