The pulses are received at stations within the operating area of the 

 transmitters . Time -difference measurements are made by utilizing both the 

 pulse envelope and the phase of the carrier cycles incorporated within the en- 

 velope. After the envelopes are matched to within ± 5 microseconds, a cycle 

 match is made using the 100- kc sine waves that make up the pulse. In this man- 

 ner, the pulse envelope match produces a coarse time-difference reading as in 

 Loran-A, and the cycle match produces a fine time-difference reading. Thus, 

 the time -difference readings of two or more master- slave combinations estab- 

 lish a Loran-C fix. Special tables and charts are required to help locate the 

 geographic position of the mobile receiver. 



a ■ Ground Station Operation 



Loran-C transmitters utilize a peak pulse power of 250 kw - 1 mw and 

 transmit a series of pulses in groups of eight with 1000 microsecond separation 

 between pulses . Each pulse is about 200 microseconds long. The master sta- 

 tion transmits a ninth pulse for identification purposes . The pulse groups are 

 repeated at relatively long periods of time, such as 50, 000 microseconds. When 

 pulses are transmitted in groups the receiver integrates the pulses, thus adding 

 together the signal from each pulse of the group and greatly increasing the signal- 

 to-noise ratio. By increasing the S/N ratio at the receiver in this manner, one 

 may use lower power at the transmitter and may increase the area of coverage. 



To permit identification of master and slave station chains certain 

 pulses within each pulse group are in phase opposition to others according to a 

 predetermined code. Also, the transmission from different stations is very 

 closely synchronized and a coding delay is used at each slave station as in 

 Loran-A. Station coding is outlined in Loran-C tables and charts. 



Each station in a chain of three or four stations transmits one pulse 

 group per recurrence interval. Pulse groups are transmitted at six basic repe- 

 tition rates. Seven additional specific rates can be generated for each basic 

 rate, to provide a total of 48 different usable repetition rates. The master sta- 

 tion transmits first, and is, foil owed by the slave station. Signals are simul- 

 taneously displayed at the receiver as the envelope and phase-time difference 

 readings for the two signal pairs, i.e. , master and one slave, master and the 

 other slave . 



Each transmitter uses 100-kcps phase -coded signals from its timer 

 synchronizer and in turn generates a 200-microsecond pulse of 100-kc sine 

 waves. These pulses are amplitude -modulated, giving an envelope of the proper 

 rise time and shape . 



72 



arthur ai.^ittlcilnr. 



