scholarly journals The Low frequency Space Array (LFSA)

1988 ◽  
Vol 129 ◽  
pp. 459-460
Author(s):  
K. W. Weiler ◽  
B. K. Dennison ◽  
K. J. Johnston ◽  
R. S. Simon ◽  
J. H. Spencer ◽  
...  
Keyword(s):  
High Resolution ◽  
Milky Way ◽  
Low Frequency ◽  
High Sensitivity ◽  
Electromagnetic Spectrum ◽  
Low Frequencies ◽  
Frequency Space ◽  
Physical Limit ◽  
Path Lengths ◽  
Fundamental Physical

At the lowest radio frequencies (≤30 MHz), the Earth's ionosphere transmits poorly or not at all. This relatively unexplored region of the electromagnetic spectrum is thus an area where high resolution, high sensitivity observations can open a new window for astronomical investigations. Also, extending observations down to very low frequencies brings astronomy to a fundamental physical limit where the Milky Way becomes optically thick over relatively short path lengths due to diffuse free-free absorption.

scholarly journals Low Frequency Radio Astronomy from Above the Ionosphere

2002 ◽  
Vol 199 ◽  
pp. 488-489
Author(s):  
D. L. Jones
Keyword(s):  
High Resolution ◽  
Radio Astronomy ◽  
Coronal Mass Ejections ◽  
Solar Radio ◽  
Low Frequency ◽  
Dramatic Improvement ◽  
Radio Bursts ◽  
Solar Radio Bursts ◽  
Low Frequencies ◽  
High Resolution Images

The GMRT represents a dramatic improvement in ground-based observing capabilities for low frequency radio astronomy. At sufficiently low frequencies, however, no ground-based facility will be able to produce high resolution images while looking through the ionosphere. A space-based array will be needed to explore the objects and processes which dominate the sky at the lowest radio frequencies. An imaging radio interferometer based on a large number of small, inexpensive satellites would be able to track solar radio bursts associated with coronal mass ejections out to the distance of Earth, determine the frequency and duration of early epochs of nonthermal activity in galaxies, and provide unique information about the interstellar medium.

scholarly journals Bioinspired Low-Frequency Material Characterisation

2012 ◽  
Vol 2012 ◽  
pp. 1-12 ◽  
Author(s):  
C. Hopper ◽  
S. Assous ◽  
P. B. Wilkinson ◽  
D. A. Gunn ◽  
P. D. Jackson ◽  
...  
Keyword(s):  
Sound Velocity ◽  
Medical Physics ◽  
Low Frequency ◽  
High Sensitivity ◽  
Prediction Errors ◽  
Low Frequencies ◽  
Longitudinal Sound Velocity ◽  
Velocity Prediction ◽  
Thickness Measurements

New-coded signals, transmitted by high-sensitivity broadband transducers in the 40–200 kHz range, allow subwavelength material discrimination and thickness determination of polypropylene, polyvinylchloride, and brass samples. Frequency domain spectra enable simultaneous measurement of material properties including longitudinal sound velocity and the attenuation constant as well as thickness measurements. Laboratory test measurements agree well with model results, with sound velocity prediction errors of less than 1%, and thickness discrimination of at least wavelength/15. The resolution of these measurements has only been matched in the past through methods that utilise higher frequencies. The ability to obtain the same resolution using low frequencies has many advantages, particularly when dealing with highly attenuating materials. This approach differs significantly from past biomimetic approaches where actual or simulated animal signals have been used and consequently has the potential for application in a range of fields where both improved penetration and high resolution are required, such as nondestructive testing and evaluation, geophysics, and medical physics.

Effects of Frequency on the Directional Auditory Sensitivity of Northern Saw-Whet Owls (Aegolius acadicus)

2021 ◽  
pp. 1-11
Author(s):  
Eileanor P. LaRocco ◽  
Glenn A. Proudfoot ◽  
Megan D. Gall
Keyword(s):  
Auditory Evoked Potentials ◽  
Transfer Functions ◽  
Low Frequency ◽  
High Sensitivity ◽  
Directional Hearing ◽  
Directional Sensitivity ◽  
Low Frequencies ◽  
Aegolius Acadicus ◽  
Spatial Locations ◽  
Prey Items

Many animals use sound as a medium for detecting or locating potential prey items or predation threats. Northern saw-whet owls (<i>Aegolius acadicus</i>) are particularly interesting in this regard, as they primarily rely on sound for hunting in darkness, but are also subject to predation pressure from larger raptors. We hypothesized that these opposing tasks should favor sensitivity to low-frequency sounds arriving from many locations (potential predators) and high-frequency sounds below the animal (ground-dwelling prey items). Furthermore, based on the morphology of the saw-whet owl skull and the head-related transfer functions of related species, we expected that the magnitude of changes in sensitivity across spatial locations would be greater for higher frequencies than low frequencies (i.e., more “directional” at high frequencies). We used auditory-evoked potentials to investigate the frequency-specific directional sensitivity of Northern saw-whet owls to acoustic signals. We found some support for our hypothesis, with smaller-magnitude changes in sensitivity across spatial locations at lower frequencies and larger-magnitude changes at higher frequencies. In general, owls were most sensitive to sounds originating in front of and above their heads, but at 8 kHz there was also an area of high sensitivity below the animals. Our results suggest that the directional hearing of saw-whet owls should allow for both predator and prey detection.

scholarly journals Machine-learning based data recovery and its contribution to seismic acquisition: simultaneous application of deblending, trace reconstruction and low-frequency extrapolation

Geophysics ◽  
2020 ◽  
pp. 1-62
Author(s):  
Shotaro Nakayama ◽  
Gerrit Blacquière
Keyword(s):  
Machine Learning ◽  
Incomplete Data ◽  
Field Data ◽  
Low Frequency ◽  
Cost Effective ◽  
Complete Data ◽  
Prediction Errors ◽  
Low Frequencies ◽  
Simultaneous Application ◽  
Frequency Space

Acquisition of incomplete data, i.e., blended, sparsely sampled, and narrowband data, allows for cost-effective and efficient field seismic operations. This strategy becomes technically acceptable, provided that a satisfactory recovery of the complete data, i.e., deblended, well-sampled and broadband data, is attainable. Hence, we explore a machine-learning approach that simultaneously performs suppression of blending noise, reconstruction of missing traces and extrapolation of low frequencies. We apply a deep convolutional neural network in the framework of supervised learning where we train a network using pairs of incomplete-complete datasets. Incomplete data, which are never used for training and employ different subsurface properties and acquisition scenarios, are subsequently fed into the trained network to predict complete data. We describe matrix representations indicating the contributions of different acquisition strategies to reducing the field operational effort. We also illustrate that the simultaneous implementation of source blending, sparse geometry and band limitation leads to a significant data compression where the size of the incomplete data in the frequency-space domain is much smaller than the size of the complete data. This reduction is indicative of survey cost and duration that our acquisition strategy can save. Both synthetic and field data examples demonstrate the applicability of the proposed approach. Despite the reduced amount of information available in the incomplete data, the results obtained from both numerical and field data cases clearly show that the machine-learning scheme effectively performs deblending, trace reconstruction, and low-frequency extrapolation in a simultaneous fashion. It is noteworthy that no discernible difference in prediction errors between extrapolated frequencies and preexisting frequencies is observed. The approach potentially allows seismic data to be acquired in a significantly compressed manner, while subsequently recovering data of satisfactory quality.

scholarly journals Low Frequency Radio Astronomy from Earth Orbit

1990 ◽  
Vol 123 ◽  
pp. 508-508
Author(s):  
Kurt W. Weiler ◽  
Namir E. Kassim
Keyword(s):  
Wavelength Range ◽  
Radio Astronomy ◽  
Low Cost ◽  
Low Frequency ◽  
Earth Orbit ◽  
The Moon ◽  
Low Frequencies ◽  
Frequency Space ◽  
Current Technology

AbstractLow frequency radio astronomy for the purpose of this discussion is defined as frequencies ≲100 MHz. Since the technology is fairly simple at these frequencies and even Jansky’s original observations were made at 20.5 MHz, there have been many years of research at these wavelengths. However, though radio astronomers have been working at low frequencies since the first days of science, the observing limitations and the move of much of the effort to ever shorter wavelengths has meant that most areas still remain to be fully exploited with modern techniques and instruments. In particular, the possibilities for pursuing the very lowest frequencies by interferometry of ground to space, in Earth orbit, or from the Moon promises a rebirth of work in this wavelength range.We present concepts for space-ground VLBI and a fully space-based array in high Earth orbit to pursue the astrophysics which can only be probed at these frequencies. An Orbiting Low Frequency Radio Astronomy Satellite (OLFRAS) and a Low Frequency Space Array (LFSA) are two concepts which will open this last, poorly explored area of astronomy at relatively low cost and well within the limits of current technology.

scholarly journals Μελέτη, ανάλυση και βελτίωση της λειτουργίας αερομεταφερόμενου ραντάρ συνθετικής απεικόνισης (SAR) χαμηλών συχνοτήτων

2002 ◽  
Author(s):  
Αθανάσιος Πότσης
Keyword(s):  
Data Processing ◽  
High Performance ◽  
Dynamic Range ◽  
Low Frequency ◽  
Low Noise ◽  
Electromagnetic Spectrum ◽  
System Operation ◽  
Low Frequencies ◽  
Technological Advances ◽  
Sar Data

Because of its high resolution, frequency scattering properties and indifference to day/night or cloud cover, Synthetic Aperture Radar (SAR) has become into vogue in the last years. The field of SAR remote sensing has changed dramatically with the operational introduction of new high performance signal processing techniques and new operational modes, like the polarimetry in 1980’s and the interferometry in 1990’s. Additionally, technological advances in antenna design, low noise amplifiers, band-pass filters, digital receiver technology and high frequency digital sampling devises, increase the availability and the performance of airborne as well as spaceborne SAR sensors. All these technological advances result to real time SAR system operation and in most of the frequency bands of the electromagnetic spectrum. These advanced hardware components combined with the new radar techniques result to large variety of operational and research applications. In several of the new coming applications there is the need for a SAR system to penetrate vegetation and foliage. As a result of this, a new class of SAR systems, using low frequencies, has emerged. The combination of low frequency with high bandwidth allows a variety of new military as well as civilian applications. In the frame of this thesis, several hardware and software modifications made in the E-SAR P-Band system operated by DLR aiming the improvement of the collected and processed data quality is described. The basic P-Band inherent problems like the low Signal-To-Noise-Ratio (SNR), the presence of Radio Frequency Interferences (RFI) as well as the high dynamic range of the backscattered signal are addressed. A new mode of operation called “Listen Only” (LO) channel mode gave us the unique opportunity to study and analyze the special characteristics of the interfering signals and the nature of the low frequency backscattered signal. Based on this analysis new RFI suppression algorithms have been developed and the system operation parameters have been set to the correct value resulting to high quality collected data. The effect of RFI signals in fully polarimetric SAR data processing and applications are analyzed in detail. One of the principal items of this thesis is the development of a new robust sub-aperture algorithm for improved Motion Compensation (MoCo) in wide azimuth beam SAR data processing. The new algorithm is incorporated to the Extended Chirp Scaling SAR data processing algorithm. The improved MoCo algorithm results to focused images with high SNR, contrast, higher resolution and better geometric correctness. The performance and the correction accuracy of the proposed algorithms are analyzed by using mainly real data collected by the E-SAR system of DLR.

scholarly journals The Astronomical Low-Frequency Array (ALFA)

1998 ◽  
Vol 164 ◽  
pp. 393-394 ◽  
Author(s):  
D. L. Jones ◽  
K. W. Weiler ◽  
R. J. Allen ◽  
M. M. Desch ◽  
W. C. Erickson ◽  
...  
Keyword(s):  
High Resolution ◽  
Solar System ◽  
Angular Resolution ◽  
Low Frequency ◽  
Low Frequencies ◽  
Wide Range ◽  
High Resolution Images ◽  
Coherent Emission ◽  
Age Of The Universe ◽  
The Universe

AbstractThe ALFA mission is designed to map the entire sky at frequencies between approximately 0.3 and 30 MHz with angular resolution limited by interstellar and interplanetary scattering. Most of this region of the spectrum is inaccessible from the ground because of absorption and refraction by the Earth’s ionosphere. A wide range of astrophysical questions concerning solar system, galactic, and extragalactic objects could be answered with high resolution images at low frequencies, where absorption effects and coherent emission processes become important and the synchrotron lifetimes of electrons are comparable to the age of the universe.

scholarly journals A High-Through Technique to Measure DNA Methylation

2010 ◽  
Vol 3 ◽  
pp. GEG.S5035 ◽  
Author(s):  
Zhiming Zhang ◽  
Jian Gao ◽  
Cheng Qin ◽  
Li Liu ◽  
Haijian Lin ◽  
...  
Keyword(s):  
Dna Methylation ◽  
High Efficiency ◽  
Simple Procedure ◽  
Point Mutations ◽  
Low Frequency ◽  
High Sensitivity ◽  
Snp Analysis ◽  
Allelic Series ◽  
Low Frequencies ◽  
Powerful Approach

MethyLight is a sodium-bisulfite-dependent, quantitative, fluorescence-based, real-time PCR strategy that is used to detect and quantify DNA methylation in genomic DNA. High-throughput MethyLight allows the rapid and sensitive detection of very low frequencies of hypermethylated alleles in populations of alternated individuals. The high sensitivity and specificity of MethyLight can be applied not only to make it uniquely suited disease clinical but also quantitatively assessed of these low-frequency methylation events. Owing to its full of advantages of simple procedure, high efficiency and high sensitivity, MethyLight provides a powerful approach for clinical examination, Gene expression analysis, SNP analysis and allele analysis. Coupled with other techniques, MethyLight can be used immediately in identifying allelic alterations in genes exhibiting expressions correlating with phenotypes, Locating an allelic series of induced point mutations in genes of interest. The development of this technique should considerably enhance our ability to rapidly and accurately generate epigenetic profiles of samples.

scholarly journals Geostrophic Turbulence in the Frequency–Wavenumber Domain: Eddy-Driven Low-Frequency Variability*

2014 ◽  
Vol 44 (8) ◽  
pp. 2050-2069 ◽  
Author(s):  
Brian K. Arbic ◽  
Malte Müller ◽  
James G. Richman ◽  
Jay F. Shriver ◽  
Andrew J. Morten ◽  
...  
Keyword(s):  
High Resolution ◽  
Kinetic Energy ◽  
General Circulation Model ◽  
General Circulation ◽  
Low Frequency ◽  
Circulation Model ◽  
Satellite Altimeter ◽  
Low Frequencies ◽  
Low Frequency Variability ◽  
Geostrophic Turbulence

Abstract Motivated by the potential of oceanic mesoscale eddies to drive intrinsic low-frequency variability, this paper examines geostrophic turbulence in the frequency–wavenumber domain. Frequency–wavenumber spectra, spectral fluxes, and spectral transfers are computed from an idealized two-layer quasigeostrophic (QG) turbulence model, a realistic high-resolution global ocean general circulation model, and gridded satellite altimeter products. In the idealized QG model, energy in low wavenumbers, arising from nonlinear interactions via the well-known inverse cascade, is associated with energy in low frequencies and vice versa, although not in a simple way. The range of frequencies that are highly energized and engaged in nonlinear transfer is much greater than the range of highly energized and engaged wavenumbers. Low-frequency, low-wavenumber energy is maintained primarily by nonlinearities in the QG model, with forcing and friction playing important but secondary roles. In the high-resolution ocean model, nonlinearities also generally drive kinetic energy to low frequencies as well as to low wavenumbers. Implications for the maintenance of low-frequency oceanic variability are discussed. The cascade of surface kinetic energy to low frequencies that predominates in idealized and realistic models is seen in some regions of the gridded altimeter product, but not in others. Exercises conducted with the general circulation model suggest that the spatial and temporal filtering inherent in the construction of gridded satellite altimeter maps may contribute to the discrepancies between the direction of the frequency cascade in models versus gridded altimeter maps seen in some regions. Of course, another potential reason for the discrepancy is missing physics in the models utilized here.

scholarly journals LoTSS/HETDEX: Optical quasars

2019 ◽  
Vol 622 ◽  
pp. A11 ◽  
Author(s):  
Gülay Gürkan ◽  
M. J. Hardcastle ◽  
P. N. Best ◽  
L. K. Morabito ◽  
I. Prandoni ◽  
...  
Keyword(s):  
Star Formation ◽  
High Resolution ◽  
Low Frequency ◽  
Radio Continuum ◽  
Continuum Emission ◽  
Host Galaxies ◽  
Low Frequencies ◽  
Sky Survey ◽  
Open Question ◽  
The Impact

The radio-loud/radio-quiet (RL/RQ) dichotomy in quasars is still an open question. Although it is thought that accretion onto supermassive black holes in the centre the host galaxies of quasars is responsible for some radio continuum emission, there is still a debate as to whether star formation or active galactic nuclei (AGN) activity dominate the radio continuum luminosity. To date, radio emission in quasars has been investigated almost exclusively using high-frequency observations in which the Doppler boosting might have an important effect on the measured radio luminosity, whereas extended structures, best observed at low radio frequencies, are not affected by the Doppler enhancement. We used a sample of quasars selected by their optical spectra in conjunction with sensitive and high-resolution low-frequency radio data provided by the LOw Frequency ARray (LOFAR) as part of the LOFAR Two-Metre Sky Survey (LoTSS) to investigate their radio properties using the radio loudness parameter (R =L144 MHz/Li band). The examination of the radio continuum emission and RL/RQ dichotomy in quasars exhibits that quasars show a wide continuum of radio properties (i.e. no clear bimodality in the distribution of ℛ). Radio continuum emission at low frequencies in low-luminosity quasars is consistent with being dominated by star formation. We see a significant albeit weak dependency of ℛ on the source nuclear parameters. For the first time, we are able to resolve radio morphologies of a considerable number of quasars. All these crucial results highlight the impact of the deep and high-resolution low-frequency radio surveys that foreshadow the compelling science cases for the Square Kilometre Array (SKA).

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