Single Frequency Measurements Research Articles

Electromagnetic source localization in shallow waters using Bayesian matched-field inversion

The propagation of an electromagnetic signal in a marine environment cannot be modelled as a plane wave due to the high attenuation in seawater and the interactions with the ocean boundaries. Consequently, conventional beamforming techniques are not applicable for electromagnetic source localization. In this work, the Bayesian approach to matched-field processing is used to localize an electromagnetic source and estimate the environmental parameters. In this formulation, the solution to the inverse problem is given by the a posteriori probability distribution calculated here using the Gibbs sampling method. Bayesian inversion theory provides the formalism for estimating parameters, their uncertainties and verification of the estimates convergence. Two situations were investigated for the case where the single frequency measurements represent the magnitudes of two orthogonal horizontal electric field components: (1) all environmental parameters known and (2) unknown seabed conductivity. The objective function that relates the array data to the propagation model and environment parameters was chosen for the practical situation considered.

A comparative study of non-polar solvents effect on dielectric relaxation and dipole moment of binary mixtures of mono alkyl ethers of ethylene glycol and of diethylene glycol with ethyl alcohol

Dielectric relaxation and dipole moment of binary mixtures of homologous series of mono alkyl ethers of ethylene glycol and of diethylene glycol, i.e., mono methyl, mono ethyl and mono butyl ethers of ethylene glycol (ROCH 2CH 2OH) and mono methyl, mono ethyl and mono butyl ethers of diethylene glycol (ROCH 2CH 2OCH 2CH 2OH) with ethyl alcohol (C 2H 5OH) of different concentrations were studied in dilute solutions of benzene, dioxane and carbon tetrachloride at 35 °C. Permittivity ( ε′) and loss ( ε″) at 10.1 GHz, static dielectric constant ε o at 1 MHz and high frequency limiting dielectric constant ε ∞ = n D 2 at optical frequency of these molecules and their binary mixtures at different concentration were measured in dilute solutions of non-polar solvents. The average relaxation time τ o, relaxation times corresponding to overall molecular reorientation τ 1 and group rotations τ 2 were determined using Higasi's single frequency measurement equations for dilute solutions. The evaluated values of relaxation times and free energy of activation Δ F were used to explore the solvent effect on molecular dynamics of these polar binary systems in non-polar solvents. The excess inverse relaxation time and excess free energy of activation were determined to confirm the existence of hydrogen-bonded heterogeneous cooperative domains of the ethers and alcohol molecules at different concentration their binary mixtures in non-polar solvents. The dipole moment of the binary mixtures was evaluated using Higasi's and Guggenheim's equation for dilute solutions. The evaluated values of dipole moments and computed dipole moment values using a simple mixing equation of the polar molecules binary mixture were used to explore the effect of non-polar solvent environment on heterogeneous molecular interactions between ethers and alcohol molecules. The effect of number of carbon atoms in the molecular structure of these homologous series molecules was also considered for the interpretation of various evaluated dielectric parameters.

Cardiorespiratory monitoring equipment interferes with whole body impedance measurements

Bioelectrical impedance measurements are widely used for the study of body composition. Commonly measurements are made at 50 kHz to estimate total body water or at low frequencies (<10 kHz) to estimate extracellular fluid volume. These measurements can be obtained as single measurements at discrete frequencies, or as fitted data interpolated from plots of measurements made at multiple frequencies. This study compared single frequency and multiple frequency (MF) measurements taken in the intensive care environment. MF bioimpedance (4–1000 kHz) was measured on an adult with and without cardiorespiratory monitoring, and on babies in the neonatal intensive care unit. Measurements obtained at individual frequencies were plotted against frequency and examined for the presence of outlying points. Fitted data for measurements obtained at 5 kHz and 50 kHz with and without cardiorespiratory monitoring were compared. Significant artefacts were detected in measurements at approximately 50 kHz and at integral divisions of this frequency as a result of interference from cardiorespiratory monitors. Single frequency measurements taken at these frequencies may be subject to errors that would be difficult to detect without the aid of information obtained from MF measurements.

Reduced dynamic orbit determination using GPS code and carrier measurements

The three-dimensional nature of Global Positioning System (GPS) measurements provides a unique opportunity for accurately determining the position and velocity of satellites in low Earth orbit (LEO). For optimum results a reduced dynamic technique is commonly preferred, which combines the merits of kinematic positioning techniques with those of a fully dynamic trajectory modeling. As part of the present study two different approaches to reduced dynamic orbit determination are compared, both of which involve the estimation of empirical accelerations on top of a precise deterministic force model. In the batch least-squares estimator piece-wise constant accelerations are adjusted in consecutive sub-intervals that are sufficiently short compared to the orbital period. The extended Kalman filter/smoother approach, on the other hand, estimates the empirical accelerations using a first-order Gauss–Markov process noise model. Software implementations of both estimation methods have been used with GPS measurements of the GRACE mission to assess the individual merits of the different filtering schemes. Both approaches are shown to provide accurate and compatible results, which match an external reference solution to better than 5 cm when processing dual-frequency data and better than 10 cm when using single-frequency measurements. While the extended Kalman filter/smoother requires less computer resources in terms of memory and processing time, the batch least-squares estimator ensures a better smoothness of the resulting trajectory and was found to be more robust in case of data gaps.

ANTENNA MODELING BY INFINITESIMAL DIPOLES USING GENETIC ALGORITHMS

The binary Genetic Algorithm (GA) optimization method is used to simulate antennas from their near-field distribution by a set of infinitesimal dipoles. The infinitesimal dipoles could be of electric and/or magnetic types that produce the near field of the actual antenna and thus the same far field. The method is verified using near fields from known infinitesimal electric and/or magnetic dipoles. Some simple antennas have been simulated by infinitesimal dipoles such as dipole, loop, waveguide, and dielectric resonator antenna. The obtained equivalent dipoles from single frequency measurements are found to be valid for certain frequency band.

Cation and Water Content Effects on Dipole Rotation Activation Energy of Smectites

In soil science, single frequency permittivity measurements are used to determine soil water content, and single frequency bulk electrical conductivity is used to determine soil salinity. The accuracy of these measurements may be influenced by complex interactions between frequency, temperature, and water that is tightly bound to clay surfaces. The purpose of this study is to determine the effect of temperature, saturating cation, water content, smectite properties, and frequency on electrical properties of humidified clays by analyzing three different activation energies for dipole rotations, which are calculated from the temperature dependence of electrical properties. Four reference smectites saturated with K, Na, Ca, and Mg and equilibrated at relative humidities ranging from 56 to 99% were investigated over a frequency range from 3 × 105 to 1 × 109 Hz. Two of the three activation energies were found to decrease slightly as water content increased. Higher activation energies were found for smectites saturated with Mg and K and lower values for smectites saturated with Na and Ca. Trends for type of clay were variable but appeared to be influenced by both the total water content and the distribution of water between the interlayers and the external surfaces of the smectite quasi‐crystals. Changes in quasi‐crystal orientation induced by thermal cycling were also found to influence the activation energies. The results indicate complex frequency and temperature dependent interactions impact electrical properties of the clays. We conclude that no simple equation will correct for temperature and clay content effects on single frequency measurements of permittivity or bulk electrical conductivity.

A Simple Methodology For Determining Electrical Conductivity of Soils

Abstract Electrical conductivity of soils has been demonstrated to be an important parameter for determining their basic properties. As measurement of soil conductivity using the DC field is limited by irreversible changes in the state of the soil and the errors due to electrode polarization, application of an AC field has been recommended. However, it has been noted that researchers have resorted to the ‘single frequency measurements’, considering that impedance is purely resistive, which may not always be true. Hence, it becomes essential to study electrical conductivity of soils in a broad frequency range. With this in view, using the concept of Impedance Spectroscopy (IS), conductivity measurements were carried out on different type of soils in the 20 Hz to 1 MHz frequency range, and details of the study are presented in this paper. Measured data has also been analyzed using Nyquist and Bode formats, and a good matching of the results has been noted. This demonstrates utility of the developed methodology and the ease with which electrical conductivity of soils can be determined in the laboratory.

Graphical representation and multicomponent analysis of single-frequency fluorescence lifetime imaging microscopy data.

Graphical representation of fluorescence lifetime imaging microscopy data demonstrates that a mixture of two components with single exponential decays can be resolved by single frequency measurements. We derive a method based on linear fitting that allows the calculation of the fluorescence lifetimes of the two components. We show that introduction of proper error-weighting results in a non-linear method that is mathematically identical to a global analysis algorithm that was recently derived. The graphical approach was applied to cellular data obtained from a lifetime-based phosphorylation assay for the epidermal growth factor receptor and yielded results similar to those obtained by a global analysis algorithm.

Separation of foreground and background signals in single frequency measurements of the CMB polarization

The polarization of the Cosmic Microwave Background (CMB) is a powerful observational tool at hand for modern cosmology. It allows to break the degeneracy of fundamental cosmological parameters one cannot obtain using only anisotropy data and provides new insight into conditions existing in the very early Universe. Many experiments are now in progress whose aim is detecting anisotropy and polarization of the CMB. Measurements of the CMB polarization are however hampered by the presence of polarized foregrounds, above all the synchrotron emission of our Galaxy, whose importance increases as frequency decreases and dominates the polarized diffuse radiation at frequencies below ≃50 GHz. In the past the separation of CMB and synchrotron was made combining observations of the same area of sky at different frequencies. In this paper, we show that the statistical properties of the polarized components of the synchrotron and dust foregrounds are different from the statistical properties of the polarized component of the CMB, therefore one can build a statistical estimator which allows to extract the polarized component of the CMB from single frequency data also when the polarized CMB signal is just a fraction of the total polarized signal. Our estimator improves the signal/noise ratio for the polarized component of the CMB and reduces from ≃50 to ≃20 GHz, the frequency above which the polarized component of the CMB can be extracted from single frequency maps of the diffuse radiation.

Kinematic GPS positioning of LEO satellites using ionosphere-free single frequency measurements

A method for kinematic point positioning of satellites in low Earth orbit (LEO) based on single frequency GPS measurements is discussed. It makes use of a ionosphere-free combination (GRAPHIC) of C/A code and L1 phase observations and can thus be employed with the majority of non-geodetic spaceborne GPS receivers. Other than dynamical Kalman filter schemes estimating position, velocity and clock parameters along with a set of biases for all tracked channels, a purely kinematic approach is examined in this study. It comprises a global adjustment of positions, clock offsets and pass-by-pass biases from the observations. As such, it is free of any assumptions on the motion of the user vehicle and can be applied for free-flying as well as maneuvering spacecraft. Sample results are shown for LEO GPS data collected with a low-cost single frequency receiver in a signal simulator test bed as well as flight data from the CHAMP mission. In both cases, a purely kinematic 3D position accuracy of 1–1.5 m could be demonstrated in comparison with concise reference trajectories.

Measurements of ambient bubble populations with a multibeam sonar

A 240 kHz multibeam sonar was deployed on the research platform FLIP off the coast of San Diego for two weeks in July, 2002. The sonar was oriented to measure backscattering from the ambient bubble field near the ocean surface. A bubble population density spectrum shape described by Novarini et al. [IEEE J. Ocean. Eng. 23, 423–438 (1998)] is assumed, and the spectrum level is shifted up or down to fit the backscatter measurements. This allows the bubble population to be inferred from the single frequency measurement. The wide field of view for the sonar (1.5° beams distributed over a 150° swath) allowed the spatial structure of the bubble field to be observed over the time period, and compared with environmental measurements including wind/wave conditions and the spatial variation of temperature and salinity. [Work supported by ONR Code 321US.]

Orbit Determination of Spacecraft Using Global Positioning System Single-Frequency Measurement

The dynamic orbit determination of a low Earth orbiter using global positioning system single-frequency measurements has been implemented. Currently two methods are being applied to eliminate or reduce ionospheric path delay in single-frequency measurement. One is a group and phase ionosphere calibration technique using code pseudorange and L1 carrier phase, and the other is application of total electron content values from an ionospheric model using only L1 carrier phase to determine the orbit. A new method based on the latter has been developed, which estimates the scale factors of total electron content values in the location of a low Earth orbiter once per each measurement time. Orbit determination using actual global positioning system measurements of the TOPEX/POSEIDON and the Challenging minisatellite payload was conducted to verify the accuracy of the new method. It is verified that, if the total electron content's scale factor estimation technique were applied, 1-m level position accuracy (1σ) for low Earth orbit below 500-km altitudes could be achieved using precision orbit determination based on the global positioning system double-differencing method.

Ionospheric Correction for GPS Tracking of LEO Satellites

This paper describes an ionospheric correction technique for single frequency GPS measurements from satellites in low Earth orbit. The fractional total electron content (TEC) above the receiver altitude is obtained from global TEC maps of the International GPS Service network and an altitude dependent scale factor. By choosing a suitable effective height of the residual ionosphere, the resulting path delay for positive elevations is then computed from a thin layer approximation. The scale factor can be predicted from the assumption of a Chapman profile for the altitude variation of the electron density or adjusted as a free parameter in the processing of an extended set of single frequency measurements. The suitability of the proposed model is assessed by comparison with flight data from the Champ satellite that orbits the Earth at an altitude of 450 km. For the given test case, a 90% correction of the ionospheric error is achieved in a reduced dynamic orbit determination based on single frequency C/A-code measurements.

A ground-based, multi-frequency ice-penetrating radar system

AbstractTo better understand how ice sheets respond to climate, we designed a new multi-frequency ice-penetrating radar system to investigate subsurface structures of ice sheets. The system is mounted on a single platform and handled by a single operator. Three radio frequencies, 30,60 and 179 MHz, were used. An underlying principle of these multi-frequency observations is that the lower frequencies are more sensitive to electrical conductivity changes, whereas the higher frequencies are more sensitive to dielectric permittivity fluctuations in the ice. The system is composed of three single-frequency pulse radars, a trigger-controller unit and a data-acquisition unit. The trigger controller is the key component of this system. It switches transmitters on at different timings to prevent mixing of signals among the three radars. The timing difference was set as 50 μs, which is equivalent to the two-way travel time for radio waves reflecting from 4250m below the surface. A field test was done along a 2000 km long traverse line in east Dronning Maud Land, Antarctica. The multi-frequency system successfully acquired data that are equivalent in quality to our earlier single-frequency measurements along the same traverse line. The details of the system and preliminary data are described.

Grading of internal carotid artery stenosis: validation of Doppler/duplex ultrasound criteria and angiography against endarterectomy specimen.

duplex ultrasound has replaced angiography prior to carotid endarterectomy (CEA) in many institutions. However, the indications for CEA are based on angiographically controlled studies and widely accepted ultrasound criteria do not exist. Consequently, the reliability of Doppler and/or duplex ultrasound to predict a high-grade ICA stenosis has to be proven. prospective validation study. one hundred and fifty carotid bifurcations assessed by ultrasound and selective angiography and 68 acrylat outcasts of carotid specimen after eversion CEA. ICA stenosis was measured angiographically according to the ECST criteria. Combined Doppler acoustic standard criteria (CDASC), peak systolic frequency (PSF), peak systolic velocity (PSV) and end-diastolic velocity (EDV) served as criteria for the ultrasound assessment. These criteria and the results of angiography were compared to the degree of ICA stenosis determined by specimen measurements. the median degree of ICA stenosis as assessed by angiography (82%, range 56-97%) and CDASC (83%, range 50-99%) corresponded well to the specimen measurements (80%, range 50-95%). The sensitivity of angiography and CDASC to predict a 70-90% ICA stenosis (ECST criteria) compared to the specimen measurements was 88% and 95%, respectively. The positive predictive value (PPV) reached 92% and 96%, respectively. CDASC were equivalent to angiography and were superior to the best single frequency or velocity parameters. If CDASC do not indicate a >/=70% ICA stenosis in spite of a PSV >/=180 cm/s and/or an EDV >/=50 cm/s, angiography may detect patients with a >70% ICA stenosis. CDASC are valid in the quantification of high-grade ICA stenosis. They are more reliable than single velocity and/or frequency measurements. However, if velocity criteria and CDASC do not agree, angiography should be performed.

A test of magnetospheric radio tomographic imaging with IMAGE and WIND

Theoretical studies have shown the potential scientific value of multi‐spacecraft radio tomographic imaging of the magnetosphere. The &lt;10 RE WIND perigee passes during August 2000 afforded a unique opportunity to test and verify the capabilities of radio tomography by measuring interspacecraft electromagnetic wave propagation parameters using the Radio Plasma Imager (RPI) on IMAGE as the transmitter and the WAVES instrument on WIND as the receiver. The primary goal of this experiment was to measure Faraday rotation variations in the RPI signal and interpret them in terms of the path‐integrated magnetic field and electron density. A special 6 W linearly‐polarized 828 kHz RPI signal was clearly detected by WAVES more than 6 RE away and showed a distinct signature of time‐varying Faraday rotation. We show how changes in the path‐integrated electron density/magnetic field product can be unambiguously measured from this continuous, low signal to noise ratio, single frequency measurement.

Ionospheric tomography using Ørsted GPS measurements - preliminary results

Data from the Turbo-Rogue GPS receiver onboard the Danish Ørsted Satellite are used to derive satellite-to-satellite Total Electron Content (TEC). We present preliminary results obtaining Electronic Density fields from ionospheric tomography using TEC Ørsted data. TEC is based on single frequency measurements, since only the L1 signal is of good quality. The C/A pseudo range (C1) and the L1 phase are used to obtain the TEC measurement. Electron density profiles from individual occultations are derived, based on the Abel transform, and compared with the tomographical solution. The assumption of spherical symmetry in the Abel transform limits the accuracy of the profiles and often results in a bias. We select occultations observed along the orbital plane to minimize the spherical assymetry effects. The tomographic grid is also confined to a narrow torus close to the orbital plane. The TEC calculations are based on data from 6 hour (3.6 orbits) periods. Two days have been selected for this study, December 5, 1999 and February 12, 1999.

Double relaxation times, dipole moments, energy parameters and molecular structures of some aprotic polar molecules from relaxation phenomena.

Abstract The real ɛ′ij and imaginary ɛ″ij parts of complex dielectric constant ɛ*ij of some aprotic polar molecules (j) like dimethylsulphoxide (DMSO) : N,N-diethylformamide (DEF) : N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMA) in benzene (i) for different weight fractions wj's of solutes are located at different temperatures under nearly 10 GHz electric field together with the static and infinite frequency dielectric constants ɛoij and ɛoeij. All the molecules show the double relaxation times τ1 and τ2 for rotations of their flexible parts and the whole molecules by the single frequency measurement technique. The τ1 in agreement with reported τ signifies that a part of the molecule is rotating under 10 GHz electric field. It is also supported by energy parameters ΔHτ, ΔSτ and ΔFτ measured from ln (τ T) against 1/T curves for DMSO. The relative contributions c1 and c2 towards dielectric relaxations for τ1 and τ2 are ascertained from Fro¨hlich's equations and graphical technique. The corresponding dipole moments μ2 and μ1 of the whole and the flexible part of the molecule are obtained from τ2 and τ1 and the slope β of high frequency conductivity σij against wj in order to compare with the measured static μs and reported μ's. μ1's are found to agree with μj and static μs. The molecular conformational structures are obtained by μcal from μs/μtheo by considering mesomeric and inductive effects of the substituent polar groups. μtheo's are obtained from available bond moments and bond angles. The comparison of all the μ's shows little dependence on electric field frequency.

Microwave characterization of dielectric materials from 8 to 110 GHz using a free-space setup

In this paper, we present a free-space setup for the high- and low-temperature dielectric characterization of materials from 8.0 to 110 GHz along with the measured results. Using an HP 8510 XF single-connection, single-sweep network analyzer along with three pairs of spot-focused horn lens antennas, it is possible to perform broadband in-situ and real-time dielectric characterization of materials. Presently, all other methods have the disadvantages of either being narrowband or single-frequency measurement, and the difficulties associated with accurate sample preparation for different frequency bands. This method, based on a microwave beam focused to a measurement plane, is noncontact, so it can be adapted easily for measurements in high-temperature and hostile environments. The frequency dependence of the complex dielectric properties of a number of common samples is measured as a function of temperature and frequency, and is compared with existing data. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 26: 100–105, 2000.

Active feedback microstrip leaky wave antenna-synthesiser design with suppressed back lobe radiation

The design of an active leaky-wave antenna which integrates the antenna with a feedback synthesiser is introduced. The measurement result shows that the antenna has very low back lobe radiation compared with that of traditional single-terminal feeding leaky-wave antennas. Single frequency measurement shows that the radiated power difference between the main beam and back lobe is &gt; 15 dB at 8.2 GHz. For the designed feedback synthesiser antenna, the measured radiated power difference between the main beam and back lobe is &gt; 15 dB at 9.25 GHz, and the scanning angle is ~5° as the synthesiser frequency sweeps from 9.25 to 9.37 GHz.