Radio leakage and eavesdropping

The possibility of the detection of extraterrestrial technological civilizations through observations of the inadvertant leakage of their radio emissions is considered. The characteristics of the radiation leaked into interstellar space from the earth are examined, and considerations of transmitter power, frequencies and bandwidth are used to show that Ballistic Missile Early Warning System radars may serve as acquisition signals and television transmissions may serve as information signals for detection by extraterrestrials. The characteristics of television broadcasting signals are discussed in order to illustrate the value of the carrier signals as informational signals (program material is most likely not detectable), and information obtainable about the size and orbit of the earth is indicated. On the basis of these considerations, and of observations of the leakage radiation of the earth as reflected from the moon, it is concluded that both leakage and direct emissions must be considered in the search for extraterrestrial signals.