730 



AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



FREQUENCY ^ 



Fig. 14-5 Typical Doppler Spectrum. 



It is the function of the/requency tracker of the doppler radar, which will 

 be discussed later, to determine the center of area of the doppler spectrum, 

 i.e. to determine the single frequency which is proportional to the desired 

 velocity component. The half-power width of this spectrum A/^ is quite 

 important with regard to velocity accuracy considerations. It is dependent 

 on the antenna beamwidth and several other radar parameters and can be 

 expressed mathematically by the formula 



^fd = ^ sin 7A7 



(14-2) 



where A/^ is the half power spectrum width, and A7 is the two-way beam- 

 width of the antenna beam in the 7-direction. 



In order to measure both of the horizontal velocity components of the 

 aircraft or both ground speed and drift angle, it is necessary to have at least 

 two beams radiating energy toward the ground. This is shown schemat- 

 ically in Fig. 14-6. The along-heading velocity component (or ground speed 



Isodops 



Fig. 14-6 Two-Beam Doppler System Configuration. 



