14-16] 



BASIC PRINCIPLES 



773 



extent on the characteristics of the antenna beam pattern used to paint the 

 ground. 



The antenna beam pattern usually employed in ground mapping systems 

 is narrow in one dimension and has wide angular coverage in the other 

 dimension. (This type of beam is known as -a. fan beam; see Fig. 10-6.) It is 

 usually oriented so that the narrow dimension is horizontal, thus illumi- 

 nating a long narrow strip of ground from beneath the airplane to some 

 maximum range (see Figs. l-5a and l-5b.) Thus, for a given pointing 

 direction, the radar beam illuminates (or "paints") targets at many 

 different ranges and depression angles. Variations in the brightness are 

 therefore functions of range and the angle at which the ground is viewed 

 as well as the reflective properties of the terrain. This condition — if not 

 corrected — would complicate the correlation between the radar map and 

 terrain. To compensate for the effects of range and viewing angle, the 

 vertical gain pattern of a radar ground mapping antenna is designed to be a 

 function of the depression angle at which a given patch of ground is viewed. 

 The characteristics of this type of pattern, known as a cosecant-squared 

 bea772, are discussed in Chapter 10 and Paragraph 14-18 below. 



Scanning of the antenna beam is usually accomplished either by rotating 

 the antenna about a vertical axis or by positioning the antenna along the 

 fuselage of the aircraft, so that the motion of the airplane provides the 

 scanning. When the beam is rotated a full 360° about the vertical axis, the 

 map is usually in the form of a plan position map with the airplane at the 

 center. Most systems employing this type of scan use a sector scan, i.e. 

 less than the full 360°. When the sector is directed forward of the airplane, 

 the system is known as a forward-looking mapping radar. Systems em- 

 ploying the velocity scanning technique, where the beam is directed to the 

 side of the airplane, are known as side-looking radars. The maps obtained 

 with this system are in the form of 

 strip maps along each side of the 

 airplane track (see Fig. 1-5) and are 

 especially adapted to the use of 

 photographic techniques to obtain a 

 permanent record. 



Fig. 14-17 shows the simplified 

 block diagram of a typical mapping 

 system. In addition to the usual 

 components — i.e. antenna, trans- 

 mitter, receiver, and display — of 

 the typical radar system, a recorder 

 is included. In some systems the 

 recorder may be used merely to 

 preserve a permanent record of the 



Recorder 



Fig. 14-17 Block Diagram of a Typical 

 Ground-Mapping Radar System. 



