344 GENERIC TYPES OF RADAR SYSTEMS AND TECHNIQUES 



The speed of scan is controlled by the time constant t of the detector. To 

 define a practical upper limit we shall use the time required for the in- 

 stantaneous field to move its own width o-, equal to t. To go faster would 

 result in considerable smearing of the display pattern, with resultant loss of 

 resolution. In this sinusoidal scan the velocity is not constant, but for 

 simplicity we use the average value. The time required to scan one com- 

 plete spoke (27 radians) is 



;= ] r \ — = n — '■ — seconds/spoke. (6-57) 



lotal no. or spokes Iirsmy 



Motor 1 then turns clockwise at — ( ) rps and motor 2 turns counter- 



11- 1 /^TT sin7 A 

 clockwise at — I ~~ W ''P^- 



Since a complete spoke is 27 radians, the average scan rate is 



-. seconds/radian. (6-58) 



4x7 sm 7 



and the time required to scan a- radians is 



4x7 sin 7 



= r seconds. (6-59) 



which we accordingly equate to the time constant r of the detector. 

 Consider an actual case in which we want: 



^ = ly^o = 0.0058 rad 



^ = 20° = 0.35 rad 



T = 10~^ sec 



Then the time required to scan a complete field is, from Equation 6-57: 



^ 4x7 sin yr 4x X 0.35 X 0.34 2 X lO"'^ . , .. ,^. 



^ = a-^ = mossy = ^-^ '''■ ^^-^^^ 



For many practical cases, this is obviously too long. A modern aircraft 

 will have changed course considerably in this time. Therefore the instan- 

 taneous field must be enlarged, the total field reduced, or a faster detector 

 sought — possibly all three. Perhaps the scan mode would have to be 

 abandoned in favor of a more economical one without the multiple retrace 

 encountered in the center of this field — say a raster scan similar to that 

 used in television. 



Target Tracking. If the system is required also to track a target, 

 this could be accomplished by orienting the aircraft so that the target image 

 falls in the center of the screen, setting angle a = 0, and reducing angle /3 



