Radar System Engineering

SEC.6.12] HEIGHT-FINDING WITH A FREE-SPACE BEAM 191
lobes are not objectionable in this antenna, it displays considerable frequency
sensitivity. A standing-wave voltage ratio of 1.33 (Chap. 11) is
encountered for a frequency change of 2 per cent. The beamwidths are
1° in the vertical dimension and 6° in the horizontal dimension. The
pulse width is 2 psec, the output power 500 kw.
In operation, the antenna is moved up and down in the expected
azimuth of search at a rate of one oscillation in ten seconds. An RHI
display is used (Fig. 6.23). Because of the slow rate of Scan, it is imperative
to use a second radar to determine the approximate direction of the
target. The Type 7 equipment was widely used for this function. The
azimuth position of the Type 13 radar is shown on the PPI of the Type 7
by means of a beam of light pr~
20° 15” 10” 8“
jected from behind the phospho- 6“
rescent screen of the PPI. This
5“
light is yellowish and does not affect
4“
the persistence of the radar signals
3“
on the PPI. In addition there is a
2“
color contrast bet ween it and the
1“
echoes. By rotating the antenna
of the Type 13 until this’ ‘azicator”
line cuts the center of the signal on
the PPI, the Type 13 can be correctly
enough pointed to clisplay the
desired target. The beamwidth cf
0“
Ground
echoes
IkG. 6.23.- -l~ange-height indicator of
Type 13.
6° is chosen to be wide enough to permit Type 13 uperation with a PPI
radar having very poor resolution.
A similar American set (A Y/TPS-10) is lighter and simpler since it
operates at 3 cm. This enables the antenna size to be reduced to 10 by
3 ft with the very high gain of 18,000. The range is over 50 miles on a
four-motored aircraft, although the radar set uses only 60-kw output
power at l-~sec pulse width and 1000 pps. The antenna can be oscillated
once per second. The beamwidths are 0.7° in elevation and 2° in azimuth.
This last is much smaller than was at first thought feasible; it
was argued that so small an azimuth beamwidth would render search for
the target laborious. It has proved, however, to be no operational
limitation.
Height-finding by such a set is reasonably accurate. The results of a
calibration flight with an experimental model of the AN/T PS-10 are
shown in Fig. 6.24. Relative height is accurate to about ~ 300 ft under
favorable conditions. Absolute height is more difficult to find accurately
by radar, because of the bending of the beam by atmospheric refraction,
which varies from time to time. Fortunately, relative height is usually
sufficient for aircraft control. Further, it is generally possible to keep