770 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



STC. Sensitivity time control is extremely useful in weather mapping. 

 This circuit operates on the receiver sensitivity in such a manner as to 

 maintain constant returns from a constant target for a moderate range of 

 distances. Its inclusion is virtually mandatory if iso-echo contouring is to 

 have any significance. 



AFC. Automatic frequency control is generally desirable, particularly 

 in that it frees the operator from receiver tuning for more important tasks. 

 It needs to be sufficiently precise and fast to follow any predictable trans- 

 mitter frequency variation. The advent of small ferrite load isolators (see 

 Sec. 10-16), as well as ferrite duplexers with their inherent load isolation, 

 has alleviated this problem by buffering the magnetron against load 

 mismatch fluctuations caused by the antenna as it rotates. 



Antenna Stabilization. The stabilization of the antenna against 

 pitching and rolling of the aircraft is generally desirable. It prevents a 

 smearing of the display when the aircraft is in a climb or turn and is 

 particularly valuable if the radar is also to be used for ground mapping. 

 The two systems most generally used for this function are line of sight 

 stabilization and platform stabilization. Line-of-sight stabilization utilizes 

 the information derived from the plane's vertical gyro to operate on the 

 antenna tilt servo system in such a manner as to compensate for variations 

 in the plane's aspect. Platform stabilization essentially floats the active 

 portion of the antenna free of the airframe and, through its own separate 

 gyro, tends to maintain this platform level with the horizon regardless of the 

 motion of the plane. Of the two systems, platform stabilization is inherently 

 more accurate, but is considerably more complex and requires more costly 

 parts and components. Stabilization accuracy must be related to antenna 

 vertical beamwidth, which is in turn a function of dish size and frequency. 

 If care is exercised in design and utilization, line-of-sight stabilization is 

 generally considered adequate for beam widths down to 3°. 



Csc^ Antenna Pattern. In order to improve the radar's utilization 

 for ground mapping, several X-band systems have included means of 

 spoiling the antenna's normal pencil beam into a fan-shaped beam in the 

 elevation cross section, following essentially a cosecant squared radiation 

 pattern over the vertical angle of interest. This provides approximately 

 equal power return from ground scatterers over a considerable distance to 

 give a uniform mapping of ground. A typical method of accomplishing this 

 is to provide the antenna with a spoiler grid and a means of rotating the 

 polarization of the microwave energy relative to the direction of the grid 

 lines. When polarization is perpendicular to the grid lines, the spoiler is 

 essentially invisible to the radiated energy and hence a pencil beam results. 

 When parallel, the spoiler is seen by the radiated energy and the pattern is 

 spoiled downward. 



