RFI Mitigation

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 Dt x Dn 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. L-Band Radar Blanking & Canceling 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/F to PC SDP, Integrate 1K FFT I/Q Conv., LPF, Pulse Blanker Implemented completely in Altera Stratix FPGAs • Time window blanking is hard to beat, if you can tolerate the gaps and loss of integration time • Typically limited by pulse detection performance. Before: Radar pulses corrupt spectrum After: Radar pulses excised (~4% of the data is blanked)

6. Mitigation of Iridum – Blanking vs. Nulling • Argus • Array of 24 spiral antenna elements • Tsys ~ 215 °K per element • 1200-1700 MHz Tuning Range • Ellingson, Hampson & Childers (2007), IEEE Trans. Ant & Prop., in press. • Top: RFI mit off • Middle: Nulling • Bottom: Blanking • Detector: • Total power pulse • W = 8 ms • b = 10s 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: • Dn = 100 Hz • Dt = 10 ms • = 58.3 s Time Series Rank Detector Matched Filter Output Pulse Detector Blanking: Better Performance; Nulling: No Data Loss. More info: Proc. RFI2004 http://www.ece.vt.edu/swe/rfi2004

7. “Model & Subtract” Method vs. GLONASS 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 Technique achieves > 20 dB canceling of the GLONASS interferer as follows: • 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 Ellingson, Bunton, and Bell (2001), ApJS., 135, 87.

8. Digital TV (ATSC) 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 ATSC NTSC FM Ch 6 Ch 5 Ch 4 Ch 3 Ch 2 From Clegg briefing to CORF, 2006 Fall Ch 3 ATSC MJD Craig County VA – Oct 2005 ATSC test transmission spotted at VLA – Jul 07

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 • 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. * Work of Kyehun Lee, VT

12. Broadcast FM Canceller: Demonstration Complete parametric model Simple “Chirp” Model Before / After Simulated signal (no channel effects) Before / After Off-the-air signal (includes channel effects) Toxicity to simulated spectral line ([0..1]; 1 is perfectly safe) 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. * Work of Kyehun Lee, VT

13. 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.