1. An RFI Mitigation Strategy for the Allen Telescope Array Geoffrey C. Bower UC Berkeley

2. The Allen Telescope Array 350 element interferometer 6.1 m offset Gregorians 0.5-11.2 GHz Science goals – SETI – HI survey machine – Transient radio sources

3. Strategy Overview Know the enemy Put many arrows in the quiver – No Magic Bullets Exploit large N capability Implement at all levels

4. Available Tools Modeling of environment Measurement Real time strategies

5. RFI Modeling: Hat Creek Terrain

6. RFI Modeling: Free Space 3 x 3 degree region centered at Hat Creek Contours of path loss Visualyse simulation

7. RFI Modeling: ITU-R P.526 Model includes topography, diffraction, tropospheric scatter, ducting/layer reflection, fade Future modeling: frequency, scale, model parameters, Longley-Rice

8. RFI Monitor Simple design – Discone antenna – Broadband amp – Spectrum analyzer – Linux PC on ‘net 0.1 to 10 GHz – 0.3 to 3 GHz currently Isotropic sensitivity

9. … (One of) the Enemies is Us

10. Is Glonass Present at 1612 MHz?

11. Tracking Glonass

12. …The Enemy is Us, Part 2

13. And Tracking GPS

14. Aircraft DME

15. RFI Sources Detected at Hat Creek Self-interference Satellites --- GPS, Iridium, DARS Aircraft Radar TV, cell phone, pager Microwave ovens

16. Real Time RFI Mitigation Monitoring Artificial horizons around known interferers Time Blanking Adaptive canceling Interferometric Nulling Postcorrelation Analysis

17. Time Blanking: Radar

18. Time Blanking: DME

19. Time Blanking Implementation Done in backends Correlator will have – 100  sec precision – External & Internal Triggers – Channel, Antenna Configuration Undetermined

20. Adaptive Canceling

21. Rapid Prototyping Array

22. Cancellation of Glonass

23. Cancellation of GPS

24. ATA Implementation of Canceling Multiple reference antennas (~10) Correlator/black box operating on phased- array signal Wiener or LMS solutions

25. Interferometric Nulling: Two Slits

26. Interferometric Nulling

27. Nulls with the ATA

28. Muliple Nulls

29. Nulls in Frequency Space

30. The Wages of Nulling Requires large-N to be effective Loss in SNR with number of nulls Must be able to update complex gains at ~100 Hz Matrix inversion is computationally expensive

31. Postcorrelation Analysis Examine output of correlator/visibility matrix ~100 element PC cluster Data rate: – 350x349/2 – 4 Stokes – 1024 Channels – 16 bit – 100 Hz sampling  400 Gbits/sec: too much!

32. How to Use the Tools Blanking – Radar, Aircraft, intermittent signals Adaptive Canceling – Reference antenna(s) on transmitter Interferometric Nulling – Predictable trajectories: satellites, fixed transmitters Postcorrelation Analysis – Broadly adaptable

33. Future Steps Algorithm development More modeling of environment Implement canceller, external blanker Angle of arrival methods