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RFI Mitigation
Steve EllingsonVirginia Polytechnic Institute & State University
“Frontiers of Astronomy with the World’s Largest Radio Telescope” Meeting September 13, 2007
RFI Problems
• Ourselves (not the topic of this talk, though…)
• Aviation Systems (UHF, L-, S-, and X-Band)• DME, Air Surveillance Radar, …
• Navigation Satellites (L-Band)• GPS (Three 20 MHz channels), GLONASS (16xx MHz)
• Communication Satellites (L-, C-, and K-Band)• Iridium (16xx MHz), INMARSAT, etc.
• TV: NTSC (analog) is becoming ATSC (digital, worse)
• Harmonic and IM products of FM, TV, mobile telephony, etc…• “Crud”
Lines of Defense
• Regulation / Frequency Coordination
• Avoidance Avoiding contaminated frequency bands, Scheduling to avoid satellites
• Analog Filtering / High Dynamic Range Receivers
• “Pre-detection” Signal ProcessingExcision: Detection & Blanking, Adaptive Filtering, Null Forming
Canceling: -- “look through”; e.g., Model-and-subtract methods
• “Post-detection” Signal ProcessingPost-correlation & Cross-spectral techniques
• Post observation: Data Editing, Anti-Coincidence
Real-TimeTechniques
Focus ofthis talk
“Emerging” Techniques• Radar Pulse Blanking
• Actually, “old school”; e.g., Arecibo pulse-pattern-synchronous blanker
• Many others have explored this; useful to some extent right now
• Effectiveness limited by detection sensitivity, multipath spread,
“mangled” (semi-correlated multipath) pulses – room for improvement
• More General: Time-Frequency Blanking• Useful esp. against “crud” if t x can be 1 ms x 1 kHz or better
• • Spatial Nulling (Arrays)
• Especially vs. satellites (ATA, some FPA concepts)
• Canceling• “Reference antenna” and “Model & subtract” methods – even for radar
• The low frequency “renaissance” is driving the testing /
implementation of all kinds of techniques
Real time mitigation of the notorious 1330/1350 MHz radar @ Arecibo using a digital receiver
Ellingson & Hampson (2003), ApJS, 147, 167.
Other good examples:Zheng et al. (2003), AJ, 126, 1588.Zheng et al. (2005), AJ, 129, 2933.Dong, Jeffs & Fisher (2005), Radio Sci., 40, RS5S04.
Against mobile telephony:Various papers by Leshem, van der Veen & Boonstra
200 MSPSA/Ds
I/Q Conv., LPF,Pulse Blanker
1K FFT
I/F to PC
SDP,Integrate
Implemented completely inAltera Stratix FPGAs
Before: Radar pulses corrupt spectrum
After: Radar pulses excised(~4% of the data is blanked)
L-Band Radar Blanking & Canceling
• Time window blanking is hard to beat, if you can tolerate the gaps and loss of integration time
• Typically limited by pulse detection performance.
Mitigation of Iridum – Blanking vs. NullingArgus
• Array of 24 spiral antenna elements• Tsys ~ 215 °K per element• 1200-1700 MHz Tuning RangeEllingson, Hampson & Childers (2007), IEEE Trans. Ant & Prop., in press.
Time Series
Matched Filter
Output
Rank DetectorPulse Detector
Top: RFI mit offMiddle: NullingBottom: Blanking
Detector:Total power pulseW = 8 ms= 10 at PMF output
Nulling:Projecting out estimated spatial signature of burstCancel 56 ms windowStart 16 ms before triggerNo data loss
Blanking:Blank 56 ms windowStart 16 ms before trigger~ 20% of data is blanked
PSD calculation: = 100 Hzt = 10 ms = 58.3 s
Blanking: Better Performance;Nulling: No Data Loss. More info: Proc. RFI2004
http://www.ece.vt.edu/swe/rfi2004
“Model & Subtract” Method vs. GLONASS
Ellingson, Bunton, and Bell (2001), ApJS., 135, 87.
Technique achieves > 20 dB canceling of the GLONASS interferer as follows:
Australia Telescope Compact Array (ATCA)Narrabri, NSW
Observations of OH MaserIRAS 1731-33
Corrupted by weak (-20 dB SNR) RFI from the Russian GLONASS
satellite system
• GLONASS signal is tracked (in the same fashion as a GLONASS receiver),
• Instrumental responses are adaptively estimated
• A noise-free model of the RFI is synthesized• RFI model is coherently subtracted from the
telescope output
Digital TV (ATSC)
ATSC NTSC
ATSC test transmissionspotted at VLA – Jul 07
Ch 3
ATSC
Craig County VA – Oct 2005
US federal law banishes NTSC in favor of ATSC by Feb 2009
ATSC is “spectrallywhite” – blots outentire 6 MHz
ATSC is being rolledout now
Also, TV Ch 52-69 (698-746 MHz)moving to Ch 2-51
Emerging threat forVHF- and UHF-bandradio astronomy
Ch 2
Ch 4
Ch 5
FM
Ch 3
Ch 6
MJD
From Clegg briefing to CORF, 2006 Fall
Preliminary Work on ATSC Model & Subtract
* Work of Kyehun Lee, VT
Mitigation of Broadcast FM?
• Strong source of RFI in 88-108 MHz (U.S.); surrogate for a very broad class of difficult-to-handle RFI across the VHF & UHF bands
• Bandwidth ~ 200 kHz
• Baseband is analog audio + many other components, total ~75 kHz: Processing gain!
• Simpler version used to convey audio in NTSC
• Prone to multipath; especially apparent in weak signal areas
A Canceller for Broadcast FM
* Work of Kyehun Lee, VT
Architecture
• Analyze band; determine # of signals & form coarse estimate of associated center frequencies
• Extract carriers one at a time, demodulate, estimate model parameters
• Reconstruct noise-free version using extracted model parameters
• Subtract synthesized carriers from telescope output.
Estimation Block
Looks complicated, but is only slightly more complex than a high-performance commercial FM receiver.
This version does not account for channel characteristics, such as mutlipath.
Broadcast FM Canceller: Demonstration
* Work of Kyehun Lee, VT
Before / AfterSimulated signal
(no channel effects)
Before / AfterOff-the-air signal
(includes channel effects)
Somewhat toxic: Current algorithm suppresses uncorrelated in-band spectral content (i.e., underlying radio astronomy) by about 40%.
Note detailed model knowledge helps a lot with this.
Prospects good for further improvement, especially with site diversity: Using information from sites closer to the transmitter.
Toxicity to simulated spectral line
([0..1]; 1 is perfectly safe)
Simple “Chirp” Model
Complete parametric
model
For More Information
Recent summary paper for ITU on mitigation techniques (see Lewis or Ellingson) & references
RFI2004 on-line proceedings / Radio Science Special Section http://www.ece.vt.edu/swe/rfi2004
A.-J. Boonstra, Ph.D. Dissertation, T.U. Delft
Non-technical discussion: Ellingson (2004), “RFI Mitigation and the SKA,” Exp. Astronomy, 17, 261. Reprinted in The Square Kilometre Array: An Engineering Perspective, P.J. Hall (ed.), Springer, 2005.