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

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

Adaptive Filter (schematic)

Single Dish - Autocorrelation Reference antenna Parkes 64m

Synthesis Array Filtering

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.

Adaptive Filter – hardware version

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 :

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

Post-Correlation filter

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:

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 -

A2I = 1. (A2I*Tsys)

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

A2I = 1.

Post-Correlation Filter in an ARRAY

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.

ATCA - 1503 MHz; 4 MHz BW

Before and after images

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

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

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.

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