How air attacks were detected before radar.
Beginning in the middle of the 1930s, engineering labs in the U.S. and Europe were experimenting with radar systems. Early radars did not have the slick plan position indicator (PPI) displays that modern systems use for plotting target movement for indication of azimuth (direction) and range (distance). Instead, oscilloscopes showed radar returns as amplitude blips along a time base that represented range. Azimuth was determined by where the operator pointed the antenna (rotating versions came later). Since radar cross section stealth technology had not been invented yet, the amplitude of the signal was useful a measure of the size of the target. (Pics)