726 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



3. Active ground mapping systems 

 (a) Forward look 



(i?) Side look 



4. Passive ground mapping systems 

 (a) Radiometric 



(^) Infrared 



I 



14-1 INTRODUCTION TO DOPPLER NAVIGATION SYSTEMS 



Doppler radars may be designed for measuring an airborne vehicle's 

 velocity with respect to the earth's surface. One of the primary applications 

 of the measured velocity is its use in a self-contained navigation system in 

 which the velocity is integrated to obtain distance traveled and combined 

 with heading information to obtain the vehicle's present position in earth 

 coordinates — for instance, as latitude and longitude or as deviations from 

 a desired course. Present position coordinates may then be compared with 

 desired destination coordinates, and the distance and the required course to 

 the destination computed and furnished to the pilot or autopilot as a vehicle 

 steering signal. Such a system indirectly measures the effects of unknown 

 winds, drift errors, etc., and thus achieves an improvement in navigation. 



In such an application the doppler radar becomes one of the components 

 of a self-contained navigation system. A typical system consists of three 

 basic components: (1) a velocity sensor^ for instance a doppler radar, 

 (2) a reference which relates the information to the earth, such as a magnetic 

 compass, astrocompass, or gyrocompass, and (3) a computer, which operates 

 on the information obtained from the velocity sensor and the reference 

 (which is really a form of sensor also) and computes the desired output 

 information. It is important to recognize that in such a system the overall 

 accuracy performance is determined by the accuracies of all three of the 

 components, and is essentially limited by the accuracy of the weakest of 

 the three. 



A basic self-contained navigation system is shown in block diagram form 

 in Fig. 14-1. A system of this type can be considered a dead reckoning 

 navigation system since it is based on navigating from a known point and 



Sensor 



Computer 



Display 



Fig. 14-1 Basic Self-Contained Navigation System. 



