794 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



the velocity and altitude, together with the instantaneous field, determine 

 the scan line rate. In turn, this rate, together with the number of scan lines 

 per revolution of the scanning element, as fixed by the total angular field 

 of view, determines the rotation frequency of the scanner. 



For example, consider the reasonable case where the mission requirements 

 have in turn determined the following parameters: 



Aircraft velocity v = 1000 ft /sec (relative to ground) 

 Aircraft height h = 30,000 ft 

 Instantaneous field A(j) = 1 milliradian 

 Total field of view = 150° 



Let n = scan lines /revolution ^ 2, in view of the total field size. Then the 

 rotation frequency of the scanner (Q,) in revolutions per minute is clearly 



n = 60jinAcf>)-' rpm. (14-39) 



Assuming the above conditions to impose the most severe v /h requirements 

 the system must meet, then the scanner must be capable of 



1000 



" = ^«X3-xTo^^^x^^-^)-^ 



fi = 1000 rpm. (14-40) 



Note that the effect of detector time constant and electronic bandwidth 

 have not been included at this point, and therefore nothing can be said 

 concerning data rate or resolution until these effects are evaluated. These 

 considerations are discussed below. 



The scan rate (revolutions per unit time) must be variable with velocity 

 and altitude as the above dependency indicates. The time required per 

 scan line to assure adjacent scan lines in object space is readily shown to be 

 inversely proportional to the velocity-to-height ratio. The most stringent 

 requirements on the sensor and scanning optical element are encountered 

 for a high-velocity aircraft flying at low altitude. In this case it may be 

 desirable to somehow increase the instantaneous angular field of view. 

 Carried to the other extreme, high velocities at very high altitudes pose less 

 of a problem from the scan rate point of view. It is also noteworthy that 

 the idea of range is meaningless in its usual sense when applied to recon- 

 naissance. If sufficient system sensitivity is available to detect the ambient 

 temperature ground while at ground level, then except for atmospheric 

 attenuation there will be just as much energy falling into the system at one 

 altitude as at another until the instantaneous field of view includes the 

 entire earth. The important point is that the energy from a target within 

 the instantaneous field is averaged over the whole instantaneous field, and 

 when this average is not distinguishable from the average over an adjacent 



