Aliens Viewing The House SETI Debate!

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Some years ago it was shown that the narrow-band video carriers from terrestrial Amplitude Modulated (A.M.) TV transmitters would be detectable at distances of many light years as a substantial increase in the radio brightness of Earth's planetary limb. The following is an exercise to see if Frequency Modulated (F.M.) satellite TV transmissions would also be detectable.

On June 28, 1990, in the House of Representatives, Congressmen Conte and Matchtley participated in a shameful debate which removed the remaining $6 million that had been earmarked by NASA for the 1991 Microwave Observing Project. Ridiculous tabloid newspaper materials on UFOs as are often found at supermarket checkouts, were inserted into the Congressional Record of this debate. It seems fitting to run some calculations to see if a technical alien civilization at 10 light years distance will eventually be able to view that debate.

In about five years, the C-SPAN broadcasts of the June 28 debate on SETI and the Microwave Observing Program will be reaching the nearest star systems. Within a few years of that, the broadcast may be detected by other intelligent civilizations. From what they would see (and hear) if they can detect the signal, they cannot be blamed for concluding that humans are rather backward, particularly ones that claim to be their leaders.

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Yes, Members of Congress should ponder the scenario that when they rise to speak on the floor of the Senate or the House, the audience may be somewhat larger and more distant than they could ever imagine! How fortunate for Representatives Conte and Matchtley that these aliens, which aren't supposed to exist, don't have the vote!

The House of Representatives is broadcast on C-SPAN I. This is relayed across the nation on Transponder 24 of the domestic satellite known as Galaxy 3, which is positioned at 93.5^o W longitude. The downlink frequency for Transponder 24 is 4.180 GHz. The uplink frequency for that transponder is 6.405 GHz. At Alexandria, Virginia, C-SPAN uses a 10-meter diameter dish for uplinking their F.M. signal with a transmitter power of 44 W.

Only a minute fraction of that uplink beam is actually received by Galaxy 3, the rest of the power is wastefully radiated into space. As the Earth rotates, and with it Galaxy 3 in its 22,300 mile high geostationary orbit, the uplinked beam sweeps through the cosmos. In ten years time, could this much weakened signal be detected by an alien civilization? To answer this question we must calculate the C-SPAN I signal strength at that range.

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First we will use the theory previously given to calculate the signal strength at geostationary orbit. The gain of the transmitting uplink dish:

G = 5.73 x 10⁵ (57.6 dB)

The Effective Isotropic Radiated Power is calculated to be:

EIRP = 25.2 MW

The geosychronous orbit is at an altitude of 22,300 miles or 35,786 km. The slant range from Alexandria, Virginia to Galaxy 3 is R = 37,630 km. Using this range, the intensity of the received beam (EIRP/4.pi.R²) at the satellite can be shown to be:

I_r = 1.42 x 10^-9 W/m²

This signal strength is sufficient to fully saturate the Galaxy 3 receiving satellite transponder.

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Let us now extend this result to a range of 10 light years. Since 10 light years = 9.461 x 10¹⁶ m, the intensity we have just calculated for geostationary orbit will be attenuated by a further factor of (37,630)²/(9.461 x 10¹³)². Thus, the intensity at 10 light years will be 2.24 x 10^-28 W/m². This intensity is more than an order of magnitude below the sensitivity limit of the 300 meter diameter Arecibo telescope (area = 7.07 x 10⁴ m²) on a 1 Hz bandwidth basis, so clearly the aliens will require a radio telescope of much greater area to receive the signal.

We can now estimate the area of the alien receiving antenna required to obtain a reasonable CNR. The Carrier-To-Noise Ratio is given by:

                EIRP.Ar
CNR = ---------------
              4piR2kTsBe

where:

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The above equation can be rearranged so that we can determine the receiving area required to obtain a 10 dB CNR in a 30 MHz bandwidth. An 8 dB CNR is typically the F.M. threshold for a high-quality receiver, so that a 10 dB CNR represents as excellent picture, as good as most people will receive on their cable or satellite receiver systems. Employing the parameter values given above, we find that:

A_r > 1.85 X 10¹⁵ m² !

Such a large antenna is unlikely even for a very technically advanced civilization, though a much smaller antenna would yield sufficient signal in a much reduced bandwidth to allow spectral analysis of the signal and a determination of modulation format. Such a modulation format determination might cause the alien civilization to use a similar modulation technique, even if they transmit back to Earth via an optical beam, as they will logically surmise that we would know how to demodulate the signal. Thus, even if one didn't think that F.M. modulation would be a universally accepted modulation technique, the fact that terrenes make use of this modulation format might not be unknown to alien civilizations.

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If the uplink power was increased to 100 kW and the 300 meter diameter Arecibo dish used for transmission, the EIRP would increase from 25.2 MW to 51.5 TW, and the required area of receiving antenna would fall from 1.85 x 10¹⁵ m² to 9.04 x 10⁸ m². This would be equivalent to a receiving dish only 34 km in diameter! This might be a reasonable "listening" dish for an advanced technical civilization.

Note that the C-SPAN uplink beamwidth is about 0.27^o, equivalent to a diameter of 3,610 Astronomical Units (A.U.) at a range of 10 light years. This means that for the two C-SPAN uplinks (C-SPAN I & II), the beams will completely sweep across the receiver in about 1 minute during every 24 hour period. Obviously, it is relatively unlikely that alien receiver antennas would be optimally positioned in space to observe most of the uplinked satellite signals which are directed at many points in equatorial geosynchronous orbit from various terrene latitudes.

Clearly, aliens would have a very difficult time constructing such a large receiving array of dishes to collect the C-SPAN uplink signals. So perhaps, to the relief of some Members of Congress, we can say that it is unlikely that alien civilizations at distances of ten or more light years could observe their deliberations, even for fleeting 1 minute periods.

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This fun piece of analysis has indirectly demonstrated how hard it is to send high bandwidth signals across the cosmos using microwave technology. One day, uplinked signals to satellites may use laser beams. More likely, laser beam links will be employed for direct communications between geostationary satellites, by-passing terrestrial relay stations. A decade after this happens, much stronger signals (though for shorter periods of time) might be detectable at nearby star systems.

See also the 1993 Congressional debate that lead to the cancellation of NASA's HRMS Project.