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What is the difference between
an All-Sky Survey and a Targeted Search?

Before its funding was terminated by Congress in 1993, NASA's SETI (Search for Extra-Terrestrial Intelligence) program consisted of two distinct but complimentary research elements: a targeted search of nearby sun-like stars, and an all-sky survey for interesting signals of unknown origin. The former, which involves aiming at likely candidate stars for long periods of time, is well suited to large, steerable dishes with their narrow beamwidths and high sensitivities. If we guess right as to which stars constitute likely candidates, the targeted search will provide us with the greatest likelihood of immediate success. But since only a limited number of relatively nearby candidate stars is known to us, concentrating our search in their direction may cause us to miss an equally good star of which we happen to be unaware.

An all-sky survey, on the other hand, makes no a priori assumptions as to the most likely direction to explore. The sky survey attempts to sweep out the entire sky which can be seen from a given location. No antenna tracking is required, since it is the entire sky, rather than individual stars, which we seek to scan. While target searching antennas must be constantly moved, sky survey radiotelescopes are operated in what's called meridian transit mode, in which it is the Earth's rotation which turns them.

NASA's late targeted search has been resurrected by our colleagues at the non-profit California-based SETI Institute. Their Project Phoenix employs some of the world's best radiotelescopes. But since large antennas have quite narrow beamwidth, they see only a small portion of the sky at a given time. The sky survey component would be better performed with antennas of moderate size. Smaller antennas can see more sky within their beam patterns, but have less gain. We achieve reasonable sensitivities through digital signal processing, but the antennas need to scan for extremely long periods of time. The sky survey approach seems ideally suited to the community of radio amateurs and microwave experimenters. This is the area in which The SETI League is concentrating its efforts, through our own Project Argus.

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