During the last month we've been searching the science data base for radio signals seen two or more times. Since the SETI@home participants have already analyzed 1.5 years of data from the Arecibo telescope, there are already many places on the sky we've observed two or more times.

We search for signals that recur at almost exactly the same place on the sky and almost exactly the same frequency. We also search for signals whose frequency may have drifted significantly over the many months between observations. Signals might drift in frequency because of the "doppler effect" - a transmitter on a rotating and revolving planet will move toward and away from earth, and this acceleration produces a shift in the observed frequency. Another civilization might correct for their motion if they are deliberately transmitting a signal towards us; then their transmitter's frequency would remain steady, assuming we account for our own planet's motion. But we don't know if extraterrestrial civilizations will correct for their motion, so we look for both drifting and non-drifting signals.

So far, we've identified two pairs of strong gaussian shaped signals, both seen on two different observations. We've also found three trios of weaker gaussian signals, each signal seen on three different observations. We are examining these data further, and also examining some even weaker signals. But these multiple detections are almost certainly from noise, and not signals from another civilization. We are searching through a data base of 50 million gaussian signals, and a few of these 50 million signals will randomly occur at almost the same place and the almost the same frequency. We've examined thousands of these kinds of multiple detections in our 25 years of searching, and so far they've always turned out to be noise or radio pollution from terrestrials, not extraterrestrials.

2. Pulse Detection

We've also been developing a new version of the SETI@home screensaver program that searches for pulsing radio signals. We hope to beta test this new program next month and make it available for all SETI@home participants sometime this summer.

The new pulse software searches for regularly spaced radio pulses. Much like a light house sends out regular optical flashes, another civilization might send out regular radio pulses. If we converted these radio waves into sound waves, you might hear "click, click, click...".

The pulse hunting software uses two different algorithms - one algorithm searches for three pulses evenly spaced in time. This "triplet" algorithm doesn't take much computer time, but it can only detect powerful pulses. The second algorithm, developed by Eric Korpela, can find very weak pulses if they are regularly repeated tens, hundreds or thousands of times.