1270 
The R Scope. The R scope presents the same infor- 
mation as the A scope, but greatly expands the hori- 
zontal coordinate so that instead of some 100 miles 
being represented on the scope, only 5 or 10 miles of 
any desired portion of the A scope is displayed. Figure 3 
illustrates the appearance of this presentation. 
AIRCRAFT 
ECHO 
y 
ELECTRON 
"NOISE" 
PRECIPITATION ECHO 
(SMALL) 
ELECTRON 
"NOISE" 
RANGE 
MARKS 
ECHO SIGNAL 
INTENSITY 
MILES 
Fic. 3.—Diagram of R scope of SCR 615B radar (modified), 
illustrating difference in appearance of aircraft and precipi- 
tation echo signals. 
The Plan Position Indicator or PPI Scope. The PPI 
scope, as it is popularly known, is one of the most 
favored scopes for storm detection purposes because of 
its graphic and easily interpreted display. The presen- 
tation is circular and therefore employs the entire area 
of the cathode ray tube face in contrast to the rather 
small portion utilized by the A and R scopes. When 
the radar beam is directed horizontally and rotated in 
azimuth, the PPI display takes the form of a circular 
map with the position of the radar represented at the 
center of the scope. Targets are displayed in position 
relative to the radar as if viewed from an infinite height 
directly above the radar. Range from the radar to any 
target may be determined from the relative distance 
from its echo signal to the center of the scope. To aid 
in range determination, an electronic circuit injects 
marks at proper intervals which correspond to range 
increments of 1, 10, or 20 miles or any other number 
and unit of length desired. As the sweep rotates, these 
marks draw concentric circles about the center of the 
scope. The appearance of the scope is illustrated in 
Fig. 4. The electron beam is so controlled that if the 
radar is not detecting a target at a given position, the 
face of the scope is not brightened at the corresponding 
point. When a target is detected, the electron beam is 
intensified so that it brightens the face of the scope at 
the proper position in azimuth and range from the 
radar. This process of target indication is called “‘in- 
tensity modulation” of the electron beam, as contrasted 
to the beam deflection method employed by A and R 
scopes, the beams of which are always on and are of 
constant intensity. 
If the antenna is directed horizontally, the indicated 
range will correspond closely to the actual horizontal 
distance between target and radar, and its geographical 
position may be determined by reference to a map of 
the area. If the map is drawn on transparent material 
and is of proper scale, it may be laid directly over the 
face of the scope and geographical positions of targets 
can be determined directly. 
RADIOMETEOROLOGY 
Another version of the PPI scope may be controlled 
so that the indicated position of the radar may be dis- 
placed from the center in any direction to distances of 
{NORTH) 
PRECIPITATION 42, 
My 
EGHO SW LOCATION OF 
RADAR IN 
CENTER OF 
GROUND 
CLUTTER 
(WEST) (EAST) 
AIRCRAFT 
RANGE MARKS ECHOES 
AT 10 OR 20 
MILE INTERVALS 
(SOUTH) 
Fic. 4.—Diagram of features of a PPI scope. Some detail 
has been omitted to clarify the drawing. In all PPI-scope 
photographs which follow, north is at the top of the scope, 
east to the right, etc. Elevation angle in all cases is zero de- 
grees. 
about two diameters of the scope face. This scope is 
called an ‘off-center PPI” and enables close examina- 
tion of selected areas by virtue of the great sweep ex- 
pansion which can be employed. 
The Range-Height Indicator or RHI Scope. This scope 
is particularly valuable for storm-detection purposes, 
but is found only on radars designed to scan a vertical 
plane. The electron beam is intensity-modulated like 
that of the PPI scope. The radar antenna scans cycli- 
cally from the horizon upwards to any angle, depending 
upon control settings or the design of the radar. In 
order for the display on the scope to be intelligible, the 
azimuth of the beam must remain nearly constant dur- 
ing the scanning process. Targets in the vertical cross 
section along a given azimuth are displayed on the face 
of the scope in coordinates of range versus altitude or 
elevation angle. Because practically all targets, whether 
storms or aircraft, will be found at altitudes less than — 
10 miles, and since the horizontal range of the radar is 
nearly always more than 50 miles, it is usually desirable 
to expand the vertical scale in order to utilize the maxi- 
mum available area of the scope face. This results in 
more precise altitude determination due to the vertical 
scale increase. For this reason, RHI scopes are usually 
designed to have an altitude distortion of about 10:1. 
The appearance of the scope of a typical height-finding 
radar (AN/TPS-10A) is shown in Fig. 5. 
Tt will be noted that the range and height of a target 
may be determined directly from this scope by means 
of properly calibrated scales. In order to fix the target 
in space, its bearing or azimuth must also be known. 
This information cannot be obtained from the RHI 
display alone; consequently it must be read from a dial 
\ 
AA i ae 
i 
