OCEAN ELECTRONIC NAVIGATIONAL AIDS 5 



determined through the use of "ground waves," but, nevertheless, is still of a 

 high order of accuracy. 



One of the surprising facts about Loran is that in a matter of 2 to 3 min- 

 utes' time a navigator at sea can determine his position with an accuracy 

 comparable to that obtained from good celestial observations, which require 

 considerably longer to make and which entail somewhat laborious mathe- 

 matical computations. 



The accuracy of Loran fixes varies considerably, depending on the relative 

 position of the navigator and the transmitting stations, the angle at which 

 the Loran lines of position intersect and several other factors. 



A very rough rule of thumb has been stated to be that a Loran line of posi- 

 tion has an accuracy of better than 1 percent of the distance of the navigator 

 from the stations; thus a navigator 1,000 miles away from the stations would 

 expect the line of position to be well within 10 miles of the proper position. 

 As the stations are approached, the accuracy increases greatly, and along the 



Ionosphere 



Figure 1-4. — Ground wave and sky wave paths. 



imaginary line between the two stations, or "base line", a line of position may 

 have an accuracy of the order of several hundred feet. This feature has par- 

 ticular practical value, inasmuch as the physical arrangement of Loran sta- 

 tions is such that a navigator making a landfall usually will approach the 

 shore in this highly accurate area of Loran service. Figure 1-5 shows the 

 pattern that a family of Loran lines of position make with respect to their 

 transmitting stations and points out the regions of accuracy. Figure 1-6 

 shows a vessel approaching a harbor along a line of position. 



Another important feature of Loran to the navigator is the reliability of 

 the signals and the consequent removal of doubt in the navigator's mind as 

 to the dependability of Loran fixes. Loran signals can be received under all 

 ordinary conditions of storms, gales, and other severe weather. This is 

 possible because the ordinary electrical interferences that accompany these 

 conditions obscure the Loran signal for only a few seconds at a time and the 



