14-4] MAJOR CHARACTERISTICS AND COMPONENTS 739 



cross-linear and circular-even (same rotation received) have also been 

 considered. Originally the main advantage of circular-odd polarization was 

 more efficient duplexing; ferrite duplexers have now removed this advan- 

 tage over linear polarization. The same is true for cross-linear polarization; 

 however, the greatly increased return power loss, particularly over water, 

 is a considerable disadvantage of this polarization. To date, there is no 

 experimental evidence of any appreciable difference in performance 

 between linear and circular-odd (opposite rotation received) polarization. 

 Circular-even polarization (same rotation received) has very desirable 

 and well-known rain discrimination properties. However, like cross-linear 

 polarization, it suffers from an appreciable scattering loss over the sea, 

 which has discouraged designers from using this polarization in practical 

 systems. 



Beam Configuration. The beam configurations of doppler radars 

 can be classed into two general categories, namely Janus (two-way looking) 

 and non-Janus (one-way looking). Janus systems use three or four beams 

 of radiation, while non-Janus systems use two beams of radiation. Four 

 beams are used in some systems, primarily because of the symmetry of this 

 configuration (which usually results in greater computer simplicity) and 

 also because it is naturally produced by certain types of antennas, such as 

 planar arrays. While two beams are the minimum number for determina- 

 tion of ground speed and drift angle (or along-heading and cross-heading 

 velocity components), three beams are the minimum number for deter- 

 mination of along-heading, cross-heading, and vertical velocity components. 

 However, as indicated briefly in the beginning of this section, even for 

 determination of only the horizontal velocity components the Janus 

 configuration has important advantages, at the cost of one extra beam, 

 over the non-Janus configuration — namely, much less velocity error 

 dependence on the knowledge of the vertical attitude and vertical rate of 

 the aircraft. The mathematical expressions for this dependence are easily 

 derived and are 



Ev = — = tan 7(67) non-Janus (pitch only) (14-12a) 

 Ey = — = ] — cos (8y) Janus (pitch only) (14-12b) 



V 



£„ = — = 1 - cos (57) + sin (57) tan a (14-1 2c) 



Janus (pitch and vertical velocity) 

 where £„ = fractional velocity error 

 8v = error in velocity 



