192 REFLECTION AND TRANSMISSION OF RADIO WAVES 



4-7 RADAR CROSS SECTIONS OF AIRCRAFT 



Because all aircraft have dimensions large in terms of the wavelengths 

 used in airborne radar, the radar area of an aircraft target is very sensitive 

 to its instantaneous aspect. Because of air turbulence, the aspect is subject 

 to statistical variations of roll, pitch, and yaw. Consequently the radar area 

 is a statistically fluctuating quantity and it is not possible to give a single 

 number for the radar area of such a target. The quantities of chief interest 

 are the probability distribution of amplitudes, the aspect and frequency 

 dependencies, and the time characteristics, or spectra, of the fluctuations. 



The amplitude distributions and aspect and frequency dependencies of 

 certain aircraft will be presented in this paragraph, while the fluctuations 

 and their effect on tracking systems will be discussed in Paragraph 4-8. 

 A summation of the echo characteristics and their association with the 

 physical structure and dynamic behavior of the aircraft will then be 

 presented in Paragraph 4-9. This should make it possible to predict, with 

 useful accuracy, the main features of the radar properties of a new or 

 unmeasured target aircraft. 



An appreciation of the complicated nature of the radar area of an aircraft 

 and its association with the physical structure of the aircraft can be 

 obtained from some of the results of basic investigations into the properties 

 of radar echoes from aircraft carried out by the Naval Research Laboratory, 

 and recently made available. ^~^^ Pulse-to-pulse measurements were made of 

 both fighter and bomber categories, with propeller-driven and jet-propelled 

 models in each category. The measurements were made on three fre- 

 quencies, 1250, 2810, and 9380 Mc/sec, with the radars searchlighted on 

 the target by an optical tracker, and pulsed simultaneously. No antenna 

 scanning was used, so that the observed fluctuations were all attributable 

 to the target. The data were analyzed to determine amplitude distributions, 

 median radar area versus aspect, and frequency spectrum of the amplitude 

 fluctuations. The particular series of measurements to be discussed was 

 made at elevation angles less than 15°. These measurements will be 

 discussed in some detail, since comparable data have not been published 

 before. Many of the characteristics observed can be explained in terms of 

 physical processes, so that from these it should be possible to predict the 

 principal characteristics to be expected in other situations. 



9F. C. MacDonald, Quantitative Measurements of Radar Echoes from Aircraft III: B-16 

 Amplitude Distributions and Aspect Dependence, NRL Report C-3460-94A/51, 19 June 195J. 



low. S. Ament, M. Katzin, F. C. MacDonald, H. J. Passerini, P. L. Watkins, Quantitative 

 Measurements of Radar Echoes from Aircraft V: Correction of X-Band Values, NRL Report 

 C-3460-132A/52, 24 Oct. 1952. 



"W. S. Ament, F. C. MacDonald, H. J. Passerini, Quantitative Measurements of Radar 

 Echoes from Aircraft VIII: B-45, NRL Memorandum Report No. 116, 28 Jan. 1953. 



12W. S. Ament, F. C. MacDonald, H. J. Passerini, Quantitative Measureynents of Radar 

 Echoes from Aircraft IX: F-5I, NRL Memorandum Report No. 127, 4 March 1953. 



