The conventional Decca lane identification system does not lend itself 

 to mobile equipment where light weight and compactness are desired, but a 

 modified technique known as the Lambda method (Low Ambiguity Decca) which 

 overcomes this difficulty has been evolved and is incorporated in a Decca sur- 

 vey system operating on the two-range principle. Lane identification is 

 achieved as in standard Decca by superimposing on the fine grid a coarse 

 circular pattern with a lane width of about 10 km. 



3. ACCURACY 



Errors in position as obtained with the Decca Two-Range Survey 

 system are both systematic and random in nature. The systematic errors can 

 generally be corrected to the degree required . More important are the random 

 errors which may be caused by the instability of wave propagation and by in- 

 strumental variations . 



Systematic errors usually involve incorrect knowledge of the mean 

 velocity of propagation, and corrections must be made where the highest system 

 accuracy is to be obtained . Corrections for variations in ground conductivity 

 are usually applied in high precision work. This is especially important where 

 part of the transmission path is over water and part over land. Figure V-2 

 illustrates the amount of phase correction required for the red and green sta- 

 tions for a typical seawater transmission path. With proper correction these 

 errors can be reduced to about 1 or 2 parts in 10,000. 



An additional phase shift caused by placing the receiver in close prox- 

 imity to the transmitter must also be accounted for. This correction is usually 

 determined experimentally. 



Random errors often occur because of variations in skywave inter- 

 ference . This type of interference generally appears during the period from 

 sunset to sunrise at all seasons of the year. It becomes detectable with this 

 equipment at ranges beyond about 40 miles and increases in magnitude as the 

 range increases . For this reason the survey activity is generally confined to 

 daytime operation. During summer daylight, 9S% rms radial errors run from 

 25 to 60 feet as the range increases. Typical random-error contours for day- 

 light operation are illustrated in Figure V-3. 



107 



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