Fall 2009 Issue
DR. SETI’s STARSHIP
The Day the Earth Called Out
By Dr. H. Paul Shuch, N6TX
In a December 12, 2008 publicity stunt, 20th Century Fox, producers of The Day the Earth Stood Still (the 2008 cheesy remake, not the 1951 cheesy original) beamed its science fiction opus toward Alpha Centauri, our nearest stellar neighbor. A Fox spokesman called it the first “galactic motion-picture release.” Wide discussions ensued within the SETI community as to the technical feasibility and societal implications of such interstellar transmissions. Many opined that the transmission could not possibly be detected (four years hence) by its intended audience (four light years distant). Others argued that we cannot place limits on extraterrestrial technology. I argue that we won’t know until we run the numbers. However, since the purpose of this supposed interstellar transmission was to promote the release of a motion picture, one could argue that the intended audience was in fact human, rather than extraterrestrial.
This column concentrates on the medium, not the message. Although I am decidedly not a film critic, I cannot resist the temptation to comment briefly on the motion picture itself. One wonders why an advanced extraterrestrial would travel all the way to Earth with a message of warning, and then fail to meet with a single scientist, diplomat, or head of state. Had Klaatu, as portrayed by Keanu Reeves, ever bothered to watch any terrestrial television (which question is, in fact, the very focus of this exercise), he surely would have learned how to say “take me to your leader.”
As reported in the media, the transmission in question emanated from “NASA’s Deep Space Communications Network at Cape Canaveral.” Those news reports are factually flawed; Deep Space Communications Network (henceforth DSCN) is a private company, in no way affiliated with NASA, that transmits private messages into space for a fee. Whether by geographical coincidence or marketing design, this company’s uplink facility is located in the municipality of Cape Canaveral, FL, USA, but by no means is it on site at the Kennedy Space Center, which shares that address. Mr. Jim Lewis, proprietor of the company in question, asserts that it was never his intention to imply otherwise, a claim which I am inclined to accept at face value.
DSCN’s transmission equipment is in fact a standard, commercial-grade C-band remote uplink facility, such as is commonly used for remote news and entertainment broadcasts via satellite. It consists of a trailer-mounted 5.5-meter diameter parabolic reflector and redundant 1-kW klystron FM video transmitters operating in the 5925–6425 MHz TVRO uplink allocation. The transmitters are typically operated at 500 watts average power (+57 dBm), using 10.25-MHz peak deviation, 30-Hz dithering, and a highest modulating frequency of 6.8 MHz, that being the highest available audio subcarrier frequency. These specifications yield a 99% power bandwidth on the order of: 2³f + 2fm = 34 MHz, which is wholly compatible with a full 40-MHz DOMSAT transponder, allowing a reasonable guardband for non-significant sidebands.
Note that DOMSAT video being frequency
modulated, the signal’s energy components are spread out as sidebands
across this entire 34 MHz of spectrum. Thus, to recover and demodulate the
transmission, a suitable receiver must be designed with a 34-MHz
intermediate frequency (IF) bandwidth.
l = c/n = (3 ¥ 108 m/s)/(5945 ¥ 106 Hz) = 5.0 cm
At that wavelength, the manufacturer’s stated gain of the 5.5-meter offset-fed parabolic reflector is +48 dBi, suggesting a commercial-standard 55% illumination efficiency. (The gain of this antenna could be improved by nearly 2 dB through the use of a more highly optimized feed geometry, but that is a subject for another occasion.) The computed antenna half-power beamwidth is on the order of:
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