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RFI Subtraction with a reference horn… F. Briggs & M. Kesteven

RFI Subtraction with a reference horn… F. Briggs & M. Kesteven. illustrations from Parkes Telescope… (Jon Bell et al.) origins in Int-Mit group at CSIRO – ATNF. S. g A S. S. - g 1 I. I. g 1. Signal Processing. X. g 3. g 3 I. I. g A S. + g 1 I. R x. Cross Correlation. R F I.

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RFI Subtraction with a reference horn… F. Briggs & M. Kesteven

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  1. RFI Subtraction with a reference horn…F. Briggs & M. Kesteven • illustrations from Parkes Telescope… (Jon Bell et al.) • origins in Int-Mit group at CSIRO – ATNF

  2. S gAS S - g1I I g1 Signal Processing X g3 g3I I gAS + g1I Rx Cross Correlation

  3. R F I

  4. I Signal radiated by transmitter … with DELAY ta Impulse Responseh(t) for each path a Convolution operator Reference Antenna I(t) = Sha(t) *io(t-ta)

  5. I(t) = Sha(t) *io(t-ta) Time Domain a Frequency Domain Reference Antenna I I(f) = (SGa(f) e-i2p taf ) Io(f) = G(f) Io(f)

  6. G1 G1 G1G4* | Io2| = ?? G3 G3 G3G4* | Io2| Cross correlation: single polarization feed with 2 reference signals 1. Pol A …G1G4* | Io2| 3. Ref 1 G3G4* | Io2| 4. Ref 2

  7. C14 (f) = C34 (f) Cross correlation: single polarization feed with 2 reference signals 1. Pol A = C14 …G1G4* | Io2| 3. Ref 1 = C34 G3G4* | Io2| 4. Ref 2 G1G4* | Io2| G1 = G3 G3G4* | Io2|

  8. g1g3*|I2| g4g1*|I2| g4g3*|I2| C13(f)C14*(f) C34*(f) = Advantage: Cross Correlation Spectra a NO BIAS due to NOISE power Practical Application: Auto-Correlation Spectrometer Power Spectrum P(f) S |gA2||S2| + |g12||I2| A/C spectrometer I |g12||I2| =

  9. Cross correlation: single polarization feed with 2 reference signals 1. Pol A G1G3* | Io2| = C13 = C14 G1G4* | Io2| 3. Ref 1 = C34 G3G4* | Io2| 4. Ref 2 C13(f)C14*(f) C34*(f) A/C Spectrum Contamination |g12||I2| =

  10. Cross correlation: dual polarization feed with 2 reference signals 1. Pol A = C13 G1G3* | Io2| 2. Pol B = C14 G1G4* | Io2| = C23 G2G3* | Io2| = C24 G2G4* | Io2| 3. Ref 1 = C34 G3G4* | Io2| 4. Ref 2

  11. Pol A frequency Pol B Duration 56 s with 0.1 s steps Bandwidth 5 MHz Raw Dynamic Spectra: Time

  12. Pol A frequency RFI not in reference horn is NOT subtracted ! Pol B Canceled Dynamic Spectra: Time Non-Toxic to celestial signals

  13. Estimate { g1I(t) } = x13(t) *g3I(t) 1 Df Update every second C14(f) C34(f) stable on 0.1 second time scale Ref Horn Spectral Domain Contamination: the VOLTAGE spectrum Estimate { g1(f) I(f) } = X13(f) g3(f) I(f) Time Domain Contamination: Effectively… FIR filter coefficients

  14. FIR Coefficients Delay 0.1ms steps + Time 0.1 sec steps

  15. Pol A Pol B using Ref. “4” using Ref. “3”

  16. Average Spectra Dynamic Pulsar Spectra Pol. B Pol. A Phase Frequency

  17. Cancellation Applied Average Spectra Dynamic Pulsar Spectra Pol. B Pol. A Phase Frequency

  18. SNR RAW uncorrected self- normalized Self-Norm cancelled 1 Cancelled 2 Cancelled INTEGRATED PULSE PROFILES Pulse Phase

  19. Self-Normalized byTotal Power Spectra Dynamic Pulsar Spectra Pol. B Pol. A Phase Average Spectra Frequency

  20. VLBI

  21. VLBI Jodrell Bank Westerbork

  22. nearby TV station HI absorption z = 0.68 Power Spectrum at 840 MHz at Jodrell Bank Cross Correlation Spectrum Jodrell Bank x Westerbork 1504+377

  23. RFI VLBI recorder off-line correlation VLBI recorder Celestial Signal + post- correlation processing ….

  24. Conclusions • complications of multi-path are contained in • complex gains G(f) • adaptive filtering with correlation functions • preserves phase information…. • … equivalent to subtraction of the voltage waveform

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