Showing papers in "Radio Science in 2011"

Statistics of total electron content depletions observed over the South American continent for the year 2008

Abstract: [1] This paper presents for the first time regional plots of total electron content (TEC) depletions derived from GPS observations over the South American continent with a coverage of over 45° longitude (i.e., 35°W to 80°W). We introduce a new numerical algorithm that has been developed to automatically detect TEC bite-outs that are produced by the transit of equatorial plasma bubbles. This algorithm was applied to TEC values measured by the Low Latitude Ionospheric Sensor Network (LISN) and by receivers that belong to 3 other networks that exist in South America. The general characteristics of the TEC depletions are provided along with their temporal length, local time distribution and depletion depth. The regional day-to-day and seasonal variability of the TEC depletions are also presented for 2008, a year of low solar activity. The regional day-to-day variability of TEC depletions is highly dynamic, but their seasonal distributions follow the longitudinal characteristics of plasma bubbles presented by other authors. During the equinoxes, TEC depletions are mainly observed on the west coast of South America, and during the December solstice they mostly occur on the east side of the continent. However, in all seasons, we observe days when depletions extend all over the continent. We place these new results in the context of theories of plasma bubble seeding.

Total electron content models and their use in ionosphere monitoring

Abstract: At L band frequencies used in Global Navigation Satellite Systems (GNSS), the ionosphere causes signal delays that correspond with link related range errors of up to about 100 meters In a first order approximation the range error is proportional to the integral of the electron density along the ray path (Total Electron Content - TEC) Whereas this error can be corrected in dual frequency measurements by a simple linear combination of L1 and L2 phases, single frequency measurements need additional information for mitigating the ionospheric error This information can be provided by TEC maps deduced from corresponding GNSS measurements or by model values In this talk we discuss the development and use of background models for reconstructing reliable TEC maps distributed via an operational space weather and ionosphere data service (http://swaciwebdlrde ) to the international community, To reconstruct TEC over a selected region we assimilate the observation data into the specific background model of TEC This approach has the advantage that in case of only a few measurements or even in case of total loss of input data, the operational data service is maintained by providing model values Since ground based GNSS data are often uneven distributed, the inclusion of a background model in the TEC reconstruction helps to overcome such data gaps which naturally occur over the oceans The Neustrelitz TEC Model (NTCM) is a basic approach for a family of regional and a global TEC models The model approximates TEC variations depending on the input of location, local time and solar activity The model coefficients are deduced from calibrated TEC measurements by least squares methods The European TEC model NTCM-EU is a polynomial with 60 coefficients Since the European monitoring and modelling activities started in 1995, the data cover more than a full solar cycle, thus forming a data set that is needed for developing a full solar cycle TEC model Reported are also regional models from both polar areas as well as a global TEC model All these models serve as background models in which observation data are assimilated Operational TEC maps primarily deduced from European IGS and EUREF GNSS data area are available via the SWACI service at an update rate of 5 minutes Accuracy of various background models and corresponding TEC maps obtained after data assimilation are compared with other models and reconstructions such as the Klobuchar and NeQuick models and IGS TEC maps

Localization, tracking, and imaging of targets in wireless sensor networks: An invited review

Abstract: [1] Wireless sensor networks (WSNs) have shown many attractive features in a lot of real-world applications that motivate their rapid and wide diffusion. One of the most challenging topics when dealing with WSNs is the localization and tracking of objects from measurements collected by the nodes themselves. Once distributed in a region without the knowledge of their positions, the nodes actively take part in the localization of the network as well as to the detection and monitoring of the presence and movements of targets lying within the sensed area. This paper reviews state-of-the-art systems and approaches developed for WSN-based localization and tracking of active as well as passive targets. The main focus is on systems that exploit the strength of the received signal, always available at the WSN nodes, without ad hoc or additional hardware. Recent strategies for WSN-based imaging are discussed as well.

GPS radio occultation for climate monitoring and change detection

Abstract: [1] Observation of the atmospheric climate and detection of changes require high quality data. Radio Occultation (RO) using Global Positioning System (GPS) signals is based on time measurements with precise atomic clocks. It provides a long-term stable and consistent data record with global coverage and favorable error characteristics. Highest quality and vertical resolution is given in the upper troposphere and lower stratosphere (UTLS). RO data exist from the GPS/Met mission within 1995–1997, and continuous observations are available since 2001. We give a review on studies using RO data for climate monitoring and change detection in the UTLS and discuss RO characteristics and error estimates, climate change indicators, trend detection, and comparison to conventional upper-air data. These studies showed that RO parameters cover the whole UTLS with useful indicators of climate change, being most robust in the tropics. Refractivity is most sensitive in the lower stratosphere (LS) and tropopause region, pressure/geopotential height and temperature over the UTLS region. An emerging climate change signal in the RO record can be detected for geopotential height of pressure levels and for temperature, reflecting warming of the troposphere and cooling of the LS. The results are in agreement with trends in radiosonde and ERA-Interim records. Climate model trends basically agree as well but they show less warming/cooling contrast across the tropical tropopause. (Advanced) Microwave Sounding Unit LS bulk temperature anomalies show significant differences to RO. Overall, the quality of RO climate records is suitable to fulfill the requirements of a global climate change monitoring system.

A review on array mutual coupling analysis

Abstract: An overview about mutual coupling analysis in antenna arrays is given. The relationships between array impedance matrix and embedded element patterns, including beam coupling factors, are reviewed while considering general-type antennas; approximations resulting from single-mode assumptions are pointed out. For regular arrays, a common Fourier-based formalism is employed, with the array scanning method as a key tool, to explain various phenomena and analysis methods. Relationships between finite and infinite arrays are described at the physical level, as well as from the point of view of numerical analysis, considering mainly the method of moments. Noise coupling is also briefly reviewed.

GNSS-R ground-based and airborne campaigns for ocean, land, ice, and snow techniques: Application to the GOLD-RTR data sets

Abstract: [1] Several ground-based and airborne data sets taken with the Global Navigation Satellite System Reflectometry (GNSS-R) technique are made available to the research community. This paper reviews the potential applications of these bistatic radar observations, including a list of possible approaches and algorithms described in the literature for oceanic measurements (altimetric and scatterometric), soil moisture sensing, and sea ice and snow characterization. A list of applicable models complements the review. The paper continues with descriptions of the campaigns included in the initial data set, together with the basic information required to understand the instrumental issues of the data. Finally, some parameters and observables provided in the data are detailed.

Bipolar climatology of GPS ionospheric scintillation at solar minimum

Abstract: [1] High-rate sampling data of Global Navigation Satellite Systems ionospheric scintillation acquired by a network of GPS Ionospheric Scintillation and TEC Monitor receivers located in the Svalbard Islands, in Norway and in Antarctica have been analyzed. The aim is to describe the “scintillation climatology” of the high-latitude ionosphere over both the poles under quiet conditions of the near-Earth environment. For climatology we mean to assess the general recurrent features of the ionospheric irregularities dynamics and temporal evolution on long data series, trying to catch eventual correspondences with scintillation occurrence. In spite of the fact that the sites are not geomagnetically conjugate, long series of data recorded by the same kind of receivers provide a rare opportunity to draw a picture of the ionospheric features characterizing the scintillation conditions over high latitudes. The method adopted is the Ground Based Scintillation Climatology, which produces maps of scintillation occurrence and of total electron content relative variation to investigate ionospheric scintillations scenario in terms of geomagnetic and geographic coordinates, interplanetary magnetic field conditions and seasonal variability. By means of such a novel and original description of the ionospheric irregularities, our work provides insights to speculate on the cause-effect mechanisms producing scintillations, suggesting the roles of the high-latitude ionospheric trough, of the auroral boundaries and of the polar cap ionosphere in hosting those irregularities causing scintillations over both the hemispheres at high latitude. The method can constitute a first step toward the development of new algorithms to forecast the scintillations during space weather events.

Comparative testing of four ionospheric models driven with GPS measurements

Abstract: In the context of the European Space Agency/European Space Operations Centre funded Study A¢Â�Â�GNSS Contribution to Next Generation Global Ionospheric Monitoring,A¢Â�Â� four ionospheric models based on GNSS data (the Electron Density Assimilative Model, EDAM; the Ionosphere Monitoring Facility, IONMON v2; the Tomographic Ionosphere model, TOMION; and the Neustrelitz TEC Models, NTCM) have been run using a controlled set of input data. Each model output has been tested against differential slant TEC (dSTEC) truth data for high (May 2002) and low (December 2006) sunspot periods. Three of the models (EDAM, TOMION, and NTCM) produce dSTEC standard deviation results that are broadly consistent with each other and with standard deviation spreads of ~1 TECu for December 2006 and ~1.5 TECu for May 2002. The lowest reported standard deviation across all models and all stations was 0.99 TECu (EDAM, TLSE station for December 2006 night). However, the model with the best overall dSTEC performance was TOMION which has the lowest standard deviation in 28 out of 52 test cases (13 stations, two test periods, day and night). This is probably related to the interpolation techniques used in TOMION exploiting the spatial stationarity of vertical TEC error decorrelation.

Refractivity estimation from sea clutter: An invited review

Abstract: [1] Non-standard radio wave propagation in the atmosphere is caused by anomalous changes of the atmospheric refractivity index. In recent years, refractivity from clutter (RFC) has been an active field of research to complement traditional ways of measuring the refractivity profile in maritime environments which rely on direct sensing of the environmental parameters. Higher temporal and spatial resolution of the refractivity profile, together with a lower cost and convenience of operations have been the promising factors that brought RFC under consideration. Presented is an overview of the basic concepts, research and achievements in the field of RFC. Topics that require more attention in future studies also are discussed.

Review of crop growth and soil moisture monitoring from a ground-based instrument implementing the Interference Pattern GNSS-R Technique

Abstract: [1] Reflectometry using Global Navigation Satellite Systems signals (GNSSR) has been the focus of many studies during the past few years for a number of applications over different scenarios as land, ocean or snow and ice surfaces. In the past decade, its potential has increased yearly, with improved receivers and signal processors, from generic GNSS receivers whose signals were recorded in magnetic tapes to instruments that measure full Delay Doppler Maps (the power distribution of the reflected GNSS signal over the 2-D space of delay offsets and Doppler shifts) in real time. At present, these techniques are considered to be promising tools to retrieve geophysical parameters such as soil moisture, vegetation height, topography, altimetry, sea state and ice and snow thickness, among others. This paper focuses on the land geophysical retrievals (topography, vegetation height and soil moisture) performed from a ground-based instrument using the Interference Pattern Technique (IPT). This technique consists of the measurement of the power fluctuations of the interference signal resulting from the simultaneous reception of the direct and the reflected GNSS signals. The latest experiment performed using this technique over a maize field is shown in this paper. After a review of the previous results, this paper presents the latest experiment performed using this technique over a maize field. This new study provides a deeper analysis on the soil moisture retrieval by observing three irrigation-drying cycles and comparing them to different depths soil moisture probes. Furthermore, the height of the maize, almost 300 cm, has allowed testing the capabilities of the technique over dense and packed vegetation layers, with high vegetation water content.

Data ingestion into NeQuick 2

Abstract: [1] NeQuick 2 is the latest version of the NeQuick ionosphere electron density model developed at the Aeronomy and Radiopropagation Laboratory of the Abdus Salam International Centre for Theoretical Physics (ICTP) - Trieste, Italy with the collaboration of the Institute for Geophysics, Astrophysics and Meteorology of the University of Graz, Austria. It is a quick-run model particularly designed for trans-ionospheric propagation applications that has been conceived to reproduce the median behavior of the ionosphere. To provide 3-D specification of the ionosphere electron density for current conditions, different ionosphere electron density retrieval techniques based on the NeQuick adaptation to GPS-derived Total Electron Content (TEC) data and ionosonde measured peak parameters values have been developed. In the present paper the technique based on the ingestion of global vertical TEC map into NeQuick 2 will be validated and an assessment of the capability of the model to reproduce the ionosphere day-to-day variability will also be performed. For this purpose hourly GPS-derived global vertical TEC maps and hourly foF2 values from about 20 ionosondes corresponding to one month in high solar activity and one month in low solar activity period will be used. Furthermore, the first results concerning the ingestion of space-based GPS-derived TEC data will be presented.

Numerical modeling of ionospheric effects in the middle‐ and low‐latitude F region during geomagnetic storm sequence of 9–14 September 2005

Abstract: [1] This study presents the Global Self-Consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP) numerical simulations of the 9–14 September 2005 geomagnetic storm effects in the middle- and low-latitude ionosphere. Recent modifications to the GSM TIP model include adding an empirical model of high-energy electron precipitation and introducing a high-resolution (1 min) calculation of region 2 field-aligned currents and a cross-cap potential difference. These modifications resulted in better representation of such effects as penetration of the magnetospheric convection electric field to lower latitudes and the overshielding. The model also includes simulation of solar flare effects. Comparison of model results with observational data at Millstone Hill (42.6°N, 71.5°W, USA), Arecibo (18.3°N, 66.8°W, Puerto Rico), Jicamarca (11.9°S, 76.9°W, Peru), Palmas (10.2°S, 48.2°W, Brazil), and San Jose Campos (23.2°S, 45.9°W, Brazil) shows good agreement of ionospheric disturbances caused by this storm sequence. In this paper we consider in detail the formation mechanism of the additional layers in an equatorial ionosphere during geomagnetic storms. During geomagnetic storms, the nonuniform in height zonal electric field is generated at the geomagnetic equator. This electric field forms the additional layers in the F region of equatorial ionosphere.

Three months of local sea level derived from reflected GNSS signals

Abstract: By receiving Global Navigation Satellite System (GNSS) signals that are reflected off the sea surface, together with directly received GNSS signals (using standard geodetic‐type receivers), it is possible to monitor the sea level using regular single difference geodetic processing. We show results from our analysis of three months of data from the GNSS‐based tide gauge at the Onsala Space Observatory (OSO) on the west coast of Sweden. The GNSS-derived time series of local sea level is compared with independent data from two stilling well gauges at Ringhals and Gothenburg about 18 km south and 33 km north of OSO, respectively. A high degree of agreement is found in the time domain, with correlation coefficients of up to 0.96. The root‐mean‐square differences between the GNSS‐derived sea level and the stilling well gauge observations are 5.9 cm and 5.5 cm, which is lower than for the stilling well gauges together (6.1 cm). A frequency domain comparison reveals high coherence of the data sets up to 6 cycles per day, which corresponds well to the propagation of gravity waves in the shallow waters at the Kattegat coast. Amplitudes and phases of some major tides were determined by a tidal harmonic analysis and compared to model predictions. From the GNSS‐based tide gauge results we find significant ocean tidal signals at fortnightly, diurnal, semi‐diurnal, and quarter‐diurnal periods. As an example, the amplitudes of the semi‐diurnal M2 and the diurnal O1 tide are determined with 1σ uncertainties of 11 mm and 12 mm, respectively. The comparison to model calculations shows that global ocean tide models have limited accuracy in the Kattegat area.

Global prediction of the vertical total electron content of the ionosphere based on GPS data

Abstract: [1] Although vertical total electron content (VTEC) forecasting is still an open subject of research, the use of predictions of the ionospheric state at a scale of several days is an area of increased interest. A global VTEC forecast product for two days ahead, which is based exclusively on actual Global Positioning System (GPS) data, has been developed in the frame of the International Global Navigation Satellite Systems (GNSS) Service (IGS) Ionospheric Working Group (IGS Iono‐WG). The UPC ionospheric VTEC prediction model is based on the Discrete Cosine Transform (DCT), which is widely used in image compression (for instance, in JPEG format). Additionally, a linear regression module is used to forecast the time evolution of each of the DCT coefficients. The use of the DCT coefficients is justified because they represent global features of the whole two‐ dimensional VTEC map/image. Also, one can therefore introduce prior information affecting the VTEC, for instance, smoothness or the distribution of relevant features in different directions. For this purpose, the use of a long time series of final/rapid UPC VTEC maps is required. Currently, the UPC Predicted product is being automatically generated in test mode and is made available through the main IGS server for public access. This product is also used to generate two days ahead preliminary combined IGS Predicted product. Finally, the results presented in this work suggest that the two days ahead UPC Predicted product could become an official IGS product in the near future.

Estimating ionospheric delay using kriging: 1. Methodology

Abstract: [1] The Wide Area Augmentation System (WAAS) is an augmentation of the Global Positioning System (GPS) that provides safe and reliable use of GPS signals for airline navigation over much of North America. Ever since WAAS was first commissioned in July of 2003, the vertical delay estimate at each node in the WAAS ionospheric grid has been determined from a planar fit of slant delay measurements, projected to vertical using an obliquity factor specified by the standard thin shell model of the ionosphere. In a future WAAS upgrade (WAAS Follow-On Release 3), however, the vertical delay will be estimated by an established, geo-statistical technique known as kriging. When compared to the planar fit model, the kriging model is generally found to match better the observed random structure of the vertical delay. This paper presents the kriging methodology to be used to estimate the vertical delay and its uncertainty at each ionospheric grid point. In addition, it provides examples of the improvement in delay accuracy achieved. Under disturbed conditions, the implementation of kriging reduces the magnitude of the root mean square fit residual by up to 15%.

A new global empirical NmF2 model for operational use in radio systems

Abstract: [1] The ionospheric F2 region around the peak electron density height hmF2 of about 250–500 km causes the most pronounced impact on transionospheric radio wave propagation. Therefore, the peak electron density of the F2 layer NmF2 is a key parameter for characterizing the ionosphere. We present an empirical model approach that allows determining global NmF2 with a limited number of model coefficients. The nonlinear approach needs 13 coefficients and a few empirically fixed parameters for describing the NmF2 dependencies on local time, geographic/geomagnetic location and solar irradiance and activity. The model approach is applied to a vast quantity of global NmF2 data derived from GNSS radio occultation measurements by CHAMP, GRACE and COSMIC satellite missions and about 60 years of processed NmF2 data from 177 worldwide ionosonde stations. The model fits to these input data with the same standard and root mean squared (RMS) deviations of 2 × 1011 m−3. The proposed Neustrelitz global NmF2 model (Neustrelitz Peak Density Model - NPDM) is climatological, i.e., the model describes the average behavior under quiet geomagnetic conditions. A preliminary comparison with the electron density NeQuick model reveals similar results for NmF2 with RMS deviations in the order of 2 × 1011 m−3 and 5 × 1011 m−3 for low and high solar activity conditions, respectively.

Assimilation of autoscaled data and regional and local ionospheric models as input sources for real-time 3-D International Reference Ionosphere modeling

Abstract: [1] This paper describes how the joint utilization of autoscaled data such as the F2 peak critical frequency foF2, the propagation factor M(3000)F2, and the electron density profile coming from two reference ionospheric stations (Rome and Gibilmanna), and the regional (Simplified Ionospheric Regional Model Updated) and global (International Reference Ionosphere) ionospheric models can provide a valid tool for obtaining a real-time three-dimensional (3-D) electron density mapping of the ionosphere. Preliminary results of the proposed 3-D model are shown by comparing the electron density profiles given by the model with the ones measured at three testing ionospheric stations (Athens, Roquetes, and San Vito).

A newly developed UHF radiotelescope for interplanetary scintillation observations: Solar Wind Imaging Facility

Abstract: [1] A large-aperture radiotelescope called the Solar Wind Imaging Facility (SWIFT) has been developed at the Toyokawa Observatory of the Solar-Terrestrial Environment Laboratory (STEL), Nagoya University. The SWIFT is dedicated to interplanetary scintillation (IPS) observations of the solar wind at 327 MHz, the same frequency as that of the existing STEL IPS system. The aim of this instrument is to improve the spatial and temporal resolutions of tomographic reconstructions from STEL IPS observations by increasing the number of usable lines of sight within a given time period. The SWIFT consists of a pair of asymmetric cylindrical parabolic reflector antennas with an aperture size of 108 m (N-S) by 19 m (E-W), and a 192-element phased array receiver system which forms a single beam steerable between 60°S and 30°N with respect to the zenith. Since the antenna beam is fixed in the local meridian, IPS observations are taken around the time of meridian transit for each source. The performance of the SWIFT has been tested using preliminary observations for strong discrete sources and diffuse galactic background.

Multiple phase screen modeling of ionospheric scintillation along radio occultation raypaths

Abstract: [1] We present the Radio Occultation Scintillation Simulator (ROSS), which uses the multiple phase screen method (MPS) to simulate the forward scatter of radio waves by irregularities in the equatorial ionosphere during radio occultation experiments. ROSS simulates propagation through equatorial plasma bubbles which are modeled as homogeneous electron density fluctuations modulated by a Chapman profile in altitude and a Gaussian window in the magnetic east-west direction. We adjust the parameters of the density model using electron density profiles derived from the ALTAIR incoherent scatter radar (9.4°N, 167.5°E, 4.3° north dip), and space-to-ground observations of scintillation using VHF and GPS receivers that are colocated with the radar. We compare the simulated occultation scintillation to observations of scintillation from the CORISS instrument onboard the C/NOFS satellite during a radio occultation occurring near ALTAIR on 21 April 2009. The ratio of MPS predicted S4 to CORISS observed S4 throughout the F region altitudes of 240–350 km ranged between 0.86 and 1.14.

Statistics of GPS scintillations over South America at three levels of solar activity

Abstract: [1] This study characterizes low-latitude scintillations at L-band frequency in South America on daily, monthly, and seasonal time scales at three levels of solar activity: high, moderate and low levels. Three years (November 2001–October 2002, 2004 and 2008) of amplitude scintillation data from GPS receivers at three locations: Cuzco (14.0°S, 73.0°W, dip 1.0°S), Iquitos (3.8°S, 73.2°W, dip 7.0°N), and Bogota (4.4°N, 74.1°W, dip 16.0°N) were used for the investigation. These data were grouped into daily, monthly, and seasonal sets. We introduced tests on the data to reject signal fluctuations from non-ionospheric sources, such as multipath from terrestrial objects. The scintillation events from the data were further classified into three levels: weak (0.3 ≤ S4 < 0.4), moderate (0.4 ≤ S4 < 0.7), and intense (S4 ≥ 0.7) scintillations and their monthly percentage of occurrences and durations were determined. We also used the seasonal averages of daily (1900 LT–2000 LT) TEC values to observe the variability of the equatorial anomaly using a chain of ten GPS receivers that are located at a latitudinal span of 10°N–40°S along the west coast of South America. This study concludes that GPS scintillations in this longitude sector are post-sunset events and decay before or around local midnight, with intense activity and longer durations in the months of March and January. On seasonal time scales, the highest frequencies and longest durations of events were recorded during March Equinox and December Solstice. The months of May–July (June Solstice) had the least frequencies of occurrence and durations of events. Scintillation activity also increases with solar activity. Finally, scintillations increase in frequencies of occurrence and durations from Cuzco (near the magnetic equator) toward Bogota (near the crest of the equatorial anomaly) during solar maximum. However, a gradual collapse of the anomaly crest, away from Bogota toward Iquitos was observed as solar activity decreases, and as a result, the occurrence frequencies of scintillations at Iquitos increase relative to those at Bogota.

Observation of planetary radio emissions using large arrays

Abstract: [1] Planetary radio astronomy mostly concerns plasma phenomena at low frequencies (i.e., below a few hundred MHz). The low frequency limit for ground-based observations of these phenomena is given by the Earth's ionosphere, which limits ground based radio observations to frequencies ≥10 MHz. We give an overview and update on the status of a few representative ground-based radio arrays that have been used for planetary studies within the frequency range 10–200 MHz, and we discuss their potential for the four types of planetary radio emissions that can be observed within this frequency range: (1) synchrotron emission from Jupiter's radiation belts, (2) radio bursts caused by solar system planetary lightning, (3) Jupiter's magnetospheric emission, and (4) magnetospheric radio emission from extrasolar planets, for which we also give an update to previous predictive studies. Comparing the four emission modes with the characteristics of existing ground-based radio arrays, we show that the Low Frequency Array (LOFAR) has the potential to bring considerable advances to those four fields of planetary radio science.

Full polarimetric beam‐forming algorithm for through‐the‐wall radar imaging

Abstract: [1] Development of an imaging algorithm that accurately models the wave propagation physical process has become an important topic in through-the-wall radar imaging (TWRI). In this paper, a full polarimetric beam-forming algorithm for through-the-wall radar imaging for a general multilayer wall case is presented and applied to various 2D and 3D simulated and measured scenarios. Polarimetric TWRI not only enhances the target characterization but also mitigates the wall ringing effect in cross polarizations. The far field layered medium Green's function is incorporated in the proposed TWRI algorithm for the quad-polarization of the target returns, namely, VV, HV, VH and HH. Due to the incorporation of the layered medium Green's function, the imaging algorithm not only takes into account the wall reflection, bending, and delay effects but also accounts for the complex scattering mechanism due to the presence of the wall. Numerical and experimental results show that the proposed full polarimetric beam former can provide high quality focused images in various wall-target scenarios, in particular, when the technique is combined with a wall parameter estimation technique.

On the accuracy of approximate techniques for the evaluation of lightning electromagnetic fields along a mixed propagation path

Abstract: In this paper we review simplified analytical expressions derived by Wait using the concept of attenuation function for the analysis of the propagation of lightning-radiated electromagnetic fields over a mixed propagation path (vertically stratified ground) Two different formulations proposed by Wait that depend on the relative values of ground surface impedances are discussed It is shown that both formulations give nearly the same results for the time domain electric field However, depending on the values of the normalized surface impedance for each ground section, the use of one of the two formulations is computationally more efficient The accuracy of the Wait formulations was examined taking as reference full-wave simulations obtained using the finite difference time domain technique It is shown that Wait's simplified formulas are able to reproduce the distant field peak and waveshape with a good accuracy

A new approach for identifying ionospheric backscatter in midlatitude SuperDARN HF radar observations

Abstract: [1] The Super Dual Auroral Radar Network (SuperDARN) is a network of HF radars that are traditionally used for monitoring phenomena in the Earth's ionosphere at high latitudes The radar backscatter is due primarily to reflections from plasma irregularities in the ionosphere, known as ionospheric scatter, and to signal reflected from the ground, known as ground scatter In recent years, SuperDARN has expanded to midlatitudes to provide improved coverage of the auroral region during times of enhanced geomagnetic activity In addition to high-speed auroral flows, the radars commonly see a variety of low-velocity plasma drift associated with the quiet time midlatitude ionosphere The traditional method of distinguishing between scatter types in SuperDARN data was developed for high latitudes and depends solely on the Doppler velocity and Doppler spectral width of each data point This method has proven inadequate for identifying quiet time midlatitude ionospheric scatter In this paper, we present a new technique for the classification of SuperDARN data, which operates on a distributed range time basis and involves procedures similar to “depth first search” Using the new method for classification of ground and ionospheric scatter, we show a dramatic improvement in the determination of ionospheric scatter within extended (>1 h) events Compared to the traditional method, the number of ionospheric measurements resolved increases by more than 50% The new classification algorithm identifies discrete events of ionospheric scatter and can be applied to statistical analysis of event occurrence and characteristics

Evaluation of the orbit altitude electron density estimation and its effect on the Abel inversion from radio occultation measurements

Abstract: [1] In this paper, the observations from CHAMP radio occultation (RO) and Planar Langmuir Probe (PLP) during 2002–2008 and Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) observations during 2007.090–2007.120 are used to evaluate the orbit altitude electron density estimation and its effect on the Abel inversion from RO measurements. Comparison between PLP observed and RO estimated orbit electron density on board CHAMP shows that RO estimation tends to overestimatethetrueorbitelectrondensityby10%averagely.Theaveragerelativedeviation is ∼20% and decreases slightly with the increase of the ionospheric peak height and the satellite orbit. It is larger at nighttime than daytime and peaks around sunrise time. Simulations based on COSMIC observations using NeQuick model indicate that the solar activity and the satellite orbit altitude variations will not influence the ratio of the successfully retrieved electron density profiles to the observed occultation events and the relative Abel inversion error of the electron density as well. Different orbit electron density derivation methods, including estimation by the RO total electron content, given by an independent onorbitobservation, andassumed to beequalto thetopmostpoint, willhaveno essential influence on the Abel retrieved electron density. Adding an on orbit observation even has a negative effect on the Abel retrieved electron density around the orbit altitude, which is contrary to our imagination.

Comparison of spherical harmonic and B spline models for the vertical total electron content

Abstract: [1] Most of the geodetic ionosphere models describe the electron density of the Earth's atmosphere by global maps of the vertical total electron content (VTEC) for fixed time intervals (e.g., 2 h) assuming a single-layer model and using a spherical harmonic expansion up to a specific degree n. However, it is well known that spherical harmonic models are not suited for representing data of heterogeneous density and quality. As a consequence, data gaps cannot be handled appropriately. In this paper we present a different approach for modeling VTEC generally depending on space and time defining the 3-D system of base functions as tensor products of trigonometric B spline functions for the longitude and two sets of endpoint-interpolating B spline functions for latitude and time, respectively. We compare this approach to a spherical harmonic expansion with similar resolution and show how data gaps influence the accuracy of VTEC maps even in areas with good data coverage.

On the use of the method of moments in plasmonic applications

Abstract: [1] In the last 40 years the method of moments (MOM) has been a cornerstone in the field of computational electromagnetics. Traditionally, this method has been used to solve integral equations formulated for antennas and other components in the microwave frequency range and below. In this paper, the application of MOM in the field of plasmonics is briefly reviewed. First, existing literature is referenced. Then the differences with the classical implementations are pointed out. Finally, its applicability, advantages, and disadvantages are discussed. This is done by comparing it with a numerical finite difference time domain solver well-known in the plasmonics research community for a number of example structures. It is shown that also at these higher frequencies, namely in the IR and optical range, MOM is a very powerful technique.

Role of thermosphere‐ionosphere coupling in a global ionospheric specification

Abstract: [1] In this paper we present our recent research development in the area of ionospheric specification by means of data assimilation of ground-based observations NOAA's Total Electron Content specification methodology (namely, a Gauss-Markov Kalmanfilter with an empirical model of the ionosphere as a background model) over the continental United States has lately been expanded to the multiregional domains and to the entire globe Analyses of the global TEC maps reveal clear signatures of thermosphere-ionosphere coupling, in both dynamical and compositional nature, even though the underlying specification methodology does not take thermospheric effects into account This suggests that ground-based observations of electron density contain some information about the state of the thermosphere By using a thermosphere ionosphere general circulation model in a prototype Ensemble Kalman filter (EnKF), we examine the role of thermosphere-ionosphere coupling in a global ionospheric specification Observing system simulation experiments, designed for a global network of ionosondes, suggest that ionospheric data assimilation considerably benefits from self-consistent treatment of thermosphere-ionosphere coupling in a forecast model as well as in assimilation schemes, both of which can be achieved inherently by using the EnKF

Attenuation of radio signals by the ionosphere of Mars: Theoretical development and application to MARSIS observations

Abstract: [1] We investigate the ionospheric conditions required to explain Mars Express Mars Advanced Radar for Subsurface and Ionosphere Sounding topside radar sounder observations of ionospheric attenuation in excess of 13 dB at 5 MHz during solar energetic particle events. We develop theoretical expressions for the attenuation caused by a layer of ionospheric plasma in cases of high, intermediate, and low radio frequency relative to the electron-neutral collision frequency at the ionospheric layer. We apply these relationships to four layers: the M2 layer produced at 120 km by solar extreme ultraviolet photons, the M1 layer produced at 100 km by solar X-ray photons and associated electron impact ionization, the meteoric layer produced at 85 km by meteoroid ablation, and a putative plasma layer produced at 35 km by cosmic rays. Attenuation is weaker in the M2 layer than in the M1 layer. Attenuation in the M1 and meteoric layers are comparable, although their properties are quite variable. The greatest attenuation for radio frequencies above 50 MHz occurs in the predicted plasma layer at 35 km, but its effects are relatively small at lower frequencies. If optimally located with a peak altitude of 50 km, a layer with a peak plasma density of 109 m−3 is sufficient to explain the observed 13 dB attenuation. Although the electron densities produced by solar energetic particle events at Mars have not been directly simulated, the required electron densities are plausible. However, the altitude at which solar energetic particles produce plasma is uncertain.

Analytical synthesis technique for linear uniform-amplitude sparse arrays

Abstract: A novel analytical approach to the synthesis of linear sparse arrays with uniform?amplitude excitation is presented and thoroughly discussed in this paper. The proposed technique, based on the auxiliary array factor concept, is aimed at the deterministic determination of the optimal array element density and excitation phase tapering distributions useful to mimic a desired radiation pattern. In particular, the developed antenna placement method does not require any iterative or stochastic optimization procedure, resulting in a dramatic reduction of the antenna design times (typically on the order of a few seconds). Antenna mutual coupling is not taken into account within the developed methodology; hence, to assess the sensitivity of the design procedure to such nonideality, a dedicated numerical investigation has been carried out by using a rigorous full?wave electromagnetic field prediction technique. The obtained results prove the effectiveness and the versatility of the proposed array synthesis approach even in operative scenarios where a significant deviation from desired antenna operation is observed.