202 



ECHOES AND TARGETS 



be agciin essentially iiidepeudeut of the scauuiug speed, 

 as long as it is high enough to cause pulse-to-pulse and 

 sean-to-scan integration. The limit, however, may 

 occur at 20 rpm instead of 10 rpm. 



One variable has been omitted which has proven 

 puzzling. This is the target speed. What really con- 

 stitutes a scan-to-scan integration ? If the target moves 

 the distance of one spot diameter in a scanning period, 

 is it still integrated ? It would seem to be so integrated 

 provided the observer is able to perform as an aided 

 tracker, i.e., can appreciate a change in linear motion. 

 If it is not integrated, one would expect to find a 

 difference in signal threshold depending on the target 

 speed, probably in direct proportion to the square root 

 of target speed. Some experiments have been made on 

 simulated echoes of this variety, and there was some 

 indicatioji that targets of higher velocity are definitely 

 harder to see, but this cannot be considered quantita- 

 tively established. Signal fluctuation, however, is im- 

 portant, and it is felt that, in general fluctuating tar- 

 gets with cross sections defined on the basis given in 

 the following paper are harder to see by perhaps 2 db 

 but that this estimate is not affected by any arguments 

 aliout scanning. 



There was another inquiry concerning the explana- 

 tion of tlie Watson ett'ect observed occasionally at very 

 close ranges when the background noise was so large 

 the normal signal could not be detected. This con- 

 sisted of an inverted signal smaller in amplitude than 

 the background noise which could be observed to a 

 range of almost 100 yd in sets which had a direct wave 

 extending to 3,000 to 4,000 yd. In these cases the signal, 

 instead of appearing as a small inverted V, showed up 

 as a small upright V, approximately Vs the amplitude 

 of the initial noise. This effect had been often reported 

 on short-range targets. The author considered this to 

 bo a form of receiver saturation. Another group had 

 ))een troubled by the same phenomenon and had at- 

 tributed it to receiver saturation in which there was 

 blocking of the i-f amplifier during a portion of the 

 time. 



la-* RADAR SCATTERING OVER 



CROSS-SECTION AREA* 



It is of great interest to determine the cross-section 

 values of aircraft, not only in order to attempt predic- 

 tion of ranges on these aircraft, bnt also to make pos- 

 sible the design of radar equipment which will utilize 

 these factors a little l)etter. The instantaneous pattern 



"^By J. L. Lavvson, Radiation Laboratory, MIT 



of reflection properties of an airplane is very complex. 

 It depends upon the frequency, type of aircraft, and 

 certain other factors, such as propeller rotation. The 

 pattern has an extremely complex lobe structure which 

 depends essentially upon the lengths of the plane's 

 structure in terms of wavelength and upon the areas 

 of specular reflection, that is, reflection from fairly 

 large, flat, mirror-like surfaces found in most aircraft, 

 such as the sides, bottoms, or wing surfaces. 



It would be possible to define the instantaneous 

 cross-section area as a fnnction of the angle from the 

 airplane, but this kind of thing would be purely aca- 

 demic, since actually the airplane is moving. In the 

 early part of this work an attempt was made to derive 

 a cross-section numl^er which would apply to the actual 

 radar performance on an airplane in flight. The scat- 

 tering cross section may l^e calculated from the re- 

 lation 



_ {JwyPrR' 

 " ~ PtG^y- ' 

 where the quantities are measured in free space. The 

 symbols are defined in Section 12.3. They are all easily 

 measurable except Pr, the received power. This was 

 measured by injecting into the system, with a signal 

 generator, an artificial echo which was nmtched to the 

 size of the airplane echo. 



In pratice, u is necessarily a function of time, and 

 for lack of a better criterion the following procedure 

 was adopted. The signal generator reading was con- 

 tinuously nuitched to the size of the aircraft echo and 

 recorded for successive 3-sec intervals. The signal 

 measured in decil)els above receiver noise power was 

 plotted against range. On a logarithmic scale such a 

 plot should be a straight line whose variation is 40 db 

 for a factor 10 in range. This is actually found, pro- 

 vided one draws a line through the average of the 3-sec 

 interval points. From moment to moment the fluctua- 

 tion is rather high, but nevertheless a good average 

 line can be drawn. 



It is now possible to define a cross section by the 

 condition that its value is exceeded in one-half of 

 these 3-sec intervals, and this appears to be an easy 

 operational way of obtaining cross sections. However, 

 this still does not represent what could be called the 

 average value for each 3-sec interval. It was found very 

 early that it was very difficult to adjust a signal gen- 

 erator to the average value of the signal. It is much 

 easier to adjust to the top value that has occurred dur- 

 ing an interval. The reason for this is that the signal 

 is quite often fuzzy and filled in by pi'opeller modula- 



