other is designated the slave station. In the system shown in Figure IV-9, the 

 master station is a double-pulsed station common to two shore stations. 



The accuracy of Loran-A is largely dependent upon the transmitting 

 stations' keeping their signals correctly timed and synchronized. For this pur- 

 pose, the master station utilizes a stabilized 100-kc crystal oscillator and preci- 

 sion timer to set the pulse recurrence rate . A precisely spaced series of pulses 

 is thus transmitted. On reception of these pulses at the slave station, a corre- 

 sponding series of pulses is transmitted. 



The crystal oscillator also provides precise 1 -microsecond timing 

 pulses . Synchronization of the slave to the master is maintained by an operator 

 at the slave station who monitors these radiated pulses and continuously makes 

 the proper timing adjustments . 



Loran signal pulses are approximately 40 microseconds in length. 

 The number of pulses per second' from each station is the recurrence rate. 

 There are three basic rates, each of which is divided into seven specific rates 

 differing by 100 microseconds. This system provides separation of signals 

 from transmitters in the same area . 



Figure IV- 10 indicates the way in which the master and slave station 

 pulses are spaced in time. The coding delay inserted at the slave station is a 

 fixed value and insures that the time between reception of the master and slave 

 pulses is always greater than one -half the recurrence interval at all receiver 

 locations. An operator thus can distinguish between the master and slave station 

 pulses and make a nonambiguous identification. The synchronization of slave 

 to master station is presently ± 1 microsecond and is being reduced to ± 0.25 

 microsecond at U. S. operated stations. 



When trouble occurs and the two stations are not properly synchronized, 

 the operator "blinks" the signal by shifting the signal to the right and left at in- 

 tervals of about 1 second. Blinking is a fail-safe feature of Loran and indicates 

 to the navigator at a receiving station that the transmitter is malfunctioning and 

 that readings should not be taken at that time. 



b. Mobile Station Operation 



Any mobile station having a Loran type A receiver can use the basic 

 Loran navigation system. In this receiver, the pulses from the slave and master 

 stations are displayed on a cathode -ray tube indicator. The display is divided 

 into two equal horizontal segments one above the other, so that received pulses 

 produce a vertical deflection on the screen. The operator matches the master 



61 



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