740 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



V = velocity 



57 = uncertainty in pitch angle 



7 = angle between horizontal velocity component and direction of 

 radiation 



a = angle of climb or descent. 



It is seen from these expressions that for a 7 angle of 70° (a typical value 

 for many doppler radars) the non-Janus system suffers approximately 

 4.7 per cent error in velocity per degree of uncertainty in pitch angle, 

 whereas the Janus case suffers approximately 0.014 per cent per degree of 

 uncertainty. Similarly, the error incident to vertical velocity is very small 

 in the Janus case, while it must be compensated for in the non-Janus case, 

 with uncertainties in vertical velocity appearing as a direct factor. 



The choice of 7 angle for a doppler system represents a compromise 

 between high velocity sensitivity (cycles-per-second per knot) which 

 increases with smaller 7 angles, and high signal return over the sea which 

 increases with larger 7 angles (or at least larger antenna depression angles, 

 which generally increase with 7 angles). Most early equipment designers 

 adopted a 7 angle of 70° as the optimum compromise value. Another 

 disadvantage of 7 angles larger than this value is a larger land-to-water 

 bias error (discussed in more detail in Sec. 14-5); however, the use of land- 

 sea switches has reduced the importance of this factor to a very consider- 

 able extent, resulting in the use of larger 7 angles in later designs. 



The choice of the /3 angle depends largely on th.e application of the system 

 and certain technical considerations such as drift angle sensitivity and 

 signal return. Typical /3 angles range all the way from 20° to 90°. Fig. 14-9 



(a) X Configuration 



(b) Four ■ Beam Configuration 



(c) Three - Beam Configuration 

 Fig. 14-9 Typical Doppler Janus System Beam Configuration. 



shows three examples of azimuth beam configurations of three typical 

 Janus system configurations. Fig. 14-9a is the X configuration already 



