(1) Sky- Wave Interference 



The first-hop sky wave limits the range of the useful ground wave. 

 At ranges of about 1300 miles the signal strength of the sky wave exceeds that 

 of the ground wave and varies with the time of day and season of the year. For 

 example, at this range during the day the sky-wave-to-ground-wave- signal ratio 

 is about 10/1 in winter and about 3/1 in summer. Sky waves do not affect 

 ground-wave reception when they are less than about 10 times ground-wave 

 amplitude. This is true during all seasons of the year. However, at night the 

 ground wave is complicated by the very large sky-wave component at distances 

 greater than 1000 miles during all seasons of the year. 



The Loran-C receiver's ability to utilize the ground wave is largely 

 dependent upon this component's arriving at the receiver slightly ahead of the 

 sky wave. The nighttime sky-wave delay over sea is about 54 microseconds 

 beyond 1300 nautical miles. This provides time for the ground-wave signal to 

 be built up to a suitable S/N ratio before sampling. The sampling point is 30 

 microseconds after arrival . 



(2) Atmospheric Noise Interference 



The accuracy of the cycle -time -difference measurement is practically 

 independent of atmospheric noise for a S/N ratio greater than 1 (0 db). At lower 

 S/N ratios, the mean indicated cycle -time difference is little affected by noise, 

 even though the short-term deviations from the mean increase in magnitude. 



At low values of S/N ratio the cycle accuracy can be improved by 

 averaging the indicated time difference for a few minutes. Reliable measure- 

 ments of envelope-time difference require a S/N ratio of about 3 (10 db). How- 

 ever, for special applications such as surveying and mapping, satisfactory re- 

 sults are obtained with S/N ratios of -20 db. 



(3) Factors Affecting Propagation 



The pulse transmission time from transmitter to receiver depends 

 upon the velocity of propagation. The factors affecting this are the conductivity 

 of the earth's surface and the index of refraction of the atmosphere. An as- 

 sumed constant propagation velocity is normally used. Some measurements of 

 propagation time over the Bermuda base lines have been made. Variations no 

 greater than a few hundredths of microseconds are expected over all seasons of 

 the year . 



75 



arthur ZH.IlittlcIlnr. 



