14-14] DESIGN FEATURES 769 



targets. This action can be heightened by increasing pulse rate or slowing 

 scan rate, the first being limited by the maximum range to be displayed, the 

 second by tube persistence and rate at which the display changes. The new 

 storage tube indicators will give some small improvement incident to their 

 greater integration action (see Chapter 12). 



14-14 DESIGN FEATURES 



Inasmuch as servicing and adjustment cannot normally be made in the 

 aircraft in flight and reliability is of utmost importance (dual installations 

 being highly impractical), the design must be simple and straightforward in 

 its approach. Critical adjustments and circuits should be avoided if at all 

 possible. For this reason virtually all present-day weather radars utilize 

 more or less standard design techniques. However, a few special design 

 features are highly desirable in order to enhance the system's utilization, 

 principally for weather mapping. Some of the more important of these are 

 the following: 



Iso-Echo Contour Circuitry. As mentioned in Sec. 14-13, one of 

 the most valuable pieces of weather information from the pilot's standpoint 

 is knowledge of areas of highest rainfall gradient and rainfall rate. The 

 iso-echo contour circuit is a video inversion device which causes a black hole 

 to occur within a displayed storm cell wherever the rainfall rate is above a 

 predetermined level, as determined from relative back-scattering (see Fig. 

 14-15). The thinner the line between the outer edge of the displayed cell and 

 the contour "hole," the higher is the rainfall gradient, thus indicating areas 

 the pilot definitely should avoid. 



CONTOUR OFF CONTOUR ON 



Fig. 14-15 Effect of Iso-Echo Contouring On Storm Display. 



