14-6] MODIFYING THE RADAR RANGE EQUATION 751 



ground speed. This was shown in Equation 14-2 to be 



IV 



-— sin 7A7, where A7 is the two-way beamwidth of the 

 A 



antenna. In pre-tracker-mixed Janus systems, there is an 



additional factor of -\f2 in this expression due to the spectrum 



broadening in the Janus mixing process 



h = aircraft altitude 



;■ = Janus factor; applicable only to Janus (two-way looking) 

 systems and consisting of the following three factors, any or 

 all of which may exist and be practically significant: (1) 

 js = signal suppression factor, which exists in any pretracker 

 mixed Janus system but which becomes significant only at 

 extremely small IF ^/A^ ratios; it can cause the S /N ratio 

 altitude fall-off to increase to as much as 12 db per octave 

 (double the altitude); it is the result of certain undesirable 

 signal-cross-noise and noise-cross-noise products. (2) 

 jt = time spread factor, which may become significant in 

 coherent or incoherent pulse systems at high altitudes due 

 to the time spread of the echoes (with respect to the pulse 

 repetition period); it can cause the ^/A^ fall-off to increase 

 from 6 db per octave at the low altitudes to 9 db per octave. 

 (3)io = overlap noise factor, which may become significant 

 in incoherent pulse systems at very high altitudes and which 

 results from the so-called pulse overlap noise (noise pro- 

 duced by the beating of the returns of successive trans- 

 mitter pulses) 



J = antenna efficiency (frequently near 0.55) 



^ = incidence angle of beam at the ground, i.e. the angle between 

 the vertical and the direction of radiation. This angle is 

 normally somewhere between 10° and 30° 



F = antenna pattern parameter. This factor depends on the type 

 of antenna — parabola, rectangular aperture, linear array, 

 etc. — and for an extended target such as the ground usually 

 lies between 0.5 and 0.67. 



One additional factor not included in Equation 14-13 is that of atmos- 

 pheric attenuation, i.e. absorption in or back-scattering from rain, clouds, 

 etc. The data presented in Chapter 4 can be used to account for these 

 phenomena in calculating the theoretical signal-to-noise ratio. 



Several other factors tend to degrade the signal-to-noise ratio in an actual 

 equipment. For example, pure CW systems are affected by reflection noise 



