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purposes as 
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SURVEY NAVIGATION, CORTEN 69 
5. processing of this frequency shift uoneurrogurinto terms of velocity components of 
the vehicle (airplane) with respect to the diffusing surface (earth). 
Main components. 
A complete Doppler system consists of 
1. a Doppler radar supplying the components of aircraft motion with respect to the 
earth; 
2. a heading reference supplying the direction of the coordinate frame, e.g. magnetic or 
geographic coordinates; 
3. a computer processing the aircraft velocity vectors into terms of horizontal ground 
speed, drift, wind speed, and capable of integrating the velocity information over the 
time: in such a way the “distance travelled” is presented as output; if the computer 
has a dead reckoning capacity, the “present position” can be supplied as output. 
DISTANCE DISPLAY 
POSITION DISPLAY 
TO COMPUTER 
  
  
  
  
RADIO FREQUENCY 
FREQUENCY LI} DISCRIMINATOR LS. AMPLIFIER FREQUENCY af 
MEASUREMENT 
SOURCE of al 
  
  
  
  
  
  
  
  
  
  
  
  
EARTH 
Fig. 12. 
Principle of operation. 
The principle of Doppler radar measurements is shown in figure 12. 
Microwaves of frequency f and wavelength 4, generated by a radio frequency source, 
are transmitted by the antenna A under angle y to the earth’s surface. The return signal 
will arrive at A with a Doppler shifted frequency f$. The frequency shift Af — f, —f 
is a measure for the airplane ground speed V according to the relation 
2V .cosy 
Af m faim TM 
This frequency shift is measured, transformed into speed units and displayed or fed 
(13). 
into a computer. 
If the antenna is rotated about a vertical axis, the maximum frequency shift will be 
found in the flight direction and the angle between this flight direction and the air- 
plane's longitudinal axis is the drift angle. In order that the three airplane velocity com-