Transcript RFI Mitigation Techniques for RadioAstronomy

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
Outline
• Introduction
• Adaptive Filter details
• Results
• Conclusions
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.
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.
RFI mitigation. Groningen, 2010
The basic narrow-band adaptive filter
RFI mitigation. Groningen, 2010
Filter details
V64 m  V64astm  V64rfim  V64sysm
Vref  Vrefrfi  Vrefsys
filter.gain 
INR 
rfi
V64rfimVref

Pref
rfi 2
(Vref
)
sys 2
(Vref
)
1
filter.attenuation 
(1  INR)
RFI mitigation. Groningen, 2010
= residual RFI / unfiltered RFI
The broad band filter, as implemented
RFI mitigation. Groningen, 2010
PDFB3: Real-time RFI Mitigation
64 MHz at 50 cm (655 – 717 MHz)
RFI
Mitigation
Off
RFI
Mitigation
On!
Filter output
Ref ant
Cross-Correlation
RFI mitigation. Groningen, 2010
• Reference antenna pointed at
RFI source
• 2048 channels across band
• Signal*reference crosscorrelation subtracted from data
• Filter updates at 1ms intervals
to follow tropospheric multipath 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
PSR J0437-4715 at 50cm
Stokes I
RFI Mitigation On
S/N ~ 1710!
RFI Mitigation Off
S/N ~ 1040 in 4 min
• 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
RFI mitigation. Groningen, 2010
Filter OFF
RFI mitigation. Groningen, 2010
Filter ON
(operational details)
RFI mitigation. Groningen, 2010
Operational details
The filter gain is computed as :
g n  g n 1  
1

t 
*
V
V
 filt ref dt
t
 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.
RFI mitigation. Groningen, 2010
Propagation Issues - movie
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.
RFI mitigation. Groningen, 2010
Filter performance (1)
RFI at the filter output
RED : filter OFF
BLUE : filter ON
BLACK : 10% of the receiver bandpass
RFI mitigation. Groningen, 2010
RFI in the reference antenna
Filter performance(2) : the RFI attenuation
RED : the measured attenuation
BLUE : the attenuation predicted from the INR in each channel
RFI mitigation. Groningen, 2010
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 :
z = (Area of 0 dBi antenna) / (Area of the reference antenna)
z = (l/2pR)2
[= 0.0016 at 600 MHz, 4m ref antenna]
2. The residual power (due to the RFI) in the filtered output :
resid = z * Tsys * INR / (1 + INR)
3. resid tends to z *Tsys for large INR. (~0.0016*Tsys)
4. The filter starts to fade out at INR ~ 1, with resid ~ z *Tsys/2
5. Thereafter resid falls gracefully to 0 as the INR decreases.
NOTE : Tsys is the system temperature of the reference antenna
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.
RFI mitigation. Groningen, 2010
New 50 cm environment : Peak Hill
RFI mitigation. Groningen, 2010
WHITE: Unfiltered
RED: Reference
Peak Hill: unfiltered
RFI mitigation. Groningen, 2010
filtered
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.
Australia Telescope National Facility
Michael Kesteven
Phone: 61 2 9372 4544
Email: [email protected]
Web: www.csiro.au/group
Thank you
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RFI mitigation. Groningen, 2010