RFI Mitigation Research Articles

A Two-Stage Approach for TSNB and ITWB RFI Mitigation in P- and L-Band SAR Data

A Two-Stage Approach for TSNB and ITWB RFI Mitigation in P- and L-Band SAR Data

RFI Mitigation of Radio Astronomical Receiver Using a Low-Profile Metasurface-Loaded Antenna

RFI Mitigation of Radio Astronomical Receiver Using a Low-Profile Metasurface-Loaded Antenna

A Principal Component Analysis Perspective for RFI Mitigation in Synthetic Aperture Interferometric Radiometer

A Principal Component Analysis Perspective for RFI Mitigation in Synthetic Aperture Interferometric Radiometer

Range-Frequency Domain Iterative Notch Filtering Method for RFI Mitigation in SAR

Range-Frequency Domain Iterative Notch Filtering Method for RFI Mitigation in SAR

Sparsity and M-Estimators in RFI Mitigation for Typical Radio Astrophysical Signals

Sparsity and M-Estimators in RFI Mitigation for Typical Radio Astrophysical Signals

An Advanced RFI Mitigation Scheme for Phase Synchronization of Bistatic SAR Based on Blind Source Separation

An Advanced RFI Mitigation Scheme for Phase Synchronization of Bistatic SAR Based on Blind Source Separation

First Demonstration of RFI Mitigation in the Phase Synchronization of LT-1 Bistatic SAR

First Demonstration of RFI Mitigation in the Phase Synchronization of LT-1 Bistatic SAR

Progress of radio search for extraterrestrial intelligence technosignatures

<p indent="0mm">Based on the Copernican principle, Drake’s formula and the discovery of a large number of exoplanets, most scientists believe that there must be life outside the Earth, especially advanced intelligent life (or civilization). Because of the advantages of radio technology, it has become one of the best means to search for extraterrestrial intelligence (SETI). China’s “eye in the sky”, Five-hundred-meter Aperture Spherical radio Telescope (FAST), is the largest single-aperture radio telescope in the world, and SETI is one of its five scientific goals. Its extremely high sensitivity in low frequency radio L-band is of great significance to SETI. Based on the backend equipment of FAST for SETI observation and observing time for FAST, we carry out SETI backend commensal survey observations and exoplanet targeted observations. We find two groups of candidates that demonstrate this RFI mitigation and candidate selection, and validate our SETI instrumentation techniques as well as our data processing pipeline. Cooperating with SETI@home and Breakthrough Listen Team at Berkeley, we develop multibeam coincidence matching (MBCM) strategy and polarization and frequency drift criteria that make the ETI signal identification process more scientific and complete, and conduct observation and data processing to screen ETI candidate targets. We also find a signal at <sc>1140.604 MHz</sc> detected from the observation toward Kepler-438 originally piqued our interest that is roughly consistent with assumed ETI technosignatures. However, its polarization characteristics are able to eliminate the possibility of an extraterrestrial origin. Our observations achieve an unprecedented sensitivity. In addition, we also study the drift rate that is contributed by the rotations and orbits of Earth and the exoplanet in the celestial mechanics perspective, and obtain the expected pseudosinusoidal drifting result with both long-term observations and shorter orbital periods of exoplanets. The characteristics of pseudosinusoidal curves can be applied to long-term SETI reobservations in the future research as another new criterion for extraterrestrial signals. This paper mainly introduces the history of SETI observation in the world, especially the status quo and latest progress of SETI research conducted by FAST, and its future prospects.

RFI Detection and Mitigation for Advanced Correlators in Interferometric Radiometers

This work presents the first RFI detection and mitigation algorithm for the interferometric radiometers that will be implemented in its correlator unit. The algorithm operates in the time and frequency domains, applying polarimetric and statistical tests in both domains, and exhibiting a tunable and arbitrary low probability of false alarm. It is scalable to a configurable number of receivers, and it is optimized in terms of quantization bits and the implementation of the cross-correlations in the time or frequency domains for hardware resource saving. New features of this algorithm are the computation of the Stokes parameters per frequency bin in the Short-Time Fourier Transform and a new parameter called Polarimetric Kurtosis. If RFI is detected in one domain or in both, it is removed using the calculated blanking masks. The optimum algorithm parameters are computed, such as length of the FFTs, the threshold selection for a given probability of false alarm, and the selection of the blanking masks. Last, an important result refers to the application of Parseval’s theorem for the computation of the cross-correlations in the frequency domain, instead of in the time domain, which is more efficient and leads to smaller errors even when using moderate quantization levels. The algorithm has been developed in the framework of the ESA’s technology preparation for a potential L-band radiometer mission beyond SMOS. However, it is also applicable to (polarimetric) real aperture radiometers, and its performance would improve if more than one bit is used in the signal quantization.

RFI Mitigation via Auto-Encoder Framework in SAR Data

RFI Mitigation via Auto-Encoder Framework in SAR Data

Automatic RFI Identification for Sentinel-1 Based on Siamese-Type Deep CNN Using Repeat-Pass Images

Since the start of the Sentinel-1 mission, numerous cases of severe image degradation caused by RFI have been reported, which puts forward an urgent need for RFI identification and mitigation. In this paper, an automatic RFI identification method is proposed based on a siamese-type deep convolutional neural network (Siam-CNN-RIM). The Siam-CNN-RIM can be served as a pre-processing step before RFI mitigation to identify whether an S-1 image is RFI-contaminated or not. Different from traditional RFI identification networks which only use a single image as input, an additional image in the repeat-pass time-series is also fed into the input of Siam-CNN-RIM as a reference. Both of the input images correspond to the same illuminated area, and pass through the same convolutional layer followed by an energy function, such that the different features caused by RFI can be extracted and the background terrain features can be ignored. This is beneficial for distinguishing the real RFI signatures and the similar terrain signatures that may cause false positives, and thus improving the RFI identification performance. Experimental results show that the proposed method is robust in different scenarios and can achieve more than 97% RFI identification accuracy, even for the open-set task where the test scenarios are not included in the training set.

Passive Microwave Remote Sensing of Snow Layers Using Novel Wideband Radiometer Systems and RFI Mitigation

Passive Microwave Remote Sensing of Snow Layers Using Novel Wideband Radiometer Systems and RFI Mitigation

Pinta: The uGMRT data processing pipeline for the Indian Pulsar Timing Array

Abstract We introduce pinta, a pipeline for reducing the upgraded Giant Metre-wave Radio Telescope (uGMRT) raw pulsar timing data, developed for the Indian Pulsar Timing Array experiment. We provide a detailed description of the workflow and usage of pinta, as well as its computational performance and RFI mitigation characteristics. We also discuss a novel and independent determination of the relative time offsets between the different back-end modes of uGMRT and the interpretation of the uGMRT observation frequency settings and their agreement with results obtained from engineering tests. Further, we demonstrate the capability of pinta to generate data products which can produce high-precision TOAs using PSR J1909 $-$ 3744 as an example. These results are crucial for performing precision pulsar timing with the uGMRT.

The High Time Resolution Universe Pulsar Survey – XVI. Discovery and timing of 40 pulsars from the southern Galactic plane

ABSTRACT We present the results of processing an additional 44 per cent of the High Time Resolution Universe South Low Latitude (HTRU-S LowLat) pulsar survey, the most sensitive blind pulsar survey of the southern Galactic plane to date. Our partially coherent segmented acceleration search pipeline is designed to enable the discovery of pulsars in short, highly accelerated orbits, while our 72-min integration lengths will allow us to discover pulsars at the lower end of the pulsar luminosity distribution. We report the discovery of 40 pulsars, including three millisecond pulsar-white dwarf binary systems (PSRs J1537−5312, J1547−5709, and J1618−4624), a black-widow binary system (PSR J1745−23) and a candidate black-widow binary system (PSR J1727−2951), a glitching pulsar (PSR J1706−4434), an eclipsing binary pulsar with a 1.5-yr orbital period (PSR J1653−45), and a pair of long spin-period binary pulsars which display either nulling or intermittent behaviour (PSRs J1812−15 and J1831−04). We show that the total population of 100 pulsars discovered in the HTRU-S LowLat survey to date represents both an older and lower luminosity population, and indicates that we have yet to reach the bottom of the luminosity distribution function. We present evaluations of the performance of our search technique and of the overall yield of the survey, considering the 94 per cent of the survey which we have processed to date. We show that our pulsar yield falls below earlier predictions by approximately 25 per cent (especially in the case of millisecond pulsars), and discuss explanations for this discrepancy as well as future adaptations in RFI mitigation and searching techniques which may address these shortfalls.

Improving the Epoch of Reionization Power Spectrum Results from Murchison Widefield Array Season 1 Observations

Abstract Measurements of 21 cm Epoch of Reionization (EoR) structure are subject to systematics originating from both the analysis and the observation conditions. Using 2013 data from the Murchison Widefield Array (MWA), we show the importance of mitigating both sources of contamination. A direct comparison between results from Beardsley et al. and our updated analysis demonstrates new precision techniques, lowering analysis systematics by a factor of 2.8 in power. We then further lower systematics by excising observations contaminated by ultra-faint RFI, reducing by an additional factor of 3.8 in power for the zenith pointing. With this enhanced analysis precision and newly developed RFI mitigation, we calculate a noise-dominated upper limit on the EoR structure of Δ2 ≤ 3.9 × 103 mK2 at k = 0.20 h Mpc−1 and z = 7 using 21 hr of data, improving previous MWA limits by almost an order of magnitude.

Spectral Kurtosis-Based RFI Mitigation for CHIME

We present the implementation of a spectral kurtosis-based Radio-Frequency Interference detection system on the CHIME instrument and its reduced-scale pathfinder. Our implementation extends single-receiver formulations to the case of a compact array, combining samples from multiple receivers to improve the confidence with which RFI is detected. Through comparison between on-sky data and simulations, we show that the statistical properties of the canonical spectral kurtosis estimator are functionally unchanged by cross-array integration. Moreover, by comparison of simultaneous data from CHIME and the Pathfinder, we evaluate our implementation’s capacity for interference discrimination for compact arrays of various sizes. We conclude that a spectral kurtosis-based implementation provides a scalable, high cadence RFI discriminator for compact multi-receiver arrays.

Mitigation of non-narrowband radio-frequency interference

The rapid development and implementation of wireless communication standards put increasing pressure on spectrum allocation, and therefore threaten the efficacy of radio astronomy. For example, digital audio broadcasting (DAB) is a wide-bandwidth broadcast technology that is now being implemented and has spectrum allocated in the L band. We demonstrate that standard narrowband subspace subtraction methods may provide insufficient suppression of such signals. We therefore propose two algorithms that take into account the non-narrowband nature of these signals. The first proposed algorithm is based on a flat-frequency-response model. The second is an approximation of the first that makes use of two frequency-shifted monochromatic sources (Zatman's model). An experimental demonstration of both proposed algorithms yielded an increase of approximately a factor six in bandwidth per channel that can be processed when compared to conventional narrowband techniques (for the same attenuation of the RFI signal). The performance of the two methods was identical for LOFAR station configurations with bandwidths between 763 Hz and 195 kHz. However, the model based on Zatman's algorithm required fewer operations (a speed-up of 1.3 was achieved). An equation is derived that gives the direction of arrival for a far-field non-narrowband signal that causes the greatest distortion of the visibilities. Another equation is derived for the appropriate frequency channel bandwidth at which to implement the proposed non-narrowband RFI mitigation algorithms.

Subarray Processing for Projection-based RFI Mitigation in Radio Astronomical Interferometers

Abstract Radio Frequency Interference (RFI) is a major problem for observations in Radio Astronomy (RA). Adaptive spatial filtering techniques such as subspace projection are promising candidates for RFI mitigation; however, for radio interferometric imaging arrays, these have primarily been used in engineering demonstration experiments rather than mainstream scientific observations. This paper considers one reason that adoption of such algorithms is limited: RFI decorrelates across the interferometric array because of long baseline lengths. This occurs when the relative RFI time delay along a baseline is large compared to the frequency channel inverse bandwidth used in the processing chain. Maximum achievable excision of the RFI is limited by covariance matrix estimation error when identifying interference subspace parameters, and decorrelation of the RFI introduces errors that corrupt the subspace estimate, rendering subspace projection ineffective over the entire array. In this work, we present an algorithm that overcomes this challenge of decorrelation by applying subspace projection via subarray processing (SP-SAP). Each subarray is designed to have a set of elements with high mutual correlation in the interferer for better estimation of subspace parameters. In an RFI simulation scenario for the proposed ngVLA interferometric imaging array with 15 kHz channel bandwidth for correlator processing, we show that compared to the former approach of applying subspace projection on the full array, SP-SAP improves mitigation of the RFI on the order of 9 dB. An example of improved image synthesis and reduced RFI artifacts for a simulated image “phantom” using the SP-SAP algorithm is presented.

Ultra-low loss lumped element cryogenic coolable high pass filter and cryogenic measurement method

This letter presents an ultra-low loss cryogenically-coolable lumped element high pass filter and the measurement method of its S parameters and noise temperature at cryogenic temperature. This filter is going to be used in RFI mitigation of FAST wideband single pixel receiver. It adopts 5 poles Chebyshev high pass structure, and ultra-low loss lumped elements are employed. At cryogenic temperature, its 3 dB cutoff frequency is 248 MHz and the high-end of passband with ultra-low insertion loss is up to 3.5 GHz. Specially, the novel mechanical and microwave design ensure effectively cryogenic cooling to all components of the filter. This filter can be operated at temperatures of 300-4 K. S parameters are measured at room and cryogenic temperatures. At 26 K, its insertion loss in passband is smaller than 0.1 dB. A direct measurement to noise temperature at 24 K physical temperature is performed in cryogenic Testing Dewar, and noise temperature smaller than 0.55 K is achieved. The stopband rejection at 95 MHz is larger than 43 dB. In passband, the return loss of 2 ports is larger than 23 dB.

RFI Mitigation in Aperture Synthesis Radiometers Using a Modified CLEAN Algorithm

For aperture synthesis radiometers, sparse samplings on the $u$ - $v$ frequency plane cause undesirable sidelobes in the synthesized beam. Through these sidelobes, artificial sources emitting in the protected 1400–1427 MHz band contaminate the retrievals of the soil moisture and ocean salinity (SMOS) from MIRAS measurements. One effective way to correct the artificial interferences is to create a synthetic signal to compensate for the interference’s impact. Based on the similar idea, in this letter, we describe an algorithm to compensate for the interference’s impact by constructing an artificial signal as close as possible to the Gaussian beam. Numerical studies using synthetic and real SMOS data have been carried out to demonstrate that the proposed algorithm outperforms the classical CLEAN algorithm in correcting the impact of the extended radio frequency interference source.