728 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



speed meter in the previous system yields a block diagram of a typical 

 doppler navigation system as shown in Fig. 14-3. The system operates in the 

 following manner. The doppler radar furnishes aircraft ground velocity as 

 ground speed and drift angle (or as along-heading and cross-heading 

 components) to the present position computer. The computer combines 

 this information with heading information (from the heading reference) to 

 convert it to earth coordinates and continuously integrates the velocity to 

 obtain miles traveled. This results in North-South and East-West miles 

 traveled and ground track being made good. Further conversion, using the 

 secant of latitude, finally results in the generation of present position in 

 coordinates of latitude and longitude, assuming those are the coordinates 

 desired. The present position information is then furnished to the course 

 and distance computer, where it is compared with the desired destination 

 coordinates, and the course and distance to the destination are automatical- 

 ly and continuously computed. These are displayed to the pilot or fur- 

 nished to an autopilot for automatic control of the aircraft. 



It bears emphasizing that a doppler navigation system is not a complete 

 system unless it contains all three elements — the doppler radar, the 

 computer, and the heading reference. The performance of the overall 

 system is again dependent upon the performance of its weakest component. 

 However, the component which is weakest in the true air speed system — 

 namely the velocity source — has been replaced by a highly accurate 

 source, the doppler radar. The navigation computer can be built with 

 accuracies compatible with the doppler radar; thus the heading reference is 

 usually the weakest component of the system. 



A doppler velocity measuring radar may be used for many applications 

 other than as a component in a self-contained navigation system. These 

 applications include radar display ground speed stabilization (as in AEW) 

 photographic stabilization, approach and landing, and north-seeking 

 compass correction. A large number of the design criteria for a doppler 

 velocity measuring radar are invariant, regardless of the application, while 

 certain other ones, such as frequency tracker time constant and degree 

 of attitude stabilization may be greatly dependent on the application 

 intended. 



14-2 BASIC PRINCIPLES OF DOPPLER RADAR 

 NAVIGATION 



If we mount a transmitter and a receiver on an aircraft, transmit energy 

 toward the ground, and receive the back-scattered energy, the difference 

 between the received and transmitted frequency is the doppler shift caused 

 by the fact that the transmitter (and also the receiver) are moving with 

 respect to the earth, from which the energy is reflected. If we measure this 



