L76 



SITING AND COVERAGE OF GROUND RADARS 



working part of the scale over a sufficiently wide 

 angle. A common difficulty with this method is lack 

 of suitable permanent echoes (Section 15.5.3). 



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 

 plotted against range, counting very weak signals 

 or stronger as a hit, as in Figure 75. The data may be 



5000 FEET 

 INCOMING 



ANTENNA 

 15 RPM 



25 30 



15 



20 



RANGE- MILES 



Figure 75. Test flight data for PPI scope. 



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. 



15.7.4 



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. Flights 

 should be carefully planned and full provision made 



for various contingencies. Changes from prearranged 

 plans should be held to a minimum. Close liaison 

 should be maintained with the flight section and 

 every effort made to avoid hazardous flying. 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. 



157 5 . Analysis of Test Data 



Test data should be accompanied by a complete 

 description of the conditions of the test. Data should 

 be analyzed promptly, and every effort should be 

 made to extract the full amount of useful information. 



In Figure 76 is shown a signal-to-noise graph fol- 



ic 



a. 4 



S 2 



O 20 40 60 80 I 



DISTANCE- MILES 



Figure 76. Typical signal-to-noise test data. 



a station similar to Example 20. The noise is set at 

 a relative height of 1, and the signals are read in 

 proportion as the plane comes in. The weak signals 

 at medium ranges are due to shore line diffraction. 

 The peaks correspond to lobe maxima and ranges at 

 S/N = 1 to the locations of the lobe contour at 

 12,000 ft. The receiver in this case is of the "linear" 

 type, and the lobe maxima may be obtained by 

 extrapolation. Along a line of constant path difference 

 such as the maxima of the lobes the signal-to-noise 

 ratio varies as the inverse square of distance. Thus 

 the fourth lobe has a peak S/N ratio of 6 at 68.5 

 miles, and the lobe length isL = 68.5 y/Q = 167.5 

 miles. 



In practice the length computed in this manner 

 would be compared to those obtained from tests at 

 other altitudes. Notes made during the test and other 

 factors would be considered and the data weighted 

 accordingly. For example, at 90 miles the S/N ratio 



