When entering the region of coverage, a ship would have to know its 

 position within half a lane; thereafter, a counter would record the number of 

 lanes traversed. This system operates quite adequately in many cases, but is 

 often inconvenient. The difficulty is avoided by superimposing a coarser -scale 

 hyperbolic net over the basic Decca net . TTie transmitting stations radiate a 

 coarse hyperbolic pattern confocally with each fine pattern sequentially, e.g., 

 red, green, purple. The coarse pattern corresponds to the fundamental frequency 

 f; hence, one of its lanes, called zones, corresponds to 18 green, 24 red, or 30 

 purple lanes . This effect is achieved in the case of the master by transmitting, 

 together with the 6f signal, a 5f (purple) signal once a minute during the 1/2 sec 

 period occupied by each identification transmission; simultaneously the normal 

 purple slave is shut off. An f signal can thus be obtained by subtraction at the 

 receiver. Similarly, momentary transmissions of 9f and 8f together from each 

 slave provide a signal of frequency f at the receiver. 



The lanes of the fine hyperbolic grid are numbered in away that avoids 

 confusion between the three patterns; the zones are identified by letters of the 

 alphabet and are of uniform width (about 10 km on the base line) . Figure IV -6 

 illustrates the lanes and zones of the English Decca chain. 



50a 



Arthur ZD.'^LittlcKnr. 



