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 FormingCanceling: -- “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 ∆ν 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 @ Arecibousing a digital receiverEllingson & 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

1KFFT

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 msβ = 10σ at PMF output

Nulling:Projecting out estimatedspatial signature of burst

Cancel 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:∆ν = 100 Hz∆t = 10 msτ = 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 3ATSC

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 KilometreArray: An Engineering Perspective, P.J. Hall (ed.), Springer, 2005.