744 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



are used and does not afford any cancellation of incidental FM in the 

 transmitter, which can result in appreciable spectrum broadening. 



The Navigation Computer. The computer of the navigation system 

 is required to do primarily two jobs: 



1. Computation of present position by integration of the velocity and 

 by transformation of the data to directionally oriented earth 

 coordinates by use of data from a heading reference. 



2. Computation of course and distance to a destination by comparing 

 destination coordinates with present position coordinates and thus 

 solving the navigational triangle. 



The discussion which follows will treat these two functions of the computer 

 separately. 



Mathematically, all present position computers are essentially equiva- 

 lent, since they must integrate the velocity components after converting 

 them into earth coordinates. There are, however, several types of com- 

 puters, which may be classed in several ways. One division is that oi generic 

 analogue computers and general purpose digital computers. 



Generic analogue present position computers can be classed as: 



1. Electromechanical analogue computers 



2. Electrical analogue computers 



3. Operational digital computers 



The electro-mechanical analogue computer contains ball-and-disk integra- 

 tors and other analogue components to perform the analogue computation. 

 Several modern navigation computers are of this type. They are probably 

 the oldest of the three types listed and therefore represent fairly well- 

 established techniques. Their major characteristics include reasonable 

 simplicity, medium size and weight advantage, good reliability and 

 maintenance characteristics, and fair accuracy. 



The electrical analogue computer contains various forms of electronic 

 integrator circuits to perform the computations and uses such components 

 as motor tachometers, synchros, and resolvers which are considered to be 

 primarily electrical rather than mechanical. Several modern computers are 

 of this type and their characteristics include medium size and weight, good 

 reliability and maintenance characteristics, and medium accuracy. 



The operational digital computer, is probably the most recent development 

 of the three types. In a generic or conceptual sense, it is an analogue 

 computer which performs the computation in real time. The computer 

 converts the doppler frequencies representing the velocity components into 

 pulse trains, the rates of which are now proportional to the velocity 



