Strong Radio Frequency Interference Research Articles

A Cryogenic Wideband (2.5–14 GHz) Receiver System for the Arecibo Observatory 12 m Telescope

AbstractIn this paper we present details of the construction of a wideband, cryogenic receiver and its successful commissioning on the Arecibo Observatory 12 m telescope. The cryogenic receiver works in the 2.5–14 GHz frequency range. We upgraded the current narrow band, room temperature receivers of the telescope with the new wideband receiver. The current receiver is built around a Quadruple‐Ridged Flared Horn (QRFH) developed by Akgiray et al. (2013, https://doi.org/10.1109/tap.2012.2229953). To mitigate strong radio frequency interference (RFI) below 2.7 GHz, we installed a highpass filter before the first stage low noise amplifier (LNA). The QRFH, highpass filter, noise coupler and LNA are located inside a cryostat and are cooled to 15 K. The measured receiver temperature is 25 K (median value) over 2.5–14 GHz. The system temperature measured at zenith is about 40 K near 3.1 and 8.6 GHz and the zenith antenna gains are 0.025 and 0.018 K/Jy at the two frequencies respectively. We recommend the following improvements to the telescope system: (a) Upgrade the highpass filter to achieve better RFI rejection near 2.5 GHz; (b) Improve aperture efficiency at 8.6 GHz; (c) Upgrade the intermediate frequency system to increase the upper frequency of operation from 12 to 14 GHz.

Low Complexity Radio Frequency Interference Mitigation for Radio Astronomy Using Large Antenna Array

With the ongoing growth in radio communications, there is an increased contamination of radio astronomical source data, which hinders the study of celestial radio sources. In many cases, fast mitigation of strong radio frequency interference (RFI) is valuable for studying short lived radio transients so that the astronomers can perform detailed observations of celestial radio sources. The standard method to manually excise contaminated blocks in time and frequency makes the removed data useless for radio astronomy analyses. This motivates the need for better RFI mitigation techniques for array of size M antennas. Although many solutions for mitigating strong RFI improves the quality of the final celestial source signal, many standard approaches require all the eigenvalues of the spatial covariance matrix ([Formula: see text]) of the received signal, which has [Formula: see text] computation complexity for removing RFI of size d where [Formula: see text]. In this work, we investigate two approaches for RFI mitigation, (1) the computationally efficient Lanczos method based on the Quadratic Mean to Arithmetic Mean (QMAM) approach using information from previously-collected data under similar radio-sky-conditions, and (2) an approach using a celestial source as a reference for RFI mitigation. QMAM uses the Lanczos method for finding the Rayleigh–Ritz values of the covariance matrix [Formula: see text], thus, reducing the computational complexity of the overall approach to [Formula: see text]. Our numerical results, using data from the radio observatory Long Wavelength Array (LWA-1), demonstrate the effectiveness of both proposed approaches to remove strong RFI, with the QMAM-based approach still being computationally efficient.

A Simulation Experiment of a Pipeline Based on Machine Learning for Neutral Hydrogen Intensity Mapping Surveys

We present a simulation experiment of a pipeline based on machine learning algorithms for neutral hydrogen (H i) intensity mapping (IM) surveys with different telescopes. The simulation is conducted on H i signals, foreground emission, thermal noise from instruments, strong radio frequency interference (sRFI), and mild RFI (mRFI). We apply the Mini-Batch K-Means algorithm to identify sRFI, and Adam algorithm to remove foregrounds and mRFI. Results show that there exists a threshold of the sRFI amplitudes above which the performance of our pipeline enhances greatly. In removing foregrounds and mRFI, the performance of our pipeline is shown to have little dependence on the apertures of telescopes. In addition, the results show that there are thresholds of the signal amplitudes from which the performance of our pipeline begins to change rapidly. We consider all these thresholds as the edges of the signal amplitude ranges in which our pipeline can function well. Our work, for the first time, explores the feasibility of applying machine learning algorithms in the pipeline of IM surveys, especially for large surveys with the next-generation telescopes.

Extragalactic H i Survey with FAST: First Look at the Pilot Survey Results

We present the first data release of H i sources extracted from a pilot extragalactic survey using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). We extracted sources from three-dimensional (3D) spectral data cubes datacubes to perform interactive searching and computing, yielding global parameters for each source, extending redshift ranges of H i emission up to z = 0.04. A total of 544 extragalactic H i sources has been detected by the pilot FAST H i drift scan survey covering part of the sky region in R.A. (R.A. or α) and decl. (decl. or δ) range 00h47m < R. A. (J2000) < 23h22m and +24° < decl. (J2000) < +43°. Of which, 528 sources are matched with optical counterparts via examination of digital optical survey databases collected from NED and Vizier data center, and 449 of them have optical velocities. Furthermore, we detect 36 galaxies with H i mass <108 M ⊙, which is significant for the study of low-mass systems in the local universe. We present catalogs for all H i detections with signal-to-noise ratio (S/N) greater than 5.1. The data are classified into four categories based on their S/N and baseline qualities, which are flagged with code 1 to 4: (1) 422 sources with S/N > 6.5; (2) 61 sources with 5.1 ≲ S/N ≲ 6.5; (3) 28 sources with relatively poor baselines; (4) 33 sources that are partly masked by strong radio frequency interferences (RFIs). In addition, we find 16 H i sources that have not been matched with any counterparts in the existing galaxy catalogs. This data release can provide guidance for the future extragalactic H i survey with FAST.

Radio Frequency Interference Mitigation and Statistics in the Spectral Observations of FAST

In radio astronomy, radio frequency interference (RFI) becomes more and more serious for radio observational facilities. The RFI always influences the search and study of the interesting astronomical objects. Mitigating the RFI becomes an essential procedure in any survey data processing. The Five-hundred-meter Aperture Spherical radio Telescope (FAST) is an extremely sensitive radio telescope. It is necessary to find out an effective and precise RFI mitigation method for FAST data processing. In this work, we introduce a method to mitigate the RFI in FAST spectral observation and make a statistic for the RFI using ∼300 h FAST data. The details are as follows. First, according to the characteristics of FAST spectra, we propose to use the Asymmetrically Reweighted Penalized Least Squares algorithm for baseline fitting. Our test results show that it has a good performance. Second, we flag the RFI with four strategies, which are to flag extremely strong RFI, flag long-lasting RFI, flag polarized RFI, and flag beam-combined RFI, respectively. The test results show that all the RFI above a preset threshold could be flagged. Third, we make a statistic for the probabilities of polarized XX and YY RFI in FAST observations. The statistical results could tell us which frequencies are relatively quiescent. With such statistical data, we are able to avoid using such frequencies in our spectral observations. Finally, based on the ∼300 h FAST data, we obtained an RFI table, which is the most complete database currently for FAST.

Coherent Combining and Long Coherent Integration for BOC Signal Acquisition under Strong Interference

<h3>Abstract</h3> A coherent combining and long coherent integration (CCLCI) scheme is presented for standalone direct acquisition of binary offset carrier (BOC) signals under strong radio frequency interference (RFI). To mitigate the ambiguity of BOC signals, a split-spectrum method extracts the upper and lower sidebands of a BOC signal, treats them separately as two binary phase shift keying (BPSK) signals, and finally combines the results to recover the loss due to splitting. The CCLCI scheme burns through strong interference by building up the desired weak signal while averaging out noise and interference. It exploits all information available (L1 and L2, upper and lower sidebands, odd and even chips, and I- and Q-components) by applying coherent combining across signal components and long coherent integration over time, followed by noncoherent accumulation if necessary. Issues and enabling techniques are described. The results of an embedded implementation in demonstration with a GPS RF simulator are analyzed.

Low- and Moderate-Level RFI Detection Using Point-Source Ripple for Synthetic Aperture Interferometric Radiometer

Finding out and turning off the radio-frequency interference (RFI) sources as many as possible is an important work for synthetic aperture interferometric radiometer (SAIR). While the impact of strong RFIs on reconstructed brightness temperature (BT) image is clearly observable, the presence of low- or moderate-level RFIs is often hard to be aware of, leaving the RFI detection and localization a difficult problem. In this article, an improved RFI detection method is proposed by making use of the point-source ripple to give prominence to the RFIs presented in the SAIR imagery. The method tests based on Soil Moisture and Ocean Salinity satellite (SMOS) visibility samples and the performance evaluations by numerical simulations validate the correctness and effectiveness of the developed method.

Fourier domain excision of periodic radio frequency interference

The discovery and study of pulsars and fast radio bursts (FRBs) in time-domain radio data is often hampered by radio frequency interference (RFI). Some of this terrestrial RFI is impulsive and bright, and relatively easy to identify and remove. Other anthropogenic signals, however, are weaker yet periodic, and their persistence can drown out astrophysical signals. Here we show that Fourier-domain excision of periodic RFI is an effective and powerful step in detecting weak cosmic signals. We find that applying the method significantly increases the signal-to-noise ratio of transient and periodic pulsar signals. In live studies we detected single pulses from pulsars and FRBs that would otherwise have remained buried in background noise. We show the method has no negative effects on pulsar pulse shape, and that it enhances timing campaigns. We demonstrate the method on real-life data from a number of large radio telescopes, and conclude that Fourier-domain RFI excision increases the effective sensitivity to astrophysical sources by a significant fraction, which can be even larger than an order of magnitude in the case of strong RFI. An accelerated implementation of the method runs on standard time-domain radio data formats and is publicly available.

Detection and Analysis of C-Band Radio Frequency Interference in AMSR2 Data over Land

A simplified generalized radio frequency interference (RFI) detection method and principal component analysis (PCA) method are utilized to detect and attribute the sources of C-band RFI in AMSR2 L1 brightness temperature data over land during 1–16 July 2017. The results show that the consistency between the two methods provides confidence that RFI may be reliably detected using either of the methods, and the only difference is that the scope of the RFI-contaminated area identified by the former algorithm is larger in some areas than that using the latter method. Strong RFI signals at 6.925 GHz are mainly distributed in the United States, Japan, India, Brazil, and some parts of Europe; meanwhile, RFI signals at 7.3 GHz are mainly distributed in Latin America, Asia, Southern Europe, and Africa. However, no obvious 7.3 GHz RFI appears in the United States or India, indicating that the 7.3 GHz channels mitigate the effects of the C-band RFI in these regions. The RFI signals whose position does not vary with the Earth azimuth of the observations generally come from stable, continuous sources of active ground-based microwave radiation, while the RFI signals which are observed only in some directions on a kind of scanning orbit (ascending/descending) mostly arise from reflected geostationary satellite signals.

Novel RFI Mitigation Methods in the Square Kilometre Array 1 Mid Correlator Beamformer

The Square Kilometre Array (SKA) Phase 1 Mid Correlator Beamformer (Mid.CBF) adopts two novel methods to mitigate radio frequency interference (RFI) at the various stages of its signal chains. First, the pioneering Sample Clock Frequency Offset (SCFO) sampling suppresses interference which leaks into individual ‘Frequency-Slice’ (FS) (sub-bands) in the cross-correlations. Second, the ‘Shift-Resample-Shift-Back’ method minimizes the addition of noise due to strong clustered RFI. Empirical studies conducted with simulation of the systems confirm that the proposed methods significantly reduce the impact of RFI on the output of the radio telescope.

Iridium Satellite Signals: A Case Study in Interference Characterization and Mitigation for Radio Astronomy Observations

Several post-detection approaches to the mitigation of radio-frequency interference (RFI) are compared by applying them to the strong RFI from the Iridium satellites. These provide estimates for the desired signal in the presence of RFI, by exploiting distinguishing characteristics of the RFI, such as its polarization, statistics, and periodicity. Our data are dynamic spectra with full Stokes parameters and 1[Formula: see text]ms time resolution. Moreover, since most man-made RFI is strongly polarized, we use the data to compare its unpolarized component with its Stokes I. This approach on its own reduces the RFI intensity by many tens of dBs. A comprehensive approach that also recognizes non-Gaussian statistics, and the time and frequency structure inherent in the RFI, permits exceedingly effective post-detection excision provided full Stokes intensity data are available.

Interference and multipath suppression with space-time adaptive beamforming for safety-of-life maritime applications

In this work, we present mitigation algorithms to protect GNSS receivers against malicious interference. Maritime applications with an antenna array-based receiver are considered as a use case. A two-stage mitigation algorithm, that tackles multipath and radio frequency interference (RFI), caused by personal privacy devices (PPD) or additive white Gaussian noise (AWGN) interferers is presented. Our approach consists of a pre-whitening step, followed by a space-time adaptive principle component analysis (PCA) beamformer that uses a dimensionality reduction (i.e. compression) method based on Canonical Components (CC) with a bank of signal-matched correlators. The algorithms are capable of suppressing strong RFI and separating highly correlated and even coherent multipath signals, thus achieving a reliable time delay and, therefore, pseudorange estimation performance. Finally, we evaluate and compare the proposed algorithms not only via numerical simulations but also with real data collected from a measurement campaign performed at DLR’s maritime jamming testbed in the Baltic sea. A complete description of the test platform and the scenarios is provided.

RFI Cancellation for an Array Radio Astronomy System With Receiver Nonlinearity

Radio frequency interference (RFI) is an important problem for a radio astronomy system (RAS) due to the very weak radio astronomical signals. Rapid expansions of active wireless systems (AWSs), as well as more demands for spectrum access by both AWS and RAS will result in scenarios with strong RFI. This will cause RAS's low-noise amplifier (LNA) to operate in a nonlinear region and impose a more challenging nonlinear RFI issue. We address this issue for RAS with the antenna array. We develop a new signal model at the output of RAS array processing when the LNA at each RAS receiver behaves nonlinearly. We show that the two existing methods to address RFI in RAS array systems fail to deal with the nonlinear RFI. Next, we propose two modified methods to address the nonlinear RFI. Analysis of the squared error of the RAS signal correlation estimate is also presented. The numerical results show the severe effects of nonlinear RFI and the capability of the proposed methods to keep the squared error almost as low as that of a linear system.

Ionogram RFI Rejection Using an Autoregressive Interpolation Process

AbstractDefence Science and Technology Group's Digital Oblique Receiving System (DORS) is a direct‐digital high‐frequency ionosonde receiver, capable of collecting high‐quality ionograms simultaneously on multiple oblique ionospheric paths. One of the key signal processing steps that run on board the receiver is a novel technique for the detection and removal of radio frequency interference (RFI). In the down‐converted narrow‐band time series, external RFI sources manifest as impulsive noise, often many orders of magnitude stronger than the ionosonde signal of interest. The RFI rejection technique applies a threshold detector to the whitened data, to locate the corrupt samples, and then replaces the bad data using forward and backward linear prediction, based on an autoregressive model of the desired ionosonde signal. Algorithm performance is evaluated using a set of simple statistics and ~6,000 DORS ionograms from central Australia. Compared to a more traditional approach of clipping the strong RFI impulses, the new technique significantly reduces instances of undersuppression and recovers more of the weaker propagation mode content in the ionogram. The result is a much cleaner image for the DORS automatic feature extraction and parameterized fitting technique.

The first detection of neutral hydrogen in emission in a strong spiral lens

We report HI observations of eight spiral galaxies that are strongly lensing background sources. Our targets were selected from the Sloan WFC (Wide Field Camera) Edge-on Late-type Lens Survey (SWELLS) using the Arecibo, Karl G. Jansky Very Large Array, and Green Bank telescopes. We securely detect J1703+2451 at z=0.063 with a signal-to-noise of 6.7 and W50=79+/-13 km/s, obtaining the first detection of HI emission in a strong spiral lens. We measure a mass of M(HI)= 1.77+/-0.06(+0.35/-0.75) x 10^9 M_(sol) for this source. We find that this lens is a normal spiral, with observable properties that are fairly typical of spiral galaxies. For three other sources we did not secure a detection; however, we are able to place strong constraints on the HI masses of those galaxies. The observations for four of our sources were rendered unusable due to strong radio frequency interference.

Impact of Nonlinear RFI and Countermeasure for Radio Astronomy Receivers

Radio frequency interference (RFI) has become a critical issue to radio astronomy system (RAS) due to fast-expanding active wireless systems. A recent solution based on a coexistence paradigm offers some RFI-free durations for RAS but further expansion of RAS observation needs to cope with strong RFI. To enhance spectrum access opportunities for both wireless communications and RAS, we explore a new approach where both the systems simultaneously access the spectrum for some durations and RAS exploits RFI cancellation. The main challenge is the nonlinear distortion caused by strong RFI. We develop a signal model incorporating the nonlinearity effect and study its impact on RAS observation performance. We propose an auxiliary receiver assisted nonlinear-RFI cancellation scheme and a modified RAS signal detection scheme. Furthermore, we develop two channel estimators (unscented Kalman filter-based approach and nonlinear-to-linear mapping approach) for nonlinear systems which we apply in our RFI cancellation scheme. Analytical and numerical performance evaluation results show substantial performance advantage of our proposed methods and illustrate importance of incorporating nonlinearity in the channel estimation, RFI cancellation, and RAS signal detection.

Desired Features of Adaptive Antenna Arrays for GNSS Receivers

Adaptive antenna arrays have become the system of choice for protecting Global Navigation Satellite Systems (GNSS) receivers in strong radio frequency interference environments. GNSS receiver adaptive antenna arrays have small aperture (of the order of a wavelength) and few (e.g., less than 10) elements. In the near future, even smaller vehicles will be equipped with protected GNSS receivers. Therefore, future GNSS receiver antenna arrays will be even smaller in size. At the same time, requirements for availability, accuracy, and integrity monitoring will increase. Novel array signal processing techniques need to be developed and deployed to meet these requirements. This paper presents state-of-the-art GNSS adaptive antenna array signal processing techniques. Some drawbacks of the current GNSS antenna signal processing techniques are also discussed in this paper, and new ideas for further research and development in GNSS adaptive antennas are presented.

Aquarius Active/Passive RFI Environment at L-Band

Active/Passive instrument combinations (i.e., radiometer and radar) are being developed at L-band for remote sensing of sea surface salinity and soil moisture. Aquarius is already in orbit and SMAP is planned for launch in the Fall of 2014. Aquarius has provided for the first time a simultaneous look at the Radio Frequency Interference (RFI) environment from space for both active and passive instruments. The RFI environment for the radiometer observations is now reasonably well known and examples from Aquarius are presented in this manuscript that show that RFI is an important consideration for the scatterometer as well. In particular, extensive areas of the USA, Europe and Asia exhibit strong RFI in both the radiometer band at 1.41 GHz and in the band at 1.26 GHz employed by the Aquarius scatterometer. Furthermore, in areas such as the USA, where RFI at 1.4 GHz is relatively well controlled, RFI in the scatterometer band maybe the limiting consideration for the operation of combination active/passive instruments.

Nonlinear Behavior of the Radio Frequency Interference (RFI) Sources at Faculty of Applied Sciences, MARA University of Technology

In this article, we describe and compare several sources of the nonlinear of Radio Frequency Interference (RFI) based on classification methods. It is very important to characterize and understand the nature of interference in as much of the candidate spectrum as possible. Preliminary analysis has been done in 2011. As data sizes of observations grow with new and improved solar monitoring system, the need for completely automated, robust methods for RFI mitigation is highlighted. The current status of RFI noise level is being compared at two different sites (i) indoor and (ii) outdoor. The main objective is to evaluate and find the best range of low frequency in MHz for the solar monitoring purpose. Our findings are consistent with those of previous studies. There is not much different in terms of the sources of RFI. However, the level of RFI is become increase. Based on the results, it was found that the distribution of RFI sources in indoor site is in the range from -(80-105) dBm. A strong and moderate RFI can be identified in the range of -100 dBm. The dominant sources in this region are due to the fixed mobile signal with 10 points of this signal from 1-2000 MHz. If we compare with outdoor site, the distribution of RFI sources in indoor site is in the range from -(75-105) dBm. It means that the signal of noise is larger compared with indoor site. While new sources strive to remain the increasing of RFI signal levels, numerous factors interact to influence the pattern of this noise. Reporting to the authoritative body should be made to make sure the allocation of the solar monitoring frequency region was not used by other applications. This work is a current scenario of the nonlinear RFI level at our site.

Nonlinear Behavior of the Radio Frequency Interference (RFI) Sources at Faculty of Applied Sciences, MARA University of Technology

In this article, we describe and compare several sources of the nonlinear of Radio Frequency Interference (RFI) based on classification methods. It is very important to characterize and understand the nature of interference in as much of the candidate spectrum as possible. Preliminary analysis has been done in 2011. As data sizes of observations grow with new and improved solar monitoring system, the need for completely automated, robust methods for RFI mitigation is highlighted. The current status of RFI noise level is being compared at two different sites (i) indoor and (ii) outdoor. The main objective is to evaluate and find the best range of low frequency in MHz for the solar monitoring purpose. Our findings are consistent with those of previous studies. There is not much different in terms of the sources of RFI. However, the level of RFI is become increase. Based on the results, it was found that the distribution of RFI sources in indoor site is in the range from -(80-105) dBm. A strong and moderate RFI can be identified in the range of -100 dBm. The dominant sources in this region are due to the fixed mobile signal with 10 points of this signal from 1-2000 MHz. If we compare with outdoor site, the distribution of RFI sources in indoor site is in the range from -(75-105) dBm. It means that the signal of noise is larger compared with indoor site. While new sources strive to remain the increasing of RFI signal levels, numerous factors interact to influence the pattern of this noise. Reporting to the authoritative body should be made to make sure the allocation of the solar monitoring frequency region was not used by other applications. This work is a current scenario of the nonlinear RFI level at our site.