Showing papers in "Radio Science in 2007"

Mapping the GPS multipath environment using the signal-to-noise ratio (SNR)

Abstract: [1] GPS multipath, where a signal arrives by more than one path, is a source of positioning error which cannot be easily neutralized. Better understanding of the multipath environment, i.e., the direction of and distance to reflecting objects, is important for multipath mitigation during the site construction phase as well as discerning the impact of multipath on positioning estimates for existing sites. This paper presents a tool called power spectral mapping that visually represents the multipath environment of a GPS site. This technique uses the spectral content (frequency and magnitude) of signal-to-noise ratio (SNR) time series to determine which satellites, and therefore which portions of the antenna environment, contribute significant multipath error and at what frequencies. Wavelet analysis is used to extract the time-varying frequency and magnitude content of various multipath constituents, and these data are projected onto a map representing the GPS antenna surroundings. Power spectral map examples from stations with very different multipath environments are presented. The maps are interpreted in terms of potential sources of multipath reflections, and how these multipath signals contribute to positioning error at each station is also assessed.

Polarity asymmetry of sprite‐producing lightning: A paradox?

Abstract: [1] Sprites and halos in the mesosphere are produced electrostatically by lightning ground flashes whose polarity is positive, by a margin of at least 1000 to 1 in collected observations. The initiation of these events is controlled by the vertical charge moment change of the flash. Schumann resonance ELF methods have been used to measure the charge moments of millions of flashes worldwide. The bipolar distributions of these events show stronger positive than negative tails, consistent with the predominance of “positive” sprites, but the negative tail of supercritical events is still of the order of 10% of the total supercritical population, more than 1 order of magnitude larger than the observed fraction of “negative” sprites. This juxtaposition constitutes a paradox. The suggested resolution of the paradox is that the more impulsive population of supercritical negative flashes is producing dim halos that are not readily detected in conventional video imagery. Additional sensitive, high-resolution, and high-speed imager (<1 ms) studies of halos and their lightning parents are needed to verify this hypothesis.

Automatic scaling of critical frequency foF2 and MUF(3000)F2: A comparison between Autoscala and ARTIST 4.5 on Rome data

Abstract: [1] The performances of Autoscala and ARTIST 4.5 were comparatively evaluated using a large database of 6098 ionograms recorded from September 2005 to June 2006 by the digisonde DPS4 at the Rome ionospheric station. Results of comparisons between automatically and manually scaled data are shown for both programs highlighting the different behaviors. The Autoscala and ARTIST 4.5 values of foF2 and MUF(3000)F2 both agree with the hand-scaled values for ∼95% of ionograms. For the other ∼5% of ionograms, which the manual scaler classed as unscalable, ARTIST 4.5 usually gave invalid results, whereas Autoscala usually gave no result. The data recorded by the ionosondes DPS4 (interpreted by ARTIST 4.5) and AIS-INGV (interpreted by Autoscala) during three geomagnetic storms were also analyzed. Ionograms with typical errors both for Autoscala and ARTIST 4.5 are displayed.

Scintillation modeling using in situ data

Abstract: [1] Satellite in situ measurements of plasma (electron) density fluctuations provide direct information about the structure and morphology of irregularities that are responsible for scintillation of radio waves on transionospheric links. When supplemented with the ionosphere model and irregularity anisotropy model, they can be applied to model morphology of scintillation provided a suitable propagation model is used. In this paper we present a scintillation climatological model for the Northern Hemisphere high-latitude ionosphere, which makes use of the Dynamics Explorer 2 retarding potential analyzer plasma density data, IRI ionosphere model, and the phase screen propagation model. An important aspect of our work is that we derived from the satellite data not just the turbulence strength parameter Cs but also the spectral index p, which influences the scintillation level as well. We discuss the magnetic activity, season, magnetic time, and latitude dependence of these parameters. We were able to reproduce successfully the observed scintillation intensity diurnal and seasonal variations. The model satisfactorily describes the expansion of the scintillation zone under magnetically disturbed conditions and reproduces the dawn-dusk asymmetry in the scintillation intensity. The results demonstrate the usefulness of the proposed approach.

Comparison of CHAMP and Digisonde plasma frequencies at Jicamarca, Peru

Abstract: [1] Ionospheric plasma frequencies at the altitude of the CHAMP satellite have been deduced from ionosonde true-height profiles for Jicamarca, Peru, and have been compared with the in situ measurements made by CHAMP The differences between the plasma frequencies have been found to be well within the uncertainties associated with the ionosonde profiles, confirming the validity of the CHAMP measurements For satellite-ionosonde separations of less than 250 km and for satellite altitudes below the peak of the F2 layer, the average discrepancy between the two plasma frequencies is 025 MHz or 4% For the most reliable ionosonde measurements, the average discrepancies reduce to 018 MHz (or 17%), with a standard deviation of 016 MHz (or 15%) Given the validity of the CHAMP plasma frequencies, corresponding ionosonde and CHAMP observations have been used to support the practice of extending the ionosonde profile above hmF2 by assuming a Chapman layer with a constant scale height equal to that of the lower side of the F2 layer peak The average discrepancy for CHAMP passing above the peak of the F2 layer is 022 MHz (or 26%), and the standard deviation is 08 MHz (or 133%)

Traveling waves on two- and three-dimensional periodic arrays of lossless scatterers

Abstract: [1] The kd–βd (dispersion) equations are found for traveling waves on two- and three-dimensional infinite periodic arrays of small lossless acoustic monopoles, electric or magnetic dipoles, and magnetodielectric spheres. Using Floquet mode expansions and then expressions for the rapid summation of Schlomilch series, prohibitively slowly convergent summations are converted to forms that can be used for the efficient calculation of the kd–βd equations. Computer programs have been written to obtain the kd–βd diagrams for all the arrays treated, and representative numerical results are presented and discussed. Expressions, more accurate than the Clausius-Mossotti relations, are obtained for the effective or bulk permittivity and permeability of the arrays utilizing quantities readily available from the solutions of the kd–βd equations. Exact computable expressions for the fields of three-dimensional lossless or lossy magnetodielectric sphere arrays that are finite in the direction of the array axis, illuminated by a plane wave parallel to the array axis, are obtained from the analyses performed to obtain the kd–βd curves for the infinite arrays.

Fredholm factorization of Wiener‐Hopf scalar and matrix kernels

Abstract: [1] A general theory to factorize the Wiener-Hopf (W-H) kernel using Fredholm Integral Equations (FIE) of the second kind is presented. This technique, hereafter called Fredholm factorization, factorizes the W-H kernel using simple numerical quadrature. W-H kernels can be either of scalar form or of matrix form with arbitrary dimensions. The kernel spectrum can be continuous (with branch points), discrete (with poles), or mixed (with branch points and poles). In order to validate the proposed method, rational matrix kernels in particular are studied since they admit exact closed form factorization. In the appendix a new analytical method to factorize rational matrix kernels is also described. The Fredholm factorization is discussed in detail, supplying several numerical tests. Physical aspects are also illustrated in the framework of scattering problems: in particular, diffraction problems. Mathematical proofs are reported in the paper.

Statistical maritime radar duct estimation using hybrid genetic algorithm–Markov chain Monte Carlo method

Abstract: [1] This paper addresses the problem of estimating the lower atmospheric refractivity ( M profile) under nonstandard propagation conditions frequently encountered in low-altitude maritime radar applications. This is done by statistically estimating the duct strength (range- and height-dependent atmospheric index of refraction) from the sea surface reflected radar clutter. These environmental statistics can then be used to predict the radar performance. In previous work, genetic algorithms (GA) and Markov chain Monte Carlo (MCMC) samplers were used to calculate the atmospheric refractivity from returned radar clutter. Although GA is fast and estimates the maximum a posteriori ( MAP) solution well, it poorly calculates the multidimensional integrals required to obtain the means, variances, and underlying posterior probability distribution functions of the estimated parameters. More accurate distributions and integral calculations can be obtained using MCMC samplers, such as the Metropolis-Hastings and Gibbs sampling (GS) algorithms. Their drawback is that they require a large number of samples relative to the global optimization techniques such as GA and become impractical with an increasing number of unknowns. A hybrid GA-MCMC method based on the nearest neighborhood algorithm is implemented in this paper. It is an improved GA method which improves integral calculation accuracy through hybridization with a MCMC sampler. Since the number of forward models is determined by GA, it requires fewer forward model samples than a MCMC, enabling inversion of atmospheric models with a larger number of unknowns.

Direction of arrival estimation for nonuniform linear arrays by using array interpolation

Abstract: [1] A new approach is proposed for DOA estimation in nonuniform linear arrays (NLA) based on array interpolation. A Wiener formulation is presented to improve the condition number of the mapping matrix as well as the performance for noisy observations. Noniterative and iterative methods for DOA estimation are proposed. These methods use an initial DOA which is then significantly improved by the subsequent processing. Partially augmentable nonredundant arrays (PANA) and partly filled NLA (PFNLA) are considered and initial DOA is found in a different manner for each of these arrays. PANA are used for noncoherent sources whereas PFNLA are employed when the sources are coherent. Array interpolation is used to map the sample covariance matrix of the NLA to the covariance matrix of a uniform linear array (ULA) with the same aperture size and root-MUSIC is employed as a fast subspace algorithm for DOA estimation. Proposed methods overcome some of the limitations of the conventional array interpolation. DOA estimation problem is considered for correlated and coherent sources. Proposed approaches are compared with two previous methods as well as the spectral MUSIC and root-MUSIC algorithms applied on the NLA and ULA, respectively. It is shown that DOA performance is significantly improved for a wide variety of DOA scenarios.

Recursive Bayesian electromagnetic refractivity estimation from radar sea clutter

Abstract: [1] Estimation of the range- and height-dependent index of refraction over the sea surface facilitates prediction of ducted microwave propagation loss. In this paper, refractivity estimation from radar clutter returns is performed using a Markov state space model for microwave propagation. Specifically, the parabolic approximation for numerical solution of the wave equation is used to formulate the refractivity from clutter (RFC) problem within a nonlinear recursive Bayesian state estimation framework. RFC under this nonlinear state space formulation is more efficient than global fitting of refractivity parameters when the total number of range-varying parameters exceeds the number of basis functions required to represent the height-dependent field at a given range. Moreover, the range-recursive nature of the estimator can be easily adapted to situations where the refractivity modeling changes at discrete ranges, such as at a shoreline. A fast range-recursive solution for obtaining range-varying refractivity is achieved by using sequential importance sampling extensions to state estimation techniques, namely, the forward and Viterbi algorithms. Simulation and real data results from radar clutter collected off Wallops Island, Virginia, are presented which demonstrate the ability of this method to produce propagation loss estimates that compare favorably with ground truth refractivity measurements.

Multiple scattering of waves by dense random distributions of sticky particles for applications in microwave scattering by terrestrial snow

Abstract: [1] We investigate the multiple scattering of waves by dense random distribution of particles. Maxwell equations are put in the form of Foldy-Lax multiple-scattering equations which are solved numerically. The positions of the particles are generated by random shuffling and bonding. Simulations are performed for applications in microwave scattering by terrestrial snow. The results are illustrated for the copolarization and cross-polarization scattering phase matrices and the extinction coefficients for sticky particles. We consider concentrations of particles up to 40% by volume. Results of dense media simulations depart from the predictions on the basis of classical theory of independent scattering and are applicable for very low concentrations. The simulation results agree with those of quasi-crystalline approximation (QCA) for concentration up to 20%. However, they start to deviate from those of the QCA for higher concentrations as QCA underestimates the extinction. Simulation results also predict strong cross polarization in the phase matrix of densely packed spheres, a result that is not predicted by classical independent scattering nor by QCA.

ULF/ELF magnetic field variations from atmosphere induced by seismicity

Abstract: [1] Local variations of the magnetic field in the ULF-ELF frequency range associated with seismicity are studied with the data of more than 3 a observations at Karimshimo complex observatory (latitude 52.83°N, longitude 158.13°E, Kamchatka, Russia). A wideband emission is found to start about 5 d before an earthquake and last until 5 d after it. Seismic ULF/ELF emission in the frequency range of 4–6 Hz as compared with the seismically quiet background has enhanced Phh/Pdd spectral ratio and reduced standard deviation of ellipse orientation angle and the ellipticity, and it has a more linear polarization. Parameters of this emission are studied for more than 30 individual earthquakes and statistically with the superposed epoch method. The reliability of the earthquake predicting hypothesis is verified, and the favorable parameters for the earthquakes together with those for ELF magnetic field are selected. The following earthquake parameters are favorable for this emission: depths H 5.5, and epicenter distances R < 300 km. The changes of natural ULF/ELF emissions during the periods of enhanced seismic activity are interpreted as the result of the excitation of additional ULF/ELF emissions in the seismic zone to the east of the observatory or the redistribution of lightning discharges with their possible concentration near the active crust fault. The earthquake prediction hypothesis is verified for the complex field parameter ΔS and proved to be successful.

Focal waveform of a prolate‐spheroidal impulse‐radiating antenna

Abstract: [1] This paper develops some analytical approximations for the transient focal waveform produced at the second focus of a prolate-spheroidal reflector due to a pulse TEM wave launched from the first focus. This is extended to consider the spot size of the peak field near the second focus.

Effect of ducting on radio occultation measurements: An assessment based on high‐resolution radiosonde soundings

Abstract: Recent studies have shown that the presence of elevated ducts in the lower atmosphere has an adverse effect on the inversion of GPS radio occultation data. The problem arises because the microwave refractivity within and below an elevated duct is no longer uniquely determined by the bending angle profile. Applying Abel inversion without a priori knowledge of the duct will introduce a negative bias in the retrieved refractivity profile within and below the duct. In this work, high vertical resolution radiosonde data are used to give a quantitative assessment of the characteristics and effects of ducts, including their frequency of occurrences, heights, and thicknesses at different latitudes and seasons. The negative bias from the Abel-retrieved refractivity profiles resulting from these ducts is also computed. The results give a strong indication that ducting in the lower troposphere is a frequent phenomenon over the tropics and midlatitudes. The ducts are shown to be predominantly caused by sharp changes in the vertical structure of water vapor. The majority of the ducts are found to be below 2 km, with a median duct layer thickness of about 100 m. The negative refractivity bias is shown to be largest below 2 km, with a median value of about 0.5-1% in the tropics and 0.2-0.5% in midlatitudes. The bias is about a factor of 2-3 smaller between 2 to 3 km and is negligible above 4 km.

Initial results from SKiYMET meteor radar at Thumba (8.5°N, 77°E): 1. Comparison of wind measurements with MF spaced antenna radar system

Abstract: [1] In the present communication, initial results from the allSKy interferometric METeor (SKiYMET) radar installed at Thumba (8.5°N, 77°E) are presented. The meteor radar system provides hourly zonal and meridional winds in the mesosphere lower thermosphere (MLT) region. The meteor radar measured zonal and meridional winds are compared with nearby MF radar at Tirunalveli (8.7°N, 77.8°E). The present study provided an opportunity to compare the winds measured by the two different techniques, namely, interferometry and spaced antenna drift methods. Simultaneous wind measurements for a total number of 273 days during September 2004 to May 2005 are compared. The comparison showed a very good agreement between these two techniques in the height region 82–90 km and poor agreement above this height region. In general, the zonal winds compare very well as compared to the meridional winds. The observed discrepancies in the wind comparison above 90 km are discussed in the light of existing limitations of both the radars. The detailed analysis revealed the consistency of the measured winds by both the techniques. However, the discrepancies are observed at higher altitudes and are attributed to the contamination of MF radar neutral wind measurements with Equatorial Electro Jet (EEJ) induced inospheric drifts rather than the limitations of the spaced antenna technique. The comparison of diurnal variation of zonal winds above 90 km measured by both the radars is in reasonably good agreement in the absence of EEJ (during local nighttime). It is also been noted that the difference in the zonal wind measurements by both the radars is directly related to the strength of EEJ, which is a noteworthy result from the present study.

A simple and efficient FDTD/PBC algorithm for scattering analysis of periodic structures

Abstract: [1] An efficient finite difference time domain (FDTD) algorithm with a simple periodic boundary condition (PBC) is developed to analyze reflection and transmission properties of general periodic structures with arbitrary incident angles. In this new approach, the FDTD simulation is performed by setting a constant horizontal wave number instead of a specific incident angle. The principle of the FDTD/PBC algorithm is discussed and two important implementation issues are addressed, namely, the excitation of plane wave and the suppression of horizontal resonance. The validity of the approach is demonstrated through several numerical examples on dielectric slabs and frequency selective surfaces.

Software developed for obtaining GPS-derived total electron content values

Abstract: This paper describes a complex technique with its built-in cycle slip correction procedures that have been developed for ionospheric space research to obtain high-quality and high-precision GPS-derived total electron content (TEC) values. Thus, to correct GPS anomalies while the signatures of space weather features detected in the dual-frequency 30-s rate GPS data are preserved is the main aim of this technique. Its main requirement is to complete fully automatically all the tasks required to turn the observational data to the desired final product. Its major tasks include curve fitting, cycle slip detection and correction in the slant relative TEC data, residual error detection and correction in the vertical TEC data, and vertical TEC data filtering for quantifying data smoothness and GPS phase fluctuations. A detailed description of these two data correction methods is given. Validation tests showing weaknesses and strengths of the methods developed are also included and discussed. Versatility and accuracy of the methods are demonstrated with interesting and real-world examples obtained from smooth midlatitude and from dynamic low- and high-latitude data. Results indicate that errors can be detected and corrected more reliably in the vertical TEC data than in the slant TEC data because of the lower rate of change of vertical TEC over a 30-s sampling period. Future work includes the development of a complex software package wherein the individual FORTRAN algorithms, described in this paper, will be incorporated into one main (FORTRAN, Matlab, or C++) program to provide professional and customized GPS data processing for ionospheric space research.

A method for automatic scaling of sporadic E layers from ionograms

Abstract: [1] A method for automatic scaling of the maximum frequency and virtual height of a sporadic E layer is presented. A set of ionograms recorded at the ionospheric observatory of Gibilmanna was used to test the performance of the algorithm. The test was performed by comparing the data obtained automatically with the values scaled by an operator.

Synopsis of the historical development of Schumann resonances

Abstract: [1] The life and work of Winfried Otto Schumann and the historical development of the main ideas leading to the hypothesis of Earth-ionosphere cavity electromagnetic oscillations are reviewed. The so-called Schumann resonances are a set of frequencies of electromagnetic waves in the natural cavity formed by a planet's (moon's) surface and its ionosphere, in the extremely low frequency (ELF) range, caused by natural electrical activity of the planet (moon) and/or its atmospheric environment. Additionally, the reception of his work by the contemporary scientific community as well as the experimental evidence for the postulated ELF resonance oscillations is described.

Wavelet analysis of GPS amplitude scintillation: A case study

Abstract: [1] Amplitude scintillations on GPS links due to diffraction by ionospheric irregularities are traditionally described, in their strength, via the standard deviation of the signal power normalized to its average within 1 min, the S4 index. One should be very careful in using S4 and always remember that this quantity contains useful information when the signal is stationary for 60 s. In this work, amplitude scintillations are described in a multiscale approach; a wavelet decomposition of the amplitude time series is performed, and the evolution of the signal is studied through its scalogram. What is found is that sudden turbulent events in the amplitude time series, potentially very harmful for the stability and reliability of the radio link, are singled out very clearly in scalograms and cannot be confused with segments of quiet signals. This technique is found to be very useful in those cases in which the signal cannot be considered as stationary over a minute. This new approach is a powerful technique for identifying events that are particularly disruptive to GPS operations. Applications of the new approach in identifying short-duration fading and rapid phase fluctuations are discussed.

Differential validation of the US-TEC model

Abstract: [1] This paper presents a validation and accuracy assessment of the total electron content (TEC) from US-TEC, a new product presented by the Space Environment Center over the contiguous United States (CONUS). US-TEC is a real-time operational implementation of the MAGIC code and provides TEC maps every 15 min and the line-of-sight electron content between any point within the CONUS and all GPS satellites in view. Validation of TEC is difficult since there are no absolute or true values of TEC. All methods of obtaining TEC, for instance, from GPS, ocean surface monitors (TOPEX), and lightning detectors (FORTE), have challenges that limit their accuracy. GPS data have interfrequency biases; TOPEX also has biases, and data are collected only over the oceans; and FORTE can eliminate biases, but because of the lower operating frequency, the signals suffer greater bending on the rays. Because of the difficulty in obtaining an absolute unbiased TEC measurement, a “differential” accuracy estimate has been performed. The method relies on the fact that uninterrupted GPS data along a particular receiver-satellite link with no cycle slips are very precise. The phase difference (scaled to TEC units) from one epoch to the next can be determined with an accuracy of less than 0.01 TEC units. This fact can be utilized to estimate the uncertainty in the US-TEC vertical and slant path maps. By integrating through US-TEC inversion maps at two different times, the difference in the slant TEC can be compared with the direct phase difference in the original RINEX data file for nine receivers not used in the US-TEC calculations. The results of this study, for the period of April–September 2004, showed an average root mean square error of 2.4 TEC units, which is equivalent to less than 40 cm of signal delay at the GPS L1 frequency. The accuracy estimates from this “differential” method are similar to the results from a companion paper utilizing an “absolute” validation method by comparing with FORTE data.

On modeling the lower characteristic ELF altitude from aeronomical data

Abstract: [1] Improved observations in the Schumann resonance frequency range have expanded interest in improved models for the Earth-ionosphere waveguide, particularly models that incorporate day-night asymmetry. Aeronomical data are used here to construct daytime and nighttime analytic conductivity models of the lower characteristic layer, one of two vertically separated regions governing ELF propagation in the waveguide. The models have the form of double-exponential functions that represent the “knee”-like transition from ion- to electron-dominated conductivity in a physically realistic manner. On the basis of these profiles, analytic approximations of the lower complex characteristic altitude are obtained for the frequency range 3–100 Hz. The approximations are accurate and simple to use for parametric analysis.

Data assimilation of incoherent scatter radar observation into a one-dimensional midlatitude ionospheric model by applying ensemble Kalman filter

Abstract: [1] In this paper, electron densities during 25-28 September 2000 observed by the Millstone Hill incoherent scatter radar (ISR) are assimilated into a one-dimensional midlatitude ionospheric theoretical model by using an ensemble Kalman filter (EnKF) technique. It is found that (1) the derived vertical correlation coefficients of electron density show obvious altitude dependence. These variations are consistent with those from ISR observations. (2) The EnKF technique has a better performance than the 3DVAR technique especially in the data-gap regions, which indicates that the EnKF technique can extend the influences of observations from data-rich regions to data-gap regions more effectively. (3) Both the altitude and local time variations of the root mean square error (RMSE) of electron densities for the ensemble spread and ensemble mean from observation behave similarly. It is shown that the spread of the ensemble members can represent the deviations of ensemble mean from observations. (4) To achieve a better prediction performance, the external driving forces should also be adjusted simultaneously to the real weather conditions. For example, the performance of prediction can be improved by adjusting neutral meridional wind using equivalent wind method. (5) In the EnKF, there are often erroneous correlations over large distance because of the sampling error. This problem may be avoided by using a relative larger ensemble size.

Analytical and numerical investigation of the radiation from concentric metamaterial spheres excited by an electric Hertzian dipole

Abstract: [1] The canonical problem of an electric Hertzian dipole radiating in the presence of a pair of concentric double negative metamaterial spheres is investigated analytically and numerically. The spatial distribution of the near field as well as the total radiated power are examined. The results are compared to those for the corresponding structures made of conventional double positive materials. It is shown that electrically small concentric metamaterial spheres can be designed to be resonant and that these resonant designs lead to significant changes in the field radiated by the electric Hertzian dipole and, in particular, to significant enhancements of the total power radiated by it. The impact of the location of the electric Hertzian dipole in both radiating and scattering configurations is studied in detail. Furthermore, the influence of dispersion and loss is investigated. Finally, a few results for larger size metamaterial concentric spheres are given.

Solar corona–induced fluctuations on spacecraft signal amplitude observed during solar superior conjunctions of the Cassini spacecraft

Abstract: [1] During the superior solar conjunctions of interplanetary spacecraft, the emitted radio signals undergo increased amplitude scintillation, phase scintillation, and spectral broadening due to the intervening charged particles of the solar corona. The magnitude of these parameters generally increases as the angle between the spacecraft and the center of the Sun (Sun-Earth probe (SEP) angle) decreases as observed by the tracking station on Earth. During periods of quiescent background, the scintillation and spectral broadening measurements of spacecraft in the ecliptic follow well-defined trends of established models as a function of SEP angle and link frequency. The amplitude data acquired from the Cassini solar superior conjunctions of May 2000 and June 2001 during the maximum of Solar Cycle 23 were analyzed. The X band (8.4 GHz) and Ka band (32 GHz) scintillation measurements from the signal amplitude, the autocorrelation function of the amplitude, and the histogram of the amplitude samples acquired during quiescent periods were found to be consistent with models based on previous studies with other wavelengths. Amplitude autocorrelation functions of the signal amplitude yield information on the limiting scale sizes of the charged-particle blobs which constitute the solar corona. Evidence for very fast fluctuations at X band (∼3 ms) was observed at SEP angles near 0.6°. The histograms of the open-loop receiver amplitude samples were found to be consistent with the Rice distribution. The transition from strong to weak scintillation at Ka band was clearly captured in a Cassini 2001 solar conjunction pass that occurred near an SEP angle of 0.6° (2.3 RS) as was expected for this frequency. The existing scintillation models were validated for Ka band, which is planned as a link frequency for deep space communications for upcoming missions. These results from this study are useful in providing information on solar activity on spacecraft signals at both X band and Ka band, which can be used to develop telecommunication strategies during solar superior conjunctions of planetary spacecraft.

Ionospheric Doppler and echo phase measured by the Wuhan Ionospheric Oblique Backscattering Sounding System

Abstract: [1] Wuhan Ionospheric Oblique Backscattering Sounding System (WIOBSS) is a high-frequency (HF) sky wave over-the-horizon radar developed for ionospheric research and HF channel management This radar, located in Wuhan (3035°N, 11433°E) has been under construction by the Ionosphere Lab of Wuhan University since 2002 WIOBSS uses long coded pulses and pulse compression to achieve good sensitivity and measures the bitemporal ionospheric response, channel scattering function, and phase of echoes simultaneously and then displays the processed data as ionograms and Dopplionograms in real time Bitemporal ionospheric response is a matrix composed of the ionospheric transmission function with delay and time variables, while the scattering function shows the spectrum of the received signals as a function of the delay and Doppler coordinates, and is obtained by the Fourier Transform of the bitemporal ionospheric response over the time variable The application of pulse compression and coherent integration allows WIOBSS to measure ionospheric Doppler with high resolution and very low power A method is developed to get phase data directly from the bitemporal ionospheric response The measured phase is strung in a series and then divided into linear and nonlinear components The angular frequency of the linear component is in direct proportion to ionospheric Doppler velocity while the fluctuation of the nonlinear component includes the information of Doppler velocity spread The phase method based on pulse impression is able to obtain the statistic Doppler information from weak backscattered echoes in short detection time

Relative importance of the day-night asymmetry in Schumann resonance amplitude records

Abstract: [1] Computations of Schumann resonance (SR) field amplitudes from a point source in the uniform and nonuniform cavities are performed to estimate the relative importance of the day-night asymmetry in observed SR amplitudes. Additional simulations with globally distributed sources, representing diurnal and seasonal variations of global lightning activity, are performed in uniform and nonuniform cavities to evaluate the impact of the day-night asymmetry on the diurnal and seasonal amplitude variations. The results show that the effect of diurnal ionosphere changes on the first mode SR amplitudes is ∼10%. Model results indicate that the source properties (the level of activity and proximity to the observer) play the dominant role in the diurnal variations observed in SR field records, with the day-night asymmetry being secondary in importance.

Testing regional vertical total electron content maps over Europe during the 17-21 January 2005 sudden space weather event

Abstract: [1] The intense level of solar activity recorded from 16 to 23 January 2005 led to a series of events with different signatures at the Earth's ionospheric distances Measurements of the critical frequency of the F2 layer foF2 and the vertical total electron content (VTEC) are used to describe the temporal and spatial electron density distributions during this space weather event, which gives an excellent opportunity to test regional VTEC maps over Europe under such disturbed solar-terrestrial conditions In this context, the tests used to validate the International GNSS Service (IGS) VTEC maps have been applied to assess the accuracy of the European Rutherford Appleton Laboratory (RAL) VTEC maps Thus the self-consistency test and the Jason altimeter test have been used to compare such performances with the IGS and Universitat Politecnica de Catalunya global ionospheric maps The results show discrepancies between the RAL maps and the IGS ones, which leads to significant RMS and bias values of several total electron content units Moreover, in this work a kriging technique to improve the accuracy of any regional VTEC map is also considered, with relative improvements of the RAL VTEC maps up to more than 20% at the peak of the storm

Validation of ionospheric weather predicted by Global Assimilation of Ionospheric Measurements (GAIM) models

Abstract: [1] The ability of the Utah State University (USU) Global Assimilation of Ionospheric Measurements (GAIM) model to specify the day-to-day variability of the ionosphere is assessed by comparison of specified hourly values of foF2 with observations that were not assimilated by the model. If the specifications are to be an improvement over a climatological model with zero average error, the interdecile width of errors of the specifications at each hour must be less than the interdecile width of the observations. It has been found that the median specified values of foF2 for undisturbed days are quite accurate, and that the interdecile width of errors of the specifications is less than that of the observations most of the time for midlatitude sites, provided there is a GPS TEC site sufficiently close to the ionosonde site. The errors in GAIM values of foF2 for Australian locations tend to be larger for larger distances from a GPS site, and for low latitudes. GAIM also tracks the values of foF2 on disturbed days, but tends to be conservative, not reproducing the full extent of the departures of the disturbed values from the background values.

Analytical and numerical investigation of the radiation and scattering from concentric metamaterial cylinders excited by an electric line source

Abstract: [1] An antenna configuration consisting of an arbitrarily located electric line source radiating in the presence of a pair of concentric metamaterial cylinders is investigated analytically and numerically. The near- and far-field properties of these structures are analyzed through an investigation of such parameters as the total radiated power, directivity, and total as well as differential scattering cross sections. The results obtained for these metamaterial structures are compared to those for the corresponding structures made of conventional materials. It is shown that electrically small concentric metamaterial structures can be designed to be resonant and to possess advantageous radiation and scattering characteristics in contrast to the corresponding structures made of conventional materials. More specifically, it is shown that metamaterial structures lead to significant enhancements of the total radiated power as well as the total and differential scattering cross sections. Moreover, the feasibility of controlling the directivity pattern of the electrically small metamaterial structures through appropriate locations of the electric line source is demonstrated. The effects of the dispersion and loss present in the metamaterials are taken into account to study the bandwidth properties of these resonant configurations.