86 NAUTICAL CHART MANUAL 



I 



Electronic Navigational Systems 



Included within this category are those electronic navigational systems which require the 

 cartographer to prepare the necessary lines of position for overprinting on selected charts. 

 Currently this category includes Loran "A", Loran "C" and Consol. 



Loran "A" provides accurate fixes at sea to ranges of 700 nautical miles by day and about 

 1400 nautical mUes by night, regardless of weather. Loran depends upon measuring in micro- 

 seconds (millionths of seconds) the time interval between the reception of short pulses trans- 

 mitted from pairs of radio stations on shore. One transmitter of a loran pair, the "master 

 station", emits a number of uniformly spaced pulses each second. Several hundred miles away 

 a second transmitter, the "slave station", emits a corresponding series of pulses which are 

 kept accurately synchronized with those from the master station. 



The time difference between the reception of a master pulse and the corresponding slave 

 pulse establishes one point. If a constant value of time difference is observed at a series of 

 points, and if these points are plotted on a chart and connected, the resulting smoothed curve 

 will be a spheroidal hyperbola, and a loran line of position. A minimum of two "families" or 

 "rates" of these loran lines of position are overprinted on a given chart to provide intersections 

 for fixes. 



Loran "C" provides very acciu-ate fixes at sea to ranges of 1400 nautical miles day and 

 night by using ground wave signals. Fixes up to 2500 nautical miles may be obtained day and 

 night using sky wave signals; however, the acciu'acy is then of the order of ±5 nautical miles. 

 The method of observation is basically similar to Loran "A" except that all stations in a chain 

 operate on the same pulse repetition rate. 



At the present Loran "C" is being used as a special purpose highly accurate navigation 

 system for the Department of Defense. In order to obtain the ultimate accuracy out of the 

 system, especially programed shipboard computers are used. Due to receiver costs, few sets 

 other than by the military, are in use. 



In preparing for this overprinting, the cartographer is concerned with these steps: 



1. Determining which rates will provide the best coverage on the given chart. 



2. Determining which lines of position for a given rate will result in the most satisfactory 



and usable spacing. 



3. Plotting the tabular intercepts on a copy of the chart printed on a stable medium, and 



connecting like points with smooth curves. 



4. Preparing overlays as necessary to indicate color separations. 



