Since the transmissions are pure unmodulated cw, a very narrow 

 receiver bandwidth maybe used- -typically ± 30 cps at half -amplitude. Further- 

 more, a large number of stations occupy only a small part of the frequency 

 spectrum . 



The maximum working range is generally set by the operating authority 

 at 240 nautical miles from the master. The principal limiting factor is a tendency 

 for the amplitude of the sky wave reflected from the ionosphere by night to reach 

 equality with the ground wave at around 240 nautical miles . However, by day 

 useful range may be extended by a factor of two or more, though accuracy at 

 these larger ranges will be low due to the large lane width and small intersection 

 angles . 



3. ACCURACY 



Random errors in the standard Decca system include errors in read- 

 ing the phase meter, random changes in circuit parameters, and changes in local 

 velocity of propagation. The standard deviation in phase readings from all such 

 causes amounts to less than .01 mean lane; a meaiji lane being defined as one 

 whose base-line width is 500 meters . 



Systematic errors in Decca involve incorrect knowledge of the mean 

 velocity of propagation and, more important, the tendency for the sky wave 

 reflected from the ionosphere to reach equality with the ground wave at longer 

 distances . When this situation obtains , ionospheric fluctuations will produce 

 variations in phase -meter readings and lead to position errors . These errors 

 increase with range and are more serious at night than at day, and in winter 

 than in summer. The 95% radial errors to be expected under various conditions 

 are reproduced in Figure IV -7. The tabulation in the figure lists the errors 

 unlikely to be exceeded in one out of 20 readings . 



The sky wave also has an important effect on lane identification errors . 

 The probability of correct lane identification is a function of range, season, and 

 time of day. Some typical results: During summer daylight over ground of 

 conductivity 10"-'^^ e.m.u., lane identification is about 95% successful at all 

 working ranges; by night, the 95% probability contour extends out only some 150 

 miles, while the 67% probability level is at about 250 miles . 



54 



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