14-4] MAJOR CHARACTERISTICS AND COMPONENTS 745 



components, and performs the integration by simply counting these pulses, 

 so that the total number of pulses represent total miles traveled. The 

 coordinate transformation (with reference to heading) and roll and pitch 

 stabilization can be done by means of binary multipliers. The major 

 characteristics of this type of computer include very high potential accu- 

 racy, because of the digital techniques used, and medium size and weight. 

 Among the disadvantages of this type of computer is its high component 

 count and the necessity of using analogue-to-digital converters for con- 

 verting such quantities as heading and roll and pitch information into 

 digital form. However, small-size analogue-to-digital converters of this 

 type have recently been developed for this purpose. 



It is the function of the course and distance computer to compare a 

 desired destination with the aircraft's present position and thus by solving 

 the navigational triangle, to compute the desired course and distance to 

 destination. Mathematically there are three types of computations 

 possible,^ and all three types are represented in modern course computers. 



These are: 



1. Great-circle computation 



2. Rhumb-line computation 



3. Midlatitude planar computation 



Existing present-position computers are of either the analogue or the 

 digital type. Their accuracies range from 0.2 per cent to 2 per cent peak 

 error. Their weights range from 15 lb to approximately 50 lb depending 

 largely on the accuracy, the form of outputs, and, particularly, on the 

 number of special functions and outputs required. 



Existing course and distance computers are most frequently analogue 

 type. Great-circle, rhumb-line, and midlatitude types of computers are in 

 use; the trend is in the direction of great-circle computation for long- 

 distance navigation in view of the great accuracy of this computation for 

 long distances and improvements in computer techniques, making small 

 weights and volumes possible. Course computer accuracies are of the order 

 of 0.5 per cent peak error, and weights vary greatly, again depending on 

 the type of outputs required. 



Presently there is considerable development effort on general-purpose 

 as well as differential analyzer types of digital computers (and combinations 

 of these) for airborne applications, which can be programed for the present 

 position and course computation. Such computers promise to have high 

 potential accuracy and versatility, although they tend to exhibit consider- 

 able complexity. 



^See Locke, Guidance, Chap. 3 for a discussion of terrestrial and celestial navigation. 



