lew (11/01/82)
This is a synopsis of "Eavesdropping: The Radio Signature of the Earth", by W.T. Sullivan, S. Brown, and C. Wetherill in the 27 Jan 1978 issue of "Science". The authors report on an extensive and thorough analysis of the probable radio appearance of the earth from ~1 light year. The principal conclusions: 1) The most distantly detectable radio emission is from BMEWS (Ballistic Missile Early Warning System) radar. An Arecibo-like antenna could detect it at ~18 light years. 2) The richest source of distantly detectable radio emission is UHF and VHF TV broadcasting detectable at ~1.8 light years by Arecibo (carriers only, program demodulation would require ~2e4 times the sensitivity.) The authors analyze the appearance of about 2000 major stations due to scheduling, geography, and diurnal and orbital motion of the earth. Broadcasts at frequencies less than 20Mhz are contained by the ionosphere. In the 27 October 1978 issue of "Science", there is a letter from Carl Sagan arguing the implications for SETI policy. W.T. Sullivan, in his reply, mentions that he had since learned of the U.S. Navy Space Surveillance radar, which illuminates the sky from 0 to 33 degrees for a period of about 7 seconds once a sidereal day. It would be detectable at a distance of 60 light years by the Arecibo antenna. I had erroneously recalled this article as being in Scientific American. After an exhaustive search of the indexes from 1974, I turned to the "Readers Guide to Periodical Literature" at my local library. This turned out to be a great resource. I started looking under "Earth" before moving to "Life on other planets", and finally finding it under "Interstellar Communication". There were ~10 entries under this for 1978, many of them reports of and comments on that "Science" article. Lew Mammel, Jr. ihuxr!lew