Chapter 4 

 VISIBILITY ON RADAR SCOPES 



STANLEY B. WILLIAMS^ 



The Johns Hopkins University 



Introduction 



A novice taking his first look at a radar 

 scope finds that it yields no meaningful in- 

 formation. Even after considerable practice 

 and training, radar operators find that scope 

 reading is not easy. Part of the difficulty 

 lies in the fact that cathode-ray tubes are at 

 best rather poor purveyors of visual informa- 

 tion, being made, as thej' are, of a combina- 

 tion of curved glass surfaces, uncertain chem- 

 icals, and delicate electrical circuits. Other 

 difficulties arise if the user does not properly 

 adjust certain aspects of scope function 

 and presentation. The possibilities of im- 

 provement are great, and many of them are 

 contingent upon knowledge of the human 

 operator and of his capacity for visual dis- 

 crimination. 



Cathode-Ray Tubes Used as Radar 

 Scopes 



General Design 



A cathode-ray tube (CRT) is a type of 

 vacuum tube designed for the visual display 

 of electrical events. The display is a phos- 

 phor screen, a thin coating of inorganic salts 

 on the inside surface of the top of the glass 

 tube. From an external source a high po- 

 tential is put on the anode to attract the 

 electrons being emitted by the cathode to the 

 region of the phosphor screen. All except 

 the screen is called the electron gun. The 

 number of electrons it fires toward the screen 

 is measured as beam current and is primarily 

 governed by variations in bias applied to the 



1 The writer is deeply indebted to Beverly 

 Richards for invaluable bibliographic and re- 

 search assistance. 



control grid of the tube. The direction of 

 the beam is governed by either magnetic or 

 electrostatic deflecting coils collared around 

 the neck of the tube, one set of coils being 

 used for vertical deflection, the other for hor- 

 izontal, the two together being sufficient for 

 locating the luminous spot at any position on 

 the phosphor screen. 



Types of Cathode-Ray Tubes 



There are two basic types of cathode-ray 

 tubes: deflection modulated and intensity mod- 

 ulated. 



In the deflection-modulated type, the in- 

 formation or "signal'" to be displayed is put 

 into the CRT gun as a spatial deflection of 

 the electron beam. This is the type used in 

 the familiar laboratory oscilloscope, and in 

 radar it is called an A-scope (or A-scan). 

 The observer sees the signal as a momentary 

 vertical line or "spike," whose extent is usu- 

 ally a linear function of signal strength. 



In the intensity-modulated type, the sig- 

 nal is put in as a variation of the beam cur- 

 rent, i.e., the beam is momentarily intensified 

 so that more electrons reach the screen and 

 locally excite a spot to a brightness greater 

 than its surround. Beam intensification is a 

 linear function of the voltage drop in the grid 

 bias caused by the signal. The observer sees 

 the signal as a local bright spot whose area is 

 determined by other fixed parameters and 

 whose incremental brightness may not be — 

 and usually is not — a linear function of signal 

 strength, depending on the type of phosphor, 



2 The following words are used almost inter- 

 changeably to denote the actual or potential 

 screen appearance of a radar echo: echo, trace, 

 image, target, signal, blip, and pip. 



101 



