The conductivity of the earth has the greatest effect on the phase of 

 the low-frequency ground wave. With a service area completely over water, 

 conductivities can be predicted with good accuracy. Service areas over land, 

 however, cause large variations in conductivity, which cause large distortions 

 in the observed lines of position. 



The greatest use of Loran-C equipment is for paths over water, where 

 accurate prediction of conductivity can be made. Here the greatest cause of 

 signal-phase variation is changes in the refractivity of the atmosphere. 



b ■ Sky Wave 



Many of the factors affecting ground waves also affect sky-wave prop- 

 agation . A minimum S/N ratio is required for either mode of operation . There 

 is also the possibility of contaminating by greater and lesser modes. 



(1) Mode Interference 



At times overlap by sky-hop pulses causes distortion of the pulse 

 envelope at the receiver and malfunction of the time -difference -measuring cir- 

 cuits. Even small distortions will cause errors in phase measurement. The 

 maximum phase error occurs when the (N-1) sky hop arrives at the receiver 

 90° out of phase with the Nth hop. The maximum phase error in this case 

 would be 0.32 microsecond if the ratio of desired-to undesired signal were 

 20 db. For this reason, the amplitude of the Nth hop is required to be at least 

 10 db greater than the (N-1) hop before the Nth hop can be used. 



Table IV-6 gives the minimum distance at which the amplitude of the 

 Nth hop sky wave is 10 db greater than the next lower mode . 



TABLE IV-6 



SKY-WAVE RANGE DATA 



Nautical Miles 





Hops 





Winter 







Summer 



No. of 



Day 





Night 



Day 



Night 



1 





877 





610 



1300 



610 



2 





2180 





1900 



3200 



1900 



3 





3050 





3200 



4300 



3200 



76 



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