LORAN-C 



1. GENERAL 



Loran-C is a pulsed, low-frequency, long-range navigation system . 

 It operates in a frequency band of 90-110 kc which was allocated on a world- 

 wide basis for long-range radio navigation purposes by the ITU Conference of 

 1947. The first Loran-C installations were made and successfully tested in 

 1957 . Coverage is now available for the greater part of the ocean areas of the 

 northern hemisphere. (See Figure IV- 11.) 



Loran-C, like Loran-A, is a pulsed, hyperbolic navigational system 

 which uses the time difference of arrival of rf signals from three shore -based 

 stations of known location to establish a navigational fix. A phase-difference 

 measurement system which provides a much higher degree of accuracy than is 

 available by the pulse -difference method alone is also used. In addition, the 

 Loran-C system uses a lower frequency than basic Loran, and this makes pos- 

 sible a large increase in range . 



Under good conditions, position fixes to within 1500 feet and ranges 

 of 1000- 1400 nm can be expected 95% of the time when ground wave transmission 

 is used. Ranges of 1800-2300 miles and fix accuracies of 2 nautical miles or 

 better are generally available with sky wave reception. 



2. DESCRIPTION 



Loran-C stations are generally used in groups of three, a master and 

 two slave stations. When 360° coverage is desired, a fourth station is added, 

 and the system is called a star of "Y" configuration. Pulses are transmitted in 

 a precisely timed sequence from the master and the slave stations, which are 

 located on a base line of known length. The optimum base line is a compromise 

 between geometry and signal strength considerations . Because of the low fre- 

 quency (90-110 kc) used and the long range possible, base lines as long as 800 

 miles are used. (See Figure IV- 12.) 



Both the master and slave stations transmit on the same frequency on 

 a shared time basis; i.e., the master station transmits its pulse group and is 

 followed in succession by each of the slaves. A coding delay at each slave sta- 

 tion eliminates ambiguity. 



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