102 



RADAE SCOPES 



its excitation history, the amount of space- 

 charge collected near the screen capable of 

 repelhng electrons, etc. Examples of this 

 type in radar are the PPI, or Plan Position 

 Indicator, the B-scope and Eagle scans. In- 

 tensity modulated scopes usually use medium 

 or long-persistent phosphors, i.e., phosphors 

 which glow after electronic bombardment 

 has ceased. 



Types of Radar Displays 



There are three useful dimensions on a 

 scope: the two space coordinates and inten- 

 sity. Displays are denoted by type accord- 

 ing to the use made of these dimensions. 



A-Scan or A -Scope 



This uses only the space coordinates, by 

 deflection modulation, and keeps intensity 

 essentially constant.^ The horizontal di- 

 mension is a time sweep; since radar target 

 distances are measured only by the time con- 

 sumed by a pulse's travel to the target and 

 back, they can be displayed as distances 

 along a cahbrated baseline. Usually this is 

 made to read from left to right, the left side 

 of the scope representing the pulse source 

 and the right side the limit of the range being 

 scanned. The seeing task places a heavy 

 burden on acuity functions, on the percep- 

 tion of form, on judgments of small differ- 

 ences in linear extent, and on linear inter- 

 polation. 



B-Scan 



The radar B-scope uses all three dimen- 

 sions. The vertical dimension measures 

 sweep time or range (distance from source). 

 The horizontal dimension represents the suc- 

 cessive angular orientations of the rotating 

 antenna, and therefore indicates the relative 

 bearing of the target. The intensity or con- 

 trast of the pip is a rough index of signal 

 strength. The seeing task usually is an 

 alignment of the pip with a reference spot or 



^ Pictorial descriptions of the several types of 

 radar scan are given in Chapter C. 



Hne, but brightness fluctuations must also be 

 appreciated. 



PPI-Scan 



The Plan Position Indicator presents a cir- 

 cular (polar coordinate) map with the center 

 the position of the radar antenna (Fig. 1). 

 Radial distance is range; angular orientation 

 is bearing; and pip intensity is signal 

 strength. The angular width of the pip may 

 also furnish useful inforaiation about the 

 echoing object, but it mainly reflects only 

 the dispersion (lobe) pattern of the trans- 

 mitted pulse energy. As the antenna ro- 

 tates, the sweep trace follows it, painting a 

 succession of lines whose discreteness varies 

 with (a) the frequency of the pulse transmis- 

 sion (Pulse Repetition Frequency, or PRF) 

 and with (b) the antenna's rotation rate. 

 Usually the combination of the two is suffi- 

 cient to paint in a fairly uniform background, 

 but one which always retains a brightness 

 gradient, highest at the sweep line and fading 

 off around the circle behind it — much hke a 

 circular optical wedge. The critical seeing 

 task is brightness discrimination, especially 

 for weak signals, but form discrimination and 

 acuity increase in importance with the 

 amount of "noise" (clutter of unwanted pips 

 or pseudo-pips due to atmospheric interfer- 

 ence and intra- tube emission). 



Other Scans 



Most of the others are classifiable as Eagle 

 scans, portions of a PPI-scan, for which the 

 seeing task is similar to that of the PPI. 

 (See Chapter C.) Television, of course, is a 

 special case of intensity modulation in which 

 the vertical and horizontal dimensions are 

 made to correspond to the dimensions of the 

 object being televised. 



Types of Phosphor Screens 



Upon excitation, either by electrons or by 

 light of short wavelengths, the screen mate- 

 rial fluoresces momentarily. The lumines- 

 cence may last only long enough to be seen 

 or it may excite another phosphor compo- 



