Radio Frequency Interference Environment Research Articles

L-Band RFI Detected by SMOS and Aquarius

Ocean salinity and soil moisture are key parameters for understanding the global water cycle, weather, and climate. These parameters are being measured with spaceborne radiometers operating in the L-band window at 1400–1427 MHz. Although man-made activity in this band is prohibited, radio frequency interference (RFI) is still a problem over significant portions of the earth. This paper reports a comparison of the RFI environment in this window as observed by two L-band radiometer systems, Aquarius and Soil Moisture and Ocean Salinity. The observed RFI environment depends on the sources and also on the characteristics of the instrument. Comparing the observations provides insight into the extent of the problem (actual sources), the influence of the instrument on the observation of RFI, and on potential ways of mitigating the effects. As this report shows, the global distribution of RFI is largely consistent between the two instruments, but the details, especially at low levels of RFI, depend on the characteristics of the instrument.

A Search for Meteoroid Lunar Impact Generated Electromagnetic Pulses

Lunar white light flashes associated with meteoroid impacts are now regularly observed using modest optical instrumentation. In this paper, we hypothesize that the developing, optically-dense hot ejecta cloud associated with these hypervelocity impacts also produce an associated complex plasma component that rapidly evolves resulting in a highly-transient electro magnetic pulse (EMP) in the VHF/UHF spectral region. Discovery of the characteristics and event frequency of impact EMPs would prove interesting to meteoroid flux and complex plasma physics studies especially if EMPs from the same event are detected from at least two locations on the Earth with relative delays appropriate to the propagation paths. We describe a prototype observational search, conducted in May 2014, for meteoroid lunar-impact EMPs that was conducted using simultaneous, overlapping-band, UHF radio observations at the Arecibo (AO; Puerto Rico) and Haystack (HO, Massachusetts, USA) Observatories. Monostatic/bistatic lunar radar imaging observations were also performed with HO transmitting and HO/AO receiving to confirm tracking, the net delay, and the pointing/timing ephemeris at both observatories. Signal analysis was performed using time–frequency signal processing techniques. Although, we did not conclusively identify EMP returns, this search detected possible EMPs and we have confirmed the search paradigm and established the sensitivity of the AO–HO system in detecting the hypothesized events. We have also characterized the difficult radio-frequency interference environment surrounding these UHF observations. We discuss the wide range of terrestrial-origin, Moon-bounce signals that were observed which additionally validate the observational technique. Further observations are contemplated.

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.

Adaptive Control of Undetected Radio Frequency Interference With a Spaceborne Microwave Radiometer

In microwave radiometric remote sensing, undetected radio frequency interference (RFI) can adversely affect the accuracy of the science products. A method is presented to adaptively tune the parameters of an RFI detection algorithm which controls the equivalent brightness temperature of undetected RFI. The method is adaptive in the sense that it adjusts to variations in the RFI environment, e.g., from high RFI conditions near some population centers to low RFI conditions in the tropical Pacific Ocean. The RFI environment is characterized by inferring the distribution of low-level undetected RFI from that of high-level detected RFI using appropriate scaling arguments. The resulting tuned algorithm adjusts its detection threshold to equalize the brightness temperature calibration bias due to RFI at the expense of the now variable measurement precision (noise equivalent delta temperature). This tradeoff between calibration bias and measurement precision can be represented as a modified version of the classic receiver-operating-characteristic curve. The radiometer on the Aquarius/SAC-D mission is used as an example.

The Low-Frequency Environment of the Murchison Widefield Array: Radio-Frequency Interference Analysis and Mitigation

AbstractThe Murchison Widefield Array is a new low-frequency interferometric radio telescope built in Western Australia at one of the locations of the future Square Kilometre Array. We describe the automated radio-frequency interference detection strategy implemented for the Murchison Widefield Array, which is based on the aoflagger platform, and present 72–231 MHz radio-frequency interference statistics from 10 observing nights. Radio-frequency interference detection removes 1.1% of the data. Radio-frequency interference from digital TV is observed 3% of the time due to occasional ionospheric or atmospheric propagation. After radio-frequency interference detection and excision, almost all data can be calibrated and imaged without further radio-frequency interference mitigation efforts, including observations within the FM and digital TV bands. The results are compared to a previously published Low-Frequency Array radio-frequency interference survey. The remote location of the Murchison Widefield Array results in a substantially cleaner radio-frequency interference environment compared to Low-Frequency Array’s radio environment, but adequate detection of radio-frequency interference is still required before data can be analysed. We include specific recommendations designed to make the Square Kilometre Array more robust to radio-frequency interference, including: the availability of sufficient computing power for radio-frequency interference detection; accounting for radio-frequency interference in the receiver design; a smooth band-pass response; and the capability of radio-frequency interference detection at high time and frequency resolution (second and kHz-scale respectively).

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.

ARECIBO PULSAR SURVEY USING ALFA. III. PRECURSOR SURVEY AND POPULATION SYNTHESIS

The Pulsar Arecibo L-band Feed Array (PALFA) Survey uses the ALFA 7-beam receiver to search both inner and outer Galactic sectors visible from Arecibo (32° ≲ ℓ ≲ 77° and 168° ≲ ℓ ≲ 214°) close to the Galactic plane (|b| ≲ 5°) for pulsars. The PALFA survey is sensitive to sources fainter and more distant than have previously been seen because of Arecibo's unrivaled sensitivity. In this paper we detail a precursor survey of this region with PALFA, which observed a subset of the full region (slightly more restrictive in ℓ and |b| ≲ 1°) and detected 45 pulsars. Detections included 1 known millisecond pulsar and 11 previously unknown, long-period pulsars. In the surveyed part of the sky that overlaps with the Parkes Multibeam Pulsar Survey (36° ≲ ℓ ≲ 50°), PALFA is probing deeper than the Parkes survey, with four discoveries in this region. For both Galactic millisecond and normal pulsar populations, we compare the survey's detections with simulations to model these populations and, in particular, to estimate the number of observable pulsars in the Galaxy. We place 95% confidence intervals of 82,000 to 143,000 on the number of detectable normal pulsars and 9000 to 100,000 on the number of detectable millisecond pulsars in the Galactic disk. These are consistent with previous estimates. Given the most likely population size in each case (107,000 and 15,000 for normal and millisecond pulsars, respectively), we extend survey detection simulations to predict that, when complete, the full PALFA survey should have detected normal pulsars and millisecond pulsars. Identical estimation techniques predict that normal pulsars and millisecond pulsars would be detected by the beginning of 2014; at the time, the PALFA survey had detected 283 normal pulsars and 31 millisecond pulsars, respectively. We attribute the deficiency in normal pulsar detections predominantly to the radio frequency interference environment at Arecibo and perhaps also scintillation—both effects that are currently not accounted for in population simulation models.

Radio frequency interference measurements in Indonesia

We report the first measurements of radio frequency spectrum occupancy performed at sites aimed to host the future radio astronomy observatory in Indonesia. The survey is intended to obtain the radio frequency interference (RFI) environment in a spectral range from low frequency 10 MHz up to 8 GHz. The measurements permit the identification of the spectral occupancy over those selected sites in reference to the allocated radio spectrum in Indonesia. The sites are in close proximity to Australia, the future host of Square Kilometre Array (SKA) at low frequency. Therefore, the survey was deliberately made to approximately adhere the SKA protocol for RFI measurements, but with lower sensitivity. The RFI environment at Bosscha Observatory in Lembang was also measured for comparison. Within the sensitivity limit of the measurement equipment, it is found that a location called Fatumonas in the surrounding of Mount Timau in West Timor has very low level of RFI, with a total spectrum occupancy in this measured frequency range being about 1 %, mostly found at low frequency below 20 MHz. More detailed measurements as well as a strategy for a radio quiet zone must be implemented in the near future.

SMOS Radiometer in the 1400–1427-MHz Passive Band: Impact of the RFI Environment and Approach to Its Mitigation and Cancellation

The Soil Moisture and Ocean Salinity (SMOS) radiometer operates within the Earth Exploration Satellite Service passive band at 1400-1427 MHz. Since its launch in November 2009, SMOS images are strongly impacted by radio frequency interference (RFI). So far RFI sources distributed worldwide have been detected. Up to 42% of these RFIs could be suppressed thanks to the co-operation of the National Spectrum Management Authorities. Some of the strongest RFI sources might mask other weaker sources underneath, hence it is expected the total number of RFI detected may increase as strong ones are progressively identified and switched off. Most RFIs are located in Asia and Europe, which together hold ~73% of the active sources and of the strongest interference. The areas affected by RFI may experience either an underestimation in the retrieved values of soil moisture and ocean salinity or data loss, with the associated detrimental impact on the scientific return. ESA and the teams participating in SMOS mission have put in place different strategies to alleviate this RFI situation.

RFI Characterization and Mitigation for the SMAP Radar

The Soil Moisture Active-Passive (SMAP) mission will launch in late 2014 and will carry a combined L-band radiometer/radar instrument for the retrieval of global soil moisture and surface freeze-thaw state. Radio frequency interference (RFI) is a known challenge for Earth remote sensing in the L-band portion of the spectrum. This paper addresses efforts to characterize and mitigate RFI for the SMAP radar. A model for the RFI environment due to surface-based emitters is developed, and is shown to agree well with the observations of currently operating L-band radar systems. An analysis of the environment due to space-based emitters is also presented. Techniques to mitigate RFI in the radar band are described, and are shown to perform sufficiently well to meet the stringent SMAP measurement requirements. A companion paper addresses the different issues encountered with RFI in the radiometer band.

Intentional GNSS Interference Detection and Characterization Algorithm Using AGC and Adaptive IIR Notch Filter

A Ground Based Augmentation System (GBAS) is an enabling technology for an aircraft’s precision approach based on a Global Navigation Satellite System (GNSS). However, GBAS is vulnerable to interference, so effective GNSS interference detection and mitigation methods need to be employed. In this paper, an intentional GNSS interference detection and characterization algorithm is proposed. The algorithm uses Automatic Gain Control (AGC) gain and adaptive notch filter parameters to classify types of incoming interference and to characterize them. The AGC gain and adaptive lattice IIR notch filter parameter values in GNSS receivers are examined according to interference types and power levels. Based on those data, the interference detection and characterization algorithm is developed and Monte Carlo simulations are carried out for performance analysis of the proposed method. Here, the proposed algorithm is used to detect and characterize single-tone continuous wave interference, swept continuous wave interference, and band-limited white Gaussian noise. The algorithm can be used for GNSS interference monitoring in an excessive Radio Frequency Interference environment which causes loss of receiver tracking. This interference detection and characterization algorithm will be used to enhance the interference mitigation algorithm.

THE PRECISION ARRAY FOR PROBING THE EPOCH OF RE-IONIZATION: EIGHT STATION RESULTS

We are developing the Precision Array for Probing the Epoch of Re-ionization (PAPER) to detect 21 cm emission from the early universe, when the first stars and galaxies were forming. We describe the overall experiment strategy and architecture and summarize two PAPER deployments: a four-antenna array in the low radio frequency interference (RFI) environment of Western Australia and an eight-antenna array at a prototyping site at the NRAO facilities near Green Bank, WV. From these activities we report on system performance, including primary beam model verification, dependence of system gain on ambient temperature, measurements of receiver and overall system temperatures, and characterization of the RFI environment at each deployment site. We present an all-sky map synthesized between 139 MHz and 174 MHz using data from both arrays that reaches down to 80 mJy (4.9 K, for a beam size of 2.15e−5 sr at 156 MHz), with a 10 mJy (620 mK) thermal noise level that indicates what would be achievable with better foreground subtraction. We calculate angular power spectra (Cℓ) in a cold patch and determine them to be dominated by point sources, but with contributions from galactic synchrotron emission at lower radio frequencies and angular wavemodes. Although the sample variance of foregrounds dominates errors in these power spectra, we measure a thermal noise level of 310 mK at ℓ = 100 for a 1.46 MHz band centered at 164.5 MHz. This sensitivity level is approximately 3 orders of magnitude in temperature above the level of the fluctuations in 21 cm emission associated with re-ionization.

L-Band RFI as Experienced During Airborne Campaigns in Preparation for SMOS

In support of the European Space Agency Soil Moisture and Ocean Salinity (SMOS) mission, a number of soil moisture and sea salinity campaigns, including airborne L-band radiometer measurements, have been carried out. The radiometer used in this context is fully polarimetric and has built-in radio-frequency-interference (RFI)-detection capabilities. Thus, the instrument, in addition to supplying L-band data to the geophysicists, also gave valuable information about the RFI environment. Campaigns were carried out in Australia and in a variety of European locations, resulting in the largest and most comprehensive data set available for assessing RFI at L-band. This paper introduces the radiometer system and how it detects RFI using the kurtosis method, reports on the percentage of data that are typically flagged as being corrupted by RFI, and gives a hint about geographical distribution. Also, examples of polarimetric signatures are given, and the possibility of detecting RFI using such data is discussed.

Alternative Adaptive Filter Structures for Improved Radio Frequency Interference Cancellation in Radio Astronomy

In radio astronomy, reference signals from auxiliary antennas that receive only the radio frequency interference (RFI) can be modified to model the RFI environment at the astronomy receivers. The RFI can then be canceled from the astronomy signal paths. However, astronomers typically only require signal statistics. If the RFI statistics are changing slowly, the cancellation can be applied to the signal correlations at a much lower rate than is required for standard adaptive filters. In this paper we describe five canceler setups; precorrelation and postcorrelation cancelers that use one or two reference signals in different ways. The theoretical residual RFI and added noise levels are examined and are demonstrated using microwave television RFI at the Australia Telescope Compact Array. The RFI is attenuated to below the system noise, a reduction of at least 20 dB. While dual-reference cancelers add more reference noise than single-reference cancelers, this noise is zero-mean and only adds to the system noise, decreasing the sensitivity. The residual RFI that remains in the output of single-reference cancelers (but not dual-reference cancelers) sets a nonzero noise floor that does not act like random system noise and may limit the achievable sensitivity. Thus, dual-reference cancelers often result in superior cancellation. Dual-reference precorrelation cancelers require a double-canceler setup to be useful and to give equivalent results to dual-reference postcorrelation cancelers.

Examination of a simple pulse‐blanking technique for radio frequency interference mitigation

Radiometry at L band can be adversely impacted by radio frequency interference (RFI) due to the presence of numerous sources, especially pulsed RFI from radars operating below 1400 MHz. RFI mitigation is very important to deal with this problem. A simple strategy for reducing pulsed RFI, termed “asynchronous pulse blanking” (APB), has been implemented in a digital receiver developed at Ohio State University. This paper presents results from a simulation of the APB algorithm. Several aspects of algorithm use and performance are reported, including means for choosing the algorithm's parameters and the robustness of the method in a realistic RFI environment. Effects of the blanking process on the final output are also examined.

Radio frequency interference identification and mitigation using simultaneous dual‐station observations

Radio frequency interference (RFI) mitigation is a critically important issue in radio astronomy using existing instruments as well as in the development of next generation radio telescopes, such as the Square Kilometer Array (SKA). Most designs for the SKA involve multiple stations with spacings of up to a few thousands of kilometers and thus can exploit the drastically different RFI environments at different stations. As demonstrator observations and analysis for SKA‐like instruments, and to develop RFI mitigation schemes that will be useful in the near term, we recently conducted simultaneous observations with Arecibo Observatory and the Green Bank Telescope. The observations were aimed at diagnosing RFI and using the mostly uncorrelated RFI between the two sites to excise RFI from several generic kinds of measurements such as giant pulses from Crab‐like pulsars and weak H I emission from galaxies in bands heavily contaminated by RFI. This paper presents observations, analysis, and RFI identification and excision procedures that are effective for both time series and spectroscopy applications using multistation data.

RFImitigation methods in radio astronomy

Various methods of radio frequency interference (RFI) mitigation methods at radio astronomy tele- scopes are being considered. Special attention is given to real-time processing algorithms. Computer simulations and observational results are used to describe the applicability of these methods. Best results can be achieved when the RFI mitigation procedures are adapted to the particular radio telescope, the type of observations, and the peculiarities of the RFI environment. A combination of dierent linear and non-linear methods in the tem- poral and frequency domains, with and without the use of reference antennas, may give considerable suppression of strong RFI.

A New Approach to Interference Excision in Radio Astronomy: Real-Time Adaptive Cancellation

Every year, an increasing amount of radio-frequency (RF) spectrum in the VHF, UHF, and microwave bands is being utilized to support new commercial and military ventures, and all have the potential to interfere with radio astronomy observations. Such services already cause problems for radio astronomy even in very remote observing sites, and the potential for this form of light pollution to grow is alarming. Preventive measures to eliminate interference through FCC legislation and ITU agreements can be effective; however, many times this approach is inadequate and interference excision at the receiver is necessary. Conventional techniques such as RF filters, RF shielding, and postprocessing of data have been only somewhat successful, but none has been sufficient. Adaptive interference cancellation is a real-time approach to interference excision that has not been used before in radio astronomy. We describe here, for the first time, adaptive interference cancellation in the context of radio astronomy instrumentation, and we present initial results for our prototype receiver. In the 1960s, analog adaptive interference cancelers were developed that obtain a high degree of cancellation in problems of radio communications and radar. However, analog systems lack the dynamic range, noised performance, and versatility required by radio astronomy. The concept of digital adaptive interference cancellation was introduced in the mid-1960s as a way to reduce unwanted noise in low-frequency (audio) systems. Examples of such systems include the canceling of maternal ECG in fetal electrocardiography and the reduction of engine noise in the passenger compartments of automobiles. These audio-frequency applications require bandwidths of only a few tens of kilohertz. Only recently has high-speed digital filter technology made high dynamic range adaptive canceling possible in a bandwidth as large as a few megahertz, finally opening the door to application in radio astronomy. We have built a prototype adaptive canceler that consists of two receivers: the primary channel (input from the main beam of the telescope) and a separate reference channel. The primary channel receives the desired astronomical signal corrupted by RFI (radio-frequency interference) coming in the sidelobes of the main beam. A separate reference antenna is designed to receive only the RFI. The reference channel input is processed using a digital adaptive filter and then subtracted from the primary channel input, producing the system output. The weighting coefficients of the digital filter are adjusted by way of an algorithm that minimizes, in a least-squares sense, the power output of the system. Through an adaptive-iterative process, the canceler locks onto the RFI, and the filter adjusts itself to minimize the effect of the RFI at the system output. We have designed the adaptive canceler with an intermediate frequency (IF) of 40 MHz. This prototype system will ultimately be functional with a variety of radio astronomy receivers in the microwave band. We have also built a prototype receiver centered at 100 MHz (in the FM broadcast band) to test the adaptive canceler with actual interferers, which are well characterized. The initial laboratory tests of the adaptive canceler are encouraging, with attenuation of strong frequency-modulated (FM) interference to 72 dB (a factor of more than 10 million), which is at the performance limit of our measurements. We also consider requirements of the system and the RFI environment for effective adaptive canceling.

Performance degradation of pseudonoise code lock detector due to radio frequency interference

This paper computes the probabilities of detection and false alarm for a pseudonoise (PN) sequence code lock detector when a strong radio frequency interference (RFI) hits the communications link. A soft-limiter, a hard-limiter, and no limiter at the front-end of the receiver are considered. Both continuous wave (CW) type RFI and pulse type RFI are analyzed. It is shown that a soft-limiter or a hard-limiter can protect the PN-code lock detector against a typical pulse RFI, but in a CW RFI environment, the PN-code lock detector performance with a soft-limiter or a hard-limiter will be worse than that with no limiter. >

An importance sampling analysis of a noninterleaved Viterbi decoder in an RFI environment

This paper presents an importance sampling simulation model which analyzes a communications system consisting of a noisy channel, a transmitter/receiver, and a convolutional encoder/Viterbi decoder. The model determines the amount of signal degradation caused by any noise environment that can be modeled as a Markov chain. The specific example of a Radio Frequency Interference (RFI) noise environment is discussed in detail. The model uses importance sampling to determine low bit error rates (BERs) for a wide range of noise environments. It is faster than a conventional simulation because the required run time is independent of the BER. It is more flexible than existing analytic models, as these make major assumptions, such as that symbol errors are independent (interleaving), or that all bursts have infinite power. The model increases the simulation efficiency by biasing the channel statistics so that more codeword errors occur and adjusts for this by using a weighting function whose value is calculated through the use of a Markov chain. The results show that using interleaving results in a significant performance improvement when the lengths of the interfering bursts are long relative to the data symbol length.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>