762 BELL SYSTEM TECHNICAL JOURNAL 



nonlinear function of time; its exact time function being dependent on the 

 altitude of the aircraft. The altitude information is obtained from the air- 

 craft altimeter and may be manually or automatically introduced into the 

 radar receiver to produce the proper form of sweep function. These modi- 

 lied forms of PPI presentation were employed extensively in the large bomb- 

 ing through overcast radar program which attained a status of major 

 importance toward the later portion of World War II. 



The elevation versus azimuth classification of display forms is essentially 

 restricted to fire-control and aircraft interception radar applications. As 

 previously noted, the C-type display was developed early in the military 

 radar program and has somewhat the same characteristics as the B scan in 

 terms of the distortion which results in the display of polar coordinate data 

 in a rectangular coordinate field without proper mathematical conversion. 

 In the case of aircraft interception radar applications, this type of display 

 is quite satisfactory and has been employed quite extensively for this 

 purpose. 



The moving spot (MS) form of radar display is usually associated with 

 a radar system in which conical scanning or lobing is employed. Here the 

 source of radiation of the antenna is arranged and rotated so as to provide 

 a beam whose path describes a cone. If the target is located on the axis of 

 this cone of radiation the signal response will be essentially constant for all 

 instantaneous positions of the beam. If the target is positioned to one side 

 of the cone's axis the received radar signal will be modulated at the fre- 

 quency of the conical scanning process and the degree of modulation will be 

 related to the angle between the conical axis and the bearing toward the 

 target. This modulation information is utilized within the radar receiver 

 to position an optical image on the face of the indicator screen in accordance 

 with the direction of the target. In radar systems employing this form of 

 indication the observer positions his radar antenna, and accordingly the 

 associated weapon, to center the target image on the indicator. Mechanical 

 or electronic cross hairs are employed as the reference axis. A measure of 

 range to target information is often introduced into this form of display 

 by assigning an arbitrary but distinctive size or shape to the target spot 

 which can be varied in accordance with the range to the target being ob- 

 served. 



2.42 The Cathode-Ray Tube 



The cathode-ray tube is without serious competition as the ideal radar 

 indicator, i)rimarily because of its unique high-frequency electro-optical 

 response characteristic. Since the radar presentation requirements are not 

 unlike those encountered in television practice, it is natural that the cathode- 

 ray tube development for radar purposes should have progressed along simi. 



