Transcript Adaptive Filters for RFI Mitigation in Radioastronomy M. Kesteven Australia Telescope National Facility

Adaptive Filters for RFI
Mitigation in Radioastronomy
M. Kesteven
Australia Telescope National Facility
[email protected]
IVS Symposium In Korea
New Technologies in VLBI
Nov, 2002
Outline
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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
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INR : the ratio of Interference power to System noise,
in the reference channel.
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tc : the time scale of the stability of the coupling:
relative delay; multi-pathing; changing sidelobes.
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A2I : the ratio of interference power in the astronomy
channel to the interference in the reference channel.
Adaptive Filter – hardware version
Adaptive Filter - theory
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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 :
unfiltered RFI
Min pwr 
1.  INR
Adaptive Filter - features
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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
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The interference in each channel can be written:
V(t)  c(t) I (t)
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The coupling terms c(t), vary slowly, so can be
extracted from each cross-product:
 Va V   c c  II 
*
r
*
a r
*
Post-Correlation filter
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We combine three cross-products to get a good estimate
of the interference in the Astronomical channel.
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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 -
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RFI
SNR ~
Bt c
Tsys
A2I  1.
(A2I*Tsys)
Post-Correlation Filter
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Cancellation is exact (but noisy)
RFI in ast channel
Added noise ~ 2Tsys
RFI in ref channel
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Cancellation starts to fail when
INR ~
1.
Bandwidth * Integ cycle
A2I  1.
Post-Correlation Filter in an
ARRAY
Connected-element Arrays
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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
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RFI generally does not correlate over VLBI baselines,
so is less of a problem.
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It will appear as increased noise, and so degrade the
SNR.
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An adaptive filter, by removing the RFI, will improve
the SNR
Application to VLBI
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RFI is only a problem when strong –> adaptive filter
quite suitable
RFI
Tsys 
 Tsys 1 
Tsys
.. no filter
RFI
Tsys 
 Tsys 1 
.. filter
(1.  INR)Tsys
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The planned wide bandwidths (eg, 1 GHz) will be a
serious challenge to a simple adaptive filter.
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At the observatory : Filter just the affected sub-bands.
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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
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Adaptive filters work well in radioastronomy
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Post-correlation filters are preferred for
– Single dish spectroscopy
– Imaging arrays (connected element)
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Hardware adaptive filters suitable for VLBI