Radio communication via the Moon

Radio reflections from the Moon were first detected (in the United States and Hungary) in 1946 using VHF radar constructed from military radar equipment. These and subsequent observations at HF (in Australia) and VHF (in England) revealed that the echoes were subject to both rapid and slow fading. By 1954 it had been established, by means of experiments at Jodrell Bank, that the slow fading was caused by the rotation of the plane of polarization of the radio waves in the Earth's ionosphere, the so-called Faraday effect. The rapid fading was believed to be caused by interference between the many scattering centers on the surface of the Moon, whose relative distance from the Earth is constantly changing owing to its libration. Studies of this fading at Jodrell Bank in 1956 confirmed this theory and revealed that most of the power in the reflected signals arose from scatterers lying near the center of the visible disk. The range extent of these returns was less than 1 ms, that is, much less than the full radar depth of the Moon (approximately 10 ms). Accordingly, it was recognized that radio waves modulated by speech (or music) could be reflected from the Moon and remain reasonably intelligible. Subsequent experiments, supported by the Pye Company, explored this form of communication as a possible alternative to overseas HF broadcasting.