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SITING AND COVERAGE OF GROUND RADARS 125 
A SCOPE SENSITIVITY —LOW 
SCOPE DEFLECTION IN INCHES 
RECEIVER INPUT IN LV 
Ficure 74. Typical receiver characteristics. 
input voltages. Test data may be conveniently 
plotted as decibels above noise after this conversion. 
It should be noted that the calibration depends upon 
the type and percentage of modulation. 
A third method involves calibration of the gain 
control dial by comparison of permanent echoes. 
Three lines are drawn on the scope face such as 44, 
1, and 11% in. from the baseline. The position of the 
gain dial with the noise at 14 in. is marked 0 db. A 
permanent echo is selected which comes to the 1-in. 
line at this setting. The gain dial is then turned to 
bring this echo to the 14-in. mark, and this position 
of the dial is marked 6 db. Another echo is then 
selected which is 1 in. high, and it is brought down 
to 14 in. by further adjustment of the gain dial. This 
position is marked 12 db. In this manner the gain 
dial may be calibrated over the full range of adjust- 
ment. It may be necessary to change the series 
resistor on the gain potentiometer to spread the 
working part of the scale over a sufficiently wide 
angle. A common difficulty with this method is lack 
of suitable permanentsechoes (see page 92 ). 
A fourth method is suitable for microwave gear 
where search is conducted with a PPI scope. As the 
beam sweeps past the target a hit or miss is recorded. 
If desired, additional note may be made such as miss, 
very weak, weak, or hit. In analyzing the data the 
percentage of hits in an arbitrary period of 30 sec is 
5000 FEET 
INCOMING 
a ANTENNA 
= 15 RPM 
ee 
55 
Ww 
# MAXIMUM 
a RANGE 7 
i} 
° 
(o} 15 20 25 30 
RANGE~ MILES 
Ficure 75. Test flight data for PPI scope. 
plotted against range, counting very weak signals 
or stronger as a hit, as in Figure 75. The data may be 
scattered, but it is not difficult to decide the range 
at which the percentage of hits is 50 per cent. This is 
taken as the, maximum range. At lower altitudes 
a lobe structure may be detected, indicating ground 
reflection. 
Conduct of Test Flights 
The test planes should have two or more engines. 
Slow-speed, high-ceiling, long-range planes are most 
desirable. They should be equipped with navigational 
aids such as radio compass, DF system, and loran, 
and full complement of communication sets, trans- 
ponders, and altimeters. For positive identification 
in regions of high traffic density, a distinctive IFF 
(identification friend or foe) response is essential. 
Mark II transponders may be readily modified in 
the VHF band to give a double pulse by shifting 
the condenser rotors. 
Tests are conducted by flying out from the station 
and returning at a Specified altitude to a range 
estimated to be about 10 per cent beyond the maxi- 
mum of the lobes. Suitable altitudes are from 5,000 
to 20,000 ft. Little is learned from tests below 1,000 
ft since nonstandard propagation effects are most 
pronounced in this region. 
Data should be taken by specially trained opera- 
tors as considerable judgment is required. Where 
feasible, flights over sea should pass near landmarks, 
etc., to check navigation. Other radars and agencies 
should be employed to assist the test plane in holding 
its course. The permanent echoes should be noted 
during the test and compared with average condi- 
tions so that an estimate of nonstandard propagation 
may be made. Similar checks should be made at other 
nearby radars. 
