Adaptive Filters for RFI Mitigation in Radioastronomy

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Adaptive Filters for RFI Mitigation in
Radioastronomy

• M. Kesteven
• Australia Telescope National Facility
• michael.kesteven_at_csiro.au

IVS Symposium In Korea New Technologies in
VLBI Nov, 2002
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Outline

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

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Adaptive Filter (schematic)
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Single Dish - Autocorrelation
Reference antenna
Parkes 64m
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Synthesis Array Filtering
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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.

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Adaptive Filter hardware version
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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

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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

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Post-Correlation filter
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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

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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.
• NoiseRFI products are also removed.
• The signal/noise is set by the ratio of
  Correlated RFI to noise products -

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A2I 1.
(A2ITsys)
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(No Transcript)
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Post-Correlation Filter

• Cancellation is exact (but noisy)
• Cancellation starts to fail when

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A2I 1.
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Post-Correlation Filter in an ARRAY
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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.

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ATCA - 1503 MHz 4 MHz BW
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Before and after images
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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

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Application to VLBI

• RFI is only a problem when strong gt adaptive
  filter quite suitable

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• 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 (NoiseRFI) products
  from the visibilities.

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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