SETI efforts have been plagued from the start by the reception of intelligently generated signals of decidedly terrestrial origin. In 1960, early in the listening phase of Project Ozma, Frank Drake received a rather strong signal from the apparent direction of Epsilon Eridani. After tracking it through several orbits, he concluded that he was most likely receiving interference from a military satellite. It was operating right on the Hydrogen Line, now a supposedly protected spectral region (but try to tell the military that!). Harvard University's Project META and BETA sky surveys have mapped numerous Radio Frequency Interference (RFI) sources from navigation satellites. Ironically, BETA uses these same interfering satellites to generate an accurate time and frequency reference! These signals (see Figure 1) are many MHz wide, and right in the middle of the SETI Water-Hole. During its 1995 Australia deployment, the SETI Institute's Project Phoenix targeted search experienced scores of hits, which had to be weeded out by a sophisticated Follow-Up Detection Device (FUDD).
And now, The SETI League's Project Argus sky survey has a bona fide RFI hit of its own. On May 10, Trevor Unsworth, one of our British members, detected an anomalous signal at 1471.5 MHz (see Figure 2), using his homemade 3.5 meter dish. The signal exhibited some sort of digital modulation, with a 270 Hz bandwidth, and its Doppler shift of -25 Hz/min clearly marked it as RFI from a Low Earth Orbit (LEO) satellite.
At the recent SARA meetings in Green Bank, I showed a slide of Figure 2, and asked for help identifying the source of the signal. Several others had seen signals of this type in that same spectral region. One radioastronomer informed me that the US Navy apparently has LEO satellites using that particular frequency region, but no further information was available. The military doesn't publicize its classified satellites to our satisfaction!
Actually, I am greatly encouraged by Trevor's "hit" for several reasons. The signal is from a relatively weak source, which indicates that his system has reasonable sensitivity. Our member contacted SETI League headquarters first, not the BBC or the London Times, which suggests that our verification protocols are being respected. And the Doppler shift (indicated by the slope of the line in the spectral display) was clearly not consistent with Earth rotation rates, which told us that the signal was not moving with sidereal time, hence must be considered RFI. This first test of our ability to weed out RFI by its Doppler signature was entirely successful. But the situation will only get worse. Despite its FUDD technology, there were wide gaps in Project Phoenix's frequency coverage, regions where satellite RFI made monitoring for ETI sources impossible. New communications and navigation satellites, operating smack in the middle of the Water-Hole, are being launched every month. Barney Oliver used to fear that politics and paranoia might cause us to draw a curtain across the sky. That curtain is forming, but it's being caused by our own technology.
At the Observatorie de Paris, Jean Heidmann is putting forward a proposal for a SETI observatory on lunar farside, in Saha Crater. In Italy, Claudio Maccone has for years been proposing a Solar Sail mission to 550 AU, which would use our Sun as a gravitational lens for surveying the cosmos. These attempts to escape Earth's RFI are serious scientific proposals which deserve our full support. But they are also missions for well into the next century. In the interim, it falls to us in The SETI League to pioneer techniques for working around the ever-increasing RFI which surrounds our planet, cutting right across our prime radioastronomy band. Your thoughts and suggestions are encouraged.
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this page last updated 4 January 2003
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