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SETI
- Allen Telescope Array
- Optical SETI
- Interstellar Message Construction
- Project Phoenix 1995-2004
- Background Science
- History of SETI
- Center for SETI Research Team
- Other people doing SETI
- Hollywood's View of SETI
Finding Radio Frequency Interference
by Mike Davis, Director of SETI Projects
Observations began Monday evening. As usual, we pointed the telescope straight up, turned off the drive system, and spent the entire first night filling the data base with signals from our own planet -- otherwise known as radio frequency interference.
This session, we moved our observing to a new receiver that covers 1,750 to 3,000 MHz. To a large extent, this is unexplored territory, as until now, we have concentrated on lower frequencies (1200 1750 MHz).
Over the past few decades this frequency range has become increasingly filled with signals from commercial and government transmitters, so we weren't quite sure what to expect. We had some warning, however, as the observatory provides filters to let us suppress the regions with the strongest interference. We were pleasantly surprised then, to find that at the very high resolution of our spectrum analyzers, much of the spectrum is still usable outside these highly saturated regions.
We still have to identify and screen out many hundreds of narrow ranges that contain terrestrial or satellite transmitters. To ensure that we didn't include rare cases in our screening, we went through the whole frequency range eight times, and looked only for signals that showed up repeatedly.
We constantly upgrade the database of the signals we find with new ones as they appear during normal observing. A signal that fails to reappear after a week is removed from the database so that we don't eliminate usable frequencies permanently on the basis of a single night's detection. The database slowly matures, and after two weeks it is unusual to see a new signal that has not already been detected.
At present, we find we have to initiate this process each time we return to Arecibo. Once we shift observing to the Allen Telescope Array in Hat Creek, well be free from this chore, as we will not be stopping and restarting at several-month intervals.