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RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 1 -
LOFAR RFI Mitigationspatial filtering at station level
Albert-Jan BoonstraMark Bentum
Mathheijs Eikelboom
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 2 -
Contents
• LOFAR overview• Spectrum environment• RFI mitigation in LOFAR• Data model, spatial filtering algorithm• Spatial filtering in LOFAR, considerations• Spatial filter results• Conclusion
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 3 -
LOFAR signal processing, overview
LOFAR: fsky ~ 30 – 240 MHz
BlueGenecentral
ProcessorCEP
(correlator)
LBA
HBA
RSP
receiver
antennabeam
stationbeams
synth.beams
1 x 32 MHz
High Band Antenna
Low Band Antenna
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 4 -
LBA antenna layout
Operational Oktober 1, 2006 with “final” prototype hardware at Exloo96 dual-dipole LBA antennas distributed over ~500m:• one cluster with 48 dipoles • three clusters of 16 dipoles
Total 24 microstation, 4 dipoles each
Goal: emulate LOFAR with 24 micro-stations at reduced bandwidth or act as a single station at full BW
Exloo
R.Nijboer 2006
LOFAR CS1 configuration 2006-2008 – Exloo
LBA CS10
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 5 -
LOFAR status
Stations 18 core stations + 18 remote stations + 8 int.
Validated: 14 CR, 6 RS
In progress: 6 CS, 1 RS, 3 German, 1 French
Next: 9 RS, 1 UK, 1 Germany, 1 Sweden
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 6 -
LOFAR numbers
Number of sensors on the various fields:Core station fields (18)
96 Low Band Antennas, 2 x 24 High Band Antenna Tiles (HBA field is split)Remote station fields (18)
96 Low Band Antennas, 48 High Band Antenna TilesMicrobaromater (infrasound)
Geo-Remote station fields (10)Geophones & Microbarometers International station fields (8)
96 Low Band Antennas, 96 High Band Antenna Tiles
Numbers for the LOFAR telescope performanceFrequency range: 30- 80 MHz and 120 - 240 MHzPolarisations 2Bandwidth 32 MHz (currently 48 MHz investigated)Stations: 18 core, 18 remote, 8 internationalBaseline length: 100 m to 1500 kmSimult. dig. beams: 8Sample bit depth: 12Spectral resolution: 0.76 kHz
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 7 -
Some LOFAR imaging results
High-resolution LOFAR 3C61.1 imageCredit: Reinout van Weeren (Sterrewacht Leiden) 8 feb 2010
Cas A, Sarod Yatawatta 23 Dec. 2009
LOFAR HBA tile all-sky imageMichiel Brentjens, 22 nov. 2007
20102009
2008
2007
LOFAR all-sky imageStefan Wijnholds19 Nov. 2008
LOFAR LBA alll-sky image Sarod Yatawatta & Jan Noordam 20 April 2007
Deep LOFAR HBA Image Sarod Yatawatta, 21 Feb. 2008
LOFAR all-sky imageStefan WIjnholds25 June 2006
20062007
2008
2004
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 8 -
Spectrum environmentSpectrograms 2009 FM band IIAM
TV band I
Lopik
weathersat.
DABTV#6,7,...
pagerambu-lance,taxi
mariphone
geostat.mil.
satelliteTV band III / DAB (DVB)
FM band II
aviationRAS
RASland
mobileland mobilemobileland
mobile
frequency (MHz)
frequency (MHz)
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 9 -
R
LOFAR overview
spectral estimation:multiply one arm per interferometerwith:
eit
Rclean = R-R
spectral estimation:
derive for AM from R
Spatial filtering:wnew = P w
LOFAR stationon-line correlator
One covariance matrix R per second
512 subbands correlated in ~8.5 min.
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 10 -
The radio spectrum: occurrence of weak RFI
1 minute: ~ 0.02 dB
relatively few weak RFI sources:“horizon effect”
LOFAR High Band Antennavar(R11) = 4 / N
Tsky = 333 KNf = 256Nt = 300 = 60 st = 5 hf = 0.76 kHz
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 11 -
The radio spectrum: effect on data transport rate
Purpose: - increase the number of beams from 8 to 24 (both for 24 MHz bw)- Without increase of station output data rate- Solution: reduce data rate to the LOFAR central processor
from 16 to 4 bits (complex) for each beamLoss when using 4 bit could be solved by spatial filters at stations, but only for fixed transmitters (“fast moving nulls” would hamper calibration)
Experiments in cleanest part of the spectrumindicated that < 10% of the data would be lost(no spatial filtering applied).
e.g. L2007-0189525 HBA bands:In 3 of 23 bands:loss @ 16 bits: 0% @ 4 bits ~ 50%In 20 of 23 bands:no lossAverage loss: 6.5%
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 12 -
Requirement: “smooth” station beamshape changes
Credit: Sarod Yatawatta, ASTRON
LOFAR CS1 calibration/imagingObservation:
• 16 single-dipole stations, 48 h• 20 subbands, each 0.14 MHz
“Calibrated”: removing phase drift (uv)“Residual”: peeling CasA and CygA
LOFAR ITS 2004 observations60 antennas, 26.75 MHz, basel.<200 m• with transmitter (left)• after subtraction filtering (right)• after projection filtering (middle)
LOFAR spatial filtering• At stations, filters for fixed directions (at subband level, ~ 200 kHz)• Post correlation: offline spatial filtering (in ~1 kHz channels)
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 13 -
Data model – signal model
Consider an array of p antennas with baselines :
bij = ri-rj
Array output signals xi(t) and the noise signals ni(t) (from LNAs, spillover etc) are stacked in a vector:
x(t) = [x1(t), … , xp(t)]t, n(t) = [n1(t), … , np(t)]t
Suppose there is one source (astronomical or RFI) with signal s(t) from direction s, and with spatial signature vector a:
The signal vector is defined by:
x(t) = a s(t) + n(t)
t
t
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 14 -
Data model – covariance model
Define the signal covariance sample estimate (observational data):
R = xn xnH with xn = x (nTs)
Given i.i.d. noise vector n(t), E{n(t)n(t)H} = n2 I, and E{s(t)}2 = 2:
R = E{R} = 2 a aH + n2 I
Data model easily exended to multiple sky sources and multiple RFI sources
Complication: low frequency sky contains strong extended structures
Solution: extend model, use baseline restrictions, factor analysis algorithmsHowever: this is not always a problem, e.g. in estimating DOA of strong RFI
n=1
N
^ ^
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 15 -
Data model with sky sources having covariance Rv, and interference with power r2
and signature vector ar: R = Rv + n
2 I + r ar arH
Spatial filtering using projections
Projection matrix: P = I – ar (arHar) -1 ar
H note: Par = 0, Pa ≠ 0
Applying projection: R = P R P
Spatial filtering using subtraction:
R = R – r2 ar ar
H
Note: the subtraction filter can be rewritten as a projection filter by adding a scaling factor , dependent on the noise and on the RFI power:
P = I – ar (arHar)-1 ar
H
cf. A. Leshem, A.J. van der Veen, and A.J. Boonstra. Multichannel interference mitigation techniques in radio astronomy. The Astrophysical Journal Supplement Series, 131(1):355–373, November 2000.
Spatial filtering after correlation
~ ^
~ ^
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 16 -
Beamforming and spatial filtering
narrow band beamforming
Source power: IB = Ryy = E{yyH} = wHE{xxH}w = wHRw=> station sky map Ib(s)
Recall data model: R = s2 aaH + n
2 I
Maximum if w = a: wHRw = s2 wHa aHw + n
2 wHw
beamfomer output y=wHxto central processor (BlueGene)
local processing: station correlator, one second integrated R every 512 seconds- used for station calibration and RFI mitigation
Spatial filtering (beamformer impl.), with Pa spatial filter: w’ = P wAnd: IB = wH PRP w
… y = wHx
x1
x2
xp
w1
w2
wp
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 17 -
How to find DOA and time-occupancy
DOA:• From a subspace analysis, R =U U:
• Finding maxima in sky maps• Transmitter locations may be known• Using factor analysis, efficient rank-one methods
ITS data
Credit: M.Tanigawa& M.Moren
How to assess the time-occupancy oftransmitters: sorting eigenvalue spectra and make daily percentile plots of number of eigenvalues above threshold
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 18 -
Spatial filtering, LBA station at 50.2 MHz
One-hour LOFAR LBA spectr (left) and one-day duration Frobenius norm spectrogram (right).
Data: 1 second integrated LOFAR station subbands, every subband is updated once per 512 seconds
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 19 -
Spatial filtering, LBA station at 50.2 MHz
LBA station eignevalues @ 50.2 MHz (upper)
LBA station spectrogram (upper left) and same data after spatially filtering, based on first time slot at 50.2 MHz (lower left)
After one hour a second transmitter at a different direction emerges
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 20 -
Integration time = 60 s
Filter: one dimension is projected out
Spatial filter suppession: 20 dB (right figure)
2nd obs: second 60 sample @ filter of previous time slot, result: no suppression
Note: HBA station data is correlated by CEP, forming ~1kHz channels
Spatial filtering, HBA station at 143.75 MHz
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 21 -
Spatial filtering, HBA station at 131.25 MHz
Fixed spatial projection filter estimated from and applied to first 60 s integration time (right)• 16 dB suppression, one subspace dimension removed• 38 dB suppression after two dim. Removed (not shown)• 4 dB supp using spatial filet of first time 60 s slot (not shown)Air traffic band: moving transmitter or strong changes in propagation
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 22 -
Spatial filtering, HBA station at 185.02 MHz
Fixed spatial projection filter estimated from and applied to first 60 s integration time (right)• 10 dB suppression, one subspace dimension removed• 6 dB supp using spatial filter of first time 60 s slot (not shown)• 1 dB supp using spatial filter after one hour (not shown)Somewhat erratic suppression numbers over time
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 23 -
fixed spatial projection filter (one dim) estimated from and applied to first 60 s integration time (upper right), and applied after 8 hours (right)
Spatial filtering, HBA station at 225.04 MHz
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 24 -
Channel without RFI(phase fluctuations partly due tot sky) Channel with RFI
Spatial filtering, HBA station at 225.04 MHz
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 25 -
LOFAR Core Station, LBA antennas
subspace analysis:eigenvalues
Frobenius norm spectrogram
October 2009
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 26 -
Core station: off-line spatial filtering
spectra before filtering spectra after filtering
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 27 -
Core station: off-line spatial filtering
before filtering after filtering, filter update every snapshot
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 28 -
Core station: off-line spatial filtering
after filtering, only one filter settingafter filtering, filter update every snapshot
RFI2010 Workshop, Groningen, Nl, March 29-31, 2010 - 29 -
Conclusions and next steps
• No accumulation observed of weak RFI @ -240 dBWm-2Hz-1 levels (horizon effect)
• Experiments indicated that station output signal data rate reduction (16 to 4 bit) would lead to < 10% data loss in cleanest parts of the spectrum
• Fixed spatial filters can be applied at station level to suppress fixed transmitters– LOFAR systems are stable enough, performance will be
improved by applying station calibration
• Coming year: RFI direction inventory
• At some later stage: reconsider station spatial filter updates every second using filters with constraints at the direction of the strongest “peeling sources”