50 Years of Arecibo Lunar radar mapping

William E. Gordon conceived the giant radar/radio telescope at Arecibo, Puerto Rico (Figure 1) in 1958 as a backscatter radar system to measure the density and temperature of the ionosphere at altitudes of up to a few thousand kilometers [1, 2]. The backscattered ionospheric echo was calculated to be weak, requiring a large antenna with a diameter of 1000 ft (305 m) to measure it. From the beginni...