RFI mitigation at ATA
From setiquest wiki
These are the steps used to mitigate the RFI problem at the ATA. When a signal is detected by SonATA, it is sent through a number of tests to determine whether the source of the signal is terrestrial (RFI) or not:
- Look to see if this signal has been previously observed (within the last week) in another pointing direction. As described in step 3. below, this proves the signal is RFI. All of the signals observed in the prior week are distilled down to a few parameters that are stored in a database. This database needs to be "seeded" initially using steps 2 and 3 below, but after seeding it becomes a very effective filter for RFI.
- Look for persistent signals. Signals that appear only in one observation and never again could be the result of a rare but expected noise spike in the data. Once in a while, the noise in our receivers accidentally line up in phase with one another just enough to generate the appearance of a signal. Also, human RFI (like a LEO satellite) may accidentally run through our telescope field of view, never to return. For reasons like this, we must insist that the signal appear more than once.
- Point the telescope away from the direction where the signal is observed. Almost all RFI that appears in our telescope is not coming from the direction where we point. This is because it is impossible to build a perfect telescope unless it is infinitely large in diameter. The ratio of sensitivity in our pointing direction to the sensitivity everywhere else is inversely proportional to the diameter of our telescope. Since the ATA is relatively small (both in dish diameter and in array diameter), we can see some satellites no matter where we point.
- The good news is that for such satellites, moving the telescope away from the intended source does not make the RFI go away. So if we move and the signal does not change, we know that the signal is not coming from the pointing direction and we classify it as RFI. This is fair, because if there were a SETI signal as strong as a satellite, it would have been discovered a long time ago!
- If the signal disappears in the first "off" measurement, perform many on/off trials done a few hundred different ways (100 ways times 100 seconds per trial = about a day) to be really really sure the signal is coming from the target direction. in the history of the SETI Institute, we have never gone beyond this stage. In fact, it is exceedingly rare (once a year?) that any signal persists after 3 on/off trials.
- Contact other observatories and ask, "Do you see what we see?"
Beyond step 5 we enter the realm of the international SETI protocol which lays out what to do if you find a very interesting signal.