Recently on the SETI email discussion list there has appeared some good dialog regarding possible detection of Earth TV broadcasts at ET locations. [These discussions were stimulated by the question: could beings on worlds 40 light years distant really be viewing I Love Lucy? -- Ed.] This is not a novel idea. An older book titled Communication With Extraterrestrial Intelligence (editor Sagan, 1973, MIT Press, ISBN 0-262-69037-3) mentions the possibility of eavesdropping by ETI in the chapter Techniques of Contact. I have seen this idea mentioned many times in other writings.
Discussion on the list has been centered on detection of UHF television broadcasts, which in the US encompasses the band 470-806 MHz (minus the astronomy allocation at channel 37, 608-614 MHz). Why has interest been focused on UHF television?
Some possible reasons:
In the US, all analog television channels are split across three bands; low VHF (54-88 Mhz), high VHF (174-216 MHz) and UHF. Maximum permitted EIRP is 100 KW for low VHF, 316 KW for high VHF, and 5 MW for UHF. Horizontal polarization is the rule, with circular polarization being allowed at the broadcaster's discretion. It would thus be possible to radiate up to 10 MW EIRP from a single facility. The varied limits across bands help compensate for differences in terrestrial propagation, in theory placing the UHF broadcaster at parity with the VHF broadcaster.
Because UHF TV stations radiate more power, and because the UHF band is nearest to the so called "microwave window" of 1.2-50.0 GHz, it would seem to some that these broadcast signals have the best chances of being detected at astronomical distances. There are hundreds of UHF TV stations in the US alone, each radiating day and night, year after year. Could these act as unintentional beacons to an interested ETI?
This writer's opinion is that there are far better candidates for Earth's "unintentional beacons". Here are my reasons:
One goal of a TV broadcaster is to deliver modulated RF as cheaply as possible to as many receivers as possible. Five MW of power at UHF frequencies is not easy to come by. The typical way of doing so is to generate 60-240 KW of power at the transmitter, then excite an antenna having 10-14 dB of gain. The 60-240 KW of power needed at the transmitter output is obtained with much difficulty, usually by generating a modulated carrier at low levels and passing that through class-A vacuum tube (klystron, IOT or tetrode) amplifiers with attendant low efficiencies.
Electricity is not free. The engineer responsible for design and/or operation of a transmission facility would not think of squandering RF by intentionally directing it into the sky. The TV audience makes their homes near ground level. Gain of a TV broadcast antenna is obtained by concentrating the radiation into one direction. From a bird's eye view, the signal radiates in all directions, equally well North, East, South and West. From a side view, the main lobe is carefully engineered to equally illuminate receiving antennas on the ground, both close in and near the horizon. What is often done is to aim just below the horizon, and design in some null-fill to accomodate the close in receivers. From a typical transmitting antenna height of 1000 feet, the desired beam tilt will be something on the order of 0.5 degrees negative (that is, below horizontal). Any energy radiated above the horizon is wasted, as far as the broadcaster is concerned.
In an imperfect world, some energy does radiate above the horizon, but this side lobe is much reduced in amplitude. Likewise, some of the energy striking the ground reflects back into space, but it will be scattered and specular.
There are better "unintentional beacon" candidates for study and speculation. Consider SPASUR (space surveillance radar) at 217 MHz, or varied other defense radars operated by the military (see the Raytheon home page for the names of some of these systems). Perhaps even HARRP transmissions (2.8-10.0 MHz at 3 MW+) may be detected at great distances, although the frequencies employed are far from ideal.
Some of you might be interested to know that the US has already orbited what amounts to a sensitive spectrum analyzer which has observed the Earth. Project Blackbeard was established to study TIPP (trans ionospheric pulse pairs), events possibly associated with lightning storms. Frequencies ranging from 25-100 MHz are observed. Spectrograms may be viewed on the project's home page (http://sst.lanl.gov/nis-projects/blackbeard). They show strong RF carriers of Earth TV and FM broadcasts. Keep in mind that these carriers have been detected from a satellite a few hundred miles above the Earth, not from interstellar distances.
I do feel strongly that reception and demodulation of Earth television and radio by ETI is still highly possible, though not from interstellar distances. How?
Many people study extinct cultures such as the ancient Egyptians, just to choose one at random. We know of the Egyptians by their writings and artifacts. What if they had television and we could send a probe back in time to capture a few dozen terabytes of their transmissions? We would obviously know more about them than we now do. Radio and TV broadcasts can quickly deliver massive amounts of information about the society which generates them.
Somewhere in the galaxy, there might exist a society that has lasted for many thousands of years. This race may feel confident that intelligence will someday arise on a small watery planet orbiting a yellow star in their sky. This culture prides itself in the accumulation of wisdom, and projects that might take a few thousand years to bear fruit are conducted with the same vigor as short-term projects. They therefore send robot probes to promising worlds. If such a probe were to detect the presence of non-natural RF emanating from it's target world, it would perform a relay transmission back to the home planet. If ETI is going to watch our TV broadcasts, this is how it will happen, in my opinion.
I hope that this stimulates further discussion along these lines, and I welcome differing points of view.
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this page last updated 4 January 2003
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