1. RFI Mitigation Steve Ellingson Virginia Polytechnic Institute & State University “Frontiers of Astronomy with the World’s Largest Radio Telescope” Meeting September 13, 2007

2. 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”

3. Lines of Defense Regulation / Frequency Coordination Avoidance Avoiding contaminated frequency bands, Scheduling to avoid satellites Analog Filtering / High Dynamic Range Receivers “Pre-detection” Signal Processing Excision: Detection & Blanking, Adaptive Filtering, Null Forming Canceling: -- “look through”; e.g., Model-and-subtract methods “Post-detection” Signal Processing Post-correlation & Cross-spectral techniques Post observation: Data Editing, Anti-Coincidence Real-Time Techniques Focus of this talk

4. “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

5. 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 MSPS A/Ds I/Q Conv., LPF, Pulse Blanker 1K FFT I/F to PC SDP, Integrate Implemented completely in Altera 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.

6. Mitigation of Iridum – Blanking vs. Nulling Argus Array of 24 spiral antenna elements T sys ~ 215 °K per element 1200-1700 MHz Tuning Range Ellingson, Hampson & Childers (2007), IEEE Trans. Ant & Prop., in press. Time Series Matched Filter Output Rank Detector Pulse Detector Top: RFI mit off Middle: Nulling Bottom:Blanking Detector: Total power pulse W = 8 ms  = 10  at PMF output Nulling: Projecting out estimated spatial signature of burst Cancel 56 ms window Start 16 ms before trigger No data loss Blanking: Blank 56 ms window Start 16 ms before trigger ~ 20% of data is blanked PSD calculation:  = 100 Hz  t = 10 ms  = 58.3 s Blanking: Better Performance; Nulling: No Data Loss. More info: Proc. RFI2004 http://www.ece.vt.edu/swe/rfi2004

7. “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 Maser IRAS 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

8. Digital TV (ATSC) ATSC NTSC ATSC test transmission spotted 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 “spectrally white” – blots out entire 6 MHz ATSC is being rolled out now Also, TV Ch 52-69 (698-746 MHz) moving to Ch 2-51 Emerging threat for VHF- and UHF-band radio astronomy Ch 2 Ch 4 Ch 5 FM Ch 3 Ch 6 MJD From Clegg briefing to CORF, 2006 Fall

9. Preliminary Work on ATSC Model & Subtract * Work of Kyehun Lee, VT

10. 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

11. 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.

12. Broadcast FM Canceller: Demonstration * Work of Kyehun Lee, VT Before / After Simulated signal (no channel effects) Before / After Off-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

13. For More Information l Recent summary paper for ITU on mitigation techniques (see Lewis or Ellingson) & references l RFI2004 on-line proceedings / Radio Science Special Section http://www.ece.vt.edu/swe/rfi2004 l A.-J. Boonstra, Ph.D. Dissertation, T.U. Delft l 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.