Eavesdropping: the radio signature of the Earth.

In addition to searches for purposeful signals, those attempting interstellar communication should also consider the possibility of eavesdropping on radio emissions inadvertently "leaking" from other technical civilizations. To understand better the information which might be derivable from radio leakage, the case of planet earth is considered. The most detectable and useful escaping signal arise in a few BMEWS-type radar systems and in normal television broadcasting. A model including over 2000 television transmitters is used to demonstrate the wealth of astronomical and cultural information available from a distant observer's careful monitoring of frequency and intensity variations in individual video carriers (program materials is not taken to be detectable).