Radio Frequency Interference

RFI - Radio Frequency Interference

SETI@home is certainly one of the most exciting internet projects ever. Using a simple screensaver program, you can participate in the grandest search of all - looking for intelligent life elsewhere in the cosmos. Your SETI@home screensaver takes data from the world's most sensitive radio telescope and does a very detailed analysis of the signals received. Being so sensitive in both the telescope sensing and the data analysis also makes it possible to catch signals that are not from extraterrestrial sources. Many of our participants have seen these signals and written or called us brimming with excitement about their dramatic discovery. Unfortunaley it's a whole lot easier to detect the earthbound signals than it is the extraterrestrial ones. Let's take a look at some examples of these interfering signals.

Narrowband Interference

Remember that the Arecibo Radio Telescope is fixed in place. Although the receiving antennas can be made to move and follow objects for a short period, SETI@home does not do this. SETI@home is a piggy-back system and uses the telescope when no one is using it or even when other astronomers are doing their work. SETI@home prefers (actually requires) that the SETI@home antenna NOT track the sky. SETI@home is specifically looking for signals that start off weak, get stronger, and then fall off again in a 12 second period. Why 12 seconds? That's the time it takes an object to pass through the "beam" (angle of view) of the telescope. If a signal is of constant strength over time, we know that it is interference from a ground-based source. Click either thumbnail to see what this would look like on your screensaver. This is a narrowband signal, that is, its frequency is restricted to a very narrow range. You can see this because the "wall" that the signal forms within the graph is only one pixel wide. Note that the signal is continuous throughout the entire 107 seconds of the data - proof of its terrestrial origin. In this case, not what we are interested in.

Broadband Interference

Narrowband interference is not the only kind of RFI (Radio Frequency Interference) that you will see in your graphs. Broadband signals - signals that cover a wide frequency range - can also be generated on the earth and detected by SETI@home. We do not believe that these are types of signals that extraterrestrial beings would send. To cover a broad range of frequencies with the necessary power would be unneccessarily wasteful of their energy resources. The broader the range of frequencies, the less energy you can put into a signal-frequency that sticks up out of the noise demanding attention. Some times, earthbound interference produces just these kinds of signals. Click the thumbnail to see an example of one of these. In this case a powerful radar signal completely swamped the antenna and detector system. This is not what we are looking for either.

Testing and Verification

Most likely, if there is an intelligent signal lurking around, it will be almost buried in the noise of your graph, probably invisible to the unaided eye, but glaringly obvious to the detailed analysis that your computer is doing. The billions of calculations you do will hopefully expose the message that we all dream is there. If you see some evidence of a signal on your screen it is very important that you NOT contact the media until the signal is verified and checked. False alarms severly damage the credibility of SETI@home and all serious SETI programs. Be patient. If there is a real signal, we promise to inform you and keep you "in the loop" concerning the progress of the search.

Click the thumbnail. If you see anything like this one, then you can get a little excited (and maybe a little frightened or disappointed, or both). You still can't get too excited though. Your discovery still has to go through a rigorous set of tests and verifications to prove that it's truly of extraterrestrial origin. If your signal passes all of the tests and re-tests, then we'll all get excited!