2 OCEAN ELECTRONIC NAVIGATIONAL AIDS 



lines of position. These Loran lines can be crossed with other Loran lines, 

 sun lines, star lines, soundings, Radar ranges or bearings to provide fixes. 

 Loran lines are fixed with respect to the earth's surface ; their detemination 

 is not dependent upon the ship's compass, chronometer, or other mechanical 

 or electronic devices. Loran shipboard equipment requires no special calibra- 

 tion and is not affected by the arrangement or disarrangement of shipboard 

 antennas, cargo booms, ventilators, etc., as in the case of radio direction 

 finders. 



Loran signals are on the air and available to navigators for 24 hours per 

 day, and cover the major ocean shipping lanes of the world. Developed as 

 a wartime necessity, the system is now at the disposal of private shipping — 

 any nation, any line, all may make free use of it. 



PRINCIPLES OF OPERATION 



Loran operates on the following principles : 



1. Radio signals consisting of short pulses are transmitted from a pair of 

 shore-based transmitting stations. 



2. These signals are received aboard the ship or plane by a Loran radio 

 receiver. 



3. The difference in times of arrival of the signals from the two radio sta- 

 tions is measured on a special Loran indicator. 



4. This measured time-difference is utilized to determine directly from 

 special tables or charts a line of position on the earth's surface. 



5. Two lines of position, determined from two pairs of transmitting stations, 

 are crossed to obtain a Loran fix. 



Since radio signals travel at a constant speed, a direct relationship between 

 time of travel and distance traveled exists. Thus, measurement of intervals 

 of time is, in essence, a measurement of distance itself. 



The radio signals which are transmitted by Loran stations are not con- 

 tinuous transmissions such as those of everyday commercial broadcasting 

 stations, but are "pulse" signals, or short bursts of radio energy transmitted 

 at regular intervals. The use of "pulse" signals permits the individual 

 signals to be identified in order that time measurements can be made. This 

 would not be possible if the transmissions were of a continuous character. 



Because the basic Loran measurement evaluates the difference in the dis- 

 tances between the navigator and each of two fixed transmitting stations 

 and not the individual distances themselves, there are many points at which 

 the difference would be the same even though the distances varied widely. 

 These points fall along a smooth curve (hyperbola) which is known as a 

 Loran line of position. Therefore, when a navigator has obtained a Loran 

 reading from a pair of transmitting stations he has determined that his true 

 position lies at some point on a particular Loran line of position. By making 

 Loran measurements on a second pair of stations, a second line of position 

 has been identified and the navigator's true position of "fix" has been estab- 

 lished at the point of intersection of the two lines. 



In order to simplify the navigator's problem of interpreting the Loran data 

 in terms of coordinates of latitude and longitude, Loran charts are available 

 which picture the electronic lines of position with respect to some convenient 

 chart of the region in which the ship is sailing. The same information is 

 available in the form of Loran tables for the convenience of navigators who 

 desire to plot Loran lines of position directly on their regular navigators' 

 chart. 



The diagram of figure 1-2 illustrates the basic principles of the determina- 

 tion of position by means of Loran. 



EQUIPMENT USED BY THE NAVIGATOR 



The Loran equipment used by the navigator on shipboard or aircraft at sea 

 in the determination of his position is known as a receiver-indicator. The 

 receiver performs the functions of an ordinary radio receiver, but delivers its 

 output to a visual indicator rather than to a loudspeaker, and is designed for 

 the reception of pulsed signals rather than ordinary radio signals. The indi- 

 cator is essentially an "electronic stop-watch" capable of measuring, in micro- 

 seconds, the difference in times of arrival of the pulse signals from the two 

 stations of a pair. In the indicator, horizontal traces or lines of light on the 

 screen of a cathode ray oscilloscope form the equivalent of the dial of a watch. 



