In the event of improper transmitter operation, a standardized blink- 

 ing procedure is used to notify receivers in the area . The blink consists of 

 shifting the entire pulse group by a specified amount once each second. This 

 operation is visible to the receiver operator and activates automatic alarms . 



Master and slave station transmitters are similar, and both feed 625- 

 foot antennas. Each station has a certain amount of redundancy in equipment 

 to minimize downtime caused by equipment failure . 



b. Receiving Equipment 



Specialized receiving equipment is required to take full advantage of 

 the Loran-C positional accuracy. A coarse position is obtained by the measure- 

 ment of the time difference of the envelope signals from two master-slave pairs. 

 This measurement is made to within ± 5 microseconds. A second measurement 

 of the phase of the signals is made to within a few hundredths of a microsecond, 

 thus making possible a very accurate line of position. 



Sky-wave contamination is avoided by making the time -difference and 

 phase -difference measurements before the sky wave arrives. Thus, these 

 measurements are made automatically at the Loran-C receiver within the first 

 30 microseconds after receipt of the ground wave from the master station and 

 each slave station. 



The coarse position measurement is made automatically within the 

 receiver. Only the leading edge of the envelope is used. By electronically 

 processing the master and slave signals the receiver establishes a precise 

 base line crossing at the 30 microsecond point of the leading edge for each sig- 

 nal and makes a time -difference measurement between them. A readout estab- 

 lishing a coarse line of position is automatically made on vernier dials to within 

 5 microseconds. 



Ilie fine measurement is made by comparing the phase of the individual 

 sine waves contained in the pulse of the master and slave signals with the phase 

 of a precisely controlled reference oscillator in the receiver . Electronically 

 the receiver locks the frequency of the local oscillator to the frequency and 

 phase of the incoming signal . A sampling gate allows the receiver to be turned 

 on and to "look at" the signal at the 30-microsecond sampling point, thus avoid- 

 ing sky-wave interference. The measurement is automatically presented on a 

 vernier dial which reads from to 10 microseconds in increments of 0.01 micro- 

 second. The total time difference is the sum of the envelope and phase readings. 

 For example, if the coarse (envelope) measurement were 19374.2 microseconds 

 and the fine (phase) were 1 .36 microseconds, the actual measurement would be 



73 



arthiir Sl.ltittlfJInr. 



