800 



AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



temperatures, is multiplied by the Instantaneous field of view of the system 

 A</), and the reciprocal of the product is plotted against effective scan rate. 

 The same is done for different black body temperatures, so that a family of 

 curves each of a given temperature T is obtained. The A<^ for a multi- 

 element system is the A(^ of any one detector. 



The effective scan rate is the actual revolutions per minute (for conven- 

 ience) times the number of scan lines per revolution. For a front- and 

 back-surface scanning mirror (double primary) the effective scan rate would 

 be 2 times the actual revolutions per minute; for a 10 element linear array, 

 the effective rate would be 10 times the actual rate, and so on. 



The system itself is doing the actual measuring, so that any data are 

 modified in accordance with system characteristics. If bandwidth is 

 insufficient, (AT X A0)~^ drops off at smaller effective scan rates. If the 

 bandwidth is too large, the AT will be so affected that the ordinate value 

 (AT X A</>)~^ is smaller than it otherwise would be. If A(^ is decreased too 

 much, the additional bandwidth requirement and possibly time constant 

 limitation will reduce sensitivity, thus increase AT, and cause the curve to 

 break at lower effective scan rates. If A0 is increased, AT will increase and 

 thus the ordinate value will again be lower. If a slow detector is used, the 

 break will of course occur at low effective scan rates. The use of fast 

 detectors will allow the curve to remain flat to high effective scan rates with 

 both the ordinate value and deviation from flatness occurring as a result of 

 50 



40 



20 



10 



1000 



6000 



2000 3000 4000 5000 



EFFECTIVE SCAN RATE (scan lines/ minute) 

 Fig. 14-29 A Method of Comparison for IR Mapping Systems. 



7000 



