between 19369.2 and 19379.2 microseconds. Tlie exact time difference measure- 

 ment is therefore 19371.36 microseconds. 



A method of statistical signal integration which helps extract the multi- 

 pulse signal train from background noise is used. A series of sampling rates 

 with a spacing corresponding to the 1000-microsecond spacing between transmit- 

 ter pulses is used. In this manner only the coherent transmitter pulse train will 

 integrate to a value other than zero. In many areas a signal-to-noise ratio of 

 1/10 can provide a useful output from the receiver. 



A system of automatic signal research which makes use of the phase 

 coding of the transmitter pulses is used in the Loran-C receiver. With this 

 method of synchronous phase detection, the receiver yields an integrated output 

 signal only when the received signal is of the same phase coding as contained 

 in the receiver phase coder. The receiver searches and tracks the incoming 

 signals automatically by maintaining the sampling gates in coincidence with the 

 sampling points of the signals. The phase coding is determined when the opera- 

 tor selects the Loran-C stations to be used. 



Sky-wave discrimination or selection is provided in the receiver by 

 an additional sampling mechanism 30 microseconds ahead of the sampling gates. 

 If the search results in the setting of the receiver at a sky-wave signal, this 

 additional sampling mechanism causes a voltage to be generated through the 

 integrator; to use the ground-wave signal the operator advances the position of 

 the sampling mechanism . 



The receiver may comprise a visual oscillographic display which 

 facilitates signal search and allows checking to see if the signals are correctly 

 followed by the receiver during its automatic operation. There are also pro- 

 visions for manual operation. The receiver has protection and alarm circuitry 

 which help the operator monitor proper operation. 



3. ACCURACY 



a. Ground Wave 



The principal factors affecting radio-wave propagation, which in turn 

 affects the range and accuracy of Loran-C, in the ground- wave areas are: the 

 amplitude ratio of ground wave to sky wave and sky-wave delay, the S/N ratio, 

 and the interference from other frequencies within the 90-110 kcps band. Inter- 

 ference from other frequencies is minimized by the synchronous detection and 

 filtering techniques used in Loran-C. These tend to reduce the susceptibility 

 to nonsynchronous interference caused by refractivity of the earth's atmosphere 

 and by the conductivity of the earth . 



74 



arthur ai.^ittlfjnr. 



