Using Multiple Beams to Distinguish RFI from ETI Signals

1. Using Multiple Beams to Distinguish RFI from ETI Signals Gerry Harp Allen Telescope Array, SETI Institute, Mountain View, CA 4. Trick: Four Simultaneous Beams 7. How does it work with fewer beams? 10. Fewer Antennas: ATA-32 1. Allen Telescope Array (ATA-350) 350 x 6m dishes RF = 0.5 – 11.2 GHz IF = 4 x 100 MHz Funded by Paul Allen and Nathan Myhrvold Developed jointly by SETI Institute and U. C. Berkeley RAL Do SETI on 4 different stars at once. Sidelobes from each beam are uncorrelated. RFI will usually appear in more than one beam – can be discarded. Rejection Ratio vs. Declination Even with only 2 beams, RFI is well identified. ATA-3 (now), ATA-32 (2005) ATA-350 (2008) Sidelobes are fatter and less complex, but technique is still fairly effective. 8. Frequency Dependence 2. Synthetic Beam 5. Snapshot Observation • 11. ConclusionsMultibeam correlation is a great way to exclude RFI from SETI observations.ATA supports up to 4 simultaneous beams, gives high confidence that RFI will appear in more than one beam.Not shown: It is important to average power, not amplitudes, when averaging snapshots (please refer to paper). 0 dB Rejection Ratio vs. Frequency Place Beam on Star P = -5 db  50% chance that RFI is 3 dB higher in one beam than all others. RFI Enters In the Sidelobes Lower is better. Circle: ATA Primary Beam -50 dB If sidelobe is high, RFI looks like ETI signal from star. Better at higher frequencies since sidelobes are smaller. 3. How often does this happen? 6. Better: Take many snapshots while source moves. 9. Declination Dependence Rejection Ratio vs. Declination P = -50 db  0.001% chance that RFI is 3 dB higher in one beam than all others. Gather statistics from beam simulations. It is very unlikely that RFI can masquerade as ETI signal. Best at low declination since source moves faster.