Showing papers in "Radio Science in 2006"
TL;DR: In this paper, a new radar-imaging technique, called polarization coherence tomography (PCT), was introduced, which employs variation of the interferometric coherence with polarization to reconstruct a vertical profile function in penetrable volume scattering.
Abstract: [1] In this paper we introduce a new radar-imaging technique, called polarization coherence tomography (PCT), which employs variation of the interferometric coherence with polarization to reconstruct a vertical profile function in penetrable volume scattering. We first show how this profile function can be efficiently represented as a Fourier-Legendre series, with tomographic reconstruction reducing to estimation of the unknown coefficients of this series from coherence data. We then show that we can linearize this inversion by using a priori knowledge of two parameters, namely, volume depth and topographic phase. We further propose a new algorithm based on polarimetric interferometry to estimate these two from the data itself. To assess stability, we investigate both the single- and dual-baseline conditioning of the associated matrix inversion and then concentrate on the single-baseline case to demonstrate that for sufficient multilooking (around 50), stable retrievals of profiles can be obtained in the presence of coherence noise. Finally, we apply the technique to simulated L band coherent radar data to demonstrate its potential for new applications in radar remote sensing.
180 citations
TL;DR: In this paper, the authors established a statistical database of more than 14,400 equatorial plasma bubble (EPB) observations from polar-orbiting Defense Meteorological Satellite Program (DMSP) spacecraft for 1989-2004.
Abstract: [1] After examining evening sector plasma density measurements from polar-orbiting Defense Meteorological Satellite Program (DMSP) spacecraft for 1989–2004, we have established a statistical database of more than 14,400 equatorial plasma bubble (EPB) observations. EPBs are irregular plasma density depletions in the postsunset ionosphere that degrade communication and navigation signals. In general, the DMSP observations support Tsunoda's (1985) hypothesis that EPB rates peak when the terminator is aligned with the Earth's magnetic field, but unpredicted offsets are also evident in many longitude sectors. Plots of EPB rates for solar cycle phases: maximum 1989–1992 and 1999–2002, minimum 1994–1997, and transition years 1993, 1998, and 2003 reveal significant differences in the climatologies for solar maximum and minimum, between the two solar maxima, and in the transition years. To assess local time effects on EPB rates, we also compare observations from F12, F14, F15, and F16 at slightly different postsunset local times for 2000–2004. This study was undertaken to facilitate improvements in ionospheric models in preparation for the Communication/Navigation Outage Forecasting System (C/NOFS) mission.
97 citations
TL;DR: In this paper, the authors developed a model called the Middle East Technical University Neural Network (METU-NN) model, which is a data-driven neural network model of one hidden layer and several neurons.
Abstract: [1] Near-Earth space processes are highly nonlinear. Since the 1990s, a small group at the Middle East Technical University in Ankara has been working on a data-driven generic model of such processes, that is, forecasting and nowcasting of a near-Earth space parameter of interest. The model developed is called the Middle East Technical University Neural Network (METU-NN) model. The METU-NN is a data-driven neural network model of one hidden layer and several neurons. In order to understand more about the complex response of the magnetosphere and ionosphere to extreme solar events, we chose this time the series of space weather events in November 2003. Total electron content (TEC) values of the ionosphere are forecast during these space weather events. In order to facilitate an easier interpretation of the forecast TEC values, maps of TEC are produced by using the Bezier surface-fitting technique.
89 citations
TL;DR: In this article, the authors reviewed the different approaches, their implications for IRI, and their current status, and special emphasis is given to the coupling between topside and plasmaspheric models.
Abstract: [1] Shortcomings of the representation of the topside electron density profile in the International Reference Ionosphere (IRI) model have been noted in comparison with recently analyzed topside sounder data and also with total electron content (TEC) data. Various studies have proposed corrections of the IRI formulas or have introduced a new formalism. This paper reviews the different approaches, their implications for IRI, and their current status. An important challenge for topside modeling is the truthful representation of profiles in the equatorial anomaly (EA) region over the whole range of the EA fountain. This means that the latitudinal representation has to reproduce the merging of the double-peak signature at F region heights into a single peak at the top of the fountain. In this context, special emphasis is given to the coupling between topside and plasmaspheric models.
89 citations
TL;DR: Aperture synthesis radar imaging has been used to investigate coherent backscatter from ionospheric plasma irregularities at Jicamarca and elsewhere for several years, and the algorithm was improved recently by incorporating the antenna radiation pattern in the prior probability and estimating and including the full error covariance matrix in the constraints.
Abstract: [1] Aperture synthesis radar imaging has been used to investigate coherent backscatter from ionospheric plasma irregularities at Jicamarca and elsewhere for several years. Phenomena of interest include equatorial spread F, 150-km echoes, the equatorial electrojet, range-spread meteor trails, and mesospheric echoes. The sought-after images are related to spaced-receiver data mathematically through an integral transform, but direct inversion is generally impractical or suboptimal. We instead turn to statistical inverse theory, endeavoring to utilize fully all available information in the data inversion. The imaging algorithm used at Jicamarca is based on an implementation of the MaxEnt method developed for radio astronomy. Its strategy is to limit the space of candidate images to those that are positive definite, consistent with data to the degree required by experimental confidence limits; smooth (in some sense); and most representative of the class of possible solutions. The algorithm was improved recently by (1) incorporating the antenna radiation pattern in the prior probability and (2) estimating and including the full error covariance matrix in the constraints. The revised algorithm is evaluated using new 28-baseline electrojet data from Jicamarca.
75 citations
TL;DR: The Coherent Electromagnetic Radio Tomography (CERTO) constellation of radio beacons is available for measurements of ionospheric total electron content and radio scintillations.
Abstract: [1] The new constellation of radio beacons called Coherent Electromagnetic Radio Tomography (CERTO) will be available for measurements of ionospheric total electron content and radio scintillations. These beacons transmit unmodulated, phase-coherent waves, VHF, UHF, and L band frequencies. A fixed radio of 3/8 is used between successive frequencies. Total electron content (TEC) can be measured using the differential phase technique. The range between beacon and receiver is removed from the phase measurements, leaving a differential phase that is proportional to TEC. The three CERTO frequencies cover a wide range for determination of the radio scintillation effects caused by diffraction after propagation though ionospheric irregularities. All of the CERTO beacons are in low Earth orbit with inclinations ranging from equatorial to polar. Each satellite that carries CERTO has other plasma instruments that complement the beacon data. In addition, a Scintillation and Tomography Receiver in Space (CITRIS) instrument will be placed in orbit to detect signals from the CERTO beacons and from the array of 56 Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) VHF/S band radio beacons placed around the word by the French Centre National D'Etudes Spatiales. CITRIS will record ionospheric occultations and radio scintillations with a unique occultation and ground-to-space geometry. New algorithms have been developed for the multifrequency CERTO and CITRIS data to provide improved acquisition and analysis of TEC and scintillation data in ionospheric studies. The data from the CERTO constellation of beacons and receivers may be used to update space weather models.
74 citations
TL;DR: Validation of US-TEC indicates an accuracy of the line-of-sight electron content of between 2 and 3 TEC units, equivalent to less than 50 cm signal delay at L1 frequencies, which promises value for GPS users.
Abstract: [1] The potential of data assimilation for operational numerical weather forecasting has been appreciated for many years. For space weather it is a new path that we are just beginning to explore. With the emergence of satellite constellations and the networks of ground-based observations, sufficient data sources are now available to make the application of data assimilation techniques a viable option. The first space weather product at Space Environment Center (SEC) utilizing data assimilation techniques, US-TEC, was launched as a test operational product in November 2004. US-TEC characterizes the ionospheric total electron content (TEC) over the continental United States (CONUS) every 15 min with about a 15-min latency. US-TEC is based on a Kalman filter data assimilation scheme driven by a ground-based network of real-time GPS stations. The product includes a map of the vertical TEC, an estimate of the uncertainty in the map, and the departure of the TEC from a 10-day average at that particular universal time. In addition, data files are provided for vertical TEC and the line-of-sight electron content to all GPS satellites in view over the CONUS at that time. The information can be used to improve single-frequency GPS positioning by providing more accurate corrections for the ionospheric signal delay, or it can be used to initialize rapid integer ambiguity resolution schemes for dual-frequency GPS systems. Validation of US-TEC indicates an accuracy of the line-of-sight electron content of between 2 and 3 TEC units (1 TECU = 1016 el m−2), equivalent to less than 50 cm signal delay at L1 frequencies, which promises value for GPS users. This is the first step along a path that will likely lead to major improvement in space weather forecasting, paralleling the advances achieved in meteorological weather forecasting.
68 citations
TL;DR: In this paper, an iterative ray-tracing approach is described to recover the parameters of a quite sophisticated three-dimensional (so-called wedge-stratified ionosphere) model of the local electron density distribution, characterizing its actual vertical Ne(h) profile together with horizontal gradients and general tilts.
Abstract: [1] The problem of electron density inversion of digital ionograms is reconsidered from the viewpoints of new possibilities and of modern requirements. The data processing system of an advanced ionosonde (the dynasonde) provides accurate measurements not only of echo group range but also of direction of arrival, among other physical parameters, thus yielding the three-dimensional distribution of apparent echolocations in each ionogram recording. An iterative ray-tracing approach is described here to recover the parameters of a quite sophisticated three-dimensional (so-called wedge-stratified ionosphere) model of the local electron density distribution, characterizing its actual vertical Ne(h) profile together with horizontal gradients and general tilts. The power of a contemporary PC is sufficient to accomplish this analysis quickly. This approach is implemented in the algorithm introduced here, is named “NeXtYZ,” and is pronounced “next wise.”
63 citations
TL;DR: In this paper, a near-real-time electron density retrieval technique based on the knowledge of the model driving parameter Az (ionization level) for the location considered is presented.
Abstract: [1] NeQuick is a three-dimensional and time-dependent quick run electron density model specifically designed for transionospheric propagation applications. It allows calculation of electron concentration values at any location in the ionosphere and the total electron content (TEC) along any ground station–to–satellite ray path. After specific adaptations, the model has been used to develop a near-real-time nontomographic electron density retrieval technique able to provide the electron density of the ionosphere above the geographic area of interest. The technique relies on the knowledge of the model driving parameter Az (ionization level) for the location considered. In the present study, the necessary Az values have been obtained through direct ingestion of Global Positioning System (GPS)–derived slant TEC data in two different ways: using data from a single GPS receiver and using data from multiple ground stations. Statistical comparisons between experimental and reconstructed slant TEC values and between experimental and retrieved maximum electron concentration values are shown.
61 citations
TL;DR: The conclusion is that GPS σϕ values depend critically on the circumstances of measurement and are difficult to compare among observations without additional knowledge, particularly of relative ionospheric drift and irregularity orientation, that may not be available from an isolated GPS receiver.
Abstract: [1] The phase scintillation index (σϕ), equal to the standard deviation of measured phase, is often used to characterize Global Positioning System (GPS) observations in ionospheric environments that may be scintillated. Since σϕ is dominated by large-scale fluctuations, questions of usage and interpretation exist as will be illustrated here. Beyond traditional concerns with detrending, multipath and receiver phase noise, there are at least two issues to be considered. The first is the marginal suitability of σϕ to characterize a power law phase screen with a poorly defined low-frequency component (e.g., outer scale). Second, observed σϕ parameters may not be relevant to GPS receiver tracking impacts. These arguments are outlined here in greater detail and are illustrated with simple one-dimensional phase screen propagation modeling results. The conclusion is that GPS σϕ values depend critically on the circumstances of measurement and are difficult to compare among observations without additional knowledge, particularly of relative ionospheric drift and irregularity orientation, that may not be available from an isolated GPS receiver. The development of suitable alternative measures requires careful consideration of the elements of GPS scintillation and its impacts. The broader GPS scintillation community should take an active role in developing suitable replacement measures for σϕ.
61 citations
TL;DR: In this paper, two functions with linear variations along the triangulation edges in both tangential and normal directions (linear normal and linear tangential (LN-LT) type) were defined and compared to the previously employed divergenceconforming Rao-Wilton-Glisson (RWG) and curl-conforming × RWG functions.
Abstract: [1] Basis functions with linear variations are investigated in terms of the accuracy of the magnetic field integral equation (MFIE) and the combined-field integral equation (CFIE), on the basis of recent reports indicating the inaccuracy of the MFIE. Electromagnetic scattering problems involving conducting targets with arbitrary geometries, closed surfaces, and planar triangulations are considered. Specifically, two functions with linear variations along the triangulation edges in both tangential and normal directions (linear normal and linear tangential (LN-LT) type) are defined. They are compared to the previously employed divergence-conforming Rao-Wilton-Glisson (RWG) and curl-conforming × RWG functions. Examples are presented to demonstrate the significant improvement in the accuracy of the MFIE and the CFIE gained by replacing the commonly used RWG functions with the LN-LT type functions.
TL;DR: In this article, two assimilative models of the ionosphere are described: the Electron Density Assimilative Model (EDAM), developed by QinetiQ, and the IonoNumerics model developed by Fusion Numerics, Inc.
Abstract: [1] Two assimilative models of the ionosphere are described: the Electron Density Assimilative Model (EDAM), developed by QinetiQ, and the IonoNumerics model developed by Fusion Numerics, Inc. Output from each technique has been compared to independent validation data measured by oblique and vertical ionosondes. Results indicate that when tested against vertical ionosondes, both models can reduce RMS errors compared to a median model. For example, the daytime RMS errors in the F region critical frequency above the Eglin Air Force Base ionosonde are 1.2 MHz for the Parameterised Ionospheric Model, 0.7 MHz for EDAM, and 1.0 MHz for IonoNumerics. Testing conducted against an oblique ionosonde has helped to expose a potential problem with ionospheric hmF2 due to poorly specified empirical driver models within IonoNumerics. The testing shows the usefulness of ray-tracing experiments for improving numerical models and the limitations of only testing models against independent total electron content measurements.
TL;DR: In this article, the authors analyzed the directional characteristics of HF signals reflected from the northerly ionosphere, and considered the propagation effects in the form of a channel simulator suitable for the testing of new equipment and processing algorithms.
Abstract: [1] The northerly ionosphere is a dynamic propagation medium that causes HF signals reflected from this region to exhibit delay spreads and Doppler shifts and spreads that significantly exceed those observed over midlatitude paths. Since the ionosphere is not perfectly horizontally stratified, the signals associated with each propagation mode may arrive at the receiver over a range of angles in both azimuth and elevation. Such large directional spreads may have a severe impact on radio systems employing multielement antenna arrays and associated signal-processing techniques since the signal environment does not comprise a small number of specular components as often assumed by the processing algorithms. In order to better understand the directional characteristics of HF signals reflected from the northerly ionosphere, prolonged measurements have recently been made over two paths: (1) from Svalbard to Kiruna, Sweden, and (2) from Kirkenes, Norway, to Kiruna. An analysis of these data is presented in this paper. The directional characteristics are summarized, and consideration is given to modeling the propagation effects in the form of a channel simulator suitable for the testing of new equipment and processing algorithms.
TL;DR: In this article, the authors developed a restriction operator to map the transient electromagnetic (TEM) field and the material properties from a fine to a coarser finite difference mesh during a forward field simulation with an explicit time-stepping scheme.
Abstract: The fact that the transient electromagnetic (TEM) field is smoothed gradually in space with time allows for a reduced spatial sampling rate of the EM field. On the basis of concepts known from multigrid methods, we have developed a restriction operator in order to map the EM field and the material properties from a fine to a coarser finite difference mesh during a forward field simulation with an explicit time-stepping scheme. Two advantages follow. First, the grid size can be reduced. Field restriction involves reducing the number of grid nodes by a factor of 2 for each Cartesian direction. Second, as can be seen from the Courant-Friedrichs-Levy condition, the larger grid spacing allows for proportionally larger time step sizes. After field restriction, a material averaging scheme is employed in order to calculate the underlying effective medium on the coarse simulation grid. Example results show a factor of up to 5 decrease in solution run time, compared to a scheme that uses a constant grid. Key to the accuracy of the approach is knowledge of the proper time range to restrict the fields. An adequate criterion to decide during run time when to restrict involves an error measure for the locations of interest between the fields on the fine mesh and the restricted fields.
TL;DR: In this paper, the authors defined the transfer functions as a transmitting antenna transfer function, a receiving antenna transfer functions, and an antenna system transfer function to characterize UWB antenna systems readily and accurately.
Abstract: [1] Transfer functions are useful parameters for ultrawideband (UWB) antenna characterization. The response of an antenna system to any excitation can be completely determined in terms of the transfer functions (or the “impulse response” in time domain) of the antennas. In this paper, the transfer functions are defined as a transmitting antenna transfer function, a receiving antenna transfer function, and an antenna system transfer function to characterize UWB antenna systems readily and accurately. The transfer functions are expressed by ABCD and S parameters when the antenna system is considered as a two-port network. As examples, an antipodal Vivaldi antenna and a disc cone antenna are investigated to validate the proposed method. The measured results are further verified by XFDTD simulation.
TL;DR: In this article, a three-dimensional finite difference time domain modeling for the extremely low frequency propagation is employed to study the Schumann resonance problems on Titan, Venus, and Mars, and the atmospheric conductivity profiles for these studies are derived from the previously reported ionospheric models for these planets.
Abstract: [1] The conducting ionosphere and conducting surface of Titan, Venus, and Mars form a concentric resonator, which would support the possibility of the existence of global electromagnetic resonances. On Earth, such resonances are commonly referred to as Schumann resonances and are excited by lightning discharges. The detection of such resonances on other planets would give a support for the existence of the electrical discharges in the lower atmosphere on these planets. In this paper, a three-dimensional finite difference time domain modeling for the extremely low frequency propagation is employed to study the Schumann resonance problems on Titan, Venus, and Mars. The atmospheric conductivity profiles for these studies are derived from the previously reported ionospheric models for these planets. The Schumann resonance frequencies and Q factors on these planets are calculated and are critically compared with those obtained from the previously published models.
TL;DR: In this article, an expert system for validating ionograms autoscaled by the University of Massachusetts-Lowell system Automatic Real Time Ionogram Scaler with True Height (ARTIST) is described.
Abstract: [1] An expert system for validating ionograms autoscaled by the University of Massachusetts-Lowell system Automatic Real Time Ionogram Scaler with True Height (ARTIST) is described. The expert system is implemented as a FORTRAN program QualScan. QualScan reads in the standard archiving output file that is created by ARTIST, which is an ASCII summary of the salient information about the scaled trace, and attempts to determine if the scaled trace is a valid representation of the actual ionogram. It provides an overall quality figure for the scaled trace, based mainly on its reasonableness, and defines error bars for the main F2 layer parameters and for the F2 plasma frequency profile. QualScan can be used on a single ionogram for real-time operations or on a batch of ionograms for qualifying data to be stored in ionospheric databases. QualScan typically “fails” one-third of ionograms from low-latitude and midlatitude sites.
TL;DR: In this article, a method to remove the nonatmospheric changes of the beacon signals and to establish the reference levels from which to measure the excess and total attenuation has been presented in detail.
Abstract: [1] Long-term statistics of tropospheric attenuation were derived from almost 4 years of measurements made in the south of England using the ITALSAT F1 beacon signals at 49.5, 39.6, and 18.7 GHz; coincident rainfall rate measurements were made at the site of the receiving ground station. A method to remove the nonatmospheric changes of the beacon signals and to establish the reference levels from which to measure the excess and total attenuation has been presented in detail. The accuracy of fade level retrieval is estimated to be ∼±0.5 dB. A new method for predicting the annual total attenuation statistics has been proposed and validated against our data and data collected in Italy at 18.7, 39.6, and 49.5 GHz. For both locations, the new proposed method gives much better predictions compared with the established International Telecommunication Union recommendation method. A significant monthly and seasonal variation was observed in the attenuation and rainfall statistics and should be taken into consideration when planning the design and use of future slant path systems. We have seen that the attenuation statistics are subject to diurnal variations; however, for the period analyzed, this variation does not seem to follow a particular pattern.
TL;DR: In this article, a review of the status of past work and a summary of TEC average patterns from pre-GPS data sets is presented, and suggests avenues of research needed to advance the GPS-era yield from TEC storm studies.
Abstract: [1] Prior to the GPS era of global diagnostics for the ionosphere's total electron content (TEC), a remarkable degree of insight into TEC storm effects was achieved using low Earth orbit and geostationary satellite radio beacon observations. Long-term studies at individual stations, networks of stations, and campaign-mode case studies revealed the complex seasonal, local time, and latitude and longitude effects upon TEC during geomagnetic storms. Theory and simulations were used to successfully explain the roles of electrodynamical mechanisms, Joule heating, thermospheric dynamics, and neutral composition changes within the general context of solar wind–magnetosphere–plasmasphere–ionosphere/thermosphere coupling during storms. This paper gives a brief review of the status of past work, presents a new summary of TEC average patterns from pre-GPS data sets, and suggests avenues of research needed to advance the GPS-era yield from TEC storm studies.
TL;DR: A system that processes phase and group delay time series from a network of dual-frequency GPS receivers and produces a dynamic ionospheric model that is consistent with all the input data and automatically estimates the measurement noise and receiver-transmitter biases.
Abstract: [1] We present a system that processes phase and group delay time series from a network of dual-frequency GPS receivers and produces a dynamic ionospheric model that is consistent with all the input data. The system is intended for monitoring the ionosphere over a fixed geographical area with dimensions of the order of several thousand kilometers. The inversion technique utilized in this system stems from the inversion technique previously developed by our group within the Coordinate Registration Enhancement by Dynamic Optimization (CREDO) project (a software package for inverting the vertical sounding, backscatter sounding, and satellite total electron content (TEC) data for over-the-horizon radar). The core of this technique is Tikhonov's methodology for solving ill-posed problems. We extended the method to multidimensional nonlinear inverse problems and developed techniques for fast numerical solution. The resulting solution for the ionospheric distribution of electron density is guaranteed to be smooth in space and time and to agree with all input data within errors of measurement. The input data consist of time series of absolute TEC and relative TEC (directly calculated from the raw dual-frequency group delays and phase delays, respectively). The system automatically estimates the measurement noise and receiver-transmitter biases. We test the system using archived data from dual-frequency GPS receivers in the southern California Scripps Orbit and Permanent Array Center (SOPAC) network and data from a vertical sounder.
TL;DR: In this paper, the structure and dynamics of the main ionospheric trough in terms of a set of defined parameters were investigated. But the primary aim of the investigation was to represent the position and shape of the trough in such a way that the experimental observations can be used for direct comparison and validation of ionosphere models.
Abstract: [1] Measurements of total electron content, obtained by monitoring satellite transmissions at stations in the United Kingdom during a period of 12 months, have been used to characterize the structure and dynamics of the main ionospheric trough in terms of a set of defined parameters. The primary aim of the investigation was to represent the position and shape of the trough in such a way that the experimental observations can be used for direct comparison and validation of ionospheric models.
TL;DR: In this article, the capabilities of advanced techniques for routine monitoring of the ionosphere and studying the ionospheric perturbations are discussed, and it is concluded that the permanent monitoring of ionosphere-plasmasphere system can play a significant role in mitigating adverse space weather effects on GNSS-based positioning.
Abstract: To gain a more comprehensive view of the powerful and dynamic plasma processes occurring during ionospheric storms, ground- and space-based observations using Global Navigation Satellite System (GNSS) signals have been analyzed. Several case and statistical studies of ionospheric storms are presented, clearly showing the generation and propagation of ionospheric disturbances. Discussed are the capabilities of advanced techniques for routine monitoring of the ionosphere and studying the ionospheric perturbations. It is concluded that the permanent monitoring of the ionosphere-plasmasphere system can play a significant role in mitigating adverse space weather effects on GNSS-based positioning.
TL;DR: In this article, a narrowband VLF receiver was used to monitor the behavior of the amplitude of several high-power transmitters located in the Northern Hemisphere under the influence of the solar proton events (SPE) of October/November 2003.
Abstract: [1] At Ny Alesund, Svalbard (78°54′N, 11°53′E, L ∼ 18), a narrowband VLF receiver was used to monitor the behavior of the amplitude of several high-power transmitters located in the Northern Hemisphere under the influence of the solar proton events (SPE) of October/November 2003. We have used Sodankyla ion chemistry (SIC) atmospheric model profiles calculated at the midpoint location of the propagation paths in the northern wintertime polar region to investigate the radio propagation properties of several high-latitude paths. Different paths showed different responses to the proton precipitation, but propagation modeling was broadly able to account for most of the positive and negative responses observed. Using the SIC-based electron density profiles, we have been able to develop models of ionospheric effective height (h′) and sharpness (β) in order to describe the D region behavior as a function of proton flux, extending previous work which reported β and h′ values as functions of the X-ray flux from solar flares. As a result of these models, our understanding of VLF propagation influenced by SPEs is such that VLF observations might be used to predict changes in the ionospheric D region electron density profiles during other particle precipitation events.
TL;DR: In this article, a methodology to simulate two-dimensional rain rate fields at large scale (1000 × 1000 km2, the scale of a satellite telecommunication beam or a terrestrial fixed broadband wireless access network) is proposed.
Abstract: [1] A methodology to simulate two-dimensional rain rate fields at large scale (1000 × 1000 km2, the scale of a satellite telecommunication beam or a terrestrial fixed broadband wireless access network) is proposed It relies on a rain rate field cellular decomposition At small scale (∼20 × 20 km2), the rain field is split up into its macroscopic components, the rain cells, described by the Hybrid Cell (HYCELL) cellular model At midscale (∼150 × 150 km2), the rain field results from the conglomeration of rain cells modeled by HYCELL To account for the rain cell spatial distribution at midscale, the latter is modeled by a doubly aggregative isotropic random walk, the optimal parameterization of which is derived from radar observations at midscale The extension of the simulation area from the midscale to the large scale (1000 × 1000 km2) requires the modeling of the weather frontal area The latter is first modeled by a Gaussian field with anisotropic covariance function The Gaussian field is then turned into a binary field, giving the large-scale locations over which it is raining This transformation requires the definition of the rain occupation rate over large-scale areas Its probability distribution is determined from observations by the French operational radar network ARAMIS The coupling with the rain field modeling at midscale is immediate whenever the large-scale field is split up into midscale subareas The rain field thus generated accounts for the local CDF at each point, defining a structure spatially correlated at small scale, midscale, and large scale It is then suggested that this approach be used by system designers to evaluate diversity gain, terrestrial path attenuation, or slant path attenuation for different azimuth and elevation angle directions
TL;DR: In this article, the authors report the first determination of ionospheric distortion, comprising the simultaneous characterization of both multipath and Doppler, on wideband, transionospheric VHF (158 MHz) and UHF (422 MHz) signals.
Abstract: [1] To the best of our knowledge, we report the first determination of ionospheric distortion, comprising the simultaneous characterization of both multipath and Doppler, on wideband, transionospheric VHF (158 MHz) and UHF (422 MHz) signals. The measurements took place as part of the test phase of the United Kingdom–United States Wideband Ionospheric Distortion Experiment during the evening (∼1000 UT) of 18 January 2005. This characterization has been achieved using the ALTAIR radar at the Ronald Reagan Ballistic Missile Defense Test Site on Kwajalein Atoll (9.395°N, 167.469°E (12.87°N, 237.16°E corrected geomagnetic)) in the Pacific, in conjunction with a low Earth orbiting, constant radar cross-section, passive satellite (calibration sphere). During the period when the two-way S4 index was above ∼0.8 on both channels, the median coherency times were 43 and 96 ms at VHF and UHF, respectively (at 1.5σ). The corresponding median coherency bandwidths were 0.8 and 2.1 MHz.
TL;DR: A comprehensive set of technical solutions has been developed for the distributed network of digisonde ionospheric sounders to make their data available in a timely and user‐friendly fashion and lessons learned from the system operations are discussed.
Abstract: [1] A comprehensive set of technical solutions has been developed for the distributed network of digisonde ionospheric sounders to make their data available in a timely and user-friendly fashion. In a significant engineering effort, 34 digisondes operating worldwide stand as a modern sensor network with on-site intelligent data interpretation, data delivery to multiple locations for the automated storage and dissemination to end users, and a software suite for interactive data visualization/editing and remote network maintenance and sounding schedule control. The paper discusses lessons learned from the system operations and outlines future efforts.
TL;DR: In this article, a collocated digisonde at the DF site measures the vertical electron density profile and the local ionospheric tilt, providing, in real time, the inputs for the construction of the 3D Ne distribution.
Abstract: [1] High-frequency (HF) direction-finding (DF) systems measure the angles of arrival of signals at selected frequencies. With this information, ray tracing can accurately determine the location of the HF transmitters if the three-dimensional (3-D) electron density (Ne) distribution between the DF site and the transmitters is known. The usual approach is to use an ionospheric model like the International Reference Ionosphere (IRI) as a proxy for the density distribution. We describe a more realistic approach developed in cooperation with Codem Systems in Merrimack, New Hampshire. A collocated digisonde at the DF site measures the vertical electron density profile and the local ionospheric tilt, providing, in real time, the inputs for the construction of the 3-D Ne distribution. The vertical profile is automatically obtained from the Automated Real Time Ionogram Scaler with True Height (ARTIST)-scaled ionogram and the local tilt from the sky maps recorded after each ionogram. The characteristics of each layer, for example, critical frequencies and peak heights, are expressed as a function of latitude λ and longitude Ψ. In the neighborhood of the DF site each characteristic, for example, foF2, is given as foF2(λ, Ψ) = foF2m (1 + C7Δλ + C8ΔΨ) (1 + CλΔλ + CΨΔΨ). The coefficients C7 and C8 for any given azimuth direction are determined with the use of the Union Radio Scientifique Internationale/CCIR coefficients (which are also used in IRI), and the calculation of Cλ and CΨ makes use of the measured ionospheric tilt data; foF2m is the local, measured foF2 value. When the measured density profile and tilt data are available, the derived 3-D density distribution represents the instantaneous ionosphere structure near the site. The numerical ray tracing includes the effects of the magnetic field and properly treats the spitze effect, making the ray-tracing program especially useful for small distances. Ray tracing through simulated tilts shows that the differences in ground distances for one-hop high-frequency (HF) propagation vary from about 1 to 100 km depending on the assumed tilts and distances. Operational tests for distances up to approximately 100 km have demonstrated good results in determining the transmitter location in real time and have illustrated the importance of using the actual ionospheric profiles and tilts in the ray tracing.
TL;DR: In this paper, the diurnal variability of raindrop size distribution (DSD) in precipitating clouds over Kototabang, West Sumatra, Indonesia (0.20°S, 100.32°E), is studied using three types of Doppler radars, operated at VHF (47 MHz), UHF (1.3 GHz), and X band (9.4 GHz) frequencies.
Abstract: [1] The diurnal variability of raindrop size distribution (DSD) in precipitating clouds over Kototabang, West Sumatra, Indonesia (0.20°S, 100.32°E), is studied using three types of Doppler radars, operated at VHF (47 MHz), UHF (1.3 GHz), and X band (9.4 GHz) frequencies. Two precipitation events from 5 to 6 May 2004 in the first observation campaign of the first Coupling Processes in the Equatorial Atmosphere (CPEA-I) project reveal a difference between clouds precipitating in the early afternoon and clouds precipitating in the nighttime. In the early afternoon, the precipitating clouds were dominated by shallow convective types with high rainfall rate at the surface. In the nighttime, precipitating clouds were dominated by stratiform types with small rainfall rate at the surface. A diurnal variation of horizontal wind was observed over this area. The westerly in the lower troposphere and the easterly in the middle troposphere began to be enhanced in the afternoon (1400–1700 LT). DSD parameters were retrieved from VHF band Doppler radar data. A modified gamma distribution was used to model DSD parameters. The shape parameter (μ) was larger during stratiform precipitation than during shallow convective precipitation events, as shown by previous studies. During stratiform rain events on 5 May 2004, the median volume diameter (D0) was dominantly greater than 1 mm, which is larger than D0 during shallow convective rain events. Results presented in this paper indicate that DSD has a diurnal cycle over the mountainous region of Sumatra.
TL;DR: In this article, the results of observations with a combination of high-speed and telescopic imaging to capture high spatial resolution images of sprite streamers and beads at high frame rates, revealing the evolution and propagation of these structures on a decameter scale at millisecond and higher resolution.
Abstract: [1] We report on the results of observations with a combination of high-speed and telescopic imaging to capture high spatial resolution images of sprite streamers and beads at high frame rates, revealing the evolution and propagation of these structures on a decameter scale at millisecond and higher resolution. In July and August 2004, sprites were observed from Langmuir Laboratory, in the mountains of New Mexico, using a >1000 frames-per-second intensified CCD imager mounted to a Dobsonian reflecting telescope. We present a number of examples of sprite features, along with photometric data on sprites and sprite halos, taken with the Wide-angle Array for Sprite Photometry (WASP). Results show a variety of structures, including evidence of formation and evolution of both streamers and beads. Examples and statistics presented indicate that most bead structures have sizes of 10–300 m, similar to previous telescopic observations, and endure typically for a few milliseconds to 10 ms, with rare cases of up to 50 ms. Similarly, streamer-like structures are observed to have diameters of 10–300 m but persist for shorter timescales, typically 2–3 ms, with occasional cases of up to 10 ms. Attempts to measure propagation of streamers indicate that higher frame rates are required in order to observe speeds on the order of previous wide-field-of-view observations and modeling predictions.
TL;DR: In this article, the meteor radar system, the data detection and collection process, and the postprocessing software that was developed to extract information from the meteor echoes collected with the interferometer that is part of the radar system are described.
Abstract: [1] A VHF meteor radar system was installed at the geographical South Pole in 2001. The purpose of this system is to measure the horizontal wind field in the mesosphere–lower thermosphere (MLT) region and to understand the large-scale dynamics of the Antarctic polar region. The radar operated for a few months in 2001 and with minor interruptions since that time. In this paper we will describe the meteor radar system, the data detection and collection process, and the postprocessing software that was developed to extract information from the meteor echoes collected with the interferometer that is part of the radar system. Finally, the main features of the meteor distribution will be presented and discussed. Our results show that the meteor activity peaks during the Antarctic summer. Furthermore, it occurs mostly in a small region around the ecliptic plane roughly ∼20° wide in terms of elevation angle spread.