RADAR SCATTERING OVER CROSS-SECTION AREA 



203 



tioii. Therefore, tlie figures represented here are iu 

 general the highest values that occur during tlie 3-sec 

 interval. For this reason we have attempted to see how 

 the value of a depends upon the interval timing and 

 whether or not it is permissible to put this value into 

 range formulas in the usual way. A rough working 

 model is the following: If these cross-section values 

 are reduced to 60 per cent, they may be used in the 

 range formula presented in the previous paper to ob- 

 tain the correct operational radar range. The cross 

 section averaged over the lobe structure in the front 

 aspect or tail aspect of a plane would be lower than 

 these values by probably 50 per cent. 



Some representative figures are as follows : Fighter 

 aircraft usually vary from 1 to 200 sq ft; medium 

 bombers, B-IS, Beaufighter and similar aircraft range 

 from 4 to 600 sq ft; and heavy bombers, B-17, 800 

 sq ft. The larger bombers such as the B-29 have not 

 been measured but are estimated to be of the order 

 of 1,300 sq ft. 



Discussion 



To a question regarding the wavelength dependence 

 of aircraft cross sections, the reply was that such a 

 dependence was a function of the structure of the air- 

 craft. Outside surfaces having rounded structures such 

 as wings, wires, and similar members have a cross 

 section which is essentially independent of wavelength 

 and produce random scattering, provided the frequen- 

 cy is high enough. As the frequency is lowered, reso- 

 nances in the structure of the airplane and differences 

 in the wings may apjsear. This might possibly cause 

 diffei'ences with regard to polarization. At S-band and 

 higher frecpiencies there seems to be little dependence 

 upon frequency. These figures have been cheeked at 

 S and X bands with essentially the same results. No 

 sensible dependence on jjolarization was observed, in- 

 dicating that at S-band or higher frequencies, this sort 

 of cross-section value will apply. 



Ohio State University is conducting an extensive 

 program of cross-section measurements on various 

 types of aircraft for a variety of frequencies up to 500 

 mc. Measurements are made for all aspects of the air- 

 craft and for both vertical and horizontal polarization. 

 The procedure used is to scale the aircraft down to a 

 convenient model size and to use a corresp)ondingly 

 higher frequency. 



The results of these measurements exhibit a confus- 

 ing lobe structure. In order to give an overall descrip- 

 tion of the behavior of the cross section, a reasonable 

 procedure must be found for averaging the data. This 



has been attempted. At 100 mc, the specular reflections 

 are not particularly marked, though the cross section 

 does increase in directions perpendicular to the axis 

 of the aircraft. There is still fairly strong scattering 

 in all directions. At 500 mc the echoes are almost en- 

 tirely due to specular reflection. The dependence on 

 polarization is stronger at the lower frequencies. 



The author commented that simultaneous measure- 

 ments of average values for different polarizations 

 showed them to be about the same but that instan- 

 taneous pulse-to-pulse photographs of a single target 

 with two different polarizations showed them to be 

 quite different at a given instant. 



An inquiry was made as to whether any correlation 

 had been made between radar cross section and type 

 and dimensions of aircraft. A report was mentioned 

 which attempted to show that scattering cross section 

 was proportional to wingspread. The results of the 

 author's group did not appear to correlate with wing- 

 spread, but the fuselage is important, and both factors 

 must be significant. Experiments had been made with 

 controlled flights in which the aircraft was flown 

 straight toward or away from the radar site. It was 

 believed that, because of normal wind conditions and 

 such factors as yawing in flight, the results obtained 

 represented an average over an angle of about 10° for 

 both front and rear aspects. Some measurements at 

 45° aspects were made which showed a drop of about 

 1 or 3 db for most aircraft. Some aircraft showed a dif- 

 ference between average head and average tail aspect 

 of about 1.5 to 1, and the figures previously quoted 

 represented an average between the two aspects. When 

 the aircraft in turning presents a broadside, specular 

 reflection occurs, and this side flash often exceeds the 

 ordinary signal by 100 times or more. 



The comment was made that the measurements de- 

 scribed seemed to have been made entirely with track- 

 ing radars using A-scope presentation. What would be 

 the probable effects of such fluctuations ou radars with 

 search type presentation? The author believed that 

 such fluctuations M^ould affect search-type radars 

 when scanning slowly but that no serious effect had 

 been observed at scanning speeds as low as 3 rpm. 

 When the cross-section figures gi^•en were used with 

 a 2-db reduction for average values, the predicted 

 ranges were in agreement with the observed ranges 

 even on scanning or search type radars. This is prob- 

 ably not true at certain longer wavelengths for which 

 the lobe structure is such that an aircraft can "ride" 

 a null for an appreciable time interval. At micro wave- 



