14-11 



INTRODUCTION TO DOPPLER NAVIGATION SYSTEMS 



727 



computing the track being made good and a new position by means of 

 airborne data, namely, ground velocity and aircraft heading. It is signifi- 

 cant that these data are obtained without the need for visual access to the 

 ground or the use of radio or radar stations located on the ground; hence 

 this method has been given the name self-contained navigation. 



One example of such a self-contained dead reckoning navigation system 

 is the true air speed syste?n shown in Fig. 14-2. Here the basic velocity 



Fig. 14-2 Typical True Air Speed Dead Reckoning System. 



information is composed of air speed from a true air speed meter and wind 

 vector (magnitude and direction) information. The latter is usually 

 obtained from a meteorological source and inserted manually. The heading 

 reference is most frequently a magnetic compass. The weakest links in this 

 system are probably the air speed sensor and the poorly known wind 

 information, so that accuracies of this type of system have been quite 

 limited. The system obviously does not employ a radar. 



A doppler radar is a highly accurate sensor of aircraft ground velocity 

 (ground speed and drift angle). It measures actual ground velocity, that is, 

 the velocity of the aircraft with respect to the ground (not the air mass); 

 hence all wind motion is included in this measurement and external wind 

 data are no longer needed. Substitution of doppler radar for the true air 



Doppler 



Ground Speed 

 Drift Angle 



or Ground 



Velocity 



Components 



Present 

 Position 

 Computer 



Present Position 



Track 



Heading 



Heading 

 Reference 



Course and 

 Distance 

 Computer 



Course, Distance 



Display 



Autopilot 



Desired 

 Destination 



Fig. 14-3 Block Diagram of a Typical Doppler Navigation System. 



