14-3] 



SYSTEM CONSIDERATIONS 



733 



Since a condition of no pitch or roll was assumed for the above expres- 

 sions, the quantities Fh, Fd, and Fv can be defined as the three orthogonal 

 velocity components in aircraft coordinates. 



Ground Speed and 



Drift Angle or 



Ground Velocity 



Components 



Fig. 14-8 Block Diagram of Typical Doppler Radar. 



The block diagram of a typical doppler radar is shown in Fig. 14-8. 

 It consists of four major components — the antenna system, a radar 

 transmitter, a radar receiver, and one or more frequency trackers. The 

 outputs of the doppler radar are frequencies, voltages, or pulse rates 

 proportional to the orthogonal aircraft velocity components or to the 

 ground speed, and /or a shaft angle proportional to drift angle of the 

 aircraft. 



14-3 SYSTEM CONSIDERATIONS 



Typically, doppler navigation systems are employed where the navi- 

 gation accuracy requirements are of the order of 1 per cent of the distance 

 traveled. Because of certain limitations which will be explained in more 

 detail, accuracies appreciably better than this are not obtainable, partic- 

 ularly for extended operations over water. Since a doppler navigation 

 system is an active system, its use is restricted to applications where the 

 transmission of electromagnetic radiations does not seriously endanger 

 aircraft survival or mission accomplishment. Other important system 

 considerations governing the choice of a particular type of doppler naviga- 

 tion system are (1) trade-offs between accuracy — as it affects mission 

 accomplishment — and the size, weight and complexity of the system, 

 (2) mission profiles (maximum and minimum speeds, altitudes, range, 

 times of flight, and geographical latitudes), and (3) types of terrain and 

 weather. 



A doppler navigation system may take many forms. It may be a 

 combination of various existing components or it may be a highly integrated 

 system. The basic thesis, advanced earlier, that the system performance, 

 primarily as regards accuracy, is a function of the performance of its 

 weakest component, holds in either case. 



