Real-time RFI Mitigation for the Upgraded GMRT Kaushal D. Buch - - PowerPoint PPT Presentation

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Real-time RFI Mitigation for the Upgraded GMRT Kaushal D. Buch - - PowerPoint PPT Presentation

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Real-time RFI Mitigation for the Upgraded GMRT Kaushal D. Buch Digital Backend Group, Giant Metrewave Radio Telescope, NCRA-TIFR, Pune, India kdbuch@gmrt.ncra.tifr.res.in The (Upgraded) GMRT Giant Metrewave RadioTelescope (GMRT) Array

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Real-time RFI Mitigation for the Upgraded GMRT

Kaushal D. Buch Digital Backend Group, Giant Metrewave Radio Telescope, NCRA-TIFR, Pune, India kdbuch@gmrt.ncra.tifr.res.in

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The (Upgraded) GMRT

Giant Metrewave RadioTelescope (GMRT)

Array consisting of thirty 45m diameter parabolic reflector antennas A highly sensitive radio receiver system for observing astrophysical phenomena at

low radio frequencies

Upgraded GMRT (uGMRT)

8/27/2017 CASPER-2017, Caltech, USA 2

Upgraded GMRT (uGMRT)

Nearly seamless frequency coverage from 50 to 1450 MHz Instantaneous receiver bandwidth from 32 MHz to 400 MHz Increased bandwidth and receiver sensitivity to encounter increasing levels of man-

made radio frequency interference (RFI)

uGMRT 250-500 MHz and L-band released for observations on shared-risk basis http://gmrt.ncra.tifr.res.in/gmrt_hpage/Users/doc/GMRT-specs.pdf

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Major Sources of RFI at GMRT

Broadband RFI

Sparking on power-lines Corona Discharge Automobile sparking

11 kV transformers around S-arm of GMRT array

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Narrowband RFI Communication

transmitters

Broadcast TV / Radio Satellites

Data Courtesy: Pravin Raybole, RFI Group, GMRT

8/27/2017 CASPER-2017, Caltech, USA

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SLIDE 4

RFI at GMRT

Broadband RFI RFI Broadband RFI is stronger at lower radio frequencies RFI

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Narrowband RFI Examples showing impulsive time and frequency domain RFI observed at the GMRT

CASPER-2017, Caltech, USA

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Need for Real-time Excision

Temporally impulsive RFI: Energy spreads post-FFT hence

excision is needed before FFT.

Power-line RFI: Low duty cycle but high spectral occupancy RFI is correlated in closely spaced antennas

Spectrally impulsive RFI: RFI excision useful for low time

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Spectrally impulsive RFI: RFI excision useful for low time

  • ccupancy

Best possible time resolution: reduction in loss of astronomical

data due to flagging (trade-off)

Leads to improvement in the receiver sensitivity

8/27/2017 CASPER-2017, Caltech, USA

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Location of Real-time Excision blocks

FFT: Fast Fourier Transform MAC: Multiply & Accumulate

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RFI excision at multiple locations in the receiver chain (This talk will focus on real-time RFI for the uGMRT correlator) IA: Incoherent Array PA: Phased Array

8/27/2017 CASPER-2017, Caltech, USA

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SLIDE 7

GMRT Wideband Digital Backend (GWB)

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Real-time broadband RFI Mitigation is implemented on ROACH-1 FPGA board GWB: Reddy et al., JAI 2017

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Robust Detection using MAD

  • RFI in astronomical data – outliers make Gaussian distribution heavy-tailed

(Fridman, 2008)

  • Excision (Baan, 2001, 2010) assumes that RFI is much stronger than the

astronomical signal

  • Robust threshold using Median Absolute Deviation for RFI detection

(Fridman, 2008) σMAD = 1.4826(med(|x(i) – med(x)|))

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(Fridman, 2008) σMAD = 1.4826(med(|x(i) – med(x)|))

  • Impulsive RFI detection and filtering (Buch et al., JAI, 2016)
  • Excision (filtering) by replacing the RFI affected samples by constant value
  • r noise or threshold

Robust threshold: median ± n*σMAD

  • Implemented in temporal and spectral domains for the uGMRT (Buch et al., RFI-

2016, IEEE, 2016)

8/27/2017 CASPER-2017, Caltech, USA

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SLIDE 9

Broadband RFI Excision

FFT: Fast Fourier Transform MAC: Multiply & Accumulate

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Broadband RFI filtering is carried out in real-time on Nyquist-sampled digital time- series (for each antenna) at 800 MHz IA: Incoherent Array PA: Phased Array

8/27/2017 CASPER-2017, Caltech, USA

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Estimation – Detection - Filtering

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Filtering can be bypassed

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MAD-based RFI Detection and Filtering

  • Computation of real-time

Median using the histogram method

  • Non-linear filtering

med(x+y) ≠ med(x) + med(y)

  • Values outside the range of

[median ± n*σMAD ] are treated as RFI Available filtering options:

  • 1. Clipping

Replacement with

  • 2. Constant value
  • 3. Digital noise (Buch et al.,

JAI, 2014)

  • 4. Median

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  • RFI Detection is followed by

filtering

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Real-time Implementation

MAD = med(|x(i) – med(x)|) Resource utilization: (8-bit input) 16k window MAD: 20% slices 4k window MoM: 18% slices

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Uses the histogram method for median computation – explores parallelism on FPGA Window size is 2TR/TS) samples where TR is the (worst case) duration of RFI and TS is

the sampling interval Long-lasting RFI:

Hold MAD values from consecutive windows in a memory buffer and compute the

median (M) i.e. median of MAD (MoM) values (Mm) Mm =M(MAD1,MAD2,...,MADn)

Buch et al., RFI-2016, IEEE, 2016 https://casper.berkeley.edu/wiki/Impulsive_RFI_Excision:_CASPER_Library_Block

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Real-time RFI Filtering: Features

Filter one or both the polarizations

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Includes option to bypass the filter

8/27/2017 CASPER-2017, Caltech, USA

Two main parameters – threshold value (N) and filtering options First Median (External input)

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RFI Count

Broadband RFI filter counts the RFI samples in a given time period

along with the total number of samples

64-bit counters Two control signals – reset and hold Manual hold : Asynchronous mode, Automatic hold: Synchronous mode

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Manual hold : Asynchronous mode, Automatic hold: Synchronous mode Flagging fraction = No. of RFI samples / Total samples Timestamp along with the count for each antenna Takes about half a minute to read data from all the 16 ROACH boards

Works even if the filtering is OFF (bypass mode)

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Test Procedure

  • a. RFI emulator (controlled testing)
  • b. Antenna signals
  • c. System-level tests

Outputs: raw voltage (1.25 ns), beam mode (1.31 ms) and correlator

mode (671 ms)

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mode (671 ms)

Simultaneous comparison between unfiltered and filtered data (different

threshold and replacement options), beam and correlator outputs

Improvement characterized by the mean/rms ratio Preliminary tests carried out for imaging radio sources with and without

broadband RFI excision

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Simultaneous Testing – Unfiltered and Filtered

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Digital copy of one antenna fed to two or four inputs for simultaneous comparison

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Test Results – Raw voltage

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Comparison between MAD and MoM on unfiltered and filtered raw voltage Pulse input provided through RFI emulator, on-period 32μs total-period 240μs

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Test Results – MAD vs MoM

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Total samples in 30 min. duration = 1440000000000 Samples flagged as RFI = 6146025673 (MAD)(0.43%) and 6270107796 (MoM)(0.44%)

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Test Results – MAD vs MoM

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I (dB) = 10log(MF/MU) MF, MU : running mean/rms for filtered, unfiltered beam output

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Test Results – Beam and Correlator

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Simultaneous recording of beam mode (1.3 ms) and correlator mode (671 ms) with different thresholds replacement by zeros.

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First Image

Image Courtesy: DharamVir Lal Unfiltered RMS: 2.26 mJy Filtered RMS: 1.29 mJy

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250-500 MHz, uGMRT, 16 antennas, Pol.-1 (left, without filter) & Pol.-2 (right, with broadband RFI filter), a factor of two (approx.) improvement in noise RMS

8/27/2017 CASPER-2017, Caltech, USA

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Spectral Domain RFI Filtering

FFT: Fast Fourier Transform MAC: Multiply & Accumulate

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Real-time Narrowband RFI filtering is carried out at 0.671s integration on multi-core CPUs IA: Incoherent Array PA: Phased Array

8/27/2017 CASPER-2017, Caltech, USA

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Narrowband RFI Excision

  • Two approaches to Spectral domain MAD filtering – 1. MAD-based filtering of each channel
  • ver time (MFAT) and 2. MAD-based filtering across the spectral channels (MFAC)
  • Estimation and filtering across channel is more suitable for real time applications – with

additional correction required for across the band gain variations.

  • Current system: Normalization of spectral power is followed by MFAC

8/27/2017 23

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Narrowband RFI – uGMRT (250-500 MHz)

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Summary

Broadband RFI filtering is available for the uGMRT user

community

Long-term tests and imaging experiments show improvement in

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the performance of the system

Narrowband real-time RFI excision will be released soon! Fine-tuning of broadband and narrowband RFI filtering techniques

is in progress

8/27/2017 CASPER-2017, Caltech, USA

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SLIDE 26

Acknowledgements

Current members

Kishor Naik Sanjay Kudale Ajithkumar B. Yashwant Gupta

Imaging tests and data analysis

DharamVir Lal Ruta Kale Sanjay Kudale Ishwara Chandra C.H. Viral Parekh (RRI)

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Yashwant Gupta

Past members

Swapnil Nalawade Shruti Bhatporia Short-term Engineering Interns

8/27/2017

Viral Parekh (RRI)

GMRT groups

GMRT Backend Team Control Room and Operations

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8/27/2017 CASPER-2017, Caltech, USA 27

Thank You!

For queries, suggestions or comments, email:

kdbuch@gmrt.ncra.tifr.res.in