ECHOES AND TARGETS 317 
procedure must be found for averaging the data. This 
has been attempted. At 100 me, 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 
‘Tepresented 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 2 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 on radars with 
search type presentation? The author believed that 
such fluctuations would affect search-type radars 
when scanning slowly but that no serious effect had 
been observed at scanning speeds as low as 2 rpm. 
When the cross-section figures given were used with 
a 2-db reduction for average values, the predicted 
Tanges 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- 
lengths, the lobes are so close together that it becomes 
practically impossible for an aircraft to remain in a 
null for an appreciable time. 
It was inquired whether drops of more than 2 db 
were to be expected for other aspects, such as 45 
degrees. While a complete. series of measurements had 
not yet been made by the author’s group, measure- 
ments on three or four aircraft in various aspects had 
revealed no drops below 2 db. Although the calibration 
had been carried out entirely with signal generators, 
standard targets consisting of corner reflectors and 
spheres later produced results in substantial agree 
ment with the theoretically predicted values. 
