(2) Sky- Wave Correction 



The ability of sky waves to increase the area of coverage is depend- 

 ent upon the stability and the predictability of the delay. In addition, the re- 

 ceiver must be able to identify the sky hop the receiver is using for each of the 

 transmitters . 



Many factors must be considered and special corrections applied to 

 obtain good accuracy in Loran-C fixes using sine waves. Matching two one- 

 hop sky waves is generally preferred to matching a sky wave with a ground 

 wave. Sky-wave corrections can be computed or are available in tables and 

 graphs . 



Range 



The maximum usable ground-wave range over water for db S/N 

 ratio for 100 kw of radiated power is about 1400 nautical miles in the daytime 

 and about 900-1000 nautical miles at night. Increasing the radiated power to 

 i mw increases the range an additional 260 miles. Line -of -position accuracy 

 at these extreme distances should be to within 0. 1 microsecond plus 0.1 micro- 

 second for seasonal variations. Operationally the Loran-C system has recently 

 demonstrated an ability to supply positional information to an accuracy of 

 0.1-0.5 nautical mile with ground-wave transmission in the North Pacific. 



The nighttime sky-wave range under the same conditions should be 

 about 1800 nautical miles for the first hop and 2300 nautical miles for the second 

 hop. Increasing the power level to 1 mw radiated increases both figures ap- 

 proximately 300-400 nautical miles . 



Position fixes with sky waves can be obtained with a probable error of 

 1-1 .5 microseconds. About a 10% decrease in range can be expected for paths 

 over land for both ground-wave and sky-wave operation. 



4. SUMMARY 



a ■ Type of System 



(1) Pulsed time -difference hyperbolic plot plus phase- 

 difference measurement for high accuracy. 



(2) Length of base line: 500-800 miles 



77 



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