OCEAN ELECTRONIC NAVIGATIONAL AIDS 



15 





Figure 1-13. — Loran signal building 



sented is in tabular form and may be of interest only to those readers who 

 desire electrical system information.) 



LORAN. — A long-range aid to navigation for the determination of position, 

 based upon the difference in times of arrival at the navigator's position of 

 radio signals transmitted from two fixed Loran transmitting stations. 



TYPE OF TRANSMISSION. — Loran transmissions are "pulse" transmissions. 

 Very short pulses of radio frequency energy are radiated at periodic intervals 

 which are very long compared to the duration of the pulse. During the 

 interval between successive pulses of a station, no radio signal from that 

 station is on the air. 



RADIO FREQUENCY. — Loran Signals are transmitted at the present time on 

 two frequency channels in the United States : 

 1950 kilocycles. 

 1850 kilocycles. 



PULSE DEFINITION. — The Loran pulse signal is of approximately 80 micro- 

 seconds duration and the rectified envelope has a shape similar to that of a 

 sine wave when viewed on a Loran receiver. It is customary to measure 

 pulse width at one-half amplitude; standard width is thus considered to be 

 40 microseconds. 



PULSE REPETITION RATES. — Loran pulses are transmitted at a number of dif- 

 ferent repetition rates. Differences in repetition rate are the means of 

 identification of particular pairs of stations. Specific pulse repetition rates 

 are assigned in several general categories with the particular rates of each 

 category being only slightly at variance with a convenient rate known as 

 the "base rate." Present "base rates" and "specific repetition or recurrence 

 rates" are tabulated on the following page. 



LORAN TIMING SEQUENCES. — (Loran timing sequences are shown in fig. 1-8.) 

 Master and slave stations are pulsed at exactly the same pulse recurrence 

 rate. The master pulse is transmitted first and, after traveling the distance 

 of the base line, arrives at the slave where it is received and used as a timing 

 reference. After waiting for a predetermined length of time which is neces- 

 sary to establish correct Loran synchronization, the slave station transmits 

 its pulse. The amount of time that the slave waits, or delays, is always fixed 

 at an amount greater than one-half of the recurrence interval plus a 

 coding delay (usually 1000 u/s). As a consequence, the master signal is 

 always transmitted during the first half of the recurrence interval and the 

 slave transmission always occurs during the second half of the interval, 

 regardless of the navigator's position with respect to the stations. The sig- 

 nals are received and viewed by means of a cathode ray oscilloscope using a 

 time base equal to the recurrence interval, but divided into two equal half- 

 interval traces so placed on the scope by the sweep circuits that two half- 



