Adaptive Filters for RFI Mitigation in Radioastronomy

1. Adaptive Filters for RFI Mitigation in Radioastronomy M. Kesteven Australia Telescope National Facility michael.kesteven@csiro.au IVS Symposium In Korea New Technologies in VLBI Nov, 2002

2. Outline • What is an adaptive filter? • How can it help radioastronomy? • Some implementations • Application to VLBI

3. Adaptive Filter (schematic)

4. Single Dish - Autocorrelation Reference antenna Parkes 64m

5. Synthesis Array Filtering

6. Critical Parameters • INR : the ratio of Interference power to System noise, in the reference channel. • tc : the time scale of the stability of the coupling: relative delay; multi-pathing; changing sidelobes. • A2I : the ratio of interference power in the astronomy channel to the interference in the reference channel.

7. Adaptive Filter – hardware version

8. Adaptive Filter - theory • Balance Increasing filter gain to improve RFI cancellation • Against Decreasing filter gain to reduce added noise from reference channel receiver. • Optimum leads to residual power with the RFI signature :

9. Adaptive Filter - features • Robust, automatic tracking of changing propagation characteristics • No added noise when RFI disappears • Multi-pathing handled correctly • Can treat multiple sources of RFI provided there is no frequency overlap. • Cancellation starts to fail when INR ~ 1

10. Post-Correlation filter

11. Post-Correlation filter • The interference in each channel can be written: • The coupling terms c(t), vary slowly, so can be extracted from each cross-product:

12. Post-Correlation filter • We combine three cross-products to get a good estimate of the interference in the Astronomical channel. • No total power products in the cross-products, thus no bias. • Noise*RFI products are also removed. • The signal/noise is set by the ratio of Correlated RFI to noise products -

13. A2I = 1. (A2I*Tsys)

15. Post-Correlation Filter • Cancellation is exact (but noisy) • Cancellation starts to fail when

16. A2I = 1.

17. Post-Correlation Filter in an ARRAY

18. Connected-element Arrays • Correlator requirements: the reference antenna amounts to one additional station in the array. • The cancellation is enhanced by the phase tracking machinery. • The RFI mitigation is most important on the short calibration observations.

19. ATCA - 1503 MHz; 4 MHz BW

20. Before and after images

21. RFI and VLBI • RFI generally does not correlate over VLBI baselines, so is less of a problem. • It will appear as increased noise, and so degrade the SNR. • An adaptive filter, by removing the RFI, will improve the SNR

22. Application to VLBI • RFI is only a problem when strong –> adaptive filter quite suitable

23. The planned wide bandwidths (eg, 1 GHz) will be a serious challenge to a simple adaptive filter. • At the observatory : Filter just the affected sub-bands. • Transport the reference IF to the correlator: only two additional cross-spectra required for each RFI source. Remove (Noise*RFI) products from the visibilities.

24. Conclusions • Adaptive filters work well in radioastronomy • Post-correlation filters are preferred for • Single dish spectroscopy • Imaging arrays (connected element) • Hardware adaptive filters suitable for VLBI