14-2] 



BASIC PRINCIPLES OF DOPPLER RADAR NAVIGATION 



731 



in a gimbaled and track-directed antenna) can then be obtained by adding 

 the doppler shift obtained from the two beams. 



The cross-heading velocity component is obtained by taking the dif- 

 ference between the doppler shift from the two beams. In a gimbaled 

 antenna system, the drift angle can be obtained by servoing the antenna 

 until equal doppler shifts are obtained from the two beams. It is clear that 

 the measurement of along-heading and cross-heading velocity components 

 is equivalent to the measurement of ground speed and drift angle, since the 

 former two are simply components of the ground velocity vector, of which 

 the ground speed is the magnitude and the drift angle is the angle of the 

 vector with reference to the aircraft heading line. 



The hyperbolas appearing on the right side of Figs. 14-6 and 14-7 are lines 

 of constant doppler shift on the ground ("isodops") for a velocity vector 



Isodops 



Fig. 14-7 Three-Beam Doppler System Configuration. 



which is coincident with the heading line of the aircraft. By considering 

 these lines, it is easy to see how the doppler shifts from the left and right 

 antenna (of a fixed antenna system) will be different if the velocity vector 

 is not coincident with the heading line, i.e. when a drift angle exists. The 

 difference between these two doppler shifts is then proportional to the 

 cross-heading (or drift) component of velocity. 



For a number of reasons it turns out that the use of two beams is not 

 optimum for most applications, but rather the use of three or four beams, 

 as shown schematically in Figs. 14-7 and 14-9. This configuration has been 

 given the name Janus, after the Roman god who was said to have the 

 facility of looking forward as well as backward. Such a Janus system has 

 the following advantages over the one-way looking (or non-Janus) system: 



1. It provides for inherent cancellation of the vertical velocity compo- 

 nents, when the forward and rearward looking beams are combined 

 to measure horizontal velocity components. 



