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RFI Mitigation. Steve Ellingson Virginia 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)

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rfi mitigation

RFI Mitigation

Steve Ellingson

Virginia Polytechnic Institute & State University

“Frontiers of Astronomy with the World’s Largest Radio Telescope” Meeting

September 13, 2007

rfi problems
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
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

emerging techniques
“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
l band radar blanking canceling
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)

slide6

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

model subtract method vs glonass
“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.

digital tv atsc
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

mitigation of broadcast fm
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
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

broadcast fm canceller demonstration
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

for more information
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.