field trials of an rfi adaptive filter for pulsar observations m. kesteven, r.n. manchester, g....
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Field Trials of an RFI Adaptive Filter for Pulsar Observations
M. Kesteven, R.N. Manchester, G. Hampson & A. Brown
Australia Telescope National Facility
Groningen, 28 March 2010
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RFI mitigation. Groningen, 2010
Outline
• Introduction
• Adaptive Filter details
• Results
• Conclusions
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RFI mitigation. Groningen, 2010
Introduction
• The RFI mitigation work at the Parkes Observatory was given greater emphasis when one of the Parkes Pulsar observing bands became heavily polluted with RFI (digital TV).
• The prospects were favourable:• The levels were substantial, but not so large as to overload the
system.
• Some of the critical hardware needed was already in place.
• A new digital backend with reserve capacity came on-line.
• An adaptive filter works well with pulsar observing.
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RFI mitigation. Groningen, 2010
Introduction (II)
• The Parkes Observatory recently commissioned a new pulsar processing machine. This a digital polyphase filterbank that is capable of 8-bit sampling at a rate of 2GHz for an observational bandwidth of up to 1GHz. It supports a range of configurations including an online-folding mode with up to 2048 pulse phase and radio frequency bins.
• For observations in the 50cm (~700 MHz) band there is spare capacity which could readily be deployed for RFI mitigation.
• This takes the form of an adaptive filter in conjunction with a reference antenna directed towards a TV tower, the source of RFI for the receiver’s band.
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RFI mitigation. Groningen, 2010
The basic narrow-band adaptive filter
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RFI mitigation. Groningen, 2010
Filter details
)1(
1.
.
2
2
64
)(
)(
64646464
INRnattenuatiofilter
INR
gainfilter
VVV
VVVV
sysref
rfiref
ref
rfiref
rfim
V
V
P
VV
sysref
rfirefref
sysm
rfim
astmm
= residual RFI / unfiltered RFI
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RFI mitigation. Groningen, 2010
The broad band filter, as implemented
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RFI mitigation. Groningen, 2010
PDFB3: Real-time RFI Mitigation
Filter outputRef ant
RFI Mitigation Off
Cross-Correlation
• Reference antenna pointed at RFI source
• 2048 channels across band
• Signal*reference cross-correlation subtracted from data
• Filter updates at 1ms intervals to follow tropospheric multi-path propagation
• Single reference can be applied to both polarisations of telescope signal
• Main application (so far): Digital TV signals in 50cm band
• 4-m reference antenna pointed toward Mt Ulandra, 200 km south of Parkes
RFI Mitigation On!
64 MHz at 50 cm (655 – 717 MHz)
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RFI mitigation. Groningen, 2010
PSR J0437-4715 at 50cm
RFI Mitigation Off
RFI Mitigation On
S/N ~ 1040 in 4 min
S/N ~ 1710!
• Pulsar signal under RFI is recovered with no (evident) perturbation
• Improvement in S/N will be greater with weaker pulsars
• Expect to apply to regular 50cm observations with APSR soon
Stokes I
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RFI mitigation. Groningen, 2010
Filter OFF Filter ON
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RFI mitigation. Groningen, 2010
(operational details)
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RFI mitigation. Groningen, 2010
Operational details
dtVVgg ref
t
t
filtnn*
1
1
determines the speed of the filter in responding to changing conditions. In effect it sets the rate at which gn converges on the optimum setting. A value of[0.1/(mean IF power)] works well for us
needs to be smaller than time scale of the changing conditions in the propagation path. We use 1 msec.
The filter gain is computed as :
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Propagation Issues - movie
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RFI mitigation. Groningen, 2010
How well does the filter work?
1. Compare the observed and predicted attenuation of the RFI at the filter output.
There is a good match – the filter is working as designed.
2. Relate the residual RFI to the science requirements:
The rms in the pulse /phase plot has dropped to close to the rms of the RFI-free condition. The residual RFI after filtering is less than 10% of the receiver bandpass.
3. The pulse shape is not affected by the filter, neither inside nor outside the bands affected by RFI.
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RFI mitigation. Groningen, 2010
Filter performance (1)
RFI at the filter output
RED : filter OFF BLUE : filter ONBLACK : 10% of the receiver bandpass
RFI in the reference antenna
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RFI mitigation. Groningen, 2010
Filter performance(2) : the RFI attenuation
RED : the measured attenuationBLUE : the attenuation predicted from the INR in each channel
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Adaptive Filter Performance (1)
Adopting the RA-769 criteria we can estimate the limitations of the adaptive filter.
1. The ratio of the RFI powers in the main and reference antennas is given by :
= (Area of 0 dBi antenna) / (Area of the reference antenna)
= (R)2 [= 0.0016 at 600 MHz, 4m ref antenna]
2. The residual power (due to the RFI) in the filtered output :
resid = * Tsys * INR / (1 + INR)
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3. resid tends to *Tsys for large INR. (~0.0016*Tsys)
4. The filter starts to fade out at INR ~ 1, with resid ~ *Tsys/2
5. Thereafter resid falls gracefully to 0 as the INR decreases.
NOTE : Tsys is the system temperature of the reference antenna
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RFI mitigation. Groningen, 2010
Current Status
• The filter is embedded in the pulsar processor, and is available for on-line operations.
• However, the receiver has been tuned to a new frequency –
Planned nearby transmitters would violate the linearity requirements, so for continuity in the pulsar timing, prudence dictated a re-tune.
• We still have RFI to contend with, from a different direction, and with less RFI.
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RFI mitigation. Groningen, 2010
New 50 cm environment : Peak Hill
WHITE: Unfiltered RED: Reference
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RFI mitigation. Groningen, 2010
Peak Hill: unfiltered filtered
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Conclusions
0. The filter works well.
1. The filter meets RA.769 in that the residual RFI is below the 10%Tsys.
2. It would satisfy RA.769 for VLBI, and for most pulsar work.
3. It is not so clear for a pulsar blind search mode : low frequency modulation in the range 1 ms to 5 sec could compromise the period search machine.
4. It would probably not work for spectroscopy.
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RFI mitigation. Groningen, 2010
Contact UsPhone: 1300 363 400 or +61 3 9545 2176
Email: [email protected] Web: www.csiro.au
Thank you
Australia Telescope National FacilityMichael Kesteven
Phone: 61 2 9372 4544Email: [email protected]: www.csiro.au/group