14-24] SYSTEM CONSIDERATIONS 793 



redirected into the image-forming or primary reflector. The resultant 

 converging beam is redirected by a second flat and is finally incident on the 

 detector. Rotation of the scanning flat, as shown, provides a linear scan 

 pattern (neglecting distortions incident to change in slant range and 

 deviation of the scanning flat from the 45° position shown) in object space. 

 If the total field is to be 60°, five scan lines per revolution are possible with 

 a single element sensor. If 150° is to be scanned, two lines per revolution is 

 the maximum. This becomes more clear by referring to Fig. 14-27 and 

 picturing (for example) the scanning flat as replaced by a four-sided 

 pyramid reflector with the base attached to the rotating shaft as shown in 

 insert A. If an opening is provided so that the system may look down over a 

 total field angle of 360° /4 or 90° (±45° from vertical), then one of the four 

 pyramid sides is always receiving radiation through the opening and 

 directing it into the system. With the understanding that the other three 

 mirrors are providing no fluctuating radiation to the system, the total 

 signal variations enter by way of the reflector that is "looking" through the 

 opening. It is clear, then, that in one revolution of the shaft each of the 

 four mirrors will in turn provide a 90° total field scan transverse to the 

 flight path. Four scan lines per revolution are thus generated. The so-called 

 dead time between scan lines may conveniently be used for processing and 

 transmission via telemetry links. 



The required resolution in general establishes the instantaneous field of 

 view. Assuming no other system limitations (most notably the optics and 

 recorder), the instantaneous field and electronic bandwidth determine 

 system resolution. At this point the concept of data rate is quite useful. 

 Data rate is the number of picture elements per second that are collected 

 and used by the system. While a given instantaneous field of view may 

 make possible a given data rate, the eff'ect of the detector response, elec- 

 tronic bandwidth, and recorder range will generally limit the system data 

 rate. In terms of the present discussion, the instantaneous field of view 

 is not necessarily equivalent to the system resolution (actually, system 

 resolution can be better than the instantaneous field of view, although this 

 more sophisticated approach is not discussed in this text). With the 

 instantaneous angular field established, the angular width in the direction 

 of flight path of one scan line is fixed. A fixed angular width corresponds to 

 nonuniform linear width in object space because of change in slant range 

 with change in scan angle. In order that there may be no gaps in object 

 space mapping, the scan line width used in calculations is the width 

 measured directly below the aircraft. 



With fixed angular scan line width, the aircraft altitude then determines 

 the linear scan line width in object space (see above). The velocity deter- 

 mines the number of scan lines of this given width required per unit time 

 in order that each line may lie adjacent to the preceding one. Therefore, 



