Broadband Radio Frequency Interference Research Articles

Research on RFI Mitigation Methods for Fast Radio Burst Observational Data

To quickly search for rare fast radio bursts (FRBs) from massive astronomical observational data, Radio Frequency interference (RFI) mitigation is one of the key and challenging task. RFI will cause search algorithm to output a large number of false positive candidates, and even submerges real astronomical events. Due to the complexity of the sources and types of RFI. there is currently no universal method to solve this problem. In order to reduce the impact of RFI on FRB search, the RFI in L band observational data of the Nanshan 26m radio telescope (NSRT-26m) was analyzed and studied. A three-layer RFI mitigation procedure was established for the main narrowband RFI and broadband RFI, which effectively alleviated the RFI pollution of observational data. Embedding this procedure into the FRB DDSS (Dispersion Dynamic Spectra Search) pipeline, experimental results show that the detection rate and accuracy of the search algorithm have been further improved. This method provides valuable reference for RFI mitigation of FRB observational data.

A Search for Extraterrestrial Technosignatures in Archival FAST Survey Data Using a New Procedure

The search for extraterrestrial intelligence (SETI) commensal surveys aim to scan the sky to find possible technosignatures from an extraterrestrial intelligence (ETI). The mitigation of radio frequency interference (RFI) is an important step, especially for the most sensitive Five-hundred-meter Aperture Spherical radio Telescope (FAST), which can detect more weak RFI. In this paper, we propose several new techniques for RFI mitigation and use our procedure to search for ETI signals from the archival data of FAST’s first SETI commensal survey. We detect the persistent narrowband RFI by setting a threshold of the signals’ sky separation and detect the drifting RFI (and potentially other types of RFI) using the Hough transform. We also use the clustering algorithms to remove more RFI and select candidates. The results of our procedure are compared to the earlier work on the same FAST data. We find that our methods, though relatively simpler in computation, remove more RFI (99.9912% compared to 99.9063% in the earlier work) but preserve the simulated ETI signals, except for those (5.1%) severely affected by the RFI. We also report more interesting candidate signals, about a dozen of which are new candidates that were not previously reported. In addition, we find that the proposed Hough transform method, with suitable parameters, also has the potential to remove the broadband RFI. We conclude that our methods can effectively remove the vast majority of the RFI while preserving and finding the candidate signals that we are interested in.

Performance Analysis Techniques for Real-Time Broadband RFI Filtering System of uGMRT

Electromagnetic radiation from human activities, known as man-made Radio Frequency Interference (RFI), adversely affects radio astronomy observations. In the vicinity of the Upgraded Giant Metrewave Radio Telescope (uGMRT) array, the sparking on power lines is the major cause of interference at observing frequencies less than 800[Formula: see text]MHz. A real-time broadband RFI detection and filtering system is implemented as part of the uGMRT wideband signal processing backend to mitigate the effect of broadband RFI. Performance analysis techniques used for testing and commissioning the system for observations in the beamformer and correlator modes of the uGMRT are presented. The concept and implementation of recording simultaneous unfiltered and filtered data along with data analysis and interpretation is illustrated using an example. For the beamformer mode, spectrogram, single spectral channel, and its Fourier transform is used for performance analysis whereas, in the correlator mode, the cross-correlation function, closure phase, and visibilities from the simultaneously recorded unfiltered and filtered is carried out. These techniques are used for testing the performance of the broadband RFI filter and releasing it for uGMRT users.

A Comparative Analysis to Deal with Missing Spectral Information Caused by RFI in Cosmological H i 21 cm Observations

We investigate the effect of radio-frequency interference (RFI) excision in estimating the cosmological H i 21 cm power spectrum. Flagging of RFI-contaminated channels results in a nonuniform sampling of the instrumental bandpass response. Hence, the Fourier transformation of visibilities from frequency to delay domain contaminates the higher foreground-free delay modes, and separating the spectrally fluctuating H i signal from spectrally smooth foregrounds becomes challenging. We have done a comparative analysis between two algorithms, one-dimensional CLEAN and least-squares spectral analysis (LSSA), which have been used widely to solve this issue in the literature. We test these algorithms using the simulated SKA-1 Low observations in the presence of different RFI flagging scenarios. We find that, in the presence of random flagging of data, both algorithms perform well and can mitigate the foreground leakage issue. But CLEAN fails to restrict the foreground leakage in the presence of periodic and periodic plus broadband RFI flagging and gives an extra bias to the estimated power spectrum. However, LSSA can restrict the foreground leakage for these RFI flagging scenarios and gives an unbiased estimate of the H i 21 cm power spectrum. We have also applied these algorithms to observations with the upgraded GMRT and found that both CLEAN and LSSA give consistent results in the presence of realistic random flagging scenarios for this observed data set. This comparative analysis demonstrates the effectiveness and robustness of these two algorithms in estimating the H i 21 cm power spectrum from data sets affected by different RFI scenarios.

Power-Based Broadband RF Interference Detector for Wireless Communication Systems

As broadband radio frequency interference (RFI) affects various systems operating radio frequencies, it has to be detected and mitigated. Accordingly, a computationally simple power-based detector (PD) is investigated. By assuming reception over the Nakagami- ${m}$ fading channels, exact closed-form expressions for the probabilities of RFI detection and of false alarm are derived and validated via simulations. Simulations also demonstrate that PD outperforms kurtosis detector.

Implementing and Characterizing Real-Time Broadband RFI Excision System for the GMRT Wideband Backend

ABSTRACTThe Giant Metrewave Radio Telescope (GMRT) is being upgraded to increase the receiver sensitivity. This makes the receiver more susceptible to man-made Radio Frequency Interference (RFI). To improve the receiver performance in the presence of RFI, real-time RFI excision (filtering) is incorporated in the GMRT wideband backend (GWB). The RFI filtering system is implemented on FPGA (Field Programmable Gate Array) and CPU (Central Processing Unit)–GPU (Graphics Processing Unit) platforms to detect and remove broadband and narrowband RFI, respectively. The RFI is detected using a threshold-based technique where the threshold is computed using Median Absolute Deviation (MAD) estimator. The filtering is carried out by replacing the RFI samples by either noise samples or constant value or threshold. This paper describes the status of the real-time broadband RFI excision system in the wideband receiver chain of the upgraded GMRT. The methodology for carrying out various tests to demonstrate the performance of broadband RFI excision at the system level and on the radio astronomical imaging experiments is also described.

The GMRT Epoch of Reionization experiment: a new upper limit on the neutral hydrogen power spectrum at z≈ 8.6

We present a new upper limit to the 21cm power spectrum during the Epoch of Reionization (EoR) which constrains reionization models with an unheated IGM. The GMRT-EoR experiment is an ongoing effort to make a statistical detection of the power spectrum of 21cm neutral hydrogen emission at redshift z~9. Data from this redshift constrain models of the EoR, the end of the Dark Ages arising from the formation of the first bright UV sources, probably stars or mini-quasars. We present results from approximately 50 hours of observations at the Giant Metrewave Radio Telescope in India from December 2007. We describe radio frequency interference (RFI) localisation schemes which allow bright sources on the ground to be identified and physically removed in addition to automated flagging. Singular-value decomposition is used to remove remaining broadband RFI by identifying ground sources with large eigenvalues. Foregrounds are modelled using a piecewise linear filter and the power spectrum is measured using cross-correlations of foreground subtracted images.

The GMRT EoR experiment: limits on polarized sky brightness at 150 MHz

The GMRT reionization effort aims to map out the large scale structure of the Universe during the epoch of reionization (EoR). Removal of polarized Galactic emission is a difficult part of any 21 cm EoR program, and we present new upper limits to diffuse polarized foregrounds at 150 MHz. We find no high significance evidence of polarized emission in our observed field at mid galactic latitude (J2000 08h26m+26). We find an upper limit on the 2-dimensional angular power spectrum of diffuse polarized foregrounds of [l^2 C_l/(2 PI)]^{1/2}< 3K in frequency bins of width 1 MHz at 300<l<1000. The 3-dimensional power spectrum of polarized emission, which is most directly relevant to EoR observations, is [k^3 P_p(k)/(2 PI^2)]^{1/2}< 2K at k_perp > 0.03 h/Mpc, k < 0.1 h/Mpc. This can be compared to the expected EoR signal in total intensity of [k^3 P(k)/ (2 PI^2) ]^{1/2} ~ 10 mK. We find polarized structure is substantially weaker than suggested by extrapolation from higher frequency observations, so the new low upper limits reported here reduce the anticipated impact of these foregrounds on EoR experiments. We discuss Faraday beam and depth depolarization models and compare predictions of these models to our data. We report on a new technique for polarization calibration using pulsars, as well as a new technique to remove broadband radio frequency interference. Our data indicate that, on the edges of the main beam at GMRT, polarization squint creates ~ 3% leakage of unpolarized power into polarized maps at zero rotation measure. Ionospheric rotation was largely stable during these solar minimum night time observations.

Postcorrelation ripple removal and radio frequency interference rejection for Parkes Telescope survey data

The 21 cm multibeam receiver on the Parkes telescope has been used for many neutral hydrogen (HI) imaging projects, for example, the HI Parkes All Sky Survey (HIPASS) and the Southern Galactic Plane Survey. In many experiments to date, basic radio frequency interference (RFI) rejection has been accomplished on a per‐feed basis, using the median statistic throughout the processing, which eliminates outliers in a nonparametric fashion at the expense of increased statistical noise. The classic 21 cm baseline ripple problem has been largely ignored, except for the subtraction of a coarse template which is calculated independently of feed, time, telescope elevation, and receiver rotation. The ripple is especially strong during the daytime, when the Sun acts as a broadband RFI noise source. Here we report on new techniques that we are developing, for application to raw, postcorrelation multibeam data, which handle RFI rejection and ripple suppression in more sophisticated ways. RFI can be identified by the presence of coincident outliers on multiple feeds, by sharp increases or decreases in flux time series that are inconsistent with a beam‐smeared sky, and by the presence of highly polarized flux. We present our progress with this approach and highlight some remaining difficulties. We also present a novel technique for modeling the variation during telescope scans of 21 cm baseline ripple caused by stray continuum radiation undergoing multiple reflections in the telescope cavity and demonstrate its application to HIPASS data; this may increase the usefulness of daytime spectral line observing.

A baseband recorder for radio pulsar observations

ABSTRA C T Digital signal recorders are becoming widely used in several subfields of centimetrewavelength radio astronomy. We review the benefits and design considerations of such systems and describe the Princeton Mark IV instrument, an implementation designed for coherent-dedispersion pulsar observations. Features of this instrument include corrections for the distortions caused by coarse quantization of the incoming signal, as well as algorithms that effectively excise both narrow-band and broad-band radio frequency interference. Observations at 430 MHz, using the Mark IV system in parallel with a system using a 250-kHz filter bank and incoherent dedispersion, demonstrated timing precision improvement by a factor of 3 or better for typical millisecond pulsars.