E. LORAN-A 



1. GENERAL 



Standard Loran (Loran-A) was developed at M.I.T. Radiation Labora- 

 tories during World War IL Loran stations were established in the Atlantic 

 and Pacific theaters and operated under security conditions as a navigational 

 aid. Commercial equipment has been made available since the war and installed 

 world-wide to increase the system's usefulness and coverage. (See Figure IV- 

 8.) It now enables properly equipped ships and aircraft to plot their position 

 with good accuracy out to 800 miles or more. 



Loran-A uses a pulsed hyperbolic method of fixing a ship's position 

 by the reception of pulsed radio signals in a timed sequence from transmitting 

 stations of known position. The time difference of arrival of two or more pairs 

 of signals is measured,' and these measurements are transformed into radial 

 distances from the transmitter. The Loran-A system does not require a 

 knowledge of the ship's heading or dead-reckoning position and is independent 

 of other mechanical devices, including the compass and chronometer. 



2. DESCRIPTION 



Loran-A operates in the frequency band from 1800 to 2000 kc . With 

 present techniques the ground waves may be used to distances of 600-900 nauti- 

 cal miles in the daytime, and the sky waves are used out to 1400 nautical miles 

 at night . 



In this system, radio signals consist of short pulses which are trans- 

 mitted in a precisely timed sequence from a pair of shore stations . The shore 

 stations are carefully located on a base line of known length . The pulses are 

 received on special receivers within the operating area of the transmitters, 

 and the time difference of arrival of the pulses is displayed and measured on 

 a cathode -ray-tube indicator. This measurement is used to determine from 

 Loran-A tables or charts a line of position on the earth's surface. Two or 

 more lines of position from two or more Loran station pairs are used to deter- 

 mine a position fix. 



a . Ground Station Operation 



A Loran position fix requires that the difference in time for pulses 

 to arrive from a pair of transmitting stations be measured to a precision of 

 ± 1 microsecond. One station of a Loran pair is a master station, and the 



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